A model of zooplankton diel vertical migration off the Canary Islands: implication for active carbon flux

A mathematical model of the diel vertical migration of zooplankton off the Canary Islands is presented. It simulates the time and space variations of zooplankton biomass and allows calculations of metabolism over the diel cycle in the water column from 0 to 1000 m depth. The results are used to estimate the active carbon flux due to respiration by diel vertical migrants. This model depends mainly on the influence of absolute light intensity and of the rate of irradiance change. Swimming responses to the properties of the light field described the diel vertical migration of zooplankton. The simulated vertical distribution of animals in near-surface waters (75-112 m) during the night and in deeper layers during the day (428-436 m) was in good agreement with the in situ data used to initialise the model. The daytime respiration at depth obtained was compared with in situ estimations of respiration from ETS (electron transfer system) activity in a previous study and from published empirical equations relating temperature and metabolism of epiplanktonic zooplankton. We found that the latter procedure tends to overestimate active flux while the opposite was observed with ETS-derived assessments. Our results show that carbon consumption in the shallow layers estimated from metabolic rates and the subsequent production of large faecal pellets should be considered in the assessment of active carbon flux in the ocean.