Submitted to: Journal of the North American Benthological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2010
Publication Date: 6/1/2011
Citation: Murdock, J.N., Dodds, W.K., Gido, K.B., Whiles, M.R. 2011. Dynamic influences of nutrients and grazing fish on periphyton during recovery from flood. Journal of the North American Benthological Society. 30(2):331-345 (doi:10.1899/10-039.1). Interpretive Summary: Stream flow patterns in many parts of the U.S. are predicted to change with a changing climate, with one key result being more frequent and intense floods. As stream hydrology is changed, alterations in stream organism abundance and diversity are also likely, especially for larger organisms such as fish. It is unclear how changes in community structure will alter stream recovery patterns from flood, and how biodiversity changes will interact with other major human-induced modifications such as nutrient enrichment, and ultimately affect stream services. We measured stream recovery dynamics for 35 days after a simulated flood in large outdoor mesocosms under a gradient of six nutrient loadings crossed with six densities of a locally dominant grazing fish. We found that nutrient effects were generally greater than those of fish, but the influence of both were not constant during recovery. Also, fish influence on stream recovery increased with lower nutrient input. Our results suggest changes in consumers in streams may exacerbate future changes in stream hydrology, but biodiversity shifts will be in part mediated by increased nutrient input. Given that current human-induced changes to streams include increased nutrients and alteration of abundances of large consumers, these results are also directly applicable to understanding how current changes in stream biodiversity are affecting stream services such as retaining and processing nutrients.
Technical Abstract: Nutrients and grazers can regulate benthic structure and function in streams. However, the relative strength of each factor is largely dependent on stream environmental conditions. The abundance of stream organisms and nutrient loading can change rapidly after a flood, suggesting that nutrient and grazer influences on algal development, and how they interact, may also vary temporally during recovery. We measured benthic algae structural and functional development for 35 days after a simulated flood in large outdoor mesocosms under a gradient of six nutrient loadings crossed with six densities of grazing fish (Southern redbelly dace, Phoxinus erythrogaster). Experimental results were used to parameterize a model of post-flood controls of algal growth and nutrient fluxes, and further investigate grazer and nutrient interactions during recovery. Nutrients influenced algal development more than fish, with nutrients correlating better with algal function (area-specific primary productivity and nutrient uptake) than with structure (biomass). Dace effects were relatively weak and only observed early in recovery, but nonetheless influenced all structural variables and biomass-specific gross primary productivity. Dace influence weakened and nutrient influence increased during recovery, differing from typical steady state conditions where grazers often regulate algal biomass. Model results suggest that ambient nitrogen concentrations < 100 µg TN L-1 are necessary for nutrient remineralization by dace to stimulate algal growth rates. In frequently disturbed systems such as headwater streams, understanding the context-dependent relationships in post-disturbance community dynamics is imperative to predict ecosystem responses to future biotic and abiotic alterations. Given that anthropogenic influences include increases of nutrients and alteration of abundances of large consumers, our results suggest that both can influence stream ecosystems, even under non-equilibrium conditions.