The nearshore food web: Detritus

Detritus, or dying or decaying matter, is a central component of the nearshore food web in Puget Sound. This article was prepared as part of the 2015 Puget Sound Fact Book produced by the University of Washington Puget Sound Institute. 

Dead salmon. Photo: Boris Mann (CC BY-NC 2.0) https://www.flickr.com/photos/boris/3037705761
Dead salmon. Photo: Boris Mann (CC BY-NC 2.0) https://www.flickr.com/photos/boris/3037705761

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

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):

  1. Eelgrass: 79,360 metric tons (22610 ha eelgrass, Christiaen et al., 2015). This is equivalent to 7936 school buses (at 10mt each).
  2. 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 school buses. This doesn’t include the shorelines within Puget Sound or its sub-basins, which together cover nearly 8000 km.
  3. Mudflats (benthic microalgae): 31.3 metric tons (mt). Mudflats, which we think are barren zones, actually produce 3 school buses worth of organic material per year.
  4. Emergent marshes (estuarine mixing zone): 62.7 mt.  6.2 school buses.
  5. Scrub-shrub tidal wetlands: 36.3 mt. 3.6 school buses.
  6. Tidal freshwater swamps: 16.3 mt. 1.6 school buses.
  7. Marine riparian vegetation: Unknown
  8. Terrestrial/Riverine organic matter: Cumulative measure unknown. The total organic carbon exported Skagit and Snohomish alone contribute 18000-56000 mt/yr (Mullholland & Watts, 1982).  1800-5600 buses.

Primary productivity is exceptionally high for these ecosystems (range: 350 – 1800 g C m2/yr; Thom, 1990; Ewing, 1986), rivaling that of tropical rainforests, which are often thought to be the most productive ecosystems in the world (2200 g C m2/yr).  The other source of food energy for Puget Sound food webs comes from water column production by phytoplankton (planktonic algae), which exhibit comparatively lower productivity rates than marsh ecosystems (465 g C m2/yr Winter et al., 1975).

Primary productivity estimates for detrital sources contributing to Puget Sound are strong for:

  1. Eelgrass ecosystems: 351 g C/m2/yr (Thom, 1990). 50% of annual primary production due to epiphytic algae, 2% to Z. japonica, and 48% to Z. marina (Padilla Bay). This compares to 303 g C/m2/yr in Grays Harbor (Thom, 1984).
  2. Emergent marsh ecosystems: 443-878 g C/m2/yr (Ewing, 1986).
  3. Brackish wetlands: 1115-1742 g C/m2/yr (Ewing, 1986), 1629 g C/m2/yr (Disraeli & Fonda, 1978), 1390 g C/m2/yr (Burg et al., 1976), 1355 g C/m2/yr (Levings & Moody, 1976).

Primary productivity estimates are poor or unavailable for Puget Sound/ Salish Sea for the following detrital sources:

  1. Benthic microalgae: 229 g C/m2/yr in Hood Canal, WA (Simenstad & Wissmar, 1985). 50-250 g C/m2/yr, measured in the temperate Ems-Dollard estuary, Denmark (Colign & de Jonge, 1984).
  2. Tidal freshwater marshes: 1530 g C/m2/yr, mean value from North American review (Findlay et al., 1981).
  3. Kelps and macroalgae: 350-1500 g C/m2/yr, Macrocystis pyrifera, 600-1300 g C/m2/yr for Laminaria in coastal California (Dayton, 1985). Macroalgae productivity strongly depends on dissolved inorganic nitrogen, which varies with coastal upwelling cycles associated with El Nino and La Nina events. No Puget Sound primary productivity rates were available.
  4. Riverine inputs (limited data Puget Sound) and marine riparian vegetation: Nanaimo River = Dissolved organic carbon (DOC): 2000 g C/ m2/yr, fine particulate organic carbon (FPOC): 56 g C/ m2/yr (Naiman & Sibert, 1978), similar to marsh ecosystems. Skagit River 3.8 g C/m3/yr Much of this material is thought to be refractory, and therefore unavailable to consumers (Canuel et al., 2009; Mueller-Solger et al., 2002).

