Size matters: The contribution of mega-infauna to the food webs and ecosystem services of an Oregon estuary

Background/Question/Methods

Large-bodied invertebrates (bivalves, polychaetes, burrowing shrimps) are common to infaunal communities of NE Pacific estuaries, but their contribution to estuarine community structure, function and ecosystem services is poorly understood because they are difficult to sample and quantify.  In a study of Yaquina estuary (OR) food webs, particular effort was made to quantify intertidal and subtidal mega-infauna using suction-excavated 40-cm diameter corers in addition to conventional sampling of macro-infauna. The abundance and biomass of floral and faunal guilds were directly quantified or estimated from published studies, and C and N stable isotopes were measured for abundant species.  Inverse analysis was used to generate models of C flow within food webs of the lower and upper reaches of the estuary, including or excluding mega-infauna in appropriate guilds.  The latter scenario represented the too-common practice of not sampling mega-infauna during benthic community surveys.

Results/Conclusions

Benthic invertebrates dominated the biomass and respiration among faunal guilds in both reaches, whereas biomass and respiration of birds and fish were two orders of magnitude smaller.  Mega-infauna, particularly intertidal burrowing shrimps and bivalves, constituted most of the benthic invertebrate biomass, respiration, and secondary production in both reaches, although they were only a small fraction of the infaunal abundance.  Mega-infauna were dominant consumers of phytoplankton, major contributors of C to sediment organic matter, and facilitated C and N flux between sediments and the water column.  However, meio- and macrofauna were relatively more important as prey for fish, crabs and birds.  Mega-infauna contributed directly to estuarine ecosystem services as fishery species (clams, burrowing shrimp) and indirectly by supporting fish, crab and bird populations, by accelerating C and N cycling, and by improving water clarity through filter feeding.  Under-sampling of mega-infaunal invertebrates resulted in substantial underestimation of C flow in the food webs and production of key ecosystem services.