Research Communications Reconstruction of Lake Phosphorus Loading and Dynamics Using the Sedimentary Record
the DI-TP concentration and lake outflow volume records the output; the sum of these two components records the input. We can present this more formally using a continuous stirred tank reactor model of a lake (4, 20). Assuming that phosphorus sedimentation depends on lake concentration, the steady-state lake model is
TP ) L/z(F + σ)
BRIAN RIPPEY* Freshwater Laboratory, University of Ulster, Traad Point, Ballyronan BT45 6LR, Northern Ireland
N. JOHN ANDERSON
where TP is the lake total phosphorus concentration (mg m-3), L is the total phosphorus loading on the lake (mg m-2 yr-1), z is the lake mean depth (m), F is the hydraulic flushing coefficient (yr-1), and σ is the phosphorus sedimentation coefficient (yr-1). Rearranging
Geobotany Division, Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark
Introduction Eutrophication is a widespread environmental problem that now affects rivers, lakes, estuaries, and in some areas the nearshore ocean. At present, the phosphorus loading on a lake is established by monitoring or desk evaluation, and loading-lake response models are the main tools of lake management. There have, however, been three recent developments in the understanding of freshwater eutrophication. First, diffuse sources of phosphorus are now known to be important (1-3); second, the theoretical basis of phosphorus loading-lake response models (4), particularly the influence of loading and lake concentration on phosphorus sedimentation, has been improved (5); finally, diatom-total phosphorus transfer functions have been developed that allow the changes in phosphorus concentration in a lake to be reconstructed from fossil diatom communities in the sediment (6-8). Lake sediment cores have been used for 2 decades to produce a qualitative interpretation of changes in lake phosphorus concentration and loading (9-12), and while whole-lake-basin sedimentary phosphorus accumulation rates accurately record the phosphorus retained in a lake (13-15), the problems of using the sedimentary record to quantitatively interpret lake concentration and loading history have been recognized (10, 16-18,). The main problem is the change in phosphorus sedimentation as a result of anoxia (19). We present a new development in the field of environmental reconstruction using lake sediments that allows the total phosphorus load on a lake over the last 150 years to be accurately reconstructed. The key is that the total phosphorus load can now be reconstructed by combining the diatom-inferred total phosphorus (DI-TP) concentration with the whole-lake-basin sedimentary phosphorus accumulation rate. As input ) (output + sedimentation) in a lake in steady state and the sedimentary accumulation rate records the phosphorus sedimentation, the product of * Corresponding author fax: +44 1648 418777; e-mail address:
[email protected].
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L ) TPzF + TPzσ The flux through the lake outflow (TPzF, mg m-2 yr-1) and accumulation in the sediment (TPzσ, mg m-2 yr-1) can now be estimated from the sedimentary record. The product of the DI-TP concentration (an estimate of TP), lake mean depth, and the hydraulic flushing coefficient (estimated or measured) gives TPzF, while TPzσ is estimated directly from the whole-basin sedimentary phosphorus accumulation rate. If the lake is not in steady state, we should add a term for the change in lake storage of phosphorus in order to calculate the correct loading. It is the ability to infer the lake total phosphorus concentration from the diatom record in the sediment that allows this development to be made. The sedimentary record has been used before to reconstruct lake phosphorus concentrations by assuming a constant phosphorus sedimentation coefficient (10).
Methods The information necessary to reconstruct the total phosphorus loading on a lake is available for Augher Lough, Northern Ireland. This is a small lake (area 9.25 ha, mean depth 5.5 m) that became eutrophic as a result of creamery effluent disposal between 1890 and 1976. A multicore study of its sediments has provided a detailed description of the eutrophication (21, 22), including DI-TP concentrations and sedimentary phosphorus accumulation rates (23). The information, selected from seven 1 m sediment cores sectioned into 1-cm slices, was chosen to ensure that there was at least one and preferably two slices in each time interval. Intervals of 10 years from 1850 to 1930 and 5 years thereafter were chosen. The diatom-total phosphorus transfer function was developed from a set of 49 lakes in Northern Ireland that were monitored for 3 years and found to have mean total phosphorus concentrations from 15 to 800 µg L-1 (7, 23). Weighted averaging regression and calibration are now used to quantify the relationships between diatoms and water chemistry (24). The diatom species abundance in surface (0-1 cm) sediment is related to the phosphorus concentration by a unimodal response model using weighted averaging regression, as the weighted averaging optima are
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FIGURE 1. Total phosphorus loading on Augher Lough from 1850 to 1980 reconstructed from the sedimentary record. The phosphorus fluxes through the outflow into the sediment and due to changes in lake storage are also shown separately.
