Tropical Marine Ecology. Daniel M. Alongi

will increase in some regions of the wet tropics and decrease the dry tropics, and (iii) salinity will change in tandem with changes in precipitation.

      Rising atmospheric CO₂ and climate change are associated with shifts in temperature, circulation, stratification, nutrient input, oxygen content, and ocean acidification, with potentially wide‐ranging effects on the biology and ecology of marine organisms and their communities (Chapter 13).

      2.7.1 Rising Atmospheric CO₂

      Mean atmospheric CO₂ concentrations will increase to 441 ppm over the 2081–2100 period (Church et al. 2013; Collins et al. 2013). This projected increase is the net result of complex atmospheric, land, and ocean forces. While tropical atmospheric CO₂ concentrations are rising, they respond to climatic events, such as El Niño. The tropical Pacific Ocean, for instance, plays an important role in modulating changes in atmospheric CO₂ concentrations during El Niño events. ENSO is correlated with large interannual variability in global atmospheric CO₂ concentrations. During the 2015–2016 ENSO event, there was a negative CO₂ change during the development phase (spring–summer 2015) which was likely due to a reduction in local CO₂ outgassing from the tropical Pacific Ocean; a positive CO₂ change was measured during the mature phase (autumn 2015–2016), likely a reflection of an increase in atmospheric CO₂ concentrations due to a combination of reduced vegetation uptake across pan‐tropical regions and enhanced biomass burning in Southeast Asia (Chatterjee et al. 2017). Thus, although atmospheric CO₂ levels are increasing, there are still notable variations and oscillations over time and space and probably explain some of the observed variability in pCO₂ concentrations in the world ocean.

      2.7.2 Ocean Acidification

The global ocean has a large capacity to absorb atmospheric CO₂ because CO₂ dissolves and reacts with seawater to form bicarbonate (HCO₃⁻) ions and protons (H⁺). Between one‐quarter and one‐third of the CO₂ emitted into the atmosphere from the burning of fossil fuels, cement manufacturing and land‐use changes have been absorbed by the ocean (Kleypas and Langdon 2006; Turley and Findlay 2016). Over thousands of years, the changes in pH have been buffered by bases such as carbonate ions (CO₃²⁻). However, the rate at which CO₂ is currently being absorbed is too rapid to be buffered sufficiently to prevent substantial changes in ocean pH. Consequently, the relative seawater concentrations of CO₂, HCO₃⁻, CO₃²⁻ and pH have been altered. Since the Industrial Revolution, ocean pH has decreased globally by 0.1 unit (Figure 2.10 bottom), and it is predicted that ocean pH will decline by 0.4–0.5 unit by 2100 (Kleypas and Langdon 2006). It will take tens of thousands of years for these changes in ocean chemistry to be buffered through neutralization by carbonate sediments, and the level at which the ocean pH will eventually stabilize will be lower than it currently is. pH is sensitive to changes in salinity and total alkalinity and DIC concentrations, so organisms in coastal waters receiving river inputs are ordinarily exposed to greater variability in pH than organisms in the open ocean.

      CO₃²⁻ concentration directly influences the saturation, and consequently, the rate of dissolution of calcium carbonate (CaCO₃) minerals in the ocean. Laboratory experiments and field observations indicate that ocean acidification is a threat to the survival of many marine organisms, especially organisms that use CaCO₃ to produce shells, tests, and skeletons, such as corals (Andersson and Glenhill 2013). A shift in pH alters the saturation state of CaCO₃ (called the ‘aragonite saturation state’) in seawater. The saturation state is expressed as:

      where K_sp^* is the solubility product for CaCO₃ (as the mineral aragonite, arg) and [Ca²⁺] and [CO₃²⁻] are the calcium and carbonate concentrations, respectively. When Ω_arg > 1, seawater is supersaturated with respect to mineral CaCO₃ and the larger this value the more suitable the environment will be for organisms that produce CaCO₃ shells and skeletons. When Ω_arg < 1, seawater is undersaturated and is corrosive to CaCO₃. When pH decreases, Ω_arg decreases. The surface waters of the tropical oceans are currently supersaturated with respect to aragonite (mean Ω_arg = 4.0 ± 0.2). However, Ω_arg steadily declined from a calculated 4.6 ± 0.2 one hundred years ago and is expected to continue declining to 2.8 ± 0.2 by 2100 (Kleypas and Langdon 2006). The aragonite saturation state (Ω_arg) is sensitive to the partial pressure of CO₂ above the ocean as well as ocean temperature. In the latter case, the aragonite saturation of warm tropical oceans is higher than that of polar oceans because CO₂ is more soluble in cold water. As atmospheric CO₂ has increased, Ω_arg of the world's oceans has decreased.

      The acidification of the world's oceans is a complex process that is only now being understood. In the western tropical Pacific Warm Pool, acidification is proceeding, with declines in pH (−0.0013 ± 0.0001 a⁻¹) and Ω_arg (−0.0083 ± 0.0007 a⁻¹) over the 1986–2016 period (Ishii et al. 2020). Oceanographic modelling indicates that the acidification of the Warm Pool occurs primarily through uptake of anthropogenic CO₂ in the extra tropics which is then transported to the tropics through the Equatorial Undercurrent from below (Ishii et al. 2020). The rate of Warm Pool acidification can be expected to be modulated by the contribution of a long interior residence time (years to decades) acting in concert with accelerating CO₂ increases in the atmosphere.

Acidification is an even more complex process in estuarine and coastal environments as carbonate chemistry is strongly regulated by changes in biological activity related to eutrophication and the delivery of nutrients by rivers and groundwater (Wallace et al. 2014). The increased loading of nutrients into estuaries and shallow inshore waters causes the accumulation of algal biomass and subsequent decomposition of this organic material, decreasing dissolved O₂ levels and contributing towards hypoxia. Hypoxia increases pCO₂ concentrations and upwelling processes can bring CO₂‐enriched water in contact with coastal waters, amplifying the effects of acidification. Land‐use changes such as deforestation and fossil fuel combustion also produce increased dissociation products of strong acids (HNO₃ and H₂SO₄) and bases (NH₃) in coastal waters, causing decreases in surface water alkalinity, pH, and DIC. River discharge further reduces alkalinity as river waters, especially in the tropics, are typically more acidic than receiving coastal waters. Further, the decomposition of organic matter, especially in bottom sediments and wetland soils, can reduce pH and alter carbonate chemistry, producing an increase in pCO₂ mostly due to microbial respiration. Nearly all estuarine and nearshore waters in the tropics naturally exhibit wide variations in salinity, pH, and carbonate chemical parameters, especially pCO₂ and [CO₃²⁻] (Alongi 2020). Carbonate chemistry in the coastal zone thus responds more strongly to eutrophication than ocean acidification (Borges and Gypens, 2010).

      2.7.3 Rising Temperatures, Increased Storms, Extreme Weather Events, and Changes in Precipitation

      The rate of ocean heating has increased within the 0–700 m layer by 3.22 ± 1.61 ZJ (ZJ = 10²¹ joules) from 1969 to 1993 and 6.28 ± 0.48 ZJ from 1993 to 2017, representing a twofold increase in heat uptake (Bindoff et al. 2019). This has resulted in temperature rises as noted earlier. Warming has also strengthened vertical stratification, inhibiting exchange between surface and deep ocean waters. Redistribution of heat accounts for 65% of heat storage at low latitudes. Tropical warming results from the interplay between increased stratification and equatorward heat transport by the subtropical gyres, which redistributes heat from the subtropics