Brian E. Haggard and Marty D. Matlock
Ecological Engineering Group, Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, Arkansas 72701 USA
haggard@uark.edu Introduction
Lake Eucha in Oklahoma, USA has come into sharp legal, political, and
environmental focus because of a past lawsuit between the municipal drinking water supply (City of Tulsa, Oklahoma – plaintiffs) and several poultry integrators and one municipal wastewater treatment plant (defendants) in Arkansas. The lawsuit was based on the claim that poultry industry via effluent discharge and nonpoint source P loading has elevated TP concentrations in Lake Eucha over last few decades, which coincided with increased drinking water costs related to elevated algal growth and the number and magnitude of taste and odor events in the finished drinking water.
The lawsuit was settled out of court, resulting in the adoption of P management strategies to limit poultry litter application rates based on the Eucha–Spavinaw Phosphorus Index (ESPI) (Delaune et al., 2006). The purpose of this study was to evaluate the seasonal variation in algal nutrient limitation at Lake Eucha, and we focused on whether P and or N were limiting the growth of either reservoir phytoplankton and or periphytic algae.
Methods
We used floating enclosures to assess phytoplankton nutrient limitation and passive diffusion periphytometers to assess periphytic algal nutrient limitation at Lake Eucha from August 2003 through August 2005. The floating enclosures were filled with reservoir water, spiked with nutrient treatments (i.e., control, N, P and N+P) and left on site for less than one week, and the passive diffusion periphytometers with the same treatments were modified and constructed based on Matlock et al. (1998) and left floating near the air–water interface for 10 to 14 days during these experiments.
These apparatuses were deployed 10 times and are shown below as deployed.
FAR LEFT: Deployed floating phytoplankton enclosure at the riverine zone at Lake Eucha.
LEFT: Deployed passive diffusion periphytometer [with an individual bottle pictured in the inset] near the dam at Lake Eucha.
Reservoir water samples were collected at least three times during each deployment, and water samples were analyzed for concentrations of soluble reactive P (SRP), nitrate–N (NO3–N), ammonium–N (NH4–N), total P (TP), total N (TN), and sestonic chlorophyll a (chl–a). Physico–chemical parameters (including pH, dissolved oxygen (DO), conductivity, and water temperature) were measured also. Water was collected from the enclosures at the end of the on–site incubation, then analyzed for chl–a. The growth substrate (i.e., glassfiber filter) from the periphytometer bottles was collected at the end of the deployment, and then analyzed for chl–a.
Results and conclusions
Concentrations of available nutrients varied widely, where SRP, NO3–N and NH4–N varied from <0.01 to 0.06, <0.05 to 3.17, and <0.02 to 0.42 mg L-1. Nutrient availability was low (decreased concentrations) during Summer when the reservoir was stratified and nutrient availability was high (increased concentrations) during late Fall and Winter. Nutrient availability also varied spatially and was greater in the headwaters (i.e., riverine zone) compared to near the dam (i.e., lacustrine zone).
Sestonic algal growth (measured indirectly via chl–a) varied seasonally and spatially, as typically observed in other regional reservoirs; sestonic chl–a concentrations were greatest in late Summer and in the riverine zone. The control treatments within the phytoplankton enclosures typically had increased chl–a concentrations compared to that measured in the reservoir in close proximity to the enclosures. The enclosures maintain algal population near the water surface (i.e., within 1–m) and often slightly elevated temperatures which is the likely cause for increased algal growth in the enclosures.
The treatment results from the phytoplankton enclosures and periphytometers showed distinct temporal patterns in algal nutrient limitation and to a lesser degree some spatial differences between the riverine and lacustrine zone. The typical pattern would be phytoplankton are temperature limited during late Fall and Winter, P limited during Spring and early Summer and then shifting to N limitation in late Summer and early Fall. Thus, algal growth is likely limited by temperature, when nutrient availability is greatest following mixis, or turnover in this reservoir.
However, this typical annual pattern in algal nutrient limitation can be disrupted if and when large storm events occur during early to mid Summer. These storm events provide an advective supply of nutrients, particularly NO3–N, to the epilimnion of a reservoir and increase nutrient availability to algae causing nutrient limitation pattern to shift. In fact, Lake Eucha experienced a rather large storm event one Summer which shifted the pattern to P limitation during Summer and to co–limitation during late Fall. Thus, seasonal fluctuation in nutrient transport from the catchment to the
reservoir has a profound impact of the individual nutrient limiting algal growth in water column.
HIGH
LOW Stream Discharge
Algal Growth (e.g., chl–a)
WINTER SPRING SUMMER FALL Temp–
Limited P–Limited
N–Limited Typical Annual Nutrient Limitation Pattern
Overall, phytoplankton and periphytic algae exhibited similar nutrient limitation characteristics over the study period. It was noted that periphytic algae were more often P or co–limited when phytoplankton were generally N limited during Summer.
Conclusions
• The algae at Lake Eucha switch from P to N limitation seasonally and in relation to large summer storm events which replenish the nutrient supply, particularly NO3-N, in the epilimnion.
• Nutrient availability was greatest in the reservoir following mixis, but algal growth potential during this period was low because the water temperature was colder.
Acknowledgements
This project was a component of a larger project funded by the USDA Nutrient Science for Improved Watershed Management Program that developed the Nutrient Management Decision and Education Support System (NMDESS) based on watershed modeling, reservoir hydrodynamic and water quality modeling, and stakeholder engagement, analysis and deliberation.
References
Delaune, P.B., Haggard, B.E., Daniel, T.C., Chaubey, I. & Cochran, M.J., 2006. The Eucha / Spavinaw phosphorus index: A court mandated index for litter management. Journal of Soil and Water Conservation 61, 96–105.
Matlock, M.D., Matlock, M.E., Storm, D.E., Smolen, M.D. & Henley, W.J., 1998. A quantitative passive diffusion periphytometer for lotic systems. Journal of the American Water Resources Association 34, 1141–1147.