External forcing by wind and turbid inflows on a deep glacial lake and implications for primary production

Lake Coleridge is a deep monomictic lake in the Southern Alps of New Zealan d which is affected by strong adiabatic winds. It receives significant inpu ts of inorganic suspended solids from rivers diverted into the lake for hyd ro-electric generation. External inputs of wind energy and suspended solids to Lake Coleridge were quantified throughout 1 year and their impacts on h ydrodynamics and the light climate were analysed. During stratification, th e lake had a deep mixed layer, underlain by a complex metalimnion. High win d energy inputs caused large internal waves which resulted in the upwelling of hypolimnetic water, horizontal density gradients, and significant curre nts. Calculated vertical diffusivity and exchange coefficients were similar to those of much larger lakes. Analyses indicated that mixing in the metal imnion was often a result of shear stresses arising from vertical gradients in horizontal velocities. Whole lake total suspended solids (TSS) burdens were significantly correlated with the previous 13-week integrated TSS load ing from inflows. A seasonal pattern was observed where TSS from inflows en tered either below, within or above the metalimnion. Evidence of effects of sediment resuspension from the marginal lake bottom was observed only when the lake level was low and the resulting impacts on the light climate were minimal. The synergistic effects of climate, thermal structure, and inflow s resulted in the apparent light-limitation of phytoplankton photosynthesis in winter months.