Structural-functional relationships in the pelagic community of the eastern tropical Atlantic Ocean

Abstract

Sampling was carried out in the eastern tropical Atlantic during July-September, 1987 on four transects at latitudes 6, 12, 18 and 24°W, from 6°S to 4°N. A total number of 81 stations were occupied where physical, chemical and biological measurements were token. Temperature, salinity, chlorophyll a, phytoplankton, primary production (PP), bacterial production, micro-, meso-, inacrozooplankton and mesopelagic fish and squid were sampled in the upper 120 m layer. The biomass size spectrum of the plankton community was relatively flat within the divergence zone but the slope of the size spectrum increased towards the convergence zone meaning an increased contribution of the small-sized organisms. Higher turnover rates (the ratio of PP to the total community biomass) were observed in regions where the size spectra sloped more steeply. PP exceeded the community metabolism by three to five times in regions where the size spectra were sharply sloped. In regions with fiat-type spectra (with slopes from -0.2 to 0) a balance between PP and community metabolism was observed. The variation coefficient and the dispersion index (the variance to mean ratio) indicated an increase of the spatial variability of biomass in the sequence: 'phytoplankton-mesozooplankton-macrozooplankion-micronekton'. At the higher trophic level of the zooplankton community, between the biomass of squid and gelatinous zooplankton, a logarithmic linear relationship was derived. Squid actively consumed gelatinous organisms in regions of high concentrations of their prey, although large copepods, euphausiids, amphipods, flying fish, and myctophid fish also contributed to their diet.