Interacciones tróficas y migración vertical como mecanismos de transferencia del carbono orgánico hacia el océano profundo.
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Date
2025
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Universidad de Concepción
Abstract
El transporte de carbono orgánico hacia el océano profundo es un componente clave de la bomba biológica, a través del cual la materia orgánica producida en la zona fótica es transferida a capas más profundas, promoviendo así el secuestro de carbono. En los sistemas de surgencia costera, como el del Pacífico suroriental, este proceso se ve fuertemente influido por la alta productividad biológica, la variabilidad oceanográfica y las complejas interacciones entre los procesos físicos y biológicos. Dentro de este marco, el zooplancton y el micronecton desempeñan un papel esencial en el flujo vertical de carbono, no solo contribuyendo al hundimiento pasivo de partículas de sus fecas y mudas (transporte pasivo), sino también a través de su comportamiento migratorio vertical (transporte activo). En este último proceso, se ha postulado que las migraciones verticales escalonadas (escalera de migración propuesta por Vinogradov 1962) combinadas con las interacciones presa-depredador pueden exacerbar el flujo vertical de C orgánico. Esta hipótesis sugiere que el suministro más importante de alimento en el océano profundo es el transporte activo de materia orgánica mediante migraciones asincrónicas de organismos pelágicos y sus interacciones tróficas desde la zona superior a grandes profundidades en la columna de agua. A este proceso combinado lo llamaremos “escalera trófica”.
Una escalera trófica favoreciendo el transporte vertical de carbono en el sistema de surgencia del centro-sur de Chile, sugiere que las interacciones presa-depredador y las migraciones verticales de los organismos pelágicos dan lugar a un flujo vertical escalonado de C que puede acelerar su transporte desde la capa fótica hacia las capas meso y batipelágicas, integrando así procesos ecológicos y biogeoquímicos dentro del funcionamiento de la bomba biológica del carbono. En este marco conceptual, se ha desarrollado la presente tesis, planteando como objetivo central la puesta a prueba de la hipótesis nula de la existencia de dicha escalera trófica en el sistema de surgencia del Pacífico sur oriental frente a la costa de Chile considera las zonas de Iquique y Concepción como áreas de estudio.
El capítulo 1 se centró en la caracterización de estructura y dinámica trófica de un componente clave del ecosistema pelágico, el mesozooplancton. Para tales fines, se analizaron muestras de zooplancton obtenidas durante el crucero LowPhox-II del verano austral del 2018 frente a la zona norte de Chile (Iquique). Las muestras obtenidas en 5 estratos verticales entre los 900 y 0 m de la columna de agua en condiciones diurnas y nocturnas se analizaron mediante métodos automatizados para cuantificar e identificar los grupos taxonómicos de la comunidad del mesozooplancton. El análisis incluyó adicionalmente la determinación de isótopos estables de C y N en cuatro clases de tamaño de la comunidad. Durante el crucero se obtuvieron también variables ambientales hidrográficas. Se estimó la migración vertical diurna nocturna del zooplancton (DVM) y sus interacciones tróficas basadas en la composición isotópica de los diferentes grupos taxonómicos y fracciones de tamaño. El estudio propuso un Índice de Comportamiento Trófico (Trophic Behavior Index, TBI). Este índice integra tres atributos biológicos claves del zooplancton: el tamaño del individuo, la amplitud de la migración vertical diaria (DVM), y la posición trófica estimada mediante el uso de isótopos estables de nitrógeno que compone la biomasa del zooplancton. Su aplicación permitió clasificar la comunidad en cuatro grupos funcionales: consumidores primarios, predadores moderados, predadores fuertes y predadores tope; y evidenció que la estructura trófica de la comunidad está determinada principalmente por características biológicas más que por la variabilidad ambiental. Los depredadores fuertes y tope dominaron las zonas oceánicas, mientras que los consumidores primarios prevalecieron en la costa, demostrando que el TBI constituye una herramienta para evaluar la organización funcional del zooplancton en sistemas de surgencia altamente variables y otorga un criterio para definir las relaciones presa-depredador.
