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Frog eggs
Wood Frog Tadpoles
Setting up a baseline
Springtime
Hydrology
(Information adapted from: Colburn, Elizabeth A. Vernal Pools: Natural
History and Conservation. Blacksburg: McDonald & Woodward Publishing
Company, 2004.)
Hydrology is the science dealing with the properties, distribution, and circulation of water on and below the earth's surface and in the atmosphere (Merriam-Webster's Online Dictionary). An understanding of vernal pool hydrology is crucial in understanding vernal pool ecology because where the water comes from, when it arrives, how much of it there is, and how long it persists are characteristics of the pools that affect the biological community.
Depending on its origin and location within a landscape, a vernal pool may receive water from rainfall, surface runoff, intermittent streamflow, groundwater, and/or overbank flooding from nearby water bodies. Scientists agree that a correlation exists between the timing of pools filling with water and their maximum depth, with the rainfall and snowmelt. Surface runoff from rainstorms and melting snow is assumed to be the major source of water that fills vernal pools. The extent to which surface runoff contributes to a pool ultimately depends on the topography of the land. While vernal pools lack permanent connections to other water bodies, intermittent streamflow may contribute not only to the volume of the pool, but also to the regional biodiversity in vernal pool faunas. Groundwater tables have been found to intersect with vernal pool depressions, and studies suggest that the water levels in vernal pools fluctuates with the groundwater table.
| Drawing showing the potential interaction between groundwater and surface water in pools and lakes. |  |
| Drawing showing how the water table can affect pools and lakes. For example: the level of the water table affects the depth of the pools and lakes. |  |
| (Images have been reproduced with the permission of the USGS. They are available from http://pubs.usgs.gov/fs/OFR93-643) | |
Pools and ponds exist under many hydrologic conditions. Some pools, called "ephemeral" are created when depressions in the landscape fill with rainwater or runoff. Ephemeral pools are not permanent and are usually present in the landscape for only a few weeks. On the other end of the filling and drying scale are permanent ponds containing water all year long. Colburn suggests classifying vernal pools on a continuum based on their hydrology. This continuum is outlined below.
- Ephemeral or Rainwater Pools:
These pools fill in response to rainwater and do not retain water for more than a few weeks. They are capable of supporting distinct communities of protists, rotifers, crustaceans, diatoms, algae, insects, warm water crustaceans like clams and shrimp, and breeding pools for spadefoot toads. - Short-Cycle Pools:
Annual vernal pools that reach their spring maximum in depth and volume then shrink rapidly once snowmelt and spring rains are complete and plant growth begins. These pools are usually dry by late June or July. Suitable habitat for wood frogs, spring peepers, fairy shrimp, and other vernal pool indicator species. Paul's Pool is an example of a short-cycle pool. - Long-Cycle Pools:
Annual vernal pools that remain flooded longer than short-cycle pools. They hold water into the summer and dry in mid to late summer or early fall. These pools remain flooded long enough to support more diverse populations of invertebrates and vertebrates, even salamander breeding. The sediment of long-cycle pools remains saturated at or near the surface after standing water has disappeared. Les' Pool is an example of a long-cycle pool. - Partially Drying Pools:
Vernal pools that consistently retain a small area of standing water or saturated substrate. This type of pool can support species that cannot resist drawdown but cannot support species whose life cycles span multiple years. - Semi-Permanent Vernal Pools:
Vernal pools that remain continuously flooded with at least 20 cm of water, although rarely they can dry completely (usually once every 5-10 years). These pools can sustain more populations of vertebrates and invertebrates, and even introduced fish while the pool stays flooded. Shade Pond is an example of a semi-permanent vernal pool. - Permanent Fishless Ponds:
Perennially flooded and do not dry or draw down significantly. These ponds could sustain fish unless the pool is shallow enough to freeze to the bottom or become anoxic in winter. Virginia's Pond and Scout Pond are examples of a permanent fishless pond.
In addition to those characteristics by which Colburn proposes her
classification system, several other characteristics can be used to describe a
vernal pool such as: surface area, water depth, and water chemistry. Surface
areas of vernal pools fluctuate seasonally, normally reaching a maximum in
spring. In addition, the water depth of the vernal pools, although
characteristically shallow, also reaches a maximum in spring. Water chemistry
of a pool reflects its landscape position, the surrounding geology and
vegetation, the sources of water, and the land uses within the watershed. When
studying water chemistry, the factors most often studied are, temperature,
turbidity, pH, alkalinity, dissolved oxygen, and nitrite levels, each
contributing to an understanding of the water quality.
Hydrology Study
A hydrology study is being performed in a vernal pool, Les' Pool, on the
Little Property adjacent to the Graver Arboretum. The vernal pools in the area
are potentially in danger of being contaminated from nearby development. With
the aid of GLOBE protocols, the water chemistry and size of the pool are being
monitored as part of a hydrological study, in addition to macro-invertebrate and
vegetation surveys. (Protocols, field guides, and data sheets were obtained
from the GLOBE website (www.globe.gov).
If you are interested in a hydrology investigation of your own, these documents
are available for download (protocols,
field guides, and
data sheets); however, it is recommended that you become familiar with the
GLOBE website. Data collected can be posted for others to see and use all over
the world.
Materials needed for this study include LaMotte water chemistry test kits, flags, rope/twine, a measuring tape, and GLOBE protocols and data sheets.
Results
· Maps