Nutrient accumulation at different supply rates in experimental Mediterranean planktonic communities

A mesocosm experiment was conducted between June 18 and July 8, 1997, in the Bay of Blanes (NE Spain) to test for the effect of nutrient inputs on Mediterranean plankton communities. The experiment involved a gradient of daily nutrient additions to mesocosms (14 m high, 4.2 m² cross-sectional area and 33 m³ effective volume), which were scaled to the nutrient loading of the Bay of Blanes in summer (0.64 μM N d^-1, referred to as the 'business as usual' control). This paper deals with the accumulation of nutrients in different size components (0.2-2, 2-20 and 20-200 μm) of the community and the efficiency with which these size components retain the added nutrients. Summer Mediterranean plankton responded significantly to nutrient additions by increasing their biomass, up to more than 100-fold that initially present in the enclosed water, and by changing the community structure, from an initial dominance of picoplankton to increased dominance of the 2-20 μm size component of the community (which contributes to more than 90% of the accumulated N and P) as the nutrient load increased. Though the initial community was able to accumulate nutrients exceeding the already high summer loading of the Bay of Blanes and thus acted as strong nitrogen (N) and phosphorus (P) sink, it was unable to trap the nutrients above a certain threshold: 4-fold (for N) and 8-fold (for P) the 'business as usual' load. Conspicuous biomass increases were most noticeable after the nutrient inputs were increased 16-fold that value. Nevertheless, the biomass response was relatively low and involved a low efficiency (8%) in the trapping of added nutrients. As efficiencies were transiently higher (of 50% or even greater, between consecutive sampling days) we suggest factors other than a low assimilation by osmotrophs as responsible for the resilience of the plankton community for the trapping of nutrients in biomass in response to nutrient additions. The tendency of a lower biomass response to nutrient additions in the warm oligotrophic Mediterranean waters than in more eutrophic waters, if confirmed, should have profound consequences concerning the management of the coastline and the delineation of critical nutrient loadings.