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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Phytoplankon biomass, cell diameter, and species composition in the low salinity zone of northern San Francisco Bay estuary
Auteurs :	P. W. Lehman, Auteur
Type de document :	Article
Dans :	Estuaries (23(2) APR 2000)
Article en page(s) :	216-230
Résumé :	Phytoplankton chlorophyll a concentration, biovolume, cell diameter, and species composition differed across the narrow, low salinity zone between 0.6 parts per thousand to 4 parts per thousand and may influence copepod food availability in the northern San Francisco Bay Estuary. The highest chlorophyll a concentrations (range 3.2-12.3 mu g l (1)), widest cell diameters (> 5 mu m dan), highest diatom densities and highest production rates of > 10 mu m diam cells occurred at the landward edge of the salinity zone in April during a strong spring tide and May during a strong neap tide. Near optimum predator/prey ratios, large prey estimated spherical diameters, and high chlorophyll a concentrations suggest these phytoplankton communities provided good food quantity and quality for the most abundant copepods, Eurytemora affinis, Sinocalanus doerrii, and Pseudodiaptomus forbesi. At the center of the zone, chlorophyll a concentrations, diatom densities, and production rates of > 10 mu m diam cells were lower and cell diameters were smaller than upstream. Downstream transport was accompanied by accumulation of phytoplankton with depth and tide; maximum biomass occurred on spring tide. The lowest chlorophyll a concentrations (1.4-3.6 mu g l(-1)) and consistently high densities (3,000-4,000 cells ml (1)) of 10 pm diam cells, small prey estimated spherical diameters, and high predator/prey ratios suggested the seaward edge of the zone had poor phytoplankton food for copepodids and adult copepods. The seaward decrease in phytoplankton chlorophyll a concentration and cell diameter and shift in species composition in the low salinity zone were probably a function of an estuary-wide decrease in chlorophyll a concentration, cell diameter, and diatom density since the early 1980s that was enhanced in the low salinity zone by clam herbivory after 1987.

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