Shallow Coastal Ecosystems Research Articles

Long-term responses of intertidal and subtidal seagrasses to anthropogenic disturbances and sea warming in the northwestern pacific coast

Long-term ecological research is imperative for comprehensively understanding how ecosystems respond to environmental changes over time. This study examined the long-term responses of seagrasses (Zostera japonica, Z. marina, and Z caespitosa) in intertidal and subtidal zones to anthropogenic disturbances and sea warming on the southern coast of the Korean peninsula over 15 years (2008–2022). Dramatic declines in the intertidal seagrasses were observed due to human activities such as clam harvesting and oyster cultivation, resulting in the disappearance of almost all Z. japonica in intertidal zones. Subtidal seagrasses, including Z. marina and Z. caespitosa decreased gradually throughout the study period. The biomass and canopy height of the subtidal Z. marina showed an inverse trend against water temperature. In addition, water temperature anomalies such as marine heat waves (MHWs) caused abrupt declines of seagrasses within both the intertidal and subtidal zones. Annually repeated and acute human-induced disturbances were the primary threat to the intertidal seagrass species. Nevertheless, the subtidal seagrasses were primarily affected by unpredictable summer MHWs as well as gradual rising water temperatures, with MHWs causing more dramatic alterations in the coastal seagrass ecosystems compared to gradual SST rising. The continuous decline of seagrasses due to natural and human-induced disturbances will alter the structure of shallow coastal ecosystems, leading to significant changes in the ecosystem services they provide. The results of this study underscore the importance of management actions in coastal seagrass ecosystems and highlight the need to predict future impacts of ongoing climate change.

Variability in oceanographic conditions affecting Mesophotic Ecosystems along the South Eastern Pacific: Latitudinal trends and potential for climate refugia

Oceans have been changing at the fastest pace since the beginning of the Holocene. The South Eastern Pacific (SEP), including the Humboldt Upwelling Ecosystem (HUE) is subject to changes in upwelling winds, temperature, El Niño, and the ever-increasing local anthropogenic stressors, all of which have been documented for surface coastal waters where in-situ and remote observations are readily available. Temporal and spatial changes in the adjacent deeper waters where diverse Mesophotic Ecosystems are found have been scarcely documented. These marine ecosystems have been the focus of ecological studies for less than two decades. Here we provide an overview of the thermal variability at mesophotic depths and assess their potential as climatic refugia along all SEP ecoregions. We analyzed a time series of temperature and salinity from a 19 yr reanalysis based on remote and in-situ observations (CTD, ARGO, XBTs, moorings) to quantify variability in the Tropical (0–5°S), Northern Warm Temperate (5–30°S); Southern Warm Temperate (30–39.5°S) and Magellanic subregions (39.5–45°S), at two mesophotic depth strata (50 and 100 m), and a reference surface (5 m) depth. We assessed variability in the seasonal, interannual (El Niño) and ‘long-term’ (ca. 20 yr) scales, and the relationship with wind velocities. The thermal depth gradient between surface and mesophotic depths did not change smoothly with latitude but peaked within the northern portion of the warm temperate subregion, decreasing towards lower and higher latitudes. Seasonal variation in temperature was also largest in the north and south temperate subregions and minimal in the Magellanic subregion. Depth dampening of seasonal temperature variation was also strengthened at intermediate latitudes and much reduced in the tropics, where seasonal variation at mesophotic depths was similar to that at the surface. The strong interannual El Niño events were identified at all depths in tropical and temperate subregions, with stronger standardized effects at mesophotic layers than at the surface. Long-term (ca. two decades) temperature trends were significant and changed direction from warming to cooling along the SEP but were generally patchier at mesophotic layers. Spatial temperature gradients have remained relatively stable over the past two decades and were stronger at the surface than at mesophotic depths, and stronger within the tropics than in all other subregions. Surprisingly, the velocity of climate change was patchier and generally faster at mesophotic layers than at the surface. We conclude that, judging solely by physical environmental conditions, mesophotic ecosystems may be used by species with very different temperature affinities in temperate subregions, while in the tropics, more overlap in temperature affinities of component species may be found. Importantly, while the seasonal amplitude is reduced at mesophotic depth in most subregions, except the tropics, interannual disturbances affect mesophotic depths at least as strongly as they do surface waters and climate change velocities are faster at mesophotic depths than at surface. Thus, these ecosystems are not sheltered from inter-annual and longer-term forcing and their biotas might be more vulnerable to climate change than shallow coastal ecosystems.

