Composition and differential distribution of zooplankton in Arcachon Bay

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Abstract
Specific composition, distribution and seasonally of zooplankton in Arcachon Bay was studied along a salinity-temperature gradient from coastal neritic to estuarine conditions. Zooplankton numbers ranged from 23 m−3 to 127 000 m−3 and biomass from 0.08 mg dry wt m−3 to 525.5 mg dry wt m −3. The maximum density and biomass of zooplankton occurred between April and June in the three stations investigated. Copepods were the dominant taxon comprising 71.4% in number and 91.7% in biomass of the total zooplankton. The three different water masses of the bay were characterized by copepod associations of species. The most common meroplankters were Cirripedia nauplii. Other important groups were polychaete, gastropod, bivalve larvae and the larvacean Oikopleura dioica. Biomass increased significantly from the oceanic water to the inner neritic water mass while the species diversity was decreasing. As mean densities and biomass increased, fluctuations were more important. The community structure has been measured as a mathematical description of the species-abundance relations. Total zooplankton and pelagic copepods fitted both a geometric series and a log-normal model. In general, a closer fit to a log-normal distribution was due to the addition of several assemblages of species. The fit to both models was affected by the salinity-temperature gradient. The community structure was especially unbalanced in the inner bay, directly subjected to influx of fresh water.

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