Continuous Hydrologic and Water Quality Monitoring of Vernal Ponds

Summary

Understanding the ecosystem services and processes provided by vernal ponds and the impacts of anthropogenic activities on their ability to provide these services requires intensive hydrologic monitoring. This sampling protocol using in-situ monitoring equipment was developed to understand the impact of anthropogenic activities on water levels and quality.

Abstract

Vernal ponds, also referred to as vernal pools, provide critical ecosystem services and habitat for a variety of threatened and endangered species. However, they are vulnerable parts of the landscapes that are often poorly understood and understudied. Land use and management practices, as well as climate change are thought to be a contribution to the global amphibian decline. However, more research is needed to understand the extent of these impacts. Here, we present methodology for characterizing a vernal pond's morphology and detail a monitoring station that can be used to collect water quantity and quality data over the duration of a vernal pond's hydroperiod. We provide methodology for how to conduct field surveys to characterize the morphology and develop stage-storage curves for a vernal pond. Additionally, we provide methodology for monitoring the water level, temperature, pH, oxidation-reduction potential, dissolved oxygen, and electrical conductivity of water in a vernal pond, as well as monitoring rainfall data. This information can be used to better quantify the ecosystem services that vernal ponds provide and the impacts of anthropogenic activities on their ability to provide these services.

Introduction

Vernal ponds are temporary, shallow wetlands that typically contain water from fall to spring and are often dry during the summer months. The inundation period of vernal ponds, generally referred to as the hydroperiod, is primarily controlled by precipitation and evapotranspiration¹.

Vernal ponds can also be referred to as vernal pools, ephemeral ponds, temporary ponds, seasonal ponds, and geographically isolated wetlands². In the northeastern United States, vernal ponds are most often characterized by the critical habitat they provide for amphibians, serving as the breeding grounds and providing support during early life stages (i.e., tadpoles) and metamorphosis. In California, vernal ponds are characterized by the unique vegetation and endangered plant species that they support².

These habitats are increasingly threatened due to land use and climate change, and amphibian populations are experiencing a significant global decline largely due to anthropogenic activities^3,4. Water quality concerns due to pollution are also thought to be contributing factors in recent amphibian declines globally⁵. Furthermore, recent studies have revealed an increased occurrence of intersex characteristics in frogs inhabiting vernal ponds impacted by human wastewater⁶. There is therefore a need to conduct more intensive monitoring of both natural and impacted vernal ponds to better understand the contributors to the global amphibian decline.

The physical parameters of vernal ponds that need to be measured and monitored include the pond morphology and water level. The morphology is the geometry of the pond, and is developed by conducting a survey to determine changes in elevation across the pond. The survey data are then used to establish a stage-storage curve, which enables the volume of the pond to be estimated based on water level measurements. Because the water level in a vernal pond is heavily influenced by precipitation, measurements should be made at a high temporal resolution to best understand both short (i.e., on the order of minutes to hours) and long-term fluctuations (i.e., on the order of months to years) in water level.

Water quality parameters of interest that are known to affect the function of vernal ponds include temperature, pH, electrical conductivity, dissolved oxygen levels, and oxidation-reduction potential. These parameters can all be measured in situ with relatively cheap technologies and sensor networks. Some water quality parameters of interest such as some nutrient species (i.e., total Kjeldahl nitrogen) and other pollutants (i.e., emerging contaminants) require samples to be collected and brought to a laboratory for processing and analysis.

Critical parameters that affect the ability of vernal ponds to function as appropriate habitat for breeding amphibians and the early developmental stages of tadpoles include water level, pH, and dissolved oxygen concentration. Compared to vernal ponds located in relatively pristine landscapes, elevated levels of electrical conductivity, higher pH, reduced dissolved oxygen concentrations, and high nutrient concentrations have been recorded in vernal ponds impacted by anthropogenic activities^2,7. Reducing or anaerobic conditions may occur in these habitats, particularly ones that are impacted by anthropogenic activities. This can cause a shift in the microbiological community, altering the nutrient cycling within the pond and potentially reducing degradation of endocrine disrupting compounds and other pollutants^8,9.

The goal of this paper is to provide information for how to establish a station for monitoring the water quantity and quality of a vernal pond. This method can be applied to any vernal pond, but requires access to the site (i.e., the site must be on public property or have land-owner permission to install equipment).

