Dynamics of oligotrophic pelagic environments:North western Mediterranean sea and subtropical north Atlantic

Abstract

The response of phytoplankton to various ecological forcings was examined in the upper waters of the NW Mediterranean (MED) and the subtropical North Atlantic (NA) from field observations, analysis of historical data sets and numerical simulations. Particular emphasis was given to the role played by the water column structure in controlling the nitrogen diffusion and new production. Using numerical simulations, the effects of different levels of heating of surface waters and nutrient concentrations in waters below the euphotic zone were examined.<br/>Spring microplankton metabolism of surface mixed waters in MED was studied. The system was heterotrophic explained by dark community respiration (DCR) rates higher than gross primary production (GPP) rates. The GPP to DCR ratio was ~0.53, indicating that the system was far from balance between carbon production (photosynthesis) and consumption (respiration), probably caused by organic matter accumulation in the mixed layer favouring respiration over production processes. <br/>Summer primary production and nitrogen fluxes in stratified water layers in NA were analysed. The nitrogen flux fuelling new primary production was closely linked to vertical turbulent diffusion at the nitracline rather than at the thermocline. Upward diffused nitrogen inducing new production explaining 21% of total primary production. The vertical turbulent diffusion model used to estimate new production did not explain new production in the Canary Current zone, where laterally advected nutrients from coastal upwelling areas altered the vertical nitrate gradients.<br/>A numerical ecological model of the pelagic domain was developed to assess the plankton response to different environmental pressures. The model represents the vertical dimension of the upper and intermediate water layers of the open ocean. A comparative study of the plankton functioning in MED and NA sites was carried out using the model. The nitrate entering the euphotic zone through the lower boundary explained the low but continuous primary production in the two systems. The latitudinal variability of plankton scenarios implied a year round different solar heating of the upper waters, altering both phytoplankton photosynthesis and mixed layer processes in the water column, with the latter dominating over the former in controlling the phytoplankton biomass. The balance of light availability and nutrient concentration controlled the chlorophyll maximum depth, but the zooplankton grazing prevented this maximum to reach greater concentrations. The model structure and functioning makes it suitable for comparative ecological studies and is expected to be applicable to other studies related to coastal and marine environmental issues.