Control of microplankton size structure in contrasting water columns of the Celtic Sea

Abstract

Water column structure, microphytoplankton size spectra and nutrient concentration were analysed at six sampling stations in the Celtic Sea. Three types of stations were distinguished: (i) where the upper mixed layer (UML) reaches the total depth (TD), (ii) where the UML is about half of TD and (iii) where the UML is considerably less than half the TD. The UML was nutrient rich al type A stations and was nutrient depleted at type B stations. At type C stations, the UML was nitrate depleted and silicate rich. Two groups of microplankton size-abundance spectra (SAS) were found: (i) a typical linear SAS and (ii) a more complex 'atypical' SAS, with a linear part up to 160 μm and a dome at 300 μm caused by a Coscinodiscus wailesii bloom. The dome was observed at all depths at type A stations and above the pycnocline and at the seafloor of type B stations. Combining intrinsic growth rate, sinking rate and mixing layer depth, the C. wailesii dome persists only at type A stations but settles out of the UML at the remaining stations. This suggests that a large part of the perturbation at the right extreme of phytoplankton SAS does not propagate along the planktonic SAS but sinks to the seafloor. © The Author 2006. Published by Oxford University Press. All rights reserved.