engleski

Production and fate of phytoplankton : annual cycles and interannual variability

Annual cycles and interannual variability of phytoplankton biomass and primary pro-ductivity in Chesapeake Bay and the northern Adriatic Sea are strongly driven by fresh-water flow and nutrient loading from major rivers. In this chapter, we present data on prominent hydrographic properties that are responsive to flow variability in the two systems, and discuss how the distribution and abundance of phytoplankton are regulated. The Susquehanna River is the principal freshwater source for Chesapeake Bay and the Po River is the corresponding source for the northern Adriatic. The magnitudes of freshwater flow into the Chesapeake and northern Adriatic are comparable, but basin morphometry and circulation are significantly different and these characteristics generate quite distinct water quality in the two systems. The Chesapeake is clearly eutrophic and is characterized by high phytoplankton biomass, high rates of primary productivity, and rich fisheries, while the northern Adriatic is largely oligotrophic, although it among the most productive of Mediterranean waters and has supported an active pilchard fishery for years. In sum, it is clear that each system is dominated by a single, major river source whose flow and attendant nutrient loading regulate responses at the primary producer level. Receiving waters of Chesapeake Bay effectively sequester nutrients from the land and other sources into phytoplankton biomass, and efficiently retain and recycle phytoplankton biomass produced in spring to support high primary productivity in summer. These ecosystem characteristics are manifested in the extremely high secondary production of the Chesapeake, and rest on a spatial and temporal separation of biomass accumulation and maximum primary productivity that are key in how the estuary functions. In contrast, the northern Adriatic has much lower phytoplankton biomass and primary productivity, and although it is very productive by Mediterranean standards, it remains largely oligotrophic because it is less effective in retaining nutrients on time and space scales that promote primary and secondary productivity. The variability of river flow and concomitant effects on receiving waters of the northern Adriatic produce ecosystem responses not unlike those of Chesapeake Bay. These findings are presented in the context of overenrichment and a long-term stimulation of phytoplankton growth that is a common manifestation of eutrophication in estuarine and coastal ecosystems world-wide.

phytoplankton

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