Low and variable ecosystem calcification in a coral reef lagoon under natural acidification

Kathryn E F Shamberger,Steven J Lentz,Anne L Cohen

doi:10.1002/lno.10662

Kathryn E F Shamberger, Steven J Lentz + Show 1 more

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https://doi.org/10.1002/lno.10662

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Abstract

AbstractLaboratory‐based CO2 experiments and studies of naturally low pH coral reef ecosystems reveal negative impacts of ocean acidification on the calcifying communities that build coral reefs. Conversely, in Palau's low pH lagoons, coral cover is high, coral communities are diverse, and calcification rates of two reef‐building corals exhibit no apparent sensitivity to the strong natural gradient in pH and aragonite saturation state (Ωar). We developed two methods to quantify rates of Net Ecosystem Calcification (NEC), the ecosystem‐level balance between calcification and dissolution, in Risong Lagoon, where average daily pH is ∼ 7.9 and Ωar ∼ 2.7. While coral cover in the lagoon is within the range of other Pacific reefs (∼ 26%), NEC rates were among the lowest measured, averaging 25.9 ± 13.7 mmol m−2 d−1 over two 4 d study periods. NEC rates were highly variable, ranging from a low of 13.7 mmol m−2 d−1 in March 2012 to a high of 40.3 mmol m−2 d−1 in November 2013, despite no significant changes in temperature, salinity, inorganic nutrients, Ωar, or pH. Our results indicate that the coral reef community of Risong Lagoon produces just enough calcium carbonate to maintain net positive calcification but comes dangerously close to net zero or negative NEC (net dissolution). Identifying the factors responsible for low NEC rates as well as the drivers of NEC variability in naturally low pH reef systems are key to predicting their futures under 21st century climate change.

Highlights

Laboratory-based CO2 experiments and studies of naturally low pH coral reef ecosystems reveal negative impacts of ocean acidification on the calcifying communities that build coral reefs
Because lagoon morphology makes the application of conventional alkalinity anomaly techniques challenging, we developed two new methods to estimate coral reef Net Ecosystem Calcification (NEC) and Net Ecosystem Production (NEP) in a semi-enclosed lagoon using a combination of total alkalinity (TA), salinity, and volume budgets
Rates of Net Ecosystem Calcification in the naturally lowpH Risong Lagoon are low compared to other coral reefs worldwide, despite high coral cover and diverse coral communities that inhabit the lagoon

Summary

Introduction

Laboratory-based CO2 experiments and studies of naturally low pH coral reef ecosystems reveal negative impacts of ocean acidification on the calcifying communities that build coral reefs. Our results indicate that the coral reef community of Risong Lagoon produces just enough calcium carbonate to maintain net positive calcification but comes dangerously close to net zero or negative NEC (net dissolution). The production of biogenic CaCO3 generally decreases (e.g., Langdon et al 2000, 2003), and bioerosion (e.g., Tribollet et al 2009) and dissolution (e.g., Andersson et al 2009) generally increase as pH decreases and availability of CO232 is reduced Taken together, these findings raise concerns that coral reefs could shift from net accreting to net eroding structures within the few decades, compromising their ability to provide the ecosystem services upon which millions of people depend (Silverman et al 2009)

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