How do warmer and darker waters influence population dynamics in size-structured fish communities?

Changes associated with an increased water temperature due to global climate change have potentially large consequences for aquatic organisms. However, not only temperature but also the amount of precipitation is increasing. This increased precipitation leads to increased runoff from terrestrial ecosystems into lakes and coastal waters, introducing brown coloured humic substances containing dissolved organic carbon, leading to browner waters. This browning leads to a decrease of light in the water, which may reduce both primary production and visibility. The reduced visibility can, in turn, impact organisms dependent on light for e.g. feeding, mating, and predator evasion. Warmer and browner waters can influence aquatic ecosystems on several levels of biological organization: individuals, populations and communities. The impacts on fish populations and communities mostly arise from individual-level impacts and interactions. To understand how this works, knowledge of how food-dependent body growth and size-dependent food intake impact fish population and community dynamics is needed. Some of the separate impacts of warming and browning on fish are well studied on multiple organizational levels. It is known that both warming and browning can have considerable influences on both availability and uptake of resources in aquatic systems. This influence can have immediate impact on fish individuals and populations, but also shift competitive ability among individuals of different sizes. As a consequence, there may be changes in growth rates, mean body size, fish productivity and species composition in response to warming and browning. Climate change often results in both warmer and darker lakes. Still, the combination of warmer and darker water bodies on fish individuals, populations, and communities, have not been studied extensively. In combination, the various effects of warming and browning might even be more pronounced than individually. As fish populations and communities are important for both ecosystem function, and recreational and commercial fisheries, it is important to identify the knowledge gaps concerning the combined impact of an increase in temperature and browning. In this essay I identify big gaps in our current knowledge on the combined effects of temperature and browning on interacting fish individuals and populations. The knowledge arising from future studies on combined climate change effects on interacting fish species can, for example, be used to adapt current fisheries management strategies to a future climate characterized by warmer and darker lakes

Ecosystem heating experiment reveals sex-specific growth responses in fish

Size-specific body growth responses to warming are common among animal taxa, but sex-specific responses are poorly known. Here we ask if body growth responses to warming are sex-dependent, and if such sex-specific responses vary with size and age. This was tested with sex-specific data of back-calculated individual growth trajectories, in European perch (Perca fluviatilis) from a long-term whole-ecosystem warming experiment (6.3 C above the surrounding sea). Warming led to both size- and sex-specific differences in growth responses. Warming had a consistent positive effect on body growth of females, but negative effects on male growth at size > 10 cm and age > 2 years. These sex-specific growth responses translate to an increased degree of female-biased sexual size dimorphism (in length-at-age) with warming. Although the exact temperature-mediated effects underlying differential growth responses could not be resolved, results imply global warming may have highly different effects during ontogeny of male and female perch. Such effects should be considered in climate warming scenarios concerning fish growth, population size-structure, and dynamics of aquatic food webs that include fish exhibiting sexual size dimorphism

Can size distributions of European lake fish communities be predicted by trophic positions of their fish species?

An organism's body size plays an important role in ecological interactions such as predator-prey relationships. As predators are typically larger than their prey, this often leads to a strong positive relationship between body size and trophic position in aquatic ecosystems. The distribution of body sizes in a community can thus be an indicator of the strengths of predator-prey interactions. The aim of this study was to gain more insight into the relationship between fish body size distribution and trophic position in a wide range of European lakes. We used quantile regression to examine the relationship between fish species' trophic position and their log-transformed maximum body mass for 48 fish species found in 235 European lakes. Subsequently, we examined whether the slopes of the continuous community size distributions, estimated by maximum likelihood, were predicted by trophic position, predator-prey mass ratio (PPMR), or abundance (number per unit effort) of fish communities in these lakes. We found a positive linear relationship between species' maximum body mass and average trophic position in fishes only for the 75% quantile, contrasting our expectation that species' trophic position systematically increases with maximum body mass for fish species in European lakes. Consequently, the size spectrum slope was not related to the average community trophic position, but there were negative effects of community PPMR and total fish abundance on the size spectrum slope. We conclude that predator-prey interactions likely do not contribute strongly to shaping community size distributions in these lakes

