The effects of warming on the ecophysiology of two co-existing kelp species with contrasting distributions.

The northeast Atlantic has warmed significantly since the early 1980s, leading to shifts in species distributions and changes in the structure and functioning of communities and ecosystems. This study investigated the effects of increased temperature on two co-existing habitat-forming kelps: Laminaria digitata, a northern boreal species, and Laminaria ochroleuca, a southern Lusitanian species, to shed light on mechanisms underpinning responses of trailing and leading edge populations to warming. Kelp sporophytes collected from southwest United Kingdom were maintained under 3 treatments: ambient temperature (12 °C), +3 °C (15 °C) and +6 °C (18 °C) for 16 days. At higher temperatures, L. digitata showed a decline in growth rates and Fv/Fm, an increase in chemical defence production and a decrease in palatability. In contrast, L. ochroleuca demonstrated superior growth and photosynthesis at temperatures higher than current ambient levels, and was more heavily grazed. Whilst the observed decreased palatability of L. digitata held at higher temperatures could reduce top-down pressure on marginal populations, field observations of grazer densities suggest that this may be unimportant within the study system. Overall, our study suggests that shifts in trailing edge populations will be primarily driven by ecophysiological responses to high temperatures experienced during current and predicted thermal maxima, and although compensatory mechanisms may reduce top-down pressure on marginal populations, this is unlikely to be important within the current biogeographical context. Better understanding of the mechanisms underpinning climate-driven range shifts is important for habitat-forming species like kelps, which provide organic matter, create biogenic structure and alter environmental conditions for associated communities.

Spatial and seasonal variability in benthic impact of mussel farms: Predicting and mitigating impacts using ambient oxygen conditions.

Environmental impacts are a cause for concern when developing and expanding aquaculture and to be sustainable potential negative effects need to be addressed. The intensity and extent of these impacts likely vary among sites and seasons, depending on multiple factors including the physical and biological setting and operational aspects. Using a combination of sampling techniques, we investigated the spatial variability in epibenthic impacts in eleven commercial mussel farms, on the Swedish west coast. We found increased levels of organic content, changes in epibenthic macrofauna and increased cover of Beggiatoa sp., a documented indicator of hypoxia. The extent of these impacts was generally limited to the extent of the farms. Because the cover of Beggiatoa sp. was particularly clear and because oxygen conditions in the sediment is of great importance to the structure and function of these habitats, we analysed spatial patterns using an index of the benthic footprint (BFI) accounting for both intensity and extent of impacts. In the summer, the BFI varied strongly among farm-sites and subsequent analyses showed that it highly correlated with ambient bottom oxygen concentration. Repeated sampling during early spring, however, showed that impacts were quickly reversible also in the most impacted sites. Thus, we conclude that in Swedish coastal waters the benthic footprint calculated on the % cover of Beggiatoa sp. is highly dependent on ambient oxygen concentration. We suggest that knowledge about spatial and temporal patterns of oxygen in the bottom water can be used to predict the severity of impacts and provide an important criterion in a site-selection process aimed at developing a sustainable food industry.

Assessing the potential for sea-based macroalgae cultivation and its application for nutrient removal in the Baltic Sea.

Marine eutrophication is a pervasive and growing threat to global sustainability. Macroalgal cultivation is a promising circular economy solution to achieve nutrient reduction and food security. However, the location of production hotspots is not well known. In this paper the production potential of macroalgae of high commercial value was predicted across the Baltic Sea region. In addition, the nutrient limitation within and adjacent to macroalgal farms was investigated to suggest optimal site-specific configuration of farms. The production potential of Saccharina latissima was largely driven by salinity and the highest production yields are expected in the westernmost Baltic Sea areas where salinity is >23. The direct and interactive effects of light availability, temperature, salinity and nutrient concentrations regulated the predicted changes in the production of Ulva intestinalis and Fucus vesiculosus. The western and southern Baltic Sea exhibited the highest farming potential for these species, with promising areas also in the eastern Baltic Sea. Macroalgal farming did not induce significant nutrient limitation. The expected spatial propagation of nutrient limitation caused by macroalgal farming was less than 100-250 m. Higher propagation distances were found in areas of low nutrient and low water exchange (e.g. offshore areas in the Baltic Proper) and smaller distances in areas of high nutrient and high water exchange (e.g. western Baltic Sea and Gulf of Riga). The generated maps provide the most sought-after input to support blue growth initiatives that foster the sustainable development of macroalgal cultivation and reduction of in situ nutrient loads in the Baltic Sea.