Structural equation models suggest that bottom‐up processes override top‐down processes in boreal pikeperch (Sander lucioperca) lakes

Abstract

Inter‐relations between water quality and piscivore populations have remained poorly studied but their quantification is crucial for the ecosystem‐based management of inland waters. Here, we aimed to estimate the relative strength of bottom‐up and top‐down trophic interactions between water quality and fish community composition using a comparative approach. Our main hypothesis was that lakes with an abundant piscivore (mainly pikeperch, Sander lucioperca, Percidae) stock would have better water quality than similar lakes with a poor piscivore stock. We used structural equation models on biomass catch per unit of effort data from standard gillnet test fishing to examine the effects of predators (pikeperch, pike [Esox lucius, Esocidae] and piscivorous perch [Perca fluviatilis, Percidae]) on cyprinids (Cyprinidae), and the effects of cyprinids, nutrients, and lake depth on water quality measured as chlorophyll‐a concentration and turbidity. We derived the dataset covering a single summer snapshot of 138 Finnish pikeperch lakes from national databases. Nutrients and lake depth index, the main principal component describing positive effects of nutrient concentrations and negative effects of lake depth, had a positive effect on chlorophyll‐a concentration and turbidity. Pikeperch had a detectable negative effect on both planktivorous and benthivorous cyprinids. While pike also had a negative effect on benthivorous cyprinids, both groups of cyprinids had a positive effect on pikeperch and pike. Supporting the strong, fish‐independent bottom‐up regulation, no significant relationships were detected between the cyprinids and water quality. This study suggests that the bottom‐up effects override the top‐down effects, and despite pikeperch is an efficient piscivore, the observed stock levels did not cascade down to improved water quality. Thus, in lake restoration, reductions of nutrient loading appear more efficient than biomanipulation aimed to strengthen the top‐down links.

Energy‐based top‐down and bottom‐up relationships between fish community energy demand or production and phytoplankton across lakes at a continental scale

“…The feeding on both zooplankton and benthic macroinvertebrates by omnivorous fish may weaken top‐down cascades on pelagic zooplankton and phytoplankton relative to bottom‐up cascades where high phytoplankton biomasses induce high zooplankton biomasses, and omnivorous fish use this resource directly. The effect of piscivorous fish on top‐down and bottom‐up cascades was not significant, confirming earlier studies showing that the control of omnivorous fishes by piscivore predation is usually weak (Mehner ; Mehner et al ; Kokkonen et al ).…”

“…In a study focusing on the effect of piscivores on prey density and size and indirectly on Chl a in more than 60 lakes in northern Germany, Mehner () found a weak top‐down control of fish on Chl a . In a recent study in 138 Finnish oligo‐eutrophic lakes populated by piscivorous pikeperch ( Sander lucioperca ), Kokkonen et al () demonstrated that bottom‐up effects of phytoplankton on fish were much stronger than the top‐down effects by fish on phytoplankton. We suggest that our results complement these earlier studies and support the suggestion of a relatively weak top‐down effect of fish on phytoplankton biomass in across‐lake comparisons.…”

Can biomass distribution across trophic levels predict trophic cascades?

“…We stress that this question is not equivalent to asking whether the system is bottom-up or top-down controlled (as in e.g. Kokkonen et al, 2019), but whether bottom-up or top-down dynamical effects, relevant to predict trophic cascades, can be seen in the biomass distribution.…”

Can biomass distribution across trophic levels predict trophic cascades?

“…We stress that this question is not equivalent to asking whether the system is bottom‐up or top‐down controlled (as in e.g. Kokkonen et al ., 2019), but whether bottom‐up or top‐down dynamical effects, relevant to predict trophic cascades, can be seen in the biomass distribution.…”