Warmer and browner waters: fish responses vary with size, sex, and species

Current understanding of fish population responses to climate change is often limited to studies on the effect of temperature, ignoring potential co-occurring changes in other environmental variables. However, next to getting warmer, temperate and boreal aquatic systems are getting browner due to increased concentrations of dissolved organic carbon. Studies also generally predict mean population responses to climate change, thereby ignoring the potential for size, sex, and also species-specific responses. In this thesis, I aim to study the effects of warmer and browner waters on individual and population level responses in fish, and investigate if these responses vary with size, sex, and between species (Eurasian perch, Perca fluviatilis or common roach, Rutilus rutilus). To do this I used multiple methods, including space-for-time analyses, a wholeecosystem warming experiment, and aquaria and mesocosm experiments. I found that both warming and browning of lakes will likely decrease fish biomass production. Warming may cause a shift in size-structure towards smaller perch individuals and a lower perch population biomass, while browning will likely affect perch biomass production through lower body growth. Body growth responses to warming likely depend on body size, as small but not large individuals in my study were positively affected by high temperatures, and also sex, as males were more negatively affected by warming than females. Responses to browning may vary with body size and between species, as I found browning had a stronger negative effect on body growth of larger individuals in perch, while in roach browning only affected very small individuals. Overall, my findings suggest that future warming and browning will negatively affect fish individuals and populations, but that responses will vary with size, sex, and species, with potential consequences for ecological interactions and ecosystem functioning. This thesis highlights the importance of considering multiple climate stressors, integrating responses across several levels of biological organization, and acknowledging withinand between species variation, in order to understand and predict fish population responses to further climate change

Larval fish body growth responses to simultaneous browning and warming

Organisms are facing global climate change and other anthropogenic pressures, but most research on responses to such changes only considers effects of single drivers. Observational studies and physiological experiments suggest temperature increases will lead to faster growth of small fish. Whether this effect of warming holds in more natural food web settings with concurrent changes in other drivers, such as darkening water color ("browning") is, however, unknown. Here, we set up a pelagic mesocosm experiment with large bags in the Baltic Sea archipelago, inoculated with larval Eurasian perch (Perca fluviatilis) and zooplankton prey and varying in temperature and color, to answer the question how simultaneous warming and browning of coastal food webs impact body growth and survival of larval perch. We found that browning decreased body growth and survival of larval perch, whereas warming increased body growth but had no effect on survival. Based on daily fish body growth estimates based on otolith microstructure analysis, and size composition and abundance of available prey, we explain how these results may come about through a combination of physiological responses to warming and lower foraging efficiency in brown waters. We conclude that larval fish responses to climate change thus may depend on the relative rate and extent of both warming and browning, as they may even cancel each other out

Feeding strategies and resource partitioning of whitefish (Coregonus lavaretus) and perch (Perca fluviatilis) in the Pasvik watercourse

