Longitudinal Patterns of Nutrient Cycling and Periphyton Characteristics in Streams: A Test of Upstream-Downstream Linkage

Abstract

We studied trends in nutrient cycling and periphyton characteristics (biomass, species composition, productivity, and nutrient content) along longitudinal gradients in laboratory streams to test the hypothesis that upstream-downstream linkages produce distinct longitudinal patterns in stream ecosystems. Periphyton communities were grown under uniform light and flow conditions in two 88-m-long laboratory streams created 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 streamwater N and P concentrations were observed in each stream. Although chlorophyll a declined and the proportion of cyanobacteria in the periphyton increased with distance downstream, longitudinal trends in ash-free dry mass, gross primary productivity (GPP), and total respiration were not significant. In contrast, longitudinal trends in most of the parameters related to nutrient deficiency and cycling were significant. Chlorophyll-specific phosphatase activity and C: nutrient ratios in periphyton biomass increased from upstream to downstream, suggesting greater nutrient deficiency downstream. Ratios of net N uptake rate: GPP and net P uptake rate: GPP declined from upstream to downstream, suggesting that nutrient recycling supported a greater fraction of the algal nutrient demand downstream. Ratios of net: total P uptake rate also declined from upstream to downstream, suggesting that a larger fraction of the total P uptake from stream water downstream was met by P recycled within the segment rather than by inputs from upstream. Within-segment recycling supplied only 10-25% of P uptake from stream water in upstream segments but contributed 60-70% of the P uptake from stream water in 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 results showed that increased nutrient cycling can compensate for longitudinal declines in nutrient concentrations in stream water, preventing large longitudinal changes in periphyton biomass and productivity. Nutrient cycling and algal species composition were the characteristics showing strongest longitudinal linkage in these periphyton-dominated streams, whereas total biomass and productivity patterns were poorly related to longitudinal position.