Print PageRiverine ecosystems are a vitally important link between terrestrial and aquatic ecosystems, but the relationships between patterns of land-use/land cover (LULC), in-stream nutrient dynamics, and riverine ecosystem function are often complex. Rivers are sites of major biogeochemical processes, providing important ecosystem services, yet they are increasingly threatened by human activities. In this study we investigated the LULC – aquatic nutrient relationships in a large, subtropical riverscape. Additionally, we assessed the effect of different LULC types and scales of analysis (local riparian, watershed riparian, or full catchment LULC) on nutrient concentration relationships and measures of bacterial ecosystem function. Aquatic nutrient and bacterial ecosystem function data was collected seasonally three times at each of 37 sites in the central and lower Brazos River watershed (Texas) from early 2008 – early 2009. Sampling was conducted in main-stem and small tributary sites, as well as four large tributary sub-watersheds. GIS was used to assess LULC patterns at three different spatial scales in the watershed and tributary sub-basins. LULC and environmental gradients were related to in-stream nutrient and bacterial functional response through redundancy analysis (RDA).
Results/Conclusions
The effects and scale of land use and environmental parameters changed throughout the year, but there was generally a gradient between local scale intensive landscape modification and larger scale lower impact uses and regional environmental conditions. Land use, drainage area, and mean annual precipitation were strong drivers of nutrient dynamics over the course of the study. Additionally, total and dissolved nutrients appeared to respond to different LULC drivers and environmental gradients than particulate bound nutrients, with total and dissolved nutrients generally being associated with larger scale patterns of LULC. Whole water bacterial production (BP) and bacterial respiration (BR) correlated with particulate parameters. Bacterial growth efficiency (BGE) correlated with C:N ratio and was influenced by local urban and agricultural LULC; however, this relationship was weak. Size fractionated BR data indicate that particulate and free living bacteria are responding to different types of LULC and environmental influence. These results indicate that even in large watersheds, upstream and headwaters areas are important in controlling downstream nutrient concentrations, while parameters such as pH and dissolved oxygen are controlled on a more local scale.
