Print PageNutrient availability and stoichiometry are known to affect a broad range of biological responses in aquatic ecosystems including production and community composition of primary producers with affects that propagate throughout the foodweb. However, few studies have examined broad-scale nutrient distributions in large rivers, or examined the role water-mediated connectivity might play in determining such distributions. We therefore examined patch distributions of total nitrogen (TN), total phosphorous (TP) and TN:TP ratios using global and local measures of spatial autocorrelation in five reaches of the Upper Mississippi River from 1994-2008 during spring (high discharge) and summer (low discharge).
Results/Conclusions
TN was generally distributed as discrete patches regardless of season, but the patchiness of TP and TN:TP became more distinct during the low-flow conditions of the summer growing season. Patches of TP were generally smaller than those of TN or TN:TP. Spring and summer patches of low TN, summer patches of high TP and spring and summer patches of low TN:TP were restricted to low flow (< 0.1 m sec-1) backwater areas that were poorly connected to channel environments. In contrast, patches of high TN, low TP and high TN:TP during spring and summer were generally located in channels, where current velocities were high (> 0.1 m sec-1), or in portions of impounded areas with low current velocity, but adjacent to channels.
These results suggest that spatial and temporal variation in connectivity and hydrologic exchange, indicated by proximity to channels and local current velocities, alter rates of nutrient delivery and biochemical transformation across this riverine landscape. A series of maps and empirical relationships are presented that reveal important locations, scales and degrees of hydrologic exchange, sufficient to create patchy nutrient distributions in a large river.
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