Influences Of Watrshed Land Use On C, N, P Export In Tiaoxi Streams, Zheijiang Province, China

The elemental composition of particulate or dissolved matter (e.g., carbon, nitrogen orphosphorus), exported into downstream freshwaters or oceans (e.g., lakes and estuaries), hasimplications for downstream ecosystems. These relationships between riverine nutrientstransport and downstream ecoystems have been intensively studied by the global watershedecologists. Riverine nutrients export is mainly influenced by the climate, topography, geology,land use, and hydrology regieme of a catchment. Global human acitivities, indicated by landuse/land cover change, have made47%of the earth’s surface changed irreversibly, whichaffected not only the transport process of nutrients in streams but also downstream ecosystemshealth. Thus, throught profound analyses of influences of watershed land use pattern onriverine nutirents export, we will increase our ability to understand the biogeochemicalbehavior of kinds of nutrients from the watershed upland to downstream waters, and it isimperative for mangers to develop effective adaptation strategies to reduce riverine nutrientsexport and/or restore downstream ecosystems.The Tiaoxi Rvier, which is located in northern Zhejiang Province, typically contirbutedmuch of water and nutrients for the Taihu Lake. It not only led to the eutrophication in theTaihu Lake but also have implications for the water environmental security in the YangtzeRiver delta. In this study, the Tiaoxi River watershed was selected as the study region. Thewater quatliy indexes, river discharge and land use, incorportaing RS and GIS, and somestatistical methods, such as multistep regression, redundancy analysis and spatial multicriteriaanslysis, was used to analyze the influence of watershed land use patterns and hydrolgicprocesses on riverine nutrients export, and the scale dependence of these influences on riverinenutrients export, and the loss potential of C, N and P in each spatial unit in the Tiaoxi Riverwaterhed. Through the above research, we mainly obtained the follow conclusions: (1) Stream water C:N:P ratios were strongly related to several catchment landscapeproperties in the Xitiao River watershed. In addition, the relationships between C:N:P ratiosand catchment landscape properties were significantly affected by the variable hydrologiccondition, which was indicated by the river discharge in the Xitiao River watershed. Thestrength of these relationships changed depending on the element under consideration. Forexample, increasing river discharge was found to strengthen landscape (e.g., the percentage ofcropland)-C:P ratio relationships, but weaken landscape (e.g., percentage of waterbody)-N:Pratio relationships. Similarly, there were significant correlationships between stream waterC:N:P ratios and river discharge in each stream in the Xitiao River watershed. The direction(e.g., postive or negative) of these correlations were related with different landscape propertiesamong catchments from the Xitiao River watershed.(2) During the study period, variations of stream water nutrients (e.g., C, N and P) in theTiaoxi River watershed were more responsive to physiography vairables and catchment-scaleland use than to reach-scale anthropogenic variables and riparian-scale land use. In addition,the relative contribution of physiography variables and factors at different scales (e.g., reach,riparian and catchment) varied largely with seasons change. Especially in the rainy season, theinfluence of local-scale (e.g., reach and riparian scale) variables on variations of stream waternutrients in the Tiaoxi River watershed increased significantly and riparian was the greatestpowerful scale for the variation of nutirents in streams, while the relative influence oflarger-scale (e.g., physiography and catchment-scale) variables on variations of in-streamnutrients decreased during the same period. The results indicate that variations of nutrients (e.g.,C, N and P) in stream water of the Tiaoxi River watershed were controlled by the combinedeffect of local-scale and larger scale variables. The influence from catchment-scale variables onvariability of in-stream nutrients was large and relatively stable, while the impact from localscale (e.g., reach and riparian scale) variables to variability of nutrients was generally weak andvaried largely with seasons change. (3) Potential exports of total nitrogen and total phosphorus in the Tiaoxi River watershedgenerally followed the pattern of cropland> urban> forest> waterbody, while the losspotential of total organic carbon in the study region showed an opposite pattern as compared tothat of N and P. For example, forests distributed in the watershed upland showed generallylarger values than cropland or urban in the downstream regions. Althrough loss potentials fornutirents (e.g., C, N and P) in six counties in the Tiaoxi River watershed varied largely, therewere a relatively larger values of loss potential for nutrients and mean values of the three kindsof nutrients in each county were larger than0.6. Critical areas on large potential export of C, Nand P covered21.54%,48.49%and31.34%of the entire area in Tiaoxi basin.