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 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 , 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, and the Southern Bight of . 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 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 , 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 . 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 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 SPM levels often exceed 1 g L- 1 . BIomass values, however, are not in themselves measures of production and only in the Cornwallis Estuary is the relationship between biomass and production close to being established. In most regions the highly contagious distributions of zoo- plankton species make even estimates of mean abundance impossible,

In Minas Basin, on the seaward side of the barrage, the increased euphotic zone will have little direct effect on zooplankton production and diversity. Secondary effects, however, may well be observed, particularly during the first few years following commencement of construction. Changed patterns of water movement and current velocity will undoubtedly result in new patterns of sediment deposition. Some present intertidal areas will be subject to greater current scour, whereas others will experience new depo- sition. Consequently there will be extensive changes in the relative abun- dance of benthic organisms, many of which have planktonic larvae. At certain times of the year, and particularly over productive , the holoplankton is overwhelmed by very large numbers of meroplanktonic forms such as trochophore and veliger larvae. It is probable that during and for some years after the constructive phase notable variations will occur from year to year in release of plankton larvae. This will make generalizations about the abundance, biomass and production of zooplankton virtually impossible. At present many of these meroplanktonic forms are cropped by predatory zooplankters such-as Labidocera aesrtiva and Weomysis americana; in the latter instance, predation on such forms may compensate for the expected decrease in non-living particulate food derived from intertidal and supratidal areas.

Effects on the zooplankton-fish interaction are predictable in very general terms. Present research indicates that most fish species utilizing the Minas Basin-Cobequid Bay system as larvae subsist upon the zooplankton, particularly Eurytemora hermani, but subsequently switch to benthic forms. Within the headpond, we would expect that planktivorous forms such as Menfdia menidia, Gasterosteus aculeatus, Clupea harengus, Alosa aestivalis and A. psuedoharengus will persist as planktivores as long as planktonic foodTs available. Benthic feeders such as Liopsetta putnami, Pseudo- pleuronectes americana and Microgadus tomcod will presumably continue to utilize the benthos, although the size of the intertidal resource will be reduced by the reduction of the intertidal zone. Post-construction con- ditions should favor more planktonic feeding both because of increased light levels (allowing visual feeding) and because a greater proportion of available primary production in the headpond will be intercepted within the upper portion of the water column. For some species, at least, the probable decline of larger zoo- plankters, such as the mysids, associated with increased light penetration and decreased detritus food may well be a significant change. In particu- lar, the American shad, Alosa sapidissima, feeds largely by straining large zooplankters such as Neomysis from the extremely turbid areas such as Cumberland Basin (Dadswell et al. 1984) and may not find such suitable con- ditions behind the barrage.

CODA

As indicated above, some general statements regarding the effects of the B9 proposal on zooplankton, young fish and zooplankton-fish Inter- actions can be made. It seems certain that direct effects of the barrage will be of a small scale and may well be positive in many respects. In- creased primary production in the headpond will probably be accompanied by greater planktonic diversity and utilization by young fish. The general trend will be toward the light-based estuarine pelagic community that is more commonly encountered elsewhere and away from the strongly stressed regime of the present. Although not an accurate or adequate paradigm, studies of the Arznapolis Estuary have proved instructive (Redden et al. 1984).

It should also be noted that increases in phytoplankton production might well offer opportunities for aquaculture within the headpond that would compensate in part for some of the losses of intertidal production that are anticipated. At present Cobequid Bay appears to be free of para" lytic shellfish poisoning that affects soft-shelled clam and mussel beds En the outer Bay of Fundy. Although the winter conditions may prove difficult, a raft-type culture of mussels (for example) might be feasible in the headpond. ,

ACKNOWLED (XENTS

This account has been derived from the accumulated studies of many people that we wish to acknowledge with thanks. They include: L- Arsenault, D. Calquhoun, P. Crawford, A. Evans, M. Frame, G. Gibson, R. Gregory, D. Irmie, H. Leslie, A. McDonald, G, McQuarrie, C. Pennachetti, A. Redden, P. Reid, F. Rogers, N. Rogers, J. Slater, K. Strong, and J. Williams. Financial support has been provided through grant A9679 from the Natural Sciences and Engineering Reasearch Council, through grants 2-R41 and 4-R71 from the Canadian National Sportsmen's Fund, and through funds from the Tidal Power Corporation. To all of these we are mos fl grateful.