Water and sediment microbial communities in shallow coastal ecosystems of Lake Baikal during summer 2021-2022

The taxonomical structure of the microbial community of shallow coastal areas is closely related to environmental conditions, and also changes rapidly in response to changes determined by water level fluctuations. The taxonomic structure of microbial communities in the shallow coastal areas of the Lake Baikal was studied during the period of high water level in summer months 2021-2022. At the phylum level, Proteobacteria, Actinobacteriota, Bacteroidota, Firmicutes, Cyanobacteria, Verrucomicrobiota and Deinococcota were dominant, accounting for 98.55 to 99.46% of the total microbial diversity in water microbial community. A significant similarity was revealed in the taxonomic composition of microbial communities in the shallow coastal areas of the Lake Baikal during certain summer months of 2021-2022. Similar water microbial communities formed in June 2021/22; in July 2022 in the Enkhaluk and Sukhaya sample sites, as well as at the Goryachinsk and Sukhaya sample sites in August 2022. Sediment microbial communities were dominated by Gammaproteobacteria, Bacteroidota, Alphaproteobacteria, Actinobacteriota and Verrucomicrobiota.

Changes in sea floor productivity are crucial to understanding the impact of climate change in temperate coastal ecosystems according to a new size-based model

The multifaceted effects of climate change on physical and biogeochemical processes are rapidly altering marine ecosystems but often are considered in isolation, leaving our understanding of interactions between these drivers of ecosystem change relatively poor. This is particularly true for shallow coastal ecosystems, which are fuelled by a combination of distinct pelagic and benthic energy pathways that may respond to climate change in fundamentally distinct ways. The fish production supported by these systems is likely to be impacted by climate change differently to those of offshore and shelf ecosystems, which have relatively simpler food webs and mostly lack benthic primary production sources. We developed a novel, multispecies size spectrum model for shallow coastal reefs, specifically designed to simulate potential interactive outcomes of changing benthic and pelagic energy inputs and temperatures and calculate the relative importance of these variables for the fish community. Our model, calibrated using field data from an extensive temperate reef monitoring program, predicts that changes in resource levels will have much stronger impacts on fish biomass and yields than changes driven by physiological responses to temperature. Under increased plankton abundance, species in all fish trophic groups were predicted to increase in biomass, average size, and yields. By contrast, changes in benthic resources produced variable responses across fish trophic groups. Increased benthic resources led to increasing benthivorous and piscivorous fish biomasses, yields, and mean body sizes, but biomass decreases among herbivore and planktivore species. When resource changes were combined with warming seas, physiological responses generally decreased species’ biomass and yields. Our results suggest that understanding changes in benthic production and its implications for coastal fisheries should be a priority research area. Our modified size spectrum model provides a framework for further study of benthic and pelagic energy pathways that can be easily adapted to other ecosystems.

