Pteropods (pelagic snails) are ubiquitous zooplankton in the Southern Ocean and abundant along the Western Antarctic Peninsula (WAP), one of the most rapidly warming regions on the planet. They are important prey for higher trophic levels, grazers of phytoplankton, and contribute to particulate organic and inorganic carbon export. Pteropods are heralded as bioindicators of ecosystem health due to the vulnerability of their aragonitic shells under ocean acidification conditions, which could greatly affect their abundances in the future. Despite their importance within Antarctic food webs, few studies have analyzed the effects of climate change on pteropod physiology and biogeography in the Southern Ocean. I utilized zooplankton net tows and sediment trap samples collected as part of the Palmer Antarctica Long Term Ecological Research (PAL LTER) program to determine long-term changes in pteropod biogeography and phenology (life history). I also conducted shipboard experiments on PAL LTER research cruises to analyze the effects of shifting temperature and food conditions on pteropod metabolism. Lastly, to examine WAP pteropod feeding ecology, I utilized high-throughput sequencing techniques and analyzed pteropod gut contents at an unprecedented taxonomic resolution. Pteropod populations along the WAP from 1993-2017 either remained stable (shelled pteropods) or increased (non-shelled pteropods) and were most strongly controlled by La Niña conditions the year prior, which led to warmer, ice-free waters. There was a weak relationship between pteropod abundance and carbonate chemistry, and no detectable long-term trend in carbonate chemistry parameters (i.e., aragonite saturation), thus ocean acidification is not presently a factor influencing WAP pteropod abundance. More open-water areas the year prior also increased growth rates of the shelled pteropod, Limacina helicina antarctica, and caused earlier time of appearance in the PAL LTER sediment trap. There was considerable interannual variability in the time of appearance of a new pteropod cohort, which ranged from year day 22 to 255, but no long-term, directional change in time of appearance or growth rate. The effects of warming seawater temperatures and shifting food availability on L. h. antarctica metabolism revealed that highest respiration and usually highest excretion rates occurred under higher temperatures, but the effect of food concentration was more limited. The proportion of dissolved organic matter to total organic and inorganic dissolved constituents was high and the metabolic ratios of C, N, and P were all below the canonical Redfield ratio, which can directly affect phytoplankton growth and bacterial production in the WAP. Analysis of L. h. antarctica gut contents revealed its microbiome for the first time with Mollicutes bacteria the most abundant prokaryote. Pteropods were mainly herbivorous in summer, consuming predominantly diatoms but also supplementing their diet with microzooplankton such as ciliates. My dissertation shows that pteropods along the WAP are sensitive to changes in the environment from daily to interannual time scales. These insights into the metabolic and ecologic responses of pteropods to ocean variability increase our understanding of the role of zooplankton in biogeochemical cycles and help predict future responses to climate change.
Thibodeau P. S., 2020. Environmental controls on pteropod ecology and physiology along the Western Antarctic Peninsula. PhD thesis, Virginia Institute of Marine Science. Thesis (restricted access).