A new numerical technique is presented that allows the tracking of any chemical element from any source in a
simulated foodweb, for instance assessing the proportion of these sources (river loadings, sea entrances, point
sources) in the algal diet for the limiting nutrient. An application is shown for nitrogen in an Ulva bloom occurring
in a shallow embayment connected with a strongly tidally stirred ecosystem with various sources of inorganic
nitrogen, the Bay of Brest, Brittany, France. In a first step, a biogeochemical three-dimensional model was developed
to simulate growth and erosion–transport–deposition of free-floating ulvae; this model was able to converge on a
realistic distribution of Ulva deposits after a few months, even though it was initialized with a strongly unrealistic
distribution of settled ulvae. In a second step, and unfortunately for recovery plans, the tracking technique, applied
in this model to all the nitrogen sources entering the bay, revealed that the small, nitrate-polluted rivers flowing
directly into the eutrophicated area had a negligible effect, whereas more distant but stronger sources, a big river
and a big urban sewage plant, even after dilution, accounted for about 50% and 20%, respectively, of the algal
nitrogen content during summer. Despite its high N flux, open ocean contributes only 15% to Ulva growth. The
suppression of only one of the main nitrogen sources would not significantly decrease the Ulva bloom, because of
the high nitrogen surplus present in the site. The remaining sources would still saturate the needs of the maximum
Ulva biomass the site is able to produce. The tracking technique, however, shows that the N turnover in Ulva is
only 4 months. Thus, improvements would occur within a year following large N reductions.