Environ. Sci. Techno/. 1995, 29, 960-974
Introduction
B R I A N L. H O W E S * A N D JOHN M . T E A L Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Nutrient fluxes in surface waters and processes controlling nutrient retention and loss were measured through 1 yr in a 15-ha cranberry bog discharging to a shallow embayment of Buzzards Bay, MA. Continuous measurements of streamflow and nutrient concentrations, periodic measures of nutrient release by sediments, inputs from rainfall, and exchanges with plant pools were used to construct a nitrogen balance for the bog and to estimate dissolved phosphate fluxes. Nutrient losses were small during the growing season and greatest in the fall with plant senescence and harvest. Of nitrogen inputs to the bog, 33% was fertilizers, 8% was rain, and 59% was in inflowing streamwaters. While the bog plants and sediments were net sinks for nitrogen, the fertilizers associated with cranberry agriculture resulted in the bog serving as a source of nitrogen to outflowing streamwaters discharging to the adjacent bay. Net losses of 25 kmol of N were almost completely to the outflowing stream (93%) with harvested berries and leaves accounting for 5% and 2%, respectively. NH4+ accounted for most of the N loss to streamflow, 53%. Annual Po43- losses of 4.8 kmol were associated mainly with reduced soil oxidation due to flooding for harvest and frost protection. Net losses of total N, 1.7 kmol ha-’ yr-l, or DIN PON, 1.8 kmol ha-’ yr-l, were similar to a surface water-dominated freshwater wetland but much lower than the nitrogen contribution from an adjacent residential development.
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Eutrophication of nearshore waters resulting from nutrient inputs from upland sources is a major threat to estuarine and coastal ecosystems (1-3). Watershed land-use analysis is one method for evaluating upland nutrient sources and developing coastal nutrient management plans (4,5). This approach determines nutrient loading through land-use mapping and quantitative data on nutrient fluxes for each land-use component. Agriculture can be an important nonpoint source of nutrients in the eutrophication of coastal bays and estuaries (4, 6, 7). While nutrient outputs from cultivated upland fields have been intensively investigated (cf. ref 81,quantitative estimates of nutrient retention and loss from smaller scale activities like cranberry wetland farming are lacking. Cranberry agriculture typically consists of small individual units of less than 50 ha, which collectively can cover significant areas of individual watersheds. Cranberry agriculture is concentrated in northern regions of the United States with significant production in Massachusetts, Wisconsin, New Jersey, Washington, and Oregon with lesser contributions throughout the coastal states of New England as well as in several Canadian provinces (9). Cranberry agriculture is based upon the cultivation of the wild American cranberry, Vaccinium macrocarpon, a wetland evergreen plant. The majority of the older lowland cranberry bogs were created from kettle hole swamps, frequently cedar or red maple, or other organic rich lowlying areas (9). As a result of their peat and clay basement materials,lowland bogs have elevated water tables, minimal hydraulic connection to groundwater, and losses primarily through surface water flows (10). Bogs usually have a drainage network that discharges to a lake or coastal waters within at most a few kilometers. Cranberry bogs are best described as cultivated wetlands, although highly modified from natural wetlands and managed so that the plants are growing in drained surficial soils. Bogs are heavily irrigated during the most active growing season and are periodically flooded in connection with pest control, harvesting, and frost protection (9).As a result, the sediments have intervals of soil saturation, typical of natural wetlands. Therefore, it is likely that cranberry bogs may act as other wetlands in modlfylng nutrients transported through them. On a seasonal or annual basis, it is possible that bogs may reduce the nutrient load in waters entering from upstream sources. It is also possible that bogs serve as sources of nutrients to downstream ecosystems due to the additions of N and P in fertilizers. Therefore,in regionswith cranberry agriculture, bogs present an unknown in watershed nutrient balances. In coastal areas, there is currently concern over their potential role in the eutrophication of shallow recipient embayments (5, 11). About one-fifth of the U.S. annual cranberry harvest is produced from about 2700 ha of active bogs within the watershed of Buzzards Bay, MA. To determine the role of cranberry wetland agriculture in the nutrient economy of coastal watersheds, we developed a nutrient balance for a * T o whom inquires should be addressed; e-mail address:
[email protected].
960 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
0013-936W95/0929-0960$09.00/0
D 1995 American Chemical Society
FIGURE 1. Map of the region surrounding the study bog. Data on housing were taken from USGS topographic sheets from 1977 (portion east of Buttermilk Bay) and from 1972 (western part). Dots represent houses; the blank area is forested. Roads have been omitted for clarity. A is output stream. B is input weir and culvert. Recent development has been to the southeast of Buttermilk Bay and to the east of the study hog.
hog adjacent to Buzzards Bay. We concentrated primarily on the macronutrient, nitrogen, as it is generally thought to he the nutrient limiting phytoplankton production in temperate coastal waters (12,13) and has been found to limit production of saltmarsh grasses (14).sea grasses (13, and macroalgae in shallow waters (16). We measured nutrient inputs and outputs, through both natural and agricultural pathways, and nutrient transformations within the hog system to address potential controls of nutrient retention. Measured nutrient losses from cranberry agriculture were compared to other freshwater wetlands and alternative land-uses.
