ISOTOPIC FRACTIONATION OF AMMONIUM AND NITRATE DURING UPTAKE BY SKELETONEMA-COSTATUM - IMPLICATIONS FOR DELTA-N-15 DYNAMICS UNDER BLOOM CONDITIONS

Isotopic fractionation of ammonium (NH4+) and nitrate (NO3-) during up take by phytoplankton was examined in batch culture experiments with t he diatom Skeletonema costatum under nitrogen-enriched conditions (5-1 00 mu M). The fractionation factor (epsilon) for NO3- uptake by Skelet onema was -9.0 +/- 0.7 parts per thousand and was concentration-indepe ndent. For NH4+, epsilon was more variable and dependent on ambient NH 4+ concentration. For NH,+ concentration ranges of 100-50, 50-20, and 20-5 mu M, epsilon was -24.6 +/- 5.5, -27.2 +/- 1.6, and -7.8 +/- 3.0 parts per thousand. In these cultures, isotopic fractionation by phyto plankton caused variations in delta(15)N of up to 50 parts per thousan d for NH4+, 12 parts per thousand for NO3-, and 25 parts per thousand for particulate N. Similar variability in the delta(15)N Of both disso lved inorganic and particulate organic N pools should be expected duri ng phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used t o examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes a re decoupled in either space or time during bloom conditions.