Response of a Mediterranean phytoplankton community to increased nutrient inputs:: a mesocosm experiment

The response of the biomass and primary production of a coastal NW Mediterranean phytoplankton community to a gradient of nutrient inputs was examined in a large-scale mesocosm nutrient enrichment experiment over a period of 20 d in summer. The mesocosm units (33 m(3)) received N, P and Si at a stoichiometric ratio of 20 N:7 Si:1 P, at the normal nutrient loading rate at the site (5 mmol N m(-2) d(-1) and 0.25 mmol P m(-2) d(-1)), and at 0.5, 2, 4, 8 and 16 times the normal nutrient loading rate. The biomass and production of the phytoplanktonic community was also examined in a mesocosm unit to which no nutrients were added. Substantial differences in phytoplankton biomass were observed 4 to 12 d after the initiation of the experiment, when nutrient inputs were increased more than 4-fold above the normal nutrient loading rate. The biomass of the community increased to a maximum of 40.8 mu g chlorophyll a l(-1) (200-fold above the mean initial value) at the greatest nutrient inputs. The biomass increase was largely attributable to an increase in the niicrophytoplankton biomass (Chaetoceros sp, and Licmophora sp.), while picophytoplankton increased their biomass only during the earlier phase of the experiment, reaching values 4.7 times greater at the highest nutrient addition than at the normal loading rate. The structure of the phytoplankton community shifted from an initial dominance of picophytoplankton to dominance of microphytoplankton at the highest nutrient loadings. Primary production increased in response to increased nutrient loading, reaching a level 10-fold higher at the highest nutrient loading than at the normal loading rate. However, phytoplankton carbon turnover did not increase significantly with increasing loading (p > 0.05), except for a tendency for higher turnover at the highest nutrient inputs tested. Results from this experiment suggest that eutrophication problems are likely to become important at a loading of more than 4-fold higher (20 mmol N m(-2) d(-1) and 1 mmol P m(-2) d(-1)) than the present values supplied to the coastal Mediterranean community studied.