Small-scale zooplankton community variability in a northern British Columbia fjord system

Estuarine, Coastal and Shelf Science (1986) 22,115-142

Small-scale Zooplankton Variability in a Northern Fjord System

David

L. Mackas”

and Edward

Community British Columbia

P. Andersonb

“Institute of Ocean Sciences, P.O. Box 6000, Sidney, British Columbia V8L 4B2 and bEdward Anderson Marine Sciences, 1 O-261 4 Bridge Street, Victoria, British Columbia V8T 1 W6 Canada Received 10 September 1984 and in revised form

Keywords: community composition; British Columbia Coast

14 March 1985

zooplankton;

fjords; suspended particles;

We have used multivariate classification, ordination, and discriminant function analyses to describe the small-scale spatial pattern of zooplankton community composition in a northern British Columbia fjord system. Finely ground tailings from a molybdenum mine were discharged into one branch of the fjord (Alice Arm) during a 17-month span which included our two sampling periods (August 1981 and June 1982). Our study had two major objectives. The first was to learn whether or not the zooplankton community was strongly altered in the vicinity of the tailings discharge, and the second was to characterize the intensity and dominant spatial grain scale of ‘ normal ’ community variability in the remainder of the deep inlet system. Zooplankton were collected by water column integrating oblique net hauls. The statistical analyses examined relative and absolute changes in the contribution of individual species to the local zooplankton community. Within each time period, the range of spatial variability was small in comparison both to the between-time period (probably seasonal) differences within the inlet, and to the range of spatial variability observed by identical analysis methods in open continental shelf waters to the south. However, despite this small overall community variance, we found a reasonably consistent regional partitioning of the inlet; the samples most similar in composition tended to be those from the same or adjoining locations. Sites near the turbid heads of the branching inlet (including those near the tailings discharge) had higher concentrations of Euphausia pacifica and other large non-copepod zooplankton, and relatively low concentrations of the copepod Neocakanus plumchrus. The magnitude of the between region-differences was small and did not appear to be increasing with time; we therefore conclude that the impact of tailings discharge on zooplankton community structure was relatively minor. The distance over which neighbouring samples show significant compositional correlation is smaller in the inlet than in open coastal waters previously studied; we interpret this as due largely to the absence of meso-scale horizontal turbulent eddies within the inlet. Introduction The impacts

of environmental

perturbations

on ecosystems

- whether

by natural

causes

or by human activity - are often assessedin terms of changes to the number of species and their relative dominance hierarchy in the ‘ natural ’ (i.e. unperturbed) system. When 115 0272-7714/86/010115+28$03.00/0

0 1986 Academic

Press Inc. (London)

