Biogeochemical modelling in the Bay of Seine (France): an improvement by introducing phosphorus in nutrient cycles

Abstract

As part of the French National Programme for Coastal Oceanography, this paper focuses on improvement of biogeochemical modelling in the Bay of Seine (Eastern Channel), by introducing phosphorus in nutrient cycles. The Bay of Seine receives the Seine river, which exhibits very high nutrient concentrations, and this coastal zone constitutes a typical case of eutrophication in a river plume area. In terms of analyses, sequential extraction and analysis of sedimentary phosphorus were used in order to measure various forms of particulate phosphorus in suspended matter and sediment (calcium-bound phosphate, Fe/Al-bound phosphate, exchangeable phosphate and organic phosphorus). In the modelling approach, the Bay is divided into 42 boxes and a two-layer, vertical thermohaline model is linked with the horizontal circulation scheme to take vertical stratification into account. The previous biological sub-model, with only nitrogen and silicon as nutrients, was improved by taking account of (a) various forms of bioavailable phosphorus (dissolved phosphate, exchangeable phosphate and organic phosphorus) and (b) experimental parameters which govern the adsorption/desorption of phosphate at the solid-water interface. The ecological model accurately reproduces dissolved nutrient behaviour in the river plume, and concentrations of diatoms, flagellates and zooplankton are consistent with observed values. Simulated exchangeable phosphate and organic phosphorus in suspended matter are in the range of measured concentrations, whereas these particulate variables are simulated with less accuracy in the sediment. After evaluating the respective roles of nitrogen, phosphorus and silicon as limiting factors for phytoplanktonic growth, the model was used to estimate consequences of reduced phosphorus or nitrogen input for phytoplanktonic production in the Bay of Seine.

Seasonal evolution of the nutrient pattern on Biscay Bay continental shelf over the years 1999-2000

“…In the North Sea, Woodward and Owens (1990) pointed out that North Sea waters are phosphate depleted in spring, especially in waters under the influence of river inputs. More recently, Guillaud et al (2000) showed the limiting role of phosphate in the Seine plume during spring due to the decrease in phosphate in freshwater inputs over the last two decades.…”

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)

“…drastically change the conclusions about the controlling factor in a few years (Guillaud et al 2000). Green macrophyte blooms, on the contrary, were unanimously considered as being always nitrogen controlled.…”

“…As the ''nutrient Ͼ phytoplankton Ͼ zooplankton Ͼ detritus'' path has been already fully described when used first in a spatial box model of the bay of Seine (Guillaud et al 2000), then in a 3D model of the same area (Cugier et al 2001;Cugier et al 2005), only the new ingredients (Ulva components) will be detailed here. An overview of the structure of the whole biogeochemical model is shown in Fig.…”

Modelling the eutrophication of the Seine Bight (France) under historical, present and future riverine nutrient loading

“…1), a sediment module allowing the management of sediment compartments with erosion-deposition processes (Cugier and Le Hir, 2000) and an ecological module (Cugier, 1999;Guillaud et al, 2000;Cugier et al, accepted) taking into account nutrient cycles and two phytoplankton groups, namely diatoms and nonsiliceous algae (including harmful flagellate species). Simultaneous limitation of phytoplanktonic growth by inorganic nutrient (nitrateCammonium, o-phosphate, and silica for diatoms) is taken into account by means of hyperbolic Michaelis-Menten like functions.…”