Trophic interactions of pelagic crustaceans in Comau Fjord (Chile): their role in the food web structure.

Grazing experiments were used to study the trophic interactions among plankton components in a stratified fjord of southern Chile. A strong pycnocline (7–10 m) separating the low-salinity layer from the deeper, high-salinity, marine layer, affected the composition, abundance and distribution of phytoplankton and zooplankton every season. Nanoflagellates averaging 59% were consistently the main components of the plankton biomass in both layers. The microphytoplankton contribution was always higher within the marine layer. Cladocerans and some small copepods showed a vertical gradient with greater abundances in the low-salinity layer, whereas medium-large copepods and euphausiids were more abundant below the pycnocline. Podon leuckartiaveraged an ingestion rate of 3.1 µg C ind.⁻¹ day⁻¹, consisting mostly of particles from the nanoplankton size spectrum. Given significant clearance on ciliates and athecate dinoflagellates plus high ingestion rates of heterotrophic nanoflagellates it seems that this species selects for microzooplankton and that heterotrophic components are important in their diet. Calanus australis ingested 15 µg C ind.⁻¹ day⁻¹ on average, covering a wide spectrum of prey sizes (5–65 µm equivalent spherical diameter) and diversity, with active selection for ciliates. The average ingestion rate for Euphausia vallentini was 78.7 µg C ind.⁻¹ day⁻¹, including wide variety of prey sizes but no apparent selectivity. The potential grazing impact (11%) on primary productivity, together with high abundances, feeding selectivity on nanoplankton and production of unconsolidated fecal material indicate that cladocerans could have a double positive feedback effect on the microbial loop, thereby having an important impact by modifying the composition and structure of the planktonic community in Comau Fjord in spring-summer.