Are Subterranean Estuaries a Source of Chromophoric Dissolved Organic Matter (CDOM) to the Coastal Ocean?: A Case Study in the Northern Gulf of Mexico
Submarine groundwater discharge (SGD) is a major pathway for nutrient transport in coastal marine systems. There are indications that SGD may also release large amounts of dissolved organic matter (DOM) to the coastal ocean and thus impact coastal ecosystem functioning. DOM is usually quantified as...
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| Format: | Others |
| Language: | English English |
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Florida State University
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| Online Access: | http://purl.flvc.org/fsu/fd/FSU_migr_etd-1496 |
| Summary: | Submarine groundwater discharge (SGD) is a major pathway for nutrient transport in coastal marine systems. There are indications that SGD may also release large amounts of dissolved organic matter (DOM) to the coastal ocean and thus impact coastal ecosystem functioning. DOM is usually quantified as dissolved organic carbon (DOC), which requires discrete sampling. Therefore, detailed time series on tidally-driven SGD cannot easily be obtained for DOC. The chromophoric component of DOM (CDOM) can be monitored via specific fluorescence in high temporal resolution and in situ. Here we hypothesize that SGD is a significant source of CDOM to the coastal ocean, impacting optical properties and biogeochemical cycles of coastal waters. In this context we also evaluated the possibility of using CDOM as a proxy for DOC in a subterranean estuary. To test our hypothesis, a case study was performed in a shallow bay in the Northern Gulf of Mexico. CDOM was continuously monitored in situ for approximately three weeks in a groundwater well on a beach and 300 m offshore in the adjacent bay. The radon isotope 222Rn was also continuously measured as a conservative tracer for submarine groundwater in the bay. Discrete samples for DOC analysis and associated variables were collected to cover one tidal cycle (15 hours). In a simple SGD model, CDOM concentrations in the bay were predicted by multiplying 222Rn concentrations in the bay (as a measure for SGD) with CDOM concentrations in the well (as the groundwater endmember). This was done for each hour during the entire sampling period. If a lag-time of one hour between groundwater and bay was considered, the predicted CDOM significantly correlated (p<0.01) with the measured CDOM in the bay. Independent statistical tests, including chlorophyll a, salinity and water level data, confirm this finding and demonstrate that CDOM in the bay is mainly driven by freshwater SGD. More detailed analysis of the time series data show that short-term time series of 24 hours or less can lead to erroneous results in estimating SGD. CDOM and DOC significantly correlated for the groundwater endmember, and CDOM could thus be transformed into DOC concentrations. It was estimated that at least 0.6 Mega-mole DOC are delivered to the entire Gulf Coast of Florida in a day via SGD which is similar in order of magnitude as riverine fluxes. === A Thesis submitted to the Department of Oceanography in partial fulfillment of the requirements for the degree of Master of
Science. === Summer Semester, 2009. === April 16, 2009. === Subterranean Estuary, CDOM, Submarine Groundwater Discharge, Chromophoric Dissolved Organic Matter === Includes bibliographical references. === Markus Huettel, Professor Directing Thesis; Thorsten Dittmar, Committee Member; William Burnett, Committee Member; Jeffrey Chanton, Committee Member. |
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