Étude de la production primaire phytoplanctonique dans les eaux littorales de la côte normande (Manche orientale)

A phytoplankton community was sampled monthly between February 1978 and January 1980 in a coastal area of the Eastern Channel. During each croise, measurements of several physico-chemical and biological factors were also carried out. This data basis permitted the description of the ecosystem's behaviour in a synthetic manner by inertia analysis techniques, which draw up an ordination of the components of the spatio-temporal variability of the coastal pelagie biotope. For the given time-and space-steps of our sampling strategy, it is shown that the major source of variation is seasonal. This determines the preferential observational scale of the study, whose aim is the analysis 'Of the variability of the low-frequency processes of primary productivity. The contributions of different taxonomie and size fractions to the total chiorophyll biomass were first studied and evaluated in the context of seasonal patterns. This question is approached in an original manner, by solving a system of nonlinear equations combining counts of taxonomie units on the one hand with corresponding amounts of chi a on the other. The greater part of the total chi ais represented by centric dia toms (78 %), and secondly by the size fraction < 5 J.lm (17 %). Considerable dispersion is observed around these estimates of central tendency (mean proportions computed for the whole period February 1978-January 1980): the pennate diatoms may thus con tain more than half of the chi a measured in spring. The dinoflagellate biomass remains continuously at a negligible level ( < 1 % of the community's). A similar numerical approach has been used to specify the main fractions accounting for the photosynthetic capacity of the community: a semi-empirical nonlinear model was fitted to the measures of light-saturated primary production. It shows that 67% of this production is, on an annual average, assignable to the centric diatoms, and 31 % to the ultraplankton ( < 5 J.Lm). The assimilation number of this last size fraction oscillates around 10.5 mgC (mg chi a.h)- 1 , with its seasonal fluctuations following temperature variations (Qto ~ 1.55). The productivity index of ultraplankton is more than twice the estimated Pmax of diatoms and dinoflagellates. The discussion of our results includes a sensitivity analysis of the model, performed on the jackknifed squared C.V. matrix of the L.S. estimates of the parameters.