The Problem

Many ocean organisms with hard shells, or skeletons, construct these structures by the process of calcification. Calcification is defined as the precipitation of calcium-rich minerals over an organic matrix. The rate that organisms calcify relates to how fast they grow. Calcification is a process under biological control, but it is also affected by environmental factors such as availability of calcium, temperature, pH, etc...

The 2007 IPCC report suggests a continued increase in atmospheric carbon dioxide levels over the past decade. Increasing atmospheric carbon dioxide levels results in increased CO2 dissolving into the oceans, increasing the formation of carbonic acid, which in turn decreases the buffering ability of the oceans. Ocean acidification has been suggested to be detrimental to growth in stony corals by increasing solubility of calcium carbonate in the surrounding water.

Previous Studies

A 16-year study of declining coral calcification near the northern Great Barrier Reef was performed by the Australian Institute of Marine Science and published in 2008. The study found that calcification rates declined by approximately 21%. Skeletal density declined by ~6%. The published research states that a decline of coral calcification to this magnitude is unprecedented in recent centuries.

A study was done at Columbia University on of the effects of lowered pH on coral calcification. HCl was added to normal seawater. The results showed that corals growing in seawater at a reduced pH of 7.2 calcified at half the rate of corals in a control pH 8.0. While 7.2 is not an expected pH as a result of global warming, the study suggests “coral growth is strongly dependant on the concentration of CO3(2-) ions in seawater.”

The Hawaii Coral Reef Assessment and Monitoring Program researched the effect of ocean acidification on “crustose coralline algae” (CCA). CCA is important in the growth and stabilization of carbonate reefs. During the experiment CCA developed 25% cover in the control environment and 4% in the acidified environment. In the acidified conditions coral calcification decreased between 15% and 20%. [5]

Overview

Procedure

In this experiment, the manipulated variable was the pH of the water which was controlled by adding 1M HCl, and the response variable was the mass of coral colonies, measured over the course of four months.
Thirty-one separate Bird’s Nest colonies were prepared on plugs (small aragacrete tiles) from one mother colony (figure 2). The colonies were all approximately 1” size at the start of the experiment (figure 3).
The colonies were separated into three tanks. The tanks held only the corals, a pump, a 50W heater, and plastic racks (figure 1). The pH in two of the tanks was lowered using additions of 1M HCl. One gallon water changes in the tanks were done once a week, and the pH was adjusted using HCl . The pH of the water was tested with a Vernier LabQuest weekly.
Growth (calcification) in the coral colonies was assessed by massing each coral with a 0.001g scale digital scale once a week. Each colony was patted dry before being massed to reduce error from water droplets.
The response variable, the mass of the colonies, was calculated as a % difference from the starting colony mass to compensate for different starting masses of the colonies..

Aquarium Parameters

The corals were kept in 10 gallon tanks and lit with full spectrum T-5 fluorescent lighting (8 x 54W). The salinity was maintained at 1.023 g/ml and the temperature was maintained at 79 degrees F.

Analysis

The data was interpreted by calculating weekly average growth. The coral in Tank 1 (pH 8.4) grew on average 0.266 g each week (n=90). This tank was the control, so it is surprising that the S. hystrix colonies in Tank 2 (pH 8.3) grew an average of 0.475 g each week (n=72). This data suggests that optimum pH for growth in this corals is closer to 8.3. The data suggests that the “control” tank was not at an optimum pH. This is supported by average pH values from the oceans: the pH in today’s shallow ocean is 8.2, which is lower than 8.3 in the Pre-industrial period [1]. The coral in Tank 3 (pH 8.2) grew an average of 0.265 g each week (n=110), similar to Tank 1, which also supports that 8.3 is the optimum pH. The quantitative effect of Global Warming is found to be a 44% decrease in mass per .1 pH unit variance from the optimum pH.

Our oceans have already decreased to a pH level that is detrimental to coral growth. As expected, a small variance from optimal pH has a significant effect on coral growth.

A potential source of error in this experiment was drying the corals before weighing. The water droplets were patted dry and the coral was weighed while damp. The error from the water likely balanced out and is insignificant, because there are many data points.

Further Work

Further work on this experiment may include researching the reversibility of pH damage to coral. The investigation of other pH levels will determine the relationship between pH and calcification (linear, exponential).

It will be important to learn the minimum pH at which corals can survive before oceans reach that level. The effect of a decreased pH could also be tested on other shell building organisms.