PHYTOPLANKTON BLOOM DYNAMICS IN COASTAL ECOSYSTEMS - A REVIEW WITH SOME GENERAL LESSONS FROM SUSTAINED INVESTIGATION OF SAN-FRANCISCO BAY, CALIFORNIA

Phytoplankton blooms are prominent features of biological variability in shallow coastal ecosystems such as estuaries, lagoons, bays, and ti dal rivers. Long-term observation and research in San Francisco Bay il lustrates some patterns of phytoplankton spatial and temporal variabil ity and the underlying mechanisms of this variability. Blooms are even ts of rapid production and accumulation of phytoplankton biomass that are usually responses to changing physical forcings originating in the coastal ocean (e.g., tides), the atmosphere (wind), or on the land su rface (precipitation and river runoff). These physical forcings have d ifferent timescales of variability, so algal blooms can be short-term episodic events, recurrent seasonal phenomena, or rare events associat ed with exceptional climatic or hydrologic conditions. The biogeochemi cal role of phytoplankton primary production is to transform and incor porate reactive inorganic elements into organic forms, and these trans formations are rapid and lead to measurable geochemical change during blooms. Examples include the depletion of inorganic nutrients (N, P, S i), supersaturation of oxygen and removal of carbon dioxide, shifts in the isotopic composition of reactive elements (C, N), production of c limatically active trace gases (methyl bromide, dimethylsulfide), chan ges in the chemical form and toxicity of trace metals (As, Cd, Ni, Zn) , changes in the biochemical composition and reactivity of the suspend ed particulate matter, and synthesis of organic matter required for th e reproduction and growth of heterotrophs, including bacteria, zooplan kton, and benthic consumer animals. Some classes of phytoplankton play special roles in the cycling of elements or synthesis of specific org anic molecules, but we have only rudimentary understanding of the forc es that select for and promote blooms of these species. Mounting evide nce suggests that the natural cycles of bloom variability are being al tered on a global scale by human activities including the input of tox ic contaminants and nutrients, manipulation of river flows, and transl ocation of species. This hypothesis will be a key component of our eff ort to understand global change at the land-sea interface. Pursuit of this hypothesis will require creative approaches for distinguishing na tural and anthropogenic sources of phytoplankton population variabilit y, as well as recognition that the modes of human disturbance of coast al bloom cycles operate interactively and cannot be studied as isolate d processes.