. Effects of zebra mussels on chlorophyll, nitrogen, phosphorus and silica in north shore waters of Lake Erie . DUNNVILLE ROSEHIU Figure 16. The percentage of measured chlorophyll a data values located below the -95% confidence boundary of the ARIMA forecasts for the five Lake Erie sites. forecasts over that offered by a short-term "before and after" comparison. For example, the "before and after" analysis showed an 83% reduction in spring ammoniura-N levels associated with the establishment of zebra mussels at Dunnville. The ARIMA forecasting analysis showed that these low
. Effects of zebra mussels on chlorophyll, nitrogen, phosphorus and silica in north shore waters of Lake Erie . DUNNVILLE ROSEHIU Figure 16. The percentage of measured chlorophyll a data values located below the -95% confidence boundary of the ARIMA forecasts for the five Lake Erie sites. forecasts over that offered by a short-term "before and after" comparison. For example, the "before and after" analysis showed an 83% reduction in spring ammoniura-N levels associated with the establishment of zebra mussels at Dunnville. The ARIMA forecasting analysis showed that these lower post zebra mussel ammonium concentrations were within the 95% confidence limits of the forecast concentrations (Fig. 19). Data used to generate this forecast included apparently two cyclic {periods of about seven years duration having minima during the mid 1970's and early 1980's that were similar to the low values recorded during the post zebra mussel time period. Similar cyclic patterns were found in the summer ammonium-N data at Dunnville and Rosehill (Fig. 20). 300 ^ Q. f2 200 Q. o o JZ O 100 0- Pre-zebra mussels (1976-87) Post zebra mussels (1990-94) ^ 4? ^ , ^ # ^ Q^ ^°' Figure 17. Means + 1 St. Dev. of April to November chlorophyll a- to-total phosphorus ratios at the five Lake Erie intake sampling locations during pre- and post zebra mussel time periods. 19
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