Temperature amplification and marine heatwave alteration in shallow coastal bays

Shallow coastal ecosystems are threatened by marine heatwaves, but few long-term records exist to quantify these heatwaves. Here, 40-year records of measured water temperature were constructed for a site in a system of shallow bays with documented heatwave impacts and a nearby ocean site; available gridded sea-surface temperature datasets in the region were also examined. Water temperatures at both sites increased significantly though bay temperatures were consistently 3-4°C hotter in summer and colder in winter and were more variable overall, differences not captured in high-resolution gridded sea-surface temperature datasets. There was considerable overlap in heatwave events at the coastal bay and ocean sites. Annual heatwave exposure was similar and significantly increased at both sites while annual heatwave intensity was significantly higher at the bay site owing to the high variance of the daily temperature anomaly there. Event frequency at both sites increased at a rate of about 1 event/decade. Future simulations indicate all heatwave metrics increase, as do days above 28°C, a heat stress threshold for seagrass. Ocean temperatures on the U.S. mid-Atlantic margin have rarely exceeded this threshold, while summer bay temperatures commonly do, allowing ocean exchange with coastal bays to provide thermal relief to bay ecosystems. This will have changed by 2100, creating a thermal environment that threatens seagrass communities in these systems. Documenting such change requires development of long-term water temperature records in more shallow coastal systems.

Confronting turbidity, the major challenge for satellite-derived coastal bathymetry

Monitoring the complex seafloor morphology that drives the functioning of shallow coastal ecosystems is vital for assessing marine activities. Satellite-derived bathymetry (SDB) can provide a crucial dataset for creating the bathymetry maps needed to understand hazards and impacts produced by climate change in vulnerable coastal zones. SDB is effective in clear water, but still has limitations in application to areas with some turbidity. Here, using the twin satellites Sentinel-2A/B, we integrate water quality information from the satellite with a multi-temporal compositing method to demonstrate a potential for comprehensively operational bathymetric mapping over a range of environments. The automated compositing method diminishes the turbidity impact in addition to inferring the maximum detectable depth and removing optically deep-water areas. Examining a wide range of conditions along the Caribbean and eastern coast of the U.S. shows detailed bathymetry as deep as 30 m at 10 m spatial resolution with median errors <1 m when compared to high-resolution lidar surveys. These results demonstrate that the model adopted can provide useful bathymetry in areas that do not have consistently clear water and can be extended across multiple geographic regions and optical conditions at local, regional, and national scales.

Not a Good Place to Live for Most, but Excellent for a Few—Diversity of Zooplankton in a Shallow Coastal Ecosystem

Shallow brackish water habitats are characterized by highly unstable environmental conditions, which result in low species diversity. We performed our research in the Vistula Lagoon in three seasons in the years 2019–2021. This lagoon is characterized by extremely variable environmental conditions, and our research covered the period of hydro-technical works before opening the canal through the Vistula Spit, which could be an additional stress factor. The aim of our study was to present the particularities of zooplankton communities (Rotifera; Crustacea) in the shallow coastal ecosystem. Our results pointed out very low diversity of zooplankton which could be caused by multi-stress conditions related to increased turbidity and low salinity. Nevertheless, under these conditions, some species achieved an advantage and strongly dominated the zooplankton communities. Domination of Eurytemora affinis in the Vistula Lagoon as well as in other brackish habitats is a well-known phenomenon. Moreover, the Vistula Lagoon offered favorable conditions for the intense development of non-indigenous, or alien, species like Diaphanosoma mongolianum, D. orghidani, and Acanthocyclops americanus. Therefore, brackish waters seem to be favorable transitional areas for non-native fauna that may subsequently spread into adjacent freshwater and marine systems.

Marine heatwaves in shallow coastal ecosystems are coupled with the atmosphere: Insights from half a century of daily in situ temperature records

