Natural Communities of NH -- Photo Guide

Salt pannes and pools  (S3)



Salt pannes and pools
occur as wet depressions embedded within salt and brackish marshes. They are isolated from tidal creeks. Pools are deeper, and tend to retain water through the summer, whereas pannes do not. Together, these low, wet microhabitats vary considerably in species richness and composition depending on a variety of factors, including salinity, water levels, and substrate type. Due to high evaporation rates in some of these shallow concavities, salinity levels can be very high, sometimes over a third as much as ocean water.

These fine-scale natural communities range in size from less than 1 m2 to over 100 m2. Species composition varies with salinity, hardness of substrate, elevation, soil oxygen, hydroperiod, and other factors. Low salt marsh pannes and pools are regularly flooded and often unvegetated with a soft, silty mud substrate. Irregularly flooded high salt marsh pannes and pools vary in composition, with the highest species richness and cover generally found in the shallow and relatively dry forb pannes. Salinity fluctuates in response to tidal flooding, evaporation, and rainfall. Salinity levels in pannes found in the high salt marsh are typically in the range of 40–50(-60) ppt. Under the most extreme conditions (e.g., high salinity or low oxygen) pannes may be devoid of vegetation.

Shallow pannes are created by damage to Spartina patens (salt-meadow cordgrass) and other high marsh vegetation from ice erosion or smothering by stranded mats of Spartina alterniflora (smooth cordgrass) and other flood-deposited plant litter or trash. Other processes account for the formation of “pond holes” or deep pools.

Since the first European settlers arrived on the coast until recently, pannes were routinely drained to increase the productivity of salt-meadow cordgrass and Distichlis spicata (spike grass) for hay, pasturage, and mulch. New Hampshire’s salt marshes were also ditched and their pannes drained into tidal creeks in an effort to reduce salt marsh mosquito (Aedes sollicitans) populations.

Variants:
Numerous variants have been described for salt pannes and pools, including:

Salt marsh variants:

1. Low salt marsh panne variant
   Pannes in the low salt marsh typically lack vegetation. Species that may be present in low cover include smooth cordgrass (Spartina alterniflora), and marine algae such as knotted wrack (Ascophyllum nodosum) and rockweeds (Fucus spp.). The substrate in this variant is usually soft, silty mud.

2. Forb panne variant
   Very shallow, briefly-flooded, moderately vegetated high salt marsh forb pannes, typically dominated by arrow-grass. Lower portions may remain unvegetated.

3. Smooth cordgrass (short-form) panne variant
   Shallow anaerobic depressions with poor drainage, low nutrient availability, and high concentrations of sulfides and other plant growth inhibitors. Dominated by the short form (6-12" tall) of smooth cordgrass (Spartina alterniflora). Most often found on the high salt marsh, but can occasionally be found on the upper margins of low salt marsh.

4. Salt marsh mosquito panne variant
   Sparsely-vegetated, and most often found on the upper half of the high salt marsh. Generally deeper than both forb and smooth cordgrass pannes. Typically flooded by the higher of the two spring tides, then dries out 2-3 weeks later. When dry, female salt marsh mosquitoes lay eggs on the exposed surface. The eggs then hatch after the next time the panne floods.

5. Widgeon grass - marsh minnow pool variant
   Semi-permanently and permanently flooded pools on the high salt marsh that serve as important foraging areas for many shorebirds. Occasioanally can be found at the upper edge of low salt marsh. These deepwater pools provide habitat for stickleback "marsh minnows" and submerged aquatic widgeon-grass. Purple sulphur-bacteria is often common across the stagnant water surface.

Brackish marsh variants: These variants occur in brackish marshes (short graminoid variant). In addition to these variants, some brackish marsh pannes are dominated by the non-native and invasive narrow-leaved cattail (Typha angustifolia).

1. Mixed graminoid - forb panne variant
   These shallow depressions are ponded only for short periods and are characterized by a variable mix of graminoids and forbs. Frequent herbs include three-square rush (Scirpus pungens), stout bulrush (S. robustus), arrow-grass, marsh creeping bent-grass (Agrostis stolonifera), salt-loving spike-rush (Eleocharis halophila), and small spike-rush (E. parvula). Less frequent are red fescue (Festuca rubra), New York aster (Aster novi-belgii), silverweed, salt-meadow cordgrass, and salt marsh rush.

2. Sparsely vegetated panne variant
   These saturated to occasionally ponded, mud dominated pannes can occur adjacent to forested uplands where they are shaded by overhanging canopy branches. This is the usual habitat for the uncommon seaside crowfoot (Ranunculus cymbalaria), where prostrate colonies may form small patches over the soil surface. Other graminoids and forbs scattered across the mud, or more often around the panne edge, include marsh creeping bent-grass, Virginia wild rye (Elymus virginicus), New York aster, seaside goldenrod (Solidago sempervirens), smooth cordgrass, chaffy salt sedge (Carex paleacea), and rock plantain (Plantago major var. scopulorum).


Pool in salt marsh along Sagamore Creek (photo by Ben Kimball for the NH Natural Heritage Bureau)
Pool in salt marsh along Sagamore Creek in Portsmouth (photo by Ben Kimball)

Panne at Hampton Marsh (photo by Ben Kimball for the NH Natural Heritage Bureau)
Panne at Hampton Marsh (photo by Ben Kimball)

Pool in salt marsh along Sagamore Creek in Portsmouth (photo by Ben Kimball)
Pool in salt marsh along Sagamore Creek in Portsmouth (photo by Ben Kimball)


Salt pannes and pools between creek channels in the salt marsh system at Hampton Marsh (photo by Ben Kimball for the NH Natural Heritage Bureau)
Salt pannes and pools between creek channels in the salt marsh system
at Hampton Marsh; ditches are also evident (photo by Ben Kimball)
 

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