El Capítulo 2 se enfocó en cuantificar el transporte lateral (costa océano) y transporte activo de carbono mediado por el zooplancton y el micronecton durante el verano de 2023 frente a la zona de surgencia de Chile centro-sur (36 °S). Los análisis se realizaron utilizando muestras de zooplancton y micronecton obtenidas durante el crucero MAPUCHE. Los muestreos fueron también en estratos verticales en la zona costera en los primero 200 m de la columna de agua y la zona oceánica para condiciones diurnas y nocturnas en los primeros 1000 m, acompañados con información ambiental hidrográfica. Se analizó la composición taxonómica de las comunidades del mesozooplancton y del micronecton, así como se determinó la composición isotópica de cuatro clases de tamaño del zooplancton y desde organismos seleccionados del micronecton. El estudio además integró análisis isotópicos de aminoácidos (CSIA-AA) en ambos componentes de la comunidad pelágica. Utilizando los datos de variables hidrográficas, complementados con data satelital, el estudio estimó el transporte lateral por advección (Transporte de Ekman) durante surgencia activa. Los análisis de la composición isotópica en el aminoácido fenilalanina (δ¹⁵N-fenilalanina) mostraron una baja variabilidad entre estaciones costeras y oceánicas, lo que sugiere una fuente común de nitrógeno derivada de la producción costera que se redistribuye lateralmente hacia zonas más oceánicas. A lo largo del gradiente costa-océano, las posiciones tróficas aumentaron desde herbívoros (~2.0) hasta carnívoros de niveles superiores (>5.0). La estimación integrada del transporte activo de carbono mediado por zooplancton y micronecton hasta los 1000 m de profundidad alcanzó los 38.3 mg C m-² d-¹, lo que equivale al 2.7% de la producción primaria neta (PPN) en la zona oceánica y al 2.1% de la PPN en la zona de surgencia costera. El micronecton es uno de los principales vectores en el transporte activo de carbono desde la superficie hacia la Zona Mínima de Oxígeno, integrando el carbono previamente procesado por el zooplancton migrador y superficial-no migrante.
Los resultados proveen apoyo a la existencia de una “escalera trófica”, en la cual el zooplancton migrador y el micronecton participan de manera complementaria en la transferencia activa de carbono hacia las profundidades. Este mecanismo se ve reforzado por la circulación costera y los procesos de advección lateral asociados al transporte de Ekman y otros procesos físicos que redistribuyen la biomasa generados en la zona de surgencia hacia el océano abierto. En conjunto, los hallazgos de esta tesis sugieren que la interacción entre procesos físicas acoplados a procesos biológicos del zooplancton y micronecton, tales como la migración vertical y las interacciones tróficas, conforman un mecanismo efectivo dentro de la bomba biológica del carbono, promoviendo la conexión entre la alta productividad costera y el secuestro de carbono en el océano profundo.
The transport of organic carbon to the deep ocean is a key component of the biological pump, through which organic matter produced in the photic zone is transferred to deeper layers, thus promoting carbon sequestration. In coastal upwelling systems, such as that of the southeastern Pacific, this process is strongly influenced by high biological productivity, oceanographic variability, and the complex interactions between physical and biological processes. Within this framework, zooplankton and micronekton play an essential role in the vertical flux of carbon, contributing not only to the passive sinking of particle flux from their fecal pellets and molts (passive transport), but also through their vertical migratory behavior (active transport). In this latter process, it has been proposed that stepped vertical migrations (the migration ladder proposed by Vinogradov 1962), combined with predator–prey interactions, may exacerbate the vertical flux of organic C. This hypothesis suggests that the most important supply of food to the deep ocean is the active transport of organic matter through asynchronous migrations of pelagic organisms and their trophic interactions from the upper zone to great depths in the water column. We refer to this combined process as a “trophic ladder.” A trophic ladder favoring the vertical transport of carbon in the upwelling system off central-southern Chile suggests that predator–prey interactions and the vertical migrations of pelagic organisms give rise to a stepped vertical flux of C that may accelerate its transport from the photic layer to the meso- and bathypelagic layers, thereby integrating ecological and biogeochemical processes within the functioning of the biological carbon pump. Within this conceptual framework, the present thesis was