2019 Status of the Lake Ontario Lower Trophic Levels

Spring total phosphorus (TP) in 2019 was 3.2 µg/L (offshore) and 4.7 µg/L (nearshore), both all-time lows; however, there is no significant time trend in our data series (1995-2019 for nearshore; 2002-2019 for offshore). Apr/May – Oct mean TP concentrations were low at both nearshore and offshore locations (range, 3.7 – 6.5 µg/L). TP and SRP concentrations were not significantly different between nearshore and offshore habitats.

Chlorophyll-a and Secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll-a declined significantly 1995 – 2019. Nearshore chlorophyll-a increased 1995 – 2004 and then stabilized 2005 – 2019. In 2019, epilimnetic chlorophyll-a averaged between 1.3 and 2.9 μg/L across sites, and Apr/May – Oct concentrations were not significantly different between nearshore and offshore sites. Summer Secchi depth increased significantly in the offshore 1995 – 2019 from ~6 m to ~8 m. In the nearshore Secchi depth increased 1995 – 2004 but has remained around 6 m since 1999. Apr/May – Oct Secchi depth ranged from 3.8 m to 9.1 m (12 ft to 30 ft) at individual sites and was significantly higher offshore (7.6 m; 25 ft) than nearshore (5.7 m; 19 ft).

In 2019, nearshore summer zooplankton biomass increased to 16.7 mg/m³ after an all-time low (10.3 mg/m³) in 2017. Offshore biomass (12.0 mg/m³) was near the all-time low (8.1 mg/m³, 2006). Apr/May – Oct epilimnetic zooplankton density and biomass were not different between nearshore and offshore sites. However, zooplankton average size was significantly higher in the offshore (0.72 mm) than the nearshore (0.61 mm).

Peak (July) epilimnetic biomass of Cercopagis was 2.4 mg/m³ in the nearshore and 1.4 mg/m³ in the offshore. Peak (September) epilimnetic biomass of Bythotrephes was 2.0 mg/m³ in the nearshore and 2.9 mg/m³ in the offshore.

Summer nearshore zooplankton density and biomass declined significantly 1995 – 2004 and then remained stable 2005 – 2019. The decline was due mainly to reductions in cyclopoids copepods.

Summer epilimnetic daytime offshore zooplankton density decreased significantly 1995 – 2004, but biomass did not. Density and biomass declined significantly 1995 – 2019. Density was 3885/m³ in 2019, about one-fourth the level observed the previous year. Offshore summer epilimnetic zooplankton biomass in 2019 was 12 mg/m³—well below the mean from 2005 – 2018 (20 mg/m³).

Most offshore zooplankton biomass was found in the metalimnion in July and early-October, and in the hypolimnion in September. Limnocalanus dominated the metalimnion in July while daphnids comprised most of the biomass in October. In September, Limnocalanus dominated the hypolimnion.