very close to the unimodal optima. The error statistics of the predictions are good; RMSEBoot of the log predictedlog observed plot (r2 ) 0.73) is 0.244 as determined by bootstrapping. The inverse deshrinking option within WACALIB was used (25), and all species with an abundance greater than 2% in either lake water samples or the cores were used for the reconstruction (n ) 148). Finally, weighted averaging calibration is then used to infer the epilimnetic lake total phosphorus concentrations from the fossil diatom communities in the sediment cores. The whole-lake-basin sedimentary phosphorus accumulation rates were calculated from the arithmetic mean of cores A, AA4, AA7, AA10, AA12, AB2, and AB8. There was almost always two values in each time period in each core. Cores AA4, AB2, and AB8 have high sediment accumulation rates and so do not provide values before 1890. As sediment accumulates only below 2 m depth, 59.5% of the lake area (26), the sediment accumulation rates were converted to a whole-lake area basis using this factor. The hydraulic flushing coefficient was estimated to be 2.2 yr-1 in 1978 (22) and assumed to apply to the whole period.
Results and Discussion The reconstructed total phosphorus loading on Augher Lough from 1850 to 1980 is shown in Figure 1. A steadily increasing load from 1880 to 1945, with a stronger rise thereafter, until a small fall after 1975 provides a quantitative description of the eutrophication history of the lake. In addition to the rapid rise in external loading after 1945,
FIGURE 2. Variation of sedimentary phosphorus accumulation rate (AR) with diatom-inferred total phosphorus (DI-TP) concentration and lake total phosphorus loading in Augher Lough during the period 1850-1980 reconstructed from the sedimentary record. These two plots are appropriate for lake phosphorus models where sedimentation is proportional to lake concentration (proportionality constant is the phosphorus sedimentation coefficient, σ, yr-1) and loading (proportionality constant is the phosphorus retention coefficient, Rp), respectively (4). Values for σ and Rp are calculated for the main series of points (9) and for the period between 1946 and 1970 (0).
there was an important change in lake phosphorus dynamics during the 1946-1950 interval. As indicated by the outflow flux, the lake phosphorus concentration rose abruptly (DI-TP from 59 to 97 µg L-1) during this 5-year period. The change in lake storage represents a loading of 0.21 g m-2 y-1 and accounts for the overshoot in loading compared to 1951-1955. This indicates that the reconstructed loading should be considered the total load, i.e., the sum of external and internal loads. The sudden increase in lake phosphorus concentration and internal load were due to changes in oxygen/redox conditions around the sediment-water interface (19, 27) and a drop in sedimentary manganese concentrations at this time provides direct support for this in Augher (28). The lake phosphorus dynamics are further examined in Figure 2. There has been much debate about the basis of lake phosphorus models, particularly what controls phosphorus sedimentation (5). The utility of lake phosphorus models depends on describing the net annual phosphorus sedimentation (4), and Figure 2 shows that both the two main hypotheses, sedimentation depends on either lake concentration or loading, apply equally well to Augher. The reduction in phosphorus sedimentation efficiency between 1946 and 1970, due to changed oxygen/redox conditions (19, 27), is abrupt. Furthermore, although the evidence is
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not definitive, the phosphorus dynamics in Augher appear to eventually adjust to the increased load after 25 years. When sufficient sediment cores are available, it can be established if shallower or deeper water sediments are responsible for the reduction in phosphorus sedimentation efficiency. The results show that cores from less that 9.5 m depth are mainly responsible for the reduced sedimentation in Augher. Researchers have found the scarcity of long time series of data on lakes, in comparison to the cross-sectional information available, an impediment to understanding lake phosphorus dynamics and the development of sound models (4, 5, 29). The sediment record can now be interrogated for this information on the factors that control phosphorus sedimentation in lakes. While the basis of this development is theoretically sound, the accuracy of the reconstruction depends on three factors: diatom-total phosphorus transfer functions, wholebasin sedimentary phosphorus accumulation rates, and the ability to estimate past lake hydraulic residence time. Checks on the accuracy of diatom-total phosphorus transfer functions are necessary (30). Whole-lake-basin sedimentary phosphorus accumulation rates accurately record the phosphorus retained in a lake (13-15), and so problems due to the mobility of phosphorus in the sediment due to diagenesis are small. We have also assumed that the hydraulic residence time in Augher during 1978 applies to the whole period since 1850, but there are usable hydrological models that can reconstruct stream flow (31). A check of the accuracy of the phosphorus loading reconstruction in a lake with a known loading history is also necessary. Nevertheless, this development will allow the information on phosphorus loading, lake concentration, and dynamics contained in lake sediments to be unlocked, and in this way the links between lake palaeoecology and ecology, particularly on ecosystem disturbance and recovery, can be strengthened for mutual benefit (32, 33). Combined with the information available from other microfossils (34), this development will allow lake sediments to be used to study ecological processes, particularly eutrophication.
