Seasonal Pool Definition | Seasonal Pool Species | Geology | Hydrology
Soils | Vegetation | Examples of seasonal pool habitats | Literature Cited

SEASONAL POOL DEFINITION

Seasonal pools come in many shapes and sizes and are difficult to categorize.  Because of the complexity of this habitat, many adjectives have been used to describe them.  Names such as seasonal, intermittent, semi-permanent, temporary, and ephemeral suggest that a seasonal pool holds water for part of the year, then dries up and disappears.  Other names such as vernal, spring, and autumnal describe the time of year a pool fills with water.  Most seasonal pools dry up during the drought months of summer, then refill at different rates and times.  Other names such as fishless pond and salamander pond suggest important biological characteristics of these pools.  The dry phase of a seasonal pool prevents fish from getting established.  In a habitat free of a top predator, other animals such as the large mole salamanders have thrived. 

Seasonal pools are considered ‘isolated’ wetlands because they are not permanently connected to other water bodies.  They can be found as shallow depressions in an upland area, associated with a wetland complex in a low lying area, or in the floodplain of a stream or river.  Seasonal pools can be found in a variety of land use situations.  Historically, seasonal pools were found in forested landscapes in the northeast and are most natural in this condition.  Today, many are found in open areas such as agricultural fields and residential areas. 

SEASONAL POOL ANIMAL SPECIES

Seasonal pools are unique wetland habitats because of their annual or semi-annual dry phase.  The dry phase of a seasonal pool is important because it prevents fish from becoming established.  Other animals with an aquatic stage to their life cycle, including the mole salamanders, have evolved to use these temporary aquatic habitats.  Although there are obvious challenges to overcome when using an aquatic environment that disappears for part of the year, the benefit is a habitat free of fish, which are top predators in permanent aquatic habitats such as ponds, streams and lakes.
 
Seasonal pools support wildlife that would not be able to survive without these pools.  Animals that require seasonal pool habitats to survive and reproduce are called seasonal pool indicator species.  The three common mole salamanders of Pennsylvania, the spotted, marbled, and Jefferson’s salamanders, are all seasonal pool indicator species. Other indicators include the wood frog, eastern spadefoot, and fairy shrimp.  Seasonal pool indicators have evolved a variety of strategies to survive the periodic drying of these pools.  Please refer to the field guide for a description of Pennsylvania’s seasonal pool obligate species. 

GEOLOGY

Seasonal pools form in a variety of ways. The position of a seasonal pool on the landscape can suggest its origin.  A few common origins of seasonal pools in Pennsylvania are discussed below. 

Floodplains
Seasonal pools are often formed in the floodplains of streams and rivers.  During floods, a stream will overflow its banks and enter the floodplain where the rushing high waters scour pockets in the floodplain.  Some of these pockets retain water well and become seasonal pools.  The natural meandering of a stream channel across a floodplain over the years can also create seasonal pool habitats in the form of oxbow wetlands.

Glaciers          
Glaciers covered the northern part of Pennsylvania during the last ice age.  A map of the glaciated areas of Pennsylvania is available at http://www.dcnr.state.pa.us/topogeo/maps/map59.pdf.  Glaciers created many isolated depressions, through the actions of ice melt, scour, and erosion that fill with precipitation and snowmelt.

Sag Ponds    
Shallow depressions in limestone regions are called sag ponds.  Sag ponds form in landscapes where easily dissolved bedrock such as limestone is found underneath an impermeable soil layer derived from sandstone.  Over time, groundwater dissolves pockets and holes in the limestone.  Eventually the surface soils ‘sag’ into the depression underneath.  Surface water gathers in the depression but the impermeable layer in the soil restricts surface water infiltration and creates a perched water table. 

Pingos
In the mountainous headwaters of periglacial areas, a common feature is the pingo.  Pingos are formed by freezing and thawing processes on moist slopes.  The upper layers of soil thaw first and slowly slide over underlying frozen soils.  The resulting landscape appears to be scarred with crescent shaped wetlands.  Open canopied pingos are often full of marshy vegetation.  Pingo seasonal pools frequently have groundwater influences that extend their hydroperiod.

