The Influence of Tidal Mixing on the Timing of the Spring Phytoplankton Development in the Southern Bight of the North Sea, The English Channel and on the Northern Armorican Shelf

Abstract

Hydrographic surveys of water properties have been conducted in March for two consecutive years (1983 and 1984) to examine the timing of the spring phytoplankton development in relationship to environmental parameters. A wide range of physical conditions was encountered in the two March surveys which stretched from the clear open ocean conditions of the Bay of Biscay to the shallow turbid waters of the Southern Bight of the North Sea. Over the shelf region surveyed, the water depth (Fig. 1) changed by an order of magnitude (20 m – 200 m), the beam attenuation coefficient had a 20 fold change (0.4 m^-1 – 9 m^-1) and the magnitude of the M₂ tidal currents (Fig. 2) (Pingree,1983) varied by a factor of 25 (10 cm s^-1 to 250 cm s^-1). Vertical differences of salinity, ΔS, varied from mixed to ΔS∿3 ^°/°°. The observed chlorophyll ‘a’ distributions are discussed in terms of regional changes in these variables which effectively control the availability of light with a view to providing some sea truth for the future development of realistic models. Numerical models simulating the evolution of the temperature structure in the English Channel (see for example, Agoumi et al., 1983) are well advanced.