REGULATION OF LIGHT ATTENUATION AND EELGRASS ZOSTERA-MARINA DEPTH DISTRIBUTION IN A DANISH EMBAYMENT

The distribution of eelgrass Zostera marina L. has declined significan tly throughout a Danish embayment, the Limfjord, during the past 2 dec ades, parallel to increased nutrient input and reduced Light penetrati on. In this study monthly measurements of Secchi depth from 1986 to 19 89 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 dept h transparency was highly variable as a result of seasonal fluctuation s rather than spatial differences. Within all 10 basins the seasonal c hanges in Secchi depth were closely related to suspended particle conc entrations (R(2) = 0.408 to 0.667). Suspended inorganic matter could a ccount for more of the variability in attenuation than chi a in half o f the basins, and was closely related to wind-induced sediment suspens ion. Episodes of high water turbidity associated with high wind activi ty are, therefore, likely to influence the areal distribution of eelgr ass 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 suspen ded mineral particles, such that Secchi depth transparency, closely re lated to eelgrass depth penetration, is only likely to increase by 10. 6 to 17.5% following a 2-fold reduction in phytoplankton biomass. Cont inued resuspension following the long-term perturbation of the entire embayment is, therefore, likely to constrain eelgrass colonization fol lowing nutrient abatement.