Sources of sediment and phosphorus in stream flow of a highly productive dairy farmed catchment.

Both sediment and phosphorus (P) are important contaminants for surface water quality. Knowing the main sources of sediment and P loss within agricultural catchments enables mitigation practices to be better targeted. With this in mind seasonal loads of suspended sediment (SS), dissolved reactive P (DRP), total P (TP), and bioavailable P (BAP) were measured in a low gradient stream draining an intensively farmed New Zealand dairying catchment. Integrating in situ samplers were deployed to collect samples and the results merged with continuous flow data to calculate seasonal loads during 2005 through 2006. Flow rate, SS, and TP concentrations peaked in winter-spring and were lowest in summer-autumn. Concentrations of BAP in trapped sediment were greatest in autumn, contrasting with winter and spring when greater amounts of sediment were trapped, but with lower P enrichment. Analysis of (137)Cs and mixing model output showed that a major source of sediment and associated P in winter and spring was stream banks. Possible causes for this include trampling and destabilization by stock, channel straightening and sediment removal, and removal of riparian trees that stabilize banks. Modelling indicated that overland flow probably from topsoil (but could include sediment from lanes) contributed most sediment during summer and autumn. Remediation aimed at decreasing particulate P inputs to streams should focus on riparian protection measures, such as permanent stock exclusion and planting with shrubs and trees, ensuring runoff from lanes is minimized, and decreasing Olsen P to nearer optimum agronomic levels.

Particulate phosphorus transport within stream flow of an agricultural catchment.

There is interest in quantifying phosphorus (P) loss from intensively grazed dairy landscapes to identify key pathways and target remediation methods. The Bog Burn drains a dairying catchment in Southland, New Zealand, and has been monitored at fortnightly intervals over a 12-mo period at four sites for suspended sediment (SS), dissolved reactive phosphorus (DRP), and total phosphorus (TP). Time-integrated samplers, deployed at 0.6 median water depth at each site (calculated from previous year's flow data), collected sediment samples, which were analyzed for SS, bioavailable phosphorus (BAP), and TP. Mean concentrations of DRP and TP in stream flow and BAP and TP in sediment were generally highest in summer or autumn (0.043 mg DRP L(-1), 0.160 mg TP L(-1), 173 mg BAP kg(-1), 2228 mg TP kg(-1)) and lowest in winter or spring (0.012 mg DRP L(-1), 0.034 mg TP L(-1), 6 mg BAP kg(-1), 711 mg TP kg(-1)), while loads were highest in winter. Analysis of (137)Cs concentrations in trapped sediment, topsoil, subsoil, and stream bed and bank sediment indicated that trapped sediment was derived from topsoil and entered the stream either through tile drainage or, to a lesser extent, overland flow. Because concentrations of DRP and TP in stream flow are in excess of recommended limits for good water quality (>0.01 mg DRP L(-1), 0.033 mg TP L(-1)), management should focus on the topsoil and specifically on decreasing P loss via tile drainage. This is best achieved by decreasing soil Olsen P concentrations, especially because, on average, Olsen P concentrations in the catchment were above the agronomic optimum.

Effects of aquatic macrophytes on physico-chemical conditions of three contrasting lowland streams: a consequence of diffuse pollution from agriculture?

Three lowland streams in developed pasture catchments with different farming intensities exhibited contrasting summer diurnal variations in pH, DO and temperature. These are ascribed to differences in dominant aquatic vegetation and their respective effects on shade, and on photosynthetic production and respiration within each stream. The stream dominated by submerged macrophytes had the greatest amplitude swings in DO and pH, and DO levels of 86-128% saturation. Floating marginal macrophytes reduced photosynthetic inputs while providing additional organic loading for respiration, with consequent flat DO and pH curves and conditions not conducive to healthy stream ecosystems. The third stream was shaded by riparian plants, which inhibited photosynthetic effects on DO and pH so that diurnal variation was intermediate between the other two streams. The interaction between nutrients and increased insolation in agricultural catchments, in stimulating aquatic plants, needs to be better understood for managing the sustainability of stream habitats and ecosystems.