Abstract
In southern Patagonia, the Beagle Channel shows very low production during winter but simultaneously sustains very dense aggregations of the pelagic stage of squat lobster ( Munida gregaria ), a benthic decapod whose pelagic juveniles have the largest body size within the chitinous pelagic community. To assess the coexistence of the mesozooplankton community and the pelagic M. gregaria stage under the harsh feeding winter conditions, we conducted a research cruise at two locations connected to the Beagle Channel, Yendegaia Bay (land terminating-glacier) and Pia Fjord (marine-terminating glacier). Our results showed that the zooplankton communities were similar in these two fjords, that a single pelagic group dominated in terms of biomass (pelagic Munida gregaria ), and that differences in vertical distribution existed between most of the principal crustacean zooplankton and pelagic M. gregaria . All groups showed consumption of terrestrially derived organic matter, as revealed by their δ 13 C values. However, the isotopic composition, trophic positions (TP), and isotopic niche areas of the groups separated pelagic M. gregaria , presenting some of the lowest δ 15 N and the highest δ 13 C values, and the narrowest isotopic niche width. Pelagic M. gregaria was dominated by a single body size class along the 0–100 m water column, with no diel changes in vertical distribution, remained mostly in the upper layers (0–50 m), and benefited from the slightly higher phytoplankton concentrations at shallower depths as revealed by their higher δ 13 C values and low trophic position. In contrast, the other groups, including zoea M. gregaria stages, developed changes in distribution between day and night or remained deeper in the water column. These groups showed higher δ 15 N values, higher TP, and lower δ 13 C values, most of which probably fed on a nanoheterotrophs and terrestrial particulate organic matter mixture at deeper layers. Thus, the different vertical distributions, different trophic level food sources, and slightly different organic carbon sources apparently reduced any potential competence for food resources and form part of the feeding strategy that may facilitate the coexistence of the different large pelagic crustaceans under harsh feeding winter conditions in this high latitude austral region.