Although microbial mats are known worldwide to contribute to trophic bases in shallow marine embayments, their role has yet to be ascertained for similar habitats in the Arabian Gulf region. In this study, stable carbon isotope analysis in conjunction with faecal and gut analyses were employed to evaluate the relative importance of microbial mats in the nutrition of macrofaunal consumers and their contribution to the exceptional biodiversity, abundance and biomass of species inhabiting Sulaibikhat Bay mudflats (2000-2001) in the absence of extensive macro-vegetation. Microbial mats were mainly composed of diatoms and cyanobacteria (approx. 70 and 30% respectively), with a reduction in the latter with decreasing intertidal gradient. Faeces of 5 out of the 6 species analysed contained both mat components at varying degrees. Diatoms occurred rarely in faeces of the high intertidal consumers, while cyanobacteria dominated the faeces of those occurring at lower levels. Gut analysis revealed that 6 Out of the 9 species analysed contained mat residues. Primary producers had distinct delta C-13 values with microbial mats showing a heavy signature (-15.3 parts per thousand) in comparison to the halophyte Salicornia herbacea (-24.7 parts per thousand), phytoplankton (-21.6 parts per thousand) and the mangrove Avicennia marina (-28.1 parts per thousand). The VC values for herbivorous, omnivorous and carnivorous macrofauna ranged between -9.4 and -15.3 parts per thousand. It appears from this investigation that microbial mats form the primary source of nutrition for both benthic and pelagic macrofaunal species within the bay including commercially important species, and that selective assimilation/digestion of cyanophytes from the mats is the likely explanation for the relatively high enrichment in the delta C-13 signatures of species located along the higher intertidal zone.
