Tides, light and the distribution of Zostera marina in Long Island Sound, USA

The disappearance of Zostera marina L. (eelgrass) in western Long Island Sound has been attributed to the eutrophication-induced increase in light attenuation in the waters of that area. In this work we explore whether the much higher tidal range in the western (3 m) than in the eastern (1 m) Sound could further reduce light availability and, therefore, restrict the vertical distribution of eelgrass. Assuming that the spring low water level determines the upper limit of distribution and the depth of minimum light required for growth determines the lower limit, then the vertical zone for growth in the western Sound is limited to a 1 m fringe. Eelgrass within this narrow range would be vulnerable to exposure during storm events. In the eastern Sound, the viable range for eelgrass growth is 4 m, and similar disturbances would be less likely to affect the seagrass population (since deeper growing shoots may provide energy for shallow-growing ones). A further evaluation of tidal effects on the light availability for Z. marina in Long Island Sound was pursued by allowing surface irradiance and depth of the water column above seagrass canopies to fluctuate over 24 h periods in a Lambert-Beer Law based model. It revealed that the diel benthic light curves were skewed or had a bimodal (rather than sinusoidal) shape and that the number of hours of growth-saturating (about 300 μmol quanta m^-2 s^-1) light was smaller as light attenuation and tidal ranges increased in the western Sound. Therefore, the large tidal ranges may have contributed to the disappearance of eelgrass in the western Sound. We suggest that, due to the significant influence of tides on light availability resulting in light restrictions for benthic vegetation, tides should be taken into account when managing coastal waters with the aim of allowing for the successful growth of seagrasses.