Assessing Vertical Mixing and its Impacts on Phytoplankton in Jordan Lake

Transcription

1 Assessing Vertical Mixing and its Impacts on Phytoplankton in Jordan Lake Cape Fear River Assembly Fayetteville NC 19 May 2016 Dan Obenour, PhD, PE Tarek Aziz, PhD North Carolina State University 1

2 Outline 1. Lake mixing for phytoplankton management. 2. Jordan Lake and the SolarBee Project. 3. Characterizing vertical mixing in Jordan Lake. 4. DWR water quality study. 5. Controls on phytoplankton in Jordan Lake. 6. Summary 7. Next steps 2

3 1. Lake Mixing for Phytoplankton Management. Studied since the 1960s (at least). Few models and little quantitative guidance available; most applications follow a trial-and-error approach. Most (but not all) studies indicate an increase in phytoplankton biomass due to mixing. Effectiveness dependent on on type of mixing, phytoplankton, depth, turbidity, nutrient levels, etc. 3

4 Lake Mixing Applications In North Carolina Lake Niewe Meer, Netherlands (Jungo Engineering, Ltd.) Lake Benson Lake Benson (Raleigh) City Lake, Oak Hollow (High Point) Jordan Lake (Cary/Apex) Holding reservoirs (Winston-Salem) 4

5 Controlling cyanobacteria: Cyanobacteria may produce toxins, taste and odor. Many cyanobacteria optimize their vertical position to maximize resource exposure. Mixing should disrupt this optimization. Freshwater Bio., 1996 No mix Mix 5

6 Results from other systems: Visser et al. (2015) Artificial mixing to control cyanobacterial blooms: a review. Aquatic Ecology Complete destratification usually reduces cyano Dependent on type of mixing, types of phytoplankton present, turbidity, nutrient levels, etc. 6

7 2. Jordan Lake and the SolarBee Project. Status quo: High levels of phytoplankton year-round. Increase water treatment costs. Aesthetic issues. Promote hypoxia, leading to nutrient and manganese release from sediment. High levels of cyanobacteria in summer. Taste and odor issues. Potential for toxin production. Exacerbated aesthetic issues. 7

8 Solar-powered circulators in Jordan Lake ( ) Jordan Lake Study Area (routine monitoring sites in green) Solar-powered surfacelayer circulator on Jordan Lake 8

9 Solar mixing of epiliminion Medora Corporation 2016, 9

10 3. Characterizing Vertical Mixing in Jordan Lake 2015 Jordan Lake Field Campaign Primary field team: D. Obenour, R. Smythe, J. Smithheart, T. Aziz. Physical Measurements: SCAMP: Self-Contained Autonomous Micro Profiler. Dye Study: Rhodamine WT dye releases. Thermistor Strings: Continuous temperature profile monitoring. Water Quality Measurements: Sonde: CTD, O 2, ph, chlorophyll, and phycocyanin NCDENR DWR coordination (+nutrients +phytoplankton community) 10

11 SCAMP Study NCSU undergraduate Jeremey Smithheart working with physical limnologist (Robyn Smyth) SCAMP estimates of vertical energy dissipation (m 2 /s 3, left); Vertical temperature profiles ( C, right) for a selected monitoring session. 11

12 Dye Study Dye Study Schematic Comparison of model output to observed data in the Treatment Arm (model is used to infer vertical diffusivity rate) Dye Plume immediately following dye release (top) and 28 minutes later (bottom) 12

13 Thermistor Chains Surface buoy w/ reflective tape Sub-surface buoy NCSU undergraduate Kristen McCahill preparing for thermistor chain deployment Datalogging Thermistors Anchor Schematic of thermistor chain Thermistor chain temperature profiles for selected times on 11 July 2016 (typical) 13

14 Temp, C ly/d C, mmhg Temperature Modeling Jsn ly/d Ta, C e_a, mm Time, hr Time, hr 14

15 Preliminary mixing study results Effects of solar-powered circulators: Possible effects within 10 m No observable mixing effects beyond 10 m Vertical diffusivities in Jordan Lake: Normal Conditions (with or without artificial mixing): cm 2 /s Intensive Wind Mixing Conditions: 1-10 cm 2 /s Compare to Lake Niewe Meer diffusivities: Without artificial mixing: ~0.1 cm 2 /s With artificial mixing: ~20 cm 2 /s 15

16 4. DWR Water Quality Study: Compare Compare Compare Circulator Monitoring Site 16

17 4. DWR spatial comparisons: Source: NCDENR (2015) Preliminary Assessment of In-Lake Mechanical Circulation and Their Effects Related to Water Quality Standards in the Morgan Creek and Haw River Arms of Jordan Lake 17

18 5. Controls on chlorophyll levels in Jordan Lake (preliminary results) Multiple Linear Regression: Variables Timing Water Temperature sample Depth sample Flow Rate 2,10,30 day Wind mixing power 2,10,30 day Solar radiation 2,10 day Morgan/New Hope Arm sample Circulators present sample (30 day) +/- (2 day) 18

19 6. Summary: Previous research indicates that enhanced mixing controls cyanobacteria under certain conditions, but little evidence for control of total chlorophyll. In Jordan Lake, solar-powered circulators have not appreciably increased mixing rates. NCDENR study indicates no significant improvement in chlorophyll and ph. Preliminary analysis suggests seasonality and flow rates control a significant portion of the variability in Jordan Lake chlorophyll levels. 19

20 7. Next Steps Explore spatial and temporal variability in cyanobacteria. Build more mechanistic models to assess effects of mixing, nutrients, flushing rates, temperature, etc. on phytoplankton. Expand study to other lakes with varying mixing intensities (University Lake, High Point Lake, etc.) Toxin screening of lakes (with Astrid Schnetzer, NCSU) Develop decision support tool for use of lake mixing to control phytoplankton in NC. 20

21 Questions? 21