Response of Mediterranean Synechococcus growth and loss rates to experimental nutrient inputs

The response of Synechococcus sp. growth, primary production and loss rates was examined in a large-scale mesocosm nutrient enrichment experiment in a coastal NW Mediterranean bay community during the summer of 1997. The mesocosm units (33 m(3)) received N, P and Si at a stochiometric ratio of 20N:7Si:1P, at the normal nutrient loading rate for the site (5 mmol N m(-2) d(-1)), and at 0.5-, 2-, 4-, 8-, and 16-fold the normal nutrient loading input into the bay. Growth and primary production of Synechococcus during the early phase of the experiment increased 2- to 4-fold in the mesocosms receiving greater than or equal to4-fold the normal nutrient loading rate compared to those receiving less, providing evidence of nutrient-Limited growth of the population in the Mediterranean bay studied during that summer. Synechococcus growth was saturated at approximately 0.25 muM DIN, and was unsustained as nutrient inputs continued, showing growth inhibition at relatively high DIN levels (>8 muM). The response of loss rates of Synechococcus to the experimental nutrient inputs was similar to the growth responses, although the dynamics of Synechococcus population size seemed to be an interplay between growth and loss rates. The population size of Synechococcus increased early by almost 3-fold, indicative of Limitation of Synechococcus biomass in the Bay of Blanes during summer. The increase is suggested to result from the lag between Synechococcus growth and loss rate responses during the early phase of the experiment when growth rates exceeded loss rates, resulting in an increase in net production with increased loading. The increase of Synechococcus population size towards the end of the experiment was a result of decreased grazing pressure on Synechococcus, despite the low growth rates of Synechococcus at the high nutrient inputs. Grazing of Synechococcus is suggested to be the main loss process (>50 % of calculated loss rates) except towards the end of the experiment when grazing was only 13 % of the calculated losses.