Effects of resource quality on the evolution of trait divergence in freshwater fish along a gradient of dissolved organic carbon
In recent years, a strong increase in the flux of dissolved organic carbon (DOC) into aquatic systems has been identified due to climatic factors. This “brownification” strongly reduces water transparency and therefore has substantial effects on visually orientating organisms such as fish and their perception of various prey items. Due to light limitation and the direct changes in photosynthetic activity, brownification further has the ability to restructure aquatic food webs with potential consequences on the quality of resources available for higher trophic levels. In fish, growth is constrained by nutrient stoichiometry and especially the availability of essential biocompounds (such as polyunsaturated fatty acids, PUFAs). Perch (Perca fluviatilis) show strong intraspecific variations in stoichiometry related to both morphological and dietary specialization (appearance of littoral and pelagic morphotypes). If browning of waters have an effect on diet specialization this may in turn affect the evolution of trait divergence. Previous research has shown that perch at different levels of DOC show habitat-specific morphological adaptations that are affected by water transparency with low divergence in lakes of high DOC concentrations, and an overall decrease within the reliance on littoral resources. To test whether vision or food quality is the proximate mechanism of divergence, we conducted a field survey across a gradient of DOC in Swedish lakes and studied the quality of perch and resources (measured as PUFAs).
Preliminary results show higher concentrations of PUFAs in perch from clear water lakes compare to perch from brown water lakes. Furthermore, PUFAs from perch caught in littoral and pelagic habitats of brown water lakes were indistinguishable, whereas distinct PUFA compositions appeared in littoral and pelagic morphotypes of clear water lakes. These results imply that perch from lakes of different DOC concentration likely consume prey of different PUFA content, potentially leading to differential growth and divergence. The results of this study add important knowledge on the overall effects of brownification on aquatic food webs and will further enhance the understanding of anthropogenic and climatic influences on trophic interactions with further evolutionary consequences.