Marine heatwaves (MHWs) are extreme ocean temperature events that can have wide-ranging and pervasive effects on marine species and ecosystems. However, studies of MHW characteristics and drivers primarily focus on open-ocean environments, rather than the nearshore coastal ocean (&amp;lt;10 km from coast, &amp;lt;50 m depth). This is despite coastal waters sustaining significant commercial, recreational, and customary fisheries and aquaculture activities that are highly susceptible to the impacts of MHWs. The two longest (&amp;gt;50 year) daily in situ ocean temperature records in the Southern Hemisphere are used to investigate the variability, drivers, and trends of MHWs in shallow water marine ecosystems (SWMEs). Located at the northern and southern limits of New Zealand, both locations experience an average of two to three MHWs annually, with MHWs at the exposed coastline site generally being of longer duration but less intense than those observed within the semi-enclosed harbor site. Observed MHWs have timescales similar to synoptic weather systems (9–13 days) and are most intense during Austral summer with little seasonality in frequency or duration. An investigation of MHWs co-occurring in nearshore coastal and offshore waters suggests that MHWs in semi-enclosed waters (e.g., harbors, estuaries) are more closely coupled with local atmospheric conditions and less likely to have a co-occurring offshore MHW than those occurring on exposed coastlines. Composite analysis using a reanalysis product elucidates specific atmospheric drivers and suggests that atmospheric pressure systems, wind speed and latent heat fluxes are important contributing factors to the generation and decline of MHWs in SWMEs. Investigation of long-term trends in MHW properties revealed an increase in MHW duration and annual MHW days at the southern site and decrease in maximum intensity at the northern site. This is consistent with broad-scale warming trends previously documented at these coastal stations, with differences related to changes in large-scale circulation patterns around New Zealand. Our results highlight the importance of in situ data for the analysis of MHW events in the nearshore coastal ocean, and the role of local atmospheric forcing in modulating the occurrence of MHWs in SWMEs, which can cause decoupling of temperature dynamics with the surrounding shelf sea.

Stomach content and stable isotope analyses provide complementary insights into the trophic ecology of coastal temperate bentho-demersal assemblages under environmental and anthropogenic pressures

Assessing organic matter fluxes and species interactions in food webs is of main interest to understand the ecological functioning in bays and estuaries characterised by a wide diversity of primary producers and consumers. Demersal fish and cephalopod assemblages were studied across a network of 24 shallow subtidal stations in the bay of Saint-Brieuc for their diversity, stable isotope compositions and stomach contents. The community was composed of 21 taxa, eight species accounting for 94.4% of the total abundance. Three different assemblages were identified along bathymetric gradient and spatial patterns in fish dredging. Marine POM and SOM were the most likely bases of food webs regarding δ13C range displayed by fish and cephalopod without differences among assemblages. Amphipoda was the main prey item in stomachs leading to significant diet overlaps among fish species, with some variations in additional items. Sepia officinalis was characterised by a singular diet and very low dietary overlap with other species. Contrasted stable isotope values and niche overlaps among species were evidenced in the δ13C/δ15N space. Callionymus lyra and Buglossidium luteum, characterised by the widest isotopic niches, encompassed those of other species, except the singular 13C-depleted Spondyliosoma cantharus. Coupling taxonomic assemblages, stomach contents and stable isotope analyses help disentangling the resources uses and evidencing trophic pathways. Contrasts in fish and cephalopod demersal assemblages occurring at different depths not necessarily imply differences in the trophic resources uses in such complex shallow coastal ecosystems under anthropogenic influences.

Are fatty acids in fish the evidence of trophic links? A case study from the southern Baltic Vistula Lagoon

Most knowledge on the feeding ecology of fish has been based on the analyses of food remains from the alimentary tracks. This traditional method, however, only provides information about recently consumed food, and is burdened with a risk of incorrect assessment of the role of individual diet components due to the different rates of digestion. A method free from such limitations is the analysis of fatty acids. The objective of our study was to recognise the potential of fatty acid signatures in providing information on the diet and feeding habits of six fish species from the shallow brackish Vistula Lagoon, southern Baltic Sea (Anguilla anguilla, Abramis brama, Rutilus rutilus, Pelecus cultratus, Perca fluviatilis, Sander lucioperca). Multivariate statistical analyses of fatty acid signatures permitted relevant grouping of the fish according to species and their diet, as well as evidenced substantial ontogenetic changes in perch, roach, and bream. They might be caused by dietary changes but can also result from internal regulatory processes. The obtained results confirmed that fatty acids provide useful, time-integrated dietary information, contributing to expanding knowledge regarding the feeding ecology of fish in shallow coastal water ecosystems. They also pointed to the necessity of assessment of the invertebrates and fish's ability to perform endogenous synthesis of polyunsaturated fatty acids, particularly in research on benthic communities. To our best knowledge, this is the first attempt to investigate the feeding habits of fish and food-web relationships in the coastal waters of the Baltic Sea using fatty acids.

