The terrestrial and oceanic systems have long been recognized as important components of the global carbon cycle but the aquatic continuum that connects land and ocean has often been overlooked. This land-ocean interface is extremely heterogeneous in terms of lateral input, marine primary productivity, air-sea fluxes, benthic fluxes and sediment transport processes. Climate change and human perturbation has the potential to influence the carbon cycle along this continuum through changes in weathering, river discharge, vegetation, nutrients and organic matter delivery, and transformation processes, all mechanisms which can be changed or exacerbated by climate change and human activity. Within the last several years there has been an increased focus on carbon cycling at the land-ocean interface, especially on continental shelves and marginal seas. Oceanographers, wetland scientists, ecologists, biogeochemists, hydrologists, and modelers often have varied viewpoints regarding the importance of processes occurring along this land-ocean continuum and how they might change in future. The session aims to bring together these different communities working in freshwater, brackish and marine settings to discuss the advancement in our current understanding of these interconnected processes that drive carbon, nutrient and metal cycling and transport along this critical transition zone with special emphasis on sediment-water interactions on local, regional and global scales. This includes field-based, satellite-based, and numerical modeling studies focused on mechanisms and drivers of biogeochemical cycling across the sediment water interface and impact of benthic-pelagic coupling on burial, cycling and lateral transfers of carbon, nutrients and trace metals, as well as efforts that improve the representation of these dynamic and spatially complex shallow coastal environments in Earth System Models to improve our ability to forecast Earth’s response to a changing climate.
Lead Organizer: Marshall Bowles, LUMCON (mbowles@lumcon.edu)
Co-organizers:
Kanchan Maiti, LSU (kmaiti@lsu.edu)
Christof Meile, UGA (cmeile@uga.edu)
Cristina Schultz, Northeastern (c.schultz@northeastern.edu)
Presentations
09:00 AM
BIOGEOCHEMICAL MODELLING OF THE MARANO AND GRADO LAGOON (ITALY) WITH A BENTHIC-PELAGIC COUPLING (9157)
Primary Presenter: Isabella Scroccaro, National Institute of Oceanography and Applied Geophysics - OGS (iscroccaro@ogs.it)
The Marano and Grado Lagoon is a shallow coastal system of regional and international ecological importance, located in the northeastern coastal area of Italy. The impact of anthropogenic pressures related to local socio-economic activities and to climate variability, including climate change, requires knowledge and tools to understand the lagoon's trophic dynamics and to support comprehensive management strategies. We present a multiannual analysis of the Marano and Grado Lagoon biogeochemistry using observations and a high resolution coupled physical biogeochemical model, with an explicit benthic-pelagic coupling. The model was developed by coupling the transport model SHYFEM ( https://github.com/SHYFEM-model/shyfem) and the biogeochemical model BFM (Biogeochemical Flux Model, bfm-community.eu/). The biogeochemical component of the model includes the simulation of the biogeochemical processes of both water column and sediments and their coupling. Ten-year simulations of physical-chemical properties (2006-2015) and five-year simulations for biogeochemistry (2010-2014) were conducted. Results indicate that the distributions of the pelagic biogeochemical state variables are in reasonably good agreement with the observations. Also, the benthic state variables concentrations are simulated in line with the known characteristics of the Marano and Grado lagoon benthic ecosystem. Sensitivity analysis shows that the inclusion of the explicitly resolved benthic-pelagic coupling improves the model performances highlighting its importance in the biogeochemical model.
09:15 AM
Seasonal influences of POC fluxes to water column-benthic coupling in NGOM shelf (9202)
Primary Presenter: Mukseet Mahmood, Louisiana state university (mmahmo8@lsu.edu)
Continental shelves connecting the terrestrial and oceanic systems are being disrupted by climate change and human interventions with expected alterations to river discharge, carbon burial, and transport that are mostly unknown at present. In this study, we combined the benthic sedimentary carbon remineralization (bottom-up) and particulate organic carbon (POC) transport (top-down) approach to better understand the carbon cycling in Louisiana shelf which receives a significant discharge from the Mississippi River and is associated with one of the largest seasonal hypoxia in the global ocean. Oxygen (O2) consumption and dissolved inorganic carbon (DIC) production in sediment were investigated using ex-situ core incubation during May 2023 (peak river discharge) and July 2024 (low river discharge). DIC production rates varied between 16.51 to 33.84 mmol/m2/day and O2 consumption flux varied from 13.95 to 23.16 mmol /m2/day during the high river discharge in May 2023; whereas DIC flux and O2 consumption flux increased significantly during low river discharge period in July 2024. 234Th based water column POC fluxes and surface sediment C accumulation estimates reveal connections between benthic remineralization and accumulation, which appears to be more strongly correlated further from the river outflow. It is important to incorporate benthic carbon fluxes in the nGOM ocean carbon budget and to understand the mechanisms driving the organic carbon remineralization in the adjacent shelf to better constrain its impact on the regional carbon cycle and associated marine ecosystems.
