Abstract
Autotrophic lakes are regarded to function as net autotrophic systems in which mobilization of solar energy by phytoplankton, benthic algae, and macrophytes forms the base of primary production. However, they are also subsided by allochthonous inputs. In this paper, we tested three hypotheses explaining the role of allochthonous subsidies as a driving force of autotrophic, bacterial, and heterotrophic production. The studies for this paper were conducted every fortnight from November 2015 to October 2017 in a small, shallow, temperate, eutrophic lake supplied with waters from four streams and drained by a single outlet. Changes in plankton biomass as well as variations in environmental conditions were estimated on the basis of the samples and measurements collected vertically in the water column (every 0.5 m depth) in the central part of the lake. Loads of nutrients and organic and inorganic matter were measured in both the inlets and the outlet of the lake, and on the basis of the differences between input and output loading, the amount of allochthonous subsidies in the lake was computed. Allochthonous subsidies affected the autotrophic production directly by supplying assimilable nutrients, and indirectly by providing allochthonous organic matter that supplemented the diet of omnivores and thus hampered their grazing pressure on phytoplankton. Bacterioplankton utilized allochthonous organic matter as well as effectively competed with phytoplankton for allochthonous nutrients. Dominant species of heterotrophic plankton were feeding opportunists, and thus inputs of allochthonous subsidies indubitably fostered high biomass of these species in the periods of low availability of autochthonous sources of food. Our finding suggests that allochthonous subsidies significantly affect trophic structure and impact both autotrophic and heterotrophic mobilization of energy through food chains in freshwater ecosystems.
Highlights
The European Water Framework Directive, adopted in 2000, emphasizes the management of freshwaters, which should be achieved through holistic, catchmentscale, ecosystem management approaches
We modeled the effect of phytoplankton, allochthonous total organic carbon (TOC), and allochthonous TSS on the biomass of omnivores to estimate whether allochthonous sources of food diminish the role of phytoplankton as a food source for omnivores
We found that inputs of allochthonous nutrients resulted in increases of phytoplankton biomass
Summary
The European Water Framework Directive, adopted in 2000, emphasizes the management of freshwaters, which should be achieved through holistic, catchmentscale, ecosystem management approaches. The quality of lake water depends on natural factors such as precipitation inputs, erosion and weathering of crustal material, and biota interrelationships, and on anthropogenic influences such as urban, industrial, and agricultural activities (Noges, 2009; Wan et al, 2014; Moorhouse et al, 2018). Nutrient retention in lakes has a crucial impact on the ecosystem processes, as nutrients have a limiting effect on the development of aquatic organisms (Elser et al, 2007; Grizzetti et al, 2015). Ionic retention is determined as a difference between ionic input and output, and its calculation is considered to be the most accurate approach for the determination of the state of lake ecosystems (Powers et al, 2015)
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