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.

A Novel Modeling Approach to Quantify the Influence of Nitrogen Inputs on the Oxygen Dynamics of the North Sea

“…Compared to previous TBNT studies (e.g., Ménesguen et al, 2006;Radtke et al, 2012) this implies a different treatment of the diffusive transport. In our study its calculation bases on the spatial gradients of the bulk variable concentration.…”

“…This is done by introducing additional model state variables and processes for each property. This concept dates back to the work by Ménesguen and Hoch (1997), who introduced the age as a property, and later on has been applied to various models and ecosystems (e.g., Wijsman et al, 2004;Blauw et al, 2006;Ménesguen et al, 2006;Lacroix et al, 2007;Neumann, 2007;Timmermann et al, 2010;Radtke et al, 2012;Painting et al, 2013;Troost et al, 2013;Perrot et al, 2014;Radtke and Maar, 2016).…”

“…In fact, it is difficult to state which approach is more correct than the other as the diffusive transport according to Fick's first law is a net transport, i.e., the actual gross transports are not known. However, it has to be noted that the diffusive transports of the individual fraction variables X i according to Ménesguen et al (2006) and Radtke et al (2012) can be opposed, while those following Equation (1) are unidirectional. Despite this different treatment the resulting differences in the fraction variable distributions are expected to be generally small and noticeable differences may only occur in the case of large differences between the spatial gradients of the fraction and bulk variables.…”

“…A nutrient tagging method applicable to biogeochemical models -often referred to as "trans-boundary nutrient transports" (TBNT; e.g., Blauw et al, 2006;OSPAR, 2010) facilitates the tracing of elements (e.g., N) from individual sources throughout the entire biochemical cycle and the entire ecosystem (Ménesguen et al, 2006). By this, the TBNT method enables the quantification of the influence of these individual nutrient sources on biochemical quantities (e.g., total nitrogen; TN) and processes (e.g., NPP) in different regions of an ecosystem.…”

Effect of nutrient inputs on growth, chlorophyll, and tissue nutrient concentration of Ulva reticulata from a tropical habitat

“…They are opportunistic in both ecology and physiology. Ulva of various species are usually the first colonizers on open substrata, and their cosmopolitan presence is attributed to their tolerance of a wide range of environments and opportunistic life strategy 1,22,23 .…”