Spatial heterogeneity of stream water nutrient concentrations over successional time

Nutrient availability in ecosystems is patchy both in space and in time. Wh ereas temporal trends have often been studied, less information exists on s patial patterns of nutrient availability, particularly in aquatic ecosystem s. The goals of this study were (1) to describe and quantify patterns of nu trient concentration in surface waters of an arid land stream and (2) to co mpare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concent rations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel ev ery 25 m over a 10-km section of a Sonoran Desert stream during three perio ds: early succession (2 wk post-flood), middle succession (2 mo post-flood) , and late succession (9 mo post-flood). Nutrient concentrations were extre mely variable in space (coefficients of variations as high as 145%). Coeffi cients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-var iogram analysis showed that nutrient concentrations were spatially dependen t on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as me asured by the semi-variogram range, tended to decrease with successional ti me. The strength of spatial dependence, as measured by the slope of the asc ending limb of the semivariogram, increased with successional time. The lim iting nutrient, nitrogen, was consistently more spatially heterogeneous tha n phosphorus or conductivity, both in terms of patch size (range) and stren gth of spatial dependence. In streams, downstream transport combined with nutrient transformation prod uces patches of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentr ation is likely to affect the spatial distribution of organisms and rates o f ecosystem processes.