Responses of food web to hypolimnetic aeration in Lake Vesijärvi

Abstract

We studied the responses of a food web, especially fish and zooplankton, to summertime aeration, pumping of oxygen-rich epilimnetic water to the hypolimnion in Lake Vesijärvi, southern Finland. The aim of hypolimnetic aeration was to reduce internal loading of phosphorus from sediment. The population of smelt (Osmerus eperlanus L.), the main planktivore of the pelagial area, collapsed during the two 1st years of aeration due to increased temperature and low oxygen concentrations in the hypolimnion. The population recovered after the 4th year of hypolimnetic aeration, when oxygen conditions were improved. Despite elevated hypolimnetic temperature, smelt reached exceptionally high abundance, which led to a significant reduction in cladoceran body size. The density of perch (Perca fluviatilis L.) increased at first, but then decreased when the proportion of smelt and cyprinids increased. Biomasses of Daphnia decreased probably as a result of the disappearance of dark, low-oxygen deep-water refuge against fish predation and low availability of nutritionally high-quality algae. Occasionally filamentous cyanobacteria, such as turbulence tolerant Planktothrix agardhii (Gomont), were abundant, suggesting deteriorated food resources for zooplankton. The responses of food web were controversial with respect to the aim of the management, which was to prevent the occurrence of harmful algal blooms.

Preface: Restoration of eutrophic lakes: current practices and future challenges

“…In such cases, hypolimnetic aeration may contribute to large reductions in sediment P release (e.g., up to 90%, Walker et al, 1989). However, many studies have questioned the efficacy of aeration itself, including several in this Special Issue (e.g., Chorus et al, 2020;Niemistö et al, 2020;Ruuhijärvi et al, 2020;Taipale et al, 2020). For example, destabilization of the thermocline and increased bottom water temperatures can lead to increased sediment P release and/or transport to surface waters.…”

“…One article presents a review of existing worldwide examples of hypolimnetic withdrawal, as well as a feasibility study for its new application in Lake Borsjön, Sweden (Nürnberg, 2020). The remaining studies assess the influence of various restoration measures, including aeration and biomanipulation, on food webs and biogeochemical cycles in two heavily human-impacted Finnish Lakes, Vesijärvi and Tuusulanjärvi (Niemistö et al, 2020;Rask et al, 2020;Ruuhijärvi et al, 2020;Taipale et al, 2020).…”

Lake restoration influences nutritional quality of algae and consequently Daphnia biomass

“…Daphnia biomass decreased during the second blooming stage and the aeration stage, even though phytoplankton community structure or summer averages of EPA and DHA did not suggest such a high decrease. Therefore, some additional factors may have hampered the performance of Daphnia, such as aeration induced turbulence and the loss of the dark, oxygen-poor hypolimnetic refuge from predation by fish, a phenomenon documented as a response to lake mixing (Gauthier et al, 2014;Ruuhijärvi et al, 2020). In our study lake, small-sized smelt (Osmerus eperlanus) built up exceptionally high populations following aeration, and they obviously preyed heavily on Daphnia, evidenced by drastically reduced body size (Ruuhijärvi et al, 2020).…”

“…Therefore, some additional factors may have hampered the performance of Daphnia, such as aeration induced turbulence and the loss of the dark, oxygen-poor hypolimnetic refuge from predation by fish, a phenomenon documented as a response to lake mixing (Gauthier et al, 2014;Ruuhijärvi et al, 2020). In our study lake, small-sized smelt (Osmerus eperlanus) built up exceptionally high populations following aeration, and they obviously preyed heavily on Daphnia, evidenced by drastically reduced body size (Ruuhijärvi et al, 2020). Mixing may have also made it difficult for Daphnia to find high-quality diet (Taipale et al, 2019).…”

“…Our study site, Lake Vesijärvi, has been intensively managed with the two management measures to restore the original clear-water state from the eutrophic state characterized by cyanobacterial blooms (Anttila et al, 2013). Several studies have been published on the results of these management measures (Horppila et al, 1998;Anttila et al, 2013;Salmi et al, 2014;Ruuhijärvi et al, 2020;Niemistö et al, 2016), but none of these takes into account that successful bottom-up restoration should increase high nutritional quality algae (cryptophytes, chrysophytes, diatoms, dinoflagellates) to increase zooplankton biomass. We calculated the contribution of x-3 PUFA, x-6 PUFA, EPA and DHA, and sterols (sterol index) in autotrophic seston based on laboratory culturing of different phytoplankton genera (see supplemental material, Peltomaa et al, 2017).…”