Seagrass meadows are vital blue carbon habitats, with sedimentary organic carbon (OC) originating from both the seagrass itself and external sources. In this study, lipid biomarkers (n-alkanes), a well-known proxy for tracing OC sources, were used to indicate seagrass presence in sediment records and to correlate with sedimentary OC in cold-temperate seagrass (Zostera marina) sediments. We calculated a Zostera-ratio (seagrass/algae and terrestrial plants-ratio) using identified seagrass biomass n-alkanes (C15, C17, C19, C21, C23) as a fingerprint for seagrass-derived OC. Based on the presence or absence of seagrass plant remains in sediments, we confirmed an overall significant positive correlation (R2 = 0.49, with significant sites ranging from 0.66 to 0.81; p < 0.001) between the Zostera-ratio and OC in sediment profiles down to 2 m depth. The Zostera-ratio ranged from 0.0006 to 0.35 with higher values indicating seagrass plant material. The findings show that n-alkanes can serve as proxies for both seagrass presence and total OC levels in the sediment. 

While marine seagrass habitats are acknowledged as sinks for carbon and nutrients, much less is known about sequestration in brackish-water vegetation. Here, we quantify the amount of organic carbon (C-org) and total nitrogen (TN) in shallow bay sediments (0-25 cm) in the brackish Baltic Sea and assess how it varies with morphometric isolation from the sea, catchment characteristics and abundance of brackish-water vegetation. The sedimentary C-org and TN content per surface area varied across the bay isolation gradient (mean C-org: 2500-4600 g/m(2); mean TN: 320-570 g/m(2)), with enclosed bays having the highest percentage content of C-org and TN, but low sediment density (< 0.1 g cm(3)), while open bays had more compact sediment with lower percentage content of C-org and TN. The influence of catchment and vegetation characteristics on the sediment C-org and TN content was less clear, suggesting that coastal morphology affecting hydrodynamic exposure is an important determinant of C and TN accumulation in brackish-water bays. The results show that morphometrically isolated shallow coastal areas constitute significant sinks for carbon and nitrogen, which should be considered in management and in any regional estimates of blue carbon and nutrient sequestration functions.

Seagrass provides a crucial habitat for numerous marine species and serves as a vital food source for endangered species, like dugongs. While extensive research on restoration has been conducted on certain temperate and slow-growing climax seagrass species, limited attention has been given to tropical pioneer species. This study aimed to assess and compare two restoration methods for the pioneer seagrass Halodule uninervis and evaluate their potential for biodiversity recovery after planting. We conducted a field experiment at subtropical Inhaca Island, southern Mozambique, testing the efficiency of two planting methods (plugs and single shoots) and two planting densities (similar to 100 and similar to 300 shoots/m(2)). We monitored seagrass shoot density in two sites for 16 months, and benthic macrofauna density for 12 months. Results demonstrated that seagrass could grow in all combinations of planting methods and densities in both sites. Specifically, the single shoot method at the high-density treatment proved the most effective, resulting in approximately 1000 shoots/m(2) within a year. Faunal densities, primarily dominated by polychaetes followed by malacostraca, bivalves, and gastropods, indicated rapid colonization of the planted areas, especially in the high-density treatments. Our findings suggest that restoring H. uninervis is feasible using the two tested planting methods. This is particularly significant because H. uninervis is a preferred dugong food source, and its decline due to anthropogenic activities could be reversed through restoration efforts. Nonetheless, conserving existing seagrass should be the primary focus, and restoration approaches should be employed as a valuable tool for managing coastal areas.

Assessing historical environmental conditions linked to habitat colonization is important for understanding long-term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold-temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20- and 24-fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO2 removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services. © 2024. The Authors.

Healthy ecosystems and species have some degree of resilience to changing conditions, however as the frequency and severity of environmental changes increase, resilience may be diminished or lost. In Sweden, one example of a species with reduced resilience is the Atlantic cod (Gadus morhua). This species has been subjected to overfishing, and with additional pressures such as habitat degradation and changing environmental conditions there has been little to no recovery, despite more than a decade of management actions. Given the historical ecological, economical, and cultural significance of cod, it is important to understand how Atlantic cod respond to global climate change to recover and sustainably manage this species in the future. A multi-stressor experiment was conducted to evaluate physiological responses of juvenile cod exposed to warming, ocean acidification, and freshening, changes expected to occur in their nursery habitat. The response to single drivers showed variable effects related to fish biometrics and increased levels of oxidative stress dependent parameters. Importantly, two separate responses were seen within a single treatment for the multi-stressor and freshening groups. These within-treatment differences were correlated to genotype, with the offshore ecotype having a heightened stress response compared to the coastal ecotype, which may be better adapted to tolerate future changes. These results demonstrate that, while Atlantic cod have some tolerance for future changes, ecotypes respond differently, and cumulative effects of multiple stressors may lead to deleterious effects for this important species.

As a predatory fish that migrates between freshwater and marine environments, the sea trout (Salmo trutta) is important in linking these systems. This study investigated movement patterns of sea trout in a coastal fjord at the Swedish Skagerrak region from August 2018 to January 2019, using acoustic telemetry, while assessing these against environmental variables across different spatial and temporal scales. Six acoustic receivers were deployed in the fjord and a river, which flows into the upper reaches of the fjord, with the aim of detecting 20 sea trout that had been tagged with acoustic transmitters. Sea surface temperature and winds (east-west) affected movement patterns of the sea trout the most, while changes in atmospheric pressure were also important, but to a lesser extent. Sea surface temperature and atmospheric pressure both had a positive relationship with the number of detections, whereas stronger winds from the east (i.e. from land) resulted in more detections than stronger winds from the west (i.e. from the ocean). In addition, interesting diel (day-night) movement patterns were observed for some fish. A significant positive correlation was also discovered between the weight of the fish and the number of detections. This study offers insight in movements of sea trout that use coastal habitats and how environmental conditions can affect movement patterns in a fjord system. To further our understanding of sea trout movement patterns and connectivity, tracking from river, through fjord, out to sea and at a longer time scale with more variation in fish size would be valuable to understand more about the complex movement dynamics of this important species.

