Short-term variability in chitobiase-based planktonic crustacean production rates in a highly eutrophic tropical estuary

Abstract

Although tropical oceans are generally assumed to have low zooplankton biomass throughout the year, high copepod abundance coupled with fast growth rates can result in a significant amount of crustacean zooplankton production. Here, we use the crustacean moulting enzyme chitobiase to obtain routine estimates of community-level crustacean productivity over a 3-mo period in Guanabara Bay, Rio de Janeiro, Brazil. Chitobiase-based daily production to biomass ratios (P/B) and production rates were compared to values derived from more traditional global predictive models. We examined the abiotic and biotic factors most strongly influencing copepod biomass, daily P/B, and production rates. Mean copepod biomass was 24.0 mg C m3 over the sampling period, while daily P/B ranged between 0.15 and 1.20. Copepod biomass was negatively related to dissolved oxygen and tidal amplitude, characteristic of the highly eutrophic waters from the inner bay. Mean crustacean productivity over our sampling period was 22.0 mg C m3 d1, varying more over monthly timescales compared to weekly or daily variations. No relationship was found between production rates and biomass, suggesting that biomass, alone, does not explain productivity in Guanabara Bay. Chitobiase-based daily P/B and crustacean production rates were almost always higher than production estimates from global models. Results from this study highlight the need for accurate estimates of crustacean production rates in order to fully understand trophic relationships given that biomass, alone, did not explain the short-term variability in crustacean production. Ultimately, this study reveals that small, fast-growing copepods can contribute just as much, if not more, energy to higher trophic levels in eutrophic tropical estuaries compared to temperate regions.