Significance of pelagic bacteria as a trophic resource in a coral reef lagoon, One Tree Island, Great Barrier Reef

Abstract

Bacterial numbers and frequency of dividing cells (FDC) were estimated in surface waters at seven stations along a transeet across One Tree lagoon and reefs at 2 h intervals over one tidal cycle in May 1984 and once a day on 8 d in December 1984/January 1985. Cell numbers ranged between 1.24 and 13.10x105 ml-1, with an overall mean value of 4.81x105. There was a progressive depletion in standing stock from windward to leeward stations across the lagoon, also reflected in the FDC, which generally showed similar dynamic patterns to cell numbers throughout the study. Predator-free seawater (filtered through 2 μm pore-size filters) and mucus-enriched incubations were also used to establish the relationship between the growth rate (μ) and FDC for coral-reef populations. Average growth rates predicted from FDC values ranged from 0.062 to 0.174 h-1, which is equivalent to doubling times of 5.74 to 16.00 h, with an overall mean value of 11.5 h. These fast doubling times suggest that bacteria respond rapidly to pulses of enrichment as they float over the lagoon and reefs, characteristically achieving about one doubling per tidal cycle. This probably ensures that the bulk of labile organic matter and dissolved nutrients is conserved within the immediate coral-reef environment. Estimates of bacterial production also suggest that they may contribute up to 40% of total picoplankton production and about 25% of total microplankton production. Pelagic bacteria are therefore potentially a major food resource for benthic filter-feeders, especially for those mainly dependent on the smaller (pico) components of the plankton.