Nutrient Regeneration by Zooplankton in Southern Lake Huron

Rates of nutrient regeneration by zooplankton (μmol/mg dry wt/hr) in southern Lake Huron from April to August 1975 ranged from undetectable to 2.6 for total phosphorus (TP), undetectable to 0.8 for total soluble phosphorus (TSP), undetectable to 0.12 for soluble reactive phosphorus (SRP), undetectable to 0.97 for ammonia (NH₃), undetectable to 3.8 for nitrate plus nitrite (NO₃ + NO₂), and undetectable to 2.9 for silica (SiO₂). Two diel experiments were conducted. Times of highest rates of regeneration varied for the different nutrients on these dates. Using the average concentration of zooplankton in the surface waters during this study, the calculated average concentration of nutrients regenerated by zooplankton was 0.012 μmol P/L/ hr for TP, 0.0046 μmol P/L/ hr for TSP, 0.0016 μmol P/L/ hr for SRP, 0.0146 μmol N/L/ hr for NH₃, 0.043 μmol N/L/ hr for NO₃ + NO₂, and 0.058 μmol Si/L/ hr for SiO₂. The contribution of nutrient regeneration by zooplankton to the turnover time of the various nutrients in the surface waters was calculated to be 212 hr for TP, 239 hr for TSP, 69 hr for SRP, 62 hr for NH₃, 505 hr for NO₃ + NO₂, and 531 hr for SiO₂. Although the turnover time for most of these nutrients is fairly slow, the nutrient pools for SRP and NH₃ are replenished in less than 70 hr by nutrient regeneration. Zooplankton therefore appear to play a significant role in the cycling of SRP and NH₃ in southern Lake Huron.