Nitrogen dynamics in coastal systems: coastal system with strong vertical mixing

In the coastal domain, depending on the importance of hydrodynamics, one can differentiate between several types of systems as follows: (1) a stratified system in regions where the energy dissipated by the tidal currents is weak and a thermal (or haline) stratification becomes established, (2) a homogeneous system in the sectors where the intensity of currents is strong enough to counteract the effect of stratification, and (3) a frontal system, corresponding to a transition between the stratified and well-mixed systems. The functioning pattern in the homogeneous system contrasts with that in zones of weak hydrodynamism (stratified system). The studies carried out in recent years (results obtained mainly in the English Channel) have evidenced this specificity with respect to the nitrogen cycle. When the depth of the well-mixed layers is distinctly greater than that of the euphotic zone (deep homogeneous systems), the production cycle, in terms of nitrogen as well as in terms of carbon, has a broad maximum that extends over the whole summer. The mean light energy available for phytoplankton is low and this limits the growth of the latter throughout the year. On the other hand, nitrogen, the concentrations of which remain high throughout the seasonal cycle, is not a limiting factor for phytoplankton growth. The development of phytoplankton is based mainly on the utilization of regenerated forms of nitrogen, in particular ammonium. The high primary production in the homogeneous system is related to a rapid recycling of ammonium in the water column by the microbial loop and not to a nutrient enrichment with allochthonous inputs. When the well-mixed water column is shallow and light penetration reaches to the bottom in summer (shallow well-mixed systems), the production cycle shows a sharp maximum in spring. The nitrogen stock available in spring for the development of phytoplankton is much smaller than that in the deep well-mixed system. Consequently, in summer nitrate is sometimes near totally exhausted in the shallow well-mixed systems. The decrease in nitrate concentrations is compensated for by the prevalence of high concentrations of ammonium that prevent a marked limitation of nitrogen. Hence, light remains the main factor limiting the development of phytoplankton in this shallow sector of the homogeneous system. The shallow homogeneous system, in spite of a seasonal cycle distinctly different from that in the deep homogeneous system, also shows a primary production mainly of a regenerated nature. Most of the organic matter produced in the entire homogeneous system is decomposed in situ and mineralized; the capacity of this type of system to export organic matter therefore is reduced.