Human activities
Some seasonal pools are created deliberately by people who wish to create wildlife habitat or to replace a seasonal pool that was destroyed.  Other pools are inadvertently created during human activities.  Artificially created pools can support seasonal pool wildlife if the surrounding upland habitat is relatively intact and the water quality of the artificial pool is adequate.  Seasonal pools can form in pits dug during surface mining operations, borrow pits, roadside ditches, agricultural fields, and other artificially created shallow depressions that hold water long enough to support seasonal pool wildlife.

HYDROLOGY / HYDROPERIOD

          
Hydrology is the study of where water is found and how it moves across the surface of the earth and underground.  Hydroperiod describes the amount of time water is present in a habitat (flood duration) and the timing and frequency of flood events.
           
Seasonal pools are considered hydrologically ‘isolated’ wetlands because they are not permanently connected to other water bodies.  They receive most of their waters from rain and snow melt surface runoff.  Some seasonal pools intersect the water table and receive input from groundwater sources.  Pools may also overflow or receive water from other areas during periods of heavy rain when seasonal pools, intermittent streams, nearby wetlands or other water bodies overflow and connect. 

Seasonal pools span a wide range of flooding patterns.  Hydroperiod varies greatly from pool to pool and from year to year based on precipitation amounts and timing.  Most seasonal pools dry up during the drought months of summer, then refill at different rates and times.  Some fill with fall and winter rains, while others require larger amounts of snow melt and rain in the spring.   Seasonal pools in Pennsylvania generally reach their maximum size in the spring, regardless of whether they start filling in the fall or spring.

By definition seasonal pools must go through a drying phase annually or every few years.  The amount of time the pool is inundated is important as well because it determines which species can successfully utilize a pool.  Different animals take different lengths of time to complete the reproduction process which includes adult breeding activities, egg laying, egg development, larval development, and metamorphosis from aquatic larva to terrestrial adult.  Species such as fairy shrimp and the eastern spadefoot can reproduce quickly in pools that hold water for just a few weeks or longer at a time.  Other species such as spring peepers and wood frogs need pools with slightly longer hydroperiods that flood in the spring and hold water for several months or longer.  Other species, such as the mole salamanders, need pools with even longer hydroperiods that remain flooded for four to six months.  

SOILS

With close inspection, the seasonal pool basin will look different than the surrounding upland even during its dry phase.  Leaves in the pool basin are compacted and a dark gray color after a period of inundation.  Water stains on the trunks of trees indicate high water levels. The soils also exhibit features of hydric (wet) soils, which means they have been inundated for a period of time.  These features can be used to help locate seasonal pools when they are not filled with water.

Seasonal pools tend to have an impermeable layer that results in ponded water.  The soil texture (the amount of sand, silt, and clay particles) typically contains higher amounts of fine silts and clays that lower percolation rates.  Pools that retain water for a sufficient length of time will develop hydric soils.  Hydric soils form when the soil is saturated from flooding for extended periods of time and anaerobic conditions (lacking oxygen or air) develop. 

Hydric soils exhibit redoximorphic features, such as bright red iron accumulations and light grey iron depletions.  These features may appear in the lower horizons of seasonal pools as a result of anaerobic conditions.  A soil may also be deemed "hydric" if it meets criteria set by the USDA-NRCS (http://soils.usda.gov/use/hydric/).  Typical hydric soils in Pennsylvania meet F3 Depleted Matrix or A2 Histic Epipedon criteria.  F3 Depleted Matrix soils have a grayish tint to the surface horizon while the A2 Histic Epipedon soils have an accumulation of organic matter yielding a dark black color.  Most seasonal pools have a layer of organic soil overlying the mineral soils.  The amount of organic material that can accumulate in the pool basin increases with the length of time a pool is typically inundated. 

