Habitat Classification

Introduction

Estuaries are not a single habitat, but rather a complex and interrelated web of habitats defined and distinguished by the interplay of geology, river-flows, tides, and other factors. Together these factors affect the composition, distribution and productivity of the biological communities that make up the living part of Oregon's estuaries. A major change in any single factor can create an environment suited to a wholly different set of species. In addition, the environmental requirements of a species may vary considerably throughout its life cycle and activities. For example, the environments in which a single species feeds, rests and spawns will usually differ.

Distinguishing between different habitats is important to understanding the effects of different kinds of activities and managing their impacts. Through the estuary classification scheme discussed below it is possible to identify unique environments that tend to control the production and composition of the communities that utilize them. It is possible to classify those environments by using only a few different parameters.

Oregon estuarine habitat classification system⁶

In 1979, the Oregon Department of Fish and Wildlife (ODFW) published a series of maps and reports that classified the various habitats in each of Oregon's major estuaries. Completed soon after LCDC adopted Statewide Planning Goals concerning coastal resources, ODFS's maps were intended to be used by local governments as they developed their estuary management plans.

ODFW's classification system is based on a United States Fish and Wildlife Service (USFWS) habitat classification system (Cowardin et al., 1977) (seen here to the left) that was designed to address a large variety of parameters affecting aquatic habitats. Since the USFWS system was designed to be applicable to all types of aquatic habitats nationwide, it includes parameters that were unnecessary for describing Oregon's estuarine habitats. Consequently, ODFW modified the system to utilize only those parameters that have the greatest influence on Oregon's estuarine habitats.

Classification of habitats and their communities is useful in evaluating the potential environmental impacts of site-specific proposals on an estuary. The ODFW estuarine habitat classification system incorporates tidal regime, landform, and sediment or vegetation type. These have been identified as primary factors controlling the composition of biological communities. Although a classification system that relies heavily on benthic substrates does not address all types of estuarine communities, sessile plants and invertebrates are directly influenced by bottom types, and adaptations for burrowing, attachment, and feeding are closely linked to specific types of substrate. The distribution of fishes and other mobile species is dependent at least in part on the availability of feeding and spawning areas and protective cover along the estuary bottom.

Sediment distribution indicates both the source of the parent material and the velocity and direction of tidal or river forces transporting the sediment. Therefore, habitat distribution is also influenced by the balance of these forces. For example, river-dominated systems have a high percentage of low-salinity subtidal habitats based on terrestrial sediments. Estuaries with a greater marine influence typically have large amounts of intertidal habitat and a mixture of both marine and riverine sediments. Consequently, they offer greater diversity of habitat types and, in turn, probably support a greater diversity of species.

Finally, it is important to distinguish between sediment type and habitat type, since similar classes of substrate alone do not represent similar environments. For example, communities that inhabit subtidal sand bottoms in the lower and upper estuary often differ significantly due to variations in salinity, flow velocity, or other factors independent of substrate type. Thus the location of a substrate type within the entire estuarine system will affect the species composition utilizing that habitat.

Estuarine subsystems

It is possible to broadly define four types of subsystems in Oregon's estuaries which are distinguished by geologic, riverine, and tidal forces. These forces determine the shape and depth of the estuarine basin and the distribution of salt and other material throughout the system.

Marine

The marine subsystem is a high energy zone located near the estuary mouth. The bottom is influenced by strong currents, and the substrate is primarily coarse marine sand, cobble, or rock. Salinities are generally high due to the dominance of ocean water, but may be greatly reduced during high river flows in winter. Kelp and other algal species often cover the rock substrates and form microhabitats for many species. Benthic invertebrates may include marine and estuarine species and fish utilizing the marine subsystem are marine species.

Bay

The bay subsystem is a relatively protected environment, often characterized by a broad embayment between the estuary mouth and narrow upriver reaches of tidewater. Normally the bay subsystem has a large percentage of intertidal land. Since it is influenced by both the marine and the riverine systems, bay sediments are primarily a mixture of coarse marine sands and fine river-borne silts and clays. Salinities during the summer are moderate to high, depending on the basin size, but may vary considerably with tidal stage and freshwater flow. Most bays have a wide diversity of habitats with extensive intertidal flats, eelgrass beds, algal beds, and marshes.

Riverine

Common habitat types in Oregon estuarine subsystems The riverine subsystem includes the upper tidewater portions of the larger tributaries which enter the estuary. A large percentage of the subsystem is narrow, subtidal river channel. Current velocities exhibit dramatic seasonal changes which influence benthic communities. Salinities are low most of the year, and portions of the subsystem may be entirely fresh water. Sediments range from fine silts and clays to cobble and gravel. Small fringing marshes frequently occur on narrow, intertidal portions of the river bank; riparian vegetation typically lines river banks where there are no marshes.

Slough

The slough subsystem is a sheltered environment, which is usually a narrow, isolated arm of the estuary with a very limited freshwater flow from uplands. Salinity is influenced by the proximity of the slough to the estuary mouth. Sloughs usually have fine organic sediments and high percentages of intertidal land consisting of flats, eelgrass beds, and marshes.

Tidal regime

Tide is a major limiting factor for many species in aquatic environments. The classification system distinguishes between intertidal and subtidal habitats, since biological communities often differ significantly according to the degree of tidal influence. Special adaptations are required by intertidal species to resist desiccation and tolerate large variations in temperature and salinity associated with tidal exposure.

Subtidal habitats are below extreme low water, and thus have continuously submerged substrates. Intertidal habitats are exposed and flooded by tides as often as twice daily or as seldom as a few times a year. The upper limit of the intertidal zones is defined for regulatory purposes as the line of nonaquatic vegetation, or as mean higher high water where such a line cannot be determined.

Within intertidal areas, a marked zonation of species is often apparent due to variation in the frequency and duration of exposure between lower and upper intertidal elevations. Although modifiers indicating tidal regime may be appropriate to differentiate intertidal habitats, intertidal elevations are not presently mapped for any Oregon estuaries.

The Oregon Estuary Plan Book - p12