Buffer Capacity, Ecosystem Feedbacks, and Seawater Chemistry under Global Change

Christopher Jury,Marlin Atkinson,Robert Toonen,Florence Thomas

doi:10.3390/w5031303

Abstract

Ocean acidification (OA) results in reduced seawater pH and aragonite saturation state (Ωarag), but also reduced seawater buffer capacity. As buffer capacity decreases, diel variation in seawater chemistry increases. However, a variety of ecosystem feedbacks can modulate changes in both average seawater chemistry and diel seawater chemistry variation. Here we model these effects for a coastal, reef flat ecosystem. We show that an increase in offshore pCO2 and temperature (to 900 µatm and + 3 °C) can increase diel pH variation by as much as a factor of 2.5 and can increase diel pCO2 variation by a factor of 4.6, depending on ecosystem feedbacks and seawater residence time. Importantly, these effects are different between day and night. With increasing seawater residence time and increasing feedback intensity, daytime seawater chemistry becomes more similar to present-day conditions while nighttime seawater chemistry becomes less similar to present-day conditions. Recent studies suggest that carbonate chemistry variation itself, independent of the average chemistry conditions, can have important effects on marine organisms and ecosystem processes. Better constraining ecosystem feedbacks under global change will improve projections of coastal water chemistry, but this study shows the importance of considering changes in both average carbonate chemistry and diel chemistry variation for organisms and ecosystems.

Highlights

Roger Revelle and Hans Suess long ago recognized a feedback loop whereby the ocean’s capacity to absorb additional CO2 becomes diminished the more it takes up
Model output under present-day seawater conditions is shown in Figure 1 in comparison to measurements taken on the reef flat by Shamberger et al [10], where seawater residence time varied form ~4.5–13.6 h
Our model appears to capture the majority of this variation across the three modeled seawater residence times and is generally able to recreate the dynamic changes in reef metabolism and water chemistry parameters observed over the diel cycle

Summary

Introduction

Roger Revelle and Hans Suess long ago recognized a feedback loop whereby the ocean’s capacity to absorb additional CO2 becomes diminished the more it takes up. This property of seawater chemistry is described by the Revelle factor [1,2]. A portion of this pH decrease is buffered by consuming carbonate ions (CO32−) and other bases [3], which reduces the seawater buffer capacity [4,5,6]. The removal of CO2 has opposite effects, increasing both seawater pH and buffer capacity and resulting in progressively smaller pH increases for the same given removal of CO2

Objectives

Methods

Results

Conclusion