ZOOPLANKTON GROWTH AND MORTALITY IN THE CANARY ISLANDS WATERS.

The development of plankton communities depends on biotic (growth and mortality) and abiotic factors (physical conditions). Growth and mortality are the two key factors of zooplankton production, being influenced by mesoscale structures which in the Canary waters are island-induced eddies, fronts and filaments generated in the upwelling area off northwest Africa. To study the growth of planktonic organisms, a new enzymatic index was developed. We found a relationship between the somatic growth rate of the cladoceran Daphnia magna and the activity of the aminoacil-tRNA synthetases (ARS), which catalyse the first step of the protein synthesis. Growth, feeding and respiration of different copepod populations near the Canary Islands were analysed using ARS, gut fluorescence and electron transfer system activity (ETS), respectively. We show that the variability of metabolic rates vary in relation to the physical structures found (eddies, fronts and filaments).

On the other hand, historical data of zooplankton abundances and short-term sampling of epiplankton biomass evidenced the existence of a lunar cycle in zooplankton, due to the predation pressure exerted by diel vertical migrants on epizooplankton populations. Furthermore, it was observed that predation produced the existence of a top-down control in zooplankton similar to the one observed in lakes and contrary to the bottom-up paradigm in biological oceanography. This process should be responsible for a rather high percentage of the carbon flux to the deep ocean in oligotrophic waters. This lunar cycle in marine zooplankton could explain the 30 days periodicity registered in the particle flux and the importance of faecal pellets found in sediment traps at 900 m depth in the waters surrounding the Canaries. It is suggested that it would explain the uncoupling normally observed between primary production and the particle flux. Moreover, the regulation of predation pressure by the moon cycle could explain the synchronisation of spawning in fish and invertebrates with the full moon. The presence of mesoscale structures and the phase of the moon are suggested to be of importance in the knowledge of zooplankton distribution and trends in subtropical waters.

e-mail: lymo@mail.pml.ac.uk