Resolving the vertical distribution of the pteropod Limacina helicina from a 24-year time series off the coast of British Columbia to estimate experienced ranges in pH and aragonite saturation states

Many planktonic species perform diel vertical migrations (DVM) to search for food or to avoid predation. Literature on the euthecosome pteropod Limacina helicina commonly states that this species performs DVM, despite there being relatively little direct evidence. If this is true, then L. helicina willingly moves through a large gradient of pH and aragonite saturation states (Ω_A), which may indicate that it is more resistant to changes in these parameters than currently believed. Since L. helicina has come to be viewed as a sentinel species for understanding the effects of ocean acidification, it is important to have a thorough understanding of its vertical distribution and migration patterns. We utilized a large historical dataset of vertically stratified zooplankton abundance data collected off the west coast of Vancouver Island (British Columbia, Canada) between 1986 and 2010 using a Bedford Institute of Oceanography Net Environmental Sampling System (BIONESS) to determine the vertical distribution of L. helicina. Preliminary results indicate that DVM by L. helicina is dependent on body size. Small individuals (<2 mm) show only a very weak DVM signal, whereas larger individuals (>2 mm) display a moderate, although not well-defined, DVM signal. While L. helicina occupies a wide depth range (>250-0 m) peak abundance generally seems to occur below the mixed layer, presumably to facilitate the flux feeding method. Our results suggest that adult L. helicina encounters ranges of pH and Ω_A of about 7.5-8.3 and 0.5-3.0, respectively, off the west coast of Vancouver Island, whereas juveniles (which remain near the surface) may be more susceptible to changes in these parameters. We will investigate further the potential environmental controls on vertical distributions, such as oxygen, light, and food availability.