Intraspecific variation modifies ecological processes and ecosystem functioning. Still, we know relatively little of how the nature and strength of ecosystem effects caused by intraspecific variation may interact with climate change. We conducted a mesocosm experiment to test if, and to what extent, ocean warming modifies the ecological impacts of intraspecific variation in a predatory fish. The mesocosms consisted of a simplified coastal food web with threespine stickleback (Gasterosteus aculeatus) as the top predator, from a population where two stickleback phenotypes with either complete or incomplete lateral armour plating coexist and display differentiated predation behaviour: The completely plated phenotype often feeds more on invertebrate herbivores compared with the incompletely plated phenotype. Presence of stickleback reduced biomass of arthropod shredders (crustaceans, insect larvae). Warming (+4°C) strengthened this predation, releasing benthic primary producers (diatoms) from top-down control, causing a trophic cascade. This trophic cascade was attributed to one of the plate phenotypes: the completely plated stickleback increased their predation on shredders under warming, while the incompletely plated stickleback instead decreased their predation. Diatom biomass responded accordingly: warming increased diatom biomass in the presence of completely plated stickleback but not when incompletely plated stickleback was present. Our results suggest that different plate phenotypes of threespine stickleback differentially affect lower trophic levels and that warming may exacerbate these cascading effects. These trait-dependent effects on trophic cascades highlight the consequences of intraspecific variation on ecosystem functioning. Read the free Plain Language Summary for this article on the Journal blog.

1. Improving the health of coastal and open sea marine ecosystems represents a substantial challenge for sustainable marine resource management, since it requires balancing human benefits and impacts on the ocean. This challenge is often exacerbated by incomplete knowledge and lack of tools that measure ocean and coastal ecosystem health in a way that allows consistent monitoring of progress towards predefined management targets. The lack of such tools often limits capabilities to enact and enforce effective governance.

2. We introduce the Baltic Health Index (BHI) as a transparent, collaborative and repeatable assessment tool. The Index complements existing, more ecological-oriented, approaches by including a human dimension on the status of the Baltic Sea, an ecosystem impacted by multiple anthropogenic pressures and governed by a multitude of comprehensive national and international policies. Using a large amount of social-ecological data available, we assessed the health of the Baltic Sea for nine goals that represent the status towards set targets, for example, clean waters, biodiversity, food provision, natural products extraction and tourism.

3. Our results indicate that the overall health of the Baltic Sea is suboptimal (a score of 76 out of 100), and a substantial effort is required to reach the management objectives and associated targets. Subregionally, the lowest BHI scores were measured for carbon storage, contaminants and lasting special places (i.e. marine protected areas), albeit with large spatial variation.

4. Overall, the likely future status of all goals in the BHI averaged for the entire Baltic Sea is better than the present status, indicating a positive trend towards a healthier Baltic Sea. However, in some Baltic Sea basins, the trend for specific goals was decreasing, highlighting locations and issues that should be the focus of management priorities.

5. The BHI outcomes can be used to identify both pan-Baltic and subregional scale management priorities and to illustrate the interconnectedness between goals linked by cumulative pressures. Hence, the information provided by the BHI tool and its further development will contribute towards the fulfilment of the UN Agenda 2030 and its Sustainability Development Goals.