Carbon export through zooplankton active flux in the Canary Current

Abstract

Zooplankton vertical distribution, migrant biomass, and respiratory flux were studied in two transects performed in the Canary Current at 21°N and 26°N from the eu- and mesotrophic upwelling zone off Northwest Africa to the oligotrophic central gyre waters. Migrant biomass was estimated by sampling during day and night using a Longhurst-Hardy Plankton Recorder (LHPR) net in the 0–900 m depth water column. The electron transfer system (ETS) enzymatic activity was measured as a proxy for zooplankton respiration in the 0–900 m depth water column. Respiratory flux was obtained from the migrant biomass and the daytime respiration in the 200–800 m layer, assumed as the residence depth of migrant zooplankton. The southern transect at 21°N showed higher primary production, zooplankton biomass, migrant biomass, and lower passive flux. Zooplankton respiratory flux varied from 2.8 to 88.3% of the POC flux, being lower in the northern (5.3% ±2.7) than in the southern transect (46.1% ±32.9). Migrant biomass was significantly correlated to primary production (r2= 0.570, p < 0.05, n = 10), and the respiratory flux also showed a weak but significant relationship with primary production (r2= 0.550, p < 0.05, n = 10). Migrant biomass and the respiratory flux showed the strongest and significant correlation (r2= 0.870, p < 0.001, n = 10), allowing respiratory flux estimation from day and night zooplankton catches for a wide range of ocean productivity. Assessment of active flux considering mortality and carbon excretion estimated from published metabolic efficiencies besides respiration considerably increase the role of these organisms in the biological carbon pump, suggesting a significant transport by the pelagic fauna to the mesopelagic zone.