Hydrography, circulation and suspended particle distribution in Ushuaia Bay and the Beagle Channel

FLORES-MELO, X.; MARTÍN, J.; DURRIEU DE MADRON, X.; BOURRIN, F.; KERDEL, L.; MALITS, A.

Congreso; Latin American Physics of Estuaries and Coastal Oceans (LAPECO); 2019

The Beagle Channel is a subantarctic passage connecting the Pacific and Atlanticoceans, where mixing processes between oceanic and continental waters and strongland-to-sea fluxes of water and particulate matter are taking place. A collection ofhydrographic, hydrodynamic and nephelometric observations is presented in this workto advance our knowledge of this fragile ecosystem. The waters infilling the channelhave their origin in the Cape Horn Current and are modified along the channel courseby freshwater inputs and mixing.Although the mean surface circulation is eastwardsalong the main channel, morphological constrictions hamper the flow of the deeperwaters below the wind mixed layer. In Ushuaia Bay, a counter-clockwise residualcurrent in the surface and a weak flow towards the head of the bay are permanentfeatures regardless of the season or tidal phase. The hydrographic structure of the watercolumn in the vicinity of Ushuaia presents a distinct seasonal pattern, consisting ofthorough mixing in winter and the development of a 50-70 m deep halocline fromspring to late autumn.The thermocline tends to invert from spring towards winter and isoften disconnected from the pycnocline, which is controlled by seasonal freshwaterinputs. Nepheloid layers are recurrently observed originating on the steep sides of thesubmerged glacial valley that connects Ushuaia Bay to the Beagle Channel; dissolvedoxygen is inversely related to particle concentration along these detachments. A bottomnepheloid layer tends to develop in summer, when the organic content of suspendedparticles is also at its maximum The combination of seasonal stratification, strong inputsof organic matter from natural and urban sources, and a closed circulation pattern resultsin the depletion of near-bottom oxygen in Ushuaia Bay at the end of the stratifiedseason, before the overturning of the water column in the austral winter.