In many areas of the Baltic Sea, three-spined stickleback (Gasterosteus aculeatus) has increased several fold since the early 2000s. Two major planktivores of the Baltic Sea, Atlantic herring (Clupea harengus) and European sprat (Sprattus sprattus), may interact with stickleback via competition for food and predation, but the potential implications at the population level are still unknown. Here, we explored interactions between stickleback, herring, and sprat using (i) herring diet analyses and (ii) fish biomass estimates from hydroacoustics for 2001–2019 covering the largest Baltic Sea basins—the Central Baltic and the Bothnian Sea. The diet analyses revealed that stickleback is an important prey for large herring along the Swedish Baltic Sea coast, comprising up to 64% of the diet of individuals >22 cm. We found a negative influence of large herring (>18 cm) on stickleback in the Central Baltic, and a negative relationship between sprat and small herring (<18 cm) (pooled) and stickleback in the Bothnian Sea. The decline in herring and sprat population after the mid-1990s could have contributed to the increase in stickleback population via reduced predation and competition. Overall, herring may be an underappreciated piscivore, and high fishing pressure on herring could generate cascading effects on lower trophic levels.

Aim: Biogenic structural complexity increases mobile animal richness and abundance at local, regional and global scales, yet animal taxa vary in their response to complexity. When these taxa also vary functionally, habitat structures favouring certain taxa may have consequences for ecosystem function. We characterised global patterns of epifaunal invertebrates in eelgrass (Zostera marina) beds that varied in structural and genetic composition. Location: North America, Europe and Asia. Time Period: 2014. Major Taxa Studied: Peracarid crustaceans and gastropod molluscs. Methods: We sampled epifaunal invertebrate communities in 49 eelgrass beds across 37° latitude in two ocean basins concurrently with measurements of eelgrass genetic diversity, structural complexity and other abiotic and biotic environmental variables. We examined how species richness, abundance and community composition varied with latitude and environmental predictors using a random forest approach. We also examined how functional trait composition varied along with community structure. Results: Total species richness decreased with latitude, but this was accompanied by a taxonomic shift in dominance from peracarid crustaceans to gastropods, which exhibited different sets of functional traits. Greater eelgrass genetic diversity was strongly correlated with both richness and abundance of peracarids, but less so for gastropods. Main Conclusions: Our results add to a growing body of literature that suggests genetic variation in plant traits influences their associated faunal assemblages via habitat structure. Because peracarids and gastropods exhibited distinct functional traits, our results suggest a tentative indirect link between broad-scale variation in plant genetic diversity and ecosystem function.

In December 2022, representatives adopted the Kunming-Montreal Global Biodiversity Framework (GBF). The overarching goal of the agreement is to halt biodiversity loss and restore natural ecosystems – and a much-cited target is the “30x30” target of protecting and restoring 30% of Earth by 2030. Salt marshes are highly productive coastal ecosystems relevant for both biodiversity and climate change mitigation but have experienced historical major losses and are among the habitats in need of protection and restoration. Currently, there is little knowledge about how salt marshes are valued from a societal perspective, which has important management implications. This is particularly the case in the Nordic region where awareness of these habitats is low. Using survey data from Norway, Sweden, Finland, Denmark, and Germany, this study contributes understanding how citizens value salt marshes by examining support for different management initiatives framed as contributing to the goal of either biodiversity protection or climate change mitigation. Generally, both respondents primed with information about the importance of salt marshes for biodiversity and for climate change mitigation were supportive of the management initiatives despite having little previous knowledge of salt marshes. For one initiative, respondents who were informed of the importance of salt marshes for biodiversity were slightly more likely to support the management initiative than those informed about the importance of salt marshes for climate change mitigation. Our study provides guidance for assessment of trade-offs from a human valuation perspective, thus supporting policy makers when considering arguments for salt marsh management within the context of the 30x30 targets for protected areas.

Coastal marshes are productive and diverse habitats that act as essential linkages between marine and terrestrial ecosystems. Although the Nordic region comprises diverse, unique marsh habitats along the Baltic coast and the Atlantic coast of Norway, they are not widely recognized as coastal marshes, e.g., salt marshes in other parts of the world. This may be partly due to inconsistency in habitat classification, use of terminology, and the heterogeneity of these habitats across the region, which limit overviews of their ecology, distribution, and management. Here, we aim to improve the basis for assessments of Nordic coastal marshes by providing an overview of terminology, distribution, area, and management practices of these habitats across the region. We use the general term coastal marsh to cover the variety of habitat types in the study area, which are covered mainly by the European Union's (EU's) Habitats Directive Annex I habitat classes 1310 Salicornia and other annuals colonizing mud and sand, 1320 Spartina swards (Spartinion maritimae), 1330 Atlantic salt meadows (Glauco-Puccinellietalia maritimae), and 1630 Boreal Baltic coastal meadows. Based on input from national environmental officers, an expert survey, a workshop, and reviewing environmental databases, we compiled a comprehensive database on coastal marsh habitats regarding their distribution, classification, conservation status, management, and monitoring practices in four Nordic countries (Denmark, Finland, Norway, Sweden) and Germany, covering wide environmental gradients in, e.g., salinity and tidal range. Overall, we found that the four Annex I classes cover an area of ∼900 km² in the study region, one-fourth of the total area of these habitats in the EU, with an additional ∼450 km² of other potential coastal marsh areas and 500 km² of coastal reed beds. Beyond the variable terminology across the region, our study revealed a gap in the European habitat classification systems concerning the diversity of the Baltic Sea coastal marshes. Creating subtype(s) for the Annex I class 1630 would help to improve the conservation of the most threatened habitat types within the class. In addition, we found pronounced differences in monitoring and managing coastal marshes among countries, indicating that these important and diverse habitats between land and sea might receive insufficient attention and protection despite the multiple ecosystem services they provide. Coordinated harmonization of habitat classifications and monitoring across the Nordic countries could increase regional-scale coordination of the management of coastal marshes and facilitate comparative studies.

