Size composition and estimated phosphorus regeneration rates of the copepod community along an estuarine-offshore transect in the Inland Sea of Japan

Abstract

Hydrographic data were obtained and water, phytoplankton and zooplankton samples collected at 11 stations along a transect from the heavily-eutrophified innermost part of Osaka Bay to mesotrophic offshore shelf waters in Kii Channel in June and December 1985. In June, the copepod community consisted mainly of the small species, Paracalanus sp. and Corycaeus spp. in the inner Osaka Bay, and was dominated by the large species Calanus sinicus in Kii Channel. In December, overall copepod biomass was lower than in June and the community consisted mainly of the small species, Paracalanus sp. and Microsetella norvegica, over the entire study area. From shipboard experiments, the phosphate excretion rate of a copepod was expressed as log E = 0·486 log W + 0·377, where E is excretion rate (ng P individual −1 h −1) and W is copepod body carbon weight (μg C). Using this equation, the DIP (dissolved inorganic phosphorus) regeneration rate of the copepod community was estimated at each station, based on numerical abundance and size composition. Despite large geographical variations in ambient DIP concentration and uptake rate of the phytoplankton community, the DIP regeneration rate of the copepod community was relatively constant. The role of regenerated DIP was important at offshore stations in June when the DIP uptake rate of phytoplankton was high.