chapter  4
18 Pages

Ecosystem Health Indexed through Networks of Nitrogen Cycling

Assessing ecosystem-level status is a challenge, and several indices from information theory are used to assess the status of ecosystems. Food web networks are generally used in these assessments, but we have applied them to simpler networks of nitrogen cycling. Specifically, we indexed status in the framework of ecosystem health and three of its attributes: organization, vigor, and resilience, respectively indexed by ascendency (A), total system throughput (TST), and overhead (O). These indices were derived from seasonal networks of nitrogen cycles in two coastal lagoons: the Hog Island Bay, Virginia, and the Sacca di Goro, Emilia Romagna, Italy. The sites represent a large range of nutrient loading and subsequent trophic conditions from mesotrophic to hypereutrophic and dystrophic. As the degree of eutrophication (indexed through TN loading) increased, the cycling, efficiency of use of nitrogen, and degree of organization decreased. Indeterminacy of flows, as redundancy, increased. A problem of spurious correlations arose in the empirical analysis of ascendency vs. overhead because they share the same TST as a component. We propose that log transformations of TST can lessen the problem and might be used more generally in the empirical applications of information indices. Our results

Abstract ............................................................................................................................................ 73 4.1 Introduction ............................................................................................................................ 74 4.2 Methods .................................................................................................................................. 76

4.2.1 Study Sites .................................................................................................................. 76 4.2.2 Network Construction .................................................................................................77 4.2.3 Network Analysis and Sensitivity to Uncertainty ...................................................... 82

4.3 Results and Discussion ........................................................................................................... 83 4.3.1 Trophic Status ............................................................................................................. 83 4.3.2 Ecosystem Health Framework ....................................................................................84