Role of ammonium in Alexandrium fundyense blooms in the Gulf of Maine and on Georges Bank

This study was designed to test the hypothesis that ammonium is the more important form of dissolved inorganic nitrogen supporting annual summertime blooms of the PSP-producing dinoflagellate Alexandrium fundyense in the Gulf of Maine -- Georges Bank region. It is already known that compared to nitrate and nitrite, ammonium is the preferred nitrogenous nutrient for phytoplankton because of its chemically reduced state. Previous oceanographic surveys in the study region have shown that deep-water nitrate injections into surface waters of the eastern Gulf of Maine and on the northern flank of Georges Bank lead to diatom blooms some distance downstream in the residual flow field, followed by dinoflagellate blooms, including populations of A. fundyense. These A. fundyense populations continue to grow and to maintain high cell concentrations after nitrate is depleted in the surface waters. Rapid remineralization of particulate nitrogen to ammonium and strong coupling with subsequent uptake typically keeps ambient ammonium concentrations low; nonetheless, even very low ammonium concentrations can sustain phytoplankton production. Results of laboratory culture experiments and statistical analyses of field measurements of distributions of A. fundyense and planktonic heterotrophs are presented, which together support the contention that heterotrophic regeneration of ammonium is sufficient to sustain continued growth during summertime blooms of A. fundyense in the Gulf of Maine and on Georges Bank.