Regulation of light attenuation and eelgrass Zostera marina depth distribution in a Danish embayment

The distribution of eelgrass Zostera marina L. has declined significantly throughout a Danish embayment, the Limfjord, during the past 2 decades, parallel to increased nutrient input and reduced Light penetration. In this study monthly measurements of Secchi depth from 1986 to 1989 were related to concentrations of nutrients, phytoplankton biomass (as chi a) and resuspended particles (mineral and organic) to evaluate their influence on water turbidity within 10 different basins of the Limfjord. The overall median concentrations of total N (0.87 mg l(-1)) and total P (0.072 mg l(-1)) were high, and total N accounted for 55.6% of the spatial variability in the chi a concentrations. Secchi depth transparency was highly variable as a result of seasonal fluctuations rather than spatial differences. Within all 10 basins the seasonal changes in Secchi depth were closely related to suspended particle concentrations (R(2) = 0.408 to 0.667). Suspended inorganic matter could account for more of the variability in attenuation than chi a in half of the basins, and was closely related to wind-induced sediment suspension. Episodes of high water turbidity associated with high wind activity are, therefore, likely to influence the areal distribution of eelgrass within the Limfjord. Reduction in nutrient loading is expected to increase water clarity as suggested by the direct relationship of chi a to total N. The impact is limited, however, by high levels of suspended mineral particles, such that Secchi depth transparency, closely related to eelgrass depth penetration, is only likely to increase by 10.6 to 17.5% following a 2-fold reduction in phytoplankton biomass. Continued resuspension following the long-term perturbation of the entire embayment is, therefore, likely to constrain eelgrass colonization following nutrient abatement.