Measurement of in-vivo spectral reflectance of bottom types: Implications for remote sensing of shallow waters

Field spectroscopy data serve as a standard for the inter-comparisons, calibration and validation of remote sensing data. It also greatly assists in the detection, characterization and mapping of the earth features from the satellite imagery. Strategies and methods for field-based spectro-radiometric measurements are well-established for land and deep ocean applications, however, there are certain challenges and limitations with respect to the shallow water coastal ecosystems. This paper focuses on the background theory, concepts, and measurement techniques of in-vivo sensing of benthic reflectance in shallow water environment. We investigated various measurement methods, influences of water column, air–water interface, sources of errors (introduced by experimental and environmental factors), and the correction methods that are involved in the determination of benthic substrate reflectance. Special emphasis is provided using in-vivo measurements made in the estuarine and shelf regions, illustrating the benthic substrate reflectance at various platforms, i.e., within the water column, above the water surface, at the top-of-atmosphere, in the air, and in a controlled (laboratory) environment. As part of the correction procedure, a new water column correction method is theoretically formulated and examined for varying water optical properties and water depth. A new step correction method is also developed (based on the additive approach) to correct the spectral reflectance measured by a full range spectroradiometer (350–2500 nm) that is in-built with two or more detectors. This study will have direct implications for improving the data quality of field-based spectro-radiometric measurements that are essential for shallow water remote sensing applications.

Implementation of blue carbon offset crediting for seagrass meadows, macroalgal beds, and macroalgae farming in Japan

The “blue carbon” concept and the role of blue carbon stored in shallow coastal ecosystems in mitigating climate change have attracted attention worldwide. In addition to typical blue carbon ecosystems, such as mangroves, tidal marshes, and seagrass meadows, macroalgal beds and macroalgae farming are also gaining recognition as potential blue carbon sinks. Effective policies and methodologies are important for the conservation and expansion of blue carbon sinks, as well as the consequent reduction of atmospheric carbon dioxide (CO2). One of the most effective methods is carbon offset credit schemes. However, to date, almost all credit schemes have been implemented for mangroves and saltmarshes. None has been implemented for seagrass meadows, macroalgal beds, and macroalgae farming, although their CO2 uptake potentials are large. Here, we review three blue carbon offset credit projects for seagrass meadows, macroalgal beds, and macroalgae farming implemented in Japan. We show the blue carbon offset crediting projects of (1) Yokohama City, the first in the world; (2) Fukuoka City, the second such project in Japan; and (3) the first Japanese national governmental demonstration project. Furthermore, we highlight their characteristics by comparing them with the other blue carbon offset projects. Finally, we discuss the need to accelerate blue carbon offset credit projects and related initiatives in the future.

Responses of the Mediterranean seagrass Cymodocea nodosa to combined temperature and salinity stress at the ionomic, transcriptomic, ultrastructural and photosynthetic levels

The Little Neptune grass Cymodocea nodosa is a key seagrass species in the Mediterranean Sea, forming extensive and patchy meadows in shallow coastal and transitional ecosystems. In such habitats, high temperatures and salinities, separately and in combination, can be significant stressors in the context of climate change, particularly during heatwave events, and seawater desalination plant effluents.Despite well-documented negative, macroscopic effects, the underlying cellular and molecular processes of the combined effects of increasing temperature and salinities have remained largely elusive in C. nodosa – which are addressed by the present study. High salinity and high temperature, alone and in combination, affected ion equilibrium in the plant cells. Non-synonymous mutations marked the transcriptomic response to salinity and temperature stress at loci related to osmotic stress. Cell structure, especially the nucleus, chloroplasts, mitochondria and organization of the MT cytoskeleton, was also altered. Both temperature and salinity stress negatively affected photosynthetic activity as evidenced by ΔF/Fm’, following an antagonistic interaction type.Overall, this study showed that all biological levels investigated were strongly affected by temperature and salinity stress, however, with the latter having more severe effects. The results have implications for the operation of desalination plants and for assessing the impacts of marine heat waves.

