2017 Status of the Lake Ontario Lower Trophic Levels

Abstract

Significant Findings for Year 2017: 1) Offshore spring total phosphorus (TP) in 2017 was 4.4 µg/L; values remained stable since 2001. Offshore soluble reactive phosphorus (SRP) remained low (1.1 µg/L) in 2017; 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, 3.7 – 9.0 µg/L). TP and SRP concentrations were significantly higher in nearshore compared to offshore habitats (7.9 µg/L vs 5.3 µg/L, TP; 1.7 µg/L vs 1.0 µg/L, SRP). 2) Chlorophyll-a and Secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll-a was stable 2000 – 2017. Nearshore chlorophyll-a increased 1995 - 2004 but then declined 2005 – 2015; values were above the long-term mean for 2016 and 2017. In 2017, epilimnetic chlorophyll-a averaged between 1.2 and 2.6 μg/L across sites, and offshore and nearshore Apr/May – Oct concentrations were not significantly different. Summer Secchi depth increased significantly in the offshore 2005 – 2017 and in the nearshore 1995 – 2004. There was no trend in either habitat 1995 – 2017. Apr/May – Oct Secchi depth ranged from 4.4 m to 12.5 m at individual sites and was not significantly different between offshore (8.9 m) and nearshore (5.7 m) locations. 3) In 2017, nearshore summer zooplankton biomass was at an all-time low (10.3 mg/m3). Apr/May – Oct epilimnetic zooplankton density was not different between the offshore and the nearshore, but zooplankton size and biomass were significantly higher in the offshore. (0.7 mm vs 0.52 mm and 14.2 mg/m3 vs 7.7 mg/m3). Daphnid, calanoid copepod, and cyclopoid biomass were all higher in the offshore. 4) Peak (July) epilimnetic biomass of Cercopagis was 2.5 mg/m3 in the nearshore and 1.6 mg/m3 in the offshore. Peak (September/October) epilimnetic biomass of Bythotrephes was 0.9 mg/m3 in the nearshore and 0.5 mg/m3 in the offshore. Bythotrephes biomass has increased significantly in the nearshore, 1995 – 2017. 5) Summer nearshore zooplankton density and biomass declined significantly 1995 – 2004 and then remained stable 2005 – 2017. The decline was due mainly to reductions cyclopoids. 6) Summer epilimnetic offshore zooplankton density and biomass increased significantly 2005 – 2017, due mainly to increases in cyclopoids and daphnids. In 2017, offshore summer epilimnetic zooplankton biomass was 14 mg/m3—well below the mean from 2005 – 2016 (21 mg/m3). 7) Most offshore zooplankton biomass was found in the metalimnion in July and September, and in the hypolimnion in October. Limnocalanus and cyclopoids dominated the metalimnion in July while daphnids and cyclopoids comprised most of the biomass in September. Daphnids dominated the October hypolimnion. Whole water column samples show a declining zooplankton biomass 2015 – 2017. Bythotrephes biomass in whole water column tows is the highest it has been, 2010 – 2017.