Disentangling temporal food web dynamics facilitates understanding of ecosystem functioning.

Studying how food web structure and function vary through time represents an opportunity to better comprehend and anticipate ecosystem changes. Yet, temporal studies of highly resolved food web structure are scarce. With few exceptions, most temporal food web studies are either too simplified, preventing a detailed assessment of structural properties or binary, missing the temporal dynamics of energy fluxes among species. Using long-term, multi-trophic biomass data coupled with highly resolved information on species feeding relationships, we analysed food web dynamics in the Gulf of Riga (Baltic Sea) over more than three decades (1981-2014). We combined unweighted (topology-based) and weighted (biomass- and flux-based) food web approaches, first, to unravel how distinct descriptors can highlight differences (or similarities) in food web dynamics through time, and second, to compare temporal dynamics of food web structure and function. We find that food web descriptors vary substantially and distinctively through time, likely reflecting different underlying ecosystem processes. While node- and link-weighted metrics reflect changes related to alterations in species dominance and fluxes, unweighted metrics are more sensitive to changes in species and link richness. Comparing unweighted, topology-based metrics and flux-based functions further indicates that temporal changes in functions cannot be predicted using unweighted food web structure. Rather, information on species population dynamics and weighted, flux-based networks should be included to better comprehend temporal food web dynamics. By integrating unweighted, node- and link-weighted metrics, we here demonstrate how different approaches can be used to compare food web structure and function, and identify complementary patterns of change in temporal food web dynamics, which enables a more complete understanding of the ecological processes at play in ecosystems undergoing change.

The role of marinas in the establishment and spread of non-indigenous species in Baltic Sea fouling communities.

The study included the sampling of 12 marinas across six areas of the Baltic Sea with settlement plates and scraping of submerged structures to assess the role of marinas in the spread of non-indigenous species (NIS) via biofouling. 15 NIS were detected in the marinas and secondary spread of previously introduced NIS was detected in five out of six sea areas. Salinity and sea area significantly affected the composition of the fouling assemblages. Settlement plates appeared as the more efficient sampling method over scraping, while the seasonal analyses revealed that the monitoring effort should span over the summer and early autumn in the south-eastern, central, and northern Baltic Sea. The present findings indicate that marinas contribute to the spread of non-indigenous fouling organisms, and there is an increasing demand for the monitoring of marinas and stricter regulations regarding the biofouling management of leisure boats in the Baltic Sea.

Assessing change in habitat composition, ecosystem functioning and service supply in Latvian protected stony reefs.

Healthy and diverse marine ecosystems are a source of a whole range of ecosystem services (ES) and social, and economic benefits. To preserve and restore biodiversity, and sustain service supply, an international goal was set to protect at least 10 % of the global coastal and marine area by 2020. The goal has been achieved mainly through the designation of marine protected areas (MPAs). Whilst activities within the MPAs can be restricted to manage local pressures, the protected habitats and species are still exposed to stressors that originate outside MPA borders (e.g., non-native species, eutrophication). This study investigates the change in the protected stony reef habitat composition using underwater video observation in the coastal area of the eastern Baltic Sea known to be under the pressure of a non-native fish species. Further, assesses what the observed changes have meant for ecosystem functioning and ES supply adopting a tailor-made, expert judgement-based ES supply assessment method developed during the BONUS BASMATI project. The results suggest that the quality of the protected habitats in the case study sites has deteriorated and the transformation in species composition has altered ecosystem functioning and ES supply. The study highlights the importance of rich and diverse habitats for human wellbeing and livelihoods. Further, emphasises the need for more stringent MPA management plans, as well as a wider ecosystem-based approach to decision making in order to limit the impacts of stressors on marine ecosystems and secure ES supply.

Correction to: Zooplankton indicator-based assessment in relation to site location and abiotic factors: a case study from the Gulf of Riga.

This paper, published in Environ Monit Assess 192, 147 (2020), contains errors in Table 5.

Zooplankton indicator-based assessment in relation to site location and abiotic factors: a case study from the Gulf of Riga.

