Connections between terrestrial, freshwater, and marine habitats

Overview

The terrestrial and freshwater habitats in the Puget Sound region span high-elevation glaciers and alpine meadows. Mid-elevation forests of Douglas fir, western hemlock, red alder, and big-leaf maple drop to lower-elevation areas that historically supported stands of spruce, cedar, and Pacific madrone. The elevation of the Cascade and Olympic peaks--exceeding 4,000 m–drops dramatically to sea level on the shores of Puget Sound in a short linear distance. Powerful rivers spill from glaciers over this steep terrain and pass through the diverse forest communities. In the process, the rivers create dynamic riparian zones, and may change channel locations several times throughout a decade as they migrate throughout their floodplain.

Streams and rivers in the watersheds of Puget Sound provide ecological corridors and transport water, wood, sediment, organic matter, and nutrients downstream where they influence freshwater and estuarine ecosystems. The delivery of these building blocks of habitat create a variety of habitat types within the river system and near the river mouth, including low-elevation forests, freshwater and saltwater marshes, and numerous shoreline and beach habitats—all utilized by Puget Sound’s fish and wildlife. Circulation of water, nutrients, and sediment continues along the shoreline interface and throughout the estuary via tides and currents, gravitational forces, and freshwater inflows. Substantial quantities of nutrients are returned to the upland environment through the movement of thousands of animals, notably the returning runs of adult salmon.

Freshwater discharge in Puget Sound

Freshwater flows are important determinants of aquatic food web function, estuarine and nearshore habitat structure, and circulation in the marine waters of Puget Sound. Coastal areas within Puget Sound generally are characterized by high levels of rainfall and river discharge in the winter, while inland mountains are characterized by heavy snowfall in the winter and high snowmelt in late spring and early summer. This local weather pattern creates two major periods of freshwater runoff into Puget Sound, with maxima in December and June. The major sources of fresh water from Puget Sound river systems are from the Skagit and Snohomish watersheds in the Whidbey Basin; however, the annual amount of freshwater entering Puget Sound is only 10–20% of the amount entering the Strait of Georgia, primarily through the Fraser River.

Annual freshwater inflows from Puget Sound rivers are one of the major drivers of marine circulation patterns. Width of arrows indicates the average volume of annual fresh water flows from Puget Sound streams.

River and stream deltas

Sediments ranging from gravel to fine silts and sands are eroded from river edges and transported downstream and into estuarine and nearshore habitats. These river sediments provide important gravels for salmon spawning and rearing in the freshwater system along the way. Further downstream, sediment is deposited at the river mouth forming extensive deltas with freshwater and saltwater marsh habitats for a multitude of fish, bird, and supporting species. More far-reaching impacts of riverborne sediments also affect the Puget Sound marine environment. For example, suspended sediments carried in the Fraser River plume attenuate the light in the upper water column, thereby causing declines in primary productivity.

Sediment is not the only factor affecting habitat formation along the terrestrial-aquatic interface. While the importance of large woody debris is well known for habitat formation within rivers and streams, the delivery of wood to deltas and shorelines is also necessary to ameliorate shore erosion and enhance nearshore habitats. This is especially the case in river deltas, where the wood can break up saltmarsh and form patches of habitat for terrestrial species.

Historically, delta forests often consisted of sparse spruce, pine, and alder groves and served as important habitats for many Puget Sound species such as raptors and beavers. The beaver modified and constructed expansive freshwater wetlands, used in turn by other species including juvenile salmon. Saltwater and freshwater marshes, and sand and mud flats on deltas, were historically dominant parts of the Puget Sound landscape, providing critical habitat and transitional zones for young salmon and many other species of birds, fish, and mammals.

Loss of historical habitat types in Puget Sound (Collins, 2006)

Physical destruction of habitat resulting from humandevelopment activities along river deltas has been severe in several major river systems in Puget Sound. Extensive marsh and nearshore habitats were eliminated by levees that separated rivers from their floodplain and delta, eliminating thousands of acres of habitat. Changes in upstream hydrology have also changed circulatory patterns and sediment deposition in nearshore environments. For example, the concentration of flow into a few channels at the mouth of the Skagit River, and increased sediment delivery immediately offshore of these channels, is fragmenting eelgrass meadows and altering sea-surface salinity and turbidity that can affect the migratory pathway for outmigrating juvenile salmon. Delta habitats have also been affected by activities such as upstream culvert placement, which can create hydrological changes that accelerate upland erosion and contribute to downstream sedimentation problems. Increasing urbanization and industrialization of many river deltas, including those of the Duwamish and Puyallup, have been so altered that there virtually remains no indication of their historical extensive saltmarsh habitat. These cumulative physical changes have led to dramatic habitat loss for salmon and other species that reside in, or transit, delta habitats.

Damming of rivers has locally reduced the flow of sediments to key nearshore environments, most notably at the mouth of the Elwha River. Such reductions have resulted in significant beach erosion, costly shoreline  protection measures, and loss of nearshore habitats. Dams have further restricted the river habitat accessible to salmon, reducing habitat capacity for salmon and eliminating the return of marine nutrients to portions of river food webs.

