LB 23-239 - Declining food availability and changing habitat use drive community responses to marine hypoxia

Background/Question/Methods:

Deoxygenation in aquatic ecosystems can have severe negative impacts on fish communities. Individual fish experience direct physiological stress and may respond behaviorally, populations may see changes in overall biomass as well as age and size structure, and at the community level, differential effects of deoxygenation across species may change the outcome of species interactions. Although individuals' responses to deoxygenation are well-studied, the mechanisms by which they drive population and community dynamics are less well known. Therefore, we asked the following questions: (1) How do observed patterns of individual growth, population biomass, and yield emerge from individual-level responses to deoxygenation? (2) Which pathways are most critical for emergent community dynamics, and which, if any, can be effectively ignored? We used a multispecies size spectrum model to parameterize individual-level impacts of declining oxygen on major commercial fish species in the Baltic Sea. These impacts included changes in food availability, benthic habitat use, mortality, and rates of the `physiological' processes of consumption, assimilation, and metabolic costs. We tested all possible combinations of these types of pathways by fitting the models to observed body growth, population biomass, and fisheries yield of four major fish species in the Baltic Sea.

Results/Conclusions:

Our results revealed that declining food availability in benthic habitats was by far the most critical driver of observed patterns. Escape of cod, a benthopelagic piscivore, from hypoxic waters was also critical as a compensatory mechanism. This drives an overall shift in how energy flows to these predators, with less stemming from benthic and more from pelagic sources in scenarios of decreasing oxygen. These insights can guide targeted remediation efforts as anthropogenic climate change and habitat alteration progress.