While phytoplankton becomes available to Puget Sound food webs via punctuated seasonal blooms in the spring and fall (Winter et al., 1975), detritus is available continually throughout the year because it breaks down slowly, with decomposition ranging between 8-112 weeks (Brinson et al., 1978).

Vascular marsh plants decay at a rate of approximately 0.3%/yr (Findlay et al., 1990), although decomposition depends on temperature, aerobic conditions, microbial and detritus feeder community composition, hydroperiod (moisture), and the lability of the species decomposing (Brinson et al., 1981). Microbial conditioning of detrital material enhances the nitrogen content, and hence, the nutritional quality of the material for consumers (Sosik & Simenstad, 2013).

Sound-wide degradation of these ecosystems represents a non-trivial reduction of the amount of detritus entering Puget Sound food webs.  Most critically for detritus-based food webs, the total area of wetlands has declined dramatically in most river deltas, with the greatest losses in South Central Puget Sound and the Whidbey sub-basins. In the 16 major estuarine deltas feeding into Puget Sound, 25% of unvegetated mudflats, 45% of marshes within estuarine mixing zones, 98% of brackish marshes, and 90% of tidal freshwater wetlands have been lost (Simenstad et al., 2011). This represents over 275 metric tons/yr of detrital materials that no longer reach Puget Sound food webs just due to alterations in the deltas of 16 river systems leading into Puget Sound.  When non-delta ecosystems are included, the Sound is deprived of nearly 450 metric tons of detritus per year—equal to about 45 school buses.

Calculations for the historical change in detrital biomass emanating from Puget Sound river deltas are based on primary production estimates for each source (see Fact 4) and the estimated historical change in the areal extent of each ecosystem type according to the PSNERP Historical Change Analysis of Puget Sound nearshore ecosystems (Simenstad et al., 2011).

Estimated biomass lost due to historical change in landscape structure:

River Delta losses:

  1. Mudflats: 9,500 kg/m2/yr
  2. Emergent marshes: 25,700 kg/m2/yr
  3. Scrub-shrub tidal wetlands: 90,280 kg/m2/yr
  4. Tidal freshwater swamps: 150,000 kg/m2/yr

Non-Delta losses:

  1. Mudflats: NA
  2. Emergent marshes: 37037 kg/m2/yr
  3. Scrub-shrub tidal wetlands: 98,570 kg/m2/yr
  4. Tidal freshwater swamps: 38,250 kg/m2/yr

Approximately 47% of annual marsh primary production is exported from marsh ecosystems to estuarine food webs as detritus (Sherwood et al., 1990), feeding benthic infauna such as clams and mussels (Howe & Simenstad, 2012), gammarid amphipods, and polychaete annelid worms (Jones et al., 1990). The remainder accretes in marsh sediments or feeds marsh detritivores (Sherwood et al., 1990).

In Puget Sound, over 27% of total shoreline length is armored by some type of structure, although many regions, such as Central Puget Sound (60%), exhibit much higher percentages (Simenstad et al., 2011).  

The Puget Sound Nearshore Ecosystem Restoration Project “conducted a comprehensive and spatially-explicit analysis of net changes to nearshore ecosystems of Puget Sound – its beaches, estuaries, and deltas- since its earliest industrial development” (Simenstad et al., 2011).  Present (2000-2006) shoreline structure was quantitatively compared to the earliest land surveys of the General Land Office and US Coast and Geodetic Survey (1850-1890s).

Shoreline armoring is approaching 100% in the Skagit, Stillaguamish, and Snohomish river deltas, has reached 100% in the Duwamish and Puyallup deltas, encompasses the entire eastern shore of Bellingham and Samish Bays, and stretches across 75% of the Nisqually delta.  Shoreline length has been reduced by greater than 50% in the Nooksack and Samish deltas, and over 50% of the aquatic zone in Birch bay has been covered by fill.  Tidal barriers are prominent in the Quilcene, Hamma hamma, Duckabush, Dosewallips, and Skokomish river deltas.