Limired

116

D. L. Mackas &E.

P. Anderson

the perturbation is extremely severe, modification of the community structure can occur by direct lethality. But at lower stresslevels, major changesin the speciesbalance are at least as likely to arise through sublethal changes in the components of adaptive fitness (e.g. feeding success,reproductive rate, and vulnerability to predation). Because it is difficult or impossible to perform experimental evaluation of the full suite of responses (by several to many species, and over a range of potential stressconditions), we believe that it is useful to measurethe community composition in the temporal and spatial vicinity of the perturbation and test for variability which might reasonably be attributed to the stress. Sundberg (1983) gives a good justification for the use of multivariate statistics to characterize community change and a brief review of marine pollution applications. Any observed community change can be used to direct subsequent experimental effort to speciesthat appear sensitive and to physiological or ecological interactions that appear important. This study examines the spatial pattern of zooplankton community composition in the upper reaches of Observatory Inlet, a fjord system located on the north-west coast of British Columbia. The Amax/Kitsault Molybdenum Mine dicharged tailings into one branch of the inlet (Alice Arm) over a period of 17 months from April 1981 to November 1982. The tailings were introduced at 50 m depth in an attempt to reduce the risk of contamination or depletion of the biological production of the inlet. This largely prevents direct contact of the tailings with photosynthetic primary producers in the inlet (the euphotic zone is much shallower than 50 m due to surface layer turbidity from natural inputs of river-borne sediment and from near-surface phytoplankton populations). However, many of the resident zooplankton perform daily and/or seasonal vertical migrations which can carry them into depth zones with significant concentrations of suspended tailings (concentrations in the order of 2-100 mg 1-i US.subsurface ‘ background ’ levels of 05-l .Omg l- ‘). We designed this study to detect anomalies in the zooplankton compositional pattern in the vicinity of the tailings plume. This was accomplished by examining the intensity and spatial scale dependence of community variability throughout the inlet system. Data from unperturbed areas provide an estimate of the intensity and spatial distribution of ‘ normal ’ variability against which community shifts near the tailings discharge can be compared. We also compare the statistical characteristics of the inlet community pattern with results from earlier and similar observations (Ma&as & Sefton, 1982; Ma&as, 1984) of the open continental shelf system off southern Vancouver Island (roughly 600 km to the south). This allows us to draw somegeneral conclusions about differences in the causesand consequencesof the biological pattern between the two regions. The Observatory Inlet fiord system is located on the Pacific coast of British Columbia (Figure 1). Its mouth opens onto Portland Inlet about 40 km from open water at 55”N 13O”W. The inlet extends inland among mountainous terrain about 55 km, and terminates by branching into Alice Arm and Hastings Arm (19 and 26 km long respectively). The largest freshwater input to the inlet is from the Nass River (annual discharge about 2 x 10” m3), which enters the fjord a few kilometers from the mouth of Observatory Inlet and immediately seaward of the 60-m deep sill. Smaller rivers enter at the headsof Alice and Hastings Arms, and a number of streamsfeed in along the sidesof the inlet. All of the major sourcesof freshwater have peak discharge from snow melt in early summer and a secondary maximum associated with heavy autumn rains. The inlet system is generally steep-walled and deep. Depths in most of Observatory Inlet range from 350 to 500 m, while the main basins of Alice and Hastings Arm average about 350 and 300 m

Zooplankton

community pattern in a fjord

,z9*50’

117

129.40’

IZP30'

15'30'

ALICE

ARM

i5.20'

OBSERVA

TOR Y

Figure 1. Map of the study area showing sample locations. Shading indicates bottom depth shallower than 100 m. The tailings discharge from the AMAX/Kitsault mine is south-east of sample site M9 at a depth of 50 m.

respectively. A shallower basin, lo&150 m deep, lies between Granby Bay and Larcom Island off the deserted townsite of Anyox (formerly a copper mining and smelting centre). Sills at the south end of this basin (118 m) and at the mouths of Alice Arm (25 m) and Hastings Arm (63m) restrict deep water exchange between the basins and with Observatory Inlet.

118

D. L. Mackas Q E. P. Anderson

Freshwater inputs greatly exceed evaporation from the inlet system, giving a low salinity, seaward flowing surface layer about 5 m thick. This seaward flow is compensated by up-inlet flow in the deeper layers. However, the highest current speedobserved in Alice Arm (c. 20-40 cm s-l, Burling et al., 1983) are associatedwith tidal exchange. These currents give oscillatory horizontal displacementsof up to 5-6 km over each tidal cycle; the largest displacementsoccur near the mouth of the inlet and above sill depth. Most research on the oceanography of the region was motivated by the opening of the AMAXKitsault mining operation and is not yet in the open-literature. Pickard (1961) reports salinity, temperature, and oxygen concentration from the deep layers of Alice Arm; and Gardner (1980) included samplesfrom Portland Inlet in a survey of zooplankton communities that spanned the British Columbia (B.C.) coast. The most complete compilations to date are in assessmentdocuments by Littlepage (1978) and Burling et al. (1981,1983), and in the annual environmental reports of the mining company (AMAX, 1982, 1983). A summary and interpretation of the regional physical oceanography is in preparation by D. Stucchi of the Institute of Ocean Sciences. A recent symposium volume edited by Freeland et al. (1980) provides an overview of fjord oceanography together with a number of casestudies from other inlet systems. During its 17-month operational period, the AMAX/Kitsault mine discharged 4.1 x 10’ kg of finely ground tailings through an outfall at 50 m depth near the head of Alice Arm (see Figure 1). Tests performed for the mining company indicate that suspensionsof fresh tailings are not acutely toxic to fish. Most of the tailings appear to go directly to the bottom of the inlet as a turbidity current; however, a fraction forms plumes of suspendedsediment above the seabed and at intermediate depths of 65-125 m (deepening down inlet). Concentrations in these plumes are c. lO-20mg 1-i near the

