Long-Term Observations Of Pteropod Phenology Along The Western Antarctic Peninsula

Shifts in phenology - annually occurring life history events - have been observed among many marine organisms due to global warming. We examined if phenological changes in the pteropod (pelagic snail) Limacina helicina antarctica have occurred along the Western Antarctic Peninsula, one of the most intensely warming regions on Earth, which would have important implications for regional food web dynamics. Pteropod shell diameters were analyzed from samples collected in the Palmer Antarctica Long-Term Ecological Research (PAL LTER) program year-round sediment trap from 2004 to 2018. There was considerable interannual variability in the time of appearance of a new pteropod cohort, which ranged from day of year 22-255, but no long-term, directional change. Mean L. h. antarctica growth rate for the time series was 0.009 mm day(-1) and there was no significant long-term change in growth rate. This study represents the first in the Southern Ocean to illustrate that pteropods actively grow throughout the winter season. Sea ice was the dominant driver of pteropod phenology, with earlier sea ice retreat the year prior, lower winter sea surface temperature (SST) the year prior, and higher primary productivity in the same year leading to earlier pteropod time of appearance. Similarly, more open water with higher autumn SST, both the year prior, and elevated chlorophyll a the same year, promoted faster pteropod growth. These results indicate that while pteropods are responsive to considerable environmental variability, their phenology has remained relatively stable. The identified responses of pteropod phenology to environmental shifts are key for determining future effects of climate change on biogeochemical cycling and plankton trophic interactions in the region.