Shoreline armoring reduces detritus availability to beach organisms by 66-76%, and disrupts ecosystem connectivity between detritus-generating ecosystems and marine food webs (Heerhartz et al., 2014).  Armoring also changes the composition of wrack to exclude terrestrial sources. Beach wrack available to beach detritivores is composed of ~60% marine algae, 24% terrestrial plant materials, and 13% eelgrass. 

Shoreline armoring reduces talitrid (beach hopper) abundance (Sobocinski et al., 2010), which is an important food source for shore crabs (Hemigrapsus nudus) (Lewis et al., 2007), birds and other animals (Toweil, 1974; Vermeer, 1982).

There are many types of detritivores in estuarine and marine ecosystems.  Suspension feeders, such as mussels, littleneck clams, barnacles and oysters, filter food suspended in the water as it passes by.  By contrast, benthic-deposit feeders engulf sediments, digesting the bioavailable portions. Benthic-deposit feeders include several types of clams, polychaete worms, gastropods, sea cucumbers, crabs and sand dollars.  Grazers also depend on benthic-associated food production, but are not classified as detritivores.  Grazers consume (as a group) a combination of fresh macroalgae, epiphytic algae, and fresh detritus.  Grazers include such organisms as snails, limpits, sea urchins, and chitons (Encyclopedia of Puget Sound, Herbivores and detritiores in Puget Sound).

The benthic and nearshore communities of Puget Sound rely strongly on detritus for food web support, especially near river mouths, tidal marshes, eelgrass and kelp beds.  Suspension-feeding mussels, for example, obtain between 11-88% of their nutrition from detrital sources, depending on the season (Hoffnagle et al., 1979; Tallis, 2009; Howe & Simenstad, 2014), and a variety of estuarine and nearshore invertebrates ultimately derive their nutrition from sources other than phytoplankton (Simenstad & Wissmar, 1985).

The Salish Sea’s intertidal food webs echo those studied across the world.  From Amchitka, AK (Duggins et al., 1989), San Francisco Bay, CA (Howe & Simenstad 2011, 2011), and the tip of South Africa (Bustamante et al., 1995), strongly relying on detritus to fuel community metabolism.

Detrital food webs support keystone predators, such as the Ochre seastar (Pisaster ochraceus) which plays a key role in regulating community diversity in Puget Sound’s rocky intertidal habitats (Paine, 1980).  P. ochraceus feeds preferentially on barnacles (proportion of diet = 10-54%) and mussels (~20%).  Detritus comprises 11-88% of the diet of both prey species (Tallis, 2009).

Pisaster ochraceus populations have dramatically declined in Puget Sound as a result of seastar wasting disease.  The cause of the disease has yet to be unequivocally identified, but emerging research points to a viral infection of densovirus (Hewson et al., 2014), perhaps augmented by higher than normal water temperatures (Bates et al., 2009). The loss of this keystone predatory species indicates that nearshore food webs are dynamic in space and time, and we expect extensive restructuring of rocky intertidal communities as a result its removal via densovirus (Paine, 1980).

Juvenile chum salmon exploit detritus-based food webs by feeding selectively on harpacticoid copepods, for which the detrital carbon uptake exceeds algal carbon uptake by 9-10 fold (Sibert et al., 1977). This commercially valuable fisheries resource is usually considered planktivorous, but during the first critical weeks of estuarine life, chum rely on a detritus-based, benthically derived food web.

Stable isotope evidence suggests 12-35% of carbon assimilated by chum fry emanates from the terrestrial environment; the rest is derived from detrital macroalgae in the marine environment (Romanuk & Levings, 2005). Similarly, Chinook fry depend on terrestrial detrital pathways for 10-40% of their nutritional needs, and juvenile pink salmon depend on terrestrial detritus for 12-35% of their dietary needs. Chinook are primarily tied to detritus-based food webs by feeding on emergent insect communities (Shreffler et al., 1992), with some supplementation from epibenthic suspension feeding crustaceans, such as Corophium spp. (Shreffler et al., 1992).

