SYSTEM-LEVEL INDICATORS IN DYNAMIC ECOSYSTEMS - COMPARISON BASED ON ENERGY AND NUTRIENT FLOWS

System-wide indicators of ecosystem structure and function depend strongly on the choice of flow variable, or numeraire. I compare two such indicators, exergy and ascendency, using biomass energy or elemental nitrogen as flow variable. Comparison is for steady-state (data from Cone Spring and Tayozhny Log Bog) and for dynamic behavior (hypothesized, based on steady-state data from Tayozhny Log). In both systems, measured flows are available in terms of biomass energy; nitrogen flows are imputed by calculating dynamic nitrogen intensities (analogous to energy intensities). I find that steady-state normalized ascendency differs as much for one ecosystem using these two flow variables it does for two different ecosystems using the same flow variable. This implies that the question of flow variable choice needs resolution before strong conclusions can be made about intersystem comparisons. For dynamic behavior, nitrogen-based exergy shows no more, and sometimes less, variation over time than energy-based exergy. Both nitrogen- and biomass-based ascendency follow variations in system stock and show little additional variation. Analyzing the size, concentration, and structural components of exergy and ascendency aids in understanding these behaviors. I also briefly compare trophic positions using the two flow variables. © 1990 Academic Press Limited.