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Effects of zebra mussels on chlorophyll, nitrogen, phosphorus and silica in north shore waters of Lake Erie

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Title: Effects of zebra mussels on chlorophyll, nitrogen, phosphorus and silica in north shore waters of Lake Erie
Author: Nicholls, K. H.; Hopkins, G. J.; Standke, S. J.
Abstract: Chlorophyll a and nutrient concentrations have been measured weekly and year-round at five municipal water supply intakes in Lake Erie since 1976. The establishment of zebra mussels (Dreissena polymorpha) in Lake Erie in the late 1980's-early 1990's resulted in dramatic reductions in chlorophyll levels at all five locations in all seasons of the year. The greatest change was in the western basin (Union intake), where summer concentrations averaging 6.3 ?g/L, during a 4-year pre-zebra mussel period declined to 1 ?g/L during a 4-year post mussel time period. Declines in total phosphorus (TP) were less dramatic; the chlorophyll-to-TP ratio was three times higher before the establishment of zebra mussels than after. TP values during summer in the western and west-central basins of the lake were less than the -95% confidence band for Box-Jenkins ARIMA (autoregressive integrated moving average) forecast values based on the pre-zebra mussel data, and provide supporting evidence for the net removal of phosphorus from the water column by zebra mussels. The reduction in phosphorus was greater than the reduction in nitrogen and resulted in a doubling of summer total N-to-total P ratios after the arrival of zebra mussels. The reduction in summer TP of 20 ?g/L at the north shore, central basin sampling location (Elgin intake) was used to conservatively estimate a near shore (0-12m depth contour) summertime removal of about 6000 metric tons for the whole lake. Observed increases in fall dissolved reactive silica and dissolved reactive phosphorus were attributed to reduced phytoplankton biomass by zebra mussel filtration of the water, and consequently, a lowered rate of uptake of dissolved phosphorus and silica from the water column. Summer ammonium-N concentrations were reduced 45-77% at the five locations apparently in response to reduced biomass and decomposition of phytoplankton contributing to a reduction in availability of organic-N for ammonification, especially in the west-central basin, where summer ammonium-N concentrations were well below the lower 95% confidence values of the ARIMA prediction. Eastern basin ammonium-N data were cyclic (with a period of about seven years) coinciding with flow rates of major southern Ontario rivers. The dramatic reduction in ammonium-N detected in the short term "before and after" comparisons at the two eastern basin sampling locations (Dunnville and Rosehill) may therefore have had less to do with the arrival of zebra mussels than with the cyclic supply of ammonium-N from the watershed.
Date: 1997
Rights: Queen's Printer for Ontario, Crown Copyright, Non-Commercial Use Permitted
Rights Holder: Queen's Printer for Ontario

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