Spatial patterns and structural composition of foraminiferal assemblages from the Zanzibar Archipelago (Tanzania)
Thissen, Jens M.; Langer, Martin R.
published: Mar 22, 2017
The Zanzibar Archipelago is one of the most productive and biologically rich reef ecosystems along the eastern coast of Africa and provides valuable benefits to coastal people. It is situated between the high-diversity hotspot of the coral triangle and the cold-water areas of southern Africa with significance as recipient and redistributor of biotas via equatorial and Agulhas currents. The archipelago consists of three major islands: Zanzibar (Unguja), Pemba and Mafia. The reefal structures surrounding the islands are imperiled by intensifying local and global stressors ultimately reducing their capability to maintain their physical structure. We have conducted large-scale island wide surveys on benthic foraminifera, a group of unicellular protists that contribute significantly to the calcium carbonate budget of coral reefs. The spatial distribution, composition, species richness and environmental significance of foraminiferal faunal assemblages were examined via numerical and statistical analyses. Quantitative faunal analyses reveal the presence of seven major habitats, characterized by specific indicator taxa, assemblages and gradients of species richness. Spatial patterns of foraminiferal biotas are characterized by numerical abundances of individual taxa, cluster groups and gradients of species richness, as documented by cluster, Fisher α, and ternary plot analysis. Reef vitality was assessed via the FORAM Index, a single metric index indicative of reef health and conditions for carbonate accretion. The FORAM Index and species richness patterns show that the coral reefs around Pemba, Mafia and Zanzibar are generally in moderate to good condition, but also permits the identification of areas requiring assessment and monitoring. Lowest suitability values were recorded off Zanzibar Town, a densely populated area where anthropogenic influence continues to impact reef development. The presence of habitat specific assemblages and numerical abundance values of individual taxa show that benthic foraminifera are ideal tracers of environmental perturbations useful in paleoecological and paleoenvironmental studies.