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1 L. Quetin, 1 R. Ross, 2 M. Vernet, 2 W. Kozlowski, 2 L. Yarmey, 1 A. Lowe, 1 S. Oakes, 3 C. Fritsen, 1 *M. Amsler, 1 * O. Schofield and 1 * D. Steinberg 1 Marine Science Institute - UCSB, 2 Scripps Institution of Oceanography, 3 Desert Research Institute, Reno (*at time of larval grazing experiments) Stella model - primary production and larval krill energetics Approach We constructed a carbon-based model to explore the interaction between phytoplankton primary production and larval Antarctic krill in the Southern Ocean GLOBEC and Palmer LTER study regions west of the Antarctic Peninsula during 2001 and 2002. Questions addressed were: • What proportion of the fall decline in phytoplankton can be attributed to grazing by larval Antarctic krill? • What is the contribution of phytoplankton in the water column to the energy budget of larval Antarctic krill in the fall? We used Stella software (ISEE Systems) to couple models of primary production and larval krill energetics from March 1 to July 1. To investigate the effects of differences in latitude (light regime) and interannual differences in phytoplankton production and larval abundance we compared four scenarios: Palmer Basin (-64°S) 2001 and 2002 Marguerite Bay (-68°S) 2001 and 2002 Larval Antarctic krill are only one of the potential grazers on the phytoplankton community in the fall. Adult Antarctic krill, sporadic fall blooms of salps and pteropods can be abundant in fall in the upper water column; many species of copepods have already descended to depth. References cited: Ashjian, C.J., G.A. Rosenwaks, P.H. Wiebe, C.S.Davis, S.M. Gallager, N. J.Copley, G.L. Lawson and P. Alatalo. 2004. Distribution of zooplankton on the continental shelf off Marguerite Bay, Antarctic Peninsula, during austral fall and Winter, 2001. Deep-Sea Res II 51: 2073-2098. Dierssen, H.M., M. Vernet and R.C. Smith. 2000. Optimizing models for remote estimation of primary production in Antarctic Coastal Waters. Antarct. Sci. 12(1): 20-32. Frazer, T. K., L.B. Quetin and R.M. Ross 2002. Energetic demands of larval krill, Euphausia superba, in winter. J. Expt Mar Biol Ecol 277: 157-171. Fristen, C.H., J. Memmott and F.J. Stewart. 2008. Inter-annual sea-ice dynamcis and micro-algal biomass in winter pack ice of Marguerite Bay, Antarctica. Deep-Sea Res II 55: 2059-2067. Oakes, S.A., R.M. Ross and L.B.Quetin. Young Antarctic krill (Euphausia superba) feeding by scraping surfaces vs water column filtration. Mar Ecol Prog Ser (in revision) Initial parameters and inputs • latitude • seasonal temperature: Palmer Station, Anvers Island, tide guage, US Antarctic Program; Ryder Bay, Adelaide I., Rothera Base (courtesy of A. Clarke, British Antarctic Survey) • initial chlorophyll concentration • maximum primary production from photosynthe- sis vs irradiance experiments (Vernet) • loss factor for primary production model deter- mined empirically (fit model results to field obser- vations for fall) • larval total length, carbon and wet mass • start day of sea ice coverage > 3/10 • sea ice chlorophyll a concentration (Fritsen et al. 2008) Primary production (PP) model (Dierssen, Vernet, Smith 2000) Requires: • chlorophyll-specific primary production calculated from - maximum primary production - euphotic zone depth (Z eu) - daylength • average chl in water column - integrated to shallower of surface mixed layer (SML) and Z eu • C:chl a ratio (75 from Mar 1 - May 31, 50 from Jun 1 to Jul 1) • loss factor (sum of grazing by entire zooplankton com- munity, sedimentation and advection) Generates: • inputs to larval energetics model - average chlorophyll (mg m -3 ) in water column - integrated carbon in water column (mg C m -2 ) Larval energetics model Requires: • ingestion rate both in water column and on sea ice community (experiments conducted in laboratory in 1988 (unpublished data) and 2002 (Oakes, submitted) as function of - food concentration - larval wet mass - temperature • respiration rate (Frazer et al. 2002; personal communication) as function of - temperature - larval wet mass • larval abundance for SO GLOBEC region in 2001 and 2002: average 4920 m -2 and 84 m -2 , respectively (Ashjian et al. 2004 for 2001; P. Wiebe and C. Ashjian, personal communication for 2002); assume same for -64°S • larval behavior - diel vertical migration (unpublished data) - fall: between surface and depth - after 3/10 sea ice: between under side sea ice and water column Generates: • ingestion of individual larva (µg C larva -1 d -1 ) • ingestion by population of larvae (µg C m -2 d -1 ) Results • decline of chlorophyll in fall was variable - concentrations > in 2002 than 2001 initially - small fall blooms in 3 of 4 scenarios (not Palmer 2002) - steeper decline Palmer 2002 • larval grazing as a proportion phytoplankton carbon - higher in 2001 than in 2002 - 1.5 to 6% d -1 of water column carbon grazed by larval krill in 2001 vs < 0.25% in 2002 - varies within season - larval grazing can be 21-63% of the total loss factor in years of high larval abundance • daily ration supplied to larvae through grazing in water column - above minimum requirements in March - Palmer Basin 2002, earlier phytoplankton decline led to net carbon intake of zero by late March - net carbon intake zero by middle of May for all scenarios, chlorophyll no longer able to supply basic metabolic needs Conclusions • In years of high larval krill abundance, grazing by this component of the zooplankton community has a significant impact on the fall phytoplankton biomass. • The presence and/or timing of a fall bloom and the shape of the decline curve play an important role in whether the water column can supply the metabolic needs of larval krill until sea ice advances. Acknowledgments: Support by Office of Polar Programs, National Science Foundation (ANT 0529087) to Quetin, Ross, Vernet and Fritsen and by MSI (UCSB), SIO and DRI is gratefully acknowledged. Interactions between larval Antarctic krill, Euphausia superba and phytoplankton community may influence • variation in autumnal decline of chlorophyll a concentration and thus standing stock available for incorporation into sea ice as a basis for winter primary production • daily ration (% body carbon per day) available to larval krill in fall for their energetic needs: ~ 2% for basic metabolism (Frazer et al. 2002), with remainder for growth and possible storage Study Region of the West Antarctic Peninsula Palmer Basin 0 1 2 3 4 5 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Ave Chlorophyll (mg m -3 ) - 2001 2002 Marguerite Bay 0 1 2 3 4 5 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Ave Chlorophyll (mg m -3 ) 2001 2002 Marguerite Bay 2001 0.0 4.0 8.0 12.0 16.0 20.0 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Daily Ration (% body C d -1 ) 1.0 2.0 3.0 4.0 5.0 6.0 Grazing Pressure (% water column C d -1 ) %WC C d -1 Minimum body ration day ice greater than 3/10 % body C d -1 Marguerite Bay 2002 0.0 4.0 8.0 12.0 16.0 20.0 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Daily Ration (% body C d -1 ) 1.0 2.0 3.0 4.0 5.0 6.0 Grazing Pressure (% water column C d -1 ) %WC C d -1 % body C d -1 Palmer Basin 2001 0.0 4.0 8.0 12.0 16.0 20.0 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Daily Ration (% body C d -1 ) 1.0 2.0 3.0 4.0 5.0 6.0 % body C d -1 %WC C d -1 Grazing Pressure (% water column C d -1 ) Palmer Basin 2002 0.0 4.0 8.0 12.0 16.0 20.0 0 28 56 84 112 Day in Run (Mar 1 - Jul 1) Daily Ration (% body C d -1 ) 1.0 2.0 3.0 4.0 5.0 6.0 %WC C d -1 Grazing Pressure (% water column C d -1 ) % body C d -1 Fig. 1 and 2. Phytoplankton abundance during the fall of 2001 and 2002 in the Palmer Basin and Marguerite Bay, West Antarctic Peninsula Fig. 3-6. Larval krill daily ration in percent of body carbon ingested per day and the percent of water column phytoplankton carbon consumed per day by larval krill population in each of the four model simulations. Carbon in larva ave Chla WC mg per m3 mg wet mass per larval kril Daylength Fraction Sea Ice Ingestion Upper Water Column Ing av µgC per lar per hr Deep Water Column Ing Assimilation Efficiency delay ConFac Daylength Fraction Sea Ice Chla Ice C:Chla ratio Respiration ~ Ice Coverage latitude revAngle WC mgC per m2 declinAngle arcCosParam WC mg chl per m2 PP mgC per m2 per hr ~ WC C:Chla Delay Carbon ~ Water Temperature Day Start Day of the Year daylengthCoeff daylength Deep water Chl Water Column Ingestion Fecal Pellets Carbon per DT ~ Water Temperature mg wet mass per larval kril prop growth positive growth scenario negative growth scenario C accum per hr IMP 2 ConFac TL initial ave Chla specific PP per d Clarva C initial non graz loss facto lar krill graz mgC per m2 per wetwt initial ConFac initial TL Light factor F PB opt Integrated z growth rate ~ Zeu 1% SML Upper Water Column Ing av μgC per lar per hr ~ WC C:Chla phyto Chla To ave Chla WC mg per m3 lar krill abund no per m graz proportion Grazing by larval Antarctic krill and phytoplankton dynamics during the fall west of the Antarctic Peninsula
