Salt pannes and pools (S3)
Salt pannes and pools occur as depressions embedded within salt and brackish marshes. Pools tend to retain water through the summer, whereas pannes do not. 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.
Variants: Numerous variants have been described for salt pannes and pools, including:
Salt marsh panne and pool variants:
1. Low salt marsh panne
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. Arrow-grass (forb) panne
Very shallow, briefly-flooded, moderately vegetated high salt marsh forb pannes, typically dominated by arrow-grass. Lower portions may remain unvegetated.
3. Smooth cord-grass (short form) panne
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 cord-grass (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
Sparsely-vegetated, and most often found on the upper half of the high salt marsh. Generally deeper than both forb and smooth cord-grass 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 deepwater pool
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 panne 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
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
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 in Portsmouth (photo by Ben Kimball)
Panne at Hampton Marsh (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; ditches are also evident (photo by Ben Kimball)