EcologJy of the Hudson River Zooplankton Community

Zooplankton in thc I lut lson Ri\ ,er estuary inclut le both f rcshr.v i r ter i lnd est i lar inc spt--c ies l r . rd range in bor l l ' lengths f rorn nr icror.rs to mi l l inrctcrs. , \4easurcmcnts ol abundance ant l b ionrass as rvel l as comrnuni tv ratc processes incl icate that zooplankton clo not gerrelal l r , exert s igni f icant grazing pressLrre on phl toplankton. In aclc l i t ins unclergo rcglr lar st :asonal cycles in abundance, t tp ical lv rv i t l r increases clur ing r 'varrr . lou'fkrr ' r ' pcr iods of thc vear. The invasion of thc zebra r t tussel into the I l t rc lson hacl strong inrpacts on zooplankton in the l leshr 'vater sect ion of thc cstr , rarv i \ l icrozooplankton such as lc l ( i ters c lecl incci d larnat ical l l ' . Clad<rccr ' ans also t lecl inccl in arrnual a\erage abundancc Lre t \ 'vccn pre ant l post-zcbra nrusst l per iocls r ' rhcn thc ef l 'ects of urct r l rd dry 1 'eals arc takcn into i lccount. Zebra musscls, hon,ever, hacl l i t t le elTcr: t on larger zooplankton. Regulat ion ofzooplankton appears to bc r f tnct ion of ph1's ical fbrces that af fcct poprr lat ion resi t lence t inrcs as rvcl l as tbot l arrd pledatols. F-r , ic le l rce for tbocl I i r r r i l at ion is mixccl . Sonrc species benef i t f ronr tbocl supplcnrc.r)1\ in exper inrental t r ia ls, but the recluct ion of pht toplanklon biornirss i r r associat ion r ' r , i t l . r thc zebra nrussel in l 'asion had rro et fbct on c lacloceran cgg product ion. There arc a v l r ic t l ' of potent ia l preclators, ancl cal t 'u lat ions incl i (at t ' f ish cxcf l h igh ralc 's oI r r ro l ta l i t t ' on zooplanktorr . I ) i rect nrcasurcs of prcdatory mortal i tv ancl a bct te l understanding of berr th ic-pelagic i ntc ' ract ior . rs are im[)ortant topics to r ' futurc rcsearcl . r . ln ar ld i t iun, u 'c la istthe general c lucst ion of horv strolrgly zooplanktor] i r ) Ieract ions wi th predators and prcv arc coupled in the l i rocl n 'cb. I f interact ions are \ 'veak, changes snch as species invasiorrs or large intcrannLral var iat ion in f ish populat ions ma1' l ro i inrpact the z<roplanklon, and thereb\t propagat ion of etTccts r . rp an<l c lorvn the food rvcl l coul t l be highl t ' a l tenuatecl . Introduction Zooplankton are sur:Ll l :rnirnals that l ive suspendecl in rvater. ' l his \ \ , i l tcr r]olumn l i le-stVle ret luircs cop ing with a nunrber of d ist inct abiot ic and biot i r : f actors. Zooplankton nrovement is lalgel-v the consequence of currents and not directecl swirnrning activi t ies. lhe f lorv of the Hudson l l ivcr plrshes plankton continuoLrsh' downslrei lnl . ' f i t lal currents in the esttran' rctard this dor\,nstrei lr)r nrovenlent, but populat ions nlust st i l l contencl rvi th constant net displacernent to\, \ 'ard the sea wilh irssociatecl incrcases in sal initr ' ' . [) lankton in the Hudson arc adapted to l ive in fresh\ 'vater or nroclerate sal ini t t 'condit ions, and as a consequence thele i i re two Iair lv cl ist inct corrrnunit ies fr-cshr'r 'ater plankton and those tolerant of sal ini t l ' that w'e n' i l l cal l true estuarinc plankton. In adcl i t ion to adapting to the abiot ic features of the Huclson, zouplankton nrust acquire loocl to support glowth and reprit l rrct ior-r. Appropriate foocl rcsources l ike algae arc enrbedcled in a rnixture of organic and inorganic part icles f i 'orn r 'rrhich zooplankton nrust : lcquire el lergt ' : l luclson River zooplankton are also prevecl on bv manv f ishes ancl invertebrates that rnirv be sufl icientlv abunclant to regulatc thcir prev popr.r lat ions. There are lni lny \\ravs to organizt ' a discr.rssion of a comrnunitt , of organisnrs. Here', r ,rre dcscribe tltc composit ion of t l -rc freshn'ater attd estuarinc zooplankton conrrnunit ies including their cl istr ibr-rt ion and abundar)ce at sevcralteurpolal ancl spat iai scales. Nleasurenrents of irbuntlance al lor 'r ' us to assess the inrp()rtance ofzooplirnkton rclat ive to other groups (fbr cxample, benthos) in the I irr t lson. Our nrain contention is thal zooplanktun are not part icular lv s igni l icant in the context o l ' (he overal l f lor,r ' ' of errergv in the Huclson ecosYstenr. Zouplankton do nol account fur a large corrponent o1