Excessive nutrients select for dinoflagellates in the stratified Patapsco River estuary: Margalef reigns

Abstract

Phytoplankton composition in the mesohaline Patapsco River estuary, a tributary of Chesapeake Bay, is dominated by dinoflagellates throughout the summer. This dominance is attrib- uted to the nutrient-turbulence relationships that have been described by Margalef over the last 30 yr, a field verification of this highly respected theory. The partially stratified estuary is typified by ambient nutrient concentrations higher than those in any other mesohaline area of the northern bay and its tributaries. Mean dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations in surface waters of the stratified estuary exceed 30 and 0.5 µM, respectively, during the summer, and new and regenerated nutrient loadings ensure maintenance of these high levels through the productive growing season. Turbulence levels are suggested to be low in surface waters of the stratified water column, selecting for motile dinoflagellates throughout the summer. Dinoflagellate dominance in the Patapsco River estuary is further ensured through only minor graz- ing pressure from low rotifer demand and low standing stocks of ambient herbivorous copepod pop- ulations, the latter group being present at levels lower than in any other mesohaline system of the Chesapeake and its tributaries. The resulting high dinoflagellate biomass appears to support a large and active microheterotrophic community (microbial loop), exemplified by abundant rotifers, high oxygen demand, and high nutrient regeneration in the water column and sediments. From these traits, the dinoflagellate-dominated mesohaline Patapsco River estuary and the tidal-fresh cyano- bacteria bloom areas of the upper Potomac River appear to be analogous, with salinity defining spe- cies composition of the phytoplankton.