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 Engli sh Channel. Rates of ammonium regeneration in the < 200 mu m fraction varie d from 0.6 to 27 nmol N l(-1) h(-1). On the seasonal scale, these rates wer e relatively low (< 7 nmol N l(-1) h(-1)) in autumn and winter, increased s teadily from March to attain a maximum (27 nmol N l(-1) h(-1)) at the end o f May and thereafter decreased steadily to the seasonal minimum in December . This pattern is distinctly different from that observed in deep well-mixe d waters where the peak ammonium regeneration occurs in summer (Le Corre et al., 1996, Journal of Plankton Research 18, 355-370)Total ammonium regener ated in a year by the microheterotrophs was 15 g N m(-2), equivalent to abo ut 60% of the total nitrogen uptake. Microplankton (200-15 mu m) accounted for about 50% of the regeneration measured between early spring and late su mmer. Percent contribution of nanoplankton to total ammonium regeneration v aried considerably between the seasons, from very high (83-88%) levels in w inter to very low (2-13%) levels in summer. Contribution by picoplankton (< 1 mu m) was high (20-45%) in summer but was less than 20% in other seasons . Ammonium regeneration in micro- and nanoplankton fractions was mainly ass ociated with ciliates and in the picoplankton fraction with bacteria. Macro zooplankton dynamics appears to regulate ammonium regeneration by ciliates and bacteria. Law macrozooplankton biomass in spring may favour a high grow th 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, wo uld, in turn, lower the grazing pressure on the bacteria, thus favouring th eir development and increasing the importance of their role in ammonium reg eneration. This situation, where the macrozooplankton dynamics apparently r egulates ammonium regeneration in nano- and picoplankton fractions,appears to be different from that in deep well-mixed waters. Here. the relative con tribution of ciliates and bacteria to ammonium regeneration shows little va riation with an increase in macrozooplankton biomass. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.