Mesozooplankton dynamics in nearshore waters of the Great Barrier Reef

Zooplankton dynamics (community composition, juvenile somatic growth rate, adult egg production, secondary production) were studied in coastal waters of the Great Barrier Reef. Two sectors were compared, one adjacent to a catchment of near-pristine land use patterns, the other to a more intensively farmed catchment. Sampling was conducted in the austral winter (August) and summer (January-March) of two succeeding years. Gradients in zooplankton community composition were weak, with only moderate effects of season and sector. Overall, 37% of zooplankton biomass was in the 73-150 μm size fraction, 26% in the 150-350 μm fraction, and 38% was >350 μm. There was no biomass difference and only small differences in community composition between samples taken during the day and at night; ostracods and large calanoid copepods were occasionally more common at night. Carbon-specific growth rates averaged 0.29 d−1 for cyclopoid copepods and 0.35 d−1 for calanoid copepods, with no difference between sectors. Calanoid copepod growth showed a significant relationship to chlorophyll concentration, but cyclopoid copepods did not. Copepod egg production was low (7.9 ± 5.9 eggs female−1 d−1) and apparently food-limited. Copepod secondary production was lower in August (mean = 2.6, range 1.4-4.0 mg C m−2 d−1) than in January-March (mean = 8.5, range 2.4-15.5 mg C m−2 d−1). Secondary production by mesozooplankton in the 73-100 μm size range averaged 0.9% of total phytoplankton production.