Environ. Sci. Technol. 1997, 31, 210-213
Clouds in Southern Chile: An Important Source of Nitrogen to Nitrogen-Limited Ecosystems? KATHLEEN C. WEATHERS* AND GENE E. LIKENS Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545
Rainwater collected from remote, southern Chile is reported to be some of the most dilute in the world and is estimated to result in the deposition of e1 kg ha-1 yr-1 of nitrogen to ecosystems. Rainwater, however, is only one form of atmospheric deposition. Cloudwater deposition and the deposition of particles and gases can result in significant atmospheric inputs to ecosystems. Here we report the first data on cloudwater chemistry from remote, southern Chile. Cloud samples were collected from 1987 to 1994 using active cloudwater collectors. Average cloudwater chemistry from remote, southern Chile was dominated by ions commonly associated with seawater [e.g., Ca2+, Mg2+, Na+, Cl-), but had surprisingly high concentrations of inorganic nitrogen (NH4+ (48.3) and NO3- (19.6 µequiv/ L)] as well. Relative to volume-weighted mean concentrations of rainwater from a nearby location, cloudwater ranged from 2 (H+) to 80 (NH4+) times more concentrated. Estimated nitrogen deposition via cloudwater suggests that clouds may be a very important source of nitrogen, especially for nitrogen-limited ecosystems in this region.
Introduction The chemistry of precipitation in southern Chile is thought to reflect natural rather than anthropogenic processes. Further, measurements of rain chemistry from two sites in southern Chile suggest that this region is one of the closest approximations of pre-industrial atmospheric conditions in the world (1-4). Rainwater, however, is only one form of atmospheric deposition. Nutrients and pollutants are delivered to ecosystems from the atmosphere via several wet and dry processes. Wet deposition includes rain, snow, cloudwater, fogwater and rime ice (frozen cloud) deposition. The deposition of gases and dry particles to ecosystems is considered dry deposition (e.g., refs 5-7). The relative contribution of each process depends upon many factors, such as frequency and amount of rain and snow, frequency of cloud cover, the condition and architecture of impaction surfaces, and wind speeds (7-9). Both wet and dry processes are important for most ecosystems, often making roughly equal nutrient and pollutant contributions, and in some coastal and high elevation ecosystems, cloud deposition dominates. For example, cloudwater contributes ∼50-80% of the total sulfur and nitrogen deposited to some high elevation and coastal ecosystems in the United States (e.g., refs 7, 10, and 11). Although concentrations are low, several investigators have suggested that rain nevertheless provides ecologically significant inputs of nutrients to ecosystems in southern and * Corresponding author telephone: 914-677-5343; fax: 914-6775976; e-mail:
[email protected].
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central Chile (1-4, 12). Recognizing that cloudwater might be an additional source of nutrients to these ecosystems, in 1987 we began studies to measure cloudwater chemistry in southern Chile. Much of coastal Chile receives significant rainfall, and some areas are also subject to prolonged periods of cloud immersion (13, 14). Although clouds have been considered a source of water for northern and central Chile (13), no research has been conducted on the chemistry of cloudwater in remote, southern Chile. Here, we present the results of several years of cloud sampling and discuss the potential importance of nutrients in cloudwater for cloudimpacted, southern temperate ecosystems.
