Shift in the Antarctic Peninsula Food Web in Response to Regional Warming Oscar Schofield1, Mark Moline2, Herve Claustre3, Thomas Frazer4, Maria Vernet5 1: Rutgers University, 2: California Polytechnic State University, 3: Observatoire Océanologique de Villefranche (CNRS-INSU) , 4: University of Florida, Gainesville, 4: Scripps Institute of Oceanography Regulation of Phytoplankton Community Composition in Coastal Waters As part of the Long-Term Ecological Research Program, the waters near Palmer Station, Antarctica (64° 46.45' S, 64° 03.27' W) were sampled over a seven year period (1991-1998), in part, to describe the temporal variability in the taxonomic composition of phytoplankton communities. Air Temperature > 0 °C Salinity (p.p.t) % Cryptophytes 25 50 33.3 33.6 33.8 64°W Palmer Station Antarctic Peninsula 65°S 20 40 60 80 100 -15 -10 -5 5 Mean Daily Air Temperature (°C) % Cryptophytes 35 70 Sept Oct Nov Dec Jan Feb Mar Month % Cryptopytes 1991/92 1992/93 1993/94 1994/95 1995/96 Cryptophytes Diatoms Prymnesiophytes 32 32.5 33 33.5 34 34.5 Saliinity (p.p.t.) -1.5 -1 -0.5 0.5 1 1.5 0.2 0.4 0.6 0.8 1 Proportion of total chlorophyll a associated with cryptophytes Proportion of total chlorophyll a associated with diatoms Temperature (°C) Despite inter-annual variability in chlorophyll a, a repeated pattern in phytoplankton community succession was observed. Diatoms dominated spring phytoplankton populations. A transition from diatoms to surface populations of cryptophytes occurred during the summer of each year. The dominance of cryptophytes coincided with the initiation of glacial meltwater input. Crypotphytes were confined to the high temperature/low salinity water. Given the association of cryptophytes with meltwater, there was also a striking positive relationship with presence of cryptophytes and air temperature. In contrast, diatoms and prymnesiophytes dominated the communities in other physical domains. Shipboard transects along the Antarctic Peninsula confirmed cryptophyte dominance in low salinity near shore waters along the Antarctic Peninsula. The cryptophyte-dominated waters were a significant geographic feature ranging in size from 1100 to 4800 km2 depending on the year and as far as a 100 km offshore. 0.001 0.01 0.1 1 10 100 -5 -4 -3 -2 -1 Mean Annual Air Temperature (°C) Krill:Salp Ratio Other Grazers (copepods) Diatoms & Other Phytoplankton Cryptophytes Autotrophic Carbon Production Krill Salps Sedimentation (Microbial Loop) Respiration (Other Losses) Higher Trophic Levels (fish, penguins, whales) Autotrophic Losses (Not Grazing) 4% 62% 12% 22% 15% 33% 24% 28% Partitioning of Carbon in Peninsula System 0.0641 (Krill:Salp) 33.3 (Krill:Salp) Sedimentation (MicroLoop) From Smith (1994) -0.8 -0.4 0.4 0.8 1.2 1945 1955 1965 1975 1985 Year Air Temperatures (°C) Mean Summer Faraday Station Signy Station R2 = 0.64 R2 = 0.73 Cryptomonas cryophila Thalassiosira antarctica Corethron criophilum Palmer Cryptophytes --> 8 ± 2m 10m 100m SEM Micrographs from McMinn and Hodgson 1993 40 80 McClatchie and Boyd 1983 5-10 >15 Boyd et al. 1984 20 Quetin and Ross 1985 % Food Retention by Krill Phytoplankton Size (m) The transition from diatoms to cryptophytes represents a decrease in the size class of the phytoplankton. Bloom-forming diatoms range in size from 15-270 mm, while the cryptophytes measured microscopically at 8 ± 2 mm. This will impact the zooplankton grazing. Adult krill selectively graze larger phytoplankton cells and allow for a relative increase in the abundance of smaller cells including cryptophytes. In contrast, salps effectively graze on phytoplankton in the size range of cryptophytes. There has been a significant decrease in krill along the Antarctic Peninsula. It has been suggested this reflects poor recruitment associated with a decline in sea-ice development. The spatial heterogeneity in phytoplankton is the other determinant of krill and salp abundance. Salps can efficiently graze food particles as small as 4 mm in size, but adult krill cannot. The grazing efficiency of E. superba decreases significantly with particles < 20 mm. The mean air temperatures along the Antarctic Peninsula have increased (2-3 °C) over the past 50 years leading to alterations in ice shelf dynamics and glacial melting. An increase cryptophyte biomass is ecologically significant as it would favor grazers capable of feeding on small particles such as salps. This shift in the grazer community would imact higher trophic levels and carbon flow through the coastal ecosystems. Impact of Shifts of Phytoplankton Community Composition on the Coastal Food Webs