The Effect of Light on the Vertical Structure of Epibenthic Diatom Communities

Abstract

The development of an epibenthic diatom community was observed using light and scanning electron microscopy (SEM) on samples obtained from plastic panels immersed vertically at 2 depths in the St. Lawrence Estuary, Quebec [Canada]. Live cell immigration, cell velocity and growth form were investigated using glass slides. Some panels were either maintained at a constant depth (1 or 5 m), exposed to normal daylight, or maintained shaded at the same depths. Others were transferred from 1 to 5 m from 5 to 1 m after initial colonization had occurred. Quantitative evaluations of diatom populations were made at weekly intervals. Species composition, cell abundance and vertical stratification were dependent on depth and ultimately on light intensity. At all depths, newly immersed panels were mainly colonized by highly motile diatoms like Navicula spp. After 6 wk of immersion, the community from 1 m depth had developed into a distinctly 2-layered algal mat. The upper stratum was characterized by filamentous (Melosira, Fragilaria) and branched tube-dwelling forms (Berkeleya), together with highly motile Nitzschia and Navicula. The lower stratum consisted of cells more closely associated with the substratum such as the slowly moving Cocconeis, erect fan-shaped Synedra and arborescent Gomphonema. After 8 wk of immersion, the uppermost stratum peeled off and only the understory remained. In comparison with diatom colonies o panels directly exposed to sunlight, colonies on shaded panels from 1 m and unshaded panels from 5 m depths had a slower development, lower cell densities and were less stratified. Diatom assemblages observed on shaded (1 m) and deeper water (less illuminated, 5 m) panels were very simiar to the diatom layer intimately in contact with the substratum observed at 1 m depth. Species densities on panels transferred from 1 to 5 m and vice versa indicated that the dominance pattern was not related to differential immigration but to growth (light) conditions. The vertical structure of the community and diversity of growth forms were closely related to light availability. The effects of physical disturbances on the dynamics of community stratification are discussed in a model.