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.
Highlights
In northern latitude lakes, warming and darkening of water color caused by elevated concentrations of dissolved organic carbon (DOC) and iron are ongoing processes coupled to climate change, with potentially severe consequences (Dokulil, 2014; Larsen, Andersen, & Hessen, 2011; Monteith et al, 2007; Roulet & Moore, 2006; Solomon et al, 2015; Weyhenmeyer, Müller, Norman, & Tranvik, 2016; Whitehead, Wilby, Battarbee, Kernan, & Wade, 2009)
Our results show that both high water temperature and high absorbance during summer negatively affect fish biomass production across a large number of lakes
Considering that temperate and boreal lakes are predicted to become both warmer and browner because of climate change (Larsen et al, 2011; Roulet & Moore, 2006; Weyhenmeyer et al, 2016), our results suggest a potential drop in future lake fish biomass production in large parts of Europe, especially given that perch is the dominant fish species in many European lakes (Craig, 1987; Lehtonen et al, 2008; Tammi et al, 2003)
Summary
In northern latitude lakes, warming and darkening of water color caused by elevated concentrations of dissolved organic carbon (DOC) and iron are ongoing processes coupled to climate change, with potentially severe consequences (Dokulil, 2014; Larsen, Andersen, & Hessen, 2011; Monteith et al, 2007; Roulet & Moore, 2006; Solomon et al, 2015; Weyhenmeyer, Müller, Norman, & Tranvik, 2016; Whitehead, Wilby, Battarbee, Kernan, & Wade, 2009). Brown water (or higher DOC) has in some cases been shown to result in a lower mean length‐at‐age (Estlander et al, 2010; Horppila et al, 2010), lower fish population biomass (Finstad et al, 2014) and biomass production (Karlsson et al, 2015, 2009) Whether these observations from natural systems over relatively small spatial scales (Karlsson et al, 2015, 2009; O'Gorman et al, 2016) or theoretical predictions (Lindmark et al, 2018; Ohlberger et al, 2011) hold for how temperature and water color jointly affect fish biomass production over a large range of natural systems is still unknown. Our study shows that it is necessary to study joint individual‐ and population‐level responses to concurrent warming and browning to understand and predict shifts in fish biomass production in a changing climate
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