Fig 1.
The dominant metabolic processes on coral reefs and their influence on seawater total alkalinity (TA), dissolved inorganic carbon (DIC), and pH.
(A) The organic carbon cycle (NCP) is dominated by photosynthesis and respiration, which take up or release 1 mole of DIC for every mole of organic carbon (CH2O) produced or decomposed with little influence on seawater TA. In contrast, the inorganic carbon cycle (NCC) is dominated by CaCO3 precipitation and dissolution, which alter TA and DIC in a ratio of 2:1 for every mole of CaCO3 precipitated or dissolved. Photo credit: Yuna Zayasu, OIST. (B) Depending on the relative contribution from different metabolic processes, the resulting change in TA and DIC influences seawater pH differently (colored contours). Photosynthesis and CaCO3 dissolution increase seawater pH while respiration and CaCO3 precipitation decrease pH. If NCP and NCC are closely balanced (i.e., TA-DIC slope ~1), there is little change in seawater pH owing to net reef metabolism. This is because the slope of pH isolines within the normal oceanic concentration of seawater TA and DIC are close to 1. Therefore, when the slope of the TA-DIC vector is different from 1 the pH isolines are crossed and seawater pH can be altered considerably. The calculations for pH at each TA and DIC value assume constant temperature (25°C) and salinity (35). (C) Conceptual schematic of the biogeochemical and metabolic function of coral reefs. Net CaCO3 precipitation (+NCC, green area) vs. net dissolution (-NCC, pink/red area); net autotrophy (+NCP) vs. net heterotrophy (-NCP), and different TA-DIC slopes, as well as the resulting changes in reef seawater pH (pHr) relative to the open ocean (pHo) under constant salinity and temperature conditions.
Fig 2.
Seawater TA and DIC data from 23 coral reef locations around the world in increasing order of TA-DIC slope.
The black line is the Type II linear regression of the data. Red lines are pH isolines at 0.1 units increments with the 8.0 pH isoline indicated for reference. pH contours were calculated using the average seawater temperature and salinity of each dataset. The blue dashed line in each panel is the approximate TA of the offshore seawater determined as described in the text.
Fig 3.
Net calcification potential measured as anomalies between open ocean and coral reef TA concentrations (ΔTA) at each location.
Negative values are lower TA concentrations within the reef and represent net CaCO3 precipitation (+NCC), while positive values are higher TA concentrations within the reef and represent net CaCO3 dissolution (-NCC). Edges of the box are the 25th and 75th percentiles, the line within each box is the median, the whiskers represent the most extreme data points that are not outliers, and the red + symbols are outliers.
Fig 4.
The influence of spatial and temporal sampling scales on TA-DIC slopes.
(A) The TA-DIC slopes from the different coral reef locations calculated using a Type II linear regression (see also S1 Table). Error bars are ±1 SD of the slope. The colors indicate whether the sampling protocol had a significant spatial component (>10 km2; blue) or was predominantly sampled over time at one location (orange). (B) Comparison of the average TA-DIC slope (± standard deviation) of all temporally and spatially sampled reefs. (C) TA-DIC slopes as a function of the duration of each study in days grouped by spatial (blue) and temporal (organge) sampling protocols.