Temporal variability of the physical and chemical environment, chlorophyll and diatom biomass in the euphotic zone of the Beagle Channel (Argentina): Evidence of nutrient limitation

Abstract

Coastal, semi-enclosed aquatic ecosystems of sub-polar regions surrounded by continental glaciers are among the most sensitive environments to climate change. The Beagle Channel (Argentina-Chile, ~55°S, 71°-66°W), which is part of the Patagonian Cold Estuarine System of austral South America, receives fresh water input from rivers, glaciers and mountain ice sheets. Like in most fjord systems, its productivity relies on the input of nutrients from marine and terrestrial sources. Bathymetry and circulation patterns in the western (inner) sector of the Beagle Channel are known to differ from those of the eastern (outer) sector. Physical and chemical conditions of the water column and climatic trends in the area have been understudied, as all previous surveys were strongly limited in time, spatial coverage and water depth (especially in wintertime). For the first time and during a 2.5-year period (2012–2014) this study covers the spatial and temporal variation of hydrographic and meteorological conditions, total chlorophyll concentration and diatom biomass at three different sites within the Argentine sector of the Beagle Channel. An overall vertical homogeneity and silicate limitation for diatom growth were observed in the euphotic zone of both the inner and outer sectors. These two sectors, however, differed in other ways: salinity was always higher in the outer sector occasionally showing a west-east gradient. Conversely, total chlorophyll and diatom biomass were higher in the inner sector at every sampling date. In both sectors, biotic and abiotic variables showed a seasonal pattern with clear differences between summer and winter. Nevertheless, interannual differences may cast shadow over seasonal patterns: The warmer summer and milder winter of year 2013, together with a delay in spring stratification, led to higher nutrient concentrations, lower total chlorophyll and lower diatom biomass during that year as compared to years 2012 and 2014. Inorganic nutrients and chlorophyll levels found in this study were compared with all available literature on the Beagle Channel. The present results represent a base line to encourage further time-sustained investigation relating biogeochemical, oceanographic and phytoplanktonic processes involving mainly silicate-dependent organisms. In turn, they will allow tracking changes and responses of the Beagle Channel ecosystem to climatic-oceanographic phenomena (e.g., ENSO, SAM) and climate change.