Interspecific competition for resources is, alongside abiotic factors like climate and geography, a very important factor in shaping communities. Since competition is difficult to prove directly, resource partitioning is frequently used as an indication of the presence of interspecific competition. Over the last 20 years perch abundance in the Pasvik watercourse has increased immensely, presumably because of a rise in water temperature. In this study resource partitioning and feeding strategies of sympatric large sparsely rakered (LSR) whitefish (Coregonus lavaretus) and perch (Perca fluviatilis) in the littoral zone of two lakes in the Pasvik watercourse were studied. Stomach content and stable isotope (δ13C and δ15N) analyses were done to determine short- and long-term diets, respectively, of all four populations. In both lakes, Perch had a generalist population diet with some specialization on the individual level, and showed clear ontogenetic niche shifts. Small perch were specialized on relatively small invertebrates (crustaceans), intermediate sized perch had a more generalist diet consistent of larger invertebrates and fish, while the largest perch were specialized piscivores. LSR whitefish displayed a population specialization in molluscs. Therefore, the diet overlap between LSR whitefish and perch in Lake Tjærebukta was only 12%. Isotopic niche overlap was low as well (20.6%). In contrast, in Lake Skrukkebukta the diet overlap between the two species was relative high (53%). Overlap in isotopic niches was similar (48.5%). Here LSR whitefish had a more generalist feeding strategy at both population and individual level. In addition they had an ontogenetic shift in diet as LSR whitefish <250 mm had a mixed diet of small Eurycercus lamellatus, molluscs and insect larvae, while larger LSR whitefish had a diet dominated by different species of larger insect larvae. The low overlaps in diet and isotopic niches were a good indication of clear resource partitioning on both short- and long-term, between LSR whitefish and perch in the two study lakes. A difference in feeding strategies and ontogenetic dietary niche shifts of perch likely strengthened the resource partitioning in both lakes. The resource partitioning was more intense in Lake Tjærebukta, where perch has been present in high abundance for over a longer period than in Lake Skrukkebukta. Longstanding interspecific competition between perch and LSR whitefish in Lake Tjærebukta could have caused the distinct resource partitioning, while in Lake Skrukkebukta this process is likely still on-going, and has not fully established yet. If perch spreads to more areas/increases in density because of climate change, this might have a distinct effect on other whitefish populations as well

Zooplanktivore fish body growth responses to browning-induced light limitation vary over ontogeny, but not with fish density

Ongoing climate change is leading to browning of many lakes and coastal areas, which can impair fish body growth and biomass production. However, whether and how effects of light limitation caused by browning on fish body growth vary over early ontogeny is unknown. In this study, we set up a mesocosm experiment to test whether roach (Rutilus rutilus) body growth responses to browning depend on body size, and if findings are robust over roach densities. We also studied a potential mechanism for size-specific responses by conducting an aquaria experiment to test if size-specific prey selectivity in roach changes with browning. We found that roach body growth responses to browning-induced light limitation vary over ontogeny (independent of roach density), negatively affecting body growth of young-of-the-year (YOY) but not of 1-year-old individuals. We also show that this difference in growth response is likely a consequence of browning-induced alterations in zooplankton community composition and variation in prey selectivity between YOY and 1-year-old fish. This suggests that we should account for the diverse effects of browning over fish ontogeny, mediated via altered prey composition and ontogenetic changes in prey preference, when assessing overall impacts of browning on aquatic ecosystems

Zooplanktivore fish body growth responses to browning-induced light limitation vary over ontogeny, but not with fish density

Ongoing climate change is leading to browning of many lakes and coastal areas, which can impair fish body growth and biomass production. However, whether and how effects of light limitation caused by browning on fish body growth vary over early ontogeny is unknown. In this study, we set up a mesocosm experiment to test whether roach (Rutilus rutilus) body growth responses to browning depend on body size, and if findings are robust over roach densities. We also studied a potential mechanism for size-specific responses by conducting an aquaria experiment to test if size-specific prey selectivity in roach changes with browning. We found that roach body growth responses to browning-induced light limitation vary over ontogeny (independent of roach density), negatively affecting body growth of young-of-the-year (YOY) but not of 1-year-old individuals. We also show that this difference in growth response is likely a consequence of browning-induced alterations in zooplankton community composition and variation in prey selectivity between YOY and 1-year-old fish. This suggests that we should account for the diverse effects of browning over fish ontogeny, mediated via altered prey composition and ontogenetic changes in prey preference, when assessing overall impacts of browning on aquatic ecosystems

Warmer and browner waters decrease fish biomass production

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

Warmer and browner waters decrease fish biomass production

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 thesefindings, we observed that the effects of temperature and brown water color onindividual‐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<br/