Modelling of oxygen and nitrogen cycling as a function of macrophyte community in the Thau lagoon
Pour en savoir plus : M. Plus, A. Chapelle, P. Lazure, I. Auby, G. Levavasseur, M. Verlaque, T. Belsher, J.-M. Deslous-Paoli, J.-M. Zaldívar and C.N. Murray. Modelling of oxygen and nitrogen cycling as a function of macrophyte community in the Thau lagoon. Continental Shelf Research, Volume 23, Issues 17-19 , November-December 2003, Pages 1877-1898.
The aim of this study is to develop a model that assesses the impact of macrophytes on the nitrogen and oxygen cycles. In order to assess the macrophyte impact on the biogeochemical cycles at a small spatial scale and to cope with the water column vertical heterogeneity (phytoplankton is distributed within the water column while macrophytes are on the bottom, in contact with the sediment), a macrophyte model has been developed and coupled with the three-dimensional hydrodynamical-biological model of the Thau lagoon (Lazure, 1992; Chapelle et al, 2001). The model considers ten strata of different thickness (depending on depth) along the vertical, and 400 m regular squared meshes on the horizontal scale. Seven state variables were considered: ammonia, nitrates, phytoplankton, zooplankton, detritus, oyster biodeposits and oxygen. The model is based on nitrogen since it has been found to be the limiting nutrient in the Thau lagoon (Picot et al, 1990). Dissolved oxygen has also been considered in order to simulate photosynthesis, respiration and mineralization processes. Forcing variables are nitrogen inputs from watershed, light, water temperature, wind, oyster farming and macrophyte community.
Total macrophyte biomass was estimated as 10,073±2006 tons dry weight in autumn 1986 (Gerbal, 1994; Gerbal and Verlaque, 1995) and, until now, a total of 196 different macroalgae taxa has been recorded (Verlaque, 2001). The biomass data were processed with a Correspondence Analysis (CA) in order to represent the spatial structure of the macrophyte community and identify areas with a homogeneous macrophyte association. In summary seven zones were identified and the initial biomasses assigned to each model mesh were obtained from the zonation of the lagoon. A fixed height for each species was empirically defined in order to distribute the biomass in the vertical plan. Furthermore, with the aim of taking into account the sediment processes, an additional layer (20 cm deep) has been added on the bottom. Then, for each mesh containing phanerogams, a belowground biomass has been assigned to the sediment layer. This biomass equals 80% of aboveground biomass for Zostera marina and 50% for Z. noltii (Plus, 2001).
Several simulations with and without macrophytes were performed with the 1996 data in order to assess the relationships between macrophyte communities and oxygen and nitrogen. Year 1996 has been chosen because (i) it shows large seasonal meteorological variations (a rainy winter contrasting with a dry summer) and (ii) a quite important database (water temperature, salinity, river inputs, nutrient concentrations, phytoplankton abundance, etc.) was available in order to run and to validate the model. Two sets of 1-month simulations, referring to the winter (January) and summer (June) conditions, were run and the results compared with experimental data. Furthermore, in order to estimate the annual macrophyte total production, the model has been run for a one-year period.
Macrophytes have a considerable impact in the Thau lagoon ecosystem functioning (see Figure). From simulated results, the main effects are on the biogeochemical cycles (oxygen, ammonium, nitrates and detritus) rather than on trophic relationships, because of the lower impacts on the phytoplankton compartment. The inclusion of macrophyte in the model leads to a strong small-scale spatial structuring. This spatial pattern forms an east-west gradient particularly obvious for oxygen and organic detrital nitrogen variables. This gradient is combined with another gradient (north–south) caused by the watershed inputs and the shellfish cultivation zones: higher nutrient concentrations, fine sediments with high organic load in the north and more oligotrophic conditions with sandy sediment in the south. Both extremities (East–West) of the lagoon are characterised by high quantities of organic material which during summer can lead to oxygen depletion. Thus, these sectors can be considered as areas where risks of anoxic crisis are higher.
Simulation results at the 31 January 1996. Variables presented are: (A) dissolved oxygen concentrations (mg l-1) and (B) ammonium concentrations (mmol m-3). Both simulations with (Mac+) and without macrophytes (Mac-) are presented for bottom waters.
The simulated macrophyte annual gross production is 27,300 tons of carbon, which corresponds to 1680 tons of nitrogen. This value is 2.5 fold less than the simulated phytoplankton annual gross production in terms of nitrogen (about 4300 tons N yr-1). The latter value is close to that estimated by Chapelle et al (2000) for the phytoplankton, namely 3570 tons N yr-1. Such a difference between macrophyte and phytoplankton production can be explained by the relative important depth of the Thau lagoon, which implies that about one third of the lagoon bottom is absolutely devoid of macrophytes (Gerbal, 1994). Furthermore, macrophytes have little influence on the annual phytoplankton production, with changes of -2.4 % and +0.2 % when applying respectively -10 % and +10 % to the macrophyte biomass.