Vertical and horizontal changes in the abundance and activity of bacteria in Lake Hańcza, NE Poland

Abstract

A bacteriological study was conducted in late summer (early September 2018) in the deepest postglacial reservoir in the central lowland depression - Lake Hańcza (northeastern Poland). The aim of the study was to determine the spatial variation in the density of planktonic bacteria, the level of their hydrolytic activity and the ratio of live to dead bacterial cells. During the fieldwork 92 water samples were collected from thirteen stratified sites located in different parts of the lake. The bacterial number ranged from 0.57 to 3.97 (10⁶ cells m⁻¹). No horizontal or vertical changes in bacterial number were observed in a uniform pattern. The highest bacterial abundance was recorded at two coastal sampling points, both located in the southern part of the lake at the direct catchment area with commercial development ("agroturism" guesthouses) . There was little difference in bacterial number across the horizontal gradient (F = 1.865; P = 0.170). Many water quality parameters appeared to be significant factors correlating with bacterial abundance in Lake Hancza. There were no statistically Significant differences in hydrolitical bacterial activity and live/ dead ratios between investigated sites and across horizontal gradient. Our results indicate that bacterial activity in deep lakes is affected by many water quality parameters, such as temperature (r=0.667), Ca²⁺ (r=-0.4510), Mg²⁺(r=0.511), N0₃- (r=-0.587) and oxygen saturation (r=0.582). Taking into account previous hydrobiological studies, Lake Hancza was found to be very patchy in terms of bacterioplankton abundance.