Uncoupling between Heterotrophic Bacteria and Phytoplankton and Changes in Trophic Balance Associated with Warming of Seawater in Gyeonggi Bay, Yellow Sea

Abstract

We investigated bacterial production (BP) and respiration (BR), in combination with phytoplankton and environmental parameters, to elucidate major carbon sources regulating heterotrophic bacterial metabolic activity and to evaluate variations of trophic balance associated with seawater warming in Gyeonggi Bay (GB). BP was not significantly correlated with primary production (PP, p > 0.05), but was significantly correlated with dissolved organic carbon (DOC, p < 0.01). Bacterial growth efficiency (BGE) was generally low (average 0.06) and decreased with increasing C/N ratio of DOM and concentration of humic-like fluorescent DOM (FDOMH). This uncoupling between bacteria and phytoplankton and low BGE suggests that bacterial growth largely relied on allochthonous input of DOC, but metabolic activities of the bacteria were suppressed by the low nutritional quality of the FDOMH. High BP/PP and BR/PP ratios (generally >1) implied that trophic balance of GB represented heterotrophic conditions, and ratios showed an inverse relationship with Chl-a concentrations. In comparison to the early 1990s, increasing water temperatures were associated with significant decreases in PP and Chl-a (by a factor of 9 and 2, respectively), whereas changes in BP were insignificant; these changes resulted in a 14-fold increase in the BP/PP ratio. Therefore, our results strongly imply that warming in GB intensified the carbon flux through the microbial loop, which may reduce the efficiency of energy transfer to higher trophic levels.