discharge but decrease to c. 2-5 mg l- ’ or less within 5 km down inlet from the outfall (Burling et al., 1983). For comparison, surface layer sediment loads introduced by the rivers are c. 20 mg 1- i in the upper inlet during peak runoff periods. Only a few previous publications describe the effects of suspended sediments with high metal content on marine zooplankton communities, and their results are mildly contradictory. Paffenhoffer (1972) found reduced growth and survival of Calanuspacifycusexposed in the laboratory to suspendedbauxite tailings. Wiebe et al. (1973) compared zooplankton community composition between a dumpsite in the New York Bight receiving acid-iron waste and a nearby control area. Community resemblance was high both within and between regions; the relatively small differences in biomass and community composition were attributed to random patchiness. Hirota (1981) concluded that there would probably be no major mortality of copepods resulting from resuspensionof deep ocean sediments during mining of manganesenodules. A considerable body of evidence supports the principle that communities simplify and regress towards an earlier successionalstage in response to a generalized pollutant stress. However, the proximal mechanism and direction of specieslevel responseis often strongly modified by interactions between competing speciesand between trophic levels (e.g. Menzel, 1977). Methods Sample collection and enumeration Samples were taken during August 1981 and June 1982 with flow metered ClarkeBumpus opening/closing nets (12.5-cm mouth diameter, 0.39-mm mesh). Filtered volumes typically ranged 4-10 m3 per tow. We did not estimate capture and retention

Zooplankton community pattern in a fiord

TABLE

1. Tow depth

and number

of samples

119

per site for the August

1981 and June 1982

cruises Tow depth (m)

Site code

No. samples Aug.

1981

June 1982

Alice Arm

OB4 OB3 OB2 OBI HA1

No. samples Aug.

1981

June 1982

Hastingf Arm

c3 G5 K7 M9 013 Q19 P25 N32 M40 M53 V66 Observatory

Site code

Tow depth (m)

85 90 105 145 180 240 280 270 350 350 200

0 1 1 5 2 3 1 2 3 1 3

455 470 270 190 165

0 0 1 2 2

HA5 HA4 HA3.8 HA3.7 HA3.4 HA3 HA2.5 HA2 HA1.8 HA1.5

125 170 175 225 255 270 250 300 260 205

0 0 1 0 0 0 1 0 2 1

2 2 0 2 2 2 0 2 0 0

115 85

1 0

3 2

2

GranbylLarcom

Inlet

GRl GR2

efficiencies. However, we observed no clogging of the net by phytoplankton (which might cause variation in the effective mesh size and hence sample-to-sample variation in retention efficiency). We therefore assumedthat any gear bias was uniform between samplesand did not contribute to between-site differences in fauna1 composition. Tows were oblique from near-bottom to surface at about 2 kts (1 m s-i). Vertical integration of the water column was chosen to minimize sample-to-sample variability due to diurnal vertical migration and/or displacement by internal waves of the zooplankton vertical structure. Figure 1 shows sample locations within the study area. Table 1 gives the tow depth and the number of replicate samplesper cruise at each site. In the first series,most of the sampling effort was concentrated within the Alice Arm branch of the fiord system, and all sampleswere collected during daylight hours. The spatial distribution of effort was more uniform in the June 1982 series and sampleswere collected primarily at night. Because of the ship use scheduling, we were able to obtain day : night sample pairs at only two locations. However, the results suggests little or no light aided sampler avoidance (night : day catch ratio 1.75 for euphausiids and 0.82 for chaetognaths). Both sampling periods were during normal tailings discharge conditions (c. 8-5 x lo6 kg day-‘). The zooplankton sampleswere preserved in 5O/bborate-buffered formalin. All animals longer than 5 mm were sorted from the whole sample, identified and counted. The remaining fine fraction was reduced, if necessary, with a Folsom splitter to obtain a subsample of at least 250 individuals. These were identified to species,developmental stage, and sex, and counts were converted to abundance per cubic meter of water sampled. Data reduction and statistical analysis For each of the two cruises, the source data consist of abundance estimatesfor each of the taxa in each of the samples. These are tabulated in Anderson & Mackas (in prep.).

