Jacob Silverman

Effect of aragonite saturation, temperature, and nutrients on the community calcification rate of a coral reef

(1) In this study we investigated the relations between community calcification of an entire coral reef in the northern Red Sea and annual changes in temperature, aragonite saturation and nutrient loading over a two year period. Summer (April-October) and winter (November-March) average calcification rates varied between 60 ± 20 and 30 ± 20 mmolm � 2 � d � 1 , respectively. In general, calcification increased with temperature and aragonite saturation state of reef water with an apparent effect of nutrients, which is in agreement with most laboratory studies and in situ measurements of single coral growth rates. The calcification rates we measured in the reef correlated remarkably well with precipitation rates of inorganic aragonite calculated for the same temperature and degree of saturation ranges using empirical equations from the literature. This is a very significant finding considering that only a minute portion of reef calcification is inorganic. Hence, these relations could be used to predict the response of coral reefs to ocean acidification and warming.

Coral Calcification Under Ocean Acidification and Global Change

Coral reefs are unique marine ecosystems that form huge morphological structures (frameworks) in today’s oceans. These include coral islands (atolls), barrier reefs, and fringing reefs that form the most impressive products of CaCO3 biomineralization. The framework builders are mainly hermatypic corals, calcareous algae, foraminifera, and mollusks that together are responsible for almost 50% of the net annual CaCO3 precipitation in the oceans. The reef ecosystem acts as a huge filtration system that extracts plankton from the vast fluxes of ocean water that flow through the framework. The existence of these wave resistant structures in spite of chemical, biological, and physical erosion depends on their exceedingly high rates of calcification. Coral mortality due to bleaching (caused by global warming) and ocean acidification caused by atmospheric CO2 increase are now the major threats to the existence of these unique ecosystems. When the rates of dissolution and erosion become higher than the rates of precipitation, the entire coral ecosystem starts to collapse and will eventually be reduced to piles of rubble while its magnificent and high diversity fauna will vanish. The loss to nature and to humanity would be unprecedented and it may occur within the next 50 years. In this chapter, we discuss the issue of ocean acidification and its major effects of corals from the cell level to the reef communities. Based on the recently published literature, it can be generalized that calcification in corals is strongly reduced when seawater become slightly acidified. Ocean acidification lowers both the pH and the CO 3 2− ion concentration in the surface ocean, but calcification at the organism level responds mainly to CO 3 2− and not to pH. Most reports show that the symbiotic algae are not sensitive to changes in the carbonate chemistry. The potential mechanisms responsible for coral sensitivity to acidification are either direct input of seawater to the biomineralization site or high sensitivity of the enzymes involved in calcification to pH and/or CO2 concentrations. Increase in pH at the biomineralization site is most probably the most energy demanding process that is influenced by ocean acidification. While hermatypic corals and other calcifiers reduce their rates of calcification, chemical and biological dissolution increase and hence net calcification of the entire coral reef is decreasing dramatically. Community metabolism in several sites and in field enclosures show in some cases net dissolution. Using the relations between aragonite saturation (Ωarag) and community calcification, it is possible to predict that coral reefs globally may start to dissolve when atmospheric CO2 doubles.

Reversal of ocean acidification enhances net coral reef calcification

Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO32−]), and saturation state of carbonate minerals (Ω). This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies have shown that calcification rates of many organisms decrease with declining pH, [CO32−], and Ω. Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms. Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century. While retrospective studies show large-scale declines in coral, and community, calcification over recent decades, determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.

Potential influence of sea cucumbers on coral reef CaCO3 budget: A case study at One Tree Reef

To endure, coral reefs must accumulate CaCO3 at a rate greater or equal than the sum of mechanically, biologically, and chemically mediated erosion rates. We investigated the potential role of holothurians on the CaCO3 balance of a coral reef. These deposit feeders process carbonate sand and rubble through their digestive tract and dissolve CaCO3 as part of their digestive process. In aquarium incubations with Stichopus herrmanni and Holothuria leucospilota total alkalinity increased by 97 ± 13 and 47 ± 7 μmol kg−1, respectively. This increase was due to CaCO3 dissolution, 81 ± 13 and 34 ± 6 μmol kg−1 and ammonia secretion, 16 ± 2 and 14 ± 2μmol kg−1, respectively, for these species. Surveys conducted at a long-term monitoring site of community calcification (DK13) on One Tree Reef indicated that the density of sea cucumbers was approximately 1 individual m−2. We used these data and data from surveys at Shark Alley to estimate the dissolution of CaCO3 by the sea cucumbers at both sites. At DK13 the sea cucumber population was estimated to be responsible for nearly 50% of the nighttime CaCO3 dissolution, while in Shark Alley for most of the nighttime dissolution. Thus, in a healthy reef, bioeroders dissolution of CaCO3 sediment appears to be an important component of the natural CaCO3 turnover and a substantial source of alkalinity as well. This additional alkalinity could partially buffer changes in seawater pH associated with increasing atmospheric CO2 locally, thus reducing the impact of ocean acidification on coral growth.

The coral proto - free ocean carbon enrichment system (CP-FOCE): Engineering and development

Ocean acidification is driven by increasing atmospheric CO2 and represents a key threat to the Great Barrier Reef (GBR) and other coral reefs globally. Previous investigations have depended on studies in aquaria that are compromised by reduced ecological complexity and buffering capacity, and problems associated with containment. These aquaria studies also include artifacts such as artificial flow, light, temperature, and water quality conditions. In order to avoid these issues a new technology was needed for in situ science. This need was the driver behind development of the Free Ocean Carbon Enrichment (FOCE) approach. FOCE is similar in approach to the Free Air Carbon Enrichment (FACE) experiments pursued on land for almost two decades. FOCE as a systems concept was developed at the Monterey Bay Aquarium Research Institute (MBARI) to perform controlled in situ studies on the effects of increased carbon dioxide on ocean environments. FOCE systems inject carbon dioxide enriched water into the desired control volume to lower the environmental pH to a specified value.

Taking the metabolic pulse of the world's coral reefs

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.