No-take zones can be an effective tool for protecting fish and crustacean stocks as well as marine ecosystems. However, it is important that these closed areas are properly designed and large enough. Seasonal spawning closures may be easier to enforce, but do not have as strong positive effects as no-take zones.

Fiskefria områden kan vara ett effektivt verktyg för att skydda både fisk- och kräftdjursbestånd och marina ekosystem. Men det är viktigt att områdena utformas på rätt sätt och är tillräckligt stora. Lektidsfredade områden kan vara enklare att driva igenom, men ger inte lika goda effekter.

Ecosystem regime shifts can have severe ecological and economic consequences, making it a top priority to understand how to make systems more resilient. Theory predicts that spatial connectivity and the local environment interact to shape resilience, but empirical studies are scarce. Here, we use >7000 fish samplings from the Baltic Sea coast to test this prediction in an ongoing, spatially propagating shift in dominance from predatory fish to an opportunistic mesopredator, with cascading effects throughout the food web. After controlling for the influence of other drivers (including increasing mesopredator densities), we find that predatory fish habitat connectivity increases resilience to the shift, but only when densities of fish-eating top predators (seals, cormorants) are low. Resilience also increases with temperature, likely through boosted predatory fish growth and recruitment. These findings confirm theoretical predictions that spatial connectivity and the local environment can together shape resilience to regime shifts.

Coastal ecosystem functioning often hinges on habitat-forming foundation species that engage in positive interactions (e.g. facilitation and mutualism) to reduce environmental stress. Seagrasses are important foundation species in coastal zones but are rapidly declining with losses typically linked to intensifying global change-related environmental stress. There is growing evidence that loss or disruption of positive interactions can amplify coastal ecosystem degradation as it compromises its stress mitigating capacity. Multiple recent studies highlight that seagrass can engage in a facultative mutualistic relationship with lucinid bivalves that alleviate sulphide toxicity. So far, however, the generality of this mutualism, and how its strength and relative importance depend on environmental conditions, remains to be investigated. Here we study the importance of the seagrass-lucinid mutualistic interaction on a continental-scale using a field survey across Europe. We found that the lucinid bivalve Loripes orbiculatus is associated with the seagrasses Zostera noltii and Zostera marina across a large latitudinal range. At locations where the average minimum temperature was above 1 °C, L. orbiculatus was present in 79% of the Zostera meadows; whereas, it was absent below this temperature. At locations above this minimum temperature threshold, mud content was the second most important determinant explaining the presence or absence of L. orbiculatus. Further analyses suggest that the presence of the lucinids have a positive effect on seagrass biomass by mitigating sulphide stress. Finally, results of a structural equation model (SEM) support the existence of a mutualistic feedback between L. orbiculatus and Z. noltii. We argue that this seagrass-lucinid mutualism should be more solidly integrated into management practices to improve seagrass ecosystem resilience to global change as well as the success of restoration efforts.

In the last three decades, quantitative approaches that rely on organism traits instead of taxonomy have advanced different fields of ecological research through establishing the mechanistic links between environmental drivers, functional traits, and ecosystem functions. A research subfield where trait-based approaches have been frequently used but poorly synthesized is the ecology of seagrasses; marine angiosperms that colonized the ocean 100M YA and today make up productive yet threatened coastal ecosystems globally. Here, we compiled a comprehensive trait-based response-effect framework (TBF) which builds on previous concepts and ideas, including the use of traits for the study of community assembly processes, from dispersal and response to abiotic and biotic factors, to ecosystem function and service provision. We then apply this framework to the global seagrass literature, using a systematic review to identify the strengths, gaps, and opportunities of the field. Seagrass trait research has mostly focused on the effect of environmental drivers on traits, i.e., “environmental filtering” (72%), whereas links between traits and functions are less common (26.9%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (15% of studies), including the relative importance of neutral and niche assembly processes, or the influence of trait dominance or complementarity in ecosystem function provision. These knowledge gaps indicate ample potential for further research, highlighting the need to understand the links between the unique traits of seagrasses and the ecosystem services they provide.