120

D. L. Mackas l3 E. P. Anderson

Selection of taxa to be used in the analysis (Table 2) and further data reduction followed the protocol outlined in Ma&as 81 Sefton (1982). To reduce the effect on the analysis of numerical dominance by small-bodied forms, the abundance data were converted to approximate ‘ biomass ’ estimates using taxon specific multiplicative factors roughly proportional to the cube of population mean body length (Table 2). Our goal was to examine the spatial variability of dominance hierarchy in the zooplankton commtmity, and to detect any regional anomalies coincident with the distribution of suspended tailings. To do this, the statistical analyses were directed toward the following specific questions: (a) What is the overall (i.e. average) pattern of species dominance within the zooplankton community? This was answered by ranking the taxa according to their local and mean (over all samples) contribution to total abundance and to total estimated biomass. (b) Do the deviations of the dominance pattern at individual stations from the overall average suggest clusters of biologically distinct sample groups and/or a gradient in compositional response to spatially varying environmental conditions? This was answered by multivariate cluster and ordination analyses following the methods published by Ma&as and Sefton (1982), and summarized below. (c) Given a prior definition of station groups on the basis of their location in the fiord system, for which station group comparisons are the compositionaldifferences statistically significant? Which taxa (on both an absolute and a relative abundance scale) are most important in discriminating one set of stations from another? These questions were approached through linear discriminant function analysis (Anderson, 1958). (d) Does the community variability show spatial autocorrelation? To what extent and over what range of separations does spatial proximity imply similarity in composition? These questions are important for at least two reasons. First, the grain scale of spatial patchiness determines the range and temporal stability of conditions experienced by individual animals living in and moving through a given environment. The adaptive fimess of the species forming the local community (and thus the eventual community make-up) are in part controlled by their ability to deal with this local variability. Secondly, for purposes of environmental impact assessment, it is important to know the statistical characteristics (both scale and intensity) of’ natural ’ community patchiness so that these can be compared with any variability observed in the vicinity of an artificial perturbation. We have quantified the local average relationship between sample separation and community resemblance by calculating spatial correlograms for each cruise. The methods duplicated those published by Ma&as (1984) and are briefly summarized below. The above analyses are best viewed as simplifications and summa ries of the information contained in the original sample counts. This simplification is an important aid to identification of abundance patterns that are shared by several species; dimensionality of the raw data is too high to allow a complete appreciation of these patterns simply by scanning the species lists. We have tried to avoid potential misinterpretation of the results by performing the above variety of analyses with a further variety of detailed treatments (e.g. several clustering alogorithms, absolute abundance ws. relative dominance, differing station groups for the discriminant function analyses). Classification and ordination analyses both summarize the distribution of sampling

