Cascading Trophic Impacts of Reduced Biomass in the Ross Sea, Antarctica: Just the Tip of the Iceberg? BRAD A. SEIBEL* AND HEIDI M. DIERSSEN† Monterey Bay Aquarium Research Institute, Moss Landing, California 95039 A significant reduction in phytoplankton biomass in the Ross Sea was reported in the austral summer of 2000 –2001, a possible consequence of a disruption in sea-ice retreat due to the presence of an immense iceberg, B15 (1) (Fig. 1). Our observations in McMurdo Sound suggest temporally and trophically cascading impacts of that depression in productivity. Reduced phytoplankton stocks clearly affected the pteropod Limacina helicina (Phipps, 1774) (Gastro- poda: Mollusca), an abundant primary consumer in the region (2, 3), as indicated by depressed metabolic rates in 2000 –2001. The following season, for the first time on record, L. helicina was absent from McMurdo Sound. Many important predators, including whales and fishes, rely heavily on L. helicina for food (3, 4). However, most obvi- ously impacted by its absence was Clione antarctica (Smith, 1902), an abundant pteropod mollusc (Gastropoda) that feeds exclusively on L. helicina (5). Metabolic rates of C. antarctica were depressed by 50% in 2001–2002. Both L. helicina and C. antarctica are important components of polar ecosystems and may be good indicators of overall ecosystem “health” in McMurdo Sound and perhaps in the Ross Sea. In this last austral summer, 2002–2003, sea-ice extent was much higher and phytoplankton stocks were dramatically lower than any reported previously, effects possibly associated with El Nin ˜ o conditions, and we hypoth- esize that pteropods and their consumers may be further impacted. In the Southern Ocean, phytoplankton production is linked strongly to the seasonal oscillations in the extent of the sea ice (6, 7) and survival of higher trophic levels is dependent on reproductive cycles that are synchronous with phytoplankton blooms. This is especially true of the direct food link between L. helicina and C. antarctica. L. helicina lives and feeds in the water column by extending a web of mucus that traps phytoplankton and, to a lesser extent, small zooplankton (3). L. helicina is the exclusive food source of C. antarctica throughout the life cycle, and the two species have parallel life histories. They grow in concert, with the preferred prey size increasing with predator size (3). Such specificity within the context of a highly seasonal environ- ment requires precise timing to ensure that predator and prey coexist. The coevolved predator-prey relationship be- tween L. helicina and C. antarctica provides a unique opportunity to study the ecological and trophic conse- quences of a depression in primary productivity in the Ross Sea. A 50% to 75% reduction in phytoplankton biomass, es- timated as chlorophyll a (Chl) concentrations, and high sea-ice cover was observed in December 2000 –2001 rela- tive to previous years (Table 1; Fig. 2; 8). A limited bloom did form by February, but annual primary production was still only 60% of the previous year (1). We believe that the reduced phytoplankton stocks in 2000 –2001 had pro- nounced impacts on the condition of primary consumers in the region, causing cascading effects through higher trophic levels in the following year. This assertion is supported here by a series of metabolic measurements made on L. helicina and C. antarctica between 1999 and 2002. Nutritional state is known to be among the primary de- terminants of metabolism in all organisms, including ptero- pods (3), and is especially important in the highly seasonal Antarctic environment (9, 10). Food availability will influ- ence, among other things, the rates of protein synthesis, oxygen consumption, growth, and reproduction (9 –11). We collected L. helicina and C. antarctica at four sampling stations along Ross Island (Fig. 1) and measured the oxygen Received 20 November 2002; accepted 21 July 2003. * To whom correspondence should be addressed. Current address: 100 Flagg Road, Biological Sciences Center, Biological Sciences Department, University of Rhode Island, Kingston, RI 02881. E-mail: seibel@uri.edu † Current address: Department of Marine Sciences, University of Con- necticut at Avery Point, 1080 Shennecosset Road, Groton, CT 06340. Reference: Biol. Bull. 205: 93–97. (October 2003) © 2003 Marine Biological Laboratory 93