PHYTOPLANKTON‐ZOOPLANKTON RELATIONSHIPS IN NARRAGANSETT BAY. III. SEASONAL CHANGES IN ZOOPLANKTON EXCRETION RATES IN RELATION TO PHYTOPLANKTON ABUNDANCE 1

Phytoplankton and zooplankton were sampled at three stations in Narragansett Bay over a nine‐month period at weekly or biweekly intervals. The freshly‐caught plankton was then utilized in laboratory experiments in which zooplankton oxygen consumption and phosphorus and nitrogen excretion were measured. Nitrogen excretion was minimal when phytoplankton was abundant and vice versa; this phenomenon may be due to the animals’ greater utilization of protein as an energy source when food is scarce. A similar relationship was observed between food abundance and phosphate excretion, and it was hypothesized that when food was abundant low rates of P excretion indicated that lipid was being stored or used in egg production or both. When food was scarce more lipid was used for energy and rates of P excretion increased. From the experimental results, it was estimated that the zooplankton provide only a small fraction of the required N and P to the phytoplankton during periods of phytoplankton abundance. However, during periods of phytoplankton scarcity, when plant nutrient requirements are low and zooplankton excretion rates are high, the zooplankton provide amounts of N and P in excess of phytoplankton demand.