Ocean Acidification (OA): How It Affects Shellfish

ABSTRACT

For decades evidence has shown that calcifying organisms such as several types of molluscs have been found to be vulnerable to the effects of Ocean Acidification (OA). The production of calcium carbonate which is the main component used by calcifying organisms to build their shells is predicted to decline, and with the addition of global warming and increasing sea levels the effect that OA has on the world’s oceans may only worsen over time¹. Presented here is a simple experiment which simulates how changes in pH affects shellfish alongside the chemical processes which drive acidification in the beakers of our experiment to the vast oceans on our planet. In our experiments we found that increasing the acidity of the seawater solution led to a decrease in the mass of the seashells. The seashells used for the purpose of this experiment belong to the species Austrovenus stutchburyi – a marine bivalve mollusc commonly found in New Zealand. At a pH of 8.2 we found that the seashells lost about 1.7% of their original mass and a significant loss of mass of about 15% was observed under a seawater solution with an extreme pH of 6.8.

INTRODUCTION

Previous research on ocean acidification has shown that the acidification of the oceans is due to dissolved carbon dioxide from anthropogenic greenhouse emissions². Chemical reactions that are essential for life are sensitive to changes in pH, and a small change in the pH of a marine ecosystem can have a detrimental effect on marine life, especially on shell-building organisms. Ocean Acidification (OA) is known to negatively alter the structure of skeletons in marine organisms that are sensitive to changes in pH. Shell-building organisms such as oysters utilise calcium (Ca⁺²) and carbonate ions (CO₃^-2) found naturally in seawater to form calcium carbonate (CaCO₃) shells². Therefore, it is hypothesized that as the amount of acidification of the seawater increases, the rate at which the seashells lose mass will increase as well.

EQUIPMENT LIST

• 15 bivalve shells
• 15 beakers, preferably 80ml
• pH testing meter
• Distilled water
• Hydrochloric acid (HCl)
• Sodium hydroxide solution (NaOH)
• Measuring cylinder
• Small graduated cylinder
• Test tube
• Electronic scale/electronic balance
• Ruler or tape measure
• Digital pH metre
• Universal pH indicator
• Spot plate (optional)

METHOD

Step 1: Measure the pH of seawater using a digital pH metre.
Step 2: Mix hydrochloric acid with seawater to lower the pH. Mix sodium hydroxide solution for pH 8.0 and 8.2. Check the pH with the digital pH meter. The solutions made were pH 6.8, 7.2, 7.8, 8.0 and 8.2.
Step 3: Fill beakers with 10 mL of each pH solution and label with its respective pH level. Remeasure the pH levels of the solutions – 6.8, 7.2, 7.6, 8.0, and 8.2 respectively to ensure accuracy.
Step 4: Weigh and record the bivalve shells using the electronic scale.
Step 5: Place individual seashells into the beakers and label them accordingly. Take 3 seashells (3 trials) for each of the 5 pH levels and label them as 1A, 2A, 3A then 1B, 2B, 3B and 1C, 2C, and 3C and so on for all of the pHs.
Step 6: Leave the bivalve shell in the beaker for 5 days. After 5 days, take each shell out of the beaker and dry it properly before weighing the shells. Calculate the loss in mass. After weighing, put the shells back in the same concentration of seawater.
Step 7: After 19 days, take each shell out of the beaker and dry it properly before weighing the shells. Calculate the loss in mass

RESULTS TABLE

Table 1. Loss in mass of seashells after 5 days

pH of sea water	Mass of shells After 5 days (grams)
	Trial 1	Trial 2	Trial 3	Average loss in mass (g) (3 d.p)	Average % mass loss
6.8	Mi = 2.95 ,Mf = 2.52 ∆Mass = -0.43 % loss = 14.6%	Mi = 3.19 ,Mf = 2.73 ∆Mass = -0.46 % loss = 14.4%	Mi = 3.15 ,Mf = 2.67 ∆Mass = -0.48 % loss = 15.2%	0.457g	14.7%
7.2	Mi = 4.46 ,Mf = 4.38 ∆Mass = -0.08 % loss = 1.8%	Mi = 4.05 ,Mf = 3.89 ∆Mass = -0.16 % loss = 4.0%	Mi = 3.28 ,Mf = 3.08 ∆Mass = -0.20 % loss = 6.1%	0.147g	3.9%
7.6	Mi = 4.45 ,Mf = 4.35 ∆Mass = -0.10 % loss = 2.2%	Mi = 3.71 ,Mf = 3.59 ∆Mass = -0.12 % loss = 3.2%	Mi = 2.65 ,Mf = 2.57 ∆Mass = -0.08 % loss = 3.0%	0.100g	2.8%
8.0	Mi = 4.46 ,Mf = 4.38 ∆Mass = -0.08 % loss = 1.8%	Mi = 4.28 ,Mf = 4.17 ∆Mass = -0.11 % loss = 2.6%	Mi = 2.28 ,Mf = 2.24 ∆Mass = -0.04 % loss = 1.8%	0.050g	2.1%
8.2	Mi = 4.98 ,Mf = 4.98 ∆Mass = 0 % loss = 0.0%	Mi = 4.96 ,Mf = 4.90 ∆Mass = -0.06 % loss = 1.2%	Mi = 3.49 ,Mf = 3.45 ∆Mass = -0.04 % loss = 1.2 %	0.033g	0.8%

