The role of microphytobenthic primary production in a Mediterranean mussel culture area

The production and biomass of microphytobenthos in a Mediterranean mussel farm was studied during 1991-92. Gross and net microphytobenthic production and respiration were calculated from oxygen fluxes in transparent and black bell jars at two stations; sediments under a mussel table and reference sediments, both located at 5 m depth. Net oxygen fluxes were mainly negative under the mussel tables (average 19.5 mg O₂ m^-2 h^-1, CV= 132%), and microphytobenthos production could not meet the sediment oxygen demand; in the reference sediments, microphytobenthos production was responsible for net oxygen production (average + 13.0 mg O₂ m^-2 h^-1, CV= 118%). Benthic respiration rates were, on average, 47.3mg O₂ m^-2 h^-1 (CV=82%) under the tables and 27.7mg O₂ m^-2 h^-1 (CV=45%) in reference sediments. Aerobic respiration could remineralize less than 2% of the biodeposited carbon under the tables, implying that a large amount of organic material is accumulating under the tables, and that most of the degradation will be anaerobic. Gross microbenthic production showed sharp changes between 1991 and 1992 under the mussel tables and for reference sediments (averages 20.98 mg O₂ m^-2 h^-1, CV= 135% and 33 mg O₂ m^-2 h^-1, CV=48%, respectively). Despite the negative oxygen balance in the sediments under the tables, microphytobenthos was more productive than phytoplankton in bottom waters. Per unit area, phytoplankton was more productive than microphytobenthos at both stations, especially in the area of the mussel tables, where phytoplanktonic production was enhanced by the excretion products of mussels. Microphytobenthos was composed mainly of diatoms in the sediments under the tables, while in reference sediments, the population was more diverse, with algae containing chlorophyll b also present. Chlorophyll a concentration in sediments under the tables was 207 mg m^-2 (CV=73%) and 95 mg m^-2 (CV=28%) in reference sediments; the stock of plant pigments was increased under the tables by biodeposition. Microphytobenthos constitutes a compartment with an important contribution in biomass, but also in oxygen production.