Freshwater browning is stunting fish growth of some species, shrinking populations of others and changing the composition of fish communities, McGill-led research suggests. "Browning" refers to freshwater bodies turning tea-coloured, a phenomenon driven by higher levels of dissolved organic matter and/or higher levels of iron in the water. Causes include changes in land use and climate, and reduced acid precipitation.
The findings indicate that changes in land use and climate are affecting biodiversity and how ecosystems function, with implications for fisheries management.
"Browner waters had greater abundances of northern pike and walleye, but lower abundances of lake trout, brook trout, yellow perch, largemouth- and smallmouth bass and whitefish," said Allison Roth, lead author and McGill postdoctoral fellow, now an Assistant Professor at the University of Missouri. All of those species are economically significant.
"Fish themselves affect other organisms in the lake," explained Irene Gregory-Eaves, study co-author and Professor of Biology at McGill. "Because they're predators, feeding on smaller fish, plankton and invertebrates, changing their populations can have cascading effects."
The researchers also said that browning can also have an impact on land, for example, when birds' diets are affected by changes in fish populations.
"Browning is affecting more than just fish: it's affecting the broader ecosystem. It has the power not just to affect aquatic systems, but also the terrestrial systems connected to them," Roth said.
Changes evident across North America, Europe
By assembling a large team of co-authors from across North America and Europe who were conducting research on freshwater bodies in their areas, the group pooled data and expertise.
The team reviewed existing studies to understand how freshwater browning affects fish at individual, population and community levels. (Population refers to the number of fish of a given species; community refers to the various species present in the body of water, and their relative numbers.)
The researchers also analyzed data from 871 north‐temperate lakes, covering eight economically important fish species, to look at the relationship between a lake's colour and its fish populations. The team found that for numerous species, increased browning was associated with a population decline, but it was associated with a positive impact for two species: northern pike and walleye.
Within this dataset, they further examined 303 lakes and assessed how trait composition (such as eye size, body shape and feeding traits) of fish communities changes across the browning gradient. They found that fish communities in browner lakes were "significantly more likely" to contain species with larger eyes, a trait presumed to be associated with lower-light environments.
Because the study links changes at the individual species, population and community levels, it provides a more holistic understanding of ecosystem responses than did earlier works.
The researchers say this aids in forecasting and modelling future conditions under increased browning.
"People have tackled components of what we were getting at in a series of lakes in their region, or one whole lake, or certain fish. Our job was to bring together a more synthetic portrait to be able to say, 'This is something happening in many parts of North America and Europe,'" Gregory-Eaves said.
About this study
"Differential effects of freshwater browning across fish species: consequences for individual- to community-level fish traits in north temperate lakes," by Allison M. Roth, Irene Gregory-Eaves et al, was published in Biological Reviews.
It was funded by the Fonds de Recherche du Québec Strategic Cluster - Groupe de recherche interuniversitaire en limnologie.
