Physical and Biological Controls on the Carbonate Chemistry of Coral Reef Waters: Effects of Metabolism, Wave Forcing, Sea Level, and Geomorphology

<div><p>We present a three-dimensional hydrodynamic-biogeochemical model of a wave-driven coral-reef lagoon system using the circulation model ROMS (Regional Ocean Modeling System) coupled with the wave transformation model SWAN (Simulating WAves Nearshore). Simulations were used to explore the sensitivity of water column carbonate chemistry across the reef system to variations in benthic reef metabolism, wave forcing, sea level, and system geomorphology. Our results show that changes in reef-water carbonate chemistry depend primarily on the ratio of benthic metabolism to the square root of the onshore wave energy flux as well as on the length and depth of the reef flat; however, they are only weakly dependent on channel geometry and the total frictional resistance of the reef system. Diurnal variations in <i>p</i>CO₂, pH, and aragonite saturation state (Ω_ar) are primarily dependent on changes in net production and are relatively insensitive to changes in net calcification; however, net changes in <i>p</i>CO₂, pH, and Ω_ar are more strongly influenced by net calcification when averaged over 24 hours. We also demonstrate that a relatively simple one-dimensional analytical model can provide a good description of the functional dependence of reef-water carbonate chemistry on benthic metabolism, wave forcing, sea level, reef flat morphology, and total system frictional resistance. Importantly, our results indicate that any long-term (weeks to months) net offsets in reef-water <i>p</i>CO₂ relative to offshore values should be modest for reef systems with narrow and/or deep lagoons. Thus, the long-term evolution of water column <i>p</i>CO₂ in many reef environments remains intimately connected to the regional-scale oceanography of offshore waters and hence directly influenced by rapid anthropogenically driven increases in <i>p</i>CO₂.</p></div

Effect of the ratio of metabolic forcing to wave forcing.

<p>Amplitude of diurnal <i>p</i>CO₂ variation at the backreef versus assuming (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053303#pone.0053303.e157" target="_blank">Eq. 19</a>). All other variables not listed in the legend were equal to those of the central case (‘CC’) except for <i>G</i>_net which was set equal to 20% of <i>P</i> in all simulations shown. The solid line represents the best-fit linear regression in the form of <i>y</i> = 0.16<i>x</i>+4.5, <i>r</i>² = 0.99, <i>n</i> = 9.</p

Changes in <i>p</i>CO₂ versus lagoon volume.

<p>Amplitude of diurnal <i>p</i>CO₂ variation (closed circles, left <i>y</i>-axis) and net offset in the <i>p</i>CO₂ of lagoon waters relative to offshore waters over a 24-hour period (open diamonds, right <i>y</i>-axis) versus lagoon volume per width of reef flat .</p

Effect of reef flat geometry.

<p>Amplitude of diurnal <i>p</i>CO₂ variation at the backreef versus based on simulations varying reef flat depth, reef flat width, and sea level. All other variables not listed in the legend were equal to those of the central case (‘CC’). The solid line represents the best-fit linear regression in the form of <i>y</i> = 3.8<i>x</i>+10.6, <i>r</i>² = 0.96, <i>n</i> = 11. The circle with the cross represents the shallowest reef flat simulation ( = 0.5 m) and was not included in the regression.</p

Daily rates of reef carbon metabolism worldwide.

<p>Daily rates of reef carbon metabolism worldwide.</p

Bathymetry and benthic metabolism for the central model case.

<p>(A) Bathymetry and (B) rate of hourly metabolism (<i>p</i>, <i>r</i>, <i>g</i>_net) as a percentage of the maximum rate for the entire reef system. In panel B the forereef and backreef transects defined in later analyses are indicated by the heavy black and green lines, respectively; while the lagoon zone defined in later analyses is indicated by the red box. The values shown along the <i>x</i> and <i>y</i> axes are only to illustrate the scale of the model domain.</p

Depth-dependence of bottom friction coefficient.

<p>Reef flat bottom friction coefficient versus the average depth of the reef flat.</p

Schematics of the coastal reef-lagoon system.

<p>(A) side-view depth profile of the reef flat and (B) top-view of part of the domain containing sections the forereef, reef flat, lagoon and channel. The ratio of vertical to horizontal scale in (A) is 75∶1. The main reef structures in (B) are outlined by the 1.5-m, 3-m, and 6-m isobaths. Dark grey regions represent land or solid reef basement. Light grey arrows represent the general direction of wave-driven circulation. The reference origin set at the intersection of the reef crest line and mid-channel is provided only to illustrate the scale of the reef features. All morphological dimensions shown reflect those of the central model case; however, an extra +1 m of sea level has been added to (A) to better illustrate profiles of reef flat depth and cross-reef setup. See Background and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053303#pone-0053303-t004" target="_blank">Table 4</a> for additional description of all variables shown.</p

Spatial variation in carbonate chemistry at mid-day.

<p>Changes in A) dissolved inorganic carbon, B) total alkalinity, C) pH_T, and D) water column <i>p</i>CO₂ relative to offshore values as well as E) spatial variation in aragonite saturation state at mid-day for the central case.</p

Weak and strong variation in <i>p</i>CO₂.

<p>Amplitude of diurnal <i>p</i>CO₂ variation for simulations based on the following variations in offshore wave height and reef flat length: A) <i>H</i>₀ = 3 m, B) <i>L</i>_r = 250 m, C) <i>H</i>₀ = 0.5 m, and D) <i>L</i>_r = 1000 m. Simulations shown in A and B represent conditions resulting in relatively weak variations in carbonate chemistry while simulations C and D represent conditions resulting in relatively strong variations in carbonate chemistry.</p