Significance of viruses versus heterotrophic nanofiagellates for controlling bacterial abundance in the northern Adriatic Sea

Abstract

In the present study, we aimed to investigate whether virus-mediated mortality of bacteria can prevail over grazing by heterotrophic nanofiagellates (HNFs). Data obtained from temporal and spatial trophic gradients in the northern Adriatic Sea showed that viral abundances were more closely coupled to bacterial numbers than HNF abundances. The frequency of visibly infected bacteria increased with bacterial abundance, indicating stronger virus-mediated mortality of bacteria in eutrophic systems. The high variability of I-INF abundances at high bacterial numbers shows that 1{NFs might be controlled by larger grazers in eutrophic situations. From our data, we conclude that virus-mediated mortality of bacteria can occasionally prevail over grazing by HNFs, e.g. at high bacterial abundances. Regression analysis also indicates that chlorophyll a and temperature did not contribute significantly to explaining bacterial variability in the northern Adriatic Sea.