Site Description and Methods Cloudwater samples were collected between 1987 and 1994 from two sites in southern Chile: Torres del Paine National Park (TdP) (51°10′ S, 71°58′ W) and a site 400 km south of TdP and ∼5 km west of Punta Arenas in the Magallanes Preserve (PA) (53°09′ S, 70°55′ W). Both sites are low elevation, approximately 50 and 400 m above sea level, respectively. After the first year of attempting to make cloud collections (1987-1988), we found that cloudwater events were infrequent at TdP and thus moved the site to PA. TdP is a remote, inland site that is distant from local and regional anthropogenic sources of pollution (1-3). The PA site is approximately 10 km upwind of the coastal city of Punta Arenas (population of 60 000), which borders the Straits of Magellan. The PA site is bordered by inlets of the Pacific Ocean to the west (∼10 km) and the Atlantic Ocean to the east (∼10 km). Cloudwater collectors were installed in open areas (no trees were within a >20 m diameter of the sampling point) free of woody vegetation. Forests in the nearby preserve are dominated by Nothofagus spp. Tree heights are variable with a maximum of ∼20 m. Cloudwater samples were collected on an event basis using the protocols of Weathers et al. (15): battery-powered CASC cloudwater collectors were mounted on stands 2 m aboveground and were covered between events with clean plastic bags to exclude dry particles and gases. These collectors have a lower droplet size cut of ∼3.5 µM (16). At the beginning of each event, the collector was uncovered and then “purged” by operating it for 15 min. During the purge, site operators made certain that the collection strands of the collector were well wetted, and several milliliters of water were collected in the collection bottle. The cloudwater collected during this purge was discarded. The collector then was operated for an average of 5 h, with a maximum of 7 h, or until the end of the event, whichever came first, according to the protocols of Weathers et al. (15). This sampling time period was commensurate with optimum life of the batteries that power the cloud collectors (15). After collection, the collector was covered (as noted above), samples were poured into clean, 250-mL bottles, and 0.5 mL of CHCl3 was added to prevent biological activity (17). Samples were stored in a cold, dark room until shipment to laboratories at the Institute of Ecosystem Studies (IES) (within ∼6 weeks of collection). First, samples were analyzed for pH and NH4+ (immediately after uncapping) and then for SO42-, NO3-, Cl-, Ca2+, Mg2+, Na+, and K+ using the methods and protocols of Weathers et al. (15). The data reported here (n ) 22) consist of two cloud events collected from TdP in 1987 and 20 from PA collected during 1988-1994. Not all cloud events were sampled from 1987 to 1994 due to (1) variation in the time of day that events occurred; (2) the availability of an operator; and (3) collector downtime, mostly as a result of power source limitations to recharge batteries. There were too few samples from TdP to test for statistical differences between sites; however, the data from the TdP site fell within the concen-
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TABLE 1. Chemical Concentrations of Cloudwater Collected from Punta Arenas and Torres del Paine, Chile, and Cloud-to-Rain Ratios Showing Enhancement of Cloud Chemistrya cloud concn av median SE min max ratios av cloud:VWM rain median cloud:VWM rain
K+
Ca2+
Mg2+
Na+
Cl-
SO42-
NH4+
NO3-
pH
11.7 6.14 2.96 1.79 64.0
42.4 31.4 8.77 6.49 175
60.4 43.6 11.0 3.29 169
257 169 46.8 3.92 726
310 172 46.4 23.4 886
71.9 72.9 11.1 8.33 111
48.3 42.3 7.38 2.22 116
19.6 14.7 4.37 0.16 101
4.77 5.15
29 15
39 29
19 14
20 13
18 10
26 26
81 71
39 29
4.20 6.34 1.6 1
a Rain concentrations used are volume-weighted averages (VWA) from Torres del Paine National Park, Chile (3). Cloudwater data (n ) 22) are for the period 1987-1994, rainwater data are for the period 1984-1993 (n ) 198). Data are in µequiv/L, except for pH. Cloud-to-rain ratios are based on ratios of H+ rather than pH.
tration ranges of the PA site, thus the data for the two sites were combined for this analysis. The calculated overall average ion balance [((cations - anions)/0.5(cations + anions)) × 100] was Na+ . SO42- > Mg2+ > NH4+ > Ca2+ . NO3- > H+ > K+ (Table 1). The average pH of cloudwater from these sites was 4.8. Of particular interest is the relatively large contribution (8%) of inorganic nitrogen (N) to the total ionic strength of these samples. There are no other data for southern Chile with which to compare our results; however, during 1984 and 1985, we measured cloud chemistry throughout North America using the same methods and collectors (15). Mean inorganic N concentrations in cloudwater samples collected from our Chilean site are higher than those from the sites in the Pacific Northwest, United States (Figure 1; (15)). Concentrations of nitrate and especially ammonium were much enhanced (3- and 6-fold higher, respectively) in southern Chile as compared to cloudwater collected from the Alaska (AK) site and 4- (NH4+) to 2-fold (NO3-) higher as compared to the Oregon (OR) site (Figure 1). The sites in the Pacific Northwest were chosen because we assumed that they were representative of sites relatively unimpacted by humans in North America; that they were characterized by less inorganic nitrogen than cloudwater from a more remote region in southern Chile was surprising. Cloudwater and Rainwater Comparison. Cloudwater chemistry from remote southern Chile is, on average, many times more concentrated than volume-weighted average (VWA) rainwater from TdP (Table 1), which is the closest available precipitation collection site to our cloud collection sites. Weathers et al. (15) also found that cloudwater in North
