Zooplankton
Zooplankton are a critical part of all marine food webs, and the Antarctic food web is no different.
In the Southern Ocean much is known about krill, the dominant zooplankton organism that is the food supply
for higher trophic levels like penguins, crabeater seals, and baleen whales. However, there is an amazing
abundance of other small zooplankton that can be found when sampling the open ocean in Antarctica.
This year during IVARS we have been conducting zooplankton tows to test if there is a reduced abundance of zooplankton associated with the dominance of certain phytoplankton groups. In recent zooplankton tows in the Ross Sea we have observed shelled pteropods (Limacina helicina antarctica), unshelled pteropods (Clione antarctica), numerous amphipod species, jellyfish, ctenophores, pelagic polychaetes, and fish larvae. The pteropods and amphipods are the most abundant animals found in our samples.
To collect zooplankton samples we drop a large weighted net with a closed cup at the end into the water. It descends to 200 meters and then it is brought back to the surface. A flowmeter records the actual volume of water that passes through the net. The animals are collected in the cup and emptied into a bucket. Most zooplankton samples are preserved at sea and brought back to the lab for analysis under a dissecting scope to quantify and identify species, although individual animals are available for analysis and experimentation at sea.
There are many important factors driving the diversity and abundance of zooplankton species we collect; some include temperature, sunlight, daily migration patterns, food availability and season. Overall, there is a rich zooplankton community present in the Ross Sea's cold waters. These organisms are not only interesting to study but also beautiful in form.
Some have hypothesized that pteropods may be the "missing link" in the food web of the Ross Sea. That is, Antarctic krill are largely absent on the continental shelf of the Ross Sea, but are replaced by another species called crystal krill. These are obligate ice species—their life cycle depends on the presence of ice. Because Phaeocystis commonly occurs in the Ross Sea, and because it is large with many compounds that may serve as anti-grazing devices, Phaeocystis is thought to be a dead-end in the food web. If pteropods could ingest and use Phaeocystis, it would provide a major food source to the entire food web.
Additionally, pteropods are major components of the biological pump here—the means by which organic matter, produced during photosynthesis, moves to depth and hence is eliminated from the carbon cycle for long time periods. Pteropods are often found in sediment traps, but the flux generally occurs in May or June, periods when no surface production occurs. Understanding the link between surface production, phytoplankton standing stocks, and pteropods may help improve our understanding of the cycling of carbon on the continental shelf. Our IVARS studies will hopefully provide a small piece of this big picture.