Unlike the open ocean which exhibits a clear monotonic trend in acidification and deoxygenation, long-term pH and O2 changes in estuaries and coastal systems are complicated due to a multitude of global and regional drivers. Despite a clear link between nutrient enrichment and hypoxia, the response of coastal eutrophication to a variety of diverse drivers requires improved understanding. In large stratified systems with strong benthic-pelagic coupling and where physical processes regulate oxygen supply to bottom waters, the response of hypoxia to nutrient load is often complex and nonlinear. In addition, climate change, such as warming and changing river flows, may alter key baselines impacting the functioning of coastal systems and their responses to eutrophication. Similarly, coastal pH has shown diverse long-term trends and large spatial variability, featuring acidification in some regions but basification in others. While rising atmospheric p CO2 and respiration of organic material decrease pH and weaken the buffer capacity, a number of other processes act to increase pH, including phytoplankton photosynthesis, human-accelerated chemical weathering and alkalinization in rivers, and export of alkalinity and calcium carbonate from seagrass, salt marsh and other ecosystem components. Discerning how these global and regional drivers affect the dissolved oxygen dynamics and carbonate chemistry is critical for understanding the complex long-term pH and O 2 trends in estuarine and coastal systems and developing appropriate management and adaptation strategies. In this session we solicit contributions that address the long-term pH and O 2 changes in estuaries and coastal oceans. We welcome theoretical studies that advance our conceptual understanding of the global and regional drivers of the dissolved oxygen dynamics and carbonate chemistry in coastal waters. We also welcome retrospective analysis of long-term monitoring data to distill the effects of climate change, eutrophication and land-use changes in driving the long-term trends in pH and O 2 . Finally, we encourage submission of modeling studies that investigate how warming, changing precipitation and draught patterns, storms, rising atmospheric CO 2 , river alkalinization, and export of alkalinity and organic matters from seagrass, marsh and other ecosystem components affect the dissolved oxygen and carbonate chemistry in different estuarine and coastal systems.
Lead Organizer: Ming Li, University of Maryland Center for Environmental Science (mingli@umces.edu)
Co-organizers:
Wei-Jun Cai, University of Delaware (wcai@udel.edu)
Jacob Carstensen, Aarhus University (jac@ecos.au.dk)
Hans Paerl, University of North Carolina at Chapel Hill (hans_paerl@unc.edu)
Jeremy Testa, University of Maryland Center for Environmental Science (jtesta@umces.edu)
Presentations
06:30 PM
Coastal ocean acidification variability in the Northwestern Mediterranean Sea (5308)
Primary Presenter: Maribel García-Ibáñez, Institut de Ciències del Mar - CSIC (maribel.garcia.ibanez@gmail.com)
Coastal regions exhibit a wide range of pH changes resulting from multiple factors with high spatiotemporal variability, hindering the detection of anthropogenically-driven ocean acidification signals. In this work, we evaluate the seasonal and long-term ocean acidification changes and their drivers in surface waters of two coastal time-series in the northwestern Mediterranean Sea: L'Estartit Oceanographic Station (EOS; 42.05N 3.2542E) and Blanes Bay Microbial Observatory (BBMO; 41.665N 2.805E). These two coastal time-series are located off the coast of Girona (Spain) ~60 km apart, being EOS farther from the coast than BBMO (~3.5 km and ~850 m, respectively), with the former being by a natural reserve and the latter outside a port area; therefore land-ocean interactions and anthropogenic activities should lead to higher variability at BBMO than at EOS. In these two time-series stations, measurements of total alkalinity (TA) and pH and associated variables, such as inorganic nutrients, temperature and salinity, have been performed monthly since 2010. We also assess the impact of distance to land and monitoring frequency on the detection of anthropogenically-driven ocean acidification signals in coastal time-series by comparing our results with those from the French coastal time-series station Point B (43.686N 7.316E; ~400 m from the coast of Villefranche-sur-Mer), also in the northwestern Mediterranean Sea and influenced by the same surface currents as EOS and BBMO, but where total dissolved inorganic carbon and TA are measured weekly.