Site Description The study hog is located just north of Buttermilk Bay, an emhayment of Buzzards Bay in southeastern Massachusetts (Figure 1). The hog occupies 14.97 ha (37 acre) of which 90%, 13.4 ha (33 acre), is wetland (vegetated area and ditches) and 10%consists of fringing and central dikes and upland margin. Historical maps indicate that the hog was constructed by 1890 from a natural wetland that had developed in a kettle hole depression. Natural kettle depression wetlands can he characterized as oligotrophic peat hogs that are nutrient-poor and tend to he isolated from surrounding surface water and groundwater flow systems (In. Many hogs in operation in southeastern Massachusetts and almost all hogs created prior to 1930 have been Constructed upon natural wetlands, where vegetation was removed and the surface leveled, hut the
deepernaturalwetlandpeatwasretainedasahasetorestrict water loss to the aquifer (9). Now, though wetlands protection regulations preclude the conversion of existing wetlands, construction oftwo new upland hogs 1km north ofthe studyhogutilizedthick kettle hole peat deposits ('7 m) recovered from a highway right-of-way as a lining material to prevent water loss through the new hog bases. The hog is in the lower portion of a 580-ha watershed dominated by glacial outwash and vegetated by pine and oak second growth forest. Nutrient inputs to groundwater in the upgradient watershed are relatively small due to the lack of commercial and residential development (Figure 1). The principalwatersource for thestudyhogis a2.5-ha reservoir whose water level is controlled hy a weir. The source of water to the reservoir is drainage from a comparahlecranhenyhogsystem (12.1ha) underthesame management (Figure 1). The reservoir is eutrophic with water lilies and marsh plants around the edges and submerged aquatic vegetation throughout. Water levels and outflow from the study hog are controlled by a second weir located at the outlet stream. The stream empties into Buttermilk Bay after flowing a few hundred meters through a residential area and a salt marsh. The hog is drained during most of the growing season, during which time the ditches contain only about 10-20 cm of water. The exceptions are during pesticide applications when the outlet is briefly blocked to raise water levels and to allow pumped return of water to the reservoir to limit downstream contamination. The major flooding VOL. 29. NO. 4,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY m 961
events are for harvest in fall and for protection from freezing in winter. During these flooding events, there is little outflow, but when the managed flood level is achieved, outflow resumes if inflow is continued. Maintenance of flood levels and the efficient use of water during flooding are two important reasons for constructing the bog atop a low permeability base like the compacted peat and clay layer. The bog was fertilized on four occasions: May 5,112 kg ha-’ (100lb acre-’) of 10-12-24(NPIQor 0.80 N, 0.43 P, and 0.61 K kmol ha-’; July 1 and 10,112 kg ha-’ of 10-20-20or 0.80 N, 0.72 P, and 0.57 K kmol ha-’; and Sept 12, 112 kg ha-’ of 5-15-30 or 0.40 N, 0.54 P, and 0.86 Kkmol ha-’. The nitrogen within the fertilizer was urea form. To convert kilomoles to kilograms or millimolar to milligram of N, P, or K, the molar value is multiplied by 14.0, 31.0, or 39.1, respectively. The applicationswere of slow-releasegranules broadcast by helicopter.
Hydrology. Precipitation was measured with an on-site gauge, and rain sampleswere collected adjacent to the gauge with a funnel leading to a plastic bottle. The rain gauge was read daily, and rain samples were collected following each rain event. Rain input was based upon the area occupied by the wetland and fringing upland portions of the bog. Evapotranspiration was calculated according to the Palmer and Havens (18) modification of the Thornthwaite method using temperature records from the University of Massachusetts Cranberry Experiment Station, about 1 km from the study bog. Land areas were calculated from United States Geological Survey (USGS)topographic maps using computer digitizing programs. The watershed for the greater bog system was determbed from analysis of groundwater elevations determined from water table wells and pond elevation data (19) and with limited use of topographic data. Water level in the outlet stream was monitored with a vented recording pressure sensor (ISCOModel 2100) placed at the bottom of the culvert downstream of the bog outlet weir (Figure 1). These levels were converted to discharge using flow measurements made at different water levels using propeller flowmeters and timing of semisubmersed plastic bottles and dyes. Inflow to the bog was not continuously measured but was calculated as outflow plus evapotranspiration from the bog minus rainfall onto the bog. Nutrient Concentrations. Surface water entering and exiting the bog was monitored with automated samplers (ISCO Model 27001, 3 m downstream from the inlet weir and 10 m downstream from the outlet weir (20). Water was sampled 12 and 20 times per day from the inlet and outlet, respectively. Daily composite samples varied from 1 to 2 L. The sample bottles contained sulfuric acid (5.4 N) resultingin a sample pH of ca. 2.0 to preserve the nitrogen species. The automated samplers keep the sampling tube filled with air between samples (preventingbiofouling) and have three flushes before sample collection. Water samples were returned to the laboratory every 7-12 days for analysis. Upon return to the laboratory, samples were filtered through precombusted glass fiber fdters (GFF),and the filters were dried at 60 “C. Particulate nitrogen was determined using a Perkin Elmer 2400 CHN elemental analyzer. The filtrate was analyzed for ammonium using an indophenol method (21),for nitrate + 962 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
nitrite (NO,-) by cadmium reduction using a LACHAT autoanalyzer (221,for dissolved organic nitrogen (DON) by persulfate oxidation and analysis as nitrate (231,and for orthophosphate by a molybdate-ascorbic acid method (24). Concentration standards were at a pH equal to that of the samples. To determine the effects of preserving samples with acid, unpreserved samples were collected throughout the year for comparison of concentrations of nitrogen species and orthophosphate. Experiments were also conducted to determine the effect of storage time on nitrogen speciation and orthophosphate concentration. Sediment Nutrient Flux Exchanges of dissolved inorganic nitrogen and orthophosphate across the surface of ditch bottoms and vegetated soils were measured using in-situ chambers. Measurements were conducted in ditch bottoms throughout the growing season and in vegetated areas during harvest flooding. Four chambers were deployed throughout the bog per date, each 25 cm (i.d.1, 60 cm high, and thin-walled (3 mm). The soft sediments of the ditches and the loose root mat of the vegetated areas allowed insertion (5-10 cm) with little visible disturbance. Water exchange was allowed through a 2 m long 6 mm (LdJ tube coiled around the cylinder to maintain equal levels inside and outside of the chamber. Water levels were monitored at 0.5- 1.0-hintervals. Level changes were rare, but when they occurred, the calculated rate of nutrient flux was adjusted for the measured dilution or loss of headspace volume. Cylinders were darkened to prevent photosynthesis and were gently bubbled to maintain aeration and mixing without disturbing the sediment surface. Concentrations of nutrients were determined outside the chambers before placement and in the enclosed water hourly from 0 to 6 h. Flux rate of NH4+,NO,-, and POA3-was calculated using linear regression. This procedure allowed evaluation of anomalous initial rates associated with disturbance. Samples of soil porewater (15 mL) for nutrient analysis were collected with porous porcelain lysimeters (unsaturated zone) and with sippers (saturated zone, ref 25). Lysimeters collected water under applied vacuum, the initial inflow was discarded, and the sample was collected after 18h. An initial sample from the sipper was drawn by syringe and discarded, and then the sample to be analyzed was collected. The flux, lysimeter, and sipper samples were filtered (0.45 pm) in the field, transported on ice, and analyzed immediately upon return to the laboratory using the methods described above. Plant Sampling. Samples of above-ground and belowground biomass were collectedby clipping 625 cm2quadrats and taking 30 cm long sediment cores (6.5cm i.d.). Abovegroundvegetation was sorted into live stems, leaves,berries, and litter. Below-ground biomass was assayed as total macroorganic matter (live dead roots and rhizomes). Cores were sectioned at 5-cm intervals and sorted under running water over a 0.5-mm sieve. Plant samples were weighed wet, dried to constant weight at 60 “C, ground to 40 mesh, and analyzed for total nitrogen by elemental analyzer (see above). Total nitrogen “export” in berry harvest was calculated from measured ratios of wet to dry weight, assayed nitrogen concentrations, and the actual harvested wet weight of the berries from the bog under study.