Zooplankton

community pattern in a fiord

121

sites in a multidimensional ‘ species space ’ geometry in which the co-ordinates of each sample are defined by the abundance of each species. Although classification and ordination may eventually lead to similar interpretations of the data, they assume differing structures of between-sample resemblance. Classification (clustering) methods assume that the samples group naturally into a reduced number of swarms ( = clusters), with the members of each group sharing a characteristic and relatively stable species occurrence and dominance hierarchy. In general, classification analyses are good at resolving the detailed structure of the data by identifying small groups of similar samples. Description of the coarser (low resemblance) structure is less adequate unless the data structure is disjoint (i.e. there is a pronounced natural grouping into a small number of manymember clusters). Mackas & Sefton (1982) showed that this disjoint structure was present in zooplankton and phytoplankton samples from the open coast off southern Vancouver Island (roughly a 100 x 200 km rectangle). Ordination methods such as principal components analysis allow a continuum of variability (samples need not form natural groups) and seek to identify a reduced number of component axes onto which most of the intercorrelated species variability can be projected. However, they make the occasionally restrictive assumption that the pattern of species covariation is linear. When this assumption is violated (as, for example, when one or more species have their highest abundance part of the way along an environmental gradient) a sequence of samples along the gradient will give a curve (rather than a straight line) in component space (see Whittaker, 1973, for a critique of this property of linear ordination methods). The major risk (which we have avoided) is the interpretation of a one-to-one association of component axes with separate and causal environmental gradients. In general, the major (as opposed to the detailed) features of the resemblance structure are well preserved by the separation of samples in component space (Orloci, 1978). In this sense, the classification and ordination results are often complementary. More extensive discussions of multivariate methods are available in a number of reference texts (e.g. Orloci, 1978; Pielou, 1969; Sneath & Sokal, 1973; Whittaker, 1973). The cluster and autocorrelation analyses require calculation of a measure of compositional ‘ resemblance ’ (see Sneath & Sokal, 1973) between each pair of samples. We use Orloci’s ‘ squared chord length ’ index d2 (Orloci, 1978; Mackas & Sefton, 1982). Like the ‘ percent similarity index ’ (e.g. Whittaker, 1973), d2 measures changes in relative dominance but filters out changes in biomass that are shared proportionately by all species (our choice of d2 was based largely on the fact that it is a sum-of-squares and hence more directly comparable with other variance/covariance statistics). We have also observed empirically that the relative composition transformation gives a ‘ well behaved ’ measure of between-sample variability through its tendency to normalize the strongly skewed distribution typical of single species abundance or biomass data. The d2 index is used directly in the autocorrelation analysis and in clustering of samples by the ‘ minimum variance = sums of squares ’ algorithm, (Orloci, 1978). We report clustering level from this method (hereafter labelled as ‘ SS ‘) as within-cluster mean-squared distance from cluster centroid, i.e. as the variance about the centroid. We also used centroid linkage clustering (Sneath & Sokal, 1973, Orloci, 1978) based on two different similarity measures, one derived from the d2 index (results labelled ‘ ALS ‘) and on the other Fasham & Angel’s (1975) ‘ coefficient of proportional similarity ’ (results labelled ‘ CPS ‘). (See Mackas & Sefton, 1982, for full algebraic definitions of the clustering procedures.) The three clustering methods will normally diverge slightly in their classification of the samples (particularly at the higher clustering levels in which the merging

1981

abundance

44.1 21.9 6.38 0.01 87.8 32.3 0.01 0.90 O-89 0.32 4.18 21.3 0.18 1.81 0.12 0.20 0.63 0.01 2.88 0.24 0.42 0.51

Aug.

Mean

113.4 170.7 53.7 0.84 145.1 44.1 0.69 1.35 0.70 12.3 9.11 3.43 39.3 0.68 0.63 0.17 0.84 1.0 0.05

June 1982

(no. m - 3 Aug.

100 100 100 3 100 100 3 75 88 47 97 97 16 94 16 28 41 3 84 34 31 28

1981

(% )

98 100 100 32 100 100 11 84 77 84 89 59 98 34 27 2 30 36 2 -

June 1982

Rate of occurrence

3.0 0.55 1.4 1.0 0.3 0.2 0.2 4.0 4.0 0.5 0.4 0.1 0.18 0.3 1.0 1.4 1.0 1.0 1.0 1.0 1.2 0.5

Size coeflicient

period, biomass

1982

53.0 4.8 3.6 0.0 10.5 2.6 0.0 1.4 1.4 0.1 0.7 0.9 0.0 0.2 0.0 0.1 0.3 0.0 1.2 0.1 0.2 0.2

Aug.

rate

47.5 13.1 10.6 0.1 6.1 1.2 0.0 0.8 0.4 04 0.5 0.0 1.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.0

June 1982

O/o total estimated biomass

of taxa included in the analysis and their relative importance as indicated by mean abundance within-time (fraction of samples in which the species is present), and estimated contribution to the total zooplankton

