Pamela Hidalgo

Circulación inducida por el viento en Bahía de Antofagasta, norte de Chile (23° S)

Se estudio la circulacion de la bahia de Antofagasta (23° S) sobre las base de mediciones de series de tiempo de corrientes en verano e invierno de 1999, de un crucero oceanografico realizado en agosto de 1999 y una serie de 9 meses de perfiles de corrientes a intervalos de 15 dias. La informacion de corrientes sumada a datos de temperatura, salinidad y oxigeno, e informacion de vientos, permitio analizar la variabilidad temporal y espacial de la circulacion en la bahia. La bahia de Antofagasta esta sujeta a una intensa variacion de los vientos, a la cual las corrientes se manifiestan muy asociadas y en desfases de tiempo de 1 hasta 5 dias. El ciclo que presenta mayor contribucion a la varianza total de la corriente es en la escala diaria. La bahia presenta circulacion en una capa, con velocidades medias de 30 cm s-1. El campo de corrientes sugiere que la bahia se llena a traves del sector norte y su vaciamiento ocurre en el extremo sur. La presencia de un foco de surgencia en el sector sur externo a la bahia ejerce una fuerte influencia en la circulacion, a traves del influjo de aguas de surgencia desde el sur y dando origen a una zona frontal que modula el llenado y vaciado de la bahia. El estudio presenta un modelo conceptual de circulacion sujeta a la variabilidad en la intensidad de surgencia. El modelo predice que bajo condiciones de relajacion de surgencia y el establecimiento de zona frontal en la boca sur la circulacion seria dominada por un giro ciclonico al interior de la bahia, y frente a pulsos de intensificacion de la surgencia se produciria el llenado. Se discuten las implicancias de los resultados en el contexto de la capacidad de retencion y renovacion de aguas de la bahia.

Distribución de eufáusidos en el sistema de surgencia frente a la península de Mejillones (23°S) asociada a condiciones previas y durante El Niño 1997-98

Euphausiids are abundant, although little studied components of the zooplancton in the Humboldt Current ecosystem. For instance, the effect that large scale ocean variability may exert on their population dynamics, abundance and distribution is unknown. In this work, based on 2-year time series data and two spatial surveys on December 1996 and July 1997, the distribution of euphausiids in coastal zone off Mejillones Peninsula was assessed. 10 species were identified, of which the most abundant were Euphausia mucronata, E. distinguenda and a non-identified species of the genus Euphausia. In July 1997 (El Nino conditions) fewer species were observed as compared to December 1996 (prior to the El Nino). In both situations (prior and during the El Nino) the abundance of various species was associated with surface dissolved oxygen and water density. There was no correlation between euphausiids abundance and phytoplankton biomass. Previous to the El Nino 1997-98 a Diel Vertical Migration (DVM) pattern was observed for most species, with greater abundance in the upper 0-50 m layer at night. Under the El Nino conditions DVM was apparently altered by intrusion of the warm water mass. E. mucronata, E. distinguenda, Euphausia sp. and Nematoscelis megalops, were found during the time series study. The abundance of adults of these species was significantly reduced during the warm conditions of the El Nino. By contrast, juveniles and larvae (furcilia and calyptopis), not identified to the species level, were significantly more abundant during the El Nino phase, suggesting that reproduction was continuous throughout the study and even incremented during the El Nino. During the warm conditions of the El Nino we also observed a decrease in body length of adults, juveniles and larvae. When comparing equal months in both periods (El Nino and non-El Nino), the body lengths of juveniles, larvae and at least for one species in adults, were significantly smaller during the El Nino period.

Mortalidad natural de Paracalanus indicus (Copepoda: Calanoida) en áreas de surgencia asociada a la zona de mínimo de oxígeno en el Sistema de Corrientes Humboldt: implicancias en el transporte pasivo del flujo de carbono

The in situ natural mortality (predation excluded) and growth rates (g) of the pelagic copepod Paracalanus indicus were studied under variable upwelling regi...

Intra-seasonal variation of upwelling and its effects on copepod community structure off central/southern Chile (2002–2009)

AbstractCopepods are major components of zooplankton in the Humboldt Current system. Here, intra-seasonal (monthly) variation of upwelling and its influence on the copepod community were assessed. Species abundances, species richness, diversity (Shannon–Wiener index) and dominance, were studied during four upwelling periods (2002–2009) at the Station 18 time series off of Concepción (36°S). Although 77 species were identified, most variability of the community structure was explained by few (<10) species. A negative trend in copepod abundance over the years was associated with lower oxygenation of the mixed layer. A generalized linear model test for relationships among community descriptors and environmental parameters (temperature, dissolved oxygen, chlorophyll, and water column stratification) revealed that diversity was positively correlated with stratification. Upwelling variation, reflected in alternate periods (active and relaxed), characterized two distinct communities during the spring–summer. The study concludes that upwelling interacts with copepod populations by changing stratification, and temperature and oxygenation gradients. The study also suggests that greatly increased upwelling may negatively impact copepods by reducing oxygenation, cooling down the mixed layer and causing more advection. The same mechanisms may be operating in other systems, and thus this study provides clues on how zooplankton communities can respond to climate-induced variation of upwelling.

