This project investigated the importance of top predators on oceanic food webs. The relatively pristine Ross Sea includes large populations of upper-level predators such as minke and killer whales, Adélie and Emperor penguins, and Antarctic toothfish. We focused on food web interactions of Adélie penguins, minke whales, and the fish-eating Ross Sea killer whales, all of which exert foraging pressure on their main prey, crystal krill and silver fish, in McMurdo Sound.

We quantified the abundance and distribution of phytoplankton, sea ice biota, prey, predators, and relevant habitat data in McMurdo Sound.  The sampling area encompassed “hot spots” discovered during the first year of fieldwork for type-C (fish-eating) killer whale, minke whale, and Adélie penguin feeding. At each station, data were collected with the remotely operated vehicle SCINI and a tow package FATTI that was developed specifically for this project. The sampling suite addressed 4-dimensional mapping of all levels of the pelagic food web; primary producers, intermediate consumers, and meso- and top-predators. We constructed a “map” of sea ice patterns, and added successive layers of the water column, distribution of phytoplankton, prey distribution, and predator foraging to search for interacting responses in the ecosystem.

Sea ice patterns:  Fast ice (sea ice attached to land or ice shelves) plays important physical and ecological roles: as a barrier to wind, waves, and radiation; as both a barrier and a safe resting place for air-breathing animals; and as a substrate for microbial communities. While sea ice has been monitored for four decades using satellite imagery, only recently is resolution sufficient to distinguish fast ice from mobile pack ice. Trends in fast ice distributions may differ from previously identified changes in sea ice distributions. Although human impacts on Antarctic marine ecosystems are relatively minimal, local human activities may have intense effects, including changing distributions of fast ice. We found that fast ice retreat dates have not changed, but fast ice reaches minimum extent later and begins to refreeze earlier, in partial agreement with changes in Ross Sea pack ice extent and duration. Fast ice minimum within McMurdo Sound and date of icebreaker arrival were significantly correlated. Wind velocity also influenced fast ice retreat.

Phytoplankton primary producers: Maximum concentrations of fast ice algae occurred during November, with higher concentrations on the east side of McMurdo Sound and lower concentrations on the west side in the cold water outflow from under the McMurdo Ice Shelf. In early to mid-December, warming surface water ablated the undersurface of the fast ice and ice algae sank rapidly out of the water column to provide fuel for the benthos. Also in early to mid-December, the system transitioned to a phytoplankton bloom at the fast ice edge and under the ice. The bloom sustained relatively high chlorophyll concentrations into January, providing food web support for the large abundance of top predators in this region.

Prey distributions: Crystal krill and silverfish were present in small numbers under McMurdo Sound fast ice before the summer bloom arrived. Acoustically detectible numbers appeared to arrive with the advected phytoplankton bloom. Ice edge distributions of krill and fish reflected the timing of increased predation. Fast ice habitat may provide a refuge from the most voracious, warm-blooded predators, potentially effecting trophic relationships in polar shelf waters. Growing evidence indicates that the Ross Sea, Antarctica, food web is structured as a ‘waspwaist’ system, in which krill and fish constitute the restriction. The abundance of these prey appears to be affected by top-down predation, and to have only minimal coupling with phytoplankton/primary productivity processes.

Predator patterns:  Antarctic minke whales and type-B and -C killer whales arrived in early-mid December. Arrival of whales coincided with the decrease in sea ice cover below 80%, consistent with results from other studies. Interannually, there was no trend other than continuation of a previously described decrease in maximum number of type-C killer whales. Despite primary productivity in the Ross Sea being the richest in the Southern Ocean, results indicate that a relatively small number of cetaceans, along with other abundant mesopredators, alter food availability at the sub-mesoscale sufficiently to force alterations in the foraging behavior and occurrence patterns of each other.

This project has emphasized integrating science and engineering, improving communication and recognition of synergy and overlapping goals. We developed novel tools for biological oceanographic studies in ice-covered seas. Incorporation of the latest well-tested yet inexpensive “commercial off the shelf” technology with standardized science instruments specifically built for this project enhanced transfer from engineering to science. The proliferation of remotely operated vehicles following the SCINI footprint (SCINI-Deep, MSLED, IceFin) is a testament to how initial transfer of engineering expertise is quickly spread to benefit the science community.

Our educational efforts emphasized global environmental awareness, and promoted questioning and problem-solving mindsets.  Project personnel have been successful at obtaining employment in engineering and education fields.

Last Modified: 01/05/2017
Modified by: Stacy Kim