Moreira-Saporiti, Agustín; Teichberg, Mirta; O'Sullivan, Aoife: Benthic macrofauna in a seagrass ecosystem in Changuu Island in 2017 (Unguja Island, Zanzibar Archipelago, Tanzania) [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.988036 (dataset in review)

Benthic macrofauna density was measured during a nutrient enrichment and exclusion field experiment on seagrass. A total of 24 plots (Sampling stations) were set up parallel to the shore, with presence of the three seagrass species (Syringodium isoetifolium, Thalassodendron ciliatum and Thalassia hemprichii) in each plot. The experiment was the factorial combination of two treatments: macrofauna exclusion using cages (three levels: uncaged/control, open and closed) and nutrient enrichment using garden NPK fertilizer (two levels: ambient/control and enriched). Each treatment combination was replicated four times. The macrofauna exclusion cages were made of wooden pegs (0.5 x 0.5m wide and 0.5 m high) and were covered with a plastic net (14 mm diagonal mesh size). This mesh-size excludes macrofauna, but not small fish and mesofauna. This macrofauna exclusion technique (Casini et al., 2008) simulates the consequences for the food web of losing a top predators and macrograzers. Open cages had all the side meshes halved to allow partial access from all four sides (to check for cage effects), and uncaged plots were marked with four wooden pegs. The cages were anchored by metal T bars and cleaned every 2 weeks to remove organisms and filamentous algae that would otherwise affect light penetration and water flow. Nutrient enrichment was simulated by issuing nitrogen-phosphorus-potassium (NPK) [15:9:20] fertilizer pellets. NPK fertilizer fast release pellets were packed into cotton tubes and then into a perforated plastic tubes to simulate slow release of nutrients. The tubes were filled with 75g of pellets and five tubes were placed in each of the enriched plots (375g in total per plot) following the protocol by Heck et al. (2000). The tubes were buried half-way into the sediment to ensure enrichment of both the water column and the sediment. Fertilizer was replaced every 3 weeks, making a total of 1125 grams of fertilizer released per enriched plot at the end of the experiment. All plots were separated by at least 3 metres, proven as a no-overspill area in previous nutrient enrichment experiments (Lotze & Worm, 2000). Data was collected between July 19th and September 20th of 2017: Day 0 (19.07.2017), Day 20 (09.08.2017), Day 38 (31.08.2017) and Day 63 (19.09.2017). Data collection and experiment took place in Changuu Island (Zanzibar Archipelago, Tanzania; 06˚11'S, 39˚16'E) in the area that the experiment took place (see coordinates of transects in the data table). Changu Island is located 3 km from Stone Town, Zanzibar's busiest town. Changuu remains relatively unaffected by nutrient runoff pollution, although it has a small tourist resort and a large tortoise centre with daily boat traffic bringing tourists from Stone town. The study area is characterised by a fringing reef around a multi-specific seagrass ecosystem. The substrate is primarily carbonate sediment. Average water depth is approximately 30 cm at Spring Low and 5 m at Spring high tide with an average depth of 2 m. Benthic macrofauna density was measured to assess the top down control exerted by macrofauna on seagrass and its associated meiofauna. To determine top-down control exerted by the benthic macrofauna in the study site, the density of macrofauna was quantified. We deployed four 50 m long transects within the area where the treatment plots were built. The benthic macrofauna was counted along the transects up to one-meter distance from each side of the transect tape, resulting in a total surveyed area of 400 m2. Only sea urchins and sea stars were encountered. Within the sea urchins, most of the urchins encountered belonged to the species T. gratilla, D. setosum and T. pileolus. These species were separately counted, while individuals of other species (Astropyga radiata, Echinometra spp, Toxopneustes pileolus) were included together in one category ("Other Sea urchins") due to their low abundance. Sea stars, Protoreaster linki and Pentaceraster mammilatus, were counted together as well. The counts for all benthic macrofauna were then divided by the surveyed surface to obtain individuals m-2.