Phytoplankton Dynamics in a Changing Environment

Abstract

The Northern Adriatic Sea (NAS) and Chesapeake Bay (CB) are examples of semienclosed coastal ecosystems in which primary production is dominated by phytoplankton. We update an earlier comparative analysis of phytoplankton dynamics in these two systems by comparing spatial and temporal dynamics of nutrients, chlorophyll- a biomass, community composition, and primary production. New nutrient concentrations (e.g., nitrate and nitrite) decrease with distance from riverine sources and with increasing salinity. Phosphorus is generally considered to be the principal limiting nutrient of phytoplankton growth rate in the NAS, while CB generally varies between P-limited growth in spring and N-limited growth in summer. Both systems exhibit clear annual cycles of phytoplankton biomass and productivity, with substantial interannual variability driven by variations in riverine inputs of nutrient-rich water. Both are characterized by a temporal separation between seasonal peaks in phytoplankton biomass (spring in both systems, fall in the NAS) and productivity (summer in both systems). Despite comparable nitrogen loads, phytoplankton biomass and production are much higher in CB than in the NAS, and CB is classified as eutrophic to hypertrophic over most of its extent, while the NAS is classified as oligotrophic to eutrophic depending on distance from riverine inputs of nutrients. This contrast is due primarily to the large volume of the NAS relative to the riverine input of nutrient-rich water compared with CB, and to inputs of oligotrophic water to the NAS from the southern Adriatic. Both systems have experienced reductions in nutrient loading in recent years, with associated reductions in chlorophyll- a and productivity in the NAS, but no clear changes in CB. Future comparisons of phytoplankton dynamics in these systems will need to consider ongoing efforts to reduce anthropogenic nutrient inputs concurrent with climate-driven changes in water temperature and hydrological. Such analyses will be dependent on sustained long-term monitoring of phytoplankton productivity and environmental parameters that impact plankton dynamics.