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2015 Puget Sound Factbook Book | v3.0
Food webs
The nearshore food web
Section authors: Emily Howe, University of Washington; Charles Simenstad (editor),
University of Washington School of Aquatic and Fishery Sciences
Summary
The Puget Sound food web relies on two distinct food web pathways; a phytoplankton-based
“grazer” community that directly consumes living organic matter, and a detritus-based
community that consumes dying or decaying organic materials that are first transformed by
microbes (Seliskar and Gallagher 1983). This fact sheet describes the detritus-based food webs
of Puget Sound, with an emphasis on the sources that contribute to the base of the food web,
how landscape change has affected detritus availability, and the types of organisms that
ultimately depend on detritus for their energy needs. For the most part, detritus-based food
webs are associated with benthic (sedimentary seafloor) ecosystems, with the source of energy
emanating from rooted vascular plants and their epiphytes, benthic-attached macroalgae (i.e.
kelp), or benthic microalgae. This distinguishes the detrital food web from pelagic systems
wherein the main source of food at the base of the food web is produced in the water column by
phytoplankton.
Sources of detritus and landscape change
1. Sources of detritus supporting Puget Sound food webs include: 1) terrestrial input from
watersheds, 2) estuarine wetlands (including tidal freshwater swamps, scrub-shrub tidal
wetlands, and emergent marshes, 3) seagrass beds and associated epiphytes, 4) benthic
microalgae, 5) marine macroalgae (i.e. kelp), and marine riparian vegetation (Seliskar &
Gallagher 1983). Together, these ecosystems produce the biomass equivalent of at least
15,000 school buses each year (with school buses estimated to weigh 10 metric tons).
Approximate biomass contributions to Puget Sound’s detrital pool based on areal
coverage (Simenstad et al., 2011) and annual primary productivity estimates (Fact 4):
a. Eelgrass: 79,360 metric tons (22610 ha eelgrass, Christiaen et al., 2015). This is
equivalent to 7936 school buses (at 10mt each).
b. Kelps & macroalgae: No areal estimates are available for Puget Sound, but along the
WA west coast and Strait of Juan de Fuca floating species encompass an estimated
1500 ha (Mumford & Berry, 2014). This is equivalent to 500-22000 metric tons of
biomass generated per year. Within Puget Sound, floating kelp occurs along 11% of
the shoreline and understory kelp occurs along 31% of the shoreline (ShoreZone
Inventory). So, kelps on just the outer coast produce enough material to equal 2,200
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