Aportes continentales y oceánicos de nutrientes inorgánicos al golfo de Arauco (Región del Biobío) y un análisis del estado trófico.
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Date
2024
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Universidad de Concepción
Abstract
En las zonas costeras existen ecosistemas sensibles que ofrecen hábitats críticos para muchas especies marinas, al tiempo que brindan servicios ecosistémicos de gran importancia para el ser humano. Estos ecosistemas son muy heterogéneos y están sujetos a constantes cambios provocados, en parte, por procesos físicos, químicos y biológicos externos que varían a diferentes escalas espaciales y temporales, por ejemplo, aportes de materia orgánica y nutrientes desde el continente, flujos de aguas subterráneas, fluviales y lavado del suelo, radiación solar, precipitación y surgencia costera, entre otras. Al combinarse estos procesos pueden, bajo ciertas circunstancias, crearse sistemas biológicamente muy productivos, pero vulnerables a las presiones ambientales, tanto naturales como antropogénicas. Por otra parte, las zonas costeras son altamente pobladas y, en consecuencia, son sistemas altamente vulnerables. Alteraciones biogeoquímicas producidas por ejemplo por el incremento en el ingreso de nutrientes y/o en sustancias contaminantes continentales, pueden desencadenar procesos de eutrofización y cambios en las tramas tróficas (ej. blooms de fitoplancton). El golfo de Arauco, ubicado en la zona centro-sur de Chile, presenta una orientación hacia el ecuador, siendo considerado como un enclave económico importante del país albergando importantes industrias y actividades económicas claves para la economía nacional. Entre las principales industrias, destaca la pesca, industria forestal y producción de pulpa y celulosa, industria química, minería, producción de energía, turismo y recreación, entre otras. La zona ha presentado un rápido crecimiento económico y demográfico. En la actualidad, posee una población de 419 595 personas distribuidas en cinco comunas. Desde el punto de vista oceanográfico, el golfo está bajo la influencia de la surgencia costera, debido a la predominancia de vientos sur-oeste, principalmente desde primavera hasta comienzos de otoño. Los eventos de surgencia llevan a la superficie agua ecuatorial sub-superficial (AESS) con altas concentraciones de nutrientes (20-40 de nitrato, 2,6-3 de fosfato), alta salinidad (34,4-34,9) y bajo contenido de oxígeno disuelto (2-15 % saturación). Se ha planteado, en la literatura científica, que la carga de nutrientes aportada por el río Biobío explicaría la elevada productividad primaria en el golfo de Arauco. Sin embargo, no existe suficiente información cuantitativa sobre el proceso de incorporación de nutrientes proveniente de otras fuentes externas, tales como los aportes producidos por procesos de surgencia y los flujos antropogénicos terrestres, tanto fluviales como antropogénicos. En este contexto, el presente trabajo de Tesis plantea las siguientes dos hipótesis. Primero, que la concentración de nitrógeno (N) y fósforo (P) que ingresa al golfo de Arauco producto de la surgencia generada por vientos sur-oeste es mayor que aquella proveniente de fuentes de origen terrestre, tanto naturales como antropogénicas y, segundo, que en el golfo de Arauco, durante el periodo primavera-verano la proporción N/P en la capa superficial de la columna de agua será menor que en el periodo de invierno, producto de la influencia de las aguas ecuatoriales subsuperficiales. Para contrastar estas hipótesis, se estimó el balance de nutrientes en el golfo de Arauco, determinando la importancia relativa del aporte de nitrógeno inorgánico disuelto y fósforo inorgánico disuelto generado por surgencia y fuentes terrestres fluviales y antropogénicas. Para ello, se compararon los aportes antropogénicos y el ingreso de N y P por surgencia costera en las diferentes estaciones (verano, invierno, primavera y otoño) y a escala interanual (2015 a 