The Growth of Eelgrass in Relation to Tidal Depth

Abstract

The growth of eelgrass (Zostera marina) in relation to tidal depth was studied on three areas of South Humboldt Bay, California, in 1960 and 1962. Six contours of bay bottom between +1.0 and -1.5 feet in relation to mean lower-low tide were sampled. The upper limit of eelgrass growth was at or slightly above +1.0 foot. This contour was exposed to air about 15 percent of the time. The percent of eelgrass coverage, mean turion length, and eelgrass biomass per square foot all increased with increases in tidal depth. The density of turions per square foot and the number of leaves per turion did not vary with depth. The optimum depth for eelgrass production at Humboldt Bay appears to be at about -1.0 foot. Over 90 percent of the total eelgrass biomass and about 60-70 percent of the eelgrass acreage in South Humboldt Bay occur at or below mean lower-low tide. The importance of eelgrass to waterfowl is well documented (Addy and Aylward 1944, Cottam et al. 1944, Cottam and Munro 1954). Humboldt Bay on the northern California coast supports extensive beds of eelgrass (Keller 1963, Waddell 1964) which supply the winter food for large numbers of black brant (Branta bernicla orientalis) and American widgeon (Anas americana), as well as other waterfowl (Yocom and Keller 1961, Denson 1961, Murrell 1962). Modern trends in use of tidal basins for such things as channels, oyster culture, reclamation for industrial development and agriculture, and sewage outlets tend to reduce the acreage of eelgrass beds and to disturb the conditions for growth of remaining beds. To evaluate the impact of such activities on the supply and growth of eelgrass, the effects of environmental factors on eelgrass in relatively undisturbed situations must be known. This paper describes the growth of essentially undisturbed eelgrass in South Humboldt Bay in relation to tidal (water) depth. Acknowledgment is made to Dr. Charles Cooper for his help in planning parts of this study, and to Dr. Charles F. Yocom, Dr. Richard E. Genelly, and Dr. Ernest O. Salo for reviewing early copies of this paper. Irel D. Lowe, Donald Kemp, Kenneth Sohler, and Dale Gardner assisted in the surveying of South Bay, and Kenneth Sohler assisted in the laboratory work. DEFINITIONS AND METHODS Eelgrass growth was sampled on a study ar a in South Humboldt Bay between August 7 and 22, 1960, and between July 26 and August 19, 1962. Two additional widely separated study areas were sampled during the 1962 period. On all three areas, transects were established on contour lines at each 6-inch decrease in elevation by surveying from local benchmarks. At each area, a total of six contours were chosen for study as follows (all measurements expressed in feet in relation to mean lower-low tide): +1.0, +0.5, 0.0, -0.5, -1.0, and -1.5. These contours were selected because +1.0 foot represented the elevation of the upper limit of eelgrass distribution at Humboldt Bay, and it was not physically practical to sample contours below -1.5 feet because such contours were remotely located and water depths were excessive, even at low tide. The following types of data were recorded: