Impact of nutrient loading from point and non-point sources on water quality and lotic ecosystem health in Texas' north-Bosque watershed using a bio-indicator response approach

Abstract

Nutrient enrichment in the LakeWaco/Bosque watershed in north central Texas has resulted in declining water quality, and ecological degradation of stream habitat. This study was conducted to characterize the spatial and temporal nutrient limitation and trophic status of stream sites within the Lake Waco/Bosque watershed and to examine various experimental parameters of the Matlock Periphytometer method. Periphytometric assays were conducted at eight stream sites during the period from April 1998 through February 1999. Periphytic chlorophyll a production from the Matlock Periphytometer was used as an indicator of baseline primary productivity and of maximum primary productivity (MPP) in response to nutrient enrichment (nitrogen and phosphorus). Dissolved inorganic carbon uptake (¹⁴C analysis) was compared to chlorophyll a measurements. Periphytometer fiber filters were pre-seeded with algae from stream sites to determine impact on variability of response. The most upstream site in the North Bosque River consistently exhibited the highest degree of biological degradation due to nutrient enrichment. The next downstream site exhibited relatively low MPP, which indicated active nutrient uptake and cycling. The sites furthest downstream in the watershed were the next most nutrient enriched sites. Seasonality had a significant role in the magnitude of the biological response to nutrients, suggesting seasonal differences in phosphorus assimilative capacity in the system. The highest assimilative capacity was in the late spring to early summer; whereas the lowest assimilative capacity was in the late fall. ¹⁴CO₂ analysis of dissolved inorganic carbon uptake indicated that the periphyton community is most efficient at photo-assimilating carbon under non-limiting conditions. Pre-seeding the periphytometers' fiber filters with algae had no significant impact on the variability between treatment replicates, however, pre-seeding with algae across stream sites illustrated that periphyton communities in episodically enriched environments undergo natural selection, which enables those communities to use loaded nutrients efficiently for growth.