Estimations of mesozooplankton biomass in a coastal upwelling area off NW Spain

The abundance and several estimators of mesozooplankton biomass (dry weight, ash-free dry weight, displacement volume, carbon and nitrogen) were measured in a coastal upwelling area off La Coruna (NW Spain) at monthly intervals between 1990 and 1995. Holoplanktonic copepods dominated in most of the samples, but meroplankton were also important near the coast, especially during late spring and through summer. Gelatinous organisms (medusae, siphonophores and larvaceans) reached significant abundances (similar to 10%) in shelf waters by late summer, but also near the coast in spring. Log-linear equations were computed between dry weight, displacement volume and the other biomass estimators, to allow for interconversion among different measurements. To prevent the influence of meroplankton and gelatinous zooplankton in the estimations of biomass, the equations were adjusted using only samples in which the abundance of each of these groups was <10% of total individuals. Our equations produced carbon and ash-free dry weight estimates that are comparable to those obtained with the ratios given in other studies in the region. However, the estimations of mesozooplankton carbon from displacement volume using the adjusted equation with our data are lower than those reported previously for the area, which were computed using equations adjusted with data from a different ecosystem. The use of interconversion factors taken from the literature is discussed, along with the implications of the estimations of mesozooplankton biomass in the computations of the flux of carbon through the ecosystem.