Abstract
Underwater visual monitoring methods are used broadly to evaluate coral reef conditions in the natural environment, but quantitative measurements of the coral holobiont has been largely restricted to photophysiological assessment of the endosymbionts. An underwater respirometer has been designed to make routine, diver-operated, non-invasive measurements at coral surfaces, but the realistic in-situ accuracy and precision capabilities of this device has not been critically assessed; an essential step if these measurements are to be useful for quantifying spatial and seasonal patterns of coral metabolism. We developed specific protocols for this system to survey shallow coral colonies and detect metabolic profiles (respiration, photosynthesis and biocalcification), diel cycles (day and night), and photosynthesis-irradiance curves. Analysis of data from in-situ and laboratory-controlled conditions showed good replication among coral colonies and high precision measurements of temperature, oxygen and pH fluxes over 15-minute incubation times without noticeable detrimental effects on coral health. Moreover, marked differences were observed in coral calcification rates between estuarine-influenced and coastal marine conditions, despite the absence of significant differences in visual appearance or other health indicators, revealing the system’s potential for early detection of marginally adverse conditions for coral metabolism. Its ease of operation and rapid quantification of the physiological status of the corals make this respirometer well suited for use by reef scientists, monitoring agencies, and stakeholders in biogenic reefs conservation efforts. Moreover, the high spatial and temporal resolution of these underwater respirometer data will have the potential to discriminate the effects of local stressors on coral health from those generated by broader changes associated with climate drivers.
Highlights
Warm-water coral reef systems are critical marine resources, yet their health and capacity for growth are under increasing threats from climate stressors
Our findings show that the device had no negative impact on corals, and the measured physiological rates can be combined with water column environmental data to quantify daily variations in coral metabolism
Statistical analysis was performed with IBM SPSS Statistics v26.0 (IBM Corp., Armonk, United States). Both the O2 and pH sensors were tested under temperaturecontrolled (±0.01◦C) indoor conditions in the laboratory, and achieved good precision and accuracy; the O2 sensor (PreSens) precision was ±2.33 mmol kg−1 (±1.04% saturation) in filtered (0.22 mm) seawater, and the ion-selective field effect transistor pH sensor (ISFET Durafet, Honeywell Inc.) precision was ±0.02 pH in Tris buffered seawater
Summary
Warm-water coral reef systems are critical marine resources, yet their health and capacity for growth are under increasing threats from climate stressors. Many shallow marine ecosystems will be influenced by shifting climate, the risks to coral reef systems are of particular concern due to their demonstrated sensitivity to altered environmental conditions and their central role in broadly sustaining marine biodiversity. In addition to the accumulating impacts from global change, many nearshore corals, and the urban reefs (Heery et al, 2018), are subjected to changing local stressors, including increased turbidity, nutrient loading, and as a consequence, greater dissolved oxygen fluctuations (Wong et al, 2018). Quantifying the status of coral health in present-day ocean conditions along with the early indicators of changing stress is key to understanding, forecasting, and responding to manifest effects of climate and regional anthropogenic stressors on future coral reef ecosystems (Hoegh-Guldberg et al, 2017)
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