SUSPENDED PARTICLE DISTRIBUTION IN RESPONSE TO HYDROGRAPHIC GRADIENTS IN USHUAIA BAY AND THE BEAGLE CHANNEL (TIERRA DEL FUEGO, ARGENTINA)

J. Martín1; X. Flores-Melo1; L. Kerdel2; F. Bourrin2; X. Durrieu de Madron2; C. Colloca3

ABSTRACT - The Beagle Channel and in particular Ushuaia Bay, are subantarctic environments where mixing processes between oceanic and continental waters, and strong land-to-sea exchanges of water and particulate matter are taking place. Multiple sources of particulate matter to the channel can be identified: glaciers, rivers, direct runoff from forests along steep slopes next to the seashore, marine primary production as well as an anthropogenic contribution related to navigation and human settlements, especially the city of Ushuaia. This study is a preliminary assessment of suspended matter concentrations, composition, distribution and particle size spectra in Ushuaia Bay and the neighbouring Beagle Channel for two contrasting hydrographic settings: late summer stratified and late winter unstratified conditions. The relationships of the nephelometric and particle spectra results are compared with the main hydrographic features. Intermediate nepheloid layers evolving into bottom nepheloid layers are recurrently observed on the slopes of the 150-m deep drowned glacier valley that occupies the eastern half of Ushuaia Bay. These relatively turbid layers are associated with relative oxygen minima that in certain years result in a hypoxic area developing in the deep valley. The volumic size spectra of suspended particles is dominated by aggregates of few tens to few hundreds microns. Particle size spectra show a bimodal distribution in the austral winter that tends to unimodal in the austral summer, in concordance with the vertical structure of the water column: seasonal halocline separates both particle pools in summer while winter mixing homogenizes them.

Keywords – Suspended particulate matter, particle size spectra, Beagle Channel

1) Centro Austral de Investigaciones Científicas (CADIC-CONICET), Bernardo Houssay 200, V9410CAB Ushuaia, Tierra del Fuego. Argentina, [email protected], Tel: +54 2901 422310 2) Centre d’Etude et de Formation sur les Environnements Méditerranéens (CEFREM, CNRS-UPVD) 52, avenue Paul Alduy, 66860 Perpignan Cedex, Email: [email protected]; Tel: +33 4 68 66 22 48 3) Universidad de Tierra del Fuego (UNTDF). [email protected] XIII Encontro Nacional de Engenharia de Sedimentos 1 I Partículas nas Américas 1 - INTRODUCTION

The Beagle Channel (Tierra del Fuego, Argentina), situated at the southernmost point of South America, is a tectonic valley repeatedly carved by glacial cycles (Rabassa, 2008; Bujalesky, 2011). During the last transgression, the sea inundated the valley, which in the present sea-level stand forms an uninterrupted conduit between the Pacific and Atlantic Oceans. The channel presents an irregular bathymetry with multiple shoals, islands and straits dividing relatively deeper (150-220 m) areas along its path. The tidal regime is almost microtidal (typical amplitudes 1 to 2 m; D’Onofrio, 1989). Wind waves are also moderate, with typical wave heights of 0.5 m and periods 1-3 s (Bujalesky et al., 2007), due to relative sheltering from the prevailing SW winds and to the limited fetch that the channel offers. In spite of its morphological departure from a typical fjord, it has been suggested that the Beagle Channel has in general terms a ‘fjord-like’ circulation (Isla et al., 1999). Freshwater inputs to the Beagle Channel display both a seasonal and spatial variability. The Island of Tierra del Fuego presents a marked climatic zonation owing to its orography and the prevailing westerlies that transport great amounts of humidity. Water and snow precipitations decrease from west to east. This, coupled with an additional negative gradient of freshwater inputs from west to east, determines a general surface flow in the same direction. Circulation in Ushuaia Bay displays a residual counter-clockwise circulation (Balestrini et al., 1990). This relatively closed circulation in Ushuaia Bay, coupled with enhanced inputs of organic particles from both natural and anthropic sources has been signaled as a cause for the low (3-6 mg/L) concentrations of dissolved oxygen found in the deeper sectors of the bay (Martín et al., 2016).

