RECOVERY OF INTERTIDAL BENTHIC DIATOMS AFTER BIOCIDE TREATMENT AND ASSOCIATED SEDIMENT DYNAMICS

The recovery of intertidal epipelic diatom assemblages during a sequence of weekly formaldehyde treatments was measured in the Severn Estuary. Sediment chlorophyll a concentrations fell after treatment (from 180 mug g-1 to 15 mug g-1), but recovered within six days. Grazing macrofauna did not recolonize the treated site within this time and, in the absence of grazers, final sediment chlorophyll a concentrations were significantly higher than on the control site. The fall in Chl a concentrations was matched by an increase in phaeophytin levels. Freshly deposited sediment was retained on the control site, but was removed by subsequent tides on the formaldehyde-treated site. This was attributed to the absence of sediment-stabilizing mucopolysaccharides produced by diatoms. Concentrations of both total and colloidal carbohydrate were positively correlated with sediment chlorophyll a concentrations, and fell after biocide application, but bacterial cell densities were unaffected. The development of a mucopolysaccharide matrix associated with diatom recovery was followed using low-temperature scanning electron microscopy. For the first few days after treatment, the diatoms present on the biocide-treated site were predominantly planktonic species. The recovery of epipelic assemblages after biocide treatment was mainly due to a bloom of Nitzschia epithemioides. These organisms formed visible mats over the sediment's surface during tidal exposure.