Inter-Research
Marine Ecology Progress Series

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 d¹⁵N and d¹³C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) d¹⁵N ranged between 0.8 and 5.2‰ in April and between 2.2 and 6.2‰ in June, while particulate organic carbon (POC) d¹³C 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 d¹⁵N 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, d¹⁵N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC d¹³C 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 d¹³C and d¹⁵N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low d¹⁵N) and phytoplankton production is low (low d¹³C); then primary production increases (d¹³C becomes higher) and the nitrate pool diminishes (d¹⁵N becomes higher); at a later stage, the nitrate pool is depleted (high d¹⁵N), part of the phytoplankton becomes degraded and production is still high (high d¹³C).

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

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Published in MEPS Vol. 255 (2003) on June 24
Print ISSN: 0171-8630; Online ISSN: 1616-1599. Copyright © Inter-Research, Oldendorf/Luhe, 2003