Table 2. Loss in mass of seashells after 19 days

pH of sea water	Mass of shells after 19 days ( in grams)
	Trial 1 (a)	Trial 2(b)	Trial 3(c)	Average loss in mass (g) (3 d.p)	Average % mass loss
6.8	Mi = 2.95 ,Mf = 2.47 ∆Mass = -0.48 % loss = 16.3%	Mi = 3.19 ,Mf = 2.63 ∆Mass = -0.56 % loss = 17.6%	Mi = 3.15 ,Mf = 2.79 ∆Mass = -0.36 % loss = 11.4%	0.467g	15%
7.2	Mi = 4.46 ,Mf = 4.14 ∆Mass = -0.32 % loss = 7.2%	Mi = 4.05 ,Mf = 3.72 ∆Mass = -0.33 % loss = 8.1%	Mi = 3.28 ,Mf =3.01 ∆Mass = -0.27 % loss = 8.2%	0.307g	7.8%
7.6	Mi = 4.45 ,Mf = 4.30 ∆Mass = -0.15 % loss = 3.4%	Mi = 3.71 ,Mf = 3.55 ∆Mass = -0.16 % loss = 4.3%	Mi = 2.65 ,Mf = 2.51 ∆Mass = -0.14 % loss = 5.3%	0.15g	4.3%
8.0	Mi = 4.46 ,Mf = 4.40 ∆Mass = -0.06 % loss = 1.3%	Mi = 4.28 ,Mf = 4.20 ∆Mass = -0.08 % loss = 1.9%	Mi = 2.28 ,Mf = 2.20 ∆Mass = -0.08 % loss = 3.5%	0.073g	2.2%
8.2	Mi = 4.98 ,Mf = 4.90 ∆Mass = -0.08 % loss = 1.6%	Mi = 4.96 ,Mf = 4.88 ∆Mass = -0.08 % loss = 1.6%	Mi = 3.49 ,Mf = 3.42 ∆Mass = -0.07 % loss = 2.0%	0.076g	1.7%

RESULTS

Figures 1A and 1B show that as the pH of the water decreased, the mass loss of bivalve shells increased. Changes from leaving the shells in the HCl-seawater solution for a further 14 days (for a total of 19 days represented in blue in fig. 1A below) saw little reaction take place compared to the initial 5 days. The data points of the two data sets intercept at pH 6.8 indicating that likely no reaction took place after 5 days. This was expected as the concentration of hydrogen ions is greater at pH 6.8 compared to pH 8.2. Therefore, any reaction that occurred after the initial 5 days such as that at pH 7.2 was likely due to the remaining H⁺ ions from the HCl reacting with the CaCO₃.
Upon contact with the acidified seawater solution it was observed that the bivalve shells had already started dissolving due to the CO₂ bubbles seen emerging to the surface of the beaker. The bivalve shells left in the seawater solution of pH 8.2 had lost on average 0.78% of their initial mass after 5 days and 1.7% after the next 14 days (total of 19 days).
A

B

Figure 1. Graph A shows the changes in mass (g) and graph B shows the change in mass in terms of percentage.

DISCUSSION/CONCLUSION

During the experiment it was observed that as the pH of the seawater was lowered, the change in mass of the bivalve shells increased (indicating an increase in the rate of dissolution). From data that was gathered over a period of 19 days, it can be seen that changing the pH of the water affects shellfish by dissolving the CaCO₃ that the organism needs to build new layers of shell. Since the industrial revolution, the pH of the ocean has decreased from 8.2 to 8.1, indicating a 28.8% increase in acidity (H⁺ concentration) according to the Pacific Marine and Environmental Laboratory (PMEL)³. This investigation found that at a pH of 8.0 (the ocean’s projected pH levels by 2100), the bivalve shells had lost 2.05% of its mass after 5 days and 2.2% after an additional 14 days.
In general the results of the experiment indicate that ocean acidification would have deleterious impacts on shellfish. In future studies, the methods could be improved by covering the beakers with some type of lid to prevent the CO₂ gas from escaping. Further research could also be carried out to test whether certain temperatures affect the rate at which the seashells dissolve to better understand the effects of OA combined with the effects of global warming.

REFERENCES

1. Tyrrell, T. 2007. \”Calcium Carbonate Cycling In Future Oceans And Its Influence On Future Climates\”. Journal Of Plankton Research 30 (2): 141. doi:10.1093/plankt/fbm105.
2. Bennett, Jennifer, and Ocean Portal Team. 2018. \”Ocean Acidification\”. Smithsonian Ocean. https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification.
3. \”PMEL CO2 – Carbon Dioxide Program\”. 2019. Pmel.Noaa.Gov. Accessed July 22. https://www.pmel.noaa.gov/co2/story/A+primer+on+pH.

About the Author

Bhavesh Sharma is a former student,prefect,peer mentor and Dux of Papakura High School in New Zealand. He is interested in studying the Biological Sciences and Statistics in 2020. He also enjoys writing poetry and painting in his free time.