Evaluating Oxygen Dynamics in Lake Ballard: A Stratified, Brackish Lake OEAS 442 - Field Study II Fall 2010 - Spring 2011
Participants • Jonathan Gamby • Robert Garnett • Cody Garrison • Jamie Thorpe Instructors Students Phillip Bailey Robert Bradley Eric Buzan Elizabeth Flanagan • Dr. Fred Dobbs • Dr. David Burdige • Dr. Richard Whittecar • Dr. Malcolm Scully • TA Meredith McPherson Acknowledgements • Hoffler Creek Wildlife Preserve • Chris Powell
2010-2011 Goal To assess the oxygen dynamics in Lake Ballard Determine physical and chemical water column structures What causes changes in oxygen over time?
Historical Images • Resides in a man-made borrow pit • Excavated in the late 1970’s and again in the early 1980’s by VDOT to aid in construction of local highway July 1978 April 1981
Background Information • Located at the Hoffler Creek Wildlife Preserve, Portsmouth, Virginia
Background Information • Lake Ballard • A brackish, stratified meromictic lake • Two distinct basins: one being 3 meters deep on average, and the second approximately 13 meters at the deepest spot 3 m (~10 ft) 13 m (~43 ft) Allen, 2004
Historical Data • Previous research conducted by field study classes during summer semesters (2006, 2007, 2008)
CTD • Continuous measurements ofconductivity, temperature, depth, & oxygen sensor, fluorometer added in February Thermistor Chain • Continuous measurements of temperature in entire water column YSI 85 • Discrete readings of temperature, salinity, and oxygen • Usedin conjunction with CTD drops, and water sampling Methods and Materials
Nutrient and Chlorophyll a Analysis • Standard methods were used for Nitrate/Nitrite, Phosphate, and Chlorophyll-a/Phaeophytin-a • Flow injection method used to measure Ammonium Methods and Materials
Peristaltic Field Pump Li-Cor meter Plankton Nets John Boats Methods and Materials
Overview 1 Hypolimnion remains anoxic year-round Density controls oxygen distribution • Ultimately, salinity maintains stratification. • Temperature affects dissolved oxygen concentrations • Total lake overturn is improbable • Density gradient is a barrier to mixing
Overview 2 Stratification of the water column, nutrient location, and photosynthetic organisms can be used to explain oxygen concentrations • Phytoplankton location can be explained by light and nutrient availability as well as temperature and salinity gradients • Nutrients are trapped in the deep waters by the density gradient • Nutrient flux across the density gradient exists
Winter releases its grip • Thermocline rises, decoupling from the halocline. • Upper limit of the pycnocline follows behind. • Layer of trapped water between thermocline and halocline.
Upper limit of the pycnocline follows behind thermocline beginning in the spring. April 4
Wind Speed, Air Temperature, & Thermistor Chain Data Wind Km /hr Air Temp
Weather Collected from Norfolk NAS/Chamber’s Field Weather Center via www.wunderground.com 10.6 km from Lake Ballard
36 cm net rainfall Drought
Rain=36cm • Rate of evaporation for a 0.03 ppt increase=4mm/day • Amount of fresh-water necessary for a 0.26 ppt drop: • 30 cm 08SEP= 2.86 ppt 22SEP= 2.89 ppt O4OCT= 2.63 ppt
Total lake overturn is improbable • Calculations using salt content and density led us to believe that overturn would not occur • November 17th density profile used as a comparison • Mixed layer water temperature set to 3.4oC • Salinity held constant in the surface mixed layer • Entire water column would never be isopycnal Hypothetical Profile Density (kg/m3)
Total lake overturn is improbable • CTD Data collected through the winter showed that the temperature of the surface mixed layer reached 3.5°C.
Total lake overturn is improbable • Data collected through the winter months proved that overturn was not occurring
Density gradient is barrier to mixingOxygen only mixes from surface down to pycnocline Feb. 23
In April, oxygen present above pycnocline and below thermocline April 4 April 4 Oxygen Maximum MOM
Lake Ballard has a MOM? Metalimnetic Oxygen Maximum? MOM?
Chlorophyll (μg/L) Li-Cor DATA