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 meso
cosm units (33 m(3)) received N, P and Si at a stochiometric ratio of 20N:7
Si: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 ea
rly phase of the experiment increased 2- to 4-fold in the mesocosms receivi
ng 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. Synecho
coccus growth was saturated at approximately 0.25 muM DIN, and was unsustai
ned as nutrient inputs continued, showing growth inhibition at relatively h
igh 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 be
tween 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 fro
m the lag between Synechococcus growth and loss rate responses during the e
arly phase of the experiment when growth rates exceeded loss rates, resulti
ng in an increase in net production with increased loading. The increase of
Synechococcus population size towards the end of the experiment was a resu
lt of decreased grazing pressure on Synechococcus, despite the low growth r
ates 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.