Phosphorus Dynamics Associated With Organic Carbon Mineralization by Reduction of Sulfate and Iron in Sediment Exposed to Fish Farming

The expansion of the aquaculture industry has resulted in accumulation of phosphorus (P)-rich organic matter via uneaten fish feed. To elucidate the impact of fish farming on P dynamics, P speciation, and benthic P release along with partitioning of organic carbon (Corg) mineralization coupled to sulfate reduction (SR) and iron reduction (FeR) were investigated in the sediments from Jinju Bay, off the southern coast of South Korea, in July 2013. SR in the farm sediment was 6.9-fold higher than the control sediment, and depth-integrated (0–10 cm) concentrations of NH4+, PO43–, and H2S in pore water of the farm sediment were 2.2-, 3.3-, and 7.4-fold higher than that in control sediment, respectively. High biogenic-P that comprised 28% of total P directly reflected the impact of P-rich fish feed, which ultimately enhanced the bioavailability (58% of total P) of P in the surface sediment of the farm site. In the farm sediment where SR dominated Corg mineralization, H2S oxidation coupled to the reduction of FeOOH stimulated release of P bound to iron oxide, which resulted in high regeneration efficiency (85%) of P in farm sediments. Enhanced P desorption from FeOOH was responsible for the increase in authigenic-P and benthic P flux. Authigenic-P comprised 33% of total P, and benthic P flux to the overlying water column accounted for approximately 800% of the P required for primary production. Consequently, excessive benthic P release resulting directly from oversupply of P-rich fish feed was a significant internal source of P for the water column, and may induce undesirable eutrophication and harmful algal blooms in shallow coastal ecosystems.

Microphytobenthos spatio-temporal dynamics across an intertidal gradient using Random Forest classification and Sentinel-2 imagery

Microphytobenthos (MPB) provides important ecosystem functions and services, contributing significantly to the total primary production in shallow coastal ecosystems. However, determining the factors that regulate the seasonal changes of MPB and its distribution patterns at larger scales is hindered by the considerable spatial and temporal variability in these environments. Here, we studied the dynamics of intertidal MPB biomass, cover, and net growth rates in a south European tidal flat (Cadiz Bay, Spain) over a four-year period using the Normalized Difference Vegetation Index (NDVI) calculated from Sentinel-2 satellite images. Pixels dominated by different benthic communities (MPB, Zostera sp., Caulerpa sp. and green macroalgae) were identified at a 10-m resolution using a Random Forest (RF) machine learning classification algorithm. MPB dominated the intertidal zone. MPB cover did not show a clear seasonal pattern and was clearly higher in the middle of the intertidal range of sea level. Despite interannual variability, MPB biomass was always higher during winter, coinciding with observations from other low latitude intertidal flats with temperate climate, and in the upper-middle intertidal. Net rates of MPB biomass change, calculated from the differences in MPB NDVI over time, showed maximal net growth rates from autumn to winter and maximum loss rates during spring and summer, although with high variability. Our study demonstrates that RF algorithms allow mapping MPB and other intertidal communities from Sentinel-2 multispectral satellite imagery accurately obtaining invaluable information from large areas at very high spatio-temporal resolution. The dissimilarities observed in the patterns of MPB variables over time or sea level, indicate differences in their ecological regulation, still largely unknown both here and in other temperate climate intertidal flats. High resolution remote sensing can aid in their detailed and systematic study producing a more integrated view of these systems and contributing to their science-based management and conservation.

