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 lea ves of Zostera noltii Hornem. in an intertidal area in the Banc d'Argu in (Mauritania). The effects of shading, tidal depth and grazing activ ities by the fiddler crab Uca tangeri Eydoux were evaluated. For all e xperiments, periphyton ash content was high (52-93 %) and ash-free dry weight ranged between 0. 10-0. 63 mg cm-2. Slides accumulated more pe riphyton than leaves. Artificial shading (62-99 %) for 13 days had no effect on periphyton densities on leaves. Increased tidal depth result ed in higher ash-free dry weight on slides, but in lower ash-free dry weight on leaves. Significant variation along the coastline also exist ed. 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 ef fect 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 v isual inspection during the experiment showed clear differences in app earance of leaves inside and outside the exclosures, the storm probabl y sloughed off mainly the older leaves, i.e. those on which the differ ences in periphyton cover were the highest. It is hypothesized that pe riphyton accumulation is higher with increased tidal depth, whereas fi ddler crab grazing pressure also increases in this direction. The resu lt is a decreased periphyton density with increased tidal depth. The p resently found light extinction coefficients (mean 0.8 m-1) and periph yton light attenuance (up to 25 %) in Banc d'Arguin are not likely to affect seagrass leaf growth.