Modelling Zostera marina and Ulva spp. in a coastal lagoon

We have implemented new modules of seagrass and macroalgae in the European Regional Seas Ecosystem Model (ERSEM). The modules were tested using a version of ERSEM coupled with the General Ocean Turbulence Model (GOTM) in San Quintin Bay (SQB), a coastal lagoon in Baja California, Mexico. As we are working in a region where horizontal advective transport of nutrients is important, we have included the horizontal nutrient gradients which result in nutrient advection when combined with the local currents. The addition of the Zostera marina and Ulva spp. modules to ERSEM, and the inclusion of advection results in a better simulation of the seasonal and interannual trends in nutrient concentrations and macrophyte biomasses in SQB. The differences between the simulations with and without advection are particularly apparent during the upwelling periods. Therefore, by increasing the horizontal gradients of nitrate in the model during the strong upwelling seasons a stronger advection results in higher nitrate concentrations from May to July in 2004 and 2005. The difference in the seasonal trend in biomasses between both macrophytes, with Ulva spp. reaching its seasonal maximum in June–July and Z. marina reaching it in September–October reflects the different response to the various factors controlling their primary production. Z. marina is particularly sensitive to variations in the photosynthetically active radiation (PAR) and the light limitation factor, while Ulva spp. is more sensitive to changes in the maximum uptake rates of nitrate. The model was forced using field data from the lagoon collected in 2004 and 2005.