On the ecology of larval krill, Euphausia superba, during winter: Krill-sea ice interactions

Antarctic krill, Euphausia superba, is the dominant zooplankter and a keystone species in the Southern Ocean. Krill has a broad circumpolar distribution, with highest concentrations generally restricted to areas affected by seasonal advance and retreat of sea ice. Both adults and larvae have been observed to feed on ice-associated food resources, and larval krill may be obligate consumers of ice-algae during winter. The degree of dependence of larval krill on ice and its associated food resources, however, is based largely on anecdotal information.; Chapters I and II of this work contain the first quantitative characterization of the abundance and distribution of larval krill directly associated with sea ice west of the Antarctic Peninsula during winter, and establish that a large portion of the larval population occupies the ice habitat. Samples of larvae collected by SCUBA divers and with nets are compared with respect to abundance, size and stage composition in Chapter II. Larval aggregation size and frequency of occurrence are reported to differ in early and late winter. Details of patch characteristics for larvae in the sea ice are given, and several possible explanations for the observed small-scale distribution patterns are discussed.; The use of stable isotopes as a tool to quantify the assimilation of ice-associated biota in the winter diet of larval krill is evaluated in Chapter III. Reported variation in the stable carbon and nitrogen isotope composition of larval krill and suspended particulate organic matter, particularly in early winter, has significant consequences for the interpretation of field data. A complete isotopic characterization of the early life history of krill (egg through juvenile stage) is provided in this section. Use of stable isotopes as tracers to measure carbon and nitrogen turnover in larval krill is discussed in Chapter IV. These are the first coupled measures of carbon and nitrogen turnover, using this approach, for any marine organism, and provide essential information for the interpretation of isotopic data collected from the field. The results of this research will be of interest to krill biologists as well as those studying food web interactions and/or biogeochemical cycling in a broader ecosystem context.