2019), utilizando el modelo biogeoquímico de cargas LOICZ de una caja y doble capa (Land-Ocean Interactions in the Coastal Zone, LOICZ) del programa Future Earth (Gordon et al. 1996). Nuestros resultados evidenciaron que en el golfo de Arauco existen 21 fuentes emisoras de nutrientes relacionadas con diferentes actividades o procesos industriales, incluyendo las aguas servidas, además de 4 entradas fluviales principales (ríos Biobío, Laraquete, Carampangue y Tubúl-Raqui. La mayor parte de los nutrientes que llegan a las aguas costeras del golfo de Arauco desde el continente provienen de los ríos, aportando el 65% (56.897 kg d-1) de los aportes totales (87.390 kg d-1); mientras que, los efluentes industriales aportan el 35 % (30.493 kg d-1). Cabe destacar que el aporte de los ríos incluye fuentes naturales y antrópicas. Las fuentes industriales más relevantes en cuanto a aportes de nutrientes fueron, en primer lugar, la industria y procesamiento de pescado, seguido de la industria forestal y producción de pulpa y celulosa, aguas servidas, producción de energía y, finalmente, otras industrias. Por su parte, las modelaciones indicaron que el sistema recibe flujos de agua, sal y nutrientes desde el océano adyacente hacia la capa profunda durante todo el año, con entradas oceánicas máximas durante la primavera y el verano mientras que, los suministros de origen continental hacia la capa superficial son mayores durante el invierno y primavera. Desde el punto de vista interanual, los flujos de nutrientes desde el océano abierto disminuyeron durante los eventos cálidos (El Niño) reportados durante los años 2015 y 2019 mientras que, durante los eventos fríos (La Niña) o periodos neutrales no se evidenciaron diferencias significativas. Los tiempos de residencia del sistema se estimaron en 20 días para la primavera, 35 en verano, 64 en otoño y 40 en invierno. Además, se obtuvieron flujos negativos de ΔPID y ΔNID en la capa superficial y positivos en la capa profunda, indicando producción de materia orgánica suficiente como para ser exportada al océano abierto o ser enterrada en el sedimento marino. Nuestros resultados indican que el golfo de Arauco funciona como un sumidero de carbono con predominio de la fijación de nitrógeno y un estado autótrofo en la capa superficial. La capa profunda, no obstante, mostró heterotrofia en las estaciones de primavera de 2015 y 2017, verano de 2017 y 2019 y otoño de 2017.
In the coastal zones there are sensitive ecosystems that provide critical habitats for many marine species, as well as ecosystem services of great importance to humans. These ecosystems are highly heterogeneous and are subject to constant change caused, in part, by external physical, chemical and biological processes that vary at different spatial and temporal scales, for example, inputs of organic matter and nutrients from the mainland, groundwater and river flows and soil flushing, solar radiation, precipitation and coastal upwelling, among others. The combination of these processes can, under certain circumstances, create systems that are biologically highly productive but vulnerable to environmental pressures, both natural and anthropogenic. On the other hand, coastal zones are highly populated and, consequently, are highly vulnerable systems. Biogeochemical alterations produced, for example, by increased nutrient inputs and/or inland pollutants can trigger eutrophication processes and changes in the trophic webs (e.g. phytoplankton blooms). The Gulf of Arauco, located in south-central Chile, is oriented towards the equator and is considered an important economic enclave in the country, hosting important industries and economic activities that are key to the national economy. Among the main industries are fishing, forestry, pulp and cellulose production, chemical industry, mining, energy production, tourism and recreation, among others. The area has experienced rapid economic and demographic growth. It currently has a population of 419,595 people distributed in five