Award: OCE-1041068

The impact of ocean acidification on marine ecosystems represents an important question facing both marine scientists and managers of ocean resources. The thecosomatous pteropods are a group of shell-forming planktonic molluscs, also known as æsea butterflies,Æ that are thought in many regions to play an important role in marine food webs as prey for a variety of predators, including commercial fishes, marine mammals, and seabirds. The particular mineral (aragonite) used in the pteropod shell makes them especially sensitive to the chemical changes occurring in the marine environment under ocean acidification, and especially to the shoaling of the "compensation depth" at which seawater becomes corrosive to aragonite. Relatively little is known, however, about the ecology, physiology, and behavior of these organisms, and thereby about the likely impacts on pteropod populations of ocean acidification. The goal of our project was to examine the abundance, shell condition, vertical migratory behavior, and physiology of pteropod species in the northwest Atlantic and northeast Pacific, regions of naturally very different seawater chemistry and compensation depths. An additional goal was to assess decadal changes in seawater carbonate chemistry in order to quantify the rates of ocean acidification and anthropogenic carbon dioxide invasion, and to compare these rates with previous decades to evaluate any acceleration of ocean acidification in the two regions. Our inter-disciplinary team of scientists combined expertise in zooplankton ecology, physiology, and genetics; marine chemistry; and high frequency acoustic methods for sampling zooplankton (i.e., bio-acoustics). Two research cruises were conducted in open ocean regions of the NW Atlantic (in 2011) and NE Pacific (in 2012) along transects extending between 35 and 50 degrees North. A sophisticated suite of chemical and biological sampling devices was deployed including high-frequency echosounders, underwater high-magnification camera systems, nets designed to sample multiple depth layers, and instruments for sampling seawater to depths of up to 3000 m. Live pteropods captured in nets were also used in shipboard experiments exposing them to current carbon dioxide levels and enhanced levels mimicking those predicted for the end of the century, in order to measure their metabolic response. Key findings included chemical measurements documenting the rate of ocean acidification and compensation depth shoaling in our Atlantic and Pacific study regions, as well as the rate at which anthropogenic carbon dioxide has been entering the surface ocean during the last decade. These rates are higher than the previous decade, indicating a potential acceleration of ocean acidification due to a decrease in the seawater buffering capacity. We sampled a variety of pteropod species, providing needed baseline information on their natural abundance, distribution, and diversity. Echosounder observations indicated that in some limited locations, pteropods formed impressively large and dense swarms, up to 1.4 kilometers in length and at densities in excess of 3000 animals per cubic meter. Certain species were found in both ocean basins and in some cases migrated vertically to a lesser extent in regions where the water became corrosive to aragonite at shallower depths. These data provide important clues into the possible response of pteropods to the shoaling of the compensation depth currently occurring under ocean acidification. The project also included a strong educational component. Two postdocs and two doctoral students directly advised by project scientists gained substantial experience in biological and chemical oceanography as well as the intersection of these two fields. A total of 14 other guest graduate and undergraduate students participated in the cruises or subsequent lab work, gaining a working knowledge of ocean-going research and the methods employed for studying pteropod ecology and mar...