Assessment of Cumulative Impacts to Water Quality in a Forested Wetland Landscape

One important focus of applied ecology is the assessment of environmental impacts and the recommendation of ways to avoid or minimize these impacts. Cumulative impacts result from the accumulation of many human activities whose impacts, although not individually measurable, together sum to significant adverse effects. Assessment of cumulative impacts requires a landscape approach and large-scale analysis. In our procedure for determining cumulative impacts in bottomland hardwood forests (BLHF), changes in landscape integrity over time were assessed using structural and functional ecosystem indices. In this article we present a historical analysis of water quality in the Tensas Basin, Louisiana, as part of the cumulative impacts analysis of this BLHF landscape. Historical records of suspended sediment, N, P, and turbidity from three streams in the Tensas Basin were analyzed. Significant positive relationships between water levels in these streams and concentrations of total P, total Kjeldahl N, total suspended sediment, and turbidity confirmed a loading phenomenon characteristic of watersheds in which much of the original forest cover has been cleared. Eighty-five percent of the original forest in the Tensas Basin has been converted to agricultural fields. Temporal trends in nutrient concentration show that water quality has been declining steadily since 1958 in one river, whereas in the other two the decline largely occurred before then. Nitrogen/phosphorus ratios in two of the three rivers (2.9:1 and 4.6:1 by weight, 6.4:1 and 10.2:1 by molar ratio) were lower than expected in P-limited freshwater systems, where N/P ratios are generally > 10 to 15 by molar ratio. Most of the excess in P is probably related to the geochemical adsorption of P to sediment particles and selective loading of P during erosive storm events. As a result, in much of the Tensas Basin the nutrient limiting aquatic primary productivity—and thus the most critical nutrient to control—may be N. Between 1978 and 1986, P levels exceeded 0.1 mg L⁻¹ 96% of the time (P concentrations > 0.1 mg L⁻¹ signal eutrophy in lotic systems). Total suspended sediment concentration patterns were erratic and in excess of 80 mg L⁻¹ over half the time in two of the three streams studied (1966–1986). A goal-oriented management plan for improved water quality in the Tensas Basin was devised based on this cumulative impact assessment. Improvement of the aquatic ecosystem can be achieved in a number of ways, including the reestablishment of natural hydrologic flow wherever possible, the use of agricultural practices that reduce runoff, the protection of existing forested corridors along streams, and the mitigative creation of new buffering zones. In this conservation plan, site-by-site permitting occurs within the context of a comprehensive plan for managing the renewable resources of the entire Tensas Basin landscape; a plan based on the intensity of past cumulative impacts to the ecosystem.