UBC Theses and Dissertations
The response of periphyton chlorophyll a and invertebrate drift to the end of nutrient enrichment Schell, Chris
This thesis contains two chapters concerned with the ecology of periphyton and invertebrates in streams of southwestern, coastal British Columbia. Chapter 1 deals with periphyton and addresses the hypothesis that the hyporheic zone contributes to community resistance in the face of declining nutrient levels. Two stream systems which had been enriched (target concentrations SRP 5 μgL⁻¹, NO₃ = 45 μgL⁻¹) for several months previous, were then subjected to the termination of nutrient additions. One of the systems was a natural stream, the other was a replicated, artificial, plastic lined channel apparatus with insufficient substrate for a hyporheic zone. Treatments losing enrichment were compared to controls which had never been enriched or had enrichment maintained for the duration of the experiment. Alkaline phosphatase activity (APA) and periphyton chlorophyll a were measured at regular intervals for up to two months after nutrient enrichment ended. In the natural stream, APA levels in the reach losing enrichment were similar to levels in a reach that continued to received enrichment. Chlorophyll a levels also showed no effect of the loss of enrichment. In the artificial channel experiment, one week after enrichment was terminated, APA levels increased and chlorophyll a levels decreased compared to channels which continued to be enriched. These results are interpreted to support the hypothesis. Weaknesses of the design are acknowledged and discussed. Chapter two tests the hypothesis that stream invertebrates trade - off potential food rewards and predation risk when making the decision to leave the substrate and enter the drift. Some data from a natural stream are presented, but the test of the hypothesis comes from the artificial channel experiment where fish (cutthroat trout presence/absence) and periphyton levels (continued enrichment and losing enrichment) were manipulated in a 2x2 factorial design. Artificial channels losing enrichment experienced a loss of periphyton chlorophyll a. Drift was measured during the day and night, and benthic samples were taken from the channels and used to calculate per capita drift rates. Baetid mayflies (Baetis sp.) were the only taxa to show a significant treatment effect. Large and small baetids (separated at 3.0 mm) were analysed independently. Benthic densities of baetid mayflies dropped significantly over the course of the manipulation in channels that had lost enrichment. Daytime drift rates for both large and small baetids were significantly lower in the presence of fish. Enrichment had a significant affect on the night time drift rates of larger mayflies, with increased drift rates in channels that were losing periphyton chlorophyll a due to a loss of enrichment. Smaller baetids demonstrated a significant interaction between fish and enrichment during the day. These results indicate that fish presence is more important than food during the day for larger invertebrates, causing increased drifting due to decreasing food levels to take place at night. For smaller invertebrates, who are less susceptible to predation while drifting, predation risk and food reward appear to be evaluated more equally during the day, causing a significant interaction term.
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