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Results Hydrology. Precipitation was relatively evenly distributed throughout the year, with lowest inputs in summer and
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FIGURE 2. (A) Daily precipitation on the cranberry bog. (B) Stream oulflow to Buttermilk Bay from the bog through the study year. Zero flows result from water management.
highest in winterlspring (Figure 2A). Total annual precipitation measured on-site, 990 mm, was comparable to a gauge ca. 1 km distant, 956 mm, but lower than the 20-yr average, 1229 mm (26). Mean temperature over the year of study was 9.5 "C, with a July mean of 22 "C (maximum 35 "C) and a January mean of -3 "C (minimum -9 "C). Discharge from the bog outlet stream varied from zero, during brief periods when the weir was completelyblocked, to a high of 0.20 m3s-l (Figure2B). The mean exit flow was about 0.06 m3 s-l or 5200 m3d-l. Total outflow, 1.91 x lo6 m3,was nearly equally divided between the May-October growing season (56%)and the November-April low-activity months (44%). Maximum flow was in spring due to combined rainfall and snowmelt, while over the remainder of the year, maxima were due to regulated draining of floodwater associated with harvest (Oct 2-21), freezing (Dec-Feb), or pesticide Uune-July) management practices. The latter "managed" high flows were typically of short duration ( ~ 1 . d) 5 as the bog drained. Management of surface water flow for winter plant protection required
periodic draining during warm periods to prevent anoxia (e.g.,mid-January). Unregulated flows commenced in late February when frost protection was by sprinklers. The observed increase in outflow during the few hours of spraying was always 10% of the total outflow. The combined effects of evapotranspiration losses (8.22 x lo4m3y r l ) and inputs through precipitation (14.8 x lo4 m3yr-l) were small relative to the surface water flow through the bog system, only about 4 and 8%, respectively. Estimated evapotranspiration from the bog was greatest during the most active growing season but had only aminor impact on streamflow averaging 7.4% (SE = 0.4%) of the measured outflow during nonblocked periods. Similarly, rainfall onto the bog was insufficient to result in more than a very small increase in outflow versus inflow except in mid-September when 98 mm of rain fell in 72 h and again on March 27 after 76 mm fell (Figure 2). Irrigation waters were difficultto quantify but were 4% of the water inflows. Other than atmospheric and surface water exchanges, groundwater represents the only possible pathway for both VOL. 29, NO. 4,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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water and nutrient exchange with the bog. However, four lines of evidence indicate that exchanges through the groundwater pathway were minor. First, the bog is a lowland cranberry bog, engineered to prevent water loss to the aquifer (9). While the original surface vegetation was removed, the remaining peat and muckwas graded to create a low hydraulic conductivity layer beneath newly added surficial bog sediments. At the study site, probingindicated a 2-m layer of compacted peat with some clay underlying the bog. Attempts to withdraw water from piezometers placed within this layer were unsuccessful. Hydraulic measurements in similar layers of compacted organic sediments and peats have been found to restrict vertical groundwater flows (27-29). The effectiveness of the “aquitard”underlying the bog is suggested by the limited water-level drop (ca. 10 cm) over 10 days of maximum bog water levels with virtually no surface water inflow (ca. 60 m3 d-l) or outflow (ca. 460 m3 d-9. After correcting for evaporation, if all of the “missing” water was due to groundwater recharge over the flooded surface of the bog, the volume (ca. 700 m3 d-l) is less than 15% of average daily exit flows through the year. However, even this level of water loss is a maximum, since much of the water enters the dikes surrounding to the bog and returns when the bog drains (bank storage). During this period, groundwater levels in the upland within 16 m of the flooded edge of the bog were found to be slightly elevated (ca. 17 cm) over surrounding groundwater levels, accounting for the water volume equivalent to the observedwater “loss”. Given that most of the year the bog is unflooded, it is likely that exchanges with the aquifer are at most only a few percent of the annual water balance of the system. It appears that the peat/clay layer is effective in limiting exchange with the aquifer. Second, measurements of interstitial water with sippers (permanent saturation ‘20 cm depth) indicated that the water table within the vegetated areas of the bog is elevated over water levels in the ditches. The result is a constant hydraulic gradient from the vegetated areas to the ditches during the nonflooding periods, supplying the surface water loss pathway from the bog. In addition, the lower water levels within the ditches make them the likely sites for potential groundwater inflows. Third, when the bog surfacewater inflow was stopped, outlet streamflow also stopped within hours, indicating that groundwater inflow was negligible during that period. If groundwater inflow was occurring, then during the closure of the inlet weir, the outflow should slow but baseflow from groundwater would continue or even increase slightly due to the slightly greater hydraulic gradient. The gradual slowing before complete cessation of the outflow is likely due to the return of the small amount of bank storage (ditch water levels generally were only 10-20 cm deep). Similarly, when the outlet weir was closed, the exit stream stopped flowing, indicating that groundwater flow from the flooding bog was not contributing to downstream outflow. Such groundwater flow should occur if the bog had substantial seepage loss to the aquifer. While groundwater flow can be highly time varying, these flow observations occurred throughout the active growing season (March-September) over the range of seasonal groundwater levels. Fourth, total stream outflow from the bog for the study year, 1.91 x lo6m3, and measured rainfall minus calculated evapotranspiration for the entire watershed above the outlet stream (19), 2.07 x lo6 m3, differed by 0.05, t-test) effect of preservation on nitrogen species but a nearly 8-fold increase in measured phosphate. The lack of a detectable increase in DIN levels due to acid release from organic matter likely results from the relatively low pH of the streamwater and the large DIN contribution to the total N pool. The significant increase in orthophosphate with preservation is presumably due to leaching of phosphate from organisms or weakly sorbed sites on particles. From June to September, this ratio was 3.7 (range 2.1-4.5). Measurements of the increase in orthophosphate over time indicated that leaching was complete in less than 5 h, the shortest period we tested, with no additional increase through the next 12 days. It appears that the second stage of the two stage sorption-desorption process that buffers phosphate in solution is too slow to be involved here (34). As the stream samples were always in the sampler for at least 5 h, the results are alJ comparable. Our nominal soluble phosphate budget, therefore, represents not ambient P043-in streamflow but the sum of soluble and easily leachable organic phosphate (not total organic since the samples were not digested at high temperature). In our discussion, we refer to acid-leachable phosphate for acidified samples and orthophosphate when no acid was used. Nutrient Concentrations in Surface Water Flow. Inorganic nitrogen and acid-leachable phosphate levels were consistently lower and less variable in inflowing versus outflowing surface waters (Figure 3A,B,E). In contrast, surface water inputs and outputs of organic forms (DON and PON) were similar in concentration and variability, although the highest concentrations were measured in outflowing waters (Figure 3C,D). Nitrate nitrite was generally low in both inflows and outflows, almost always below 1pM (1 pM = 0.014 mg/L) and two-thirds of the time below detection (0.05pM). The
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FIGURE 3. Nutrient concentrations (pm) in surface water inflow and outflow from tba bog over the study year (A) NH,+, (6)NO,(C) dissolved organic nitrogen, (D) particulate organic nitrogen, and (E) acid leechable orthophosphate. Concentrations of NO,in the inflow were frequently below detection (0.05 pM).