(2) Sharpley, A. N.; Withers, P. J. A. Fert. Res. 1994, 39, 133. (3) Krug, A. Hydrobiologia 1993, 251, 285. (4) Ahlgren, I.; Frisk, T.; Kamp-Nielsen, L. Hydrobiologia 1988, 170, 285. (5) Prairie, Y. T. Aquat. Sci. 1989, 51, 192. (6) Hall, R. I.; Smol, J. P. Freshwater Biol. 1992, 27, 417. (7) Anderson, N. J.; Rippey, B.; Gibson, C. E. Hydrobiologia 1993, 253, 357. (8) Bennion, B. Hydrobiologia 1994, 275/276, 391. (9) Williams, J. D. H.; Murphy, T. P.; Mayer, T. J. Fish. Res. Board Can. 1976, 33, 430. (10) Moss, B. Freshwater Biol. 1980, 10, 261. (11) Rippey, B.; Murphy, R. J.; Kyle, S. W. Environ. Sci. Technol. 1982, 16, 23. (12) Edmonson, W. T. Limnol. Oceanogr. 1991, 36, 1031. (13) Cross, P. M.; Rigler, F. H. Can. J. Fish. Aquat. Sci. 1983, 40, 1589. (14) Evans, R. D.; Rigler, F. H. Can. J. Fish. Aquat. Sci. 1983, 40, 506. (15) Johnson, M. G.; Nicholls, K. H. J. Great Lakes Res. 1989, 15, 265. (16) Bengtsson, L.; Persson, T. Pol. Arch. Hydrobiol. 1978, 25, 17. (17) Engstrom, D. R.; Wright, H. E. In Lake Sediments and Environmental History; Haworth, E. Y., Lund, J. W. G., Eds.; Leicester University Press: Leicester, 1984; pp 11-67. (18) Schelske, C. L.; Robbins, J. A.; Gardner, W. S.; Conley, D. J.; Bourbonniere, R. A. Can. J. Fish. Aquat. Sci. 1988, 45, 1291. (19) Nurnberg, G. K. Limnol. Oceanogr. 1984, 29, 111. (20) Vollenweider, R. A. Schwiez. Z. Hydrol. 1975, 37, 51. (21) Anderson, N. J. J. Paleolimnol. 1990, 3, 143 and references therein. (22) Anderson, N. J.; Rippey, B.; Stevenson, A. C. Freshwater Biol. 1990, 23, 205. (23) Anderson, N. J.; Rippey, B. Freshwater Biol. 1994, 32, 625. (24) Anderson, N. J. TREE 1993, 8, 356. (25) Line, J. M.; Birks, H. J. B. J. Paleolimnol. 1990, 3, 170. (26) Battarbee, R. W.; Titcombe, C.; Donnelly, K.; Anderson, N. J. Hydrobiologia 1983, 103, 71. (27) Davison, W. Earth Sci. Rev. 1993, 34, 119. (28) Anderson, N. J.; Rippey, B. Limnol. Oceanogr. 1978, 33, 1476. (29) Vollenweider, R. A. Mem. Ist. Ital. Idrobiol. Dott. Marco de Marchi 1976, 33, 53. (30) Bennion, H.; Wunsam, S.; Schmidt, R. Freshwater Biol. 1996, 34, 271. (31) Blackie, J. R.; Eeles, C. W. O. In Hydrological Forecasting; Anderson, M. G., Burt, T. P., Eds.; Wiley: Chichester, 1985; pp 311-346. (32) Smol, J. P. Hydrobiologia 1990, 214, 201. (33) Anderson, N. J. Freshwater Biol. 1995, 34, 367. (34) Smol, J. P. Mem. Ist. Ital. Idrobiol. Dott. Marco de Marchi 1990, 47, 253.
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Received for review September 8, 1995. Revised manuscript received January 22, 1996. Accepted January 24, 1996.
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