Carbonate sands are an integral part of coral reef environments but their role in the cycling of matter in the reef is understudied. Methods such as micro-profiling, core incubations, in-situ chamber incubations, and aquatic eddy covariance measurements can be used to study solute fluxes and biogeochemistry of these sediments. The interfacial fluxes can reveal production and degradation processes in the sands and thereby provide key information on the role of these sediments in the cycles of matter in the coral reef. To date, the eddy covariance technique is the most advanced method for measuring the solute fluxes noninvasively. The traditional eddy covariance method employs a solute sensor (e.g. oxygen sensors) and an Acoustic Doppler Velocimeter (ADV). We developed a triple O2-sensor-eddy covariance instrument that provides a hardware solution minimizing time-lag errors arising from the physical separation of the O2 sensor and the velocity sensor measuring volume, which affects the measurements in the traditional instrument. These errors are most pronounced in environments with currents and waves, and are difficult to correct through data processing. Oxygen flux measurements with the aquatic eddy covariance technique in benthic environments with wave action and/or highly varying current direction can lead to erroneous flux estimates due to the transient time-lag between the velocity and oxygen concentration measurements. By positioning the oxygen sensors in the same horizontal plane and with 120 degrees radial spacing around the center point where current flow is measured, synoptic oxygen measurements are recorded with opposing time-lags. Averaging of the three sensor signals at each time point produces an oxygen concentration closely corresponding to that at the flow measuring point, and thus, at a position that does not require time-lag correction. Field tests with the new instrument in a coral reef sand flat affected by waves demonstrated the advantages of the new instrument setup, and simultaneously allowed evaluation of the magnitude of errors that are associated with the traditional time-lag correction. Model simulations of the 3 sensors system in a known oscillating oxygen distribution field suggests that the new 3 oxygen sensor eddy covariance (3OEC) system can reduce time-lag error by at least five-fold. We conclude that the new system can improve oxygen flux estimates significantly, while simplifying the processing of the aquatic eddy covariance data. Another factor that may impose errors in the calculated fluxes by the aquatic eddy covariance method is the slow response of the solute sensors which may dampen the recorded dataset. To overcome this issue, we developed an instrument by which reliable and reproducible measurements of the response time of the sensors is possible. We used this method to select the sensors that we installed on our improved eddy covariance instrument. We used the improved aquatic eddy covariance instrument in a Florida coral reef sand flat to quantify benthic oxygen fluxes as proxy for benthic metabolism. The non-invasive measurements characterize the carbonate sands as sites of intensive organic matter production and consumption, and underline their dependency on key environmental drivers such as light, water current velocity, and significant wave height. The positive response to light and increasing light intensity were characterized by large temporal dynamics even at ~9 m water depth. Daytime fluxes reached 2.3 ± 2.0 (Mean ± SE) mmol m-2 h-1 and nighttime fluxes -2.0 ± 0.7 (Mean ± SE) mmol m-2 h-1. Spring deployments indicated net autotrophy of the sedimentary environment, while summer and winter measurements implied a metabolic balance. During summer, an increase in bottom currents correlated with an increase in sediment oxygen uptake during daytime and nighttime, reflecting enhanced benthic organic matter mineralization activity during the warm season. The oxygen fluxes reveal their role in the reef sands as hotspots of benthic carbon cycling. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / 2019 / November 8, 2019. / Aquatic eddy covariance, Benthic oxygen flux, Carbonate sediments, Ocean engineering, Permeable sediments, Sediment biogeochemistry / Includes bibliographical references. / Markus Huettel, Professor Directing Dissertation; Janie Wulff, University Representative; Amy Baco-Taylor, Committee Member; Peter Berg, Committee Member; Sven Kranz, Committee Member; Kevin Speer, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_752444 |
| Contributors | Merikhi, Alireza (author), Huettel, Markus (professor directing dissertation), Wulff, Janie L. (university representative), Baco-Taylor, Amy R. (Amy Rose) (committee member), Berg, Peter (committee member), Kranz, Sven Alexander (committee member), Speer, Kevin G. (Kevin George) (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Earth, Ocean, and Atmospheric Science (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (89 pages), computer, application/pdf |
Page generated in 0.0012 seconds