Protocol

1. Conducting a Survey of a Vernal Pond Morphology

1. Select a location to designate as the benchmark and mark it with a small survey or marking flag.
   NOTE: The location should be a higher elevation than the pond and have line-of-sight from all locations across the pond.
2. Assign the benchmark a reference elevation; the exact number does not matter, it simply provides a reference to which all other elevations can be compared.
3. Using a tape measure and marking flags, make transects at a 3 m interval over the pond area, resulting in a 3 m x 3 m grid (see example in Figure 1).
4. Determine the elevation of the bottom of the pond (i.e., the ground) at 3 m intervals along each transect by measuring the height on a leveling rod using an automatic level. Ensure that the profiles extend to the highest elevations on every side of the pond.
5. At the end of each transect, make a backsight to the benchmark and record the elevation.
6. Determine the survey error as the difference between the benchmark's assigned elevation (i.e., the reference value assigned in step 1.2) and the elevation measured from the most distant location on the profile transect.
7. Calculate the allowable error (AE) of closure for the profile as AE = K(2*M)^0.5, where K is a constant between 0.001 and 1 and M is the distance (in miles) between the benchmark and the most distant location on the profile.
   NOTE: The value of K depends on the required accuracy of the survey, which in this case can be taken as 0.1¹⁰.
8. Compare the survey error calculated in step 1.6 to the AE calculated in step 1.7. If the survey error is greater than the AE, then redo the profile leveling (steps 1.3 and 1.4) for that transect. If the survey error is less than the AE, then the profile leveling for that transect is complete, conduct the profile leveling for the next transect.
9. Repeat steps 1.4 through 1.8 to conduct profile leveling at 3 m intervals across the pond in the other direction to create a grid of known elevations (see an example of profile transects in Figure 1).
10. Develop a stage-storage curve for the pond once the elevations (with respect to the benchmark) are known across the 3 m x 3 m grid surveyed across the pond.
    NOTE: Larger intervals can be used, but the error in determining the relationship between water level and pond volume may increase.

2. Determining the Vernal Pond's Stage-Storage Curve

NOTE: Each vernal pond will have a unique relationship between water level and water volume in the pond. This relationship is called the stage-storage curve.

1. Using the elevation data gathered in Section 1, determine the highest and lowest elevations in the pond.
2. Determine the difference between the highest and lowest elevation and select an interval for which to draw contour lines; a contour interval of 0.1 to 0.2 m is recommended¹¹.
3. Calculate the surface area of each contour (A_i). This can be done either by hand using a planimeter or electronically using geographic information software (GIS).
4. Use the average-end-area method to calculate the volume between each contour interval (V_i):
   
   where E is the contour elevation.
5. Calculate the total volume (V_P) of the vernal pond as the sum of the volume between each contour interval:
   
   NOTE: Here H is the maximum depth of the pond. An example is given in Table 1.
6. Determine the stage-storage relationship for the pond by graphing the cumulative volume of the pond as a function of depth.
   1. After installing the water level sensor, use the water level as the "stage" and estimate the water volume, or storage, in the pond.
      NOTE: An example of a stage-storage curve is shown in Figure 2. If the water level sensor is installed above the lowest point in the vernal pond, an offset will be needed to convert the measured water level into the stage-storage curve (add the offset in step 3.3 to the water level recorded by the water level sensors to determine the stage).

3. Installing a Monitoring Station

NOTE: Sensors for parameters of interest for this study included a pressure transducer (measures both water level and temperature), dissolved oxygen concentration, oxidation-reduction potential, electrical conductivity, pH, and a tipping bucket rain gauge. The pH probe, dissolved oxygen sensor, and oxidation-reduction probe must be calibrated in the lab prior to deployment per the sensor's user manual. Here, a central datalogger (programmed to record data at 15 min intervals) is selected, to which all sensors are connected during deployment. A viable alternative scenario would be that each of the sensors is autonomous and do not need one central datalogger, since each sensor would record its own data.

