BACTERIAL ABUNDANCE AND ACTIVITY IN THE HIGHLY STRATIFIED ESTUARY OF THE KRKA RIVER

Temporal and spatial abundance, and activities of heterotrophic bacteria in the highly stratified Krka River estuary (central Adriatic Sea) were analyzed in spring 1987-1989. The heterotrophic activity was determined by the uptake of [C-14] glucose. The bacterial abundance (1-23 x 10(8) cells 1-1) in the estuary was higher than in the open central Adriatic waters but five times lower than in the open northern Adriatic waters. The glucose uptake rate in the Krka Estuary varied remarkably along the horizontal and vertical profiles (0.004-1.620-mu-g C l-1 h-1). Examination of the depth profiles did not reveal a significant correlation between chlorophyll a and bacterial abundance and activity, respectively. Short-time variations of microbial parameters in the freshwater and interface layer of stratified depth profiles were detected. The maximal glucose incorporation (80%) was maintained in the interface layer by small free-living bacteria. The glucose mineralization rate increased with salinity, and reached a maximum value (60%) in the lower part of the interface layer.