09:30 AM
Mercury Cycling in Coastal Ecosystems: Insights from Stable Isotopes of Dissolved Organic Carbon in Hurricane-Impacted Ecosystems (9507)
Primary Presenter: Yener Ulus, Auburn Univesity at Montgomery (yulus@aum.edu)
Mercury (Hg) is a global pollutant due to its atmospheric transport, its conversion into neurotoxic methylmercury (MeHg), and its biomagnification in food webs. Climate-driven events like hurricanes and sea-level rise can cause salinization of coastal freshwater wetlands (FW), leading to the formation of "ghost forests" (GF) and eventual conversion to salt marshes (SM). These transitions, along with storm surges, may significantly impact Hg cycling in these ecosystems, however, the effects are poorly understood. We conducted 15 field sampling to evaluate aqueous total mercury (THg) and MeHg along a salinity gradient on the Albemarle Pamlico Peninsula, North Carolina, which experienced two major hurricanes (Florence, 2018 and Dorian, 2019). Overall, THg increased from open water (OW) to FW sites, with the lowest THg level recorded in OW (1.90±1.52 ng/L) and the highest in FW (8.02±3.44 ng/L). MeHg levels were consistent across sites, except for significantly lower concentrations in OW. Post-hurricane data showed shifts in both THg and MeHg levels in which a strong positive correlation between THg and DOC was observed at FW (p<0.001) while an inverse relationship was found between THg and chloride (p<0.001). Our results suggest that habitat transitions due to salinization and storm surges increase the likelihood of Hg methylation due to the increased supplies of sulfate from seasalt and mobility. Thus, future climate scenarios likely exacerbate Hg pollution and elevates its risk in these coastal ecosystems.
09:45 AM
SEAGRASS SEDIMENT SULFUR DYNAMICS IN RESPONSE TO GRAZING BY GREEN TURTLE HERBIVORES (9322)
Primary Presenter: Robert Johnson, University of Wisconsin-Madison (robert.a.johnson@wisc.edu)
Disturbances, both natural and anthropogenic, are important drivers of ecosystem functions and services. Sediment sulfur (S) biogeochemistry plays an important role in coastal seagrass habitats, due to the prevalence of sulfate reduction and its byproduct, dissolved hydrogen sulfide (DS). Sulfide is a phytotoxin, and DS accumulation could therefore influence the health of primary producer communities and resultant ecosystem services. As marine herbivores that consume seagrasses, green turtles may influence these dynamics. Green turtle grazing decreases seagrass metabolic rates and oxygen production, which could lead to increased sediment DS. To investigate effects of grazing on sediment S dynamics, we measured porewater DS and inorganic S pools in seagrass meadows with and without active green turtle grazing along Florida’s west coast. Neither porewater DS (range 1-502 uM) nor sediment inorganic S pools (range 1-37 umol S cm-3) differed between grazed and ungrazed seagrass areas, though both varied across sites and with sediment depth. Inorganic S pool results suggest that sedimentary characteristics may drive among-site differences in DS. Additionally, porewater DS was significantly higher in disturbed, unvegetated sediments (mean 849 uM) than in seagrass meadows (p<0.01), suggesting that grazed seagrass may still produce enough oxygen to suppress sediment DS accumulation. Understanding these types of biogeochemical responses is critical for predicting how seagrass ecosystem services will be affected by future changes in herbivory and anthropogenic disturbance.
10:00 AM
Global seagrass carbon stock variability and potential emissions from seagrass loss (9703)
Primary Presenter: Johannes Krause, Florida International University (jkrause@fiu.edu)
Seagrass ecosystems are recognized for their multitude of benefits to people and particular attention is being paid to their capacity to sequester and store soil carbon. There is large variability in seagrass soil organic carbon (Corg) stocks associated with plant traits and environmental conditions, making the scaled-up carbon storage and flux inferences needed to contribute to climate change mitigation highly challenging. Here, we provide estimates of carbon stocks associated with seagrass systems through analyses of a comprehensive global database including 2200+ seagrass soil cores. The median global soil Corg stock estimate is 25.2 (13.8 – 44.1) Mg Corg ha-1 in the top 30 cm of soil, 24% lower than previous estimates. Our estimate is robust to new data addition and refines the Tier 1 soil Corg stock for places without local data. Our models of soil Corg stock correlates allow for the calculation of Tier 2 estimates by considering species traits, bioregion and geomorphic setting. We estimate that current seagrass carbon stocks at risk of degradation could emit 1,170 Tg (659 – 1757) CO2 with a social cost of $217 billion (2020 US dollars), if no action is taken to conserve these habitats.
10:15 AM
BENTHIC NITROUS OXIDE CYCLING IN A CHANGING COASTAL SEA (9021)
Primary Presenter: Dana Hellemann, Finnish Environment Institute SYKE (Dana.hellemann@syke.fi)
Nitrous oxide (N2O) is a powerful greenhouse gas that is naturally produced and partly consumed by the microbial processes nitrification, denitrification, and nitrifier denitrification. The contribution of each process to net marine N2O production is still uncertain due to methodological constrains. This is a fundamental gap for assessing present, and even more importantly, future marine N2O cycling, assuming that each microbial process responds differently to changes in environmental conditions based on its specific metabolic requirements. Coastal sediments are potential hotspots of marine N2O cycling due to seasonally changing availability of organic matter, nutrients, and oxygen, as well as high exposure to anthropogenic pressures causing eutrophication and oxygen deficiency. Hence, advanced mechanistic knowledge on N2O cycling in coastal sediments is urgently needed. By using a state-of-the-art multidisciplinary approach that combines microsensor techniques with microbial functional gene expressions and reactive transport modelling, this project aims to disentangle the present and future contribution of the different microbial processes to net benthic N2O production in the coastal Baltic Sea, northern Europe, where increasing climate change pressures meet an ecosystem struggling with long-term eutrophication. Here, we present data on the seasonal dynamics of benthic N2O cycling at a representative site in the coastal Baltic Sea over the course of one year, covering, beyond others, benthic fluxes, benthic and pelagic concentrations, and functional gene expressions.
SS37 - Benthic-pelagic coupling along the land ocean continuum
Description
Time: 9:00 AM
Date: 31/3/2025
Room: W206A