From sink to source: Dynamic of greenhouse gases emissions from beach wrack accumulations in a temperate coastal bay.

Coastal ecosystems such as salt marshes, seagrass meadows, and kelp forests contribute to climate regulation as carbon sinks. However, coastal ecosystems may act as carbon sources as beach wrack accumulations may release greenhouse gases (GHG) during decomposition. The magnitude of GHG emissions of beach wrack accumulations under natural conditions are poorly understood, hampering accurate blue carbon accountings. In this study, we assessed the spatio-temporal variability and environmental factors driving CO2, CH4 and N2O emissions from beach wrack accumulations on a temperate sandy beach. Beach wrack accumulations, dominated by Zostera marina and opportunistic brown macroalgae, presented variable spatio-temporal dynamics. Annual beach wrack GHG emissions achieved up to 77,915 mg m-2 d-1 CO2e (CO2 equivalents) and varied largely throughout the study period due to interactive effects of temperature, wave exposure, beach wrack biomass moisture, abundance, and species composition. Our findings showed that methane emissions in new, freshly deposited, and in drifting wrack in the water reached up to 100 mg m-2 d-1, representing up to 57 % of annual CO2e emissions occurring throughout the year. Nitrous oxide emissions were highly variable and comprised a minor extent (i.e., up to 4 %) of annual CO2e emissions. Together, wrack CH4 and N2O emissions provided 13.69 g CO2 m-2 per year to the atmosphere. Our findings indicate that excessive opportunistic macroalgae biomass driven by eutrophication may explain increased CO2 and N2O emissions. We conclude that whilst beach wrack depositions are a natural and essential part of coastal ecosystems, they may provide an extra source of GHG to the atmosphere, potentially counteracting the role of vegetated coastal ecosystems as carbon sinks.

Sand-capping - A large-scale approach to restore organic-enriched estuarine sediments.

Decades of eutrophication have deteriorated marine coastal habitats severely and has led to massive decline of eelgrass along European coastlines and impoverishment of benthic fauna. Although nutrient loadings were reduced and water quality increased decades ago, eutrophication in the past had enriched marine sediments in organic matter to an extent that is still affecting ecosystems today. Organic-rich sediments are readily resuspended, keeping shallow estuaries in a turbid state and benthic fauna communities remain low in density and diversity. Sand-capping of muddy sediments may restore such deteriorated estuaries and is in this study applied for the first time as a large-scale restoration approach. A 10 cm layer of sand was added on 1.0 and 1.4 ha muddy sediments at two locations in Odense Fjord, Denmark. The organic content of the mud at the sites was 8-9% and 3-5% before sand-capping. The sand-cap stabilized the mud without mixing the sand-mud interface, not even after one year. The associated lower resuspension of fine particle improved light conditions in the overlying water by up to 9 and 22% at the two locations. Benthic fauna recruitment improved after sand-capping, leading to a local shift from low to high diversity of the benthic community and increased ecosystem functionality.

Benthic bioturbation: A canary in the mine for the retention and release of metals from estuarine sediments.

After the largest mining tailings spill in Brazil, the Rio Doce estuarine ecosystem was severely impacted by metal contamination. In a 28-day laboratory experiment, we examined the effects of the polychaeta Laeonereis sp. on fluxes of oxygen and metal across the sediment-water interface. The density-dependent effect of Laeonereis sp. in the oxygen and metal fluxes was tested at low and high (74 and 222 ind m-2, respectively) densities, and compared with defaunated controls. The higher worm density had an amplified effect on the oxygen flux, sediment uptake of Al and Mn, and Fe oxidation compared with the control, but no significant effects on other metals (Ba, Cd, Co, Cr, Cu, Ni, and Zn). Higher worm density increased the oxidation of Fe phases, but no effect in the solid phase of other metals. Consequently, Laeonereis sp. bioturbation prevents the reduction of Fe phases and the release of metal-bound-contaminants to estuarine systems.

Benthic macrofauna bioturbation and early colonization in newly flooded coastal habitats.

How will coastal soils in areas newly flooded with seawater function as habitat for benthic marine organisms? This research question is highly relevant as global sea level rise and coastal realignment will cause flooding of soils and form new marine habitats. In this study, we tested experimentally the capacity of common marine polychaetes, Marenzelleria viridis, Nereis (Hediste) diversicolor and Scoloplos armiger to colonize and modify the biogeochemistry of the newly established Gyldensteen Coastal Lagoon, Denmark. All tested polychaetes survived relatively well (28-89%) and stimulated carbon dioxide release (TCO2) by 97-105% when transferred to newly flooded soils, suggesting that soil characteristics are modified rapidly by colonizing fauna. A field survey showed that the pioneering benthic community inside the lagoon was structurally different from the marine area outside the lagoon, and M. viridis and S. armiger were not among the early colonizers. These were instead N. diversicolor and Polydora cornuta with an abundance of 1603 and 540 ind m-2, respectively. Considering the species-specific effects of N. diversicolor on TCO2 release and its average abundance in the lagoon, we estimate that organic carbon degradation was increased by 219% in the first year of flooding. We therefore conclude that early colonizing polychaetes modify the soils and may play an important role in the ecological and successional developments, e.g. C cycling and biodiversity, in newly flooded coastal ecosystems. Newly flooded soils have thus a strong potential to develop into well-functioning marine ecosystems.

Carbon mineralization pathways and bioturbation in coastal Brazilian sediments.

Carbon mineralization processes and their dependence on environmental conditions (e.g. through macrobenthic bioturbation) have been widely studied in temperate coastal sediments, but almost nothing is known about these processes in subtropical coastal sediments. This study investigated pathways of organic carbon mineralization and associated effects of macrobenthic bioturbation in winter and summer (September 2012 and February 2014) at the SE Brazilian coast. Iron reduction (FeR) was responsible for 73-81% of total microbial carbon mineralization in September 2012 and 32-61% in February 2014. Similar high rates of FeR have only been documented a few times in coastal sediments and can be sustained by the presence of large bioturbators. Denitrification accounted for 5-27% of total microbial carbon mineralization while no SO4(2-) reduction was detected in any season. Redox profiles suggested that conditions were less reduced in February 2014 than in September 2012, probably associated with low reactivity of the organic matter, higher rates of aerobic respiration and bioirrigation by the higher density of small-macrofauna. Bioturbation by small macrofauna may maintain the sediment oxidized in summer, while large-sized species stimulate the reoxidation of reduced compounds throughout the year. Therefore, bioturbation seems to have an important role modulating the pathways of carbon mineralization in the area.