Shoreline Stewardship

IV. B. 3. Flats

Flats generally include gently sloping sandy or muddy intertidal or shallow subtidal areas (Figure IV-4) (Table IV-3). Because of the quiescent conditions and the input of organic matter, mudflats are usually high in organic content, and anaerobic conditions may exist below the surface.  Sandflats, which are comprised of larger sized particles, are often more aerobic.  Flats serve a wide variety of functions.

The main primary producers on flats are diatoms that inhabit the upper few mm of fine sediment of flats. Chlorophyll a concentration, used to estimate diatom biomass, ranges from 140 to 380 mg per square meter on flats in Puget Sound (Thom 1989).  Primary production measured for flats range from 22 to 59 g of carbon per square meter per year (Thom 1984; Thom 1989), and daily and seasonal inorganic nutrient flux rates can be substantial, especially on muddy flats (Thom et al. 1994a).  Nutrients released from sediments on flats may fuel algae growth on the flats and in the water column. 

Sediment-dwelling invertebrates, such as polychaete worms, amphipods, and small bivalves, can be very abundant on flats.  On two beaches dominated by sand and mudflats, Armstrong et al. (1976) recorded 203 (Richmond Beach) and 178 (Carkeek Park) species of invertebrates.  Invertebrates residing in the sand and mud can reach densities on the order of 6000 per square meter (Thom et al. 1984).  Eight or more species can be found per small core sample (5.5-cm diameter).  These animals feed on organic matter on the surface and in the sediment, and hence are dependent not only on production on the flats themselves, but also on deposition of detritus produced in other areas. 

Figure IV-4. Tide flats in Murden Cove ( © WA Dept. of Ecology 1992).

The prey (e.g., harpacticoid copepods, amphipods) of juvenile salmon can reach high densities (18,000 per square meter) on flats, and heavy consumption by salmonids can drive these prey abundances very low (Thom et al. 1989).  Studies indicate that the flats are heavily used for feeding by juvenile salmonids especially very early in spring, after which the salmon shift to feeding lower in the intertidal and shallow subtidal zone (Thom et al. 1989).  Much of the prey production on the flats is believed to be driven by benthic diatom production that occurs early in spring.  Because light drives benthic diatom growth, the flats increase their production in early spring when light increases with the onset of daytime low tides.  Later in spring and in early summer, light increases dramatically further offshore and initiates the production of prey in habitats, such as eelgrass.  Hence, the flats play an important role in the seasonal dynamics of salmon feeding within the Puget Sound nearshore landscape.

Flats often are dissected by numerous small channels, which are used by invertebrates and fish as well as shorebirds, herons, raccoons, otter, mink and other organisms for foraging.  Channels “…constitute critical interfaces within the estuary itself, linking littoral and sublittoral, riverine and marine habitats” (Simenstad 1983, page 4).  Channels transport organic matter from sources to sinks in the estuary, provide deeper wetted areas for fish (such as juvenile salmon) to reside during low tides, can be highly productive in terms of benthic infauna invertebrates, and are often used by predators, such as wading birds, as a key feeding area.  The small channels provide a conduit of access of fish to the productive portions of the intertidal system such as the edges of salt marshes.

Fish species that are common on flats include chum salmon, bay goby, Pacific staghorn sculpin, English sole, sand sole, speckled sanddab, and starry flounder (Simenstad et al. 1991).  Perhaps the most intense use of flats is by shorebirds.  Shorebirds are commonly observed in large numbers feeding on invertebrates produced on the flats.  The non-native seagrass Zostera japonica has colonized flats since its introduction in the 1930s.  It is fed upon extensively by American widgeon.  This invading species creates a three-dimensional structure to the otherwise unvegetated flats.  Prey resources for salmonids and other fish occur in high densities in Z. japonica (Simenstad et al. 1988).

Channels on the flats, which are formed by hydrological processes, can change their location and morphology dramatically.  These changes are driven by stream flow, tides, currents and wave energies.  Hence, alteration of these processes can affect natural stream number, size and location.  Sediment required to maintain flats is primarily supplied by rivers, streams, and eroding bluffs.  Nearshore currents and waves, along with river flow dynamics, act in consort to distribute and rework sediments on flats.  Although sediment composition and sediment dynamics exert primary control over the biological community that develops on flats, variations in light and temperature also appear to drive seasonal abundance of algae and invertebrate prey species (Thom et al. 1989).  Simenstad (1983) identified the following sources and mechanism of impact to channels:

• dredging and dredged-material disposal
• fillings, and land reclamation
• jetty, training wall and other construction
• urban and industrial effluent discharge
• log dumping and storage
• commercial or recreational exploitation of fauna and its artificial enhancement
• upstream water diversions and storage reservoirs.

Tidal flats are present below the beach face throughout Bainbridge Island, and are well-developed in such areas as Murden Cove, Manzanita Bay, Rolling Bay, Fletcher Bay, Blakely Harbor, and Eagle Harbor (Substrate Type Map, Appendix A).