Salp grazing:effects on phytoplankton abundance, vertical distribution and taxonomic composition in a coastal habitat

Abstract

This paper comes from a 3 yr study of environmental effects on egg and larval mortality of snapper Pagrus auratus (Sparidae), during their spring-summer spawning season in the Hauralu Gulf In the first 2 yr, the salps Thalia democratjca and Salpa fuslformls dominated mesozooplankton biomass in November and December, average chl a concentrations In the rn~xed layer were reduced and chl a maxima were well below the mixed layer. In the third year, salps were very rare, and the mixed layer was richer in chl a and the chl a maxima were at the bottom of the mixed layer. Diatoms dominated the phytoplankton community in the first 2 yr, especially below the mixed layer, but a more even mix of diatoms, dinoflagellates and flagellates was present in the third year. Averaged mixed layer depth and stratification intensity varied little across the 3 yr, suggesting that variation in vertical mixing rates did not drive the phytoplankton contrasts. Salp population grazing rates were high enough to overtake phytoplankton population growth at many sites. However, most sites had lower salp biomass than this, yet the reduction of chl a concentration and deepening of chl a maxima were widespread and temporally persistent. The vertical distributions of chl a were simulated in a nutrient-phytoplankton-zooplankton model which contrasted a salp-dominated community that had high sedimentation of organic wastes with a microzooplankton community within which wastes were recycled in situ. This model, and calculated rates of grazing, sedimentation, and water column mixing, showed that rates of nutrient incorporation into salp mass and sedimentation of waste material from salp grazing would be much greater than mixing of remineralized nutrient back up through the pycnocline. This causes formation of deep chl a maxima which can persist well after significant grazing pressure has dissipated. The variation in phytoplankton taxonomic composition is explained in terms of growth rates of the taxa under different grazing, nutrient and light regimes. The major effects of salp grazing in the coastal environment appear to be to deepen phytoplankton distributions and reduce biomass, rather than to remove phytoplankton biomass from the euphotic zone completely, as can occur in slope and oceanic waters.