Enabling Water Reuse by Treatment of Reverse Osmosis Concentrate: The Promise of Constructed Wetlands.

As more cities experience water stress, the use of reverse osmosis (RO) membranes for wastewater treatment and reuse will expand. The concentrated waste stream resulting from RO treatment can pose chronic ecotoxicity risks if discharged to surface waters or shallow coastal ecosystems. Most existing RO concentrate treatment technologies are cost prohibitive, but constructed wetlands hold promise as a viable multibenefit solution because they have the potential to provide simultaneous treatment of nutrients, metals, and trace organic contaminants at a relatively low cost. They also are popular with the public. A handful of water-stressed cities have already begun experimenting with constructed wetlands for RO concentrate treatment. However, further research is needed to reduce the land area needed for treatment and increase the reliability of constructed wetland systems.

Seagrass contributes substantially to the sedimentary lignin pool in an estuarine seagrass meadow

Shallow coastal ecosystems are reservoirs of carbon derived from allochthonous organic matter and autochthonous organic matter produced by microalgae and macrophytes. Carbon stored in vegetated coastal ecosystems has attracted broad attention as an important component of carbon sinks. Characterizing the source of carbon in sediments is essential for quantifying the carbon-sequestration function of shallow coastal ecosystems. In this study, we investigated the origins of organic matter using organic biomarkers (lignin phenols, fatty acids, cutin acids, diacids, and ω-hydroxy acids) in surface sediments in a seagrass-dominated lagoon (Furen Lagoon, Japan). Biomarkers derived from allochthonous vascular plants, such as long-chain fatty acids, showed higher concentrations near river mouths. Furthermore, biomarker signals indicated that sedimentary organic carbon originated in large part from degraded allochthonous vascular plants including roots. A Bayesian mixing model using the ratios of syringyl phenols to vanillyl phenols and cinnamyl phenols to vanillyl phenols indicated that up to about 65% of lignin in the sediments was derived from seagrass. This result indicates a substantial contribution of seagrass to the sedimentary lignin pool in an estuarine seagrass meadow. However, the percent contribution of seagrass to the lignin pool varied, with higher values near a tidal inlet and relatively low values near river mouths. Vertical profiles of organic biomarkers varied with the differences in degradability of organic compounds. Specifically, long-chain fatty acids decreased with increasing depth more than the other compounds, suggesting that they degraded more easily. Conversely, we observed a tendency for lignin phenols to be selectively preserved in the vertical sediment profiles. Our results show that sediment organic biomarkers can provide diverse information such as the composition and origins of organic carbon, the contribution of seagrass derived lignin, and the varying degrees of decomposition. This approach should bring new insights to the estimation of carbon in future blue carbon studies.

Benthic oxygen dynamics and implication for the maintenance of chronic hypoxia and ecosystem degradation in the Berre lagoon (France)

Chronic hypoxia and anoxia have strong impacts on coastal ecosystems worldwide. In shallow coastal ecosystems, such situations are essentially driven by high benthic oxygen (O2) demand resulting from organic matter mineralization in surface sediment and amplified by a low mixing of the water column. However, the benthic O2 demand may greatly vary according to the O2 availability, sediment biogeochemical properties, and bioturbation by macrobenthic fauna. Here we examined how the sediment O2 demand varies in response to seasonal and long-lasting (pluri-decadal) hypoxia in the Berre lagoon, a coastal ecosystem impacted by chronic hypoxia events since 60 years. Oxygen penetration depth, diffusive and total O2 fluxes were measured in situ using a microelectrode autonomous profiler and benthic chamber deployments at three sites impacted by quite-permanent (PA), seasonal (PI) and occasional (PO) hypoxia in August 2016. They were seasonally repeated at site PI between August 2015 and August 2016. Additional physical and chemical characteristics were also measured in surface sediment. Sediment profile images and characteristics of benthic macrofauna communities enabled to estimate the quality of the benthic ecosystem. The highest benthic O2 demand was observed after seasonal anoxia in relation to the important accumulation of reduced chemical species in surface sediment. Interestingly, both pluri-decadal hypoxia and normoxia produced relatively high benthic O2 demand related to a higher accumulation of organic matter and to the presence of reduced chemical species at site dominated by hypoxia, and to the presence of fresher organic matter and active bioturbating macrofaunal communities in normoxic site. The low benthic O2 demand at site seasonally impacted by hypoxia likely resulted from the degraded state of the macrofaunal community and from the lower accumulation of reduced chemical species. The occurrence of hypoxia and anoxia situations in the Berre lagoon was predicted from the competition between kinetics of benthic O2 demand and water column reoxygenation events induced by strong wind. The good agreement between the measured and predicted hypoxia/anoxia occurrence clearly indicates that the chronic deoxygenation events in the Berre lagoon, and the resulting degraded ecological state of the benthic ecosystem are driven both by the benthic O2 demand and by the intensity and duration of the water column stratification.

