Abstract
This research assessed the effectiveness of benthic video sleds for monitoring the impacts of dredged material disposal on epifauna at shallow and deep water disposal sites near the mouth of the Columbia River, USA. Video sleds collected visual transects at the two disposal sites and comparable reference areas during 2014 and 2015 within a Before-After, Control-Impact (BACI) experimental design. These flat, soft-bottom habitats are populated by demersal fish and benthic invertebrates, including the economically important Dungeness crab (Cancer magister). At the shallow site, results from multivariate Similarity Profile Analysis (SIMPROF) and univariate ANOVA tests on prominent species did not detect any significant differences between disposal and reference communities. At the deep site, the multivariate and univariate analyses detected differences in communities and abundances between years, rather than between disposal and reference locations. At the scale of this research, there was no detectable impact of dredged material disposal on the epifauna communities at these two Pacific Northwest disposal sites. While the species resolution of cryptic or small organisms was found to be limited, the video sled technique had sufficient power to detect potential differences in most epifaunal species densities with a BACI statistical design. We found the video sleds were an effective tool to assess potential impacts of dredged material disposal on epifauna.
Highlights
IntroductionSediment discharging from rivers nourishes ocean beaches. in highly managed river systems, dredging of this material from shipping channels is usually required to maintain safe ship passage, and the sediment is commonly deposited far from littoral zones where it is needed (USACE 2012)
Under natural conditions, sediment discharging from rivers nourishes ocean beaches
At the mouth of the Columbia River (MCR) in the Pacific Northwest of the USA, more than 3,000,000 m3 of material are annually dredged by the US Army Corps of Engineers (USACE) and placed at designated ocean disposal sites (Gailani et al 2003)
Summary
Sediment discharging from rivers nourishes ocean beaches. in highly managed river systems, dredging of this material from shipping channels is usually required to maintain safe ship passage, and the sediment is commonly deposited far from littoral zones where it is needed (USACE 2012). A new program identified a method for Bthin-layer^ deposition of sediment at nearshore disposal sites (Wilber et al 2007), where it can be entrained in littoral circulation and retard ongoing erosion (Oregon Solutions et al 2011). Note this differs from direct placement on beaches. These nearshore soft-bottom habitats are an understudied system and questions remained about the impacts of physical forces and burial of the benthos. Concerns from local fisherman and resource managers about the impacts of an expanding network of disposal sites on crab resources and navigation warranted further study (Oregon Solutions et al 2011)
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