ABSTRACT. This paper presents results on the comparat~ve ecology of 4 tidal estuaries: the Ythan In Scotland, the Ems-Dollard in The Netherlands and the Swartkops and Kromme estuaries, both located in the Cape Province, South Africa. Flow models (in carbon) were constructed for each of these and analysed by means of network analysis. Specific attention was given to the trophic structure, and to the structure and magnitude of cycling in each estuary. We found that the trophic structure is similar for all 4 estuaries, but that differences exist in the rates of primary production and its utilization, standing stocks, in the systems' trophic efficiencies, and in the detrivory:herbivory ratios. Three aspects of cycling are considered: (1) the number of cycles and their distribution against cycle lengths; (2) the Finn cycling index (FCI) and (3) the whole system average path length (APL). Cycling analysis revealed that the FCI and the APL were highest in the most polluted estuary, the Swartkops, and lower in the other less stressed systems. The composite cycled flows revealed similar structures with virtually all components participating in the recycling of material in the estuaries. Global system properties such as total system throughput, development capacity, ascendancy, relative ascendancy and indices of flow diversity have also been calculated by means of network analysis. Results show that despite the high productivity, high FCI and longer path length of the Swartkops estuary, i t exhibits the lowest relative ascendancy, ~ndicating ~t to be more impacted than any of the other 3 estuaries.

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