Adelie penguin diet composition, krill size frequency distribution, 1991-2021

The fundamental long-term objective of the seabird component of the Palmer LTER (PAL) has been to identify and understand the mechanistic processes that regulate the mean fitness (population growth rate) of regional penguin populations. Two hypotheses have guided this research, with one suggesting that population mean fitness is best explained by changes in regional krill biomass, and the other proposing that long-term changes in sea ice affects mean fitness by tipping the balance in favor of one species over another in accordance with species-specific evolved life history affinities to sea ice. Although these hypotheses are not mutually exclusive, current evidence in the PAL region tends to favor the latter over the former. Since the inception of PAL, Adélie penguin populations have effectively collapsed, while those of gentoo and chinstrap penguins have increased dramatically, trends that are spatially and temporally coherent with decreasing regional sea ice duration. Adélie penguins are an ice-obligate polar species whose life history is intimately linked to the presence of sea ice, while chinstrap and gentoo penguins are ice-intolerant species whose life histories evolved in the sub-Antarctic, where sea ice is a less permanent feature of the marine ecosystem. In contrast, although krill constitute the most important component of the summer diets by mass of these three penguin species, changes in PAL krill abundances have exhibited no long-term trends, and thus fail to explain the divergent patterns in penguin populations evident in our time series. Three species of krill occur in Adélie penguin diet samples, Antarctic krill (Euphausia superba), crystal krill (E. crystallorophias) and Thysanoessa macrura (no common name), but beyond obtaining whole subsample weights following sorting (see DIET), only E. superba is subjected to further analysis. In this case, the original subsample from each individual penguin is searched with the objective of obtaining a minimum of 50 whole krill that when measured (in millimeters) provide an index of within and between season variability in population size-frequency distributions. These data have provided critical insights into a number of ecological processes, including in particular how sea ice affects krill recruitment and, in turn, how krill recruitment impacts Adélie penguin foraging efforts. Dr. Megan Cimino took over as PI of the LTER seabird project in 2020 from Dr. William Fraser. Field data collection between 2020-2022 has remained consistent with previous years. No samples collected during the 2021-2022 season due to the Palmer Station Pier Build.