districts. From an oceanographic point of view, the gulf is under the influence of coastal upwelling, due to the predominance of south-westerly winds, mainly in the period from spring to early autumn. Upwelling events bring to the surface equatorial subsurface water (AESS) with high nutrient concentrations (20-40 nitrate, 2.6-3 phosphate), high salinity (34.4-34.9) and low dissolved oxygen content (2-15 % saturation). It has been suggested in the scientific literature that the nutrient load contributed by the Biobío River would explain the high primary productivity in the Gulf of Arauco. However, there is not enough quantitative information on the process of nutrient incorporation from other external sources, such as the contributions produced by upwelling processes and anthropogenic terrestrial inputs, both fluvial and anthropogenic. In this context, the present thesis work proposes the following two hypotheses. First, that the concentration of nitrogen (N) and phosphorus (P) entering the Gulf of Arauco as a result of upwelling generated by south-westerly winds is higher than that coming from land-based sources, both natural and anthropogenic, and second, that in the Gulf of Arauco, during the spring-summer period, the N/P ratio in the surface layer of the water column will be lower than in the winter period, due to the influence of subsurface equatorial waters. To contrast these hypotheses, the nutrient balance in the Gulf of Arauco was estimated, determining the relative importance of the contribution of dissolved inorganic nitrogen and dissolved inorganic phosphorus generated by upwelling and fluvial and anthropogenic land-based sources. For this purpose, anthropogenic inputs and the input of N and P by coastal upwelling were compared in different seasons (summer, winter, spring and fall) and on an interannual scale (2015 to 2019), using the LOICZ biogeochemical model of single-box and double-layer loads (Land-Ocean Interactions in the Coastal Zone, LOICZ) of the Future Earth program (Gordon et al. 1996). Our results showed that in the Gulf of Arauco there are 21 sources of nutrient emissions related to different activities or industrial processes, including wastewater, in addition to main four fluvial inputs (Biobío, Laraquete, Carampangue and Tubúl-Raqui rivers). Most of the nutrients that reach the coastal waters of the Gulf of Arauco from the mainland come from the rivers, contributing 65% (56,897 kg d-1) of the total inputs (87,390 kg d-1), while industrial effluents contribute 35% (30,493 kg d-1). It should be noted that the contribution of rivers includes natural and anthropic sources. The most relevant industrial sources in terms of nutrient inputs were, in first place, the fishery process industry, followed by the forestry industry and pulp and cellulose production, sewage, energy production and, finally, other industries. Modeling indicated that the system receives fluxes of water, salt and nutrients from the adjacent ocean to the deep layer throughout the year, with maximum oceanic inflows during spring and summer, while supplies of continental origin to the surface layer are greatest during winter and spring. Interannually, nutrient fluxes from the open ocean decreased during warm (El Niño) events reported during 2015 and 2019 while no significant differences were evident during cold (La Niña) events or neutral periods. Residence times of the system were estimated at 20 days for spring, 35 in summer, 64 in fall and 40 in winter. In addition, negative fluxes of ΔPID and ΔNID were obtained in the surface layer and positive fluxes in the deep layer, indicating sufficient organic matter production to be exported to the open ocean or buried in the marine sediment. Our results indicates that the Gulf of Arauco functions as a carbon sink with a predominance of nitrogen fixation and an autotrophic state in the surface layer. The deep layer, however, showed heterotrophy in the spring 2015 and 2017, summer 2017 and 2019, and fall 2017 seasons.
