LEAF DYNAMICS AND STANDING STOCKS OF INTERTIDAL ZOSTERA-NOLTII HORNEM- AND CYMODOCEA-NODOSA (UCRIA) ASCHERSON ON THE BANC-DARGUIN (MAURITANIA)

Leaf dynamics and standing stocks of intertidal seagrasses were studie d in the Baie d'Aouatif (Parc National du Banc d'Arguin, Mauritania) i n April and September 1988. Standing stocks of Zostera noltii Hornem. suggest a unimodal seasonal curve similar to what is found for populat ions at higher latitudes. Also, leaf growth rates (0.03 cm2 cm-2 day-1 on average) were similar to those found at higher latitudes in these months. Variation in leaf loss over tidal depth, time and different lo cations in the Baie d'Aouatif was larger and more often significant th an variation in leaf growth. In general, Z. noltii beds in the Baie d' Aouatif had comparable leaf growth rates and standing stocks. In both months losses were almost always higher than or equal to growth. Varia tion in leaf loss over time was much higher in the plots that were sit uated high in the intertidal than in lower plots. This is explained by differences in susceptibility to sloughing, which is presumably highe r in periods with low tide around noon for shallow depths. In an exper iment using artificial shading nets, in situ leaf growth was affected negatively from 94 % shading onwards. This shading was observed to red uce the light intensity reaching the seagrass bed to a level below the reported range of light compensation points for Z. noltii. Cymodocea nodosa (Ucria) Ascherson on average had higher leaf area and relative growth rates than Z. noltii and much lower loss rates, resulting in a positive net increase in September. Standing stocks were also higher t han for Z. noltii. A mixed seagrass bed containing the above two speci es and Halodule wrightii Ascherson had the highest observed total biom ass: 335 g m-2 ash-free dry weight.