Planktonic Food Web Structure along the Sau Reservoir (Spain) in Summer 1997

Abstract

We studied the planktonic food web in eutrophic Sau Reservoir (Catalonia, NE Spain). Along the longitudinal axis from the Ter River downstream to the dam, we characterized a microbial succession of food web dominance of bacteria‐HNF‐ciliates. The Ter River transports a large load of organic material into the reservoir, with a bacterial density of ∼9 · 106 large cells per ml. While at the first lacustrine station of the Reservoir HNF were the dominant bacterial consumers, at the others, an oligotrich ciliate, Halteria grandinella, was the main protozoan bacterivore. Most of the bacterial production in the reservoir epilimnion was consumed by grazing. The spatial succession of the reservoir microbial food webs was followed downstream by maximum densities of their potential predators among zoo‐plankters – rotifers, and early developmental stages of copepods.

Changes in the Epilimnetic Bacterial Community Composition, Production, and Protist-Induced Mortality along the Longitudinal Axis of a Highly Eutrophic Reservoir

“…Water samples were collected at nine sampling points (equidistant ∼ 1.8 km, see Fig. 1) along the longitudinal axes of the Sau Reservoir in two intensive sampling campaigns: in April 1997, when temperature of the river inflow was the same as that of the epilimnetic reservoir water (∼16°C) and in July 1997, with well developed water stratification (for details see Comerma et al [10]). In both cases, except for the shallow river inflow (station 9), mixed water samples from the epilimnetic top 3 meters were taken with a 3-meter long plastic tube.…”

“…A typical longitudinal succession of planktonic foods web structures in the Sau Reservoir, i.e., dominated by bacteria, heterotrophic nanoflagellates, ciliates, and rotifers, has been recently documented [10]. Moreover, detailed studies on protistan succession and bacterivory along the longitudinal axis of the reservoir showed distinct peaks of heterotrophic nanoflagellates and ciliate abundance, and consequently spatially changing roles of these protistan groups as bacterivores [10,37].…”

“…Canyon-shaped reservoirs represent highly specific, remarkably spatially heterogeneous systems. Selected aspects of them, i.e., hydrodynamics, chemical processes, and zoo-, phytoplankton, and protistan dynamics, have been already reported [10,18,33,35,37]. However, the dominating biogeochemical processes occurring in the upper parts of canyon-shaped reservoirs, mediated mainly by microbial activities and interactions [43], are not yet fully understood.…”

“…Armengol et al [4] and Comerma et al [10] have reported distinguishable longitudinal gradients in concentrations of nutrients, in dissolved and particulate organic and inorganic material, and in bacterial and phytoplankton biomasses, yielding marked changes in turbidity and in qualitative parameters of water from the Ter river inflow to the outflow near the dam of the reservoir. A typical longitudinal succession of planktonic foods web structures in the Sau Reservoir, i.e., dominated by bacteria, heterotrophic nanoflagellates, ciliates, and rotifers, has been recently documented [10]. Moreover, detailed studies on protistan succession and bacterivory along the longitudinal axis of the reservoir showed distinct peaks of heterotrophic nanoflagellates and ciliate abundance, and consequently spatially changing roles of these protistan groups as bacterivores [10,37].…”

“…High input of allochthonous organic material, enhanced bacterial activity, and low oxygen saturation of water are typical for the inflow parts of the reservoir [4,10], pointing to the essential role of bacteria in carbon and nutrient cycling. Given the clearly distinguishable chemical gradients and longitudinal succession of plankton populations (see above), it is conceivable that different phylogenetic groups of bacteria take part in the processes of organic matter transformation in the different parts of the reservoir.…”