An assessment of Eddy Correlation technique in marine habitats

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Oxygen production and oxygen flux play an important role in marine habitats and ecosystems. The eddy correlation (EC) technique is a key aquatic flux measurement technique to measure and calculate vertical turbulent fluxes within aquatic boundary layers without disturbing environmental hydrodynamics. This method is based on the simultaneous measurement of two parameters at the same point; the turbulent velocity fluctuations and oxygen concentration. This thesis explores the reliability of the Eddy Correlation Microelectrode system (ECE) and Eddy Correlation Optode system (ECO) for O₂ flux measurement in seagrass meadows and benthic organisms in the laboratory and field. Although EC has been used in aquatic system for over a decade, this system is still under development to gain robust results. According to its complexity, the EC and software used to analyse and calculate flux have been significantly enhanced over past three years. The method development and preliminary investigations were provided in Chapter 2. Generally, the oxygen concentration data from Eddy Correlation optode system (ECO) are converted from the raw oxygen signal based on the optode phase-shifted signal using Stern-Volmer-equation. Chapter 2 also showed a new oxygen conversion method based on the raw O₂ intensity signal using Stern-Volmer-equation. Seagrass sediment O₂ uptake was investigated in a laboratory flume under controlled flow and temperature. The calculations of sediment O₂ uptake using two oxygen conversion methods were compared. The result showed that the new oxygen conversion method can be used for EC O₂ flux measurements. Although this method has lower fluctuation in O₂ concentration which leads to better flux calculation, it needs to be considered whether the ECO is deployed under strong light or sunlight that may interfere with the O₂ intensity signal. The reliability and limitations of Eddy Correlation Optode system (ECO) were investigated by measuring seagrass sediment O₂ uptake in a laboratory flume under controlled flow and temperature (Chapter 3). The seagrass sediment was treated with temperature varying from 18°C to 28°C and flow velocity from 17 cm s⁻¹ to 51 cm s⁻¹. The EC data were validated by O₂ microprofiling technique, which can measure fluctuating O₂ concentrations at micro-scales. The results showed that O₂ microprofile and eddy correlation system provided the same range of O₂ flux where the O₂ consumption was observed due to microbial activities (respirations). It clearly showed that the eddy correlation systems using O₂ optode could be used for measuring O₂ flux in the marine system. Although the results of the ECO and O₂ microprofile were similar, the ECO results were not robust. This study provided verification for using eddy correlation system as a routine measurement for O₂ flux in situ. Temperature and light are important controls of seagrass metabolism (photosynthesis, enzyme activity and maintenance of the carbon balance in seagrass) which in turn governs their growth, survival, reproduction and distribution. Optimal temperature and light requirements for photosynthesis and respiration in the temperate seagrass Zostera muelleri were examined using the non-invasive Eddy Correlation microelectrode system (ECE) and Chlorophyll a fluorescence using PAM under control-flow environment (Chapter 3). The results showed that the ECE has a potential to quantify O₂ flux and O₂ production in seagrass meadows at different light and temperature conditions within a control-flow environment. Temperature and light have an effect on production, photosynthetic efficiency, photoinhibition and capacity for photoprotection in Z. muelleri. Optimal temperature and light for photosynthesis for this temperate seagrass is 25°C and 150-250 μmol photons m⁻² s⁻¹, respectively. The Eddy Correlation microelectrode system (ECE), Eddy Correlation optode system (ECO) and MiniProfiler MP4 system, an in situ microprofiler using a microelectrode, were deployed in Tweed River (New South Wales), Moreton Bay (Queensland) and Heron Island (Queensland) to determine the O₂ flux of marine and estuarine benthic systems composed of seagrasses, benthic microalgae, and microbes (Chapter 4). There were some situations in which the EC was unable to measure O₂ flux correctly. However, the EC system can be used to measure O₂ flux in the non-complex environment (steady environmental condition, homogeneous habitat and flat terrain). After the deployments and a series of detailed investigation of ECE and ECO along with consulting with the manufacturer, the first generation of Eddy Correlation optode system (ECO1) was replaced with the second generation of Eddy Correlation optode system (ECO2) in order to improve the O₂ flux measurement. Experimental results throughout the thesis provided a better understanding of the limitations and reliability of Eddy Correlation system for O₂ flux on seagrass meadows and benthic organisms in both laboratory flume and field. Although the EC has been developed with four major changes (from the first generation of the Eddy Correlation microelectrode system to the second generation of Eddy Correlation optode system), their stability and robustness still needs to be improved. Further investigation on the limitations and reliability of this new generation of the EC are needed.
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