Abstract

Zooplankton is the primary production consumer in the aquatic food web in fjords and channels of western Patagonia. In this study, the role of vertically migrating and not-vertically migrating crustacean mesozooplankton (collected in a 300 mu m mesh net) as potential consumers of spring phytoplankton was assessed in an inshore gulf of central Patagonia through repeated stratified zooplankton sampling during the day and night, over a 30 h cycle. Results show that while a group of copepods remained close to the surface where chlorophyll-a concentrations were higher, other copepod groups and euphausiids exhibited did vertical migrations. The maximum depth of migration appeared to be limited by very low oxygen concentrations in the water column (0.2 mL O-2 L-1). Meanwhile, the abundance of not-migrating copepods (62%) was higher than that of vertically migrating copepods and euphausiids combined (38%). The latter two groups doubled their carbon consumption at night (37.0%) compared to daytime (18.8%) but their contribution continued to be lower than that of not-vertically migrating groups at night (63%). Over the 24 h cycle, the carbon consumption carried out by all vertically migrant groups together accounted for almost one-third (28.8%) of the total daily carbon consumption by the mesozooplankton crustacean community. In most areas surveyed along Patagonia, total community respiration exceeded the estimated gross primary production when autotrophic and microheterotrophic respiration was pooled, suggesting that external subsidies of carbon (e.g., allochthonous, terrestrial) and/or recycling of fecal pellets are required to support the micro- and meso-zooplankton communities of these sub-Antarctic fjord systems.