HABITAT CLASSES

Unconsolidated bottom (1.1)

The habitat classification system identifies a range of sediment sizes that represent unique subtidal environments for benthic species. Physiological and morphological adaptations of benthic organisms allow certain species to flourish in particular types of sediment. For example, feeding adaptations of invertebrates are related to sediment size. Coarse, clean sands are generally inhabited by organisms that filter food from the water column. In quiet waters where fine, organically rich muds occur, deposit-feeding polychaetes or other invertebrates ingest the sediment directly.

Since sediments largely influence the type of invertebrates colonizing an area, activities which alter sediment characteristics have a significant impact on benthic communities. Although dredge or spoil sites can be recolonized, community structure will vary with new sediment properties. Activities or structures that alter existing currents affect patterns of erosion and deposition. Where deposition is rapid, benthic communities may be smothered, and where erosion is significant, only organisms adapted to unstable substrates may survive. An important consideration in evaluating proposed development in estuaries is its impact on current patterns and sedimentation processes, and the resulting effects on benthic habitats and communities.

habitat class: unconsolidated bottom Sand-mud bottoms are typically higher in organic content than sand bottoms, and are firmer and more aerated than mud. Mud bottoms are primarily silt and clay; organisms living in mud must be able to tolerate low oxygen concentrations. Wood and organic debris bottoms will be found where current velocities are low or where there is a continuous supply of organic material. Finally, finer sediments may be intermixed with cobble/gravel substrates.

Rock bottom (1.2)

Rock habitats in the high salinity zone near estuary mouths are highly productive environments for marine fishes and invertebrates. They are defined as being less than 30 percent covered with vegetation. Most subtidal rock habitats are located near the mouth where strong tidal currents and turbulence require that organisms be firmly attached to the substrated or seek the protection of sheltered cracks and crevices. Rock outcrops also extend into the upper estuary, particularly in the smaller systems south of Cape Blanco. Jetties have created the most extensive rock bottom habitats in Oregon estuaries.

habitat class: rock bottom Specialized and diverse fauna are adapted for attachment or browsing along rock substrates. Sucking devices such as the tube feet of star fish or more permanent methods of attachment such as the byssus threads of mussels are examples of adaptations to rocky substrates. Soft silt and sandstone outcrops in a few locations provide a unique habitat for highly specialized piddock clams capable of boring into the rock. A diversity of algal species attach to rocky substrates with a strong basal holdfast.

Aquatic bed, subtidal (1.3) and intertidal (2.3)

The aquatic bed category includes both subtidal and intertidal algal and eelgrass beds that frequently occur in bay and slough subsystems. These communities probably represent a significant portion of the primary production in Oregon estuaries. Eelgrass is the most common species of seagrass in Oregon estuaries. It grows in both sand and mud substrates. It is a rapid growing plant that provides habitat for a diverse community of estuarine plants and animals. Its leaves support large numbers of algal and invertebrate epiphytes which are consumed by fish and larger invertebrates and are the primary food of black brant during their migration along the Oregon coast. Clam beds are often associated with eelgrass. In some estuaries, eelgrass leaves provide spawning surface for herring. Thick beds of eelgrass reduce currents near the bottom and promote deposition of sediment, while roots and rhizomes bind sediments and prevent erosion. Finally, eelgrass decomposition contributes nutrients to the detrital food chain.

habitat class: aquatic bed, subtidal Algal beds occur over unconsolidated or rock substrates and also provide habitat for fish and invertebrates. Huge mats of algal species turn broad intertidal flats bright green during spring and summer. Biomass then declines as the algae decays and releases nutrients to the system. In some deeper high salinity areas where there is suitable substrate for attachment, long blades of kelp may be seen floating at the water's surface. Kelp holdfasts represent a unique microhabitat for a rich community of invertebrates.

Plant production in Oregon estuaries is highly seasonal. The timing of fish migrations, spawning, and invertebrate reproduction in estuaries corresponds closely with dramatic increases in plant production during the spring and summer.

Reduction of light penetration due to shading or turbidity can limit plant growth. Logging and road construction in the upper watershed and dredging activities in the estuary can increase turbidity. Reduced flushing of eelgrass and some algal communities may decrease nutrient and gas exchange and, as a result, plant production. Significant modification of temperature or salinity patterns from changes in freshwater flow or estuarine circulations may further threaten aquatic beds.

Shore (2.1)

habitat class: shore Shores are narrow, steeply sloped intertidal habitats that occur where river and tidal currents are relatively strong. Because these are generally high energy environments, rocky substrates or coarse sediments often predominate. Algal and invertebrate species are firmly attached to rocky shores, but waves and currents may limit plant and animal production on unstable, unconsolidated shores.

As in other intertidal habitats, there is a pronounced zonation of plant and animal species from lower to upper intertidal elevations, with generally fewer species inhabiting the upper intertidal zone. In some estuaries, mud and sandy shores are inhabited by burrowing or tube-dwelling invertebrates which are food sources for bottom-feeding fishes at high tide.

