SANCOR Newsletter Issue #171: Sept/Oct 2002
BIOGEOGRAPHY AND ESTUARY DIVERSITY: THE MNGAZI AND MNGAZANA ESTUARIES
Janine Adams1, Alan Whitfield2 and Tris Wooldridge3
Department of Botany, University of Port Elizabeth, PO Box 1600, Port Elizabeth, 6000 3Department of Zoology, University of Port Elizabeth, PO Box 1600, Port Elizabeth, 6000
2 South African Institute of Aquatic Biodiversity, Private Bag 1015, Grahamstown, 6140
The Mngazana and Mngazi estuaries are the focus of ongoing research along the former Transkei coast. Research teams have included those of the Botany and Zoology Department, University of Port Elizabeth under the leadership of Dr Janine Adams and Prof Tris Wooldridge and the South African Institute for Aquatic Biodiversity under the leadership of Dr Alan Whitfield and Dr Paul Cowley.
Research questions are addressing the following issues:
1. Comparison of the structure and function of a permanently open estuary with a temporarily open-closed estuary.
2. Biotic diversity in a biogeographic transition area.
3. Sustainable utilization of mangroves in the Mngazana Estuary.
4. Ichthyological food web structure in the two estuaries using stable carbon isotopes.
Aerial photograph of the Mngazi Estuary
Botanical importance
Earlier botanical work documented the distribution and cover of the different plant community types e.g. mangroves, salt marshes and calculated the botanical importance of the estuaries (Colloty et al. 2001). The Mngazana Estuary was recognized as an important system in South Africa in terms of biodiversity conservation, as it has the third largest mangrove area in the country (137 ha), after the Mhlathuze (652 ha) and St Lucia (279 ha) estuaries. The greatest current threat to mangroves in the estuary is removal through harvesting. The red mangrove Rhizophora mucronata is the species targeted for harvesting since the wood is termite resistant and is more useful in the construction of houses and kraals.
Population structure and utilization of mangroves
Ongoing research on the population structure and utilization of mangroves in the Mngazana Estuary has shown that there has been a loss of 17 ha of mangroves between 1961 (154 ha) and 2002 (137 ha). Only 28 % of the total mangrove area showed no signs of harvesting.
Removal of mangroves from creek banks in the Mngazana Estuary
Harvesting has created gaps in the forest canopy, which has promoted the rapid growth and establishment of seedlings. The ratio of adult trees to juveniles was 1:9 for harvested sites compared to 1:3 for non-harvested sites. Although intensive harvesting is taking place in the Mngazana Estuary there appears to be a large supply of propagules and healthy recruitment. However there is major cause for concern as harvesting has significantly decreased the density of adult trees. The number of R. mucronata adult trees at harvested sites was between 48 – 68 trees per 100m2 while the number of adult trees at non-harvested sites was 148 - 288 trees per 100 m2. The ratio of adult trees to harvested stumps was 1:1 and 1:2 at the two R. mucronata harvested sites.
Aerial photograph of the Mngazana Estuary
The continued survival of the mangroves in the Mngazana Estuary may be dependent on the area being declared a reserve with a no utilisation option. In the short term simple measures such as no removal of trees from estuary banks should be implemented.
Microalgae
The biomass and species composition of the phytoplankton and microphytobenthos of the permanently open Mngazana Estuary will be compared to that of the temporarily closed Mngazi Estuary. An understanding of the structure and function of microalgae is important in these estuaries where current development and future water demand threatens the input of freshwater to these systems. This study commenced in 2002.
Invertebrates
The invertebrate work has focused on the importance of the river-estuary interface zone (salinity less than 10 ppt) as a unique habitat for subtidal invertebrates in the permanently open Mngazana Estuary compared to the temporarily open/closed Mngazi Estuary. The study is also comparing the composition of the mangrove creeks with the main channel in the Mngazana Estuary. From 2003 the research will focus on trophic linkages and the exchange of materials in the Mngazana Estuary.
Fish assemblages in a mangrove and non-mangrove estuary
Initial results indicate that the ichthyofauna in the two estuaries are similar, with a higher number of species recorded in the Mngazana. An interesting finding was that the fish assemblages in the lower/middle reaches of the Mngazi and Mngazana estuaries were more similar to each other than they were to the nearby upper reaches of their respective systems.
Species with tropical affinities dominated the fish community during both summer and winter sampling, with only isolated representatives of a few temperate species recorded in each season. This result contrasts to that of Branch & Grindley (1979) who found as many as 12 temperate species in the Mngazana Estuary, most of which were recorded during winter. Preliminary results from the stable isotope work indicate that the same fish species are obtaining their food from similar carbon sources in the two systems. Mangrove detritus does not appear to be an important food source for any of the teleost species in the Mngazana Estuary but the mangrove creeks do offer a refuge for the juveniles of many estuarine associated taxa.
REFERENCES
Colloty, BM, JB Adams and GC Bate. 2001 The Botanical Importance Rating of the estuaries in the former Ciskei / Transkei. Water Research Commission Report TT 160/01. pp 119 + appendices.
Branch, GM and JR Grindley. 1979. Ecology of southern African estuaries. Part XI. Mngazana: a mangrove estuary in Transkei. South African Journal of Zoology 14: 149-170.
