COMPARATIVE-STUDY ON THE TROPHIC STRUCTURE, CYCLING AND ECOSYSTEM PROPERTIES OF 4 TIDAL ESTUARIES

This paper presents results on the comparative ecology of 4 tidal estu aries: the Ythan in Scotland, the Ems-Dollard in The Netherlands and t he Swartkops and Kromme estuaries, both located in the Cape Province, South Africa. Flow models (in carbon) were constructed for each of the se and analysed by means of network analysis. Specific attention was g iven to the trophic structure, and to the structure and magnitude of c ycling in each estuary. We found that the trophic structure is similar for all 4 estuaries, but that differences exist in the rates of prima ry production and its utilization, standing stocks, in the systems' tr ophic efficiencies, and in the detrivory:herbivory ratios. Three aspec ts of cycling are considered: (1) the number of cycles and their distr ibution against cycle lengths; (2) the Finn cycling index (FCI) and (3 ) the whole system average path length (APL). Cycling analysis reveale d that the FCI and the APL were highest in the most polluted estuary, the Swartkops, and lower in the other less stressed systems. The compo site cycled flows revealed similar structures with virtually all compo nents participating in the recycling of material in the estuaries. Glo bal system properties such as total system throughput, development cap acity, ascendancy, relative ascendancy and indices of flow diversity h ave also been calculated by means of network analysis. Results show th at despite the high productivity, high FCI and longer path length of t he Swartkops estuary, it exhibits the lowest relative ascendancy, indi cating it to be more impacted than any of the other 3 estuaries.