developed, with the central objective of testing the null hypothesis of the existence of such a trophic ladder in the upwelling system of the southeastern Pacific off the coast of Chile, considering the areas of Iquique and Concepción as study sites. Chapter 1 focused on characterizing the structure and trophic dynamics of a key component of the pelagic ecosystem, the mesozooplankton. To this end, zooplankton samples collected during the LowPhox-II cruise in the austral summer of 2018 off northern Chile (Iquique) were analyzed. Samples obtained from 5 vertical strata between 900 and 0 m of the water column under daytime and nighttime conditions were analyzed using automated methods to quantify and identify the taxonomic groups of the mesozooplankton community. The analysis additionally included the determination of stable isotopes of C and N in four size classes of the community. Hydrographic environmental variables were also obtained during the cruise. Diel vertical migration (DVM) of zooplankton was estimated, along with their trophic interactions based on the isotopic composition of the different taxonomic groups and size fractions. The study proposed a Trophic Behavior Index (TBI). This index integrates three key biological attributes of zooplankton: individual body size, the amplitude of daily vertical migration (DVM), and trophic position estimated using stable nitrogen isotopes that comprise zooplankton biomass. Its application allowed the classification of the community into four functional groups: primary consumers, moderate predators, strong predators, and top predators; and showed that the trophic structure of the community is determined mainly by biological characteristics rather than environmental variability. Strong and top predators dominated oceanic zones, whereas primary consumers prevailed in coastal areas, demonstrating that the TBI constitutes a useful tool for assessing the functional organization of zooplankton in highly variable upwelling systems and provides a criterion for defining predator–prey relationships. Chapter 2 focused on quantifying lateral (coast–ocean) and active carbon transport mediated by zooplankton and micronekton during the summer of 2023 off the upwelling zone of central–southern Chile (36°S). Analyses were conducted using zooplankton and micronekton samples collected during the MAPUCHE cruise. Sampling was also carried out in vertical strata: in the coastal zone within the first 200 m of the water column, and in the oceanic zone under daytime and nighttime conditions within the first 1000 m, accompanied by hydrographic environmental information. The taxonomic composition of the mesozooplankton and micronekton communities was analyzed, and the isotopic composition of four size classes of zooplankton and of selected micronekton organisms was determined. The study further integrated compound-specific stable isotope analyses of amino acids (CSIA-AA) in both components of the pelagic community. Using hydrographic variables complemented with satellite data, the study estimated lateral advective transport (Ekman transport) during active upwelling. Isotopic analyses of the amino acid phenylalanine (δ¹⁵N-phenylalanine) showed low variability between coastal and oceanic stations, suggesting a common nitrogen source derived from coastal production that is laterally redistributed toward more oceanic areas. Along the coast–ocean gradient, trophic positions increased from herbivores (~2.0) to higher-level carnivores (>5.0). The integrated estimate of active carbon transport mediated by zooplankton and micronekton down to 1000 m depth reached 38.3 mg C m-² d-¹, equivalent to 2.7% of net primary production (NPP) in the oceanic zone and 2.1% of NPP in the coastal upwelling zone. Micronekton is one of the main vectors of active carbon transport from the surface to the Oxygen Minimum Zone, incorporating carbon previously processed by migrating and surface/non-migrating zooplankton. The results provide support for the existence of a “trophic ladder,” in which migrating zooplankton and micronekton participate complementarily in the active transfer of carbon to depth. This mechanism is reinforced by coastal circulation and lateral advection processes associated with Ekman transport and other physical processes that redistribute the biomass generated in the upwelling zone toward the open ocean. Taken together, the findings of this thesis suggest that the interaction between physical processes coupled with biological processes of zooplankton and micronekton such as vertical migration and trophic interactions constitutes an effective mechanism within the biological carbon pump, promoting the connection between high coastal productivity and carbon sequestration in the deep ocean.
