Proc. Esionian Acad. Sci. Ecol., 1995,5, 112,20-25 oN rHE ROLE OF CERCOPAGIS (CERCOPAGIS) PENGOI (OSTROUMOV) IN PARNU BAY AND THE NE PART OF THE GULF OF RIGA ECOSYSTEM Henn OJAVEER and Alide LUMBERG EstonianMarinelnstitute,LaiSt.32,EE-000lTallinn,Estonia Presented by A. Jiirvekrilg Received 9 September 1994, acoepied 27 October 1994 Abstract.TheinvasionoianewCladoceraspecies,Cercopagis(Cercopagis)pengoi (Ostroumov), into Pdrnu Bay and the NE part of the Gulf. 3l.. Rigu ecosystem is reported. A briei description of its g.ogrrphi. origin and distribution, morphology, and systematics is also presented. our investigations show that ^the distribution of C. pengoi is patchy in the Gulf oi Riga' The main predator ol C' pengoi is Baltic t,eriing in the Gulf of Riga. Despite of the abundant presence of copepods, the herring diet (d_group excluded) cJnsists mainly of the cladoceran c. pengoi. In several stations up to 1000/q of herring stomachs contained this species' C. pengoi plays a minor role in the diet of the three-spined stickleback, nine-spined stickleback, and smelt. The stomachs of the o-group Baltic herring, sprat, white bream, and bleak did not contain this cladoceran species' Key words: Gulf of Riga ecosystem, Cercopagis pengoi' INTRODUCTION Investigations in the Gulf of Qiga ec.osystem were started by.K.'Frisch ,ni-ij. -ni?koja in'it. tgzo.. Duri"ng.the"past 4,0 year,s severarl changes ir lfr" Ciiff of nlgr' ""or'it.m stru"cture have be6n reported, including .irung.r- i; trr. ifivt"prr-ir.t"" species.. composition and an increase in ;;i;;;; f ioJu.tioti; iecrea.se . and disappearanc.e 9i considerable number of zooptankion, loof.nthos, and fiih species. that require -unpol- luted water; rapid develorpmenl ;t heterotrophic miciobiolog]cal communities; decre'ise i; th;'catches of severa,l fiih species, etc' (Oja- ;;;;;^,itil;;"1y. in"." changes.are mainly caused by peiiodical 'local and gi"fLi .rln-lrfd nr"irrti*i ?"a by humair activities (ntul"t"Jl:tti*,t"T* ind rcsource expl;it;tion). On ihe_other hand. thes_e changes indicatc a labiie character ti-itr.'Cutt ol Riga ecosystem. The lability is also ;"i;;;;;d'tt-; itrunp outbreak of i new cl,adoceran rspecies, never ;;i;;; f*"a'in ti'r"-Cirfi of Riga. The presence ol.Cercopag,is (Cercopa-. ;;j- pt;i;;-in tt.r" C"rf .i nie.a] rys, foi ttre lirst time stated in July. and i;;fit f992 (Lumi-erg,-uni;nt. data). Howevef, the ecological role of thaT species remained unknowln. Another outburst of C. pengoi was ob'served in JuIy .and August lgg4. In l9g4 sorne ecological inlestigations were. per.lormed in o,rder to ,.r"r.- if-r" iote of lfris clidoceran spicies in the local marine food web' 20 Bot and ,th 1-3) 1994. : the ne VET J (large NCSS A from 0 cHl The l8e7). The and ta which eye dii the ma