Sources of nutrients sustaining algal growth in a eutrophic reservoir

I compared nutrient recycling by consumers (gizzard shad and zooplankton) to watershed inputs and algal nutrient demand in a Kentucky reservoir. Average yearly nutrient loading by the primary inflow was greater than that of gizzard shad or zooplankton with total phosphorus (TP) inputs from the catchment of 0.054 μmol L−1 d−1 (67.5% as DP) while soluble reactive phosphorus (SRP) from fish and zooplankton were 0.416 and 0.030 (μmol L−1 d−1). Average yearly DIN inputs from the catchment, gizzard shad and zooplankton were 0.447, 0.115, and 0.353 (μmol L₋₁ d−1 ). Although tributary nutrient inputs may drive nutrient dynamics in this reservoir the yearly phytoplankton productivity demands for P (0.06 μmol L−1 d−1) and N (0.95 μmol L −1 d−1) were met by tributary nutrients only 27% and 9% of the time. Inter-annual variation of nutrient inputs from tributaries (26 fold for P and 61 fold for N) were large compared to that from gizzard shad and zooplankton (two fold). Even though hydrology variability is greater, volumetric loading of nutrients (P and N) by gizzard shad can at times exceed that by tributaries and thus be important. However, gizzard shad and tributary nutrient loadings were not independent of one another. Fish biomass was positively correlated with tributary nutrient loading of the previous year (R2 = .81, P = .002). On average, gizzard shad provided 23% and 10% of algal P and N demand.; I also ran three types of bioassay experiments that contributed to the understanding of factors that regulate nutrient availability for algal growth. In experiment one, phosphate and nitrate were added to determine if phytoplankton were nutrient limited. There was significant spatial and temporal variation in nutrient limitation. Phytoplankton upstream showed N-limitation while phytoplankton downstream showed both N and P limitation. In experiment two, nitrate and ammonia were added to algal bioassays to determine their effect on algal growth. Growth rates of phytoplankton with nitrate and ammonia were not significantly different from one another. This evidence supported that DIN from fish (N-NH4) and tributaries (N-NO3) are equally important to phytoplankton growth. In experiment three, gizzard shad excretions were added to algal bioassays to determine the effects of excretions on algal growth. The growth rates of algal bioassays with and without fish excretions were compared. Algal bioassays that received excretions had significantly greater chlorophyll-a concentrations (t test, P < .01). SRP became depleted in algal bioassays ran in light conditions. However, in three out of four algal experiments run in the absence of light, SRP increased. This increase may be evidence of remineralization by bacteria of dissolved organic phosphorus (DOP) into SRP.