Land use drives instream functions by influencing material and nutrient inputs. Watershed urbanization has predictable physical effects on streams, but the specific ecological responses of streams are poorly understood. In undisturbed watersheds, small streams typically act as sinks of terrestrial N exports, but urban streams may exhibit reduced NH4+ uptake rates. N demand is linked to bacterial metabolism of organic matter (OM), and varies with OM quality. This research addresses the relationships between urbanization, OM supply, and bacterial N demand.
DOM concentration and quality were measured in eleven watersheds, ranging from ~6-60% impervious surface area (IS). Streams were sampled three times during base flow for dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations. A standard microbial assemblage was incubated in filter-sterilized, unamended stream water and bacterial growth and DOM depletion were measured. A second experiment was conducted to compare microbial growth and N uptake when native bacterial assemblages were supplied with high- and low-quality DOM, and high and low [DIN] in a 2x2 factorial design.
Results/Conclusions
Stream water DOC:DON, [DOC], and [DON] varied with IS, but did not exhibit consistent patterns among dates. Four measures of DOM quality were derived from the incubations: bacterial growth rate, cell production per mmol DOC initial, change in [DOC], and change in [DON]. Cell production and growth rate increased with watershed IS, but changes in [DOC] and [DON] were inconsistent among seasons. Response variables for the N uptake experiment were bacterial growth rate, cell production per mmol DOC initial, change in [DOC], and change in [DIN]. [DOC] change was greater in low-quality DOM treatments, but bacterial growth, cell production, and change in [DIN] did not vary with DOM quality. [DIN] treatment did not affect bacterial growth, cell production, or change in [DOC]. These data suggest that OM quality and instream microbial processes do not exhibit clear-cut patterns as a function of urbanization.
