LONGITUDINAL PATTERNS OF NUTRIENT CYCLING AND PERIPHYTON CHARACTERISTICS IN STREAMS - A TEST OF UPSTREAM-DOWNSTREAM LINKAGE

We studied trends in nutrient cycling and periphyton characteristics ( biomass, species composition, productivity, and nutrient content) alon g longitudinal gradients in laboratory streams to test the hypothesis that upstream-downstream linkages produce distinct longitudinal patter ns in stream ecosystems. Periphyton communities were grown under unifo rm light and now conditions in two 88-m-long laboratory streams create d by connecting four channel segments (each 0.3 m wide and 22 m long) in series. At the end of 8 wk, large longitudinal declines in streamwa ter N and P concentrations were observed in each stream. Although chlo rophyll a declined and the proportion of cyanobacteria in the periphyt on increased with distance downstream, longitudinal trends in ash-free dry mass, gross primary productivity (GPP), and total respiration wer e not significant. In contrast, longitudinal trends in most of the par ameters related to nutrient deficiency and cycling were significant. C hlorophyll-specific phosphatase activity and C:nutrient ratios in peri phyton biomass increased from upstream to downstream, suggesting great er nutrient deficiency downstream. Ratios of net N uptake rate:GPP and net P uptake rate:GPP declined from upstream to downstream, suggestin g that nutrient recycling supported a greater fraction of the algal nu trient demand downstream. Ratios of net:total P uptake rate also decli ned from upstream to downstream suggesting that a larger fraction of t he total P uptake from stream water downstream was met by P recycled w ithin the segment rather than by inputs from upstream. Within-segment recycling supplied only 10-25% of P uptake from stream water in upstre am segments but contributed 60-70% of the P uptake from stream water i n downstream segments. Finally, total P uptake rate:GPP ratio declined from upstream to downstream, suggesting that cycling of P within the periphyton mat was greater downstream than upstream. Together, our res ults showed that increased nutrient cycling can compensate for longitu dinal declines in nutrient concentrations in stream water, preventing large longitudinal changes in periphyton biomass and productivity. Nut rient cycling and algal species composition were the characteristics s howing strongest longitudinal linkage in these periphyton-dominated st reams, whereas total biomass and productivity patterns were poorly rel ated to longitudinal position.