Two cruises (April and June 1997) were carried out in the Bay of Seine, a nitrate- and ammonium-enriched ecosystem of Western Europe, to identify the major mechanisms that control delta(15)N and VC in spring particulate organic matter (POM). Particulate organic nitrogen (PON) delta(15)N ranged between 0.8 and 5.2parts per thousand in April and between 2.2 and 6.2parts per thousand in June, while particulate organic carbon (POC) delta(13)C ranged between -24.3 and -19.7parts per thousand, and between -20.0 and -16.2parts per thousand during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON delta(15)N is due to both nitrate mixing between river and marine waters and fractionation of N stable isotopes during nitrate utilization by phytoplankton. Therefore, similarly to what was previously shown for open ocean, delta(15)N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC delta(13)C in spring is controlled by POC concentration and C:N ratio (in addition to 'temperature effects'), which are considered here as indicators of primary production and phytoplankton degradation, respectively. The co-variation of delta(13)C and delta(15)N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low delta(15)N) and phytoplankton production is low (low delta(13)C); then primary production increases (delta(13)C becomes higher) and the nitrate pool diminishes (deltadelta(15)N becomes higher); at a later stage, the nitrate pool is depleted (high delta(15)N), part of the phytoplankton becomes degraded and production is still high (high delta(13)C).