COMM-T-95-002 C2 BIOLOGICAL STUDY NONINDIGENOVS AQUATIC SPECIES IN A UNITED STATES ESTUARY: A CASESTUDY OF THE BIOI OGICALINVASIONS OF THE SAN H4%NCISCO BAY AND DELTA Andrew N. Cohen Energyand ResourcesGroup University of California at Berkeley Berke!eyCaliforrua 94720 JamesT. Carlton Maritime Studies Program Williams College Mystic Seaport Mystic Connecticut06355 A Report for the UNITED STATESFISH AND WILDLIFESERVICE, WASHINGTON D. C. and THE NATIONAL SEA GRANT COLLEGEPROGRAM CONNECTICUT SEA GRANT U NOAA Grant MLunberNA36RG0467! December 1995 .'.' l997 NtLTIGHALSU GIANTOEPOSggf PEll USRARYBOIUNg um HARIVlalLNSGTtNY Cg+g IEL CN82 Executive Suxrurtary 1 THESAN FRANCISCO BAY AND DELTA REGION IS A HIGHLYINVADED ZCOSYFIK4L TheSan Francisco Estuary can now be recognized asthe most invaded aquaticecosystem inNoxth America. Now recognized inthe Estuary are 212 introducedspecies: 69 percent of theseare invextebrates, 15percent are fish andother vertebrates, 12 percent are vascxxlar plants and 4 percentare protists. ~ In theperiod since 1850, the San Francisco Bay and Delta repon has been invadedby an average of one new species every $6 weeks. Since 1970, the ratehas been at least one aew species every 24 weeks: the first coBection recordsof over50 non-nativespecies ia the Estuarysince 1970 thus appear to reflecta significantnew pulse of invasions. ~ In additionto the212 recognised introductions, 123 species are coasidered ascryptogenic not clearly native or introdxxced!, andthe total number of cryptogenictaxa in theEstuary might weil be twice that. Thus simply reportingthe documented intxoductions andassuming that aH other species in a regionare na tix~s virtuallyall previous studies have done severely underestimatesthe impact of marineand aquatic invasions on a region's biota. Nonindigenousaquatic animals and plants have had a pi'ofoundimpact on theecology of thisxegion. No shallow water habitat aow remains uninvaded byexotic species and, in some regions, it is difficult to find any native species in abundance.In someregions of theBay, 100@ of the coinmonspecies are introduced,creating "introduced communities." In locations ranging from freshwatersites in the Delta,through Suisun and SanPablo Bays and the shallowerparts of the CentralBay to the SouthBay, intmduced species accountfor the majorityof thespecies diversity. 2. A VAST AMOUNT OF ENERGY NOW PASSESTHROUGH AND IS UlKEKD BY THE NONINDIGKhJOUS BIOTA OF THE ESTUARY. IN THE 1990S, INTRODUCED SPECIES DOMINATE ANY OF mEESTUARY'S FOOD WEBS. ~ The majorblooxn-creating, donunant phytoplankton species are cxyptogenic. Becauseofthe poor state of taxonomic and biogeographic know'ledge, it remainspossible that many of theEstuary's major primary producers that providethe phytoplankton-derived energyfor zooplankton andfilter feeders,are in fact introduced. ~ Introduced speciesare abundant ind doniinantthroughout the benthic and fouliagcouimunities of SanFrancisco Bay. Theseindude 10 speciesof introducedbivalves, most of whichare abxxndant to extremelyabundant, Intmducedfilter-feeding polychaete worms and crustaceans may occur by the thousandsper squaremeter. Oa subhttoralhard substxates, the Mediterraneanmussel Mytttlls gallopomsciNIis is abundant,while float fouliaycommunities suppoxt large populations ofintxoduced filter feeders, includingbryoxoaas, sponges and seasquirts. The holisticmle of the entire nonindigenousfilter-feeding guild iadudingdams, mussels, bryozeans, barnacles,seasquirts, spionid worms, serpulid worms, sponges, hydroids, andsea anemones in alteringand controlling the trophic dynamics of the Bay-Deltasystem remains unknown. The potenbal role of just one species, theAtlantic ribbed marsh mussel Arcuafstla dern issa, as a biogeochemical agentin theeconomy of Baysalt marshes isstriking. ~ Introducedclams are capable offiltering the entire volume otthe South Bay andthe northern estuarine regions Suisun Bay! once a day:indeed, it now appearsthat the primary inechanism controHing phytopiankton biomass duringsummer and fall in SouthSan Francisco Way is "grazing' filter feedingby the introduced Japanese clams Vetrerupis and hfrtscsdistN and the Atlanticclam Gejurna. This remarkable process has a significantimpact on thestanding phytoplankton stockin the South Bay, and since this plankton isnow utilized almost entirely by introduced filter feeders, passing the energythrough a non-native benthic fraction ofthe biota may have fundamentallyaltered the energy available for native biota ~ Droughtyear control ofJrhytopiankton byintroduced clams resultina in the failureof the summer diatom bloom to appear ln thenorthern reach of the Estuary is a remarkablephenonienon. the introducedAtlantic soft-shell clamsQLfya! alone were estiinated tobe capable attimes of filtering