Net removal of nitrogen through ingestion of phytoplankton by Atlantic menhaden Brevoortia tyrannus in Chesapeake Bay
As filter-feeding planktivores, Atlantic menhaden Brevoortia tyrannus have the potential to influence water quality through ingestion of phytoplankton and assimilation of resultant nutrients. To evaluate the influence of young-of-the-year (YOY) and age-1+ menhaden in Chesapeake Bay, rates of phytoplankton (chl a) ingestion and total dissolved nitrogen (TDN) excretion were measured experimentally across varying phytoplankton concentrations. Ingestion rates of YOY menhaden increased (0.03 to 3.85 µg chl a fish–1 min–1) in response to increasing phytoplankton concentration (4.98 to 198.22 µg chl a l–1), while age-1+ menhaden exhibited virtually no ingestion of the phytoplankton offered. For YOY menhaden, a type-III functional response model best described the relationship between ingestion rate and phytoplankton concentration. Excretion rates of TDN by YOY menhaden increased (0.93 to 3.92 µgN fish–1 min–1) across phytoplankton concentration, and the relationship was best described by an asymptotic exponential model. By contrast, excretion rates were relatively constant for age-1+ menhaden. The YOY ingestion and excretion rate models were combined and rates of net removal of nitrogen across phytoplankton concentrations ranged from –1.73 to 29.85 and –1.73 to 131.58 µgN fish–1 min–1 when the ratio of carbon-to-chlorophyll was 50 and 200, respectively. Results suggest that YOY menhaden focus their grazing on patches of elevated phytoplankton abundance and/or supplement their diet with other sources (e.g. zooplankton and detritus) to maintain a positive nitrogen balance. Population-level estimates of net nitrogen removal imply that menhaden play a minimal role regarding water quality in Chesapeake Bay.