The transport of organic carbon to the deep ocean is a key component of the biological pump, through which organic matter produced in the photic zone is transferred to deeper layers, thus promoting carbon sequestration. In coastal upwelling systems, such as that of the southeastern Pacific, this process is strongly influenced by high biological productivity, oceanographic variability, and the complex interactions between physical and biological processes. Within this framework, zooplankton and micronekton play an essential role in the vertical flux of carbon, contributing not only to the passive sinking of particle flux from their fecal pellets and molts (passive transport), but also through their vertical migratory behavior (active transport). In this latter process, it has been proposed that stepped vertical migrations (the migration ladder proposed by Vinogradov 1962), combined with predator–prey interactions, may exacerbate the vertical flux of organic C. This hypothesis suggests that the most important supply of food to the deep ocean is the active transport of organic matter through asynchronous migrations of pelagic organisms and their trophic interactions from the upper zone to great depths in the water column. We refer to this combined process as a “trophic ladder.” A trophic ladder favoring the vertical transport of carbon in the upwelling system off central-southern Chile suggests that predator–prey interactions and the vertical migrations of pelagic organisms give rise to a stepped vertical flux of C that may accelerate its transport from the photic layer to the meso- and bathypelagic layers, thereby integrating ecological and biogeochemical processes within the functioning of the biological carbon pump. Within this conceptual framework, the present thesis was developed, with the central objective of testing the null hypothesis of the existence of such a trophic ladder in the upwelling system of the southeastern Pacific off the coast of Chile, considering the areas of Iquique and Concepción as study sites. Chapter 1 focused on characterizing the structure and trophic dynamics of a key component of the pelagic ecosystem, the mesozooplankton. To this end, zooplankton samples collected during the LowPhox-II cruise in the austral summer of 2018 off northern Chile (Iquique) were analyzed. Samples obtained from 5 vertical strata between 900 and 0 m of the water column under daytime and nighttime conditions were analyzed using automated methods to quantify and identify the taxonomic groups of the mesozooplankton community. The analysis additionally included the determination of stable isotopes of C and N in four size classes of the community. Hydrographic environmental variables were also obtained during the cruise. Diel vertical migration (DVM) of zooplankton was estimated, along with their trophic interactions based on the isotopic composition of the different taxonomic groups and size fractions. The study proposed a Trophic Behavior Index (TBI). This index integrates three key biological attributes of zooplankton: individual body size, the amplitude of daily vertical migration (DVM), and trophic position estimated using stable nitrogen isotopes that comprise zooplankton biomass. Its application allowed the classification of the community into four functional groups: primary consumers, moderate predators, strong predators, and top predators; and showed that the trophic structure of the community is determined mainly by biological characteristics rather than environmental variability. Strong and top predators dominated oceanic zones, whereas primary consumers prevailed in coastal areas, demonstrating that the TBI constitutes a useful tool for assessing the functional organization of zooplankton in highly variable upwelling systems and provides a criterion for defining predator–prey relationships. Chapter 2 focused on quantifying lateral (coast–ocean) and active carbon transport mediated by zooplankton and micronekton during the summer of 2023 off the upwelling zone of central–southern Chile (36°S). Analyses were conducted using zooplankton and micronekton samples collected during the MAPUCHE cruise. Sampling was also carried out in vertical strata: in the coastal zone within the first 200 m of the water column, and in the oceanic zone under daytime and nighttime conditions within the first 1000 m, accompanied by hydrographic environmental information. The taxonomic composition of the mesozooplankton and micronekton communities was analyzed, and the isotopic composition of four size classes of zooplankton and of selected micronekton organisms was determined. The study further integrated compound-specific stable isotope analyses of amino acids (CSIA-AA) in both components of the pelagic community. Using hydrographic variables complemented with satellite data, the study estimated lateral advective transport (Ekman transport) during active upwelling. Isotopic analyses of the amino acid phenylalanine (δ¹⁵N-phenylalanine) showed low variability between coastal and oceanic stations, suggesting a common nitrogen source derived from coastal production that is laterally redistributed toward more oceanic areas. Along the coast–ocean gradient, trophic positions increased from herbivores (~2.0) to higher-level carnivores (>5.0). The integrated estimate of active carbon transport mediated by zooplankton and micronekton down to 1000 m depth reached 38.3 mg C m-² d-¹, equivalent to 2.7% of net primary production (NPP) in the oceanic zone and 2.1% of NPP in the coastal upwelling zone. Micronekton is one of the main vectors of active carbon transport from the surface to the Oxygen Minimum Zone, incorporating carbon previously processed by migrating and surface/non-migrating zooplankton. The results provide support for the existence of a “trophic ladder,” in which migrating zooplankton and micronekton participate complementarily in the active transfer of carbon to depth. This mechanism is reinforced by coastal circulation and lateral advection processes associated with Ekman transport and other physical processes that redistribute the biomass generated in the upwelling zone toward the open ocean. Taken together, the findings of this thesis suggest that the interaction between physical processes coupled with biological processes of zooplankton and micronekton such as vertical migration and trophic interactions constitutes an effective mechanism within the biological carbon pump, promoting the connection between high coastal productivity and carbon sequestration in the deep ocean.
Description
Tesis presentada para optar al grado de Doctor/a en Oceanografía.
Keywords
Oceonografía, Ecología química marina, Carbono, Zooplancton