Impact sf a T%dal Power Seation on Zospla&ton-Fish PniZteractB~~nsin Wmaa Basin Graham R. Daborn Gail S, Brown and Brian Scully Department of Biology Acadia University Wolfville, N.S. BOP 1x0 ABSTRACT Study of zooplankton populations in inner reaches of the Bay of Fundy, particularly in Minas and Cumberland Basins, has indicated that the zooplankton association is of limited diversity and highly contagious dis- tribution, All species are common estuarine ones that are either omnivor- ous or carnivorous and are utilized primarily by larval and early Juvenile stages of fish. With increasing size some fish switch to feeding on ben- thic prey which is seen as a reflection of the relatively greater abundance and/or productivity of the benthos in completely mixed waters, Construc- tion at the B9 site will reduce vertical mixing in some portions of the headpond and consequently decrease SPM levels and increase light penetra- tion. Increased phytoplankton production will compensate in part for de- creased production of benthic algae and provide a more varied food supply for zooplankton. Some increase in diversity, but not necessarily in abun- dance or production of zooplankton, is indicated as the detritus-based estuarine forms are augmented by algivorous species. Visual predation by zooplankton and fish is presently restricted by high turbidity. With de- creased SPM levels some new predators should extend into Cobequid Bey for feeding. Decreased tidal scour in many areas will allow development of a mixed deposit- and suspension-feeding benthos with indirect effects on the zooplankton. Competition for suspended food will probably limit increases in abundance of zooplankton within the headpond. A definite potential for aquaculture development is indicated. Key words: zooplankton, juvenile fish, turbulence, stratification, diversity, productivity, suspended sediments. Lq4tude de populations de zooplancton dans les appendices rentrantes de la baie de Fundy, particulisrement dans les bassins des Mines et de Cumberland, a rdvdld que l'association du zooplancton a une diversit6 lfmitge et une rdpartition fortement contagieuse. Toutes les espe'ces sont courantes dans les estuaires et elles sont soit omnivores ou carnivores; elPes servent surtout au poisson 3 l'dtat larvaire ou nouvellement juv6nfle. A mesure que leur taille augmente, certains poissons coinmencent 2 se nourrir de proies benthiques, ce qui est considgr6 come un reflet de 19abondance ou de la productivit4 relativement sup4rieures du benthos en eaux entisrement mglang4es. Les travaux de construction au site B9 rk- duiront le mdlange vertical dans certaines portions du bassin de chute, ce qui abaissera les niveaux du SPM tout en accroissant la pkngtration de la lumi3re. T4a production accrue de phytoplancton compensera en partie la baisse de production des algues benthiques, tout en fournissant une alimen- tation plus vari&e au zooplancton. On a constat6 une certaine hausse de la diversit&, mais pas ndcessairement de l'abondance ou de la production du ; qooplancton, puisque les formes estuariennes reposant sur les dktritus sont augmentges par des espdces algivores. La prgdation par le zooplancton et le poisson est actuellement emp6ch4e par une forte turbidit4 qui rend la visibilitd mauvaise. Avec la baisse des niveaux du SPM, certains nouveaux prgdateurs devraient s'avancer dans la baie Cobequid pour s'alimenter. La diminution de ltaffouillement tidal dans de nombreux secteurs favorisera la formation d'un benthos B alimentation mixte d6pdt-suspension, avec effets indirects sur le zooplancton. La comp6tition pour les aliments en sus- pension limitera probablement l'augmentation quantitative du zooplancton 2 lVint6rieur du bassin de chute. Un potentiel assur4 de d6veloppement de l'aquaculture a 4t4 constatg. INTRODUCTION At the present time, information on species compositions and bio- logical interactions in the water column of Cobequid Bay, east of the Econ- omy Point-Cape Tenny line, is extremely sparse. Jermolajev (1958) reported on a few zooplankton collections in this area and others were obtained at an anchor station in August 1979 as part of the series of cruises of C.S.S. Dawson (Daborn 1984). Much more information is available for other areas, however, notably for Cumberland Basin, Shepody Bay and the Southern Bight of Minas Basin. To the extent that dynamic processes are similar in Cobequid Bay to those in physically comparable regions elsewhere, some ten- tative predictions can be made regarding the impact of the proposed B9 bar- rage at the mouth of Cobequid Bay. Studies of zooplankton populations in the inner reaches of the Bay of Fundy system have indicated that zooplankton associations are of limited diversity and highly contagious distribution. Most