Responses of coastal osmotrophic planktonic communities to simulated events of turbulence and nutrient load throughout a year

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A year-long series of monthly experiments with laboratory enclosures were conducted with water from Blanes Bay (NW Mediterranean) to analyse the change in the short-time response of the osmotrophic planktonic community to simulated turbulence and nutrient input events. Both experimental factors triggered a relative increase of biomass in the enclosures, in terms of chlorophyll a, bacteria and particulate organic matter. Ratios of particulate organic nitrogen to phosphorus became lower in the water than in the sediment, although turbulence partially smoothed out this difference. Initial physico-chemical conditions significantly influenced the short-time responses to experimental forcing. The response to turbulence, in terms of chlorophyll a, was maximum in spring. The response to nutrient enrichment was found to be seasonal, and was correlated with photoperiod and temperature, and also in situ nitrate and silicate concentrations and Secchi depth, which are proxies of recent inputs of nutrients resulting from episodes of resuspension and river discharge. This study shows robust qualitative regularities in the response of the osmotrophic planktonic community to episodes of turbulence and nutrient enrichment, with quantitative variability throughout the year, depending mostly on the recent record of hydrodynamic forcing.

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