Participation at ASLO’23
Several lab members travelled to sunny Mallorca to present at ASLO 2023. Postdoc Rebecca James presented her ongoing work on carbon preservation in seagrass meadows. Lisbeth Fürst-Sørensen, who recently defended her MSc thesis and who is now pursuing a PhD with the HADAL Center, presented her seasonal measurements on sediment resuspension. Close collaborator Theo Kindeberg, a PhD student at Lund University in Sweden, presented our joint study on metabolism in restored seagrass meadows. The titles and abstracts can be found below.
Seagrass meadows may physically trap carbon, but do they enhance its preservation? (SS093 Coastal Blue Carbon Ecosystems: Advances and Challenges)
Rebecca James, Sebastiaan van de Velde, Sandra Arndt, Ronnie N. Glud, Karl Attard
Seagrass meadows have been identified as hotspots for coastal carbon storage. The global significance of seagrass meadows at storing organic carbon, however, depends on whether the meadows are helping to preserve carbon which would otherwise be broken down and remineralised into CO2. By combining both physical and biogeochemical measures, we elucidate the actual net effect of Zostera marina meadows on coastal carbon storage and sequestration. Under increasing bed shear stress levels, it was found that dense Z. marina meadows prevent sediment resuspension, and consequently, organic carbon remobilisation. This physical process of carbon retention results in higher sedimentary carbon loads within seagrass meadows, but does this trapping affect the preservation of the organic carbon? Using biogeochemical measures of carbon reactivity in natural sandy sediments from an established Z. marina meadow, we determined that bulk organic carbon beneath the seagrass meadow had a decay constant ranging from 0.2-0.6 year-1. When resuspended, however, the decay constant of the same bulk carbon was 0.6-0.7 year-1. Thus, while seagrass meadows physically trap organic carbon within sandy sediments of the coastal zone, this process only appears to have a minimal impact on the preservation of organic carbon. With this work we highlight an important knowledge gap in blue carbon research – the biogeochemical cycling of carbon within seagrass sediments.
Importance of sediment resuspension for benthic biogeochemical function in coastal settings: A case study from a temperate estuary (SS019 Benthic Metabolism and Fluxes in Shallow Coastal Ecosystems – Controls and Responses to Environmental Stressors)
Lisbeth Fürst-Sørensen, Karl Attard, Thorbjørn Andersen, Ronnie N. Glud
Sediment resuspension is rarely considered when assessing benthic carbon mineralization in coastal environments. Using a hydrodynamically calibrated sediment erosion device (EROMES), we quantified the impact of erosion events on benthic biogeochemistry across 4 seasons. Enhanced sediment erosion increased the benthic O2 uptake rate and the release of dissolved inorganic carbon (DIC). This was mainly associated with the reoxidation of FeSx and enhanced mineralization of organic carbon, and for the latter due to a porewater washout of DIC. Sediment resuspension changes the redox conditions and increases the oxygen penetrations depth, with implications for the benthic N and P dynamics. Sediment resuspension is thus an important player in stimulating local primary production and maintaining the sulfide buffer capacity in coastal environments. The impact of sediment resuspension on the benthic environment varied across the season and was mainly driven by the size of the FeSx pools within the sediment. The effect of sediment resuspension on the near bed O2 consumption rate as compared to pre suspension rates was highest in August and lowest in January with 4- and 2-fold increases, respectively. Repeated subsequent resuspension events gradually reduced the impact of a given resuspension event. This study shows that sediment resuspension is an important but overlooked factor for the biogeochemical function of coastal environments that needs more attention in future studies to access responses towards anthropogenic and climatic impacts.
Benthic metabolism in restored seagrass meadows – a chronosequence study using eddy covariance and benthic chambers (SS019 Benthic Metabolism and Fluxes in Shallow Coastal Ecosystems – Controls and Responses to Environmental Stressors)
Theo Kindeberg, Karl Attard, Jana Huller, Julia Muller, Cintia Quintana, Per Carlsson, Eduardo Infantes
Due to large losses of seagrass meadows, restoration is proposed as a key strategy for increasing coastal resilience and recovery. The emergence of a seagrass meadow is anticipated to substantially modify benthic metabolism through increased primary productivity and respiration. Yet, open questions remain regarding what roles benthic communities of the seagrass ecosystem play in overall metabolism and how fluxes change as a meadow develops. We utilized a chronosequence of bare sediments, adjacent Zostera marina meadows of 3 and 7 years since restoration and a natural meadow within the same bay. We combined continuous measurements of O2 fluxes using eddy covariance with dissolved inorganic carbon (DIC) and O2 fluxes from benthic chambers. Based on the ratio between O2 and DIC, we obtained site-specific photosynthetic and respiratory quotients from which we could convert eddy fluxes to continuous DIC fluxes. We assessed biodiversity parameters as potential drivers of metabolic flux variability. We observed high rates of gross primary productivity (GPP) spanning 21-96 mmol O2 m-2 d-1 between bare sediments and the oldest meadow. Community respiration (CR) mirrored the trend and all meadows displayed net heterotrophy (GPP < |CR|), which increased with meadow age. While autotrophic biomass did not increase with meadow age, species diversity did. These observations provide insights into how community composition relates to ecosystem functioning and highlights potential tradeoffs between carbon uptake and biodiversity.
