Seasonal patterns of ammonium regeneration from size-fractionated microheterotrophs

Ammonium regeneration by size-fractionated plankton was measured for 1 year at a coastal station in the shallow well-mixed waters of the western English Channel. Rates of ammonium regeneration in the <200 μm fraction varied from 0.6 to 27 nmol N l^?1 h^?1. On the seasonal scale, these rates were relatively low (<7 nmol N l^?1 h^?1) in autumn and winter, increased steadily from March to attain a maximum (27 nmol N l^?1 h^?1) at the end of May and thereafter decreased steadily to the seasonal minimum in December. This pattern is distinctly different from that observed in deep well-mixed waters where the peak ammonium regeneration occurs in summer (Le Corre et al., 1996, Journal of Plankton Research, 18, 355–370). Total ammonium regenerated in a year by the microheterotrophs was 15 g N m^?2, equivalent to about 60% of the total nitrogen uptake. Microplankton (200–15 μm) accounted for about 50% of the regeneration measured between early spring and late summer. Percent contribution of nanoplankton to total ammonium regeneration varied considerably between the seasons, from very high (83–88%) levels in winter to very low (2–13%) levels in summer. Contribution by picoplankton (<1 μm) was high (20–45%) in summer but was less than 20% in other seasons. Ammonium regeneration in micro- and nanoplankton fractions was mainly associated with ciliates and in the picoplankton fraction with bacteria. Macrozooplankton dynamics appears to regulate ammonium regeneration by ciliates and bacteria. Low macrozooplankton biomass in spring may favour a high growth of ciliates and an associated high in ammonium regeneration. In summer, the increase in macrozooplankton may exert a grazing pressure on ciliates. This, coupled with the fact that most of the flagellates are autotrophs, would, in turn, lower the grazing pressure on the bacteria, thus favouring their development and increasing the importance of their role in ammonium regeneration. This situation, where the macrozooplankton dynamics apparently regulates ammonium regeneration in nano- and picoplankton fractions, appears to be different from that in deep well-mixed waters. Here, the relative contribution of ciliates and bacteria to ammonium regeneration shows little variation with an increase in macrozooplankton biomass.