Phytoplankton productivity and size composition in Lake Simcoe: The nearshore shunt and the importance of autumnal production

Primary production by phytoplankton helps fuel hypolimnetic oxygen depletion as well as food web productivity, and is thought to have been affected by dreissenid mussel colonization in Lake Simcoe. Measured by short-term 14C uptake, areal and volumetric rates of photosynthetic carbon uptake (production) were lower in nearshore (< 15 m) than offshore sites during the study period (Aug. 2010-Aug. 2011). Chlorophyll a (Chl a) concentrations were also lower at nearshore sites, except in summer (June-Aug.). Production increased from May to an annual maximum in September (350 mg C m− 2 d− 1 nearshore and 650 mg C m− 2 d− 1 offshore) and remained high through November; its annual minimum (< 10 mg C m− 2 d− 1) was in February. A secondary spring maximum occurred in April in the offshore (450 mg C m− 2 d− 1) but not the nearshore (30 mg C m− 2 d− 1). Chl a and production of net phytoplankton (> 20 μm) was greatest from August through November at offshore sites (median contribution > 35% of total) but was much less at nearshore sites (median contribution < 10%). Production and Chl a were dominated by nanoplankton (2-20 μm), with median contributions > 40% in most seasons. The lower production at nearshore than offshore sites supports previous inferences that dreissenid mussels have caused differential loss of planktonic primary production, especially of net phytoplankton, in nearshore waters. The results from Lake Simcoe are consistent with evidence from other lakes that dreissenids can have major impacts on seasonal blooms of diatoms as the mussels continue to spread within and among North American lakes.