Fisheries impacts on lake ecosystem structure in the context of a changing climate and trophic state

Abstract

<p>Through cascading effects within lake food webs, commercial and recreational fisheries may indirectly affect the abundances of organisms at lower trophic levels, such as phytoplankton, even if they are not directly consumed. So far, interactive effects of fisheries, changing trophic state and climate upon lake ecosystems have been largely overlooked. Here we analyse case studies from five European lake basins of differing trophic states (Lake Võrtsjärv, two basins of Windermere, Lake Geneva and Lake Maggiore) with long-term limnological and fisheries data. Decreasing phosphorus concentrations (re-oligotrophication) and increasing water temperatures have been reported in all five lake basins, while phytoplankton concentration has decreased only slightly or even increased in some cases. To examine possible ecosystem-scale effects of fisheries, we analysed correlations between fish and fisheries data, and other food web components and environmental factors. Re-oligotrophication over different ranges of the trophic scale induced different fish responsesIn the deeper lakes Geneva and Maggiore, we found a stronger link between phytoplankton and planktivorous fish and thus a more important cascading top-down effect than in other lakes. This connection makes careful ecosystem-based fisheries management extremely important for maintaining high water quality in such systems. We also demonstrated that increasing water temperature might favour piscivores at low phosphorus loading, but suppresses them at high phosphorus loading and might thus either enhance or diminish the cascading top-down control over phytoplankton with strong implications for water quality.</p>

Linking atmospheric, terrestrial and aquatic environments: Regime shifts in the Estonian climate over the past 50 years

“…Thus, shifts between phases of high and low water levels are behind many of the observed RSs in biotic time series in the early 1980s and the mid 1990s. Moreover, the measures of fisheries management that target the composition of fish communities, and the balance between predatory and non-predatory fish species, cascade down through the food web [64]. Such reorganization of fisheries management of the lake food webs took place in the 1970s.…”

The Observatory on LAkes (OLA) database: Sixty years of environmental data accessible to the public

“…The unexpected trend in phytoplankton biomass observed in Lake Geneva is explained by a decrease in zooplankton grazing (Tadonleke et al, 2009), a change in phytoplankton taxonomic composition and an increase in phytoplankton biomass during winter and spring as a result of temperature increases during these seasons . Moreover, the abundance and taxonomic composition in zooplankton gradually changed with time because of decreases in food quality (Anneville et al, 2007 showed a shift from herbivorous to carnivorous copepods), changes in thermal conditions (Molinero et al, 2007) and trophic status (Molinero et al, 2006), and increased fish predation (Nõges et al, 2018).…”

The future depends on what we do today – Projecting Europe's surface water quality into three different future scenarios

“…For around 135 days per year, the lake is ice covered. As it is located in an area of flat relief the water outflow is restricted and the lake exhibits strong seasonal water level fluctuations with a maximal amplitude of 3.2 m during the snow melt (Cremona et al, 2018;Nõges et al, 2018). About half of the water in Lake Võrtsjärv comes from the river Väike-Emajõgi, which still possesses its natural flow regime (Cremona et al, 2017).…”

“…About half of the water in Lake Võrtsjärv comes from the river Väike-Emajõgi, which still possesses its natural flow regime (Cremona et al, 2017). The main land use in the basin is agriculture, therefore extensive nutrient inputs since 1961 facilitate high phytoplankton biomass, to which cyanobacteria contribute 60% to 95% (Cremona et al, 2018;Nõges et al, 2018).…”