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Lake Simcoe water quality update - LSEMS phase 2 progress report, 1995-1999

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Title: Lake Simcoe water quality update - LSEMS phase 2 progress report, 1995-1999
Author: Nicholls, K. H.
Abstract: This brief report on Lake Simcoe water quality represents the lake monitoring component of the Ministry of the Environment's contribution to the LSEMS Phase II progress report for 1995-1999. Four "indicator" variables, total phosphorus (TP), chlorophyll a, Secchi disk visibility (a measure of water clarity) and dissolved oxygen have been measured consistently at 12 sampling locations in Lake Simcoe for the May-October periods of the past 20-25 years. Results are also reported for samples collected year-round from four municipal water supply intakes in the lake and from the outflow at Atherley. Statistically significant trends in all four of these variables were found at some of the monitored Lake Simcoe stations; chlorophyll a showed the least evidence of long-term trends, while the most dramatic changes were in water clarity. TP concentrations were generally lower during the 1990's than during the 1980's as revealed by the data from the main lake (May-October) sampling stations, the water intakes and the lake outflow; however, long-term trends were complicated by shorter periods of increases and decreases over 3-6 year intervals. Decreasing TP trends for the period ending in 1999 were found only at Cook's Bay stations C1, C6 and C9, and for Station N31 at the north end of the lake, and for Station K39 in Kempenfelt Bay for both the 1975-1994 and 1975-1999 periods. Overall rates of TP decline ranged from 0.15 to 1.0 ?g L-1 yr-1, some of which could be attributed to removal of particulate TP from the lakewater by zebra mussels since their establishment in the lake in 1994-95. More specific tests of the zebra mussel effect (detection of step trends between 4-year pre- and post zebra mussel time periods) revealed significantly lower TP concentrations for the post zebra mussel time period at all sampling stations except N32 at the north end of the lake, the Kempenfelt Bay stations and main lake Station K45. At the seven sampling stations showing statistically significant change, percentage reductions in TP associated with the zebra mussel establishment in the lake ranged from 21 to 41 %. The apparent effects of the establishment of zebra mussels on Lake Simcoe water clarity were very dramatic. Water clarity during the 4-year post zebra mussel period 1996-1999 was as much as 67 and 65 % higher than during the pre-mussel period of 1991-1994 at stations E50 and E51 on the east side of the lake, and 69 % higher at Station N31 at the north end of the lake. Significant step trends in water clarity were detected for all sampling stations, even the deep-water Kempenfelt Bay and main lake sites, where water clarity increased by 22, 38 and 43 % at stations K45, K42 and K39, respectively. The volume-weighted deep-water oxygen depletion rate (normalized to a 4 degrees Celsius rate) revealed a downward trend through the 1990's (ignoring the 1999 data, which may be in error). This was in contrast to a worsening (increasing depletion rates) trend through the 1980's. Apparent improvements may relate to declining trends in TP and to the recent impacts of zebra mussels. Several more years of monitoring data may be required to determine if the apparent improvements in dissolved oxygen can be sustained.
Date: 2001
Rights: Queen's Printer for Ontario, Crown Copyright, Non-Commercial Use Permitted
Rights Holder: Queen's Printer for Ontario

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