Phase I – Sustainable Management and Valuation of Seagrass Ecosystems in the Western Indian Ocean Region

This project aimed at assessing the structural importance and ecosystem function of seagrass assemblages in the eastern Africa region.  Specifically, the project was designed to address the following objectives: i) the mapping of seagrass beds in key areas; ii) assessment of seagrass productivity; iii) assessment of the vulnerability of seagrass assemblages to environmental stresses; and iv) the ecological-economic valuation of seagrass ecosystems. Targeted beneficiaries of this project included local communities, scientific community, environmental managers, policy makers and other relevant stakeholders in the WIO region. The project was implemented in five countries – four countries from the WIO region that include Tanzania (Unguja Island at Chwaka, Jambiani, and east coast of Unguja), Mozambique (Maputo bay, Inhaca Island, Moçambique Is and Cabo Delgado), Mauritius (Le Morne, Beau Champ) and Kenya (Mombasa, Nyali and Vipingo).

This multidisciplinary research project included a variety of methodologies including direct observations, remote sensing as well as Geographic Information Systems (GIS), tagging, quadrat based techniques, hole punch, standard nutrients analysis methods, calorimetric method, UV-PAM method, corer, and acetylene reduction assay. Herbicide exposure/ toxicity studies were conducted through exposure plants to either Diuron (DCMU), 2,4 D Helamine, Fusilade or combinations thereof at varying concentrations. Furthermore, an ecological-economic approach was applied in order to evaluate the value of the seagrass ecosystem. Information on seaweed farming (test plots, area cultured, environmental parameters and production rates, involvement of the local people and exports), socio-economic and environmental impacts were collected using secondary data and field surveys.

Results indicate a loss in seagrass cover of about 588 ha (equivalent to 12.8%) between 1991 and 2003 in the studied areas in Mozambique. While the reduction in seagrass area cover at Inhaca Island was minimal (about 3.2%), a considerable loss of around 459 ha (about 86.3%) was recorded at Bairro dos Pescadores. This loss was associated mainly with removal of Nanozostera capensis due to collection of invertebrates, as well as sedimentation caused by floods that occurred in 2000. The general distribution of seagrasses at Chwaka Bay (Zanzibar) between 1987 and 2003 was fairly stable. Furthermore, satellite images comparing the distribution patterns of seagrass beds and algal communities in shallow lagoon waters of the east coast of Unguja Island, Zanzibar, as a result of degradation, removal and/or replacement to algal farms between 1987 and 2002 indicated that the seagrass beds and algal communities to be relatively stable over time.

Results further indicate overall higher seagrass abundance and biomass, cyanobacteria abundance, and a more diverse fauna community in non-seaweed areas of both Chwaka and Jambiani than in the seaweed sites, possibly due to the anthropogenic pressures resulting from seaweed farming activities. Total Nitrogen fixation rates by epiphytic and benthic diazotrophs in seagrass meadows in Chwaka Bay between non-seaweed farms and seaweed farms followed similar trend. The study on carbonate sand production by Halimeda at Chwaka Bay identified four species, namely H. macroloba, H. incrassate, H. incrassata and H. opuntia), the latter being the most common, and the major player in sand production at the area.

Leaf growth measurements done on Syringodium isoetifolium in Mozambique indicate a similar growth rate in both dry and rainy seasons. The study further recorded a variation in nutrient concentation from older to younger leaves, a possible indication of existence of translocation of N from old to younger leaves as documented elsewhere. A comparative study carried out on seagrass ecosystems at a pristine habitat (Poste Lafayette) and a degraded one (Pointe-aux- Biches) in Mauritius revealed denser vegetation, a higher plant biomass and a more diverse fauna at a pristine habitat than a degraded habitat. Toxicity study indicated that only Diuron showed a negative effect in Thalassodendron ETRmax, Fv/Fm, and gross oxygen rates. This research project also provides information on economic valuation of seagrass beds as it regards fish, invertebrates and fisheries in general.

Data gathered from this project e.g. satellite image analysis, the GIS maps produced and knowledge gained on the diversity of seagrass and algal communities, and toxicity information can contribute to coastal zone management in the WIO. Spin-offs from this project include one research work aiming at studying both aspects of structure and dynamics of the seagrass Enhalus acoroides that started in the northern Mozambique and seagrass replantation studies at NW Maputo bay. Furthermore, the existence of this project contributed in the design of students research project for the new masters course on Aquatic Biology and Coastal Ecosystems at Universidade Eduardo Mondlane, Mozambique.

Based on this report, new areas that WIOMSA can focus on include studies on: causes of seagrass area variation (i.e. sand accretion, sand revolving), relationship between seagrass removal with depletion of nutrients, faunal resources and social and economic impacts, promoting and analyzing cost-benefits of seagrass rehabilitation to help to overcome the destruction of Nanozostera capensis meadows in NW Maputo city, management issues associated with a demand for fin-fish and invertebrate food resources dependent on seagrass beds, seagrass–fisheries relationship; considering both the ecological and human dimensions, new mariculture techniques for promoting more yield and less effort per farmer in the field (new farming techniques for Eucheuma are being tested but its efficiency in seaweed production not yet conclusive) and evaluation of damage on seagrass beds by seaweed farming.