Ocean Acidification

Learning Outcomes:

1. Understanding the role of oceans in absorbing CO2 and the chemical processes of ocean acidification.
2. Identifying how CO2 uptake affects ocean chemistry, specifically pH and carbonate ion concentration.
3. Recognizing the factors influencing ocean acidification and its effects on marine life.
4. Evaluating mitigation strategies and their effectiveness in reducing acidification.
5. Analyzing short and long-term impacts on the ocean carbon cycle.

Ocean Acidification Overview

Oceans act as a major reservoir for CO2, absorbing approximately one-third of the CO2 emissions from human activities, thus mitigating climate change. Ocean acidification refers to changes in ocean chemistry, primarily the decrease in pH and carbonate ion concentration caused by the absorption of carbon compounds from the atmosphere. This process renders oceans less alkaline as they uptake more CO2, leading to increased hydrogen ion concentration, reduced pH, and a drop in carbonate ions.

CO2’s Effect on Ocean Acidification

The uptake of CO2 from the atmosphere is outpacing the ocean’s natural buffering capacity. Since the industrial revolution, ocean surface waters have experienced a pH drop of 0.1 units, equating to a 26% increase in hydrogen ions. Despite oceans being basic (pH ~8.0), the term “acidification” reflects the ongoing trend towards lower pH levels.

Forms of Calcium Carbonate:

1. Calcite: Found in the shells of organisms like planktonic algae, corals, echinoderms, and mollusks. It is less soluble.
2. Aragonite: A more soluble form, present in most corals, mollusks, and some algae species.

Influence of Other Factors

Various local factors can amplify the effects of CO2 and influence ocean acidification.

1. Acid Rain: Ranges in pH between 1 and 6, affecting surface ocean chemistry. It significantly impacts local and regional ocean acidification but has a limited global effect.
2. Eutrophication: Coastal areas receive excessive nutrients like nitrogen from agriculture and sewage, leading to large plankton blooms. Upon collapse, these blooms decompose, reducing oxygen and increasing CO2, which in turn lowers pH.

Important Note: Acidification involves multiple processes, including the formation of carbonic acid and the reduction of carbonate ions essential for marine life.

Chemical Reactions of CO2 with Seawater

1. Formation of Carbonic Acid:
   CO2 + H2O → H2CO3 → H+ + HCO3-
   Increased hydrogen ions reduce pH.
2. Production of Bicarbonate Ions:
   CO3^2- + CO2 + H2O → 2HCO3-
   This reaction decreases the availability of carbonate ions necessary for calcification in marine organisms.

Effects of Ocean Acidification

The ocean’s absorption of CO2 leads to the formation of carbonic acid, bicarbonate, and carbonate ions. Carbonate ions are vital for the calcification process in organisms like corals, mollusks, and plankton. Rising atmospheric CO2 decreases ocean pH, boosts carbonic acid, and reduces carbonate ions. As a result, calcification becomes increasingly difficult and sometimes impossible, threatening marine ecosystems and economically important species.

Mitigation Strategies

1. Reducing CO2 emissions through government policies aimed at capping carbon outputs.
2. Eliminating offshore drilling to protect marine environments.
3. Advocating for energy efficiency and promoting alternative energy sources like wind and solar power.

Saturation Horizons

Deep ocean waters naturally lack carbonate ions, leading to the dissolution of calcifying organisms’ shells. In contrast, surface waters are saturated with carbonate ions, providing a buffer for marine life. The saturation horizon marks the depth below which calcium carbonate dissolves.

1. Ocean Acidification Impact: This horizon moves upward as acidification progresses, exposing more calcifying organisms to undersaturated waters, making them vulnerable to shell dissolution.
2. Calcite vs. Aragonite Horizons: The calcite horizon occurs at a greater depth than the aragonite horizon but both are rising due to ongoing acidification.

Important Concept: Organisms below the saturation horizon possess special mechanisms to prevent shell dissolution.

Ocean Acidification and Carbon Fate

Over long timescales (greater than 100,000 years), a natural balance exists between the production and uptake of CO2, primarily through volcanic activity, organic matter production, and rock weathering. However, rock weathering is a slow process and cannot offset the current rapid anthropogenic CO2 influx.

Shorter Time Scales (>1,000 years):

1. The ocean’s internal feedback mechanism involves the carbonate-rich sediment known as carbonate compensation. The lysocline is the depth where carbonate dissolution increases, marking the boundary between surface waters (supersaturated) and deep ocean waters (undersaturated).
2. Carbonate Compensation Depth (CCD): Depth at which all carbonate dissolves. Current CO2 dissolution causes the CCD and lysocline to become shallower, dissolving shells trapped in sediments. This process may buffer ocean acidification but only over thousands of years.

Upwelling

Coastal regions occasionally experience upwelling, where deep waters circulate to the surface, bringing CO2 and nutrients to upper ocean layers. As surface waters become increasingly acidic, upwelling events expose coastal marine ecosystems to undersaturated conditions, challenging marine life accustomed to stable environments.

Winners and Losers in Ocean Acidification

1. Some marine phytoplankton and plant species may thrive with higher CO2 levels, potentially experiencing increased growth and photosynthesis.
2. However, the effects of acidification vary: for many marine organisms, elevated CO2 and acidity can negatively impact physiology or have neutral effects.
3. This variability indicates that certain species will emerge as winners while others as losers, with some showing no apparent change.

Important Note: Reducing atmospheric CO2 is critical to halting further acidification and protecting marine ecosystems.

Comprehensive Comparison Table: Forms of Calcium Carbonate

Saturation Horizon Table

Interesting Fact: India’s chameleons, predominantly arboreal, are found in most parts except regions with heavy rainfall. They prefer trees and small bushes.

Key Insights on Mitigation and Long-Term Impacts

Mitigation strategies focus on reducing CO2 emissions and promoting clean energy. While long-term natural processes (rock weathering, carbonate compensation) exist, they operate on timescales too slow to counteract current anthropogenic impacts. Immediate action is necessary to minimize harm to marine ecosystems.

Did You Know? The Odisha government in India proposed a wildlife park for UNESCO World Heritage status, recognized for its biodiversity.

Multiple-Choice Question:
Which process is primarily responsible for the initial formation of carbonic acid in the ocean?

1. Rock weathering
2. Upwelling events
3. CO2 absorption by seawater
4. Photosynthesis
   Correct Answer: 3. CO2 absorption by seawater