In the coastal zones there are sensitive ecosystems that provide critical habitats for many marine species, as well as ecosystem services of great importance to humans. These ecosystems are highly heterogeneous and are subject to constant change caused, in part, by external physical, chemical and biological processes that vary at different spatial and temporal scales, for example, inputs of organic matter and nutrients from the mainland, groundwater and river flows and soil flushing, solar radiation, precipitation and coastal upwelling, among others. The combination of these processes can, under certain circumstances, create systems that are biologically highly productive but vulnerable to environmental pressures, both natural and anthropogenic. On the other hand, coastal zones are highly populated and, consequently, are highly vulnerable systems. Biogeochemical alterations produced, for example, by increased nutrient inputs and/or inland pollutants can trigger eutrophication processes and changes in the trophic webs (e.g. phytoplankton blooms). The Gulf of Arauco, located in south-central Chile, is oriented towards the equator and is considered an important economic enclave in the country, hosting important industries and economic activities that are key to the national economy. Among the main industries are fishing, forestry, pulp and cellulose production, chemical industry, mining, energy production, tourism and recreation, among others. The area has experienced rapid economic and demographic growth. It currently has a population of 419,595 people distributed in five districts. From an oceanographic point of view, the gulf is under the influence of coastal upwelling, due to the predominance of south-westerly winds, mainly in the period from spring to early autumn. Upwelling events bring to the surface equatorial subsurface water (AESS) with high nutrient concentrations (20-40 nitrate, 2.6-3 phosphate), high salinity (34.4-34.9) and low dissolved oxygen content (2-15 % saturation). It has been suggested in the scientific literature that the nutrient load contributed by the Biobío River would explain the high primary productivity in the Gulf of Arauco. However, there is not enough quantitative information on the process of nutrient incorporation from other external sources, such as the contributions produced by upwelling processes and anthropogenic terrestrial inputs, both fluvial and anthropogenic. In this context, the present thesis work proposes the following two hypotheses. First, that the concentration of nitrogen (N) and phosphorus (P) entering the Gulf of Arauco as a result of upwelling generated by south-westerly winds is higher than that coming from land-based sources, both natural and anthropogenic, and second, that in the Gulf of Arauco, during the spring-summer period, the N/P ratio in the surface layer of the water column will be lower than in the winter period, due to the influence of subsurface equatorial waters. To contrast these hypotheses, the nutrient balance in the Gulf of Arauco was estimated, determining the relative importance of the contribution of dissolved inorganic nitrogen and dissolved inorganic phosphorus generated by upwelling and fluvial and anthropogenic land-based sources. For this purpose, anthropogenic inputs and the input of N and P by coastal upwelling were compared in different seasons (summer, winter, spring and fall) and on an interannual scale (2015 to 2019), using the LOICZ biogeochemical model of single-box and double-layer loads (Land-Ocean Interactions in the Coastal Zone, LOICZ) of the Future Earth program (Gordon et al. 1996). Our results showed that in the Gulf of Arauco there are 21 sources of nutrient emissions related to different activities or industrial processes, including wastewater, in addition to main four fluvial inputs (Biobío, Laraquete, Carampangue and Tubúl-Raqui rivers). Most of the nutrients that reach the coastal waters of the Gulf of Arauco from the mainland come from the rivers, contributing 65% (56,897 kg d-1) of the total inputs (87,390 kg d-1), while industrial effluents contribute 35% (30,493 kg d-1). It should be noted that the contribution of rivers includes natural and anthropic sources. The most relevant industrial sources in terms of nutrient inputs were, in first place, the fishery process industry, followed by the forestry industry and pulp and cellulose production, sewage, energy production and, finally, other industries. Modeling indicated that the system receives fluxes of water, salt and nutrients from the adjacent ocean to the deep layer throughout the year, with maximum oceanic inflows during spring and summer, while supplies of continental origin to the surface layer are greatest during winter and spring. Interannually, nutrient fluxes from the open ocean decreased during warm (El Niño) events reported during 2015 and 2019 while no significant differences were evident during cold (La Niña) events or neutral periods. Residence times of the system were estimated at 20 days for spring, 35 in summer, 64 in fall and 40 in winter. In addition, negative fluxes of ΔPID and ΔNID were obtained in the surface layer and positive fluxes in the deep layer, indicating sufficient organic matter production to be exported to the open ocean or buried in the marine sediment. Our results indicates that the Gulf of Arauco functions as a carbon sink with a predominance of nitrogen fixation and an autotrophic state in the surface layer. The deep layer, however, showed heterotrophy in the spring 2015 and 2017, summer 2017 and 2019, and fall 2017 seasons.
Description
Tesis presentada para optar al grado de Magíster en Ciencias con Mención en Oceanografía.
Keywords
zona costera, Ecosistemas