Aridland rivers face decreased flows and more pronounced effects of anthropogenic factors over time as water resources decline because of global warming.  Many aridland rivers are naturally nutrient-limited in a landscape that is volcanic in origin.  In the Middle Rio Grande in central New Mexico, nutrient sources, especially nitrogen, are related primarily to wastewater effluent and irrigation practices.  Nutrient dynamics and other environmental parameters can affect algal biomass and species composition, influencing food resources for higher trophic levels. Seasonal surveys were conducted during 2007-09 at five sites along a longitudinal gradient.  Rio Grande geomorphology changes from up- to downstream – from a cobbled, narrow channel to a wide-channeled river with sand/silt substrate and a large floodplain.  Physiochemical parameters, including nutrients, turbidity, and pH, as well as algal parameters (chlorophyll a and diatom species assemblages) were analyzed.
Results/Conclusions

Seasonally, shifts in turbidity played a major role in shaping algal communities. In the summer months, high turbidity associated with tributary inputs created a light-limited environment where primary production was limited to a littoral zone “bathtub ring.” Additionally, there was a gradient of nutrient inputs as the river flowed through urban landscapes. Concentrations of PO₄-P and NO₃-N were consistently low at sites furthest from urban influence, but varied seasonally at locations that were more heavily affected by anthropogenic inputs.Data indicate periphyton communities influenced by low nitrogen and turbidity levels in sites upstream of wastewater inputs. Diatom communities were dominated by cosmopolitan Cocconeis placentula var. lineata and alkalinophilous Epithemia sorex in the summer months.  During winter months when turbidity was relatively low, Diatoma vulgare, Rhopalodia gibberula, and Cocconeis placentula were most common.  Epithemia sorex, Rhopalodia gibberula, and R. gibba were common during all sampling periods, associated with low nitrogen levels and influence of alkaline soils. Substrate, flow and elevated nutrients shaped algal communities at the downstream sites.  In reaches downstream from wastewater effluents with a wide, sand-silt riverbed, epipelic diatoms (e.g. Surirella minuta, S. angustata, Nitzschia dissipata, Navicula venta, and Navicula cf. symmetrica) were abundant during summer months with high turbidity levels.  Months with lower turbidity were dominated by Nitzschia palea and several Surirella taxa. Overall, insight into diversity and drivers of periphyton dynamics is crucial to the understanding of stochasticity and seasonality in aridland riverine ecosystems.