Factors affecting microphytobenthic biomass, species composition and production in the Colne Estuary (UK)

Abstract

Biomass, species composition and primary production of the microphytobenthos (MPB) were investigated along environmental gradients in the Colne Estuary between August 1996 and March 1998. Mean monthly sediment chlorophyll a (chl a) concentrations varied between 6 and 241 mg m -2 . There was no seasonal pattern in chl a distribution. Low chl a concentrations were asso- ciated with the estuary mouth, which was characterised by exposed sandy sediments and low water column nutrient concentrations. Position on the shore affected biomass, with greater chl a concentra- tions on the high shore. Chl a concentrations followed the same pattern at both high and low shore positions. There was a significant relationship between concentrations of sediment colloidal carbo- hydrate and chl a. Epipelic diatoms dominated the MPB, with cluster analysis identifying 10 distinct species assemblages. Navicula spp. were an important component of all assemblages. Peaks in chl a biomass occurred when a single species made up >65% of the cell numbers. The distribution of the diatom assemblages was related to salinity, temperature and dissolved inorganic nitrogen (DIN) con- centrations. Net production of oxygen occurred in sediments under illumination at all sites on almost all occasions. Oxygen uptake by sediment in the dark was positively correlated with temperature at the head of the estuary, but not at any other site. Mean annual chl a concentrations correlated posi- tively with total annual dark oxygen uptake. Annual primary production was estimated by 4 models using chl a concentrations and a model based on gross O2 fluxes across the sediment-water interface. Production was estimated to be in the range of 25 to 1199 gC m -2 yr -1 , depending on site and model. There was greater production on the high shore than on the low shore, due to higher chl a concen- trations and longer emersion periods. Total production of MPB in the estuary was estimated to be between 117 and 852 tons C yr -1 . Assimilation by the MPB provided a temporary sink for DIN, equiv- alent to 12.8% of the annual 1996 load to the Colne Estuary. N assimilation rates by the MPB were in the same order of magnitude as denitrification, indicating that assimilation by the MPB is an impor- tant process affecting the flux of DIN through this mesotidal estuary.