atl of the hytoplanktonfromthe water column onthe order ofonce pei day. hytoplanktonblooms occurred only during higher flow years, when the populationsofMya and other introduced benthic filter feeders retreated downstreamtosaltier parts of theEstuary. Phytopianktonpopulations inthe northern reaches ofthe Estuary maynow' becontinuously andpermanently controlled byintroduced clams.Arriving byballast w'ater and first collected i»the Estuary in1986, by 198S the Asian clamPotamocorbula reached and has since sustained average densities exceeding2,000/m2, Since the appearance ofPofamocorbda, thesummer diatombloom has disappeared, presumably because ofincreased filter feedingby this new invasion. The Poftun ocorbtda population inthe northern reachesofthe Estuary can Filter the entire water column over the channels morethan once per day and over the shallows almost 13times per day, a rate offiltration which exceeds thephytoplankton's specificgrowth rate and approachesorexceeds thebacterioplankton's specificgrowth rate. ~ Further,the Asian clam Pottrrtt ocorbda feeds atmultiple levels inthe food chain,consuining bacterioplankton, phytoplankton, andzooplankton copepods!,andsomay substantially reducecopepod populations bothby depletionofthe copepods' phytoplankton foodsource andby direct predation.Inturn, under such conditions, thecopepodwating native opossumshrimp Ncortrysis maysuf fer a near-completecollapsein the northernreacIL Itwas during onesuch pattern thatmysid-eating juvenile stripedbasssuffered theirlowest recorded abundance. Thisexample and thelinkages between introduced andnative species mayprovide a direct andremarkable example ofthe potential iinpact ofan introduced specieson the Estllalys foodwebs. ~ Aswith the guild offilter feeders, theoverall picture ofthe impact of introducedsurface-dwelling andshallow-burrowing inggrazers anand d eliosit 't f eedersinthe Estuary isincompletely known. The Atlantic mudsnail Executive Summary 17yanassaislikely playing a significant if notthe mast ixnportant role in alteringthe diversi', abundance, size distribution, and recruitment of xnany specieson the intertidalmudflats of SanFrancisco Say. ~ The arrivaland establishment in 1989-90of theAtlantic green crab CNrciens xnaenasin SanFrancisco Bay signab a newlevel of trophicchange and alteration.The pee» crabis a foodand habitat generalist, capable of eating an extraordinarilywide variety of anixnaisand plants, and capableof inhabitingmarshes, rocky substrates, and f ouhngcoxnmunities. European, SouthAfrican, and recent Californian studies indicate a broadand striking potentialfor this crab to signi6cantly alter the distribution, density, and abundanceof preyspecies, and thusto profoundlyalter community struchire in the Bay. ~ Nearly 30 speciesof introducedmarine, brackishand freshwaterfish are now ixnportantcarnivores throughout the Bayaxld Delta. Easternand centralAmerican fish carp,mosquito6sh, catfish, green sunfish, bluegilis, inlandsilverside, layemouth and smallmouth bass, and striped bass are amongthe xnost significant predators, competitors, and habitatdisturbexs thmughout the brackishand freshwater reachesof the Delta, with often concoxnitantimpacts on native fish communities.The intmduced cray6sh Procambarusand Facifastecxxsmay play an importantrole, w hendense, in regulatingtheir prey plant and animalpopulations. Nativewaterfowl in theEstuary consume some introduced aquatic plants suchas brassbuttons! and native shazebirds feed extensively on introduced benthic invertebrates. 3. INTRODUCED SPECIESMAY 8E CAUSING PROFOUND SHtUCTULQ, CHANGES TO SOME OF THE ESTUARY'S HA8rrATS. ~ The Atlanticsalt-xnarsh cordgrass Spartxna eltcnijlora, which has converted 100sof acresof mudflatsin WiliapaBay, Washington, into grassislands, has becamelocally abundant in SanFrancisco Bay, and is competingwith the nativecordgrass. Spartinn alteruijlota has bmad potential for ecosystem alteration.Its lazger and more ripd stexns, greater stem density, and higher rootdensities may decrease habitat for nativew etlandanimals and infauna. Densestands of S.altrmijfora may cause changes in sedimentdynamics, decreasesin benthic algal production became of lower light leveb below the cordgrasscanopy, and loss of shorebirdfeeding habitat thmugh colonization of xnudflats TheAustralian-New Zealand boring isopod S'phseronxN qaroylnem creates characteristic"Splxacroma topography" on many Bay shores, with many linear metersof fringingmud banks riddled with its haM~ntixneterdianieter holes. Thisisopod may axguably play a major,if not thechief, mle in erosionof intertidalsoft rock terraces along the shore of SanPablo Bay, due to their boringactivity that weakens the mck and facilitates its removal by wave action.Sphmxanl has been burrowing inta Bay shores