A new numerical technique for tracking chemical species in a multi‐source, coastal ecosystem, applied to nitrogen causing Ulva blooms in the Bay of Brest (France)

Abstract

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.