Benthic processes significantly influence seawater chemistry, with benthic alkalinity production playing a crucial role in the oceanic alkalinity cycle, representing an important pathway for net carbon sequestration. However, direct measurements of benthic alkalinity fluxes are still scarce due to technical difficulties. It is also difficult to relate direct measurements of seawater carbonate chemistry to benthic alkalinity flux without accounting for hydrodynamics. Additionally, sediment diagenetic reactions can be very dynamic in coastal environments, leading to temporal and spatial variations in benthic alkalinity fluxes. From a modeling perspective, region-specific models that fully incorporate variabilities of coastal environments are lacking. Therefore, sediment-water alkalinity fluxes in models remain poorly constrained.
This session aims to enhance the understanding of benthic alkalinity production, from its spatial and temporal variations to its role in regional to oceanic alkalinity cycle and carbon sequestration. We invite contributions ranging from experiments to modeling (including oxygen, carbon, nitrogen, sulfur, iron, etc.) in various environments from estuaries to the open ocean.
Lead Organizer: Hang Yin, Texas A&M University-Corpus Christi (hangyin.phd@gmail.com)
Co-organizers:
Xinping Hu, Texas A&M University-Corpus Christi (Xinping.Hu@tamucc.edu)
Presentations
06:00 PM
BACK TO GRASS ROOTS: ASSESSING SEAGRASS PRODUCTIVITY AND INDUCED SEAWATER CARBONATE CHEMISTRY CHANGES IN FLORIDA BAY (9000)
Primary Presenter: Carisa MacPherson, Georgia Southern University (cm45101@georgiasouthern.edu)
Global climate change is increasing largely due to anthropogenic CO2 emissions. Ocean acidification is the reduction in seawater pH due to the uptake of excess atmospheric CO2, negatively impacting calcifying organisms. Harnessing natural changes in seawater chemistry has the potential to mitigate ocean acidification and improve the calcification processes. One approach is natural ecosystem restoration, which can use photosynthesizing flora like seagrass to capture CO2, mitigating the effects of acidification. Florida Bay is an optimal location to investigate these changes as seagrass beds primarily of Thalassia testudinum and Syringodium filiforme, make them a strong source of seawater chemistry modification via primary productivity. This work addresses research gaps and updates carbonate parameters of Florida Bay, through the following research question: To what extent does seagrass alter the carbonate chemistry in the seawater within Florida Bay? The carbonate chemistry of discrete seawater samples collected Jan/July 2024, filtered, and preserved for chemical analysis were analyzed from 40 sites within 3 basins of Florida Bay. Preliminary results indicate significant differences in total alkalinity, dissolved inorganic carbon, pH, and aragonite saturation between ground and surface water in the three basins. Future studies are planned to build on these findings to understand if seagrass coupled with enhanced coastal weathering of alkaline minerals can “supercharge” seagrass production and carbon capture while locally ameliorating ocean acidification.
06:00 PM
Anaerobic alkalinity and dissolved inorganic carbon production in the Northern Gulf of Mexico hypoxic waters (9344)
Primary Presenter: Hang Yin, The University of Texas at Austin (hangyin.phd@gmail.com)
Seawater total alkalinity (TA) and dissolved inorganic carbon (DIC) are influenced by various biogeochemical processes, such as aerobic and anaerobic remineralization, as well as carbonate precipitation and dissolution. During the summer months, bottom water hypoxia, i.e., dissolved oxygen concentration less than 2 mg L-1, has been observed in the northern Gulf of Mexico (nGoM). Using water samples collected from the nGoM between 2006 and 2019, we attempted to examine TA and DIC production from anaerobic processes, by applying a three end-member mixing model. The TA and DIC production signals were determined by calculating the difference between the observed and mixing-expected values, with corrections for photosynthesis and aerobic respiration based on dissolved oxygen changes. We found detectable TA and DIC in anoxic waters, with maximum anaerobic contributions of up to 180 and 210 µmol kg-1, respectively. In the meantime, aerobic oxidation contributes to an increase in DIC by 80-140 µmol kg⁻¹ and a decrease in TA by approximately 20 µmol kg⁻¹. This study underscores the significance of TA and DIC production in hypoxic waters of the nGoM and offers a preliminary quantification of the various biogeochemical factors influencing carbonate chemistry.
