The contribution of heterotrophic nanoflagellate grazing towards bacterial mortality in tropical waters: comparing estuaries and coastal ecosystems

Abstract

Heterotrophic nanoflagellate (HNF) grazing depends on both temperature and trophic status of an ecosystem. As most microbes already function at their temperature optimum in tropical waters, we hypothesised that HNF grazing rates would be higher in more productive sites such as estuaries than in less productive areas such as coastal waters. We sampled two estuaries and three coastal sites along the Straits of Malacca and the South China Sea near the Malaysia Peninsula. Bacterial abundance ranged 0.9–6.3 × 106 cells mL–1, whereas HNF abundance ranged 1.8–10.1 ×103 cells mL–1. Bacterial production ranged 1.1–12.7 × 105 cells mL–1 h–1, whereas HNF grazing rates were an order of magnitude lower at 1.0–78.5 × 104 cells mL–1 h–1. Bacterial abundance, net bacterial production and HNF grazing rates were higher in estuaries than coastal waters but HNF abundance did not differ between the two areas. Across all stations, HNF grazing rates increased with bacterial production, and accounted for ~33% of bacterial production. Our results suggest that in the tropical waters studied, there was a bacterial production–grazing imbalance. Other loss factors such as viral lysis, sedimentation or the presence of benthic filter feeders could account for this imbalance.