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