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

Water column structure, microphytoplankton size spectra andnutrient concentration were analysed at six sampling stationsin 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) wherethe UML is considerably less than half the TD. The UML was nutrientrich at type A stations and was nutrient depleted at type Bstations. At type C stations, the UML was nitrate depleted andsilicate 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 µmand a dome at 300 µm caused by a Coscinodiscus wailesiibloom. The dome was observed at all depths at type A stationsand above the pycnocline and at the seafloor of type B stations.Combining intrinsic growth rate, sinking rate and mixing layerdepth, the C. wailesii dome persists only at type A stationsbut settles out of the UML at the remaining stations. This suggeststhat a large part of the perturbation at the right extreme ofphytoplankton SAS does not propagate along the planktonic SASbut sinks to the seafloor. This paper was presented in a session on "Size Structure ofPlankton Communities", at the ASLO Summer International Meeting,held in Santiago de Compostela, Spain, between 19 and 24 June,and coordinated by Xabier Irigoien, Roger Harris and Angel Lopez-Urrutia.