Does the term ‘ocean acidification’ mean that seawater is becoming acidic?
In 2003, Ken Calderia coined the term ocean acidification in hopes of drawing attention to a critical marine issue. As many focused on the impacts of increased anthropogenic (human influenced or caused) carbon dioxide in the atmosphere, Ken was looking at the impact to our oceans. Less than a decade later, ocean acidification is the term used to describe the impact of increased carbon dioxide levels in the oceans. This topic has risen to the forefront of marine research, with a multitude of scientists, including our science party here on the Oceanus, trying to ascertain how marine organisms will acclimate to this environmental shift. This subject matter is important as each organism's ability to adapt will influence the food web and corresponding ecosystems.
SO, is the ocean becoming acidic?
Oceanic waters are naturally alkaline (pH scale 7.2 - 8.6 with a mean surface ocean pH of approximately 8.1). In comparison, lemon juice has a pH of 2.0 and bleach pH is nearly 13 (Fig. 5). However the ocean pH level is dropping or becoming less alkaline. All living organisms have a pH tolerance range, which is the organism's acidity tolerance. If the pH level drops or rises beyond the organism's tolerance, then it will cease to inhabit that location. Small changes in the water's pH can have enormous effects on species that have a limited pH tolerance range.
Carbon occurs naturally in seawater and without it marine life would cease to exist. There are many sources of dissolved carbon dioxide in oceanic waters including river input, hydrothermal vents and underwater volcanoes. Two significant sources are from the atmosphere and organic matter. Carbon dioxide is absorbed from the atmosphere as the Earth's oceans act as a carbon sink absorbing 1/3 of all anthropogenic carbon that comes into contact with the ocean's surface. Atmospheric gases are constantly being absorbed, dissolved and released from the ocean. The oceans hold roughly 50 times greater concentration of carbon dioxide per unit volume than the atmosphere. Another source of carbon dioxide is organic matter, derived from dissolved organic carbon decomposition, as food is broken down carbon dioxide is released. 
Fig. 1. The Carbon Cycle depicts how carbon moves among the atmosphere, the land and the oceans (Diagram courtesy of the United States Department of Energy 2011).
Too Much of a Good Thing
Since the industrial revolution, there has been a dramatic increase in atmospheric carbon dioxide (Fig. 2). The Intergovernmental Panel for Climate Change (IPCC) reported the average concentration of atmospheric carbon dioxide, as of July 2011, was 392.39 parts per million (ppm), which is 40% higher than pre-industrial levels of 280 ppm only 200 years ago (IPCC 2011). In contrast, carbon dioxide levels prior to the industrial revolution appear to have been fairly constant for at least the past thousand years.
In a natural system, without excessive anthropogenic carbon dioxide being released into the atmosphere, carbon dioxide values for the ocean and atmosphere are in equilibrium as the oceans buffer small fluctuations. But as atmospheric carbon dioxide increases, the equilibrium is disturbed as the ocean’s natural buffering system cannot accommodate the higher carbon dioxide input.
Fig. 2. Historical increase in atmospheric carbon dioxide concentrations (Diagram courtesy of the White House Inititative on Global Climate Change 2011) .
So why do we care? When carbon dioxide dissolves in seawater, carbonic acid is produced. However, carbonic acid is not stable in seawater and immediately separates or disassociates. This causes a release of hydrogen ions. This increase in hydrogen ion concentration increases the acidity of the seawater. When the hydrogen ions are released, they are able to combine with carbonate ions to form a bicarbonate ion thus reducing the amount of carbonate ions.
Calcifying marine organisms, such as corals and mussels, as well as the pteropods we are out here studying, exploit carbonate ions from calcium carbonate. When there is a decline in carbonate ions, the calcification rates decline proportionally. This means species that rely on carbonate ions to make shells are impacted.
Location, Location, Location
Carbon dioxide is not static in the water column. The movement of carbon dioxide is governed by two properties: the thermohaline circulation and carbon dioxide solubility properties in water. The thermohaline circulation pattern mixes surface waters with deeper waters through vertical and horizontal ocean currents driven by differences in water temperature and salinity levels. Colder, more saline water at higher latitudes is denser and sinks. As water temperature decreases, the solubility of carbon dioxide increases. Therefore, each cubic meter of cold water at higher latitudes carries more carbon dioxide than the same cubic meter of warmer water at the equator. The point being that the chemistry of the oceans varies with depth and latitude.
The result is that there is a depth in the ocean, the carbonate compensation depth (CCD), below which level calcium carbonate (shells) start to dissolve. The depth of the CCD is dependent on water temperature, pressure and salinity and varies within and also between oceans. The two diagrams below depict the different profiles of the Atlantic and Pacific Oceans (Fig. 3 and Fig. 4) along the study transect we are presently surveying (in the Atlantic) and along the transect we plan to survey next summer (in the Pacific). As you can see from these figures, in the modern-day North Atlantic the CCD is deeper than 2000m right up to 50 degrees North. By 2100, however, under ocean acidification the CCD is predicted to shoal to as shallow as 100m at northern latitudes. In contrast, in the modern-day Pacific, the CCD is already naturally much shallower and varies with latitude, from about 700m at 35 degrees North to about 150m at 50 degrees North. Our goal is to capitalize on these differences between and within ocean basins to understand how seawater chemistry influences the abundance, vertical range, and types of pteropods found in the water column.
Fig. 3. The Atlantic Ocean CCD Profile (Y-axis is depth in meters; the carbonate compensation depth is indicated by 1 and is shown in white. Below this depth the waters are corrosive to the form of calcium carbonate formed by pteropods.) (Diagram courtesy of Z. Wang). 
Fig. 4. The Pacific Ocean CCD Profile (Y-axis is depth in meters) (Diagram courtesy of Z. Wang). On our current R/V Oceanus cruise we are using the variety of instruments described in an earlier blog post to determine the abundance and diversity, as well as the vertical and horizontal distribution, of pteropods, while concurrently analyzing the ocean's carbonate chemistry and specifically the calcium carbonate saturation levels along a transect from 35 to 50 degrees North in the Atlantic. Next year we'll make the same measurements along the same latitudinal range in the Pacific. This research will give us baseline information on pteropod ecology, and will hopefully give us an improved understanding of how pteropods are likely to react to the continued changing chemistry of the oceans.
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