Effects of the gastropod Rhinoclavis aspera (Linnaeus, 1758) on microbial biomass and productivity in coral-reef-flat sediments

Abstract

Abundance of the deposit-feeding gastropod Rhinoclavis aspera in coral-reef-flat sediments was estimated along with numbers of bacteria, rate of bacterial production, biomass of microalgae (measured as concentration of chlorophyll a) and rates of gross primary production at two different times of the year. Standing stocks of bacteria and microalgae were greater in June 1986 (winter) than in January 1987 (summer), but rates of bacterial production and gross primary production were slower in June. Significant correlations between the abundance of R. aspera and the rates of bacterial production indicated that grazing by the gastropods may affect bacterial activity. Densities of R. aspera were manipulated in enclosures in the field to determine their effects on the number of bacteria, the rates of bacterial production and the biomass of microalgae. There were no consistent effects of gastropods on either the biomass of microalgae or rates of bacterial growth. Numbers of bacteria were, however, significantly less in enclosures with greater densities of gastropods. Physical disturbance of the sediments (to simulate the movements of the gastropods) also caused a significant decrease in the numbers of bacteria. The decrease in numbers of bacteria was not the result of slower rates of bacterial growth, indicating that cell removal was a more probable fate. It may be that disruption of sediments by the gastropods increased the abundance of other grazers, such as meiofauna, which then consumed the bacteria.