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Benthic algal and insect responses to nutrient enrichment of an in-stream mesocosm

University of British Columbia

A nutrient bioassay was conducted in the Slocan River in southeastern British Columbia to evaluate potential trophic level responses in advance of whole-stream enrichment. A mesocosm approach was used to contrast periphyton and insect responses to low-level nitrogen and phosphorus treatments over an 83 d period in late summer. The nutrient bioassay was used to describe quantitatively and qualitatively benthic algal and insect relationships to manipulations of N and P concentrations and their ratios. N:P concentrations of 1:1, 3:3 and 5:5 μg-L⁻¹ and 4:1, 12:3 and 20:5 μg-L⁻¹ were used for treatment group comparisons. The experimental design consisted of six, replicated (x2) treatments and control. Although significant differences in periphyton accrual (measured as chlorophyll a biomass) were consistently demonstrated between control and treatment groups, the 4:1 treatment group was substantially higher in algal biomass than the 1:1 treatment group. Up to 4 and 8-fold differences in periphyton accrual over background were observed for the 1:1 and 4:1 treatment groups, respectively. Differences in biomass between treatment groups suggested N limitation. With the exception of the 1:1 nutrient concentration where a cyanophyte (Oscillatoria sp.) became highly abundant, all other treatments were dominated by chlorophytes and diatoms. The benthic insect response was similar to the periphyton response with up to 3-fold differences in insect abundance and a near-doubling of biomass observed in the 1:1 and 4:1 treatment groups. Differences in the amplitude and periodicity of insect diel drift cycles suggested differences in food abundance between control and treatment groups. A reduction in total insect per capita drift rate from one-half to one-third of that observed under background conditions provided further evidence in support of delayed emigration in treated channels. Taxa-specific differences in per capita drift rate between control and treatments also occurred for baetid mayflies where a drop from one-half to one-third was observed within certain nutrient ratios of each treatment group. The higher availability of food in treatment groups was further supported by larger body size of insects in both the drift and the benthos. Over the period of study, chironomid midges and baetid mayflies were the most abundant taxa. Owing to a large emigration near the end of the experiment, chironomid midges that were initially numerically dominant were replaced by mayflies, albeit at lower densities. Although insects were not considered food-limited, grazers had substantial influence on areal algal biomass. After comparing differences in taxonomic richness and trophic level response, the 4N:1P ratio was considered optimum for whole-stream application.

Thesis/Dissertation

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2009-05-26

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http://hdl.handle.net/2429/8202

Zoology

University of British Columbia

UBCV

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