Benthic diatom biomass, production and sediment chlorophyll in langebaan lagoon, South Africa

Abstract

Direct measurements were made of the biomass of benthic diatoms in Langebaan lagoon, a sheltered lagoon on the west coast of South Africa. Highest biomasses occurred in the most sheltered areas and reached 18·9 g m −2 in the top 30 cm of sediment. A large biomass was found even at depths of 10–20 cm and 20–30 cm, often exceeding that in the 0–10 cm depth range. This contrasts with chlorophyll estimates of biomass, in which chlorophyll- a declines rapidly below the photic zone. Very high sediment turnover rates by benthic macrofauna account for the presence of microalgae at depths of 30 cm below the sediment surface. Currents or waves (and hence particle size) are inversely correlated with diatom biomass. Chlorophyll- a concentrations in the surface layers of the sediment may reach 322 μg Chl- a g −1 sediment, but generally values range from 0 to 34 μg Chl- a g −1 sediment. Like diatom biomass, chlorophyll- a concentration increases with declining waves and currents. The highest chlorophyll- a concentrations occur in the top 1 mm of sediment, declining to 35% of surface concentrations 5 cm below the surface. Rates of carbon fixation by benthic microalgae in the top 5 mm of sediment were 17·38 mg C m −2 h −1 at an exposed sandy beach, and 69·54 mg C m −2 h −1 at a sheltered sand/mud beach. Benthic microfloral production rivals phytoplankton production and ranges from estimates of 63 g C m −2 year −1 (sand) to 253 g C m −2 year −1 (sand/mud). In the lagoon ecosystem, production by benthic microalgae amounts to 22% of the total primary production of carbon.