Emergy As Ecological Indicator To Assess

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ECOSYSTEM HEALTH Reference from which these applications of emergy as ecological indicator are extracted:

Howington TM, Brown MI, Wiggington M. 1997. Effect of hydrologic subsidy on self-organization of a constructed wetland in Central Florida. Ecol. Eng. 9, 137-156.

Emergy (see Chapter 6) is used to study and explain theories concerning the effect of an external subsidy on a complex system (constructed wetland) seen by a holistic point of view.

Lake Apopka is a shallow (mean depth = 1.7 m) hypereutrophic lake in Central Florida, with an area of 124km2 (Lowe et al., 1989, 1992). In the early 1940s a hurricane removed most of the rooted macrophytes in the lake which led to the early stages of increased nutrient availability and subsequently increased algal productivity (Schelske and Brezonik, 1992). Addressing the nutrient status of this lake, the St. Johns River Water Management District (SJRWMD) constructed a 200ha freshwater marsh on former agricultural lands with the goal of reducing the nutrient levels in the lake. It was suggested that by pumping enriched lake water through a constructed marsh, filtration of phosphorus and suspended sediments could be maximized. The pump system was turned on in early 1991. The subsidized and unsubsidized marsh maintained similar average water levels (0.76m) throughout the study period varying yearly by no more than 0.2 m. Theory suggests that an external subsidy should increase the carrying capacity for wildlife of an ecosystem, all other things being equal. The increased capacity for wildlife may be an indirect result of certain self-organizational processes such as changes in vegetative cover. Other factors influencing the relationship between wetland productivity and hydro-period include nutrient inputs, export, nutrient cycling, and decomposition (Carpenter et al., 1985).

This study tested theories concerning the effect of an external subsidy on ecosystem structure and organization. Two newly established marshes (one receiving nutrient-enriched lake water and the other not receiving the subsidy) were the areas under study. The 63 ha subsidized marsh is the first of two cells that constitute the treatment wetland receiving lake water. The unsubsidized marsh, 46ha, was created as a result of being a borrow pit for building berms around the treatment wetland. Vegetative cover richness and percent cover were determined using aerial photos and GIS, and was calculated using Margalef's (1977) index for species richness. Percent cover provided a further description of the changes in structural complexity of each marsh over time. Also avifauna surveys were conducted. Shannon diversity indexes were used to compare the avian communities found in the surveyed marshes. A synoptic study on the fish population of the subsidized and unsubsidized marshes was also conducted. A model of the marsh system (see Figure 9.9 for energy symbols) was created to

Production subsystem Figure 9.9 Energy symbols used to make an energy diagram.
Figure 9.10 Diagram of constructed marsh. Removal of pump system simulated unsubsidized marsh.

describe the role of the most important components and relationships (Figure 9.10). An emergy analysis was performed to evaluate on a common basis (solar energy) the contributions of the various inputs (pumps, water, nutrients, human services, and renewable energies) driving the marshes ecosystems.

Emergy evaluation separates inputs on the basis of the origin (local or purchased) and of their renewability (see also Chapter 6). An environmental loading ratio (ratio of local and exogenous nonrenewable emergy to renewable emergy) and an investment ratio (ratio of exogenous to local emergies) were calculated to compare the quantities and qualities of the energies entering each system. Emergy analysis tables were developed separately in Tables 9.7 and 9.8 for the subsidized and unsubsidized marshes.

The environmental loading ratio showed a large contrast between the two marshes. Investment ratios for the two marshes showed a large difference in the amount of purchased energy necessary to maintain the flows of environmental inputs.

Table 9.9 contains the ratios of free to purchased energy (environmental loading) and nonrenewable energy to renewable energy (investment ratio). Renewable energy sources

Table 9.7 Annual energy, material and dollar flows, and resulting emergy flows supporting 1 ha of the subsidized marsh

Notes Quantity Emergy per unit Emergy and unit (sej unit-1) (E + 14sej)

Notes Quantity Emergy per unit Emergy and unit (sej unit-1) (E + 14sej)

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