Primary production and spatial distribution of subtidal microphytobenthos in a temperate coastal system, the Bay of Brest, France

Abstract

The main objective of this study was to define the primary production and the spatial and temporal distribution of the subtidal microphytobenthic community in a temperate coastal ecosystem, the Bay of Brest. The productivity of the microphytobenthos (MPB) was estimated in winter, spring and late summer, by a series of in situ benthic chamber incubations. Oxygen (O 2) and dissolved inorganic carbon (DIC) fluxes were measured at the sediment–water interface in light and dark conditions to determine the net and gross primary production present. Functional regression of the O 2 and DIC data demonstrated that the community photosynthetic quotient (CPQ) for the benthic community was 1. A maximal gross production ( P max) of between 0.4 and 0.8 mmol O 2 mg chl a −1 h −1 was estimated for the MPB in the Bay. E K values were low, ranging from 57.8 to 83.4 μmol photons m −2 s −1 and can be considered an adaptation of the MPB to the reduced light levels reaching the sediment–water interface. Two sampling campaigns were undertaken in winter and late summer to measure the biomass of the benthic microalgal community in the Bay. Productivity estimates were combined with this biomass to give the MPB production in all areas of the Bay. Principal components analysis revealed that stations sampled were grouped primarily as a function of their depth, highlighting the importance above all of light availability, and their sediment type, with highest biomass concentrations found in bare muddy sediments. Hierarchical classification allowed the determination of four groups of stations in the Bay defined by their biotic and abiotic differences. The importance of the gastropod Crepidula fornicata in conditioning the benthic structural and biochemical environment was also highlighted. Geographical information systems based mapping allowed the representation of the spatial and temporal distribution of biomass and primary production and consequently a determination of the overall MPB production. Average seasonal production estimates for the Bay of Brest ranged from 57 in winter to 111 mg C m −2 day −1 in late summer and represented from 12–20% of total primary production.