Factors affecting the bacteria–heterotrophic nanoflagellate relationship in oligo-mesotrophic lakes

The coupling between bacteria and heterotrophic nanoflagellates (HNF) was examined in nine lakes of low productivity for evidence of the effects of various metazooplankton (i.e. rotifers, cladocerans and copepods) on this relationship. We considered the size of cladocerans and, in contrast to most previous across-system studies, the three strata of the water column (i.e. epilimnion, metalimnion and hypolimnion). Rotifers were numerically dominant in all lakes and accounted for 45–84% of total metazooplankton abundance, while the abundance of large cladocerans was relatively low, ranging from 0.066 to 15.2 ind. L−1. The across-lake relationship between bacteria and HNF was significant in the deeper strata (meta- and hypolimnion) but not in the epilimnion and in the two groups of lakes separated on the basis of their average number of large cladocerans (5 ind. L−1, respectively). The results confirmed the negative impacts of large cladocerans on HNF, but also showed that rotifers, probably through grazing on HNF, may be an important factor causing variation in the bacteria–HNF relationship in unproductive waters. Quadratic models best described the relationships between metazooplankton and the ratio of bacteria to HNF. This ratio seemed to be a result of complex interactions between several factors, including the zooplankton composition and abundance and the depth of the lake. Indeed, this ratio significantly decreased across lakes, with increase in depth. In addition, shallower lakes (having 5 large cladocerans L−1 and substantial proportions of P. vulgaris). We suggest that the epilimnion, metalimnion and hypolimnion of lakes be taken into account when analysing the bacteria–HNF relationship as well as the cascading effects of zooplankton on microbial communities.