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2020 Status of the Lake Ontario lower trophic levels

July 1, 2021

Significant Findings for Year 2020: Note that due to covid-19 restrictions, offshore sampling was limited in 2020.

1) May – Oct total phosphorus (TP) in 2020 was 10.6 µg/L (offshore) and 7.7 µg/L (nearshore), higher than the long-term (1995-2019) average in the offshore (6.2 µg/L) and close to average in the nearshore (7.8 µg/L); mean TP values for the past decade (2010-2019) were 6.0 µg/L and 7.9 µg/L in the offshore and nearshore, respectively. In 2020, TP concentrations were significantly higher (p=0.03) in the offshore compared to the nearshore. Note that offshore duplicate samples had high relative percent difference (average 54%. 6-117%) making inferences for the offshore in 2020 uncertain.

2) May – Oct epilimnetic chlorophyll-a was similar at nearshore (1.9 µg/L) and offshore (2.1 µg/L) sites. These values were slightly higher than the average for 1995 – 2019 (1.7 µg/L, offshore; 1.5 µg/L, nearshore) and higher than for the last decade (1.4 µg/L, both offshore and nearshore).

3) May – Oct Secchi depth ranged from 3.5 m to 10.4 m (11 ft to 34 ft) at individual sites and was not significantly different between nearshore (6.3 m; 20.7 ft) and offshore (6.5 m; 21.3 ft) locations. Long-term (1995-2019) average was 7.2 m in the offshore and 6.4 m in the nearshore; means for the last decade were 7.8 m in the offshore and 6.2 m in the nearshore.

4) Despite higher TP values in 2020 than in recent years, TP, chlorophyll-a and Secchi depth are indicative of oligotrophic conditions in the offshore of Lake Ontario.

5) Nearshore summer zooplankton biomass was 10.5 µg/L, near the all-time low (9.4 µg/L, 2017) since monitoring began in 1995. Offshore epilimnetic summer zooplankton biomass was 11.7 µg/L. These values are similar to biomass in the last decade (2010-2019).

6) Peak (July) epilimnetic biomass of Cercopagis was 3.0 µg/L in the nearshore and represented 25% of the zooplankton community at that time; Cercopagis was absent from the July offshore epilimnetic samples in 2020 but was present in whole water column samples taken in August by other agencies. Epilimnetic biomass of Bythotrephes peaked in late-September in both the nearshore (1.0 µg/L) and offshore (1.8 µg/L) and represented 10% and 18% of the zooplankton community at those times, respectively.

7) Summer nearshore and offshore epilimnetic zooplankton density and biomass declined significantly 1995 – 2020. The declines were due mainly to reductions in cyclopoid copepods in both habitats.