Exploring algae – zooplankton relationships in the East Antarctic sea ice zone using Stable Isotope Mixing Models
Kerrie Swadling

Authors

Kerrie Swadling
Institute for Marine and Antarctic Studies, Hobart, University of Tasmania, PB 129, Hobart TAS 7001, Australia

Hugh Jones
Institute for Marine and Antarctic Studies, Hobart, University of Tasmania, PB 49, Hobart TAS 7001, Australia

Abstract

Sea ice provides habitat for high standing stocks of bacteria, algae, protists and invertebrates. The most commonly encountered invertebrates are crustaceans, mainly copepods, which inhabit the brine channel system of the ice-crystal matrix or live at the under-ice surface. Amphipods and krill also live in crevices beneath the sea ice where they forage and find refuge from predators. The trophic status of these ice-associated grazers remains largely unknown, including the relative importance of phytoplankton, ice algae and small metazoans to their diets. A Bayesian isotopic mixing model, SIAR (Stable Isotope Analysis in R), was used to assess the contributions of sea ice algae and phytoplankton to the diets of invertebrates inhabiting the East Antarctic pack ice. Phytoplankton can be distinguished from sea ice algae by their different isotopic profiles. Stable isotope ratios of carbon (13C/12C = del13C) show little enrichment (0.8-1‰) and usually reflect dietary composition, while stable nitrogen isotope ratios (15N/14N = del15N) exhibit a constant rate of incremental enrichment between trophic levels, typically 3 – 4‰, thereby supplying a direct measure of trophic hierarchy. Sea ice algae were generally enriched in carbon over phytoplankton, with the surface-most ice showing the largest difference. For the dominant crustacean grazers nitrogen isotopes suggested mainly herbivory for Calanus propinquus, Thysanoessa macrura and Metridia gerlachei, but omnivory for Euphausia crystallorophias, Euphausia superba and amphipods. The contribution that sea ice algae made to the diets of these species ranged from 0 to > 60% and, importantly, varied substantially in time and space.