Zooplankton and Micronekton Component

(R. Ross and L. Quetin, PIs)

Net

Zooplankton and micro-nekton, particularly those > 0.2 µm in length, provide the main trophic link between primary producers and apex predators in the southern Ocean. Within the Palmer LTER study region, both oceanic and coastal zooplankton assemblages occur. The oceanic assemblage (generally in waters > 2000m) includes salps, whereas the coastal assemblages (shelf break and slope) includes several species of euphausiids, including Antarctic krill (Euphausia superba) and Thysanöessa macrura, and the herbivorous shelled pteropod, Limacina helicina. Over much of the shelf region both the oceanic and coastal assemblages occur in varying mixes year to year with no clear boundaries between zones. In parts of the PAL grid close to the continent, two species (larval Antarctic silverfish, Pleuragramma antarcticum, and the ice krill, E. crystallorophias) have been found in a zone of cold continental shelf water influenced by summer sea ice. Climate change, in particular changes in seasonal sea ice dynamics showing trends in longer periods of open water in summer, is apt to favor species with life histories that are less dependent on sea ice.

Temporal/spatial variability in the distribution and abundances of the zooplankton assemblages, in particular any changes in distribution, has been one focus of the research on zooplankton and micronekton. Sampling tools include 2 sizes of trawl, and bioacoustic transects with a 120 kHz echosounder. Simultaneous oblique tows and bioacoustic transects are conducted at all stations during the annual PAL cruise. Dive The combination of information from the two types of sampling is particularly important for zooplankton like Antarctic krill that have patchy distributions. During the spring and early summer, bioacoustic transects are conducted semi-weekly within the nearshore sampling region from a zodiac to document the acoustic biomass and 'packaging' of prey for the foraging penguins.

Another focus is the population dynamics of Antarctic krill, an important prey item for many seabirds and seals, and environmental factors impacting its growth, reproduction and ultimately recruitment success. Seasonal sea ice dynamics have been documented as important factors in both reproduction and recruitment success of this species. Experiments measuring in situ growth rates are conducted to document variability in production (growth) on multiple time and space scales in relation to environmental variability, both seasonally (sampled by SCUBA and by net from the zodiac) and over the PAL grid (sampled by net from the ship). Spawning frequency experiments at sea document interannual and spatial variability in timing and intensity of timing. Krill Shorter-term studies to date have generally focused on two areas. The first concerns grazing in Antarctic krill, as grazing is a process that both provides energy input to the population of krill and may be a process controlling either the quantity or composition of the phytoplankton community. The second concerns the energetics of larval krill during their first winter, a critical period for recruitment success.