The Influence of River Discharge on Nutrient Export and Phytoplankton Biomass Off the Central Chile Coast (33°–37°S): Seasonal Cycle and Interannual Variability

Using in situ hydro-chemical data and MODIS-SeaWiFS ocean color images as a proxy of river plumes and phytoplankton biomass from 2000 to 2014, this study documents the temporal co-variability of river discharge, plume area, nitrate and phosphate export and phytoplankton biomass in the coastal waters off Central Chile (33°–37°S). Five major rivers (Maipo, Mataquito, Maule, Itata, and Biobío) drain into this region with annual mean discharge ranging from 120 to 1000 m3 s−1. River discharge and coastal plume area present a marked seasonal cycle, reaching maximum values during the winter rainy season (June-September). Export of riverine nutrients also peaks in winter, leading to an increase in phytoplankton biomass within the plumes that can be twice larger than the background values in coastal areas away from the river mouths. Wintertime river discharge, plume area and nutrient export are also correlated at interannual time scales. During a recent extended dry period (2010–2014), river discharges, plume areas and nutrient export clearly decreased by about 50% compared to historical values, reducing significantly the size of the chlorophyll pool within plumes off Central Chile during winter. The potential impacts of droughts are discussed in terms of coastal ecology and primary production, a highly relevant issue considering the projections of a dry climate over Central Chile in the future. Systematic evidence of mega-drought effects upon coastal productivity still does not exist, but it remains a priority to further investigate and quantify these impacts

Temporal Variability in Net Primary Production in an Upwelling Area off Central Chile (36°S)

The temporal variability of Net Primary Production (NPP) off central Chile (36°S, 73°W), an area subjected to seasonal coastal upwelling, was analyzed using monthly in situ13C incubations within the photic zone, along with bio-oceanographic variables from a fixed time series station; and satellite NPP estimations (NPPE) from the Vertically Generalized Production Model between 2006 and 2015. NPP and NPPE rates varied from 0.03 to 18.29 and from 0.45 to 9.07 g C m−2 d−1, respectively. Both rates were fairly well correlated with each other (r2 = 0.61), but when these data were separated into two periods, higher r2 value was found during winter (r2 = 0.70) with respect to the rest of the year (r2 = 0.24); the latter correlation was partially due to increased weekly NPPE variability during active and relaxed upwelling events. NPP rates along with other biophysical variables allowed for a division of the annual cycle into three distinct periods: September to January (high productivity, mean integrated NPP rates of 4.0 g C m−2 d−1), February to March (intermediate productivity, mean integrated NPP rates of 1.4 g C m−2 d−1), and May to August (basal level, mean integrated NPP rates of 0.5 g C m−2 d−1). NPP appeared to be partially controlled by nutrient inputs, either from upwelling (September-April) and river discharge (May-August), maintaining high NPP rates throughout the entire year, with an annual mean NPP rate of 1.1 kg C m−2 yr−1. In this region, El Niño Southern Oscillation events did not appear to impact the NPP interannual variability

Inter-annual variability of oceanographic conditions and phytoplankton in Valparaíso Bay (~33°S), central Chile

The inter-annual variability of oceanographic conditions and phytoplankton abundance and biomass was studied using 10 years (1986-1996) of in situ observations at a fixed station in Valparaíso Bay (~33ºS). The time series analysis revealed that strong S-SW winds drive a quasi-permanent upwelling activity that maintain the nutrients availability in the water column to fuel the phytoplankton in the bay. The most important changes in bio-oceanographic conditions were observed during 1987 El Niño event, which was characterized by higher temperatures but lower values for salinity, nitrate, and phosphate concentrations, together with a decrease of phytoplankton biomass compared to 1988 La Niña event when colder, saltier, and nutrient-rich upwelled water were observed. High Aconcagua River discharges were observed during the El Niño conditions (1987-1988 and 1993), which led to a decrease in surface salinity and a high abundance of dinoflagellates. Two periods with differences in bio-oceanographic conditions were observed; 1988-1992 showed lower temperatures but higher nutrients (nitrate), phytoplankton biomass and abundance of diatoms than 1993-1996 period. Throughout the study period, positive trends in upwelling activity was registered accompanied by a fall in phytoplankton biomass and dinoflagellate abundance. This paper provides new evidences concerning the influence of El Niño Southern Oscillation (ENSO) in phytoplankton and oceanographic conditions in the coastal upwelling off central Chile

Inter-annual variability of oceanographic conditions and phytoplankton in Valparaíso Bay (~33°S), central Chile

