Processos heterotròfics microbians a l'embassament de Sau / Heterotrophic microbial processes in the Sau reservoir

Abstract

We studied the planktonic food web in the eutrophic Sau Reservoir (Catalonia, NE Spain) taking account of both spatial as well as temporal scales. We have analysed several longitudinal transects (sampling from the river to the dam) during 1996-2000 period, covering a wide range of both seasonal and spatial water circulation patterns.<br/>The longitudinal circulation of the River Ter across the Sau Reservoir is the result of the difference between inflow and epilimnion water temperatures along the year. Horizontal patterns of stratification and river circulation in this reservoir can be combined to explain its hydrodynamics. Therefore, from these observations, an empirical annual pattern of longitudinal circulation in the Sau Reservoir was obtained for the 1996-2000 period. The general river circulation model is characterized by underflow in winter, overflow-interflow in spring and interflow in summer-autumn.<br/>We observed decreasing chemical gradients from river to dam in the epilimnion of the Sau Reservoir, caused by the inflow of River Ter, a river highly polluted with organic matter. Sau works as an efficient purification system, improving water quality from inflow to outflow. The efficiency of this system depends on nutrient loads, nutrient concentrations in the reservoir, sedimentation rates, biological activity and water flow. Water flow in the reservoir, flowing in bottom (underflow), middle (interflow) or top layers (overflow) through greatly influences the degree of mixing between river and reservoir waters.<br/>Enhanced abundances and activities of microbes were detected during the spring and summer periods. Applying a model of geometric distances, we analysed all samplings together from a longitudinal perspective (from the River Ter downstream to the dam). Along the longitudinal gradient, we characterized a downstream food-chain succession with spatial dominance of bacteria, heterotrophic nanoflagellates, ciliates, phytoplankton, and zooplankton. The river circulation pattern through the reservoir controlled this longitudinal gradient. The amplitude of microbial peaks was related to nutrient and organic carbon loads in the river inflow and the percentage of river water mixed to the epilimnion. Ciliates, not HNF, were the major consumers of the bacterial production and showed two conspicuous abundance maxima. From almost 1500 ciliates inspected, Halteria grandinella was the most abundant and the most significant bacterivore.<br/>The role of microbial and classical food chains (i. e. based directly on phytoplankon) were compared in the Sau Reservoir by analysing river-to-dam gradients in biomass and carbon and their temporal changes. The detritic metabolic pathway was more important near to the inflow, due to high allochthonous organic matter loads allowing the rapid development of the microbial food web. Protozoans (HNF and ciliates) consumed most of the bacterial production (i.e. >50 %) in the reservoir. As opposed to the systems of lower trophic status ciliate carbon biomass and bacterivory contributions were larger than those of the HNF. We estimated species-specific ciliate growing rates on bacteria and distinguished several periods with high importance of distinct ciliate communities.<br/>An experiment with differently top-down and bottom-up manipulated microcosms was conducted during spring 2000 to estimate growth rates of heterotrophic nanoflagellates (HNF) and of main groups of ciliates in the epilimnion of the lacustrian area of the Sau Reservoir (not directly influenced by the polluted river). Along with the major factors controlling the growth of protozoan populations we also studied the impact of metazooplankton on the structure of the microbial food web. Results clearly documented that, in the lacustrian area, HNF and ciliate growth rates were controlled mainly by zooplankton predation while only a limited effect of the food resource limitation could be detected in this eutrophic reservoir.