Substrate composition of shorelines may change periodically due to scouring. Smaller particles may be removed, while cobbles, boulders, and bedrock can be seasonally covered by sand or gravel.

Flat (2.2)

Broad intertidal flats commonly occur in the slough and bay subsystems of Oregon estuaries. They are generally sheltered from strong currents and wave action and their gradual slopes tend to dissipate wave and tidal energies. As a result, flats form a relatively stable environment for colonizing species. In addition, large shallow flats store heat and may have an important role in the temperature budget of the entire estuary. Ultimately, tidal flat community structure is influenced by sediment size, currents, wave action, temperature, and salinity.

Tidal flat sediments vary from fine muds to cobbles. Shallow water depths, and therefore maximum light and warm temperatures, often result in extensive algae blooms in the spring and summer, when many flats could be classified as intertidal aquatic beds.

habitat class: flat Benthic organisms in tideflats are specially adapted to sediment sizes and the temperatures and exposure of an intertidal environment. So activities which alter sediment characteristics or tidal elevations can be expected to influence benthic communities. Filling and dredging represent the most obvious threats to flat habitats. Flats have historically been filled to extend the area of level upland available for shoreland development in estuaries.

Low-tide grounding of logs stored on intertidal flats and shores has decimated benthic populations. Bark and wood debris near log storage sites can adversely affect water quality. Sewage, fish wastes, or other organic pollutants discharged over flats may also accumulate in the sediments and reduce oxygen levels. Consequently, large numbers of invertebrates that are indicative of degraded habitats colonize these areas, and species diversity decreases.

In some estuaries, logging activities in the upper watershed have tremendously increased the rate of sedimentation. Tillamook Bay has been rapidly filled since the area was first settled. This has greatly increased the acreage of flats and decreased the area of subtidal habitat.

Cockle, gaper, butter, littleneck and softshell clams and mud and ghost shrimp are frequently associated with Oregon mud and sand flats. Recreational clamming is popular in these areas during low tides, particularly in the spring and summer. Bottom-feeding fishes graze over flats during high tide. Great blue heron, great egret and a variety of shorebirds feed in the shallows as the tides recede.

Beach/bar (2.4)

Beach and bar habitats are dynamic environments subject to strong water currents in the form of tides, waves and river flow. They always have less than 30 percent vegetative cover. Bars occur within estuaries as elongated ridges of coarse sand, cobble, or gravel, and are bordered by water on at least two sides. In Oregon, bars form during summer at the mouths of smaller blind estuaries and, in some cases, prevent marine water from entering the estuaries. Shifting bars also occur near the mouths of larger estuaries or in upper riverine sections. Because bars continually shift with the currents, colonization is limited to rapidly burrowing and opportunistic species, including mollusks, crustaceans, and polychaetes.

habitat class: beach/bar Shallow intertidal bars may extend as spits from shores near the mouths of estuaries. In larger systems, these may be periodically dredged to provide a navigable channel into the estuary. Gravel removal operations have occurred on bars in the riverine sections of a few south coast estuaries.

Tidal marsh (2.5)

Tidal marshes are characterized by rooted herbaceous or woody hydrophytes that grow between lower high tide and the line of nonaquatic vegetation. These can be divided into four major subclasses: high and low salt marsh in marine and brackish areas, and fresh and shrub marshes beyond saltwater influence. Composition of these marsh communities varies with tidal elevation, sediment types, and salinity regime.

Marshes are an important habitat for invertebrates, waterfowl, small terrestrial mammals, and insects. Detritus-feeding snails, scavenging crabs, and a variety of amphipods and other invertebrates seek the food and/or protection of marshes. The well-defined channels of high marshes are heavily used by juvenile Dungeness crab and a variety of small fishes. In some areas, they may provide important rearing habitat for juvenile Chinook salmon. Marshes also provide resting and feeding areas for large populations of migrating waterfowl.

Salt marshes have been ranked among the most productive ecosystems in the world. Plant producers in salt marshes include marsh grasses, macroalgae entwined among the grass stems, microalgae on the mud surface, and phytoplankton in the water column. Organic material and nutrients stored by marsh producers are consumed directly, or transferred to other portions of the estuary as detritus.

habitat class: tidal marsh Estuarine marshes are important sediment traps that reduce the frequency of dredging required for navigation. They help to stabilize the shore, dissipate flood waters, and protect shoreland property from storms. Marshes also filter and process nitrates, phosphates, and other wastes, thus providing a pollution buffer between adjacent upland activities and the estuary.

Tremendous areas of Oregon marsh have been diked to create upland for pasture and other uses. Such diking has greatly reduced estuarine integrity and productivity. Extensive diking has resulted in altered marsh community composition, channelized estuarine water courses, reduced productive intertidal surface area, and restricted transport of organic materials and nutrients to and from the estuary. Construction of causeways and roadbeds has had identical results. Filling for shoreland development has sacrificed huge expanses of marsh in many Oregon estuaries.