Dynamics of carbon and nitrogen in the mangrove forest of Gazi, Kenya: a numerical modelling approach

Abstract

The numerical model described in this work has been designed to study the flow of carbon and nitrogen in a mangrove forest at Gazi Bay, Kenya. 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 modelling work concerned with coastal ecosystems reveals that modelling is not only a means of summarising 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 conceptualisation the modelling exercise has been handled in several 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 favourable 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 through flow (not including the import of nitrogen into 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. More information is needed on the grazing habits of detritivores.