1. Attach each of the sensors (with the exception of the rain gauge) to a cinder block or a wooden stake (Figure 3). Use hose clamps or zip ties to ensure that the sensors remain near the bottom of the vernal pond (or the depth of interest).
   1. Attach the dissolved oxygen sensor such that it is at an angle (per manufacturer instructions), to allow oxygen to diffuse across the membrane. Install the pressure transducer upright, as the pressure that it will measure is the water column above it, and the water level should be recorded in a vertical manner.
2. Install the mounted sensors at a location towards the center of the pond that is unlikely to become dry during the study period.
3. Determine the vertical distance between the sensors and the lowest point in the pond using a ruler or the surveying equipment. Record this distance for use in developing the stage-storage curve as described in step 2.6 (i.e., an offset may be needed when relating the depth measured using the pressure transducers to the total water depth in the pond).
4. While they can be submerged in the water, the sensor wires are vulnerable to mice or other animals that may chew on them when the water level is low in the pond, to prevent this use apolyvinyl chloride pipe to protect the sensor wires (optional, but recommended). Run the sensor wires up to the edge of the vernal pond through a PVC pipe (3 m long, 6.35 cm diameter), as shown in Figure 4.
   NOTE: For temporary installation (e.g., a few weeks to a few months) the PVC pipe may be deemed unnecessary.
5. Set up a tripod and mount it to the ground by inserting stakes into each of the tripod legs.
   NOTE: Some tall tripods may have a lightning rod that requires installation, too.
   1. Position the tripod near the edge of the vernal pond to ensure that it is accessible even when the pond is full of water.
6. Attach the enclosure box for the datalogger and battery (12 V) onto the tripod, leaving room above the tripod for the solar panel to be mounted above the enclosure box (Figure 4).
7. Attach a 10 W solar panel to the top of the tripod and angle it towards the sun. A solar angle calculator¹² can be used, if desired, to determine the optimum angle at which to install the panel.
8. Attach the rain gauge to the tripod if there is room. Otherwise, attach it to a wooden stake or metal pole near the edge of the pond and the tripod ( Figure 4). Ensure (if possible) that the rain gauge has tree cover that approximately represents the tree cover of the pond (if any).
9. Bring all sensor and solar panel wires into the enclosure box through the hole at the bottom of the box.
10. Connect all sensors to the datalogger's wiring panel in accordance with the sensors' instructions or the datalogger's wiring diagram. See example in Figure 5A.
11. Connect the solar panel wires to the 12V battery to recharge the battery (Figure 5B).
    NOTE: Select a battery that also has a voltage regulator (recommended) to ensure that the battery does not receive too much electricity from the solar panel.
12. Connect the battery to the power input panel on the datalogger (Figure 5B) to provide power to the datalogger and the sensors.
13. Place a desiccant pack inside the enclosure box to reduce the likelihood of moisture damage to the datalogger.
14. Recommended but optional: connect a field laptop with the datalogger communication software to the datalogger using a serial cable ( Figure 5B) to ensure that the sensor network is working properly.
15. Close the enclosure box and place clay around the hole at the bottom of the enclosure box where the wires enter to keep insects and water out of the box. If security of the equipment is a concern, secure the enclosure box with a padlock.

Results

Vernal ponds can exhibit a wide range of morphology, with profiles ranging from convex to straight slope to concave. Example morphology for a vernal pond in Central Pennsylvania is shown in Figure 1, along with the results of the stage-storage curve for this pond (Figure 2, Table 1). Maximum pond depth is not a strong indicator of surface area, as hydroperiod has only a weak correlation with pond morphology

Discussion

Significance with Respect to Existing Methods

While monitoring of streams has well-established methodologies developed by the United States Geological Survey (USGS), no such widespread monitoring program exists for understanding vernal pond dynamics. This protocol seeks to provide guidance for how to begin to approach hydrologic and water quality monitoring research at a vernal pond site, with the goal of understanding how physical and chemical factors may be changing over tim...

Materials

Name	Company	Catalog Number	Comments
CR1000	Campbell Scientific	16130-23	Measurement and Control Datalogger
ENC12/14-SC-MM	Campbell Scientific	30707-88	Weatherproof Enclosure Box (12" x 14")
CS451-L	Campbell Scientific	28790-82	Pressure Transducer
CM305-PS	Campbell Scientific	20570-3	47" Mounting Pole (Tripod)
TE525-L	Texas Electronics	7085-111	Tipping Bucket Rain Gauage (0.01 inch)
CS511-L	Campbell Scientific	26995-41	Dissolved Oxygen Sensor
SP10	Campbell Scientific	5278	10 W Solar Panel
PS150-SW	Campbell Scientific	29293-1	12 V Power Supply with Voltage Regulator & 7 Ah Rechargeable Battery
CSIM11-ORP	Wedgewood Analytical	22120-72	Oxidation-reduction potential probe
CSIM11-L	Wedgewood Analytical	22119-151	pH probe
CS547A-L	Campbell Scientific	16725-229	Water conductivity probe
A547	Campbell Scientific	12323	CS547(A) Conductivity Interface
CST/berger SAL 'N' Series Automatic Level Package	CST/berger	55-SLVP32D	Automatic Survey Level, Tripod, and 8' survey rod