Dominant estuarine/nearshore fish dependent upon detritus-based food web pathways include: (Seliskar & Gallagher, 1983).

  1. Anadromous species:
    1. Chinook fry and smolts (Oncorhynchus tshawytscha)
    2. Chum fry (Oncorhynchus keta)
    3. Pink fry (Oncorhynchus kisutch)
    4. Sockeye smolts (Oncorhynchus nerka)
    5. Longfin smelt (Spirinchus thaleichthys)
  2. Marine species:
    1. Northern anchovy (Engraulis mordax)
    2. Shiner perch (Cymatogaster aggregata)
    3. Staghorn sculpin (Leptocottus armatus)
    4. Starry flounder (Platichthys stellatus)
    5. Surf smelt (Hypomesus pretiosus)
    6. English sole juveniles (Parophrys vetulus) (80-94% detritus based; Howe & Simenstad, 2015).
  3. Freshwater species
    1. Peamouth chub (Mylocheilus caurinus)
    2. Prickly sculpin (Cottus asper)
    3. Threespine stickleback (Gasterosteus aculeatus)

Major food sources for fish in tidal marsh ecosystems include:

  1. Amphipods (especially Americorophium spp.)
  2. Harpacticoid copepods
  3. Emergent insects (adults, pupae, and larvae; Dolichopodidae, Chironomidae, Ceratopogonidae, and Ephydridae)
  4. Terrestrial insects (Hemiptera)
  5. Mysid shrimp (Neomysis mercedis)
  6. Isopods (Gnorimosphaeroma oregonensis)
  7. Flatfish larvae
  8. Cumaceans
  9. Oligochaetes
  10. Polychaetes
  11. Decapod larvae (crabs and shrimp)

*Epibenthic crustaceans are particularly important contributors to fish diets. (Seliskar & Gallagher, 1983; Levy et al., 1979; Northcote et al., 1979; Northcote et al., 1981; David et al., 2014)

Restoration efforts that have restored tidal flow to estuarine wetland ecoystems (i.e. Nisqually, Skagit, and Skokomish) via dike removals or breaches have rapidly restored ecological attributes associated with detritus-based food webs, including ecosystem capacity to support higher densities of organisms, and ecosystem connectivity in terms of sources of detritus (David et al., 2014; Howe & Simenstad, 2014; Greene and Beamer, 2011).

The importance of detritus based food web pathways differs among ecosystem types. Six primary (75-100% of total index of relative importance) direct pathways have been identified between detritus and upper trophic levels in rocky intertidal habitats. Five primary pathways have been identified in cobble littoral habitats, four primary pathways for exposed gravel-cobble habitats, and five for protected sand-eelgrass ecosystems, and four for protected mud/eelgrass systems (Simenstad et al., 1979).

Detritus-based food webs link terrestrial, estuarine, and marine ecosystems through energy flow (Romanuk and Levings 2005, Tallis 2009, Howe & Simenstad 2015). Detrital food webs in the estuary of large river systems reflect an integrated “bouillabaisse” of many types of detritus across space, while detrital food webs associated with small river systems or bays show strong associations with the detritus of immediately available vegetation (Howe 2012). 

Confining rivers between levees restricts connectivity between rivers, riparian zones, floodplains and marsh ecosystems, thereby reducing the interface where organic matter exchange occurs (Amoros and Bournette 2002, Winemiller 2003.  River channelization also focuses river discharge into a forceful jet-like plume, rather than a dispersive fan of smaller, less forceful river channels (Syvitski 2005). In the case of the Skagit River, higher river flow velocities caused by levee confinement exports detritus beyond the immediate estuarine system, functionally separating detrital marsh resources from benthic-deposit feeding organisms, such as clams. Clams on the Skagit delta derive only 30% of their diets from marsh detritus, while clams located across from the river plume on Whidbey Island derived 60% of their diets from marsh-produced detritus (Howe 2012).

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