Oceanographic sampling July 2011–July 2014; Data analyses/reporting through July 2015
Ocean Acidification in Glacier Bay
- Zooplankton in Acid? - Investigating the effect of ocean acidification in Glacier Bay.
Is ocean acidification happening in Glacier Bay? How do we know?
Did You Know?
Seawater’s alkalinity (a chemical property) gives it the ability to buffer against decreases in pH. In other words, seawater is naturally “basic” rather than “acidic”. Glacial meltwater, however, has lower alkalinity than seawater. When glacial runoff is added to the marine waters of Glacier Bay, it dilutes the ocean’s alkalinity, allowing the atmospheric CO2 taken up by the surface waters to more easily cause a decrease in pH, leading to ocean acidification (OA).
When the surface waters of Glacier Bay absorb CO2 from the atmosphere, the CO2 reacts with seawater and produces a series of chemical reactions that ultimately reduce seawater pH. This takes the form of reductions in carbonate ion concentrations and the saturation statesof the biologically-important calcium carbonate minerals calcite and aragonite (used by marine organisms in the formation of shell and bone). Saturations states serve as a numerical index to describe water chemistry in terms of the degree of OA. When the saturation state is greater than 1.0, waters are considered supersaturated with respect to calcium carbonate minerals. This means there is an abundant supply of these minerals for calcifying organisms to use. However, continued ocean acidification causes many parts of the ocean to become undersaturated with respect to these minerals, which likely impacts the ability of some organisms to produce and maintain their shells or skeletons. In some cases, undersaturated conditions can actually be corrosive to shelled organisms. Of the calcium carbonate minerals used by many marine organisms, aragonite is less stable than calcite and reacts first to decreases in pH. As a result, marine organisms that use aragonite for shell-building (e.g., many clams, snails, calcareous tubeworms) are at a higher risk under OA conditions.
While increased CO2 emissions and uptake by ocean surface waters are typically thought to be the major cause of OA in marine systems, this is only part of the story in Glacier Bay. Over the past two centuries, Glacier Bay has experienced rapid retreat and melting of its numerous glaciers, leading to an increase in the amount of fresh water entering the marine ecosystem. This input of low-alkalinity fresh water has reduced the surface water’s ability to resist decreases in pH, especially during seasons when glacial runoff is high (summer/fall).
Researchers carried out near-monthly oceanographic water sampling between July 2011 and July 2012, after which the sampling frequency was semi-annual through July 2014. We described OA-related characteristics at historical oceanographic station locations using a combination of methods. We generated vertical water profiles of basic oceanographic parameters using a Conductivity-Temperature-Depth (CTD) probe that continuously records water temperature, salinity, and depth as it is lowered through the water column from the surface to the bottom. At the same time, we collected discrete water samples from pre-determined depths.
We used the resulting data to describe water chemistry and to detect any seasonal patterns in ocean acidification events.We performed chemical analyses for dissolved inorganic carbon (including CO2), alkalinity, and the nutrients phosphate, nitrate, and silicate. Those results were then used to calculate saturation states with respect to calcium carbonate minerals.
Our data show that Glacier Bay experiences seasonal aragonite undersaturation events that vary in intensity and extent throughout the year. During the spring and summer “growing season”, phytoplankton remove dissolved CO2 from the surface waters because they use it for photosynthesis. This contributes to increases in calcium carbonate saturation states. However, during these same seasons, atmospheric temperatures are highest, which accelerates glacial melting. This low-alkalinity glacial meltwater flows into the ocean and reduces the alkalinity of surface waters, thus leading to net reductions in aragonite saturation states.
Surface waters with the highest percentage of fresh water (i.e., within the arms of Glacier Bay) directly corresponded to areas of aragonite undersaturation or near-undersaturation. This was most notable during the peak glacial melt season, and can influence waters throughout the bay. Low saturation states were well correlated with the timing of maximum glacial discharge events, and they were most prominent within the two arms where glacial discharge was highest. Saturation states with respect to aragonite reached a minimum of 0.40 (undersaturated) at the surface during the summer of 2011 before returning to supersaturated conditions (saturation states >1.0) in the winter and early spring of 2012 when glacial melt was greatly reduced. Also in spring, as phytoplankton growth (photosynthesis) accelerated with the return of longer days and more sunlight, some dissolved inorganic carbon (CO2) was removed from the surface waters. This caused an increase in the aragonite saturation states before conditions in the upper arms became undersaturated once again during the summer with increasing glacial runoff.