IS AMMONIFICATION RATE IN MARINE SEDIMENT RELATED TO PLANKTON COMPOSITION AND ABUNDANCE - A TIME-SERIES STUDY IN VILLEFRANCHE BAY (NW MEDITERRANEAN)

Observations were made near Cap Ferrat (Station B, about 80 m in water depth) France, in the water column and in the sediment, in order to e valuate to what extent variations in the ammonia and nitrate concentra tions of the sediments are related to plankton population abundance an d composition. Nitrate, nitrite, ammonia and chlorophyll a were measur ed several times during 1987 to 1989, at two depths (1 and 40 m). Cope pods and salps in the upper 75 m of the water column were counted seve ral times a week from 1987 to 1990. Ammonia and nitrate concentrations and ammonification rate were determined in the underlying sediments. During Spring 1987, phytoplankton biomass showed a maximum at the end of March; copepod populations increased regularly till the end of Apri l, and salps increased from this time to the end of May. These populat ions were not so well developed during Spring 1988 and 1989. During th e blooms, salp were mainly represented by Thalia democratica. The biom ass of phytoplankton and zooplankton was low in summer. The sequence s uggests that the copepod decline was related to reduced food levels af ter the phytoplankton decline. Salp population growth was not at the e xpense of phytoplankton and it can be assumed that the salp fed on oth er material. In 1987 and 1988, maximum organic nitrogen concentration in the bottom sediment and maximum ammonification rate directly follow ed the salp spring bloom. In 1987, the highest ammonification rate mea sured in the surficial sediment (0-2 cm) reached 0.05 mu M cm(3) day(- 1) (in June). In 1990, the rate exceeded 0.1 mu M cm(-3) day(-1) durin g an important salp bloom. Therefore, it seems that the sinking of sal p fecal pellets plays an important part in the transfer of organic mat ter to the bottom, and microbial activity in the surficial sediment le ads to mineralization of a great part of the organic nitrogen quickly after its deposition. (C) 1996 Academic Press Limited