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Dynamics of carbon and nitrogen in the mangrove forest of Gazi, Kenya: a numerical modelling approach
Ong'anda, H. (1992). Dynamics of carbon and nitrogen in the mangrove forest of Gazi, Kenya: a numerical modelling approach. MSc Thesis. FAME: Brussel. 89 pp.

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Document type: Dissertation

Keywords
    Carbon; Mangroves; Nitrogen; ISW, Kenya, Gazi Bay [Marine Regions]; Marine

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  • Ong'anda, H., more

Abstract
    This numerical model has been designed to study the flow of carbon and nitrogen in a mangrove forest at Gazi Bay, Kenya. Mangrove ecosystems, characterized by individual kind of trees belonging to some twelve genera in eight different families, occur in fringing sheltered tropical shores. They have important ecological and economic roles. These ecosystems are therefore scientifically interesting in so far as understanding the cycles and flows of matter is concerned. Several processes and storages of matter considered vital in the functioning of the ecosystem have been reviewed to help in the abstraction of the model and analysis. A numerical model represents the functioning of the ecosystem but is always limited due to the complexity of the system and lack of information on processes. A review of modeling work concerned with coastal ecosystems reveals that modeling is not only a means of summarizing data from field studies, but that it can also be an integrated part of research effort useful in formulating research hypotheses and drawing up management options. The objectives of the Gazi model are similar in this respect. The abstraction of the system is presented as a box and arrow diagram showing storages and flows of matter. Changes in each of the state variables are function of some input and output processes and are represented in the model equations as ordinary differential equations. The processes are variously formulated using published ecosystem models, field data and personal effort. For ease of conceptualization the modeling exercise has been handled in seemingly related units called submodels. The majority of the rate coefficients were estimated from field and literature values whereas a few others were chosen arbitrarily. Computer simulation of the system was done in an IBM compatible PC using a simulation package. A sensitivity analysis was carried out to investigate the sensitivity of the model to changes in input and parameter values. The favorable comparison between model output and information available points to the relative accuracy of the model. The results show that the nutrient contribution of the mangrove ecosystem to the contiguous zones is negative. The system exports carbon largely consisting of detritus poor in nitrogen. The flow of nitrogen through bacteria accounts for 72% of the total system nitrogen flow-through (not including the import of nitrogen into the system). Mangrove trees can be harvested by adopting a cycle of management. The mangrove ecosystem is critically dependent on incoming nitrogen. Of the dynamics of the inorganic nitrogen, N2-fixation and denitrification have little impact on the system. Most of the parameters and inputs are not certain but appear to be very important in the dynamics of the system. Response of detritivores to changes in the system suggests the need for more control mechanisms in the model. There are limitations to the model prediction especially with regard to changes in the system. The mangrove ecosystem strongly consumes nitrogen and compensates for this by using inorganic nitrogen in the tidal water and possibly from sheetflow and underground seepage. The grazing habits of detritivores is uncertain and more information is needed. The manner in which different ways of mangrove harvesting may affect the regeneration of forest requires a model taking into consideration all factors related to harvesting.

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