observed peak at the end of May, 17 pM, was not obviously related to any weather or management activity while the annual maximum in October, 53 pM, and elevated levels through the winter were associated with harvest and subsequent flooded conditions (Figure 3B). Inflowing waters did not show elevated NOx- ( > 1 pM)levels similar to the stream outflow. Annual concentrations of NO,- in inflows (median = 0.05 pM, range (0.05-3.2 pM) were significantly (paired t-test, p < 0.05) lower than outflows (median = 1.4pM, range 0.1-52.9 pM). Ammonium accounted for most of the dissolved inorganic nitrogen (DIN) and showed the greatest and most
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consistent differences between inflowing and outflowing surface water of all of the nitrogen forms assayed (Figure 3A). As for nitrate, inflows (median = 1.4pM, range 0.068.6 pM) were significantly (paired t-test, p < 0.05) lower thanouifiows (median=5.4pM,range0.3-57.0pM).Inflow NH4+ concentrations only exceeded 3 pM in late June, reaching the annual high of 8.6 pM concurrently with increases in NOx-, DON, and acid-leachable phosphate (Figure 3 ) . These increases were likely associated with mixing of the water column and disturbance of the sediments of the reservoir caused by the periodic pumped return of water in late June. The largest differences in VOL. 2s. NO. 4, i s 9 5 / E N V I R O N M E N T A L SCIENCE &TECHNOLOGY m gss
ammonium concentration between intlowing and outflowing waters and the major feature over the year was associated with harvest. The increased NH4+ concentrations in bog outflow were likely the result of enhanced foliar leaching due to flooding during senescence and disturbance due to harvest coupled with potential stimulation of anoxic release from the flooded bog sediments. Fertilization of the bog (see site description above) was not associatedwith “spikes” in inorganic nitrogen concentrations in stream outflows with the possible exception of ammonium and orthophosphate in early July. Un-ionized ammonia is toxic to animals. We measured total ammonia (ionized and un-ionized), and at the acid pH’s of the bog waters, almost all ( ~ 1 %was ) un-ionized and below harmful levels [Canadian Water Quality Guidelines (120pM, 32);U.S. EPA (1pM, 3311. Organic nitrogen concentrations were variable with no consistent differences between inflowing and outflowing waters (Figure 3C,D). DON levels were generally 3-fold lower than PON in both inflows and outflows, and total organic nitrogen levels were generally several fold higher than total dissolved inorganic concentrations. Median levels of DON, 9.2 pM, and PON, 23.6pM,in inflowingwaterwere nearly identical Wilconon signed rank test, p > 0.1) to outflowing waters, 8.6 and 23.72 pM. However, as in the inorganic forms, the maximum concentrations for DON and PON were found in stream outflow, 108.9 versus 48.1 ,uM and 238.5 versus 80.9 pM, respectively. Both PON and DON showed small peaks associated with harvest, but the major PON peak was associated with stream outflow during the 9.8-cm September rainstorm. Most of the variations in DON were not obviously related to anythingwe measured. There was a short period of higher values in mid-summer, one in October, and a single peak in early spring. The October high values coincide with harvest activities (in the study and upstream bogs), possibly related to plant damage and leaching during senescence. However, DON levels in outflow were significantly correlated with inflow concentrations ( r = 0.40, p < 0.05), suggesting that the bulk of DON may be refractory and merely passing through the bog. Acid-leachable phosphate was consistently higher in outflowing, median 2.1 pM (range 0.4-29.3 pM; 1 pM = 0.31 mg/L), than inflowing waters, median 0.5 pM (range 0.1-7.4 pM). The major peaks in phosphate levels in outflow were primarily associated with rainfall or bog flooding (Figures 1 and 3E). The low concentrations in inflow were similar to river waters (34)and suggest removal in the upstream reservoir to the extent that outflow from the upstream bog had similar concentrations as the study bog. The largest sustained phosphate concentration in the outflow water was during the winter flooding period with a high of 29 pM and a mean of 17.1 pM from January 14 to February 11. Nutrient Import and Export in Surface Water Flow. Net flux from the bog takes into account both the concentrations of nutrients in the water and the associated flowvolume. Given the similar inflow and outflowvolumes, the graphs of NH4+,NO,-, and Po43- concentrations (Figure 3) are quite similar to those for net flux (Figure 4). Overall, net flux was driven mostly by concentration rather than water movement. Inorganic species, NH4+, NOx-, and Po43-, were found to be consistently exported from the bog in streamflow, all registering losses for more than 94% of the dates. Am966
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monium, 14.1 kmol y ~ - accounted ~, for about 85% of the annual DIN loss of 16.8 kmol yr-l (Figures 4 and 5). In contrast, net exchange of organic fractions was variable: DON registered exports on only 44% and PON registered 74% on the dates. While PON (like DIN) had an integrated net annual export, 9.5 kmol yr-l, a small amount of DON, 1.7 kmol yr-’, was imported by the bog (Figure 4C,D). For all forms, net flux was predominantly during the periods of large concentration differences in inflowing and outflowing waters, since the volumes of water inflow and outflow generally agreed to within 10%. The dominance of surface water flow in the bog’s hydrology increases confidence in the measured net exchanges, since concentrations can be measured with high precision (kl%).In addition, if nitrogen were entering through groundwater rather than surface water, there would be little change in the calculated bog flux. Both surface water inflow and groundwater of the adjacent pineloak forest, which covers most of the bog watershed, contain similar concentrations of nitrate ( < 1pM; ref 35, Figure 3B). Combining the fluxes of each individually assayed nitrogen form on each date yields a net annual mass loss of total nitrogen in streamflow of 24.7 kmol yr-l. DIN and P043- in Porewaters of Bog Sediments. Vertical profiles of dissolved nutrients in the vegetated surficial sediments showed high concentrations of both DIN and Pod3- at depth in spring, presumably due to diffusion from below and release from roots, but declined in summer (Figure 6A,B). At the shallower depths, where the plant’s roots were concentrated, nutrient