The inter-annual variability of oceanographic conditions and phytoplankton abundance and biomass was studied using 10 years (1986-1996) of in situ observations at a fixed station in Valparaíso Bay (~33ºS). The time series analysis revealed that strong S-SW winds drive a quasi-permanent upwelling activity that maintain the nutrients availability in the water column to fuel the phytoplankton in the bay. The most important changes in bio-oceanographic conditions were observed during 1987 El Niño event, which was characterized by higher temperatures but lower values for salinity, nitrate, and phosphate concentrations, together with a decrease of phytoplankton biomass compared to 1988 La Niña event when colder, saltier, and nutrient-rich upwelled water were observed. High Aconcagua River discharges were observed during the El Niño conditions (1987-1988 and 1993), which led to a decrease in surface salinity and a high abundance of dinoflagellates. Two periods with differences in bio-oceanographic conditions were observed; 1988-1992 showed lower temperatures but higher nutrients (nitrate), phytoplankton biomass and abundance of diatoms than 1993-1996 period. Throughout the study period, positive trends in upwelling activity was registered accompanied by a fall in phytoplankton biomass and dinoflagellate abundance. This paper provides new evidences concerning the influence of El Niño Southern Oscillation (ENSO) in phytoplankton and oceanographic conditions in the coastal upwelling off central Chile.Se estudió la variabilidad inter-anual de las condiciones oceanográficas y de la abundancia y biomasa del fitoplancton en una estación fija ubicada en la bahía de Valparaíso (~33ºS) usando 10 años (1986-1996) de observaciones in situ. La serie temporal mostró vientos S-SO intensos y una cuasi-permanente actividad de surgencia que enriquecen de nutrientes la columna de agua y sustentan al fitoplancton en la bahía. El cambio más importante en las condiciones bio-oceanográficas fue observado durante el evento de El Niño 1987, con aumento de la temperatura, caída en la salinidad y en la concentración de nitrato y fosfato, así como en la biomasa del fitoplancton en comparación con La Niña 1988 cuando se detectó agua mas fría, salada y rica en nutrientes. Bajo condiciones El Niño (1987-1988 y 1993) se registraron altas descargas del río Aconcagua que precedieron la caída en la salinidad y el aumento en la abundancia de dinoflagelados. Se observaron dos periodos con diferencias en las condiciones oceanográficas; 1988-1992 presentó baja temperatura e incremento en los nutrientes (nitrato), la biomasa del fitoplancton y en la abundancia de diatomeas en comparación al periodo 1993-1996. A lo largo del periodo de estudio se registró una tendencia positiva en la surgencia, acompañada con una caída en la biomasa del fitoplancton y la abundancia de dinoflagelados. Este estudio proporciona nuevas evidencias sobre la influencia de El Niño Oscilación del Sur (ENOS) en el fitoplancton y las condiciones oceanográficas en la zona costera frente a Chile central

Dynamique des cyanobactéries fixatrices d'azote (Trichodesmium) (du Pacifique Sud-Ouest à l'océan global)

La fixation d azote, N2 (N2fix) par les cyanobactéries, avec une introduction d azote nouveau dans la couche éclairée, contrôlerait dans l océan le déséquilibre actuel du cycle de l azote, la productivité des régions les plus pauvres (oligotrophes) et la quantité de CO2 de l atmosphère pompée par l océan. Comment varie et quels sont les processus physiques et biogéochimiques qui contrôlent la distribution de cyanobactéries fixatrice d azote et leur N2fix dans l océan ? Quel est l effet de rétroaction de la fixation de N2 sur la productivité et sur la pompe biologique de CO2? Afin de répondre à ces questions, seront évaluées la distribution et la N2fix due à la plus cosmopolite de cyanobactéries : Trichodesmium. En partant d une étude focalisée dans le Pacifique sud-ouest, les résultats seront étendus à l océan global. La stratégie est pluridisciplinaire et couple l acquisition de nouvelles données in situ, in situ simulées, en laboratoire; satellitales (SeaWiFS) et le développement de modèles mathématiques. La N2fix de Trichodesmium est majoritaire dans : les régions oligotrophes de gyres anticycloniques subtropicales de grands bassins, qui associées à de convergences, restent pauvres en nutriments et en métaux et lors de conditions de forte stratification et fort éclairement (l été et jusqu à 20m). La fixation de N2 serait en conséquence plutôt contrôlée par la disponibilité lumineuse qui imposent les forts besoins énergétiques de ce mécanisme que par le fer. Se développant sous la forme d accumulations étendues, ces cyanobactéries apparaissent comme un moteur biologique de fertilisation, qui conduit à la formation des oasis étendues de relative forte biomasse phytoplanctonique et forte séquestration de CO2. La fabrication de ces oasis modifie la vision traditionnelle d extrême pauvreté et de faible pompage biologique de CO2 des océans oligotrophes.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Influence of coastal upwelling on micro-phytoplankton variability at Valparaíso Bay (~33ºS), Central Chile