Ecological consequences of sea level rise and flood protection strategies in shallow coastal systems: A quick-scan barcoding approach

Shallow coastal ecosystems have high ecological value and contribute to flood protection. Their stability is, however, sensitive to the amount and rate of future sea level rise (SLR), their ability to trap sediment which allows them to grow with rising sea level, and human response to SLR. So far, studies have focused on assessing SLR impacts using resource-intensive tools. Here, we present an approach for a first-order assessment and easily accessible ‘barcode’ visualization to rapidly assess potential impacts of both SLR and adaptation strategies on coastal ecosystems in a spatially explicit way. Our approach relates habitat types (ecotopes) to water level, morphology and salinity, allowing users to determine shifts in spatial arrangements of ecological zones under different SLR rates and strategies. We illustrate this approach for a transect in the Dutch Wadden Sea. We find that beyond a critical rate of SLR, major changes in ecotope distribution are projected to occur as this part of the Wadden Sea starts to drown due to insufficient sediment import. Even larger impacts arise from adaptation strategies. Closing the barrier islands will turn the Wadden Sea into a freshwater lake-system with the absence of intertidal areas, infilling of channels and bank erosion. A strategy that allows for inland migration of the shoreline, results in a deep tidal basin with large subtidal habitats, and a shifted intertidal zone. Our case study shows that the barcoding approach provides a rapid, quantitative and spatially explicit overview of the potential implications for coastal ecosystems under different SLR scenarios, adaptation strategies and time horizons. This can then be used to screen adaptation strategies before going into a more comprehensive analysis. The barcode visualization allows for easy dissemination of potential ecological impacts to a broad community.

Seasonal Photophysiological Performance of Adult Western Baltic Fucus vesiculosus (Phaeophyceae) Under Ocean Warming and Acidification

Shallow coastal marine ecosystems are exposed to intensive warming events in the last decade, threatening keystone macroalgal species such as the bladder wrack (Fucus vesiculosus, Phaeophyceae) in the Baltic Sea. Herein, we experimentally tested in four consecutive benthic mesocosm experiments, if the single and combined impact of elevated seawater temperature (Δ + 5°C) and pCO2 (1100 ppm) under natural irradiance conditions seasonally affected the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus. Photosynthesis was highest in spring/early summer when water temperature and solar irradiance increases naturally, and was lowest in winter (December to January/February). Temperature had a stronger effect than pCO2 on photosynthetic performance of Fucus in all seasons. In contrast to the expectation that warmer winter conditions might be beneficial, elevated temperature conditions and sub-optimal low winter light conditions decreased photophysiological performance of Fucus. In summer, western Baltic Sea Fucus already lives close to its upper thermal tolerance limit and future warming of the Baltic Sea during summer may probably become deleterious for this species. However, our results indicate that over most of the year a combination of future ocean warming and increased pCO2 will have slightly positive effects for Fucus photophysiological performance.