levels were much lower. Fertilizer applications in early Julywere most apparent at 6-7 cm in Pod3- concentrations and to a lesser extent in DIN levels. The DIN profiles of the ditch bottom sediments (Figure 6C) show a gradient driving diffusion toward the surface and into the ditch water (median 7.6 pM, maximum 60.1 pm) for most of the year. Overall, ditch bed porewaters were several fold higher than measures at comparable depths in the vegetated sediments, presumably due in part to the absence of plant uptake. A seasonal change in concentrations of both DIN and PO4 in the surface layers is suggested,with lower levels in mid-summer. Ammonium comprised almost all (96%)of the DIN pool. The high DIN levels and dominance by ammonium are consistent with active remineralization of organic nitrogen within ditch sediments. Fertilizer applications were not obvious in the ditch bed porewater measurements. Nutrient Fluxes from Bog Sediments. Ammonium accounted for almost all of the DIN flux from ditch bottom sediments (ca. 85%), consistent with the fractionation of DIN in underlying porewaters (96% NH4+). This flux of NH4+was high in early summer, then fell, and remained low through the autumn (Figure 7). The high values represent a combination of regeneration and solubilization of fertilizer since both the ditch beds and vegetated surface of the bog received slow-release fertilizer due to the helicopter application. The lower August flux was likely due to the full month since the last fertilizer application. If the combined DIN flux rate in August was taken as the baseline remineralization rate (no fertilizereffects),it would account for 220 (SE = 73) mol of DIN over the three summer months. The measured June-August rate of DIN flux from ditch sediments throughout the bog was more than 4-fold higher, ca. 1020 (SE = 180)mol or about half of the measured DIN export in outflow stream, 2200 mol, over the same
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FIGURE 4. Net flux of nutrients in streamflow over the study year. Values are the difference between the mass amount of a nutrient species in outflowing minus inflowing surface water: (A) NH,+, (B) NOJ- NOz-, (C)dissolved organic nitrogen, (0) particulate organic nitrogen, and (E) acid leachable orthophosphate. Values in the boxes represent the net annual import (-1 or export (+) of each constituent from the bog.
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period (Figure 5A). The pattern of ditch bottom orthophosphate flux was similar to DIN, reaching about 1.5 molld in mid-July. Sediment flux accounted for ~ 1 5 % of the net export of leachable phosphate during that period. However, taking into account the 3.7-fold enhancement of measured phosphate levels in the surface water flow samples due to acidification (see above), it is possible that ditch bottom flux of phosphate may be the source for about half of the orthophosphate exported from the bog duringthe summer. Measurements of nutrient flux from the vegetated surface of the bog were also conducted during the harvest flooding.
Only when flooded or in extreme rain events can the vegetated areas of the bog contribute significantlyto surface water exports. Losses from vegetated sediments were relatively high for phosphorus, comparable to the highest values from the ditch beds. This flux is consistent with the porewater concentration gradient during harvest and contributes to the pulse in net flux of phosphorus that occurred when the bogs were drained (Figure 4E). Loss of phosphate from the large above-ground plant biomass (1960 g dry wt m-2, Table 1)was also a likely source for the loss. The measured daily flux, 180 (SD= 28) mol, for the vegetated areas of the bog is consistent with the peak net losses, 200VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1967
400 350 300
h
4
250
> 200 150 100 50
4
0
600 400
200 0
-200
I000 h
2
\
750
500 250
rl
s
o -250
May Jun
Jul
Aug Sep Oct Nov Dec Jan
Feb Mar Apr
FIGURE 5. Net flux in surface water outflow of (A) total inorganic nitrogen, (B) total organic nitrogen, and (C) total nitrogen (calculated as in figure 4). Values within the boxes are the net annual import (-1 or export (+) for that nutrient form.
400 mol d-l, in outflows during this period. However, the rate of phosphate loss within the chambers, projected to the full 20 days of harvest, 3500 mol (SD = 5401, overestimates the measured outflow loss associated with harvest of 600 mol, which is 12%of the yearly loss. It is likely that conditions over the harvest period were variable and that measured losses at the outflow may be lowered by reuptake by bog sediments. The DIN losses from the vegetated sediments did not show a large increase similar to phosphate but were only slightly higher than the ditch bottom fluxes (Figure 7). Integrated DIN loss over the harvest flooded period was 53 mol, much lower than DIN losses in outflow associated with harvest (Figure 4). The higher outflow losses possibly result from the large increases in NH4+ levels due to disturbance of the senescingplants by harvesting operations (Figure 3a). High DIN levels dominated harvest-related total nitrogen losses from the bog (Figure 4c). Plant Biomass and Production. Peak above-ground biomass, 1960 dry g m-2, was composed of 54%live stems, 21% live leaves, 11%berries, and 14%litter (Table 1). The below-ground pool of macroorganic matter was 2.5 times the above-ground pool, 5110 dry g m-*, with 80% in the S68
1
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4, 1995
upper 10 cm of soil. Measured biomass and nitrogen concentrations indicated large plant nitrogen pools, 96.5 kmol and 491 kmol for the total bog above- and belowground components, respectively (Table 1). Harvest collected both berries and by accident about 5% of the live leaves, resulting in a nitrogen “export” from the bog of 4.6 and 1.9 kmol yr-l or about 8% of the living above-ground plant pool.
Discussion Controls on Nutrient Export in Surface Water Outflows. Overall, the cranberry bog was a net source of nutrients to the outlet stream. Although the bog plants and sediments served as a net sink for nitrogen, the addition of fertilizers made this land-use a nutrient source to adjacent coastal waters. Nutrient export from the bog appeared to be controlled by interactions of hydrology and biological and chemical processes within the two components of the bog system, vegetated areas and ditch bottoms. The growth cycle of the plants strongly affected both the timing and fractionation of nutrient release relative to flooding and rainfall.
