Dynamics of particulate organic matter delta super(15)N and delta super(13)C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

• Published in: Marine ecology. Progress series (Halstenbek) 2003-06, Vol.255, p.27-41
• Main Authors: Savoye, N, Aminot, A, Fontugne, M, Naulet, N, Kerouel, R
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0171-8630; 1616-1599

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