Abstract
Abstract Browning of waters, coupled to climate change and land use changes, can strongly affect aquatic ecosystems. Browning‐induced light limitation may have negative effects on aquatic consumers via shifts in resource composition and availability and by negatively affecting foraging of consumers relying on vision. However, the extent to which light limitation caused by browning affects fish via either of these two pathways is largely unknown. Here we specifically test if fish growth responses to browning in a pelagic food web are best explained by changes in resource availability and composition due to light limitation, or by reduced foraging rates due to decreased visual conditions. To address this question, we set up a mesocosm experiment to study growth responses of two different fish species to browning and conducted an aquaria experiment to study species‐specific fish foraging responses to browning. Furthermore, we used a space‐for‐time approach to analyse fish body length‐at‐age across >40 lakes with a large gradient in lake water colour to validate experimental findings on species‐specific fish growth responses. With browning, we found an increase in chlorophyll a concentrations, shifts in zooplankton community composition, and a decrease in perch (Perca fluviatilis) but not roach (Rutilus rutilus) body growth. We conclude that fish growth responses are most likely to be linked to the observed shift in prey (zooplankton) composition. In contrast, we found limited evidence for reduced perch, but not roach, foraging rates in response to browning. This suggests that light limitation led to lower body growth of perch in brown waters mainly through shifts in resource composition and availability, perhaps in combination with decreased visibility. Finally, with the lake study we confirmed that perch but not roach body growth and length‐at‐age are negatively affected by brown waters in the wild. In conclusion, using a combination of experimental and observational data, we show that browning of lakes is likely to (continue to) result in reductions in fish body growth of perch, but not roach, as a consequence of shifts in prey availability and composition, and perhaps reduced foraging.
Highlights
Fish species identity had an effect on zooplankton community composition (Figure 4b and Figure S4b, PERMANOVA: F(1,16) = 3.81, p = 0.008, for community composition on all dates see Figure S3), with mesocosms with perch being dominated by Bosmina sp. while mesocosms with roach were dominated by copepods
There was a tendency of a negative effect of brown water colour on capture rate of D. longispina by perch (ANOVA: F2,22 = 3.4156, p = 0.051, Figure 6a)
Capture rate of D. longispina by roach was not affected by water colour (ANOVA: F2,21 = 1.9256, p = 0.17, Figure 6b)
Summary
The two fish species used in this study are Eurasian perch (Perca fluviatilis) and common roach (Rutilus rutilus), two common and often cooccurring fish species in northern European lakes and coastal waters. To study fish growth responses to light limitation in a pelagic food web, we performed a mesocosm experiment in 18 open tanks (3 m diameter × 1 m water depth) that were located outside from 10 August to 10 September 2017. To separate fish growth responses caused by changes in resource availability and composition due to light limitation (shifts in prey community), and reduced foraging rates due to decreased visual conditions, we assigned three browning treatments and two fish species treatments using a factorial design (Figure 1). In the first half of the experiment, no fish were present and only BE treatment mesocosms were browned (Figure 1) This allowed the zooplankton populations to establish without predators present and us to study whether browning-induced light limitation affected phytoplankton biomass (measured as chl a) and zooplankton biomass and composition.
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