Comparative study on the trophic structure, cycling and ecosystem properties of four tidal estuaries

Abstract

Compared the Ythan in Scotland, the Ems-Dollard in The Netherlands and the Swartkops and Kromme estuaries, both in Cape Province, South Africa. Flow models (in carbon) were constructed for each of these and analysed by means of network analysis. The trophic structure is similar for all four estuaries, but 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). The FLI and the APL were highest in the most polluted estuary, the Swarkops, 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 were also calculated. Despite the high productivity, high FCI and longer path length of the Swartkops estuary, it exhibits the lowest relative ascendancy, indicating it to be more impacted than any of the other estuaries. -from Authors