Abstract: In this work 10 years of data (1986-1996) from a fixed station located in the northern part of Valparaíso Bay (33º00’S; 71º35’W) were analysed to study the influence of coastal upwelling activity on the temporal variation of micro-phytoplankton (20-200 μm) and their relationship with oceanographic conditions. The upwelling activity at the bay was associated to semi-annual wind regime with an intensification of upwelling-favourable S-SW winds from September to March followed by a decrease and the occurrence of downwelling events from April to August. Oceanographic conditions showed the ascent of cold, nutrient-rich salty water in spring (September-November). However, during summertime under highest upwelling index, thermal stratification conditions were registered. This stratification might be associated to either the solar radiation or the presence of an upwelling shadow area in the bay. The upwelling period had the highest micro-phytoplankton abundance mainly dominated by diatoms. This period was associated with an increase in biomass and richness in the bay. Meanwhile during non-upwelling period -under homogenous conditions of temperature, salinity and nutrients- an increase in diversity (but low abundance and richness) associated to dinoflagellates and silicoflagellates was noted. Therefore, the results suggest the presence of a bi-modal regime of micro-phytoplankton in the bay in response to changes in oceanographic conditions related to local wind forcing and mixing/stratification.Resumen: Se analizaron 10 años de datos (1986-1996) registrados en una estación fija ubicada al norte de la bahía de Valparaíso (33º00’S; 71º35’O), con el objetivo de estudiar la influencia de la surgencia costera en la variación temporal del micro-fitoplancton (20-200 μm) y su relación con las condiciones oceanográficas. La surgencia costera mostró un régimen bi-modal asociado al viento, con dominio de vientos S-SO favorables a la surgencia entre septiembre y marzo, seguido de una disminución en la intensidad del viento y un debilitamiento de la surgencia entre abril y agosto. Las condiciones oceanográficas mostraron el afloramiento de agua fría y salada, con alto contenido de nutrientes en primavera (septiembre-noviembre). Sin embargo, en verano bajo intensa surgencia se registraron condiciones de estratificación térmica. Esta estratificación podría asociarse tanto a la radiación solar como a la presencia de un área de sombra de surgencia en la bahía. El periodo de surgencia activa presentó el máximo en la abundancia del micro-fitoplancton, dominado por las diatomeas. Este periodo también se asoció con un aumento en la biomasa y la riqueza del micro-fitoplancton. Durante el periodo de debilitamiento de la surgencia -bajo condiciones homogéneas de temperatura, salinidad y nutrientes- se registró un incremento en diversidad (pero baja abundancia y riqueza) asociado al aumento de dinoflagelados y silicoflagelados. Los resultados muestran un régimen bi-modal del micro-fitoplancton en la bahía en respuesta a los cambios en las condiciones oceanográficas relacionados con el forzante local del viento y las condiciones de mezcla/estratificación

Influence of coastal upwelling on micro-phytoplankton variability at Valparaíso Bay (~33ºS), Central Chile

In this work 10 years of data (1986-1996) from a fixed station located in the northern part of Valparaíso Bay (33º00’S; 71º35’W) were analysed to study the influence of coastal upwelling activity on the temporal variation of micro-phytoplankton (20-200 μm) and their relationship with oceanographic conditions. The upwelling activity at the bay was associated to semi-annual wind regime with an intensification of upwelling-favourable S-SW winds from September to March followed by a decrease and the occurrence of downwelling events from April to August. Oceanographic conditions showed the ascent of cold, nutrient-rich salty water in spring (September-November). However, during summertime under highest upwelling index, thermal stratification conditions were registered. This stratification might be associated to either the solar radiation or the presence of an upwelling shadow area in the bay. The upwelling period had the highest micro-phytoplankton abundance mainly dominated by diatoms. This period was associated with an increase in biomass and richness in the bay. Meanwhile during non-upwelling period —under homogenous conditions of temperature, salinity and nutrients— an increase in diversity (but low abundance and richness) associated to dinoflagellates and silicoflagellates was noted. Therefore, the results suggest the presence of a bi-modal regime of micro-phytoplankton in the bay in response to changes in oceanographic conditions related to local wind forcing and mixing/stratification

A comparison of spring coastal upwelling off central Chile at the extremes of the 1996-1997 ENSO cycle

Southerly wind pulses are repeatedly observed in connection with upwelling off Point Curaumilla (33°S), around 20km south of Valparaíso. The remote forcing of coastal upwelling associated with intraseasonal coastal-trapped waves and the local, wind-driven upwelling forcing, are characterised there in terms of sea level, meteorological observations, surface weather charts and satellite-derived SST and chlorophyll a, from observations taken in late austral springs of 1996 (La Niña) and 1997 (El Niño). Warm and cold coastal SST periods lasting from 15 to 20 days are associated with intraseasonal (30-50-day periods) coastal-trapped waves which are detected in time series of adjusted sea level. These waves produce shoaling/deepening cycles in the thermocline depth and hence a modulation of the local wind-driven upwelling efficiency in bringing cold, nutrient-rich waters to the surface. The atmospheric forcing of the wind-driven upwelling pulses is closely related to atmospheric coastal-tra