Figure 1 – Sampling strategy. Grid of profiling stations (dots) visited in August 2017 and April 2018. The yellow star marks the station depicted in figure 2. The dashed line indicates the limit between Argentinian and Chilean territorial waters. Bathymetry data digitalized from Chart H-477 (Servicio de Hidrografia Naval). XIII Encontro Nacional de Engenharia de Sedimentos 2 I Partículas nas Américas 2 - MATERIALS AND METHODS

The present study is based on fieldwork conducted during the austral late winter of 2017 and the late summer of 2018, which comprised a network of stations centered in Ushuaia Bay (Figure 1). A self-contained CTD “Rinko ASTD-102” was used to obtain vertical profiles of temperature, salinity, dissolved oxygen, fluorescence of chl-a, and water turbidity (optical backscatter sensor). Hydrographic data was processed according to routine procedures and displayed using the Ocean Data View software (Schiltzer, 2017). In all cases, simultaneous in situ measurements of particle size distribution (PSD) were obtained by means of a Laser In Situ Scattering and Transmission (LISST-100X) and a digital holographic camera (LISST-HOLO). The LISST-100X (Type C, 2.5–500 μm range, 1 Hz) was used to estimate the in situ PSD in the water column, and also the PSD of primary particles composing the particle assemblage in water samples. Water samples were deflocculated by ultrasonification for 5 min. PSD was then estimated from 1 min average LISST-100X measurements. PSD was derived from the laser diffraction spectrum using the “randomly shaped” Mie's theory (Agrawal et al., 2008). The raw spectrum was converted in volume concentration (expressed in μL L−1) using the factory volume calibration constant. Extreme size classes (1.25–2.5 and 250–500 μm) showed typical “rising tails” explained by the presence of smaller particles (for the first class) and larger particles (for the last class) outside the measurement range. Except primary particle characterization, we excluded the extreme size classes of the PSD before calculating the derived parameters (total volume concentration). The digital holographic camera (LISST-HOLO, 0.2 Hz) was used to study the volume, and nature of large particles, within the range 20–2000 μm. PSD was estimated using an image analysis processing (based on the estimate of the spherical equivalent diameter) and the size spectrum was expressed in μL L−1. In order to compare and merge the PSD derived from the LISST-100X and LISST-HOLO, only the measurements made near the surface and the bottom were considered. Data at both levels were collected for about 2 minutes, which allowed to gather ~100 scans for the LISST-100X, and ~20 images for the LISST-HOLO. These data were averaged for each level and each station. The LISST-100 and LISST-HOLO PSD were finally merged within a 2.5–1000 μm size range to fit within the same 50 logarithmically spaced classes and the maximum volume PSD of each median size class was selected to constitute the final PSD. Water samples were collected at selected depths to calibrate optical sensors.

XIII Encontro Nacional de Engenharia de Sedimentos 3 I Partículas nas Américas 3 - RESULTS AND DISCUSSION

The two periods studied (August 2017 and April/May 2018) revealed contrasting hydrographic settings (Figure 2). In autumn, an halocline arising from freshwater inputs during spring-summer divides the water column into two water bodies with different properties in terms of temperature, salinity and dissolved oxygen. Also, two different particle size classes were identified in each of these compartments.

Figure 2. Hydrographical (top), bio-optical and nephelometric (middle) vertical profiles, and particle size spectra (lower panel) at different depth ranges (indicated by vertical arrows in middle panel) in a station representative of the deep glacial valley next to Ushuaia (see fig. 1 for location) during winter (left column) and Autumn (right).

XIII Encontro Nacional de Engenharia de Sedimentos 4 I Partículas nas Américas In winter, water column overturning in the deepest parts of the channel and Ushuaia bay results in a more homogeneous vertical distribution of suspended particles and water properties (Fig. 2). Bottom nepheloid layers of moderate turbidity (1-10 FTU) were observed along the deep (100-200 m) sectors of the study area, particularly during the stratified season. Detachments of intermediate nepheloid layers were recurrently observed along the steep slopes bordering Ushuaia Bay. Suspended particle concentration and dissolved oxygen are inversely correlated in these relatively turbid layers in winter (Figure 3), which points to strong inputs of organic aggregates from the water column and coastal sources and/or anoxic sediments being resuspended. By the last stages of the stratified season (April/May), the inputs from these intermediate turbid layers have formed a thick bottom nepheloid layer where oxygen concentrations have declined substantially with respect with the concentrations found in winter at the same depths (Fig. 3). This study helps in advancing our knowledge of particulate matter dynamics in this sensible environment and in particular the mechanisms underlying episodes of hypoxia previously detected in Ushuaia Bay (Martín et al., 2016).

Figure 3. Interpolated contours of dissolved oxygen concentration (isolines) and turbidity (color gradient) from vertical profiles taken across Ushuaia Bay (stations on right panel), during mixed (top left) and stratified conditions (bottom left).

XIII Encontro Nacional de Engenharia de Sedimentos 5 I Partículas nas Américas ACKNOWLEDGMENTS

This work was funded by a subsidy from the French-Argentinian binational progam MINCYT- ECOS/ECOS-SUD (PA17A04) and also by projects PICT 2014/3106 (FONCYT, Argentina) and P-UE 2016: 22920160100077CO (CONICET, Argentina).

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