350 300
a a
EQ
,
I
300
-
Ditches
C
250
200 150
100 50 I --r I
Jun
Jul
r
Aug
I-
Sep
I
Oct
I
Nov
6om I 1 om v IQ om v aQ om
Jun
0
Jul
Aug
Sep
Oct
Aug
""1
3
N O ~
Sep
Oct
Nov
0 0-2 om
v
40
0
Jun
Jul
0
om Q om
Sep
Oct
8-6
30-1
Jun
Jul
Aug
Nov
FIGURE 6. Nutrient concentrations during the cranberry growing season in lysimeters (6 and 11 cm) and sippers (19 and 29 cm) placed in the vegetated surface of the bog: (A) total inorganic nitrogen and (B)orthophosphate and in sippers placed in the ditch beds; (C) total inorganic nitrogen and (D) orthophosphate.
Maximum losses of inorganic N and P were related to the factors controlling outlet stream concentrations (Le., plant uptake, losses through plant senescence, storm and flooding events, and release from sediments). Inorganic N and P were generally retained by the bog during the active growing season, even with the application of fertilizers, and lost to floodwaters during periods of harvest, senescence, and decay of the plants (Figure 4A,B). This pattern contrasted with organic forms,which showed seasonal shifts from net import to net export (Figure 4C,D). Comparing the 6-month intervals approximating the growing season (May-October) and plant dormancy (November-April) shows the strong seasonality of the organic versus the inorganic N and P fluxes. Inorganic forms, NH4+, NOx-, and P043-,had nearly equal net exports between seasons, with 47%, 56%, and 48% lost during the growing season (Figure 4A,B,E). In contrast, 62% of PON loss was during the winter with almost all of the remainder occurring during harvest (October) and the late season storm (September). DON showed a small net annual import to the bog with import during the growing season (2.1kmol yr-l) and a small export in winter (0.44 kmol yr-l). The pattern of retention and loss of N forms is similar to that found in natural wetlands (cf. ref 36). Regeneration of N and P from ditch sediments played a relatively minor role in yearly outlet stream export. However, during the summer when net nutrient losses were low, benthic release accounted for about half of the net DIN and orthophosphate lost. The importance of sediment regeneration in summer likely stems from the solubilization of accidental fertilizer deposition in ditches (helicopter application) and to a lesser extent organic matter remineralization and the retention of nutrients in the vegetated areas which occupy almost 100 times the area of the ditches.
Manipulation of bog waters appears to have the largest proximate impact on net nutrient loss. Flooding of the vegetated surface of the bog for harvest and winter protection resulted in the major net nutrient losses in outflows. Flooding primarily occurs during plant senescence or dormancy. The floodwaters accelerate foliar leaching losses from senescing and decaying above-ground plant matter, provide a pathway for particulate export, and may also leach any residual fertilizer from the summer applications. In these losses, the floodwaters primarily provide a mechanism for transport of N and P from the otherwise inaccessible areas of the bog. Floodwaters may also, however, enhance nutrient losses from the emergent bog. Phosphate is solubilized at low Eh and can diffuse from sediments into overlying waters (37). Eh of bog sediments may fall duringwinter floodingwithout damaging the dormant plants. Sediment oxidation-reduction potential was not measured in the bog during the winter of study, but we did find lowered potentials as measured by platinum electrode and reduced oxygen levels in the overlying floodwaters in the following February (unpublished data). It is likely that the high sustained leachable phosphate levels (17pM) observed from mid-February to mid-March (Figure 3E) were the result of release from surficial soils under low Eh created by organic matter decomposition in saturated soils (38). While bog hydrology relating to flooding (or major summer storms) appeared to have a major impact on nutrient losses through surface water flows, net losses of total nitrogen were independent of outlet stream discharge (Figure 8A). Seasonal and flooding factors likely obscured any relationship between discharge and total N export. An exception was related to the major rain event in midSeptember during plant senescence and several days after a fertilizer application. A total of 10 cm of rain resulted in VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
969
0.8 0.7 0.6
0.5 0.4
0.3
0.2 0.1
0.0 -0.1
Jun
Jul
Oct
Nov
60
50 40
/
I\
30
20 10
I *
0
J - 4
-10
Jun
Jul
Aug
SeP
Oct
Nov
FIGURE7. Nutrientflux into water-filled chambersfrom ditch beds (solid symbols) and fromthe vegetated bog surface during harvestflooding (open symbols): (A) NH,+ (boxes) and NOJ- NO*- (triangles); (B) orthophosphate (circles). Values are mean f S E , N = 4.
+
TABLE 1
Nitmgen Storage in Plant Organic Matter at Peak Biomass (September)a total nitrogen source
dry wt (g
N
g m-*
kmolbog
Above Ground live stems live leaves berriesb litter total 0-5 cm 5-10 cm 10-15 cm 15-30 cm total
1060 (290) 410 (110) 209 280 (120) 1959 (333)
5 5
3.8 (1.0) 3.8 (1.0) 0.5 (-) 5 2.0 (0.9) 10.1 (1.7)
36.5 (10.1) 36.1 (9.7) 4.6 (-) 19.3 (8.3) 96.5 (16.2)
Below Ground (Macroorganic Matter) 2890 (630) 5 28.9 (6.3) 276.6 (60.3) 1280 (150) 4 12.9 (1.5) 123.7 (14.5) 840 (200) 2 8.5 (2.0) 81.2 (19.3) 100 (17) 2 1.0 (1.0) 9.7 (9.7) 5110 (678) 51.3 (6.91 491.2 (65.7)
total organic matter
7069 (814)
61.4 (7.1) 587.7 (67.6)
Values are meansand propagated (SD). From whole bog harvest and a measured wet/dry ratio of 0.110. a
the second highest discharge event of the year (Figure2B), increases in concentrations of each nitrogen form and 970
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4, 1995
leachable phosphate in outflowing waters (Figure 3), and large net exports. The three highest daily total nitrogen exports for the year were associated with the September storm (Figure 8A). However, factors other than rainfall are clearly at work as a similar magnitude storm, 7.6 cm, in March showed no similar pulses in export. The latter storm occurred after winter losses from the vegetated areas to floodwaters and more than 6 months after the last fertilizer addition. The cranberry bog appears to be operating as a natural wetland, retaining DIN and leachable P and organic forms during the most active growing season (June-August) with DIN (primarily ammonium) and organic nitrogen losses during the period of senescence and decay of the plants (Figures4E and 5A,B; refs 36 and 39). The combined effect of each nitrogen fraction was a pattern of net bog import of total nitrogen in the summer and loss through the rest of the year. Although daily losses and gains were highly variable, the bog consistently lost inorganic nutrients to the outlet stream, was a large net exporter of particulate organic matter, and was in balance for DON (Figures3 and 4). The net export of total nitrogen in outnow was 24.7 kmol yr-l with net retention and loss dominated by
4
I
I
I
I
1
I
I
I
I
I
I
V
I
I
I
v
0
40
I
I
I
2
4
6
I
I
12 14 Streamflow m3/d (thousands) 8
1
10
1
I
i
I
I
16
18
1
B i
F4
= *
I 20
-
-
20
-
$
-
-10
-20 0
I
I
I
10
20
1
I
1
50 60 Percent of Total Samples
30
40
I
70
80
FIGURE 8. (A) Daily net import (-1 and export (+) of total nitrogen relative to outlet stream discharge throughout the year. The three highest TN exports were during a high rainfall in September (98 mm, see Figure 2A);a March rain event (76 mm) is shown for comparison. (B) Frequency distribution of total nitrogen import (-1 and export ($1 of all samples. Ten percent of days with imports and exports accounted for 679’0 and 57% of the annual import and export, respectively.
