Seasonal cycle of nitrogen and phytoplankton biomass in a well-mixed coastal system (Eastern English Channel)

The hydrological structure of the French coastal part of the eastern English Channel is strongly linked with tidal regimes and riverine input. Two distinct water masses are separated by a frontal area and drift along the coast in SW–NE direction. These two water masses are well-mixed during the entire year. We studied the seasonal dynamic of nitrogenous nutrients, chlorophyll a and organic particulate carbon and nitrogen at two stations, characteristic of these water masses, during the year 1994. Results show (i) a winter stock of nitrate and ammonium, (ii) a pre-bloom period corresponding to the use of ammonium, (iii) a high bloom period of short duration using nitrate, (iv) a post-bloom period with little phytoplanktonic activity probably limited by nutrients other than nitrogen and (v) an autumnal period of reconstitution of stock. The essential difference between the two stations is the importance of winter stock of nutrients and of bloom chlorophyll a concentration, with the coastal station richer than the offshore one. An assumption about the nitrogen available for new production in this area gives a value of 57% of the winter stock of inorganic nitrogen.     

Growth in populations of the sea-urchin EchinusEsculentus L. (Echinodermata : Echinoidea) from the English Channel and Firth of Clyde

Growth and rate of growth curves, computed by the single logistic model, are given for populations of Echinus esculentus L. from sites near Plymouth, in the English Channel, and at Great Cumbrae, Firth of Clyde. The growth curves show a sigmoid shape, with their origins corresponding well with directly-read diameters of recently-metamorphosed juveniles, and the upper asymptote, approaching 14 cm horizontal diameter for Plymouth and 12 cm for Cumbrae, corresponding with ages of 8–12 yr and 8–10 yr, respectively. The gonopores open, and the gonads become functional, at between 0.94 and 1.66 cm ambital diameter, corresponding to an age of 1.5 to 2.5 yr. Maximum rate of growth in the Plymouth region was at mean age 4.7 yr, and at Cumbrae 4.3 yr. Results for Cumbrae are compared with those obtained there on the same species by two previous workers.