Assessment of coral ecosystem community calcifier composition using trace element cycling via ICP techniques
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Wyatt-Ngom, Sokhna Aminata
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Abstract
Coral reefs are an integral part of coastal ecosystems. They protect coastlines from storms, provide habitat for 25% of all marine life and contribute to local economies through tourism and fisheries. Unfortunately, climate change-related stressors (such as increased sea surface temperatures and acidification) associated with anthropogenic emissions of fossil fuel carbon dioxide (CO2) have contributed to declines in coverage and health of coral reefs throughout the world. Without healthy coral reefs, many coastal communities lose protection, significant amounts of marine life lose their homes and entire coastal ecosystems can collapse. Therefore, it is essential to quantify the rate of this global coral reef decline is of great concern and has been made possible through measurements of variability in seawater constituents (such as precipitation rates found via sedimentation analysis) that serve as metrics of metabolism, net ecosystem productivity (NEP) and net ecosystem calcification (NEC), on reefs. These traditional measurements are limited in their ability to measure precipitation, dissolution, and calcification rates. Studies have found precipitation rates in the same area are 40% higher than previously thought from sedimentary analysis (Steiner et al. 2014;2018). More nuanced indicators of calcification dynamic on reefs (such as trace element analysis) could be key in obtaining accurate calcification rates alongside precipitation and dissolution. Knowing this information can then greatly assist in creating adaptation and mitigation strategies for reefs under m these continued stressors.
Here, I apply inductively coupled plasma¬-optical emission spectroscopy (ICP-OES) to measurements of reef seawater strontium-to-calcium (Sr/Ca) ratios from Tetiaroa Atoll, French Polynesia collected over two complimentary diel field campaigns in October 2015 and January 2016. In this study, we look at the differences and dominance of marine organisms made up of calcite or aragonite. Calcifiers made from calcite act as “glue” for coral skeletal structures, are more soluble in acidic conditions and have a partition coefficient (KD) at or around 0.35. Calcifiers made of aragonite become the foundation for habitats on a reef, are less soluble in acidic conditions and have a partition coefficient (KD) at or around 1.02.
The next step is to apply a Rayleigh Mixing model to decompose the observed temporal variability in Sr/Ca ratios into net ecosystem partition coefficients (KD) that characterize the percent contributions of calcite and aragonite to hourly-to-daily gradients in calcification This measurement is of importance as it can give us a baseline for calcifier community dynamic within a reef, that assist in the monitoring of the reef in a time of warming and acidifying oceans. Additionally, establishing this technique in a relatively pristine reef (like Tetiaroa) allows for its calibration- for future applications in more degraded reefs- ultimately expanding our toolkit for conservation efforts.
Primary results include: 1. ICP-OES captures reproducible variability in Sr/Ca seawater ratios on par with previously published mass spectrometry techniques having RSD values of at or below 0.1 mmol/mol.
2. The temporal variability in Tetiaroa seawater Sr/Ca ratios may be seasonally influenced. Diel (24 hour) variability in data from October 2015 have a broader a range of 0.155 mmol/mol when compared to January of 2016 with range of 0.031 mmol/mol.
3. KD values found based on observed temporal variability can give great insight on calcification dynamics on seasonal timescales and implies that while corals remain the dominant calcifier throughout the seasons, crustose coralline algae may play more of a role in NEC in January (winter) in Tetiaroa Atoll, French Polynesia.
The overall results of this study suggest Sr/Ca in seawater is a promising proxy for monitoring reef calcification and community composition within rapidly warming and acidifying oceans. Further methodological advances in the development of this proxy may be made possible through the pursuit of high resolution and high precision mass spectrometry techniques.
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2024-04-29
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