Irradiance reduction: Effects on standing crops of the eelgrass Zostera marina in a coastal lagoon

Abundance of the eelgrass Zostera marina L. was studied in a coastal lagoon in southern California (USA), and was found to correlate with the level of irradiance at depths greater than 0.5 m below tidal datum. Results of controlled field experiments, using canopies to reduce downwelling illuminance by 63%, confirmed that turion density is a function of the irradiance the plants receive. By Day 18 of the experiment, turion density in the shaded experimental areas had decreased compared to the density of adjacent unshaded controls. Turion densities were continually lower throughout the 9-month study in the experimental areas, which at the end of the study had a turion density only 5% that of the control areas. Flowering in the experimental areas was also inhibited by shading. The biological implications of these findings are discussed with respect to seasonal changes in incident solar radiation, water transparency, and changes in water quality due to man's increased intervention in the natural processes of coastal lagoons.Contribution No. 4 from the Center for Marine Studies, San Diego State University, San Diego, California 92182, USA.