Ecosystems that are depicted as nodes (species groups) and links (trophic transfers) encompass well defined roles in terms of their trophic structure. Some are primary producers and furnish the system with newly bound energy or nutrients, whereas others divide themselves over various trophic levels creating a hierarchy of energy use and reuse. All waste and mortality products of nodes comprise the non-living environment of ecosystems, often supporting a considerable part of the system as detritus. All food sources are in general used, but are so to variable extents. Some links are always among the major links, e.g. flows from detritus or remineralisation from bacteria. Similarly, flows into and from high turnover nodes, which are either small species with high turnover rates, or those with disproportional high standing stock play an important role in shunting energy through the system. Ecosystem indices calculated from weighted ecological networks describing the patterns of energy flow through an ecosystem are often applied to empirical data in order to categorise the system as an efficient or inefficient energy user. We calculated flow diversity (as Shannon's Index) and average mutual information (AMI, as the degree of flow constraint) according to the method of Ulanowicz (1986). Special attention was given to the contribution of each system part to the overall holistic property. A comparison of several, mainly estuarine, ecosystems, shows that it is mostly flows involving the same species groups contributing to the bulk to the index value. These were flows from detritus, bacteria, and primary producers. The relative importance of flows from heterotrophic groups (e.g., species and species groups of macrozoobenthos, meiofauna, zooplankton, fish) compared to that of the "main" groups was comparatively minor
