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⁻¹). The mesozooplankton consumed 1–6% of the PP d⁻¹; 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⁻¹). A large part of the PP was channeled through the microbial food web, and only a small part directly to copepods via the herbivore food chain. The microbial food web transfers bacterial or small algal carbon to protozooplankton, and then to mesozooplankton, and a large percentage of this carbon is also available for the gelatinous predators. Because zooplankton are not able to feed on dissolved organic matter and cyanobacteria, the combined feeding activity of zooplankton either by direct (phytoplankton) or indirect (microbial) pathways increases the yield of PP reaching the zooplankton and, eventually, the upper trophic levels. Our findings suggest that the carrying capacity for larger omnivorous and carnivorous metazoans, and even for commercially exploitable pelagic fishes, might be considerably larger than that expected from a simple herbivore‐dominated food chain in coastal upwelling areas.

Recruitment success at the early life stages is a critical process for zooplankton demography. Copepods often dominate the zooplankton in marine coastal zones and are prey of the majority of fish larvae. Hypotheses interpreting variations of copepod recruitment are based on the concepts of “naupliar predation,” “nutritional deficiency,” and “toxic effect” of diatom diets. Contradictory laboratory and field studies have reached opposite conclusions on the effects of diatoms on copepod reproductive success, blurring our view of marine food‐web energy flow from diatoms to higher consumers by means of copepods. Here we report estimates of copepod feeding selectivity and reproduction in response to seasonally changing phytoplankton characteristics measured in a highly productive coastal upwelling area off the coast of central Chile. The variable phytoplankton diversity and changing food quality had a strong and highly significant impact on the feeding selectivity, reproduction, and larval survival of three indigenous copepod species. Seasonal changes in copepod feeding behavior were related to the alternating protozoan–diatom diets, mostly based on dinoflagellates and ciliates during winter and autumn (low highly unsaturated fatty acids [HUFA]/polyunsaturated fatty acids [PUFA] availability), but switched to a diet of centric and chain‐forming diatoms (high HUFA/PUFA availability) during the spring/summer upwelling period. Ingestion of diatom cells induced a positive effect on egg production. However, a negative relationship was found between egg hatching success, naupliar survival, and diatom ingestion. Depending on the phytoplankton species, diets had different effects on copepod reproduction and recruitment. In consequence, it seems that the classical marine food web model does not apply to some coastal upwelling systems.

A combined data set, combining data from field campaigns and oceanographic cruises, was used to ascertain the influence of both river discharges and upwelling processes, covering spatial and temporal variation in dissolved inorganic carbon (DIC) and aragonite saturation state. This work was conducted in one of the most productive river‐influenced upwelling areas in the South Pacific coasts (36°S). Additionally, further work was also conducted to ascertain the contribution of different DIC sources, influencing the dynamics of DIC along the land‐ocean range. Six sampling campaigns were conducted across seven stations at the Biobío River basin, covering approximately 200 km. Three research cruises were undertaken simultaneously, covering the adjacent continental shelf, including 12 sampling stations for hydrographic measurements. Additionally, six stations were also sampled for chemical analyses, covering summer, winter, and spring conditions over 2010 and 2011. Our results evidenced that seaward extent of the river plume was more evident during the winter field campaign, when highest riverine DIC fluxes were observed. The carbonate system along the river‐ocean continuum was very heterogeneous varying over spatial and temporal scales. High DIC and pCO₂ were observed in river areas with larger anthropogenic effects. CO₂ supersaturation at the river plume was observed during all campaigns due to the influence of low pH river waters in winter/spring and high‐pCO₂ upwelling waters in summer. δ¹³C_DIC evidenced that main DIC sources along the river and river plume corresponded to the respiration of terrestrial organic matter. We have linked this natural process to the carbonate saturation on the adjacent river‐influenced coastal area, suggesting that Ω_aragonite undersaturation in surface/subsurface waters is largely modulated by the influence of both river discharge and coastal upwelling events in this productive coastal area. Conditions of low Ω_aragonite might impact negatively physiological traits for marine organisms, such as bivalves, gastropods, and crustaceans. Therefore, local populations from river‐influenced sites could be inherently more tolerant to ocean acidification than organisms living in regions with lower Ω_aragonite variability.

Aggregate stages of the salps Thalia democratica, Cyclosalpa affinis and Salpa cylindrica collected by SCUBA diving in the Mid-Atlantic Bight were fed with naturally occurring food assemblages. This is one of the few studies where salps have been fed with natural food assemblages. The estimated clearance rate for all species based on disappearance of chlorophyll varied from 82 to 444 mL individual⁻¹ day⁻¹. Cell counts showed that T. democratica mostly ingested carbon from autotrophic nanoflagellates and autotrophic dinoflagellates. Ingestion by S. cylindrica was primarily on larger prey, such as dinoflagellates, while C. affinis ingested auto- and heterotrophic nanoflagellates. All main prey types ingested by salps corresponded to those that contributed most to biomass at each experimental station. Thus, salps fed on naturally occurring particles primarily in proportion to prey biomass and to their mechanical capacity to be retained and ingested. Feeding by salps on dinoflagellates and ciliates implies that they may act not only as potential competitors with microzooplankton, but also as consumers of them.

We present experimental data obtained from an experiment with newly hatched veliger larvae of the gastropod Concholepas concholepas exposed to three pCO₂ levels. Egg capsules were collected from two locations in northern and central Chile, and then incubated throughout their entire intra-capsular life cycle at three nominal pCO₂ levels, ∼400, 700 and 1000 ppm (i.e. corresponding to ∼8.0, 7.8 and 7.6 pH units, respectively). Hatched larvae were fed with natural food assemblages. Food availability at time zero did not vary significantly with pCO₂ level. Our results clearly showed a significant effect of elevated pCO₂ on the intensity of larval feeding, which dropped by >60%. Incubation also showed that pCO₂-driven ocean acidification (OA) may radically impact the selectivity of ingested food by C. concholepas larvae. Results also showed that larvae switched their clearance rate based on large cells, such as diatoms and dinoflagellates to tiny and highly abundant nanoflagellates and cyanobacteria as pCO₂ levels increased. Thus, this study reveals the important effect of low pH conditions on larval feeding behavior, in terms of both ingestion magnitude and selectivity. These findings support the notion that larval feeding is a key physiological process susceptible to the effects of OA.