Underwater light conditions, phytoplankton photosynthesis and bacterioplankton production in the Hungarian section of the River Danube

Abstract

Underwater light climate, phytoplankton composition and photosynthesis, bacterioplankton production and heterotrophic nanoflagellate abundance were studied in the River Danube near Budapest in 1991. During the winter months water transparency was high, the euphotic zone deep (3 m), but the phytoplankton biomass and the primary production were low as a consequence of low water temperature and reduced global radiation. An increase in phytoplankton abundance (chlorophyll-a concentration reaching 164 μg.l -1 ) during the spring low water period in April and May was observed. This large population was lost to the summer floods. The months of July and August experienced low chlorophyll-a concentrations even though water temperature increased to 20°C. This spring peak was superseded by a second peak in September. The abundance of phototrophic picoplankton in the river was low when compared to that of lakes (10 3 -10 5 cells.ml -1 ). These low concentrations suggest that their contribution towards primary production in the river is not important. The value of the vertical attenuation coefficient (Kd) was mainly influenced by the concentration of suspended solids. In winter the Z eu / Z avg ratio approached 1.0 suggesting that the algae were not light limited this period. The ratio ranged from 0.5-0.7 during the spring and autumn algal blooms and decreased to 0.2-0.3 during the summer floods. The P/R ratio of phytoplankton in the whole water column approached 1.0 in June and July and exceeded 2.0 during the rest of the period. This result supported the hypothesis that light limitation is solely sufficient to prevent algal growth during the summer floods. The mineral nutrients (N, P) do not limit algal growth in the River Danube. Bacterial production measured by 3 H-thymidine incorporation averaged 1.19 nmol.l -1 .d -1 in 1991. The annual average net bacterial production was found to be only 10% that of primary production. Bacterial production seasonal dynamics were found to be independent of phytoplankton production. Zooplankton grazing was considered unimportant in the River Danube. Heterotrophic nanoflagellates (HNF) attained high numbers and were considered to be the main consumer of bacterioplankton and so could play a determining role in the processes of the nutrient cycling in the River Danube.