Biogeochemical response of the pelagic system of Concepción Bay, Chile to atmospheric and oceanographic variability in the context of extreme natural stressors
Concepción Bay – CB is a socio-economic and ecologically important embayment, whose variability is regulated by wind-modulated seasonal upwelling during spring-summer and by freshwater from precipitation and stream flow during fall-winter. This system is subject to several anthropogenic and environmental strains due to the intense port activity and the increasing occurrence of extreme natural events. In this work, the seasonal variability of the hydrobiological pelagic conditions were characterized throughout 2018, together with the pluviometry and tributary stream flow. The seasonality of the system in response to nitrate-rich oxygen-poor water injection, that enhanced productivity, was evident. However, the system remains fertilized throughout the year, due to the occurrence of upwelling-favorable intermittent pulses during fall-winter. Furthermore, CB functions as a two-layer system: thermally stratified in spring-summer and by freshwater inputs in autumn-winter. Nevertheless, changes in the regular pattern of the rainfall that include a diminishing in precipitation and an increased frequency of extreme rainfall events during fall and spring, together with recurrent upwelling-favorable wind pulses during the non-upwelling season (fall-winter), have altered the seasonality of the physicochemical conditions and the structure of the phytoplanktonic communities, with productive and sanitary implications that affect not just the biogeochemical behavior but the ecosystem functions.
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Continuous hydrographic profiles were obtained using a Seabird 19 plus conductivity-temperature-depth device (CTD), outfitted with oxygen (Seabird 43; accuracy 2% of saturation) and fluorescence sensors (Wet Labs FLRT 3945). Discrete water samples were taken in triplicate at two strati: surface and near bottom depth, using a 5 L Niskin bottle (General Oceanics), to determine: dissolved oxygen (DO), chlorophyll (Chl-a), total suspended solids (TSS), nutrients (i.e., NO3-, NO2-, and NH4+), hydrogen sulfide (H2S), picoplankton, total and fecal coliforms, and phytoplankton abundances and structure. DO concentrations were analyzed in 125 mL glass bottles by the Winkler titration method. In order to get the total and size-fractionated Chl-a, i.e., <3, 3-20, >20 µm cells size, seawater was filtered onto combusted (12 h; 450ºC) 0.7, 3.0, and 20 µm glass-fiber filters (type GF-F), respectively, and measured by fluorometric analysis (Turner Design AU-10) according to standard procedures. Samples (up to 3 L) for total suspended solids were filtered using combusted GF-F filters (0,7 µm) and analyzed by 2540-D method. Samples for H2S were fixed by the addition of saturated ZnCl2 and determined spectrophotometrically with the methylene blue method. Nutrients (NO3-, NO2-, and NH4+) samples were filtered and frozen until further analysis.