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Biochemistry

Macrobioerosion rates and what they mean for reefs

DeCarlo, T.M, A.L Cohen, H.C. Barkely, Q. Cobban, C. Young, K.E. Shamberger, R.E. Brainard, and Y. Golbuu (2015). Coral macrobioerosion is accelerated by ocean acidification and nutrients. GEOLOGY, v. 43(1), pp. 7-10. Doi: 10.1130/G36147

Background

Reefs support diverse ecosystems and are important for sustainable fisheries. They are found in the euphotic zone where there is enough sunlight for photosynthesis. Reefs are formed from the precipitation of calcium carbonate (CaCO3) out of the water column and by organisms like corals, coralline algae, and foraminifera. Reef structures are strengthened by CaCO3 sediment that acts as cement supplied by macrobioerosion processes. Bioerosion, coupled with chemical erosion and other physically erosive actions, also degrades reef systems. Not only does this degrading involve the dissolution and breakdown of CaCO3, but it also may put the reef out of reach of the euphotic zone, hindering the success of photosynthesis.  There is a balance between formation and loss of CaCO3 in the ocean; in the modern day oceans there is a net gain (more is formed than lost), but that can change.

The formation and demise of reef systems in the ocean is complex and influenced by multiple environmental conditions like temperature, saturation state, pH, and [CO2]. For example, the precipitation of CaCO3 in the water column is a function of the saturation state; when the saturation state is low less CaCO3 can precipitate than if the saturation state is high. The saturation state, though, is a function of the pH. When the pH is lowered the saturation state is lowered too. However, pH is function of dissolved carbon dioxide in the ocean; it is lowered by the addition of CO2 into the ocean. A small change in a single environmental parameter can result in a long chain of reactions.

It has been demonstrated in laboratory studies that ocean acidification can result in the net loss of CaCO3; however, the phenomenon is difficult to observe tropical environments. This is because the saturation state of aragonite, an amorphous mineral form of calcium carbonate used to study ocean acidification, co-varies with nutrients in those areas.  Scientists from Massachusetts and Hawaii utilized the relationship between aragonite saturation state and nutrient concentration to study their impacts on macrobioerosion rates, and how a change in ocean acidification could affect CaCO3 budget through macrobioerosion. Previous studies suggest that although macrobioerosion only accounts for a fraction of reef degradation it does occur in proportion to total bioerosion.

Method         

The main goal of the scientists was to investigate the influence of natural saturation state and nutrient gradients in the Pacific Ocean on macrobioerosion rates on coral skeletons.   Their research focused on the erosion rates of live colonies of the Indo-Pacific coral Porites spp.feb4

In the field, skeletal cores were collected via drills from 11 stations within seven reef systems in the Pacific basin (Figure 1), a region with strong gradients in aragonite saturation state and nutrient concentrations. The east and central Pacific reef sites are bathed in cold upwelled water with a low aragonite saturation state and high nutrient concentrations.   The reefs in the western-central pacific are surrounded by water with high aragonite saturation state and low nutrient concentrations.   The western most reefs has a strong aragonite saturate state gradient in low nutrient conditions; it provides the opportunity to study the relationship of aragonite saturation and macrobioerosion in the absence of nutrient influence. Samples were collected over years, months, days, at different times, to capture natural variability of the system to the best of their abilities.

In the laboratory, researchers quantified the erosion using 3-D CT scans of coral cores (Figure 2). Erosion rate was defined as the product of percent volume bioeroded and coral calcification rate.  They also used the GAMLSS model to fit percent volume bioerosion values to aragonite saturation curves to predict the sensitivity of macrobioerosion to nutrients.feb3

Results

In the cores from the western Pacific, where there is a natural aragonite saturation state gradient and low nutrient levels, scientists observed that macrobioerosion rates increase when the aragonite saturation state decreased, as had been predicted in previous laboratory studies. From the other sites’ scientists calculated that elevated nutrient levels result in an order of magnitude stronger effect of saturation state on the rate of macrobioerosion than in low nutrient conditions.feb2

Discussion and Significance

Scientists found that decreased saturation state can increase macrobioerosion rates, but even more so from local stressors, like nutrients, moving towards a shift of a net removal of CaCO3 from coral reefs more quickly.

The observation that macrobioerosion increases as saturation state decreases could be explained by changes in susceptibility to physical and chemical erosion with varying saturation states. The observation that elevated nutrient levels enhance the effect of low saturation state on accelerated macrobioerosion rations could be explained by elevated nutrient concentrations sustaining filter feeding bioeroding communities.

A little bit of bioerosion is a good thing, but when it exceeds the precipitation of CaCO3 than it will hinder the survival of reefs and could cause consequences for the change in the CaCO3 budget, the diverse ecosystems the support, and related fisheries. Anthropogenic influences like nutrient loading and CO2 emission, in addition to natural environmental changes, are all factors that must be considered when discussing why and how saturation states in the ocean could change.

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