Collaborative Research: How are estuarine carbon and alkalinity dynamics influenced by macrobiota?

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The global carbon cycle consists of the processes that transform and transport carbon on Earth. Interest in the global carbon cycle stems from its intimate connection with ecological processes and its control on atmospheric carbon dioxide, an extremely important greenhouse gas. A key feature of the global carbon cycle is the transport of carbon from land to the open ocean. Before reaching the open ocean, the carbon carried by rivers must past through estuaries, where significant transformations take place, including the interconversion of organic forms (such as carbohydrates, proteins, and lipids) and inorganic forms (carbon dioxide, bicarbonate, and carbonate) through photosynthesis and respiration. In addition to carbon, alkalinity is another important chemical quantity that is relevant to climate and life on Earth. The alkalinity is the capacity of a water body to neutralize acid and determines the extent to which carbon dioxide reacts with water to create chemical species that do not interact directly with the atmosphere. While many transformations of carbon and alkalinity in the ocean are dominated by microscopic life, like phytoplankton and bacteria, these transformations are also influenced by macroscopic life (macrobiota), such as oysters, clams, salt marshes, mangroves, and seagrasses. However, macrobiota are generally ignored in conceptual and computational models of carbon transformations in estuaries. The overall objective of this project is to improve understanding of the role that macrobiota play in estuarine carbon and alkalinity dynamics. The research will also support numerous undergraduate students, two graduate students and three post-doctoral scholars.The proposed research will be carried out through a coordinated program of field measurements, laboratory experiments, historical data analysis, and numerical modeling. Two contrasting tidal tributaries of the Chesapeake Bay, the Potomac River Estuary and the York River Estuary, will be sampled because they span much of the range of carbon and alkalinity dynamics found in estuaries worldwide and hence will facilitate the generalization of the project findings. The interdisciplinary research team will evaluate four hypotheses: (1) Tidal wetlands, such as marshes and mangroves, are a source of alkalinity to estuaries and this source increases with salinity, tidal wetland productivity, and tidal range. (2) Alkalinity sinks in estuaries are favored when riverine alkalinity is high and when benthic fauna (e.g., clams and oysters) or submerged aquatic vegetation (e.g., seagrasses) are present in sufficient quantities. (3) Alkalinity sources and sinks in estuaries are highly seasonal, with summer fluxes dominated by net calcification (due to benthic fauna and submerged aquatic vegetation, an alkalinity sink) and sulfate reduction (due to tidal wetlands, an alkalinity source) and winter fluxes due to net CaCO3 dissolution (an alkalinity source). (4) Estuaries with high-alkalinity rivers and low tidal marsh areas are sinks of alkalinity and sources of atmospheric carbon dioxide while those with low-alkalinity rivers and high tidal marsh areas are sources of alkalinity and sinks of atmospheric carbon dioxide. The research plan includes seven main elements: (1) carbonate system measurements, (2) benthic fauna distribution measurements, (3) measurements of macrobiota carbon and alkalinity fluxes, (4) development of macrobiota carbon and alkalinity flux maps, (5) historical analysis of carbonate system measurements, (6) 3-D numerical modeling, and (7) a meta-analysis that extend findings to other systems. Mentoring and inclusion will occur through the development of a research affinity group of at least eight students that will connect existing regional undergraduate research programs. Students will present their research to managers and policy makers from the Chesapeake Bay Program during annual summits and we will engage estuarine managers through presentations on macrobiota influence on biogeochemistry. This research will advance the understanding of how macrobiota influence estuarine carbon and alkalinity dynamics and, ultimately, the large-scale marine cycles of carbon and alkalinity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.