General Hydrography of the Beagle Channel, a Subantarctic Interoceanic Passage at the Southern Tip of South America

The Beagle Channel (BC) is a long and narrow interoceanic passage (∼270 km long and 1–12 km wide) with west-east orientation and complex bathymetry connecting the Pacific and Atlantic oceans at latitude 55°S. This study is the first integrated assessment of the main oceanographic features of the BC, using recent oceanographic observations from cruises, moored instruments and historical observations. The waters transported into the BC are supplied mainly by the Cape Horn Current, which carries Subantarctic Water (SAAW) at depth (100 m below surface) along the Pacific Patagonian continental shelf break. SAAW enters the continental shelf via a submarine canyon at the western entrance of the BC. The SAAW is diluted by fresh, nutrient depleted (nitrate, phosphate and silicic acid) Estuarine Water (EW) from Cordillera Darwin Ice Field (CDIF) forming modified SAAW (mSAAW). Freshwater inputs from the CDIF generate a two-layer system with a sharp pycnocline which delimits the vertical distribution of phytoplankton fluorescence (PF). Two shallow sills (<70 m) along the BC contribute to EW and mSAAW mixing and the homogenization of the entire water column east of the sills, coherent with Bernoulli aspiration. The central section of the BC, extending ∼100 km toward the east, is filled by a salty (31–32) variety of EW. In winter, this central section is nearly vertically homogeneous with low nutrient concentrations (0.9–1.1 μM PO4 and 7.5–10 μM NO3) and PF. The temporal variability of seawater temperature from 50 to 195 m in the central section of the BC was found to be mostly dominated by the annual and semiannual cycles and influenced by tidal forcing. The middle section of the BC was less influenced by oceanic inputs and its basin-like structure most likely favors retention, which was observed from the weakly stratified water column at the mooring site. Toward the east, the central section bathymetry is disrupted at Mackinlay Strait where another shallow sill separates the middle channel from the shallow eastern entrance that connects to the Atlantic Ocean. In this section, a weakly stratified two-layer system is formed when the eastward surface outflow (salty-EW) flows over a deeper, denser tongue of oceanic mSAAW.

Separate Feeding Between the Pelagic Stage of the Squat Lobster Munida gregaria and the Larger Sized Zooplankton Crustacean Groups in the Beagle Channel as Revealed by Stable Isotopes

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 δ13C values. However, the isotopic composition, trophic positions (TP), and isotopic niche areas of the groups separated pelagic M. gregaria, presenting some of the lowest δ15N and the highest δ13C 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 δ13C 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 δ15N values, higher TP, and lower δ13C 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.

Are Antarctic and sub-Antarctic marine food webs different?

Aim: Food web structure plays an important role in determining ecosystem stability to perturbations. High latitude marine ecosystems are being affected by environmental stressors and ecological shifts. In the West Antarctic Peninsula these transformations are driven by climate change, and in the sub-Antarctic region by anthropogenic activities. Understanding the differences between these areas is necessary to monitor the changes that are expected to occur in the upcoming decades. Here, we compared the structure and stability of Antarctic and sub-Antarctic marine food webs. Location: Antarctic (Potter Cove, 25 de Mayo/King George Island, West Antarctic Peninsula) and sub-Antarctic (Beagle Channel, Tierra del Fuego, South America) regions. Time period: 1965 - 2019. Major taxa studied: from phytoplankton to fish. Methods: We compiled species trophic (predator-prey) interactions and calculated complexity (number of species and interactions, connectance), structure (mean trophic level, omnivory, degree distribution, modularity, species roles and traits) and stability (QSS) metrics. To be able to make statistical comparisons, we used a randomization algorithm (Strona Curveball) maintaining the number of prey and predators for each species and calculated metrics for each simulation. Results: The Beagle Channel food web presented higher values for complexity metrics (number of species and interactions), structure (mean trophic level, omnivory, modularity) but lower stability (QSS). Potter Cove fitted the exponential degree distribution, while Beagle Channel the power-law with exponential cutoff model. Both food webs presented the same connectance value (0.05), similar distribution of species in top, intermediate and top positions and topological roles, with only one network connector each. Main conclusions: Our results showed that Beagle Channel food web is more complex, but less stable and sensitive to the loss of its most connected species. While the Potter Cove food web presented less complexity and greater stability to perturbations.