06:00 PM
A Biogeochemical Perspective on Acidification and Buffering Capacity in the Piscataqua Estuary (9409)
Primary Presenter: Sanjana Varanasi, University of New Hampshire (sanjana.varanasi@unh.edu)
Coastal ecosystems are particularly vulnerable to the impacts of ocean acidification, and our understanding of the carbon system in these environments requires further exploration. The buffering capacity, the ocean’s ability to resist changes in pH, can be altered by both physical and biogeochemical processes. The correlation between changes in four biogeochemical processes (i.e., sulfate reduction, nitrification, denitrification, carbonate dissolution) and changes in seawater carbon system parameters (i.e., pH, dissolved inorganic carbon, and total alkalinity) was investigated. These biogeochemical processes are hypothesized to be significant drivers of alterations to the seawater carbon chemistry in coastal regions, such as the Piscataqua Estuary. Two experiments were conducted, one month apart, to examine temporal variation. Sediment cores were collected at the UNH Marine Research Pier in New Castle, NH, which is near the water intake site for UNH’s Coastal Marine Laboratory (CML). The sediment cores were prepared at CML and incubated until oxygen concentrations in the overlying water declined to slightly above 3 mg/L. This approach excludes the influence of physical mixing (e.g., river-ocean mixing), a primary driver in altering the buffering capacity, to focus solely on biogeochemical processes. Gaining insight into the complex feedback loops associated with benthic biogeochemical processes and the carbon system parameters will facilitate more accurate predictions of how species respond to the effects of ocean acidification.
06:00 PM
Hydrological control on benthic alkalinity flux in subtropical estuaries (9599)
Primary Presenter: Xinping Hu, University of Texas at Austin (xinping.hu@austin.utexas.edu)
Estuaries are hotspots for organic carbon remineralization and deposition. Organic carbon burial in estuarine sediments is heavily dependent on hydrological conditions, as rivers deliver both terrestrially produced organic carbon (allochthonous) and nutrients, which drive estuarine productivity (autochthonous). In three subtropical estuaries in the northwestern Gulf of Mexico, with varying hydrological conditions, the burial of organic carbon is examined in the context of reduced sulfur preservation, a product of anaerobic respiration of organic carbon, through which alkalinity is generated. This study will present results on the accumulation of pyrite sulfur in these different environments and discuss the implications of the hydrological cycle on benthic alkalinity production in estuarine environments.
06:00 PM
Evaluating the physical and biogeochemical drivers of a reef community carbonate budget (9751)
Primary Presenter: Shalimar Moreno, East Carolina University (morenosh20@students.ecu.edu)
Coral reef carbonate budgets are an important health metric that can indicate changes in community assemblages and reef complexity. In the Atlantic, a region-wide decline in coral cover and reef complexity has resulted in decreased carbonate production. This study investigated the carbonate budget of a Bermuda coral reef system within their northernmost extent in the Atlantic. Hydrochemistry and census-based approaches were used to quantify carbonate budgets. The water chemistry was evaluated using Eulerian flow respirometry to assess changes in total alkalinity between two fixed locations of known coral assemblages to estimate the net ecosystem calcification (NEC). For the census-based approach, the benthic community was quantified using orthophoto mosaics derived from video surveys. In this presentation, we compare the two approaches for measuring carbonate cycling and note the challenges in determining water residence time. These chemistry techniques are valuable in capturing temporal variability in NEC such as seasonality or changes due to disturbance events which may shift the coral community to a non-framework building taxa, reducing calcification rates. Bermuda’s reef system experiences higher seasonal variability in NEC and a reduced aragonite saturation state that limits the conditions for CaCO3 precipitation compared to tropical regions. Studying the interactions and feedback between coral biogeochemical processes and oceanic carbonate chemistry changes is challenging but crucial to detect and predict carbon cycling changes in response to future climate change.
SS13P - Benthic Alkalinity Production Across the Land-Ocean Aquatic Continuum: Experiments, Modeling, Challenges, and New Perspectives
Description
Time: 6:00 PM
Date: 29/3/2025
Room: Exhibit Hall A