Bacterial and protozoan mass accumulation in the deep chlorophyll maximum of a mesotrophic lake

Abstract

Deep chlorophyll maxima (DCM) represent high accumulations of algal biomass in the microaerobic metalimnion of mesotrophic lakes characterised by a few algal species which persist in time. We studied the accumulation of bacteria, protozoans and the mesozooplankton associated with the DCM of mesotrophic Grosser Vatersee in northern Germany over a three year period and related our findings to the pattern found in the epilimnion and the hypolimnion. More than 20 % of the bacterial and protozoan biomass was concentrated in the narrow layer of the DCM, which comprised less than 10% of the whole water column. Between 16 and 36% of the rotifers and 12-31% of the juvenile cyclopoid copepods inhabited the DCM permanently on a day/night time scale, while calanoid copepods and adult cyclopoid copepods concentrated in the upper metalimnion and migrated into the DCM occasionally. Although bacteria and flagellates were generally larger in the DCM than in the epilimnion, ratios between bacterial and protozoan biomass were similar in both lake compartments. Our findings confirm a uniform community structure in the DCM of ciliates (Dexiotricha spp. and Strobilidium spp. comprised on average 70% of ciliate biomass) and, to a lesser extent, of mesozooplankton (rotifers and juvenile cyclopoid copepods dominated in the DCM). In contrast seasonal and interannual patterns of bacterial and protozoan biomass and cell size imply variation in the effects of predation.