STABLE-ISOTOPE COMPOSITION (DELTA-C-13 AND DELTA-N-15) OF LARVAL KRILL, EUPHAUSIA-SUPERBA, AND 2 OF ITS POTENTIAL FOOD SOURCES IN WINTER

Natural abundances of C-13 (delta(13)C) and N-15 (delta(15)N) were mea sured in larval krill (Euphausia superba), suspended particulate organ ic matter (POM) and ice associated POM during early and late winter al ong the west coast of the Antarctic Peninsula. Larval krill were enric hed in C-13 (delta(13)C greater than or equal to 27 parts per thousand ) relative to both larvae and adults sampled during summer months (del ta(13)C generally less than or equal to 27 parts per thousand). Elevat ed delta(13)C values were also recorded in suspended POM (delta(13)C g reater than or equal to 21 parts per thousand) during early winter. Th ese data imply that (i) seasonal shifts in the isotopic composition of larval krill need not result from changes in diet and (ii) mechanisms other than CO2 limitation in the ice can account for C-13 enrichments in ice-associated POM. Stable carbon isotopes could not be used, in t his study, to discern between suspended POM and ice-associated POM as alternative food sources for larval krill. During one early winter sam pling period, larval krill were markedly depleted in N-15 (delta(15)N < 1 parts per thousand), suggesting that they are primarily herbivorou s prior to exploiting ice-associated food resources. Mechanisms are pr oposed to explain variation in the isotopic composition of POM and lar val krill, and will be of particular interest to those investigating f ood web dynamics and biogeochemical processes in the region.