Particle flux in the Subtropical Atlantic near the Azores: influence of mesozooplankton

The amount and composition of sinking material were studiedat two depths (200 m and base of the photic layer) in the SubtropicalAtlantic Ocean near the Azores during the AZORES I (August 1998)and II (April–May 1999) cruises. Particulate carbon andnitrogen fluxes collected in sediment traps decreased with depth,and presented maximum values near the Azores Front. However,this frontal system represents an increase of only 4.5% in theoverall regional exportation during summer. Sedimentation ratesoutside the front were higher during spring. Particulate organiccarbon exported at 200 m always represented <1.5% of watercolumn (photic zone) standing stock. Mesozooplankton faecalpellets contributed significantly to carbon flux. On average,carbon in the form of faeces represented 31% (spring) and 65%(summer) of total carbon collected at 200 m. Composition ofthe copepod community seems to be related to the pattern offaecal pellet sedimentation, with omnivorous copepods (Oithonaand Oncaea) being more abundant at stations where faecal fluxdecreased with depth. Phytoplankton sedimented at 200 m weredominated by diatoms and dinoflagellates. Phytoplankton exportedat 200 m represented <0.5% of water column (photic zone)standing stock.     

Ingestion, faecal pellet and egg production rates of Calanus helgolandicus feeding coccolithophorid versus non-coccolithophorid diets

Ingestion rates, faecal pellet and egg production were obtained in laboratory experiments with females of the copepod Calanus helgolandicus collected from the English Channel in November 1994. Five different algal monocultures were used as food: Prorocentrum micans (30 μm ESD), Thalassiosira weissflogii (13 μm ESD), Dunaliella tertiolecta (7 μm ESD), Emiliania huxleyi (5 μm ESD) and Coccolithus pelagicus (14 μm ESD). Results obtained suggest the low ingestion efficiency of the copepod when feeding on coccolithophorids during late autumn-early winter. From the five species offered, only the largest non coccolithophorid Prorocentrum micans and Thalassiosira weissflogii supported efficient feeding and calculated respiratory demand for C. helgolandicus. Both coccolithophorids, irrespective of cell size, were ingested at very low rates even when offered at high concentrations (233-468 μg C l(-1)). Besides low ingestion, no egg production was found in the copepods fed with Emiliania huxleyi, although unusual high gross efficiency (reaching 72%) was obtained in experiments performed with Coccolithus pelagicus. The late seasonal timing of the experiments (November) could explain the low ingestion and egg production rates.     

Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates wereinvestigated along a 50°S–50°Nlatitudinal transect,during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises(boreal spring–autumn 1997, boreal spring–summer1998). Copepod abundance was higher at high latitudes in spring,near northwest Africa, in the equatorial and Benguela upwellingsystems, and in the Subtropical Convergence, and lower in oligotrophicgyres. Gut contents were not related to phytoplankton biomassor production. Gut evacuation rate averaged 0.03 min^-1, andwas not related to latitude or body size. Conservative estimatesof copepod community total ingestion rates ranged between 3.4and 173 mg C m^-2 day^-1 for AMT4, 1.6–252 mg C m^-2 day^-1in AMT5 and 10–160 mg C m^-2 day^-1 in AMT6. Maximum valueswere always in the upwelling regions, the subtropical convergenceand high latitudes in the Northern Hemisphere during borealspring. Calculated ingestion rates translate into average dailyminimal consumption values of 2.07%, 1.89% and 2.6% of totalchlorophyll stock, or 8.02%, 14.5% and 12.9% of total primaryproduction ingested daily on AMT4, 5 and 6 respectively. Grazingimpact increases considerably if we consider ingestion of phytoplanktonlarger than 2 µm, especially under the influence of theEquatorial and North African upwellings, where copepod ingestionrepresents up to 30% of the biomass and >100% of productionby large cells.     

Mesozooplankton Distribution and Copepod Grazing in the Subtropical Atlantic Near the Azores: Influence of Mesoscale Structures

Mesozooplankton distribution and copepod grazing were investigatedin the Subtropical Atlantic Ocean near the Azores during theAZORES I (August) and II (April–May) cruises. Mesozooplanktonbiomass and abundance remained low throughout the region, butsignificant increases were found related to the presence ofthe Azores Front. The Azores Front also exhibited maximum valuesof copepod community ingestion, reaching 250 mg C m^–2ingested daily. This increase in ingestion was related to increasesin copepod abundance, but not in copepod gut contents. No relationshipwas found between gut contents, or ingestion, and phytoplanktonbiomass or production. Daily cycles were found in copepod gutcontents, being higher during the night, but not in copepodabundance. Multi-dimensional scaling analysis revealed differencesin copepod taxonomic composition between both sides of the front.During spring, daily copepod ingestion represents an averageof 6% of the integrated chlorophyll (Chl) a concentration and22% of the primary production. These percentages increase to15% of Chl and 61% of production if we only consider large (>2µm) phytoplankton. No clear influence of the cycloniceddy LETICIA was found in mesozooplankton biomass or grazing.A significant effect of the Great Meteor Tablemount was foundin copepod abundance and grazing, with higher values locatedwest of the mount.