Nearshore Biological Communities Prior to Removal of the Elwha River Dams 6 by Stephen P
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Chapter Nearshore Biological Communities Prior to Removal of the Elwha River Dams 6 By Stephen P. Rubin, Ian M. Miller, Nancy Elder, Reginald R. Reisenbichler, and Jeffrey J. Duda Abstract Increases in sediment delivery to (1) Bedrock/boulder reefs had the coastal waters are expected following highest kelp density and taxa richness, removal of dams on the Elwha River, and were characterized by a canopy of Washington, potentially increasing Nereocystis leutkeana (bull kelp) at the sediment deposition on the seafloor water surface and a secondary canopy and suspended sediment in the water of perennial kelp 1–2 m above the column. Biological communities seafloor; (2) Mixed sand and gravel- inhabiting shallow, subtidal depths cobble habitats with moderate relief (3–18 m) near the mouth of the provided by boulders had the highest Elwha River, between the west end of density of invertebrates and a taxa Freshwater Bay and the base of Ediz richness nearly equivalent to that for Hook, were surveyed in August and bedrock/boulder reefs; (3) Mixed sand September 2008, to establish baselines and gravel-cobble habitats lacking prior to dam removal. Density was boulders supported a moderate density estimated for 9 kelp taxa, 65 taxa of kelp, primarily annual species with of invertebrates larger than 2.5 cm low growth forms (blades close to the any dimension and 24 fish taxa. seafloor), and the lowest invertebrate Density averaged over all sites was density among habitats; and (4) Sand 3.1 per square meter (/m2) for kelp, habitats had the lowest kelp density 2.7/m2 for invertebrates, and 0.1/m2 for and taxa richness among habitats and fish. Community structure was partly a moderate density of invertebrates. controlled by substrate type, seafloor Uncertainties about nearshore relief, and depth. On average, 12 more community responses to increases in taxa occurred where boulders were deposited and suspended sediments present compared to areas lacking highlight the opportunity to advance boulders but with similar base substrate. scientific understanding by measuring Four habitat types were identified: responses following dam removal. Chapter 6 132 Coastal Habitats of the Elwha River, Washington—Biological and Physical Patterns and Processes Prior to Dam Removal Introduction (Airoldi, 2003). Sediment deposition cobble) provide some attachment space also can indirectly affect communities on their surfaces, but are subject to Two dams on the Elwha River by altering outcomes of competitive and overturning and displacement by waves have reduced sediment transport from predator-prey interactions among species and currents, and support different the upper watershed to the lower river with different tolerances and responses communities compared to more stable and coast for nearly 100 years (Duda to sedimentation (Airoldi and Cinelli, rocky substrates (Scheibling and and others, 2011, chapter 1, this report). 1997; Airoldi and Virgilio, 1998). others, 2009). Propagules of some large Increases in sediment delivery to Suspended sediment increases turbidity, seaweed species can start to grow on coastal waters are expected following which reduces light penetration and small rocks such as gravel or cobble. dam removal, potentially increasing can negatively affect photosynthetic Drag on the seaweed increases as it sediment deposition on the seafloor organisms. Seaweeds and sea grasses grows, and depending on the size of and suspended sediment in the water require high ambient light, account for a the rock, the seaweed and the rock it is column. Large increases are expected large portion of the primary production attached to may be lifted and transported initially (3–5 years after initiation of in nearshore waters, and create three- (see Miller and others, sidebar 3.2, this dam removal; Czuba and others, 2011, dimensional structures inhabited by report). A mixture of particle sizes offers chapter 2, this report) as sediments that various species (Mumford, 2007); a variety of habitats in close proximity. have accumulated behind the dams are therefore, effects on them may propagate In high current environments, large released. Thereafter, sediment supply to other parts of the community. substrates such as cobble and boulders likely will decrease but should remain Turbidity also can influence competition can dampen current speeds, allowing higher than before dam removal due to among plant species with different light retention of fine sediments that would restored transport of sediments from the requirements, or affect competitive otherwise wash away. The presence of upper watershed. Much of the sediment or predatory abilities of animals fine sediments among larger substrates currently impounded by the dams is that depend on vision (Beauchamp promotes coexistence of species adapted silt, sand, and clay, which are readily and others, 1999). Direct effects of to different particle sizes. Seafloor relief transportable (Czuba and others, 2011, suspended sediment include damage to in rocky habitats