Seagrass habitats play a major role in fisheries productivity through nursery functions and feeding grounds for diverse fish species. However, little is known about the seasonal distribution of fish larvae at large spatial scales in coastal East Africa. We investigated drivers of the seasonal fish larvae abundance and composition in seagrass habitats in Kenya and Tanzania. We found a high diversity of fish larvae (54 families) inhabiting seagrass habitats that differed between sites and seasons. Fish larvae abundance were highest in Kenya, particularly during the northeast monsoon season. Overall, total larval abundances per site were low, reaching less than 190 individuals/100 m³ in Kenya and less than 40 individuals/100 m³ in Tanzania, likely related to the low productivity and strong hydrodynamic processes in this region. Our data suggests that most of the fish spawn year-round in these tropical waters as we did not find strong seasonal patterns. All sites had a high relative abundance of larvae from demersal spawning fishes, indicating that many fish species move to coastal sites for spawning. Primary productivity and dissolved oxygen, driven by hydrodynamics conditions are positively related to fish larvae productivity both in Kenya and Tanzania. These findings indicate that the occurrence of both resident and transient fish larvae in seagrass meadows is driven by strong hydrodynamic and tidal processes that transport fish larvae across adjacent habitats.

Unsustainable harvesting practices have drastically reduced fish populations globally and developments in aquaculture have increased. Unexpectedly, Atlantic salmon farming caused the opening of a new fishery in northern European countries, where previously unharvested mesopredatory species, like the goldsinny wrasse (Ctenolabrus rupestris), are captured for use as cleaner fish in pens along the coast and fjords. The goldsinny wrasse is widespread in coastal areas where it plays an ecologically important role as a predator of small invertebrates. Since climate change effects are particularly pronounced in coastal waters, it becomes urgent to understand how fish like the goldsinny will respond to global climate change, including the increasing frequency and intensity of marine heatwaves (MHWs), ocean freshening (OF) and ocean acidification (OA). To address this, we conducted a multi-stressor experiment exposing adult goldsinny to each stressor individually, as well as to all three combined. The results indicated that the goldsinny is highly affected by MHWs and extremely sensitive to a multi-stressor environment, with 34% and 53% mortality, respectively. Additionally, exposure to a MHW event, OF and multi-stressor conditions affected fish metabolism, with the highest standard metabolic- and maximum metabolic-oxygen consumption rates observed for the MHW treatment. Increases in oxidized glutathione (GSSG) and percent oxidized glutathione (% GSSG) in the livers, indicative of oxidative stress, were also seen in the MHW, OF and multi-stressor treatments. As a single stressor, OA showed no significant impacts on the measured parameters. This information is important for conservation of coastal marine environments, given the species' important role in shallow-water habitats and for management of goldsinny or other mesopredatory fish harvested in coastal ecosystems. The sensitivity of the goldsinny wrasse to future stressors is of concern, and any potential reductions in abundance as a result of climate change may lead to cascade effects with ecosystem-wide consequences. Our findings show clear evidence of physiological stress on fish caused from exposure to global climate change, with marine heatwaves and the cumulative effects of multiple stressors leading to substantial mortality (potentially over 50%) for the ecologically important goldsinny wrasse.

Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO2. To study this aspect of the CO2 balance within tropical seagrass systems, we investigated the air-water CO2 flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO2 fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO2 were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO2 fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows.

Seagrass meadows are globally important blue carbon sinks. In northern cold-temperate regions, eelgrass (Zostera marina) is the dominant seagrass species, and although their sedimentary carbon stocks have been quantified across regions, information regarding the CO₂ withdrawal capacity as carbon sinks remains scarce. Here we assessed the carbon (C_org) accumulation rates (CARs) and stocks as well as the organic matter sources in five seagrass meadows in the Gullmar Fjord area on the Swedish Skagerrak coast. We found that the mean (±SD) CAR was 14 ± 3 g C_org m^-2 yr^-1 over the last ~120–140 years (corresponding to a yearly uptake of 52.4 ± 12.6 g CO2 m^-2). The carbon sink capacity is in line with other Z. marina areas but relatively low compared to other seagrass species and regions globally. About half of the sedimentary carbon accumulation (7.1 ± 3.3 g C_org m^-2 yr^-1) originated from macroalgae biomass, which highlights the importance of non-seagrass derived material for the carbon sink function of seagrass meadows in the area. The C_org stocks were similar among sites when comparing at a standardized depth of 50 cm (4.6–5.9 kg C_org m^-2), but showed large variation when assessed for the total extent of the cores (ranging from 0.7 to 20.6 kg C_org m^-2 for sediment depths of 11 to at least 149 cm). The low sediment accretion rates (1.18–1.86 mm yr^-1) and the relatively thick sediment deposits (with a maximum of >150 cm of sediment depth) suggests that the carbon stocks have likely been accumulated for an extended period of time, and that the documented loss of seagrass meadows in the Swedish Skagerrak region and associated erosion of the sediment could potentially have offset centuries of carbon sequestration.