MEPS 255:27-41 (2003)  -  doi:10.3354/meps255027

ABSTRACT: 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 δ¹⁵N and δ¹³C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) δ¹⁵N ranged between 0.8 and 5.2‰ in April and between 2.2 and 6.2‰ in June, while particulate organic carbon (POC) δ¹³C 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 δ¹⁵N 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, δ¹⁵N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC δ¹³C in spring is controlled by POC concentration and C:N ratio (in addition to Œtemperature effects¹), which are considered here as indicators of primary production and phytoplankton degradation, respectively. The co-variation of δ¹³C and δ¹⁵N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low δ¹⁵N) and phytoplankton production is low (low δ¹³C); then primary production increases (δ¹³C becomes higher) and the nitrate pool diminishes (δ¹⁵N becomes higher); at a later stage, the nitrate pool is depleted (high δ¹⁵N), part of the phytoplankton becomes degraded and production is still high (high δ¹³C).

KEY WORDS: C and N stable isotopes · Phytoplankton · Coastal ecosystem · Nitrate utilization · Bay of Seine