Influence of Growing Season Stream Flows on Periphyton Growth in the Lower Flint River BasinDavid L. Diaz1,2, Paul V. McCormick2, Alan P. Covich1

1Odum School of Ecology, 2Joseph W. Jones Ecological Research Center

AbstractWater extractions for irrigation in the lower Flint River Basin (LFRB) in southwestern Georgia can reduce growing season stream flows below historical levels, particularly during droughts. Anecdotal evidence suggests that these low-flow conditions promote the formation of large mats of attached algae. We conducted controlled experiments to examine the relationships between stream discharge and associated environmental factors and summertime periphyton growth, biomass, and composition in Ichawaynochaway Creek, a major Flint River tributary (Fig. 1). Results of initial experimentation are reported here.

DiscussionWe found a positive relationship between channel discharge and periphyton accumulation. We attribute this result to higher algal metabolic rates at higher flow conditions where the supply and diffusive boundary layer is thinner. This can promote greater nutrient uptake and disposal of cellular waste, allowing for greater growth potential.Results presented here suggest flow and nutrient availability have the potential to alter the composition and growth rates of periphyton in a controlled experiment. These treatment responses were consistent with observations of variation in periphyton assemblages across different flow conditions within the creek itself during the same time period. Faster flow locations within the creek were also dominated by diatoms while backwater locations were dominated by filamentous green algae.We hypothesized that slower growth rates in lower flow treatments were due to nutrient limitation resulting from reduced nutrient supply rates. Our results were inconsistent with this hypothesis. Additionally, nutrient enrichment did not strongly affect periphyton taxonomic in a given flow environment. Response to nutrient enrichment across flow regimes suggest that current velocity rather than overlying (water-column) nutrient concentrations mediate nutrient supply rates to the periphyton mat.Collectively, these findings will provide insight into how variation in summertime flow regimes affect stream ecological conditions within the LFRB.

Methods and ResultsExperiment 1: Flow Relationships

Creek water was pumped through replicate artificial stream channels to achieve 5 flow treatments ranging from 0.95 to 18.9 L/min during the summer and fall of 2014 (Fig. 4,5). Ceramic tiles were placed in each channel at the beginning of experimentation and were collected twice weekly for four weeks to quantify periphyton biomass (ash-free dry mass and chlorophyll a) and calculate growth rates using linear regression slopes.

Experiment 2: Flow and Nutrient Interactions Following completion of the first experiment, channels and tiles were cleaned to

remove remaining periphyton, tiles were returned to the channels, and previous discharge treatments for each channel were reinstated. One of three nutrient-enrichment treatments was assigned to a replicate of each discharge treatment: (1) no enrichment (control); (2) phosphorus enrichment (3) phosphorus and nitrogen enrichment.

Results• Nutrient enrichment had little to no effect on algal growth rates in the low flow

treatments. As the flow increased, growth rate also increased, particularly in treatments with nutrient additions.(Fig .8)

AcknowledgementsResearch was funded by the Joseph W. Jones Ecological Research Center and the Odum School of Ecology. Special thanks to Joe Bolton and Brian Clayton for their help in constructing the stream facility.

Research Questions

BackgroundAttached algae or periphyton is an important food base and functional component of most flowing waters. Environmental factors influencing periphyton growth and biomass include water velocity, temperature, light, nutrient availability and, grazing pressure (Fig. 2). Stream discharge (flow) can affect all these factors and thereby exert strong control over periphyton dynamics. Extremely low stream flows in the LFRB during recent droughts were associated with the formation of thick mats of periphyton in some river and stream reaches reaches of Ichawaynochaway Creek (Fig. 3). High periphyton biomass may represent a significant ecological response to flow alteration because of its influence on the benthic environment, including substrate quality, food availability, and water quality (e.g., dissolved oxygen).

Figure 3. Periphyton mats in Ichawaynochaway Creek during a summer drought period.

Figure 2. Hypothesized relationships between stream discharge and other factors affecting periphyton growth.

Figure 4. Artificial Stream Channel Facility with water pumped from the Creek.

Figure 5. Periphyton growth in different flow treatments during the experiment.

Figure 6. Mean Biomass (AFDM) over 1 month during summer growing season. Flow Index : H:High (18.9 L/min), MH:Medium High (9.5 L/min), M: Medium (3.8 L/min) ML:Medium Low (1.9 L/min), L:Low .95(L/min )

Figure 7.  Mean growth rates per flow treatment. Flow rates simulated a broad range of stream flows

•What is the relationship between summertime stream flow and periphyton growth and biomass?

•What environmental factors related to flow (e.g., light, nutrients) are responsible for this relationship?

4 9 15 18 22 25 30 330

5

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High Flow Nutrient Treatments

Control

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30Medium High Flow Nutrient Treatments

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4 9 15 18 22 25 30 3302468

1012141618

Medium Flow Nutrient Treatment

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4 9 15 18 22 25 30 33012345678

Medium Low Flow Nutrient Treatment

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4 9 15 18 22 25 30 330

0.51

1.52

2.53

3.54

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Low Flow Nutrient Treatment

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mg/

cm^2 Fig 8. Biomass AFDM

over time for flow and nutrient treatment combinations.

Figure 1. Study Site located in the Ichawaynochaway Creek basin.

Results• Noticeable growth occurred in all discharge

treatments, but growth rates and final biomass increased with discharge and were approximately 2-fold and 3-fold greater, respectively, at the highest as compared to the lowest flow treatment (Fig.6,7).

• Flow also affected periphyton taxonomic composition as higher flow promoted development of thick diatom (Class: Bacillariophyceae) mats while lower flow allowed dominance by unattached masses of filamentous green algae of the order Zygnematales.