2018 Status of the Lake Ontario lower trophic levels

Abstract

Significant Findings for Year 2018: 1) Offshore spring total phosphorus (TP) in 2018 was 7.0 µg/L; values have remained stable since 2001. Offshore soluble reactive phosphorus (SRP) remained low (1.3 µg/L) in 2018; Apr/May – Oct mean values have been stable in nearshore and offshore habitats since 1998 (range, 0.4 – 3.3 µg/L). Apr/May – Oct mean TP concentrations were low at both nearshore and offshore locations (range, 3.9 – 7.8 µg/L). TP and SRP concentrations were not significantly different between nearshore and offshore habitats. 2) Chlorophyll-a and Secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll-a was stable 2000 – 2018. Nearshore chlorophyll-a increased 1995 - 2004 and then stabilized 2005 – 2018; values were below the long-term mean in 2018. In 2018, epilimnetic chlorophyll-a averaged between 0.9 and 1.8 μg/L across sites, and offshore and nearshore Apr/May – Oct concentrations were not significantly different. Summer Secchi depth increased significantly in the offshore 2000 – 2018 and in the nearshore 1995 – 2004. Apr/May – Oct Secchi depth ranged from 4.7 m to 13.5 m (15 ft to 44 ft) at individual sites and was not significantly different between offshore (8.6 m; 28 ft) and nearshore (6.3 m; 21 ft) locations. 3) In 2018, nearshore summer zooplankton biomass increased slightly to 10.7 mg/m3 after an all-time low (10.3 mg/m 3 ) in 2017. Apr/May – Oct epilimnetic zooplankton density was significantly higher in the offshore than the nearshore, but zooplankton size and biomass were not different. Cyclopoid copepods were primarily responsible for the higher density in the offshore. 4) Peak (July) epilimnetic biomass of Cercopagis was 1.3 mg/m 3 in the nearshore and 1.9 mg/m 3 in the offshore. Peak (September-October) epilimnetic biomass of Bythotrephes was 0.4 mg/m 3 in the nearshore and 0.6 mg/m3 in the offshore. Bythotrephes biomass has increased significantly in the nearshore, 1995 – 2018. 5) Summer nearshore zooplankton density and biomass declined significantly 1995 – 2004 and then remained stable 2005 – 2018. The decline was due mainly to reductions in cyclopoids copepods. 6) Summer epilimnetic daytime offshore zooplankton density and biomass decreased significantly 2000 – 2018, but density increased marginally since 2005. Density was 16000/m 3 in 2018, three times the levels seen 2015 – 2017. Offshore summer epilimnetic zooplankton biomass in 2018 was 11 mg/m 3 — well below the mean from 2005 – 2017 (20 mg/m 3 ). 7) Most offshore zooplankton biomass was found in the metalimnion in July and early-September, and in the hypolimnion in late-September and October. Cyclopoids dominated the metalimnion in July while daphnids and cyclopoids comprised most of the biomass in September. Daphnids dominated the October hypolimnion.