REFERENCES

Bromley, J.E.C. 1979. A preliminary checklist of marine fauna of Minas Basin and Minas Channel. Proc. N.S. Inst. Sci. 29: 517-541, Daborn, G.R. 1984. Zooplankton studies in the upper Bay OF Fundy since 1976. In this report.

Daborn, G.R., A.M. Redden and R.S. Gregory. 1982. Gcologfcal studies of the Annapolis Estuary 1981-1982. Acadia Univ. Insitute Publ. No. 29, 80p.

Dadswell, M.J., J. Williams and G. Brown. 1984. Possible impact of large scale tidal power developments in the upper reaches of the Bay of Fundy on certain migratory fish stocks of the western North Atlantic. In this report.

Fish, C.J. and M.W. Johnson. 1937. The biology of the zooplankton popula- tion in the Bay of Fundy and with special reference to production and distribution. 5. Biol. Bd. Can. 3: 189-321.

Gilmurray, M.C. and G.R. Daborn. 1981. Feeding relations of the Atlantic silverside, Menidia menidia, in the Minas Basin, Bay of Fundy, Mar. Biol. Progr. Ser. 6: 231-235.

Hargrave, B.T. 1984. Barrage effects on phytoplankton production and chemosynthesis in Cumberland Basin. In this report.

Irmie, D.M.G. and G.R. Daborn. 1981. Food of some immature fish of Minas Basin, Bay of Fundy. Proc. N.S. Inst. Sci. 31: 149-154.

Jermolajev, E.G. 1958. Zooplankton of the inner Bay of Fundy. J. Fish. Res. Bd. Can. 15: 1219-1228.

Kefzer, P.D. 1984. Potential effect of tidal power development on the chemical oceanography of the Ray of Fundy. In this report.

Redden, A.M., G.R. Daborn and R.S. Gragory. 1984. Ecological aspects of the Annapolis Estuary with specific reference to operational effects of the Annapolis Tidal Power Station. In this report.

Roff, J.C. 1982. The microzooplankton of the Quoddy Region. Unpubl, manuscript, 37pp.

QUESTIONS AND COMMENTS

Unknown: What do you think causes the great abundance of Corophium in fish stornachs?

G. Daborn: It is probably due to the abundance of Corophium on tidal flats and turbulence caused by wave action and the rising tide.

M. Dadswell: Most amphipods become pelagic or semi-pelagic when they are adult. If stomach contents were examined closely for the quality of Corophium it may be that most are adults that had been swimming actively in the water column. G. Daborn: We have examined the sex ratio of the Corophium in guts but we didn't see any predominance of males or females. --Unknown: Do you think if food became a limiting factor under new con- ditions that the benthos could compete with zooplankton for it?

6. Daborn: The reality I see is that in shallow water the capacity for benthos to strain water is infinitely greater than zooplankton. If there is a limitation on food and its utilization from time to time, I see no season why the benthos would not be very effective in competing with zoo- plankton.

D. Gordon: Do you have any feeling concerning whether Cobequid Bay will ?urn out to be low as far as zooplankton production is concerned when com- pared to other parts of the upper Bay of Fundy.

G. Daborn: I believe it depends on the availability of primary production. Zooplankton can use either primary production or detritus particles from primary production for feeding. In the Cornwallis River system where SPM concentrations are up to 5 g L-I, a small animal can filter a large amount of particles but I do not think they can depend on non-living particles to survive and grow. I find it difficult to say that Cobequid Bay will be any less productive than other parts of the upper Bay of Fundy. I think in Minas Basin and the Southern Bight abundances appear to be lower than other areas, perhaps Cobequid Bay is similar.

J. Lakshminarayana: Do you think inside the headpond you will have greater diversity in the stratified regions?

6. Daborn: In general, the relationship between diversity and distance in- side the headpond ,is straightforward and dominated by salinity. However this relationship is complicated by the fact that if you have a stony or gravelly substrate diversity of benthos tends to be higher. So wherever that is situated in relation to the causeway it will change the pattern.

J. Lakshminarayana: What about diversity or faunal differences on the in- side and outside of the Annapolis Causeway?

G. Daborn: You are comparing the upstream side of the Causeway to the downstream side? Well, of course, the degree of scouring downstream is much greater than on the upstream side of the Causeway. The downstream side empties out fairly completely at low tide and current velocities are high. This adds a physical stress there which is considerably greater than above the Causeway. The causeway doesn't act as a barrier to fauna and there is considerable vertical mixing there. Stratification is mostly broken down within 1/2 km of the Causeway. There is enough turbulence to keep it mixed.