Environmental factors controlling biomass development of seagrass meadows of Zostera noltei after a drastic decline (Arcachon Bay, France)

The relative impact of light, depth, hydrology, hydrodynamics, sediment and nutrients on growth of the seagrass Zostera noltei was investigated in Arcachon Bay, where the extent of seagrass beds has been decreasing for 20 years. A comprehensive multi-parameter survey of 9 sites was conducted monthly over one year. A three-parameter logistic growth model was fitted to biomass data to unravel the relative contributions of environmental factors on seagrass growth. The present study highlights the fact that, in addition to the dominant positive effect of light on plant growth, the hydrological and hydrodynamic parameters were important drivers for Z. noltei growth at the scale of the whole Bay. At the scale of individual sites, other factors such as macroalgal mats or micro-topography (such as presence of tidal pools) may also influence seagrass growth. The statistical model also suggested that the different factors contribute differently to Z. noltei growth in different sites. Finally, as the model is not able to predict biomass accurately for sites that received very little light, and seagrasses in these sites having higher chlorophyll content and longer shoots, we suspect that adaptation to low light condition was at play. In a global context of seagrass beds decline, our study provides a framework to disentangle the site-specific effects of physical and biological drivers on seagrass seasonal growth.