No abstract

Flood pulse inundation of riparian forests alters rates of nutrient retention and organic matter processing in the aquatic ecosystems formed in the forest interior. Along the Middle Rio Grande (New Mexico, USA), impoundment and levee construction have created riparian forests that differ in their inter-flood intervals (IFIs) because some floodplains are still regularly inundated by the flood pulse (i.e., connected), while other floodplains remain isolated from flooding (i.e., disconnected).This research investigates how ecosystem responses to the flood pulse relate to forest IFI by quantifying nutrient and organic matter dynamics in the Rio Grande floodplain during three years of experimental flooding of the disconnected floodplain and during a single year of natural flooding of the connected floodplain. Surface and subsurface conditions in paired sites (control, flood) established in the two floodplain types were monitored to address metabolic and biogeochemical responses.Compared to dry controls, rates of respiration in the flooded sites increased by up to three orders of magnitude during the flood pulse. In the disconnected forest, month-long experimental floods produced widespread anoxia of four-week duration during each of the three years of flooding. In contrast, water in the connected floodplain remained well oxygenated (3-8 ppm). Material budgets for experimental floods showed the disconnected floodplain to be a sink for inorganic nitrogen and suspended solids, but a potential source of dissolved organic carbon (DOC). Compared to the main stem of the Rio Grande, floodwater on the connected floodplain contained less nitrate, but comparable concentrations of DOC, phosphate-phosphorus, and ammonium-nitrogen.Results suggest that floodplain IFI drives metabolic and biogeochemical responses during the flood pulse. Impoundment and fragmentation have altered floodplains from a mosaic of patches with variable IFI to a bimodal distribution. Relatively predictable flooding occurs in the connected forest, while inundation of the disconnected forest occurs only as the result of managed application of water. In semiarid floodplains, water is scarce except during the flood pulse. Ecosystem responses to the flood pulse are related to the IFI and other measures of flooding history that help describe spatial variation in ecosystem function.

No abstract

No abstract

A comparison of the trichopteran communities of streams associated with aspen, spruce—fir, and mixed—conifer forests demonstrated significant differences in structure. Though tricophteran species composition in aspen—associated and spruce—fir associated streams was virtually identical, relative abundances of shredder and grazer Trichoptera differed significantly in the two types of streams. The ratio of shredder to grazer trichopteran biomass in aspen streams was °1:17 while in conifer streams, the dominance was reversed with a shredder to grazer biomass ratio of 3:1. The difference in dominance of functional groups in the two types of streams was attributed to the order—of—magnitude greater standing crops of detritus found in the conifer streams. It is hypothesized that the greater accumulation of detritus in conifer streams is the result of habitat modification by logs. Five times the number of logs were found in conifer streams as compared to aspen streams. The greater accumulation of logs in the conifer streams is attributed to lower rates of decomposition of conifer wood. The data suggest a general model for long—term structural change in trichopteran communities of the southern Rocky Mountains and I suggest that these long—term changes result from changes in quantity and quality of wood inputs during forest succession.