ammonium and PON fluxes. ExcludingNO,- or DON fluxes only changes the total annual N loss by 11% and 7%, respectively. Nitrogen Balance of a Cranberry Bog. Since we measured the total nitrogen in surface water inflows and outflows and have additional data on the pools and fluxes of nitrogen within the system, we can construct a nitrogen balance for the cranberry bog (Table 2). All of the inputs and outputs relate to surface water flows or atmospheric exchanges since available data indicate that exchanges through groundwater were small (see above). Although fertilized, surface water flow dominated the inputs of fixed nitrogen to the bog at 59% of total input.
Similarly,nitrogen in stream outflow accounted for almost all (93%)of the measured losses. However, much of the nitrogen in inflow water appeared to simply pass through the bog. During most of the year when the bog was unflooded, nitrogen in surface water was isolated from contact with the emergent bog, and exchangeswere limited to ditch sediments. In contrast, the nitrogen in fertilizer was added to the entire bog surface, 0.28 mol of N m-*, so that ditches received less than 1% of the nitrogen application. The vegetation contributed to the annual cycle through seasonal nitrogen retention and loss and by permanent burial of plant tissues. The observed patterns of nitrogen retention by the bog (Figure 5) appear to be VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1971
TABU! 2
Summary of Cranbery Boy Nitrogen Balance inputs outputs kmol kmol of N yr-I (YO) N yr-I (70)
source
streamflow precipitationa Nz fixation fertilizer berry harvest leaf harvest
66.5 7.9
? 37.4
burial
denitrification totals (% of input)
-
111.8
(59) 91.2 (7) -
(93)
netb
24.7c
-
(33) 4.6 1.9
(5) (2)
? ? 97.6 (87)
-14.2d(13)
E To bog surface (89%)and fringing upland (1 1%). Negativevalues indicate import. N load contributed to outflowing waters by cranberry growing. Retention(by plants and sediments) of N relativeto all inputs. Net DON flux = 1670 rnollyr or ~ 2 of % TN loss in streamflow.
primarily controlled by uptake of inorganic nitrogen, loss of organic forms by the plants, deposition of PON, and loss of DIN by the ditch bottoms. Inputs by precipitation calculated from measured rainfall and concentrations of total nitrogen from an earlier study (39)were consistent with regional estimates (40). Precipitation inputs were small (7.9kmol) relative to surface water inputs and fertilizers but were equivalent to the losses through harvest, 6.9 kmol. Although we did not measure nitrogen fixationor denitrification,their net effect on system flux must have been small since measured inputs and outputs were nearly (87%)in balance. The apparent net annual uptake of 13%of the external nitrogen input to the bog can easily be accounted for by burial and/or potential denitrification in excess of nitrogen fixation. The 12.2kmol (0.09mol m-2) retained by the bog represents less than 3% of the nitrogen contained in the pool of below-ground macroorganic matter and is an even smaller fraction of the total soil nitrogen. Denitrification losses from the bog are probably lower than many freshwater wetlands (cf. ref 361, since the surficial soils remain aerated through most of the warmer months. Denitrification is likely limited primarily to the ditch beds and periods of bog flooding. The precise fate of the nitrogen added in fertilizerscannot be assessed from the existingbalance. However,an amount equivalent to 17%of the total fertilizeraddition was removed in harvest compared to 38% retained in the bog and 45% lost to stream outflow. Nutrient Sources and Land-Use. It is clear that the cranberry bog was a net sink for nitrogen, the sum of nitrogen inputs exceeding outputs (Table 2). However, since cranberry agriculturerequires N additions in ferilizers, the use of the land as a cranberry bog created anet nitrogen source to the outflow stream (Figure 5c, Table 2) and therefore to the adjacent bay. The bog was a source of 26.3 kmol yr-’ of DIN PON, the original source of this nitrogen probablywas the added fertilizer. However,the significance of cranberry agriculture as a nutrient source must be evaluated relative to other potential uses of the land: natural streamside wetlands (cf. ref 36), bogs (31),and residential development (35). The bog unit includes wetland area and directly associated fringing uplands and dikes (14.97 ha). The net annual export from the study bog to Buttermilk Bay was 1.65kmol ha-’. This inflates the actual impact of cranbeny
+
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4, 1995
agriculture as bog systems generally have aSSOClated expanses of surrounding forested land. The adjacent oak and pine forest loses little nitrogen to the Bay (primarily through groundwater) with an annual flux of 0.01 kmol ha-l [calculatedfrom Weiskel and Howes (4111, much less than the input through precipitation. Since the bog was constructed from a natural wetland and continues to function as a streamside wetland, fluxes from natural freshwater wetlands are most comparable. Using an automated approach similar to the present study, net annual fluxes of nitrogen were measured for 2 yr in a high marsh area of the Rhode River, MD (20). The marsh is within the tidal portion of the river, and the vegetated areas are periodically flooded and exposed. There was a net annual loss of total nitrogen of 3.36 kmol ha-’ excluding upland nitrogen inputs [calculatedfrom Jordan and Correll (201. Althoughloss from this natural wetland is comparable to the cranberry bog, the impact of the losses cannot be evaluated given the uptake of the high marsh losses by other marshes and mudflats of the Rhode River system. Even accreting sphagnum bogs with minor water outflows lose nitrogen in outflows. Thoreau’s Bog, MA, was found to have a “loss” of 0.21 kmol ha-’, one-eighth that of the surface water-dominated cranberry bog [calculated from Hemond 4211. Given the changing land-use due to expansion of residential development within the region (431, it is important to compare the source strengths of the cranberry bog and housing of varying densities. Total input of nitrogen from on-site waste disposal, lawns, and impermeable surfaces associated with residential development have been measured in an adjacent developed sub-watershed to Buttermilk Bay (35). At the measured occupancy rate (1.91 persons/home), a development of 5.8 houses ha-’ would create a total nitrogen loading to the Bay equivalent to the bog. A more general comparison based on the mean regional