In this study, we examined interactive effects of nutrient availability, river flow and food web structure on algal growth and species composition in the Middle Rio Grande, New Mexico. Algal community dynamics can be affected by both bottom-up and top-down factors. Algal production may be limited by low levels of nutrients or low light availability. Conversely, production is expected to increase when nutrient levels are high and light availability is not limited.  Top-down factors such as grazing by invertebrates can also alter periphyton growth and community composition. Generally, a reduction in grazers results in periphyton biomass increases and shifts in growth forms (to more upright taxa).

Nutrient diffusing substrates (NDS) were used in combination with grazer exclusion to investigate bottom-up and top-down effects on periphyton growth and species composition. Previously, NDS has been used in small, headwater streams and not in large sandy rivers like the Middle Rio Grande. NDS were made from inverted terracotta saucers, with four nutrient treatments – control, N, P and N+P. Large invertebrates were excluded from a subset of the saucers using an electrical field. The saucers were placed in a slow-flowing area of the Middle Rio Grande, and samples were collected weekly for a month.  

Results/Conclusions

River conditions made it challenging to conduct the NDS experiment. Water levels were variable and turbidity levels were extremely high (>4000NTU). Algal growth was extremely low on all treatments, largely due to light limitation caused by consistently high turbidity levels and a series of flooding and drying events which also decimated organismal growth on the substrates. Initial surveys of the diatom communities indicate extremely low densities and chlorophyll biomass was very low (mean ± se = 0.31 ± 0.09mg/m²). While dense periphyton communities did not develop on the NDS saucers, there was evidence of colonization by invertebrate populations. Blackfly larvae (Simuliidae) were most abundant in earlier samples whereas chironomid larvae (Chironomidae) were more abundant later in the experiment. Also, blackfly larvae were significantly more abundant on control saucers than on N+P saucers in the grazer/non-grazer experiment (P = 0.044). However, it was difficult to detect differences in the abundance of invertebrate grazers among nutrient treatments. Relationships among invertebrate population densities and the availability and diversity of periphyton for grazing will be described. These results are not definitive, but there are indications that nutrients, flow and grazers do influence algal community dynamics.