The two-way traffic of nutrient transfer processes

Nutrients originating from decomposing vegetative and animal matter are an important and necessary part of ecosystem function in Puget Sound. However, human activities have accelerated and concentrated many of these processes. Elevated levels of nutrients entering Puget Sound come from point sources such as sewage-treatment plants and paper mills, or non-point sources including fertilizers, septic systems, and animal waste. When nutrient traffic loads are excessive, and combine with low circulation rates and topographic barriers, site-specific problems may arise, such as the hypoxia conditions in Hood Canal and south Puget Sound.

Although freshwater runoff is a primary pathway for nutrient transport from terrestrial to marine environments, the thousands of mobile animals in Puget Sound, such as insects, birds, and fish, are also effective transfer agents of energy. Moreover, these transfers can occur in both directions and return nutrients from the ocean to freshwater and terrestrial environments. Birds feeding at sea and nesting and roosting on land can transport large quantities of nutrients (Cederholm et al. 1999). Anadromous\ fish such as salmon also carry nutrients back from the marine environment up into freshwater and terrestrial habitats, enriching food webs far from the sea. The life histories of these Puget Sound species reflect their biological requirements to move back and forth between terrestrial, freshwater, and marine habitats depositing substantial quantities of nutrients in the process.

Pacific salmon (Oncorhynchus spp.) accumulate most of their body mass in the sea, which is transported to freshwater lakes and streams around the Pacific Rim when these fishes return to spawn. As most species of this genus generally die after spawning, the nutrients and organic matter contained in their body tissues and reproductive products are deposited near the spawning grounds. Research has shown that the annual deposition of salmon-derived nutrients contributes to the productivity of freshwater and riparian communities throughout the Pacific coastal region. These nutrients can be incorporated into the stream food web through direct consumption of carcass tissue by fish or invertebrates or uptake of the dissolved chemicals released during decomposition of carcasses. Salmon-derived nitrogen comprised from 10% to 20% of the nitrogen in some species of fish and invertebrates in a western Washington salmon stream, and reaches much higher proportions in Alaskan systems supporting greater abundances of spawning fish.

Nutrients from salmon also enter terrestrial eco-systems. The importance of salmon carcasses for plants, insects, bears, and birds has been well docu-mented (Cederholm et al. 1989; Ben-David 1998).

Habitat conditions

Freshwater and terrestrial habitats of Puget Sound are built around the soils formed by glacial deposits and coniferous lowland forests. Changes in soil, gradient, and related variations in precipitation have given rise to diverse plant and animal communities on land. Before European settlement, lowland forests were dominated by western red cedar, western hemlock, and Douglas fir, with mixed stands of Douglas fir, Garry oak, and Pacific dogwood in drier areas. Today forest plant and animal groups coexist in a mosaic with agricultural and urban lands. Considerable attention has been placed on the need to create or preserve habitat of adequate quality, quantity, and connectivity for species migration and colonization throughout the Pacific Northwest region.

Habitat conditions are the prime determinants of the abundance of wildlife—for both the quantity of species and the number of individuals. Like estuarine, nearshore, and open-water habitats, the diverse mosaic of terrestrial and freshwater habitats in Puget Sound directly determines the ability of species to thrive, whether feeding, resting, or breeding. Late successional stands of forest are characterized by decay in living trees, downed woody material, snags, and multiple canopy layers. These mixed conditions support a greater diversity of wildlife habitats than plant communities that have been recently disturbed by fire, flood, or cutting. Several terrestrial species utilize unique habitats such as tree cavities, snags, and downed logs during some portions of their life cycle. Large areas of Puget Sound lowlands once contained prairie, oak woodland, and pine forest types, but those remaining are largely relics due to the conversion of land to urban and agricultural uses, invasive species, fire suppression, and other disturbances. The remaining forests in the region provide important habitat for reptiles, amphibians, and snails; roost sites for bats; perching and nesting sites for birds; and forage, shelter, and travel corridors for deer and other mammals. Complexity is also essential for freshwater and riparian habitats used by aquatic, amphibian, riparian, terrestrial, and avian species. Many species are totally dependent on freshwater streams, riparian areas, or wetlands and ponds, including salmon, beavers, salamanders, and frogs. The diverse vegetation layers, groundcover, and downed logs in the riparian zone produce forage material and insects for hundreds of wildlife species, areas for wildlife to breed\ and rear their young, cover for resting and migration, and thermal shelter from the extremes of summer and winter temperatures. Within freshwater streams, the “roughness” provided by large trees and boulders in the stream channel creates pockets of gravel, plunge pools, riffles, overhanging vegetation, and other features necessary for salmon and char to migrate, rest, spawn, and rear. Numerous studies have documented the impact that upland habitat modification can have on downstream, nearshore, and estuarine habitats and the slow rate of recovery once these terrestrial habitats have been modified.

Source and citation

Sound Science: Synthesizing ecological and socioeconomic information about the Puget Sound ecosystem. 2007. Mary H. Ruckelshaus and Michelle M. McClure, coordinators; prepared in cooperation with the Sound Science collaborative team. U.S. Dept. of Commerce, National Oceanic & Atmospheric Administration (NMFS), Northwest Fisheries Science Center. Seattle, Washington. 93 p.

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