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The Mediterranean Rhodes Gyre is a cyclonic gyre with high primary production due to local upwelling of nutrients and occasional deep overturning up to 1 km depth. This nutrient-rich state is in sharp contrast to other parts of the Eastern Mediterranean which are oligotrophic. Here we investigated the upwelling system central to the Rhodes Gyre and the impact of different stressors like meteorological changes, acidification and fishing pressure up to the year 2100. A water column model spanning the physical, chemical and biological system up to top predators (GOTM-ERSEM-BFM-EwE) was used to simulate the pelagic environment under single and combined stressors. Results show that due to increasing winter temperatures, deep overturning events will become more rare in the future until they stop occurring around 2060 under the Paris-agreement climate scenario (RCP4.5) or around 2040 under the business-as-usual climate scenario (RCP8.5). Stratification will become stronger as temperature effects outweigh salinity effects in the surface mixed layer. Together with the lack of deep overturning, this stronger stratification limits the nutrient supply to the euphotic zone, significantly reducing primary production. Phytoplankton species shift towards smaller species as nutrients become more scarce, mimicking the situation found currently on the edge of the gyre. Climatic changes and fishing pressure will affect higher trophic levels in an additive way for some species (sardines, dolphins), while in a synergistic way for others (anchovy, mackerel). Acidification impacts are negligible. Fish stocks will reduce significantly under all scenarios involving climate change effects: ~30% under scenarios imposing RCP4.5 and ~40% under scenarios imposing RCP8.5. The beneficial impact of maximum sustained yield-level fishing is very limited, indicating a need for mitigating measures beyond fleet control.

Climate change is affecting large-scale oceanic processes. How and when these changes will impact those reliant on marine resources is not yet clear. Here we use end-to-end modeling to track the impacts of expected changes through the marine ecosystem on a specific, small community: Cochamó, in the Gulf of Ancud wider area, Chile. This area is important for Chilean fisheries and aquaculture, with Cochamó reliant on both lower and upper trophic level marine resources. We applied the GOTM-ERSEM-BFM coupled hydro-biogeochemical water-column model to gauge lower-trophic level marine ecological community response to bottom-up stressors (climate change, ocean acidification), coupled to an existing Ecopath with Ecosim model for the area, which included top-down stressors (fishing). Social scientists also used participatory modeling (Systems Thinking and Bayesian Belief Networking) to identify key resources for Cochamó residents and to assess the community’s vulnerability to possible changes in key resources. Modeling results suggest that flagellate phytoplankton abundance will increase at the cost of other species (particularly diatoms), resulting in a greater risk of harmful algae blooms. Both climate change and acidification slightly increased primary production in the model. Higher trophic level results indicate that some targeted pelagic resources will decline (while benthic ones may benefit), but that these effects might be mitigated by strong fisheries management efforts. Participatory modeling suggests that Cochamó inhabitants anticipate marine ecosystem changes but are divided about possible adaptation strategies. For climate change impact quantification, detailed experimental studies are recommended based on the dominant threats identified here, with specific local species.

The status of an exploited population is ideally determined by monitoring changes in abundance and distributional range and pattern over time. Area of occupancy is a measure of the current distribution. Unfortunately, for many populations, scientific abundance and distribution information is not readily available. To evaluate the reliability of commercial fishing data for deriving occupancy indicators that could serve as proxies for stock abundance, we investigated four questions: 1) Occupancy changes with stock biomass, but is this change strong enough to make occupancy a sensitive indicator of population biomass? 2) Fishing boats follow fish, but when does such activity alter the positive macroecological relationship between occupancy and abundance? 3) When does the activity of pursuing fish adversely affect occupancy estimates derived from catch and effort data? 4) How does uncertainty in fishing effort data affect occupancy estimates? Spatial simulations mimicking the dynamics of four deep-water fish species showed that biomass-occupancy relationships can be weak. Fishers following fish can modify the spatial distribution of target species, even reversing the sign of the biomass-occupancy relationship in certain cases, and can affect the reliability of occupancy indicators, which can also be impaired by error in effort data. Using commercial catch and effort data and abundance indices for deep-sea fish populations to the west of the British Isles it was found that only for roundnose grenadier might occupancy provide insights into biomass changes. In conclusion, care should be taken when using occupancy for evaluating range changes in cases where fishing might have modified spatial distributions, uncertain commercial data are used or when the abundance-occupancy relationship is too flat.