An Experimental Study of the Vertical Distribution of Littoral Marine Diatoms1

Extensive covers of epilithic diatoms seldom develop on intertidal substrates in southern Oregon in the presence of grazing animals and macroscopic algae. If an area is cleared, however, a brown coating of diatoms often appears in 2 to 3 weeks, but the density depends on the season and the local conditions during tidal emergence. I have attempted to evaluate the effects of exposure on the vertical distribution of diatoms in the absence of grazers and macrophytes. The differences in exposure to insolation were emphasized. Vertical transects in the same block, facing north, east, west, and south (all with the same wave exposure) were denuded and later sampled by analyzing the species composition and estimating the density of the cover—sometimes quantitatively as chlorophyll content. Daily tidal curves were calculated and records of the daylight hours of clear and overcast skies were made. The upper limit of the diatom cover, the vertical species pattern, and the species present depended principally on the time exposed to direct sunlight during emergence and, therefore, on the sky conditions and position of the sun during low water. The diatom cover on the shaded north transect invariably extended higher than on the other transects. This maximum was usually between mean lower high water and higher high water. Some species tolerated almost no insolation‐exposure, others up to about 2.5 hr in a single exposure, and a few up to 4.0 hr or longer. The tolerance was sometimes increased by a moisture‐retaining carpet of barnacles or Ulva. The upper limit of the diatom cover was lowest on the east transect in spring and summer, since lower low water occurred in the morning, often during clear daylight hours. In the fall and winter, the limit was lowest on the west transect, since lower low water occurred after noon, often during clear daylight hours. The consequences of prolonged exposure to insolation, such as desiccation, high temperature, high light intensity, and ultraviolet radiation are discussed but were not isolated. The seasonal periodicity and tolerances of common diatom species are summarized.