Latitudinal and depth variation of microzooplancton grazing across the tropical and subtropical Atlantic Ocean

Abstract

Microzooplankton is the main phytoplankton consumer in the ocean and the grazing exerted has important implications for food-web dynamics and carbon cycling. Using the modified dilution method, we have measured growth and mortality rates for picophytoplankton populations, total phytoplankton and nanoplankton along the tropical and subtropical Atlantic Ocean at different depths (100%, 80% and 1% light). A section from 15oS in the oligotrophic waters off Brazil to 28oN was performed, crossing the areas of different productivities (the equatorial upwelling, the Guinea Dome, the oceanic upwelling of Cape Blanc in western Sahara, and the oligotrophic waters of the Canary Current). The southern basin, was characterized by high surface temperatures, low chlorophyll and deep DCM. Here we observed the highest mortality rates in contrast to lower values in the area characterized by the oxygen minimum zone (OMZ). The highest mortality rates were observed in the oceanic upwelling of northwest Africa, below the surface chlorophyll maximum. Highest mortality rates were obtained during the first 12 hours of the experiment, being lower in 24h experiments. Inn the high nutrient area mortality rates were higher during the last 12 hours of the experiment. A similar result was found in amended nutrient experiments. However, in non-amended nutrient conditions, grazing was maximum during the first 12 hours. We suggest that organisms exploit nutrients until they are depleted, whereas when they are non-limiting its assimilation is carried out gradually along the entire experiment.