provides sloped and chapter 2, this report). Spatial patterns fish and invertebrate gills, and clogging vertical surfaces that support different of suspended and deposited sediment or damage to feeding structures of filter communities than horizontal surfaces. in coastal waters will depend on local feeders (Newcombe and MacDonald, Relief also affects communities by physical processes including waves and 1991). Beds of seaweeds or sea grasses modifying flow patterns (Witman and currents (Warrick and others, 2011b, can dampen current velocities, thereby Dayton, 2001). Effects of sedimentation chapter 5, this report). The amount, increasing sedimentation and decreasing on benthic communities likely will vary timing, and temperature of river water suspended sediment within the beds among habitats with different substrate discharged into coastal waters are (Madsen and others, 2001). characteristics. expected to be little changed by dam Substrate characteristics, including particle size, stability, and relief, are removal because the dams have been Purpose and Scope largely operated as “run of the river” important for structuring benthic communities. Stable substrates of large (Duda and others, 2011, chapter 1, this This study was initiated in 2008 particle size, for example bedrock report). to characterize nearshore biological and boulders, support various species Sediment deposition and suspended communities prior to removal of the adapted to attach to the substrate’s sediment can have a variety of effects on Elwha River dams. The intent was to surface (Witman and Dayton, 2001). nearshore plants and animals. Sediment establish a baseline to measure changes Fine sediments such as sand and silt deposition can affect organisms directly following dam removal. In this chapter, do not provide attachment points but through burial, which can reduce light, two questions are addressed: (1) What do permit burrowing and support oxygen, nutrients, and waste removal; communities currently are present various species adapted to living in scour, which can injure or dislodge in shallow subtidal areas potentially the sediment (Lenihan and Micheli, organisms; and replacement of hard, affected by dam removal? (2) What role 2001). Substrates of intermediate stable substrate with finer particles that does substrate play in structuring these particle size (for example, gravel and inhibit settlement for some organisms communities? Nearshore Biological Communities Prior to Removal of the Elwha RiverStudy Dams Area 133 Study Area 2007). The length of coast that will be east of the delta, which is sheltered by affected by dam removal is not certain, the delta to the west and Ediz Hook to The study area includes shallow but we expect that the boundaries west the east. High bluffs border much of depths (3–18 m below mean lower and east of the river mouth will include Freshwater Bay and the shore between low water [MLLW]) extending from the affected area. They coincide with Dry Creek and Ediz Hook. Bathymetry the west end of Freshwater Bay to the the boundaries of the Elwha nearshore is steepest offshore of the river mouth, base of Ediz Hook, a distance of 15 km defined by Shaffer and others (2008) less steep in mid Freshwater Bay, (fig. 6.1). This range of depths should except that their eastern boundary, the and most gradual offshore of the include most depths with sufficient light tip of Ediz Hook, is 5 km farther east. eastern flank of the delta fig.( 6.1). for photosynthesis and where kelp and Prominent features of the study Geomorphology of the study area is other benthic macroalgae are dominant area are Freshwater Bay, the Elwha described in detail in Warrick and others, given suitable substrate for attachment River delta, which extends north into 2011a, chapter 3, this report. (Whitman and Dayton, 2001; Mumford, the Strait of Juan de Fuca, and the area 123°38' 123°36' 123°34' 123°32' 123°30' 123°28' Strait of Juan de Fuca 20 48° 10' Elwha River Mouth 15 Elwha River Delta 10 Freshwater Bay 5 Base Ediz Hook 48° Dry Creek 8' Study Area WASHINGTON Bathymetry and topography from Finlayson and 0 1 2 KILOMETERS others, 2005; Cochrane and others, 2008; Warrick and others, 2010. UTM Zone 10, WGS84 projection 0 1 2 MILES Bare earth LIDAR from 2001 Figure 6.1. Southern waters of the Strait of Juan de Fuca near the mouth of the Elwha River, Washington. Five-meter bathymetry contours also are shown. Chapter 6 watac11-0558_fig6-01 134 Coastal Habitats of the Elwha River, Washington—Biological and Physical Patterns and Processes Prior to Dam Removal Previous Work mid-Freshwater Bay (fig. 6.2C). Kelp or species level identifications) that also occurred frequently just to the occurred in at least one quadrate. They Sonar and video surveys were northeast of the river mouth, where also reported 57 taxa of invertebrates conducted in 2005 in Freshwater Bay the sonar survey detected little hard and fish that occurred in at least one and offshore of the Elwha River mouth substrate in 2005 (Warrick and others, quadrate and provided density estimates to map bathymetry and characterize 2008), and in a few patches between Dry for 15 invertebrate species.