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Table 9.10 Summary of fish community structure

Parameter Subsidized

Unsubsidized marsh

Significance difference

Fish density 230 m"2

165 m"2

n = 30, df = 5, p = 0.01

Fish biomass 6.44 kg m"2

4.29kgm"2

n = 30, df = 5, p = 0.03

Vegetative community richness in the subsidized marsh was lower than that of the unsub-sidized marsh. Fish biomass was also significantly different between marshes (Table 9.10).

A dynamic model was used to simulate situations in which the fuel use was increased (0% = unsubsidized marsh; 100% = subsidized marsh). Figure 9.11 shows the changes in biomass carrying capacity with different levels of fuel used. Material and energy balances, as shown in the emergy analysis, were significantly different between the subsidized and unsubsidized marshes.

Due to the nonrenewable energy sources from the lake (e.g. nutrients, phytoplankton) and the pump system itself, the subsidized system had much higher flows of available resources. This is also clearly evident in the higher densities and biomass of the avian and fish communities. On the other hand, the complexity of the subsidized marsh as measured by diversity and community structure was lower. High emergy subsidies may compromise the complexity of the system in favor of high productivity. Community structure and dynamics are likely a result of many processes including demographics, energy cycling, habitat disturbance, and the influence of other populations (Brown and Maurer, 1987; Maurer and Brown, 1988; Weins, 1989). In the case of the subsidized marsh, organization and community dynamics are also controlled by the availability of energy sources with high transformities. The importance of certain high emergy sources is their ability to facilitate the input of additional nonrenewable energies. The nutrient enrichment seemed to speed up self-organizational processes in the subsidized marsh

Month

B) Fuels at 10% of Use by Subsidized Marsh

B) Fuels at 10% of Use by Subsidized Marsh

Month

C) Fuels at 50% of Use by Subsidized Marsh

Month

D) Fuels at 100% (equal to fuel use of subsidized marsh)

D) Fuels at 100% (equal to fuel use of subsidized marsh)

Month

Figure 9.11 Percent variation of biomass over time for different rates of fuel use showing: (A) 0 fuel usage represented by unsubsidized marsh; (B) 10%; (C) 50%; (D) 100% fuel usage relative to actual usage by subsidized marsh. (100% = biomass carrying capacity of unsubsidized marsh). Vertical dashed line marks appropriate time when marsh biomass reaches a steady state.

Month

Figure 9.11 Percent variation of biomass over time for different rates of fuel use showing: (A) 0 fuel usage represented by unsubsidized marsh; (B) 10%; (C) 50%; (D) 100% fuel usage relative to actual usage by subsidized marsh. (100% = biomass carrying capacity of unsubsidized marsh). Vertical dashed line marks appropriate time when marsh biomass reaches a steady state.

Figure 9.12 Percentage variation of biomass over time as fuel usage changes monthly as sine wave between 0 and 100% fuel usage relative to actual fuel usage of subsidize marsh.

increasing the rate of vegetative coverage of the marsh. Given the higher animal densities and biomass, the external subsidy may have also increased the rate that these components reached their respective carrying capacities.

This theory seemed to be validated by the computer model simulation; it suggests that carrying capacity varies with different levels of external subsidy.

The simulation in Figure 9.12 of changing subsidy reveals that if the subsidy is pulsed, biomass will also pulse; the simulation model is sustained by studies in literature about the relationship between nutrients increase and biomass increase (Kerekes, 1990; Price, 1992). Overall, the external subsidy increased the emergy flux in the subsidized marsh by increasing the input of nonrenewable energy sources. As a result, community parameters such as density and overall biomass also increased in the subsidized marsh, but at a cost of lowered richness, diversity, and evenness.

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