Warmer and browner waters decrease fish biomass production

Abstract

Climate change studies have long focused on effects of increasing temperatures, often without considering other simultaneously occurring environmental changes, such as browning of waters. Resolving how the combination of warming and browning of aquatic ecosystems affects fish biomass production is essential for future ecosystem functioning, fisheries, and food security. In this study, we analyzed individual‐ and population‐level fish data from 52 temperate and boreal lakes in Northern Europe, covering large gradients in water temperature and color (absorbance, 420 nm). We show that fish (Eurasian perch, Perca fluviatilis) biomass production decreased with both high water temperatures and brown water color, being lowest in warm and brown lakes. However, while both high temperature and brown water decreased fish biomass production, the mechanisms behind the decrease differed: temperature affected the fish biomass production mainly through a decrease in population standing stock biomass, and through shifts in size‐ and age‐distributions toward a higher proportion of young and small individuals in warm lakes; brown water color, on the other hand, mainly influenced fish biomass production through negative effects on individual body growth and length‐at‐age. In addition to these findings, we observed that the effects of temperature and brown water color on individual‐level processes varied over ontogeny. Body growth only responded positively to higher temperatures among young perch, and brown water color had a stronger negative effect on body growth of old than on young individuals. Thus, to better understand and predict future fish biomass production, it is necessary to integrate both individual‐ and population‐level responses and to acknowledge within‐species variation. Our results suggest that global climate change, leading to browner and warmer waters, may negatively affect fish biomass production, and this effect may be stronger than caused by increased temperature or water color alone.

Does browning‐induced light limitation reduce fish body growth through shifts in prey composition or reduced foraging rates?

“…Browning of waters can strongly impact aquatic ecosystems (van Dorst et al, ; Karlsson et al, ; Kelly et al, ; Vasconcelos et al, ). However, it is unknown if browning‐induced light limitation affects food webs, and especially predators, mainly through changes in resource availability and composition or through direct effects on consumer feeding capacities.…”

“…All fish were sampled using multi‐mesh gillnets in the benthic and pelagic zones according to a standardised test‐fishing method (Appelberg et al, ). For detailed information on the fish sampling method see van Dorst et al (). All captured fish were identified to species and their total individual length was measured to the nearest mm.…”

“…Next to reduced light availability, this browning is often accompanied by increased nutrient concentrations (Creed et al, ; Findlay, ). Browning can have negative effects on many components of lake ecosystems, ranging across trophic levels, from reducing primary production and algal biomass (Ask et al, ; Vasconcelos et al, ), zooplankton production (Kelly, Solomon, Weidel, & Jones, ) and benthic invertebrate biomass (Vasconcelos et al, ), to fish growth and production (van Dorst et al, ; Karlsson et al, ). However, browning effects are rarely studied across multiple trophic levels and interacting species (but see Hansson et al, ; Vasconcelos et al, ).…”

“…For example, responses to browner waters for fish feeding on zooplankton seem to vary between fish species (Jönsson et al, ; Weidel et al, ). However, the extent to which reduced feeding rates with lower visibility contribute to observed patterns of decreased fish biomass production with browning in many temperate lakes (van Dorst et al, ; Karlsson et al, ) is unknown. Next to changes in feeding rates, lower visibility could also change prey selection, for example by reducing visual prey selectivity in some fish species (Estlander et al, ).…”

Hotspots of biotic compositional change in lakes along vast latitudinal transects in northern Canada

“…In boreal and arctic lakes, the combined effects of warmer temperatures, darker water and changes in nutrient content and quality (composition, bioavailability) linked to changes in terrestrial inputs will affect aquatic biodiversity and ecosystem functioning at various levels (Finstad et al, ; Hayden et al, ). With warming climates, these effects may include alterations in food webs and biogeochemical cycles, with loss of phytoplankton diversity (Urrutia‐Cordero et al, ), shifts to increased mixotrophy in eukaryotic phytoplankton (Wilken et al, ) and decreases in fish biomass production (Tanentzap et al, ; van Dorst et al, ). At the global scale, these changes are also likely to be observed at high altitudes (Moser et al, ).…”