occupancy rate (2.7)indicates that about 4 houses ha-’ (1.7 per acre) would equal the nitrogen contribution from the bog. The actual density of the adjacent residential development is 9.9 houses ha-’, yielding nearly double the net nitrogen discharge to the Bay from the cranberry bog. The potential role of various land-uses in coastal eutrophication is mediated by both the timing and the forms of nitrogen losses. Similar to natural wetlands, the seasonality of the nitrogen imports and exports by cranbeny bogs reduces potential negative impacts on receivingwaters. The bog retained nitrogen during the active growing season with the bulk of the net loss occurring in the fall and the winter when the assimilative capacity of the receiving systems was highest. In addition, one-third of the net nitrogen loss was as particulate organic nitrogen, which must be remineralized before uptake by phytoplankton. In contrast, nitrogen losses from residential development are nearly entirely as DIN with nearly uniform delivery via groundwater throughout the year and, therefore, are likely to play a greater role in the eutrophication of receiving waters. While we have less data on phosphorus losses through the outflow stream, the bog was clearly a source of acidleachable phosphate to recipient systems. Phosphorus is usually considered the limiting nutrient for algal productivity in freshwater systems. Wetzel (44) suggests phosphorus levels of 0.31-0.94 pM as being associated with eutrophic lakes. The measured concentration in the outflow from the bog almost always (99%) exceeded 0.6 pM
leachable phosphate. This could be of concern if the outflow was to a deep oligotrophicfreshwater pond of small volume relative to the stream discharge, since it would contribute to pond eutrophication. Relative to other landuses, phosphorus loading from bogs to oligotrophic ponds is potentially a larger problem since alternative land-uses in the region frequentlylose their nutrients to groundwaters. For example, phosphate discharges from an adjacent residential development of 524 homes on 53 ha to Buttermilk Bay (above) were similar to the forest due to the near complete retention of orthophosphate by aquifer minerals under aerobic conditions (41). However, the data from the present study indicate that phosphate losses from cranberry bogs may be greatly attenuated if the outflow passes first through a shallow pond where sedimentation and removal by rooted vegetation can occur before discharge. The surface water inflow to the bog we studied had previously passed through a 12.1-ha bog system and the reservoir pond. The inflows were below 0.6 pM 57% of the time. Assuming that the upper bog had similar losses of leachable phosphate as the study bog, passage through the pond lowered the loss from 360 to 100 kmol ha-' of vegetated bog.
Conclusions A cranberry bog at the head of Buzzards Bay, MA, was found to contribute both nitrogen and acid-leachable P043- to a shallow coastal embayment through outlet stream discharges. Nutrient losses to the bog outflow stream had a strong seasonality, similar to natural wetland systems. Nutrients were retained during the active growing season with greatest losses during senescence and harvesting disturbances in the fall. Phosphate loss was primarily during flooding for harvest and winter frost protection and appeared to be associated with low redox conditions in the surficial bog soils. Like natural wetlands, the bog loss of fixed nitrogen was primarily as NH4+ (53%)and PON (36%). Nutrient losses from ditch bottoms were small, 0111~36% of the active growing season loss and 3% of the annual loss. Nutrient release from ditch sediments appeared to be strongly influenced by fertilizer applications. A pond receiving surface water outflow from the upper bog system appeared to significantly reduce both nitrogen and acidleachable phosphate levels and may provide a method for reducing the moderate nutrient loading to downstream ecosystems associated with cranberry agriculture. The bog loses N at levels similar to that of a surface water-dominated freshwater wetland measured using a comparable approach (1.65 versus 3.36 kmol ha-', respectively) but at greater rates than a relatively hydrologically isolated sphagnum bog (0.21 kmol ha-9. Comparison of the nitrogen inputs to the adjacent bay indicated a nearly 2-foldhigher nitrogen loading from residentialhousing than cranberry agriculture, on an areal basis, but a higher phosphate loading from the bog. On average, direct discharge from the bog contributes the equivalent nitrogen loading of about 4 houses ha-'.
Acknowledyments We thank R. Van Etten and N. Millham for assistance with the field sampling; D. D. Goehringer, R. Van Etten, D. R. Schlezinger,D. S. White, and S. Brown-Legerfor laboratory analyses; and David Mann for graciously allowing the use of his bog system for study and providing fertilization and harvest data. Supported by EPA Grant CX-813548-01-2,
the Jessie B. Cox Charitable Trust, and an Independant Study award from the A. W. Mellon Foundation. Contribution No. 8734 of the Woods Hole OceanographicInstitution.
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(32) Canadian Water Quality Guidelines. Task Force on Water Quality Guidelines of the Canadian Council for Resource and Environmental Ministers, 1987. (33) U.S. EPA. Quality Criteria for Water. Office of Water Planning and Standards, US. EPA Washington, 1976; pp 10-13. (34) Froelich, P. N. Lirnnol. Oceunog. 1988, 33, 649-668. (35) Weiskel, P. K.; Howes, B. L. Water Resour. Res. 1991,27, 29292939. (36) Johnston, C. A. Crit. Rev. Environ. Control 1991, 21, 491-565. (37) Chambers, R. M.; Odum, W. E. Biogeochemisrry 1990, 10, 37. (38) Howes, B. L.; Dacey, J. W. H.; Goehringer, D. D. 4 Ecol. 1986, 74, 881-898. (39) Valiela, I.; Teal, J. M. Nature 1979, 280, 652-656. (40) Ollinger, S. V.;Aber, J. D.; Lovett, G. M.; Millham, S. E.; Lathrop, R. G.; Ellis, J. M. Ecol. Appl. 1993, 3, 459-472.
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(41)Weiskel, P. K.; Howes, B. L. Environ.Sci. Technol. 1992,26,352360. (42) Hemond, H. F. Ecology 1993, 64, 99-109. (43) Howes, B. L.; Goehringer, D. D. The Ecolosy ofBuzzards Buy; Estuarine Profile Series; US. Fish and Wildlife Service: Washington, DC, 1994, in press. (44) Wetzel, R. L. Limnologq: Saunders: Philadelphia, 1975.
Received for review June 13, 1994. Revised manuscript received December 2, 1994. Accepted December 21, 1994.@ ES9403625 @Abstractpublished in Advance ACSAbstructs, February 1, 1995.