There are a number of zooplankton parameters proposed as indicators to evaluate environmental status of marine ecosystems. Mean size and total stock (MSTS) is the only zooplankton-based and HELCOM (Baltic Marine Environment Protection Commission - Helsinki Commission)-approved core indicator. MSTS was developed to evaluate the environmental status of the Baltic Sea based on total biomass (or abundance) and mean body weight of mesozooplankton. This indicator reflects status of the food web and zooplankton biodiversity. Both are qualitative descriptors for determining good environmental status (GES) as defined by the EU Marine Strategy Framework Directive 2008/56/EC. However, the existing indicator concept is applicable to the extent that it characterizes off-shore pelagic habitats, while use of MSTS for coastal habitats remains challenging. In this case study, we aimed to assess and discuss performance of MSTS applied to mesozooplankton data from the shallow Gulf of Riga. Both off-shore and coastal communities were included in the study. MSTS responses to variable environmental factors (temperature, salinity and riverine runoff) were analysed. Temporal variations in temperature revealed response in mean size, whereas salinity covaried with total stock (both - biomass and abundance). However, spatial variations of MSTS parameters stayed unexplained. The results demonstrate difficulties with and provide possible solutions for MSTS-based assessment, with a particular emphasis on coastal waters. The use of mesozooplankton abundance as a determinant parameter for assessment of coastal waters and substitution of the corresponding 99% confidence interval of the mean as an assessment value instead of the mean have been considered.

Understanding the distribution of economic benefits from improving coastal and marine ecosystems.

The ecological status of coastal and marine waterbodies world-wide is threatened by multiple stressors, including nutrient inputs from various sources and increasing occurrences of invasive alien species. These stressors impact the environmental quality of the Baltic Sea. Each Baltic Sea country contributes to the stressors and, at the same time, is affected by their negative impacts on water quality. Knowledge about benefits from improvements in coastal and marine waters is key to assessing public support for policies aimed at achieving such changes. We propose a new approach to account for variability in benefits related to differences in socio-demographics of respondents, by using a structural model of discrete choice. Our method allows to incorporate a wide range of socio-demographics as explanatory variables in conditional multinomial logit models without the risk of collinearity; the model is estimated jointly and hence more statistically efficient than the alternative, typically used approaches. We apply this new technique to a study of the preferences of Latvian citizens towards improvements of the coastal and marine environment quality. We find that overall, Latvians are willing to pay for reducing losses of biodiversity, for improving water quality for recreation by reduced eutrophication, and for reducing new occurrences of invasive alien species. However a significant group within the sample seems not to value environmental improvements in the Baltic Sea, and, thus, is unwilling to support costly measures for achieving such improvements. The structural model of discrete choice reveals substantial heterogeneity among Latvians towards changes in the quality of coastal and marine waters of Latvia.

Indicator Properties of Baltic Zooplankton for Classification of Environmental Status within Marine Strategy Framework Directive.

The European Marine Strategy Framework Directive requires the EU Member States to estimate the level of anthropogenic impacts on their marine systems using 11 Descriptors. Assessing food web response to altered habitats is addressed by Descriptor 4 and its indicators, which are being developed for regional seas. However, the development of simple foodweb indicators able to assess the health of ecologically diverse, spatially variable and complex interactions is challenging. Zooplankton is a key element in marine foodwebs and thus comprise an important part of overall ecosystem health. Here, we review work on zooplankton indicator development using long-term data sets across the Baltic Sea and report the main findings. A suite of zooplankton community metrics were evaluated as putative ecological indicators that track community state in relation to Good Environmental Status (GES) criteria with regard to eutrophication and fish feeding conditions in the Baltic Sea. On the basis of an operational definition of GES, we propose mean body mass of zooplankton in the community in combination with zooplankton stock measured as either abundance or biomass to be applicable as an integrated indicator that could be used within the Descriptor 4 in the Baltic Sea. These metrics performed best in predicting zooplankton being in-GES when considering all datasets evaluated. However, some other metrics, such as copepod biomass, the contribution of copepods to the total zooplankton biomass or biomass-based Cladocera: Copepoda ratio, were equally reliable or even superior in certain basin-specific assessments. Our evaluation suggests that in several basins of the Baltic Sea, zooplankton communities currently appear to be out-of-GES, being comprised by smaller zooplankters and having lower total abundance or biomass compared to the communities during the reference conditions; however, the changes in the taxonomic structure underlying these trends vary widely across the sea basins due to the estuarine character of the Baltic Sea.