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Assessment of community structure and seston quality effects on plankton carbon fluxes at two contrasting coastal sites

Final Activity Report Summary - PCFLUCS (Assessment of community structure and seston quality effects on plankton carbon fluxes at two contrasting coastal sites)

This research was conducted to perform a detailed environmental characterisation of coastal waters off Montevideo at the Río de la Plata estuary, and to analyse selected carbon fluxes in the planktonic system. Key issues were the estimation of organic matter production and the exploration of zooplankton production response to the quality of the food as indicated by the fatty acid profile of seston.

On four cruises between August and November 2009, we sampled three stations at water depths that ranged between 6 and 10 m to record water column structure, including temperature, salinity, density, chlorophyll fluorescence, turbidity and photosynthetically available radiation (PAR). Sub-surface water was sampled for dissolved nutrients, chlorophyll-a, fatty acid profiles in seston, phytoplankton and zooplankton. At the land laboratory we ran incubations to estimate primary production, such as oxygen production, and copepod egg and faecal pellet production rates.

Temperature varied little between stations and vertically, however salinity always showed important horizontal changes and vertical stratification. Most remarkable was the systematic presence of subsurface chlorophyll maxima at intermediate to low light levels, usually ranging between 1.5 and 3 m depth, a feature which had not been observed before in Río de la Plata. In turn, size-fractionated chlorophyll-a showed that the fraction that was smaller than 10 µm always dominated. Oxygen production and consumption was relatively stable between stations and dates and indicated high levels of photosynthetic activity, supporting the very limited existing data on primary productivity for the Río de la Plata. The zooplankton was strongly dominated by the copepod acartia tonsa, and its Egg production rates (EPR) were always moderate to high, with some values being among the highest found in literature for that species. That result contrasted with a single earlier study on copepod production in the Río de la Plata that indicated very low production rates by this same species. Pellet production rates, which served as an indicator of feeding activity, mirrored egg production. However, EPR was systematically higher when compared in terms of biomass-specific rates.

Overall, the results suggested a high quality and nutritionally advantageous food environment for a. tonsa and higher trophic levels in the Río de la Plata.