FIGURE 1. Average chemical concentrations (µequiv/L) of cloudwater from Torres del Paine (TdP) and Punta Arenas (PA), Chile (n ) 22, collected from 1987 to 1994); southern coastal Alaska (AK); and Oregon (OR). Standard error bars are shown. Data for AK and OR, n ) 18 and 12, respectively, were collected from 1984-1985, see ref 15, and volume-weighted average rainwater chemistry (19841993) was from Torres del Paine, Chile [n ) 198 (2, 3)]. America is, on average, 3-7 times more concentrated than volume-weighted mean rainwater collected from nearby locations. The cloud:rain enhancement in southern Chile, however, is extraordinarily high for some ions: NH4+ concentration in cloudwater was 80-fold greater than volumeweighted mean rainwater from TdP (Table 1). Calcium and NO3- cloud vs rain enhancements are also very high (both 39×). Overall, relative enhancements are as follows: NH4+ . NO3- ) Ca2+ > K+ > SO42- > Na+ > Mg2+ > Cl- > H+. Median cloudwater concentrations showed similar enhancements (Table 1). Though no measurements are available for cloud droplet size distributions, the chemical composition of cloudwater has been shown to be strongly related to droplet size (e.g., refs 18 and 19). When comparing cloud:rain ratios (Table 1), there is a noticeable grouping of ions associated with sea salt (e.g., Ca2+, Mg2+, Na+, Cl-) versus acidic species (SO42-, NO3-, and NH4+). Others (e.g., ref 18) have found that sea salt aerosols may be present in larger droplet size classes while the acidic species are often associated with relatively smaller size droplets. The large differencessand similaritiessamong ratios shown in Table 1 thus largely may be a function of the heterogeneity of cloudwater chemistry and its relationship to droplet size. Again, perhaps the most surprising result was the relative enhancement of inorganic N in cloudwater vs rainwater. The
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high concentrations of nitrogen in cloudwater are particularly interesting in light of the fact that nitrogen may be a limiting nutrient in temperate terrestrial and aquatic ecosystems (e.g., refs 2-4, 12, and 20). The remainder of the discussion, therefore, will focus on three important, N-related ecological questions that arise from our data: (1) Why is there such a large enhancement of cloud:rain N? (2) How much N might be deposited to ecosystems in southern Chile via cloudwater? (3) What is the source of the N? Cloud:Rain N Enhancement. What might cause such a large difference between nitrogen concentrations in cloud and rain? There are a few possible answers to this question: (a) Different sites. Cloudwater and rainwater were collected from different sites. It is possible that cloud and rain collected from the same site would show less enhancement, although modeling estimates of N deposition via rainwater for the region (21) are consistent with those of TdP. (b) Fog vs rain. Coastal fogs often have lower LWC and higher chemical concentrations than high-elevation clouds (15). In addition, rainfall along the coasts may come from different cloud masses than fog events (22), which could result in greater chemical enhancement of cloud/fog vs rain. This latter factor is perhaps the most likely possibility for observed differences in cloudwater and rainwater chemistry. Cloud Deposition. Though we did not measure cloudwater deposition per se, it is possible to make some estimates of the potential contribution of nutrients in cloudwater to total atmospheric deposition in this region. Cloud cover in southern Chile is estimated to be high during austral spring and fall (14), although there are no estimates of cloud immersion time for these ecosystems. Galloway et al. (2) report that annual precipitation (rain only) is 75 cm yr-1 at TdP, while Hedin et al. (4) estimate 250-300 cm yr-1 precipitation for a coastal site further north in Chile (42°22′ S, 74°03′ W). Based on studies of other cloud-impacted ecosystems in the Northern Hemisphere as well as one study from northern Chile that quantified cloudwater deposition to passive collectors (13), it is likely that cloudwater deposition ranges from 10 to 80 cm annually (10, 11, 23). Using these assumptions, cloud deposition at PA would be from ∼1 to 8 kg N ha-1 yr-1. Since current estimates of nitrogen deposition via precipitation are 25% of the time for each month). When northwest, southwest, and west quadrants are combined, they account for 60% of the wind direction. As noted above, the PA site lies to the west of Punta Arenas
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where easterly winds would carry air masses from the city of Punta Arenas to the sampling site. During the months when cloud events were most frequent (April-September), winds came from the east and southeast on average only 12% of the time, ranging from 10 to 17%. We cannot say definitively what the source of nitrogen is in cloudwater collected from this region; however, Galloway et al. (2) believe that, for rainwater, there is likely a significant biologic source, either marine (e.g., upwelling) or terrestrial. Ecological Significance. Many temperate ecosystems in the Southern Hemisphere are thought to be nitrogen limited (e.g., refs 4 and 12). Likens et al. (1), Galloway et al. (2, 3), and Hedin et al. (4) have suggested that the nitrogen deposition via precipitation to temperate ecosystems in southern Chile is among the lowest in the world. Our data suggest that cloud deposition to these ecosystems may be a very importantsand as yet unaccounted forssource of this growth-limiting nutrient. Further research is necessary to test this hypothesis. In addition, inasmuch as remote locations in the Southern Hemisphere represent pre-industrial conditions (28), quantifying total nutrient inputs to these ecosystems, including the relative contribution of cloud deposition, is critical to constructing comparative input and output budgets as well as to understanding the biogeochemistry of ecosystems (4, 29). This seems especially important as atmospheric N increases worldwide due to human activities (21). Future research might include both quantifying cloud deposition and determining the extent to which natural (e.g., oceanic upwelling) vs anthropogenic (e.g., agriculture) sources contribute to inorganic nitrogen in cloudwater.
