Natural Community Systems -- Photo Guide

Subtidal system

subtidal system at the southern edge of Great Bay (photo by Ben Kimball for the NH Natural Heritage Bureau)
subtidal system at the southern edge of Great Bay (photo by Ben Kimball)


DescriptionThis system corresponds to subtidal areas that occur almost exclusively below mean low tide, although upper reaches may be briefly exposed during the lowest spring tides.  In New Hampshire, subtidal systems include eelgrass bed, and the former saline/brackish subtidal channel/bay bottom, tidal creek bottom, and oyster bed communities (these latter three are no longer considered classified natural communities by NH Heritage as more study is required to properly classify them) and other marine aquatic communities.  This system performs important ecological functions including supporting oyster, eelgrass, and flounder populations, providing refuge for fish and invertebrates that retreat from exposed eelgrass beds, intertidal flats, and estuarine marshes at low tide, and serving as spawning and nursery areas for numerous species of aquatic animals.
Vascular plants are typically absent or sparse in this system.  Seaweeds are an important component of channel/bay bottoms and their surrounding environments.  A total of 169 seaweed species have been documented as occurring in the Great Bay Estuary.  Eelgrass beds dominated by common eelgrass (Zostera marina) occur in estuarine waters on mud rich in organic matter or on sand bottoms.  This rooted aquatic vascular plant covers nearly half of the bottom of Great Bay (2,585 acres).  Eelgrass beds trap sediments, dissolved nutrients, and larval organisms flowing through the community and are an important contributor to ecosystem health and productivity.  They serve as breeding, nursery, and feeding areas for many species of fish and invertebrates.  Eelgrass beds also provide foraging grounds for waterfowl and wading birds that feed on the eelgrass or the fish and invertebrates the beds harbor.  Oyster beds occur in shallow estuarine waters of Great Bay.  Oysters (Crassostrea virginica) are an important food source for many other animals including starfish, crabs, fishes, and waterfowl.

Diagnostic natural communities:

   • Eelgrass bed (S1)

also including (unclassified) saline/brackish subtidal channel/bay bottoms, tidal creek bottoms, and oyster beds.

Landscape settings: lowest (subtidal) portions of coastal embayments

Soils: mineral sediments and mud; strongly minerotrophic, saline to brackish

Spatial pattern: large patch, extensive flats to narrow-linear (100s of acres); broad patches, linear, and irregular zonation

Physiognomy: sparsely vegetated to unvegetated

Distribution: restricted to the Great Bay estuarine complex, tidal coastal rivers, and other tidal embayments

Associated natural community systems
  Subtidal systems are bordered landward by sparsely vegetated intertidal systems, and seaward (beyond the channel mouth) by the open marine environment.

eelgrass bed in Great Bay (photo by Ben Kimball)
eelgrass bed in Great Bay (photo by Ben Kimball)

back to NH Natural Community Systems list