Dynamics of particulate organic matter d15N and d13C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

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 15 N and 13 C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) 15 N ranged between 0.8 and 5.2 in April and between 2.2 and 6.2 in June, while particulate organic carbon (POC) 13 C ranged between -24.3 and -19.7 , and between -20.0 and -16.2 during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON 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, 15 N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC 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 13 C and 15 N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low 15 N) and phytoplankton production is low (low 13 C); then primary production increases ( 13 C becomes higher) and the nitrate pool diminishes ( 15 N becomes higher); at a later stage, the nitrate pool is depleted (high 15 N), part of the phytoplankton becomes degraded and production is still high (high 13 C).