of the species are com- mon estuarine ones that are omnivorous and capable of subsisting upon non- living particulate matter and its associated microflora. A few common spe- cies are carnivorous, depending upon physical contact with prey for suc- cessful capture. In the most turbid localities such as Cumberland Basin and the Cornwallis Estuary, visual predators are uncommon or completely absent. Similarly, knowledge of the ichthyofauna of the more turbid areas is fragmentary. Although less than 50 species of fish have previously been recorded from the Minas Basin (Bromley 1979) more recent work suggests that there may be more and it is equally apparent that utilization of Cobequid Bay and other portions of the system by juvenile alosids and other migra- tory specfes is very extensive (Dadswell et al. 1984). The zooplankton association is utilized by larval and early juvenile stages of fish (Imrie and Daborn 1981, Gilmurray and Daborn 1981). With increasing size however, fish often switch to feeding mainly on benthic organisms, particularly vagile species, an apparent reflection of the much greater prod~lctionof the benthos than the plankton in these turbid macrotidal estuaries. In Minas Basin and Cumberland Basin at least, benthic animals represent a far greater and perhaps more acceptable food supply. Although the benthos of Cobequid Ray appears to be somewhat less productive than either Cumberland Basin or the Southern Bight of Minas Basin, existing data do indicate ex- tensive utilization of the benthos in that area by migrant and resident juvenile fish. IMPACT OF THE PROPOSED BARRAGE The major conclusion of our studies in recent years is that the turbid inner regions of the Bay of Fundy system are examples of extreme physical stress. High suspended particulate matter (SMP) levels are a re- sult of turbulent mixing associated with large tidal range and strong cur- rents. Consequently, any modification of physical conditions is expected to produce extensive changes in biological processes. Construction of the B9 barrage from Economy Point to Cape Tenny, and subsequent operation of the power station in an ebb-generation regime, will moderate considerably the extensive vertical turbulence now prevailing in Cobequid Bay. Much of the present-day energy of the flooding tide will be dissipated against the barrage itself and in raising the reservoir level. Consequently, it is expected that SPM levels in the upper portion of the headpond water column will be considerably lower and the euphotic zone much greater than the present. There may be some Stratification at the surface as presently occurs in the Annapolis headpond (Daborn et al. 1982). However, the annual freshwater input to Cobequid Ray is very small relative to the tidal prism and hence the surface low salinity layer may be shallow and stratification readily broken down by wind action. On the sea- ward side of the barrage the tidal range is expected to be slightly reduced and hence the euphotic zone marginally increased. Within the headpond, increased light penetration, coupled with an adequate (if not abundant) nutrient supply (Keizer 1984) should allow much greater phytoplankton production, particularly in the presently more turbid peripheral portions of Cobequid Bay (Hargrave 1984). Increased pelagic primary production, based on small flagellates and dinoflagellates, should allow an increase in zooplankton diversity as algivorous species common in the outer Bay of Fundy (cf. Fish and Johnson 1937, Roff 1983) will also be able to survive. Since the dominant zooplankton species present in Cobequid Bay are common omnivorous estuarine ones (Daborn 1984), none should be eliminated from the headpond by the changed- conditions although- the relative abundance of some (e.g. Eurytemora herdmani and Acartia tonsa) may well change. Similarly, some predatory zooplankters, such as Tortanus discaudatus and Oithona sirnilis, will become more prevalent and abundant. In all likelihood, the increased diversity of the zooplankton asso- ciation will be accompanied by a decrease in annual secondary productivity, despite the rise in primary production. Although nutrients are presently abundant, increased utilization will eventually lead to a nutrient-limited regime, exacerbated by a decrease in remineralization rates as the inter- tidal zone and tidal resuspension will be much reduced. Furthermore, the zooplankton presently exhibits an inverse relationship between light pene- tration and biomass. Minimum biomass and (presumably) production of zooplankton is found in Minas Basin and Chignecto Bay where SPM levels are less than 50 mg L-I (Daborn 1984). In contrast, maximum biomass values are found in the most turbid waters such as Cumberland Rasin and the Cornwallis Estuary where