Local Persistence of Large Benthic Foraminifera (LBF) under Increasing Urban Development: A Case Study from Zanzibar (Unguja), East Africa

Coastal marine management is vital for socio-ecological sustainability of developing, tropical ecosystems, which calls for diverse tools to monitor and assess water quality. The carbonate-dominated habitats off Zanzibar were chosen for study due to potential water quality degradation in a rapidly developing tourist destination heavily reliant on its coral reefs. These reefs are largely unmonitored and subject to local and global stressors. A widely used method for assessing reef health, as an early detection method of ecological changes, is the application of large benthic foraminiferal bioindicators, i.e., the FoRAM Index. We expected to find poor water quality conditions in the unmanaged reefs supported by stress-toelerant (opportunistic) foraminiferal assemblages. The dissolved inorganic nitrogen and phosphate values derived from untreated sewage effluent from Stone Town were highly variable (ranging 0.05–3.77 and 0.05–1.45 µM, respectively), moderate, and occasionally approached or exceeded critical threshold values for oligotrophic ecosystems. The analysis of total assemblages indicated an abundance of symbiont-bearing large benthic foraminifera, dominated by prolific Amphistegina species, comparatively low-moderate diversity, high FI values (7.6 on average), and high coral cover. A water quality gradient was reflected by subtle assemblage differences, suggesting that LBF can provide early warning signals of benthic changes, indicating the importance of long-term monitoring programs in vulnerable, rapidly developing coastal ecosystems exposed to increasing pressures.