Non-predatory mortality indices of cladoceran zooplankton in the pelagial and littoral of a shallow meso-eutrophic lake

Abstract

391 The role of topical factors, such as depth, bottom type, and the presence of water vegetation, in the functioning of plankton populations in relatively small shallow water bodies is not clear. In addition, the effect of macrophytes on plankton may be not only topical, but also metabolic (see, e.g., review [1]). The influence of macrophytes on littoral zooplankton may be studied with the use of the recently developed method of direct determination of the mortality in crustacean plankton unrelated to predation [2‐5]. The mortality due to aging, starvation, diseases, parasites, and negative physical and chemical factors is evaluated on the basis of calculation of dead individuals in samples stained with a special dye (e.g., [6]) and measurements of the sedimentation rate of dead bodies with the use of sedimentation traps. It was shown [3, 7] that the relative number of dead zooplankters is proportional to the specific mortality and, hence, is a reliable quantitative index. In this work, we measured the percentage of dead individuals in the populations of the dominant zooplankton species (Cladocera) in three biotopes of a freshwater lake—the pelagial biotope and two littoral biotopes, grown with bul rush and bare (the macrophyte-free littoral)—to determine the possible topical difference in the mortality. The work was performed in July of 2006 in the meso-eutrophic Lake Obsterno belonging to the Perebrodskaya group of lakes (northwestern Belarus). The lake area is 990 ha, the average depth is 5.3 m, and the maximum depth is 12 m. Reed and bul rush are the predominant species among littoral emergent plants. The main sediment type in the lake littoral is sand. For a detailed description of the lake morphometry, see [8, 9]. Samples of zooplankton were collected at 11 a.m. and 7 p.m. on July 25 and 26 using Apstein’s plankton net (the area of the inlet hole was 0.05 m 2 ; the mesh size was 70 µ m). In the pelagials and bul rush littorals, the net was towed from the bottom to the surface (4‐0 and 1‐ 0 m, respectively); the horizontal tow in the macrophyte-free littoral was performed over the space of 4 m; two samples were collected in each biotope (the replications and all the necessary characteristics were determined separately for each sample). The depth of sampling stations was 5.5 m in the pelagial and 1.3 m in the