06:30 PM
Alkalinity-driven changes of Ocean Acidification in the coastal zone (6626)
Primary Presenter: Karol Kulinski, Institute of Oceanology PAS (kroll@iopan.gda.pl)
Increasing atmospheric CO2 concentrations lead to an overall increase of CO2 concentrations in surface seawater and, consequently, a pH decrease – a mechanism called Ocean Acidification (OA). OA is well understood and traceable in the open ocean, where large-scale projects and actions supply an enormous amount of observations and experimental data and where the magnitude of OA is to a large extent thermodynamically consistent with the increase in atmospheric pCO2. In the coastal and shelf seas, OA is still a significantly understudied phenomenon despite their high socio-economic importance and potentially great vulnerability to acidification due to often lower salinity and corresponding lower buffer capacity of waters as compared to the open ocean. In the present study, we underline the importance of total alkalinity (TA) changes as the key factor shaping the OA dynamics, pH fields, and the variability in the aragonite saturation state in the coastal and shelf seas. The research extends from the brackish Baltic Sea to the Spitsbergen fjords affected by the high inflow of meltwaters. It revealed high variability in the marine CO2 system structure and significantly different effects of freshwater input to the investigated regions, from dilution to alkalinization. The observed overall spatial and temporal variability in TA extended in the broad range between <350 and 4,320 μmol kg-1. This makes TA a fundamental variable for studying the large-scale pH and pCO2 changes and forecasting the development of OA in the coastal zone in the future high-CO2 world.
06:30 PM
Dissolved oxygen dynamics in the deep hypolimnion of Lake Geneva during an exceptionally cold winter (6857)
Primary Presenter: Congrong Yu, Hohai University (congrongyu@126.com)
Hypoxic layers in the hypolimnion of lakes are regions where Dissolved Oxygen (DO) is < 4 mg/L, and most life cannot be sustained. In Lake Geneva, the deep hypolimnion can become hypoxic. However, the hydrodynamics of the deep hypolimnion are not well understood, since at present, they are mainly based on data from a single monitoring station located in the deepest part of the lake (309 m; SHL2; Figure 1). Lake Geneva is composed of two basins: the narrow, shallow Petit Lac and the large, deep Grand Lac. During cold winters, the Petit Lac cools much faster and its dense, oxygen rich, cold waters flow into the deep hypolimnion of the Grand Lac, which allows re-oxygenation of its oxygen poor waters. In order to quantify the amount of oxygen flowing into the Grand Lac, the three-dimensional MITgcm model was calibrated to simulate dissolved oxygen dynamics during the exceptionally cold winter of 2012. For the simulation, three different lake areas were considered: (i) the whole lake, (ii) only the Grand Lac basin, and (iii) the coastal zones of the whole lake. We quantified the contribution of oxygen originating from the Petit Lac and from the coastal zones, and demonstrated that the main source of oxygen renewal in the deep hypolimnion of the Grand Lac was water that came from the Petit Lac. During winter 2012, complete water overturning due to convective cooling did not occur in the Grand Lac. Therefore, the flow of cold, oxygen rich Petit Lac waters makes a significant contribution to the oxygen budget of the deep Grand Lac hypolimnion and can thus improve the water quality of this layer. This beneficial water exchange process between the two lake basins can also occur during other cold winters.The results of this research contribute to improving future lake ecosystem investigations.
06:30 PM
Climate and human-driven Inter-annual variability of hypoxia in the northern Gulf of Mexico (7425)
Primary Presenter: Kui Wang, Zhejiang University (kwi@zju.edu.cn)
The northern Gulf of Mexico is one of the largest hypoxic zones in global coastal oceans. Although the hypoxia area has been expanding in recent years, it remains unclear how severe the hypoxia develops during past decades and what are human and climate contribution to the variation of it. Therefore, a comprehensive index is helpful to quantify the severity of the hypoxia in terms of multiple observable properties, such as volume , thickness, longitude span etc.. On the other hand, the contribution of direct and indirect factors is also helpful to be quantified for better understanding the influencing pathways and relative importance of human activities and climate change. In this study we performed a principal component analysis (PCA) of the observable characteristics of hypoxic events in the Gulf of Mexico during summer 1985-2014 and defined the first principal component as the Hypoxia Index (HI). The thickness and longitude span are decreasing, while bottom average DO, area and volume are increasing. Then the severity in terms of HI showed a slowly alleviating trend of 0.01 yr-1. Then, we attribute three direct hypoxia drivers--- residence time, stratification and solubility@(T, S), ---as climate change-originated, and one other direct hypoxia driver---respiration---as human activities-originated. Multi-regression analysis showed that these four direct drivers explained 50% of the total variation of HI. Moreover, using a coupling model in this half variation, we revealed that climate change contributes 65% and human 35 % to the nGOM hypoxia variation during 1985-2014. The quota for the four direct drivers are 8% for DO solubility, 35% for respiration, 18% for residence time, 39% for stratification, respectively. Our model suggests reducing respiration and stratification are the two major pathways for alleviating hypoxia in nGOM under the fast-changing climate background.
SS050P Disentangling Complex Long-Term pH and O2 Trends in Coastal and Estuarine Systems From Global and Regional Drivers
Description
Time: 6:30 PM
Date: 7/6/2023
Room: Mezzanine