          
VEGETATION          

Vegetation found in and around seasonal pools can vary depending on elevation, hydroperiod, and surrounding vegetation types.  These plants are an important component of the habitat.  Vegetation influences hydrology through transpiration and by creating shade which slows evaporation and moderates pool temperatures.  Plant growth and decay builds up the organic layer in the pool basin which helps the pool retain water longer.  Organic detritus provided by plants in the form of dead leaves and woody debris is the foundation of the aquatic food web.  Plants provide physical attachment, emergence, and oviposition sites for seasonal pool animals.  They also provide protective cover where prey can hide from predators.  In all these ways, vegetation helps shape the seasonal pool community.  After a pool dries up, a new suite of plant species often appears in a pool basin.  These plants add organic detritus to the pool basin and increase the species diversity supported by a seasonal pool habitat.

The current ‘herbaceous vernal pond’ community described in ‘Terrestrial and Palustrine Plant Communities of Pennsylvania’ (Fike 1999) lists numerous plants that are often associated with seasonal pools, including three way sedge (Dulichium arundincaeum), creeping mannagrass (Glyceria acutiflora), rice-cutgrass ( Leersia oryzoides), wool-grass (Scirpus cyperinus), northeastern bulrush (Scirpus ancistrochaetus), bugleweed ( Lycopus uniflorus), pale false mannagrass (Torreyochloa pallida), smartweeds ( Polygonum sp.), marsh fern (Thelypteris palustris), nodding sedge (Carex gynandra), fringed sedge (Carex crinita var. crinita), short sedge (Carex canescens), blister sedge (Carex vesicaria), common rush (Juncus effusus), Virginia chain fern (Woodwardia virginica), St. Johnswort (Hypericum mutilum), lesser Canadian St. Johnswort (Hypericum canadense), cinnamon fern (Osmunda cinnamomea), royal fern ( Osmunda regalis), rough bentgrass ( Agrostis scabra), hiddenfruit bladderwort (Utricularia geminiscapa), marsh St. Johnswort ( Triadenum virginicum), sessilefruit arrowhead (Sagittaria rigida), broad-leaved arrowhead ( Sagittaria latifolia), and spikerushes (Eleocharis spp.).  Woody species include high-bush blueberry (Vaccinium corymbosum), he-huckleberry (Lyonia ligustrina), pin oak (Quercus palustris), blackgum (Nyssa sylvatica), red maple (Acer rubrum), willows (Salixspp.), buttonbush (Cephalanthus occidentalis), and winterberry (Ilex verticillata).  

Seasonal pools often have a component of swamp forest at the pool and upland interface.  This vegetation often closely fits Fike's ‘red maple-black gum palustrine’ forest type.  Characteristic swamp forest trees include black gum, red maple, and pin oak.  Other associates include chokeberry (Aronia sp.), cinnamon fern, swamp dewberry (Rubus hispidus), meadow-sweet (Spiraea latifolia), highbush blueberry, and alders (Alnus spp.).
           
Like animals found in seasonal pools, there are some native plant species well adapted to life in temporarily inundated habitats.  In Pennsylvania, there are five species of special concern that are known to utilize seasonal pool habitats.  They are small beggar-ticks (Bidens discoidea), lance-leaved loosestrife (Lysimachia hybrida), Oakes’ pondweed (Potamogeton oakesianus), heartleaf pondweed (Potamogeton pulcher), and northeastern bulrush.  The northeastern bulrush is a federally endangered plant species found almost exclusively in seasonal pools in Pennsylvania. 

EXAMPLES OF SEASONAL POOL HABITATS

These are examples of woodland pools photographed by PNHP staff.  We will continue to update this area with additional pictures showcasing other types of seasonal pool habitats. 

A typical unvegetated, forested,
seasonal pool     

© Betsy Leppo

Woody debris from fallen trees
and broken limbs provides good
cover for seasonal pool species. 
Many amphibians attach their egg
masses to sticks to keep them
submerged in the pool.

A seasonal pool with an open
canopy can support abundant
vegetation in the pool basin,
which provides nutritional inputs
and cover for the pond. 

All seasonal pools dry up periodically. During the dry phase, a seasonal pool may look like this. The soil may still be mucky or wet, but vegetation can grow throughout the pool.