The relative importance of microbial and classical food webs in a highly productive coastal upwelling area

Abstract

We present an analysis of seasonal variations in the trophic pathways of carbon in a highly productive coastal upwelling region in the Humboldt current system off Chile. Seasonal changes in phytoplankton, protozooplankton, and bacteria biomass, along with rates of primary production (PP), bacterial growth, secondary production, vertical particle fluxes, and feeding by protozooplankton, omnivorous mesozooplankton, and carnivorous gelatinous zooplankton were determined from July 2004 to June 2005. Phytoplankton biomass and PP were maximal during spring/summer months, associated with upwelling episodes. Heterotrophic nanoflagellates (HNF) were the principal consumers of bacteria, removing .100% of their biomass daily. During autumn/winter, the protozooplankton grazed down a large fraction of HNF production (56% to 96% d 21 ). The mesozooplankton consumed 1-6% of the PP d 21 ; the different size fractions of copepods were omnivorous mostly during autumn/winter months, and ctenophores preyed most strongly on small copepods (0.5% to 5% d 21 ). A large part of the PP was channeled through the microbial food web, and only a small part AcknowledgmentsWe thank the captains and crew of the RV Kay Kay (Universidad de Concepció n, Chile) and the many undergraduate and graduate students who participated in our cruises (L. Lizá rraga, V. Aguilera, C. Aparicio, E. Menschel, and A. Araneda). We also thank José Luis Acuñ a and Albert Calbet for their valuable suggestions that substantially improved an earlier version of the manuscript and two anonymous reviewers for their critical and helpful comments.

Efectos microbiológicos producidos por un evento de surgencia costera en Chile central

“…En los sistemas improductivos la biomasa heterotrófica es más alta que la autotrófica; en cambio, como en el caso de la surgencia, cuando los niveles de nutrientes se incrementan, se produce un cambio progresivo hacia un predominio de los organismos autótrofos sobre los heterótrofos (Gasol et al 1997, Vargas et al 2007, situación coherente con lo que hemos observado, las bacterias aumentan 6 veces y los eucariontes autótrofos, 24 veces (Tabla 1).…”

“…Los nanoflagelados heterótrofos pueden ser los principales consumidores de biomasa bacteriana (Cuevas & Morales 2006, Vargas et al 2007; en nuestro caso, no parece ser ésta la principal causa del decaimiento bacteriano debido a que las concentraciones de estos organismos son relativamente bajas. Luego, al menos parte del decaimiento bacteriano podría deberse a la lisis producida por la infección viral, lo cual es concordante, a su vez, con el gran aumento de la concentración de virus que se observa concomitantemente con la disminución de la concentración bacteriana.…”

“…Más recientemente, se analizaron las variaciones estacionales del flujo de carbono, considerando, entre otros parámetros, los cambios en la productividad primaria, las biomasas de fitoplancton, protozooplancton y organismos más complejos. Así se logró establecer la importancia relativa de la red trófica microbiana en la transferencia de carbono hacia metazoos omnívoros y carnívoros de gran tamaño (Vargas et al 2007). Como complemento a estos trabajos es preciso incorporar a los virus como protagonistas en la dinámica trófica marina y su respuesta al estímulo que significan los eventos de surgencia.…”

Scaling copepod grazing in a coastal upwelling system: the importance of community size structure for phytoplankton C flux

“…Studies on this process have long been carried out to assess the role of herbivorous zooplankton in channeling primary production (Irigoien, 1998;Båmstedt et al, 2000). Various works on different systems have suggested that herbivore-grazing exercises limited control over C flux in the marine food chain because most zooplankton are omnivorous & their feeding on heterotrophic diets predominates for a variety of situations, thus favoring microbial C pathways (González et al, 2000;Calbet, 2001;Calbet & Landry, 2004;Vargas et al, 2007). Although in strongly seasonal upwelling systems at times of the year (spring-summer) dominant zooplankton can shift their diet, such that herbivorous grazing on diatoms may greatly increase and thus serve as the C channel for higher trophic levels (Vargas et al, 2006).…”

Effect of acid Lugol solution as preservative on two representative chitineous and gelatinous zooplankton groups

“…It is common practice in limnology and oceanography to estimate biomass and evaluate carbon fluxes based on abundance and size measurements of different plankton groups (Vargas et al 2007). Fixation and preservation of samples are generally necessary for practical enumeration and biovolume estimation in subsequent laboratory analysis.…”