Abstract
Due to altered biogeochemical processes related to climate change, highly colored dissolved organic carbon (DOC) from terrestrial sources will lead to a water “brownification” in many freshwater systems of the Northern Hemisphere. This will create deteriorated visual conditions that have been found to affect habitat-specific morphological variations in Eurasian perch (Perca fluviatilis) in a previous study. So far, potential drivers and ultimate causes of these findings have not been identified. We conducted a field study to investigate the connection between morphological divergence and polyunsaturated fatty acid (PUFA) composition of perch from six lakes across a gradient of DOC concentration. We expected a decrease in the prevalence of PUFAs, which are important for perch growth and divergence with increasing DOC concentrations, due to the restructuring effects of DOC on aquatic food webs. In general, rate of morphological divergence in perch decreased with increasing DOC concentrations. Proportions of specific PUFAs (22:6n-3, 18:3n-3, 20:5n-3, and 20:4n-6) identified to primarily contribute to overall differences between perch caught in clear and brown-water lakes tended to be connected to overall decline of morphological divergence. However, no overall significant relationship was found, indicating no severe limitation of essential fatty acids for perch inhabiting brown water lakes. We further broaden our approach by conducting a laboratory experiment on foraging efficiency of perch. Therefore, we induced pelagic and littoral phenotypes by differences in habitat-structure and feeding mode and recorded attack rate in a feeding experiment. Generally, fish were less efficient in foraging on littoral prey (Ephemeroptera) when visual conditions were degraded by brown water color. We concluded that browning water may have a strong effect on the forager’s ability to find particular food resources, resulting in the reduced development of evolutionary traits, such as habitat- specific morphological divergence.
Highlights
In the face of rapid human-induced environmental changes the ability of organisms to successfully survive and reproduce is challenged, and is primarily determined by their ability to adaptively respond to these changes [1]
There was a significant difference in shape between perch caught in the littoral and pelagic zone across all lakes (DFA; MD = 1.1382, P < 0.0001) with individuals from the littoral zone having a deeper body compared to individuals from the pelagic zone that were more streamlined (Fig 2)
DistLM showed a significant relationship between dissolved organic carbon (DOC) and fatty acid composition across the whole dataset (Pseudo-F = 3.44; P = 0.0079), and 4.7% of the variance was explained by DOC
Summary
In the face of rapid human-induced environmental changes the ability of organisms to successfully survive and reproduce is challenged, and is primarily determined by their ability to adaptively respond to these changes [1]. Environmental variations will influence community interactions, alter the degree of individual resource specialization and cause displacement in the organisms’ niches [2]. This will have further effects on the development of evolutionary important traits related to resource use, potentially leading to disruptive selection [3]. When differences in resource use are stable over time, natural selection may lead to morphological adaptations and polymorphism [6, 7]. An on-going homogenization, or flattening of ecological gradients of the environment due to human alteration may weaken the effect of divergent selection [8, 9], potentially leading to reverse speciation [10, 11]. Water transparency is affected by numerous factors, such as eutrophication [13] or elevated sediment loadings [14]
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