Fish community responses to water colour : results along a large gradient from clear to brown lakes in Sweden

Abstract: An increase of dissolved organic carbon and iron in freshwater bodies across the Northern Hemisphere caused by environmental and land use changes lead to water browning with negative impacts on lake ecosystems. Effects of browning on aquatic ecosystems are complex and include reduced light penetration, thermal stratification, lower macrophyte cover, lower primary productivity, and potential shifts in species composition. However, little is known about impacts of browning on lake fish communities. In this study I analysed data from 146 lakes in Sweden along a large water colour gradient (measured as absorbance at 420 nm) to find out how fish community biomass is influenced by water colour. Furthermore, I studied if variations in species composition or mean body size contribute to differences in community fish biomass between clear and brown lakes. Therefore, I tested the relationship between absorbance and community biomass, species biomass, total a bundance of fish individuals, mean fish body length, size diversity and species richness and the relationship between community composition and absorbance. The main result of my study was a negative relationship between fish community biomass and water colour, meaning a lower fish biomass in brown lakes tha n in clear lakes. The lower community biomass is likely explained by a lower biomass of several of the most common fish species, a lower total abundance of fish in dark lakes, and a lower number of species and differences in fish community composition. In contrast, I found no relationship between absorbance and fish body length, demonstrating that a lower biomass in brown lakes was not due to smaller fish. I also found that the variation in total fish abundance and biomass wa s high for clear lakes, but low for dark lakes. Also, values for highest biomass, highest number of individuals and number of species were much lower in darker lakes than in clear ones, explaining the lower community biomass with increa sed absorbance. Based on my results and considering that resource use and production are lower in dark lakes, I suggest a limitation of fish community biomass in brown lakes due to low resource availability. My study provides novel insights into fish community responses to absorbance, but also identifies further research needed in face of ongoing climate change and anticipated further water browning.