Acknowledgments We thank the Corporacion Nacional Forestal (CONAF) staff in Punta Arenas and Torres del Paine, Chile, for their logistical assistance and field collections; Rick Artz for providing wind rose data; and the IES Analytical Laboratory staff for chemical analyses. This research was funded by grants from the Andrew W. Mellon Foundation and the National Oceanic and Atmospheric Administration. This is a contribution to the program of the Institute of Ecosystem Studies.
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Center for Atmospheric Research; Washington, DC, 1986; DOE/ Er/60085-H1. Weathers, K. C.; Likens, G. E.; Bormann, F. H.; Bicknell, S. H.; Bormann, B. T.; Daube, B. C., Jr.; Eaton, J. S.; Galloway, J. N.; Keene, W. C.; Kimball, K. D.; McDowell, W. H.; Siccama, T. G.; Smiley, D.; Tarrant, R. A. Environ. Sci. Technol. 1988, 22, 10181026. Collett, J., Jr.; Daube, B. C., Jr.; Morgan, J. W.; Hoffman, M. R. Atmos. Environ. 1990, 24A, 1685-1692. Keene, W. C.; Galloway, J. N.; Holde, D. H., Jr. J. Geophys. Res. (Oceans Atmos.) 1983, 88, 5122-5130. Collett, J., Jr.; Oberholzer, B.; Staehelin, J. Atmos. Environ. 1993, 27A, 33-42. Noone, K. J.; Charlson, R. J.; Covert, D. S.; Ogren, J. A.; Heintzenberg, J. J. Geophys. Res. 1988, 93 (D8), 9477-9482. Vitousek, P. M.; Howarth, R. W. Biogeochemistry 1991, 13, 87115. Galloway, J. N.; Levy, H., II; Kasibhatala, P. S. Ambio 1994, 23, 120-123. Kimball, K. D.; Jagels, R.; Gordon, G. A.; Weathers, K. C.; Carlisle, J. Water, Air Soil Pollut. 1988, 39, 383-393.
(23) Lovett, G. M. Atmos. Environ. 1984, 18, 361-371. (24) Asman, W. A. H. Nova Acta Leopoldina 1994, NF 70, No. 288, 263-297. (25) Dentener, F. J.; Crutzen, P. J. J. Atmos. Chem. 1994, 19, 331-369. (26) Galbally, I. E. In The Biogeochemical Cycling of Sulfur and Nitrogen in the Remote Atmosphere; Galloway, J. N., et al., Eds.; D. Reidel Publishing Co.: Dordrecht, 1985; pp 27-53. (27) Warneck, P. Chemistry of the Natural Atmosphere; Academic Press, Inc.: San Diego, 1988. (28) Galloway, J. N.; Likens, G. E.; Keene W. C.; Miller, J. M. J. Geophys. Res. 1982, 87, 8771-8786. (29) Likens, G. E.; Bormann, F. H. Biogeochemistry of a Forested Ecosystem, 2nd ed.; Springer-Verlag: New York, 1995.
Received for review April 16, 1996. Revised manuscript received August 13, 1996. Accepted August 14, 1996.X ES9603416 X
Abstract published in Advance ACS Abstracts, November 1, 1996.
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