Coastal upwelling circulation and its influence on the population dynamics of Calanus chilensis (Brodski, 1959) off northern Chile (23°S)

Abstract Marine pelagic copepods must cope with a highly heterogeneous environment in coastal upwelling systems. In addition to spatial and temporal heterogeneity in temperature and food supply, advective forces may strongly influence their populations. Off northern Chile, the population of Calanus chilensis, a dominant copepod associated with upwelling centres, is subjected to large interannual variations in abundance, not explained by changing temperature regimes occurring upon cold and warm phases of the ENSO (El Niño Southern Oscillation) cycle. During two spring and two summer periods (1998–2000), we studied the abundance, distribution and growth of the copepodids of C. chilensis. The instantaneous mean weight-specific growth rate was estimated as 0.29 day−1 and was not significantly different among cruises. Weights of late stages did not differ either. However, the numerical abundance and biomass substantially differed among cruises, such that daily production of copepodids increased with greater biomasses. Chlorophyll-a was significantly greater in spring 1999 and summer 2000, as compared to spring 1998 and summer 1999, coinciding with greater abundances of copepodids and stronger upwelling. The current field in the upper 200 m indicated that strong upwelling favoured nearshore water retention, whereas weak or moderate upwelling caused more offshore advection. This finding is opposite to the usual view that strong upwelling favours offshore advection. We further concluded that neither food or temperature had much influence on growth, abundance and production of C. chilensis, but instead seasonal and interannual variation in upwelling caused by changes in local winds may be the primary cause for the observed fluctuations in population abundance.

First measurements of euphausiid growth rates in the northern Humboldt Current (23ºS)

One hundred and five juveniles and adults of Euphausia mucronata were incubated in order to estimate their daily growth rates (DGR) using the standard instantaneous growth rate method. From 77 krill that survived after 2 days of incubation under laboratory conditions, 70 individuals molted (35.1% males, 64.9% females). E. mucronata mean intermolt period was 5.2 days (range = 4-9 d) with mean DGR of 0.01 mm d-1 for males and -0.04 mm d-1 for females. Overall mean DGR was -0.02 mm d-1. DGRs were positive (31%), zero (12%) and negative (57%). DGR<0 evidenced body shrinking process suggesting that environmental conditions in austral autumn 2010 were unfavorable for the krill population. However, because females had DGR<0 values more frequently than males it is probable that females also used part of their energy on gonad development rather than body growth.

Development of Rhincalanus nasutus (Giesbrecht, 1888) (Copepoda, Rhincalanidae) from the Humboldt Current System

Morphological changes in the exoskeleton of naupliar stages 2-6 and copepodid stages I-V of Rhincalanus nasutus are described. On the distal segment of naupliar antennule, one lateral seta and two medial setae are added during each molt to nauplius 3-6. A naupliar arthrite is a thick, distinctive structure on the coxa of antenna, and the exopod of this limb is patterned from the large proximal segment of that ramus. The coxal gnathobase of the mandible appears at nauplius 4. Buds of maxillule and maxilla are added during the molt to nauplius 5, and buds of maxilliped and swimming legs 1-2 are added during the molt to nauplius 6. The somite bearing maxilla clearly articulates completely with the somite bearing the maxilliped on the last two naupliar stages. This unusual articulation does not appear during copepodid development, and the somite bearing swimming leg 2 is the anterior articulating somite during this later phase. One articulating somite is added during molts to copepodid II-V. Segmentation of the antennule is completed by copepodid IV. Segmentation of the exopod of antenna does not change during the copepodid phase, but one segment is added to the endopod of maxilla. Segmentation and setation of the maxilliped follow the usual calanoid pattern. Buds of swimming legs 3-5 are ventrolateral on their somite. Their presumptive exopod has two attenuations; their presumptive endopod bears an attenuation and a seta. Segmentation of the rami of swimming legs 1-4 follows the common pattern for copepods except for the exopod and endopod of swimming leg 1 which are only 2-segmented.

The naupliar stages of Eucalanus inermis Giesbrecht, 1893 (Copepoda, Calanoida) from the Humboldt current system (northern Chile)

Eucalanus inermis Giesbrecht, 1893 is an abundant species of the Humboldt Current and recognized as one of the large-sized herbivorous copepods in the pelagic realm of the eastern South Pacific. However, lack of information on morphological and taxonomic characters of early stages of this species is a major shortcoming to identify them from field samples, and therefore it limits the possibility of studying its populations and assessing its ecological role in the pelagic food web of the Humboldt Current. In this work, a full description of the naupliar stages of this species is made for the first time. Thus, we provide a basic tool to assist future studies on the population dynamics of the species. Eucalanus inermis Giesbrecht, 1893 es una especie abundante en el Sistema de Corrientes Humboldt y reconocido como una de las especies herbivoras de mayor tamano del Pacifico Sur-oriental. Sin embargo, la ausencia de informacion basica sobre la morfologia y caracteres taxonomicos de sus estadios tempranos es una limitante para estudiar y comprender el ciclo vital de la especie. Esta situacion tambien limita la posibilidad de evaluar el rol ecologico de la especie en la trama trofica pelagica del sistema de corrientes Humboldt. En este trabajo se presenta una descripcion completa de los estadios naupliares por primera vez. Proveemos asi, una herramienta basica para identificar los estadios naupliares desde muestras del zooplancton y apoyar estudios futuros de la dinamica poblacional de la especie.