PERIPHYTON DENSITY AND SHADING IN RELATION TO TIDAL DEPTH AND FIDDLER-CRAB ACTIVITY IN INTERTIDAL SEAGRASS BEDS OF THE BANC-DARGUIN (MAURITANIA)

Periphyton development was studied on microscopic glass slides and leaves of Zostera noltii Hornem. in an intertidal area in the Banc d'Arguin (Mauritania). The effects of shading, tidal depth and grazing activities by the fiddler crab Uca tangeri Eydoux were evaluated. For all experiments, periphyton ash content was high (52-93 %) and ash-free dry weight ranged between 0. 10-0. 63 mg cm-2. Slides accumulated more periphyton than leaves. Artificial shading (62-99 %) for 13 days had no effect on periphyton densities on leaves. Increased tidal depth resulted in higher ash-free dry weight on slides, but in lower ash-free dry weight on leaves. Significant variation along the coastline also existed. The effect of fiddler crabs was studied using exclosures. Presence of fiddler crabs reduced periphyton density on slides, whereas light transmittance was increased. On leaves, no significant fiddler crab effect was found. This difference between leaves and slides was probably caused by a storm at the day before the end of the experiment, and by the higher periphyton density on slides as compared with leaves. As visual inspection during the experiment showed clear differences in appearance of leaves inside and outside the exclosures, the storm probably sloughed off mainly the older leaves, i.e. those on which the differences in periphyton cover were the highest. It is hypothesized that periphyton accumulation is higher with increased tidal depth, whereas fiddler crab grazing pressure also increases in this direction. The result is a decreased periphyton density with increased tidal depth. The presently found light extinction coefficients (mean 0.8 m-1) and periphyton light attenuance (up to 25 %) in Banc d'Arguin are not likely to affect seagrass leaf growth.