Investigations of Brackish Water Aquaculture in the Blackland Prairie Region of Western Alabama
Type of DegreeDissertation
Fisheries and Allied Aquacultures
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Three investigations were performed related to brackish water aquaculture in the Blackland Prairie region of western Alabama. Two of these investigations were related to pond fertilization with magnesium fertilizers needed to supplement water deficient in this cation, and the subsequent adsorption of these cations by pond bottom soils. The third investigation was performed to address the concern of salinization of local streams as the result of utilization of brackish water for culture purposes. A magnesium budget was prepared for a commercial inland brackish water shrimp farm for one production cycle. Fertilization with sulfate of potash magnesia (K2SO4·2MgSO4 or K-Mag®) was applied to three ponds. Two of the ponds had four previous production cycles and corresponding exposure to magnesium fertilizers, and the third pond had only one previous production cycle prior to the current study. Losses of iv magnesium that resulted from outflows of water failed to account for all of the magnesium applied, indicating that unaccounted magnesium inputs may have been adsorbed by the pond bottom soils. Increases of exchangeable magnesium in the upper 15-cm layer of pond bottom soils could not account for total inputs of magnesium. Magnesium unaccounted for may have been adsorbed on soil exchange sites at depths greater than 15-cm, analytical errors could have contributed to some magnesium being unaccounted for as well. Differences between the newer pond and two older ponds may indicate that the soils previously exposed to magnesium fertilizers have a diminishing affinity for the magnesium or an equilibrium concentration was being established. A study conducted in laboratory soil-water systems revealed that soils from an inland brackish water shrimp facility strongly adsorbed magnesium applied as fertilizer ? magnesium sulfate heptahydrate (MgSO4·7H2O). The rate of adsorption declined over time, indicating the systems were reaching equilibrium. Repeated exposures of soils to solutions of 40 mg Mg2+/L failed to saturate exchange sites, but rather maintained equilibrium with other base cations on soil adsorption sites. Dissolved sulfate resulting from additions of magnesium with magnesium sulfate heptahydrate (MgSO4·7H2O) was also monitored over the trial. Though difficulties of analysis occurred, it appears the sulfate is not adsorbed by the pond bottom soils. Investigation into stream salinization resulting from low-salinity aquaculture in was initiated in June 2006. Eight streams were sampled bimonthly for salinity, conductivity, and chloride concentrations. Four of the eight streams were associated with aquaculture production facilities, and received effluents from these operations. The remaining four streams were located in the area near the aquaculture facilities, but did not receive any v effluents or runoff from any aquaculture facility. The streams associated with the aquaculture production facilities were sampled at sites both upstream and downstream of the facility. Sampling through November 2007 revealed that the streams associated with the aquaculture facilities had chloride concentrations that exceeded 230 mg Cl-/L ? upper standard set by the Alabama Department of Environmental Management for freshwater streams ? at various times throughout the year. Results reveal that these facilities contributed to stream salinization.