Identification of a biomanipulation‐driven regime shift in Lake Vesijärvi: implications for lake management

Abstract

1. The biomanipulation of Lake Vesij€ arvi in southern Finland putatively changed the lake ecosystem resulting in a clear-water state, but the dynamics behind the observed change remained unverified. 2. We used a general analytical approach to identify abrupt transitions in the long-term monitoring data of Lake Vesij€ arvi. Methods included the identification of breakpoints in chlorophyll a (response variable), in total phosphorus (TP; key driver) and in large-bodied cladocerans (feedback mechanism of biomanipulation). In addition, we characterised the frequency distribution and temporal variance of the response variable, as well as the interaction between TP and chlorophyll a. 3. According to long-term records, biomanipulation resulted in two distinct states: (i) a eutrophic state, where chlorophyll a and TP were linearly correlated and (ii) a mesotrophic state, where the key driver and response variables were not significantly correlated. The changed relationship between the key driver and response variables, together with the increase in the body size of cladocerans, suggested a hysteresis-type transition phase. However, the frequency distribution and the temporal variance of chlorophyll a agreed also with a threshold-type of transition. 4. Zooplankton seemed to be more important in maintaining the clear-water state than previously reported. Our results highlight the role of long-term monitoring data and the characterisation of abrupt transitions in ecosystem interactions as tools for sustainable management of lake ecosystems.

Lake restoration influences nutritional quality of algae and consequently Daphnia biomass

“…Large-scale biomanipulation, i.e., mass removal of planktivorous fish, was conducted in 1989-1993 (Horppila et al, 1998;Kairesalo et al, 1999;. This major restoration effort was followed by the collapse of cyanobacterial biomass, which doubled water clarity and increased the body size of Daphnia (Anttila et al, 2013). However, the improved water quality turned out to be only temporary, as high cyanobacterial biomasses reappeared in the Enonselkä basin in the 21st century ([ 50% of all phytoplankton in average of summer months of 2001,2002,2005,2007,2009,2013,2016), despite the continuous management fishing .…”

“…As the concentration of TP was similar during the clear-water and the second blooming stage, it cannot explain the occasional blooms of cyanobacteria during the second blooming stage. However, there are multiple other stressors, such as intensified lakeshore building, exceptional weather conditions and hypolimnetic oxygen deficiency which probably contributed to deteriorating water quality (Nykänen et al, 2010;Anttila et al, 2013).…”

“…Increased biomass and/or body size of Daphnia and decreased concentration of chlorophyll a in, at least, some lakes after restoration suggest improved topdown control of phytoplankton (Søndergaard et al 2008;Anttila et al 2013). According to the quantitative analysis of reported biomanipulation studies by Bernes et al (2015), the increased biomass of large-bodied Daphnia following biomanipulation may well be explained not only by reduced predation by fish but also by the shift from the predominance of cyanobacteria toward high nutritional quality food for Daphnia.…”

“…We studied the impacts of biomanipulation and hypolimnetic aeration on the phytoplankton community structure and nutritional quality of phytoplankton as the diet for zooplankton, especially Daphnia and Bosmina. 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.…”

“…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).…”