Copepods Metridia okhotensis Metridia paci$ca Calanus marshallae Calanus pacificus Pseudocalanus minutus Oithona spinirostris Oithona sim’lis (= helgolandica) Neocalanus plumchrus Euckaeta elmata ( = japonica) Centropages abdominalis Aetideus divergens ( = armatus) Acartia clausi Acartia longiremis Scolecithricella minor Bradyidius sp. Candacia columbiae Ckiridius gracilis Epilabidocera kmgipedata ( = amphitrites) Gaetanus sp. Gadius variabilis Ueterokadus tyanneri Scaphocalanus brevicowais

Taxon

TABLE 2. List of occurrence

Non-crustaceans Sagitca Lima&a Larvaceans Tomopterid polychaete

and Podon)

0.18 0.82 2.41 0.07

47.2 9.20 0.06

0.25 0.03 0.01

Euphausiids Euphausia pacijica Tkysanoessa longipes Thysanoessa spinifera

Other crustaceans Conckoecia sp. Cladocera (Evadne Mysids

1.13 0.10 0.63 0.28 0.06

Amphipods Purarkemisro Euprimno Cyphocaris Calliopius Scina

3.52 15.7 6.45 0.01

25.4 1.55 0.01

I.30 0.17 0.75

044 040 0.57 0.30 0.06

28 66 91 22

100 97 25

44 9 3

72 41 59 69 28

98 82 84 7

96 29 2

96 46 69

80 25 57 46 25

2.0 0.3 0.25 4.0

0.5 0.1 8-O

10.0 10.0 10.0

8.0 8.0 8.0 8.0 8.0

0.1 0.1 0.2 0.1

9.5 0.4 0.2

1.0 0.1 0.0

3.6 0.3 2.0 0.9 0.2

1.0 0.7 0.2 0.0

1.8 0.0 0.0

10.2 0.2 0.6

1.1 0.4 0.6 0.3 0.0

124

D. L. Mackas & E. P. Anderson

groups have lower similarity). We find the multiple classification useful in providing a more balanced and robust interpretation of the resemblance structure. Ordination of the samples was by principal components analysis (PCA). This method attempts to project most of the total species variance (subject to strong covariance between species) onto a reduced number of statistically independent component axes. We performed R-mode analyses (Pielou, 1969; Orloci, 1978) on species covariance matrices calculated from both log transformed raw biomass estimates and from data normalized (as in the d2 calculation) to give relative dominance. The compositional basis for the resemblance structure can be interpreted from the species coefficients (often called ‘ loadings ‘) produced by the principal components analysis for each component axis. However, PCA attempts to maximize the ‘ explained ’ variance of the entire set of samples, rather than the separation of any particular pair of subgroups. As will be shown below, separation of regional groups is often along more than one component axis. Linear discriminant functions (Anderson, 1958) give a more direct description of the differences between one subset of samples and another. This method is usually used to assign ‘ new ’ samples to one or another of a set of ‘ known ’ multiple member classes. However, discriminant functions are also useful for summarizing and testing the statistical significance of differences between externally defined sub-groups of a set of ‘ unknown ’ samples. The method proceeds in several steps. The first is a prior (and assurhed proper) classification of some or all of the samples into the desired number of groups. These groups form a ‘ learning set ‘, and allow calculation of the most distinctive criteria (i.e. weighting factors for differences in the amounts of the various species) for pairwise separations between the defined sample groups. The method assumes between-group uniformity of the within-group variability and covariability of species concentration but is reputed to be robust to violations of this assumption (Sneath & Sokal, 1973). Differences of group means are compared with the average within-group compositional variance in a manner analogous to a univariate ‘ t ‘-test. The method also estimates the relative contribution of each taxon to the overall discrimination between clusters. To provide our ‘ learning set ‘, we divided the samples into four spatial groups on the basis of proximity and/or similarity in water depth (Figure 7). These correspond to: (1) sites near the heads of inlets and in the shallow channel west of Larcom Island, (2) ‘ inner ’ Alice Arm (near the tailings discharge), (3) ‘ outer ’ Alice Arm, and (4) the remaining locations in outer Hastings Arm and Observatory Inlet. The groups could arguably be further subdivided. However, the tests of statistical significance are dependent on having a relatively large number of individual samples in each classification category and a relatively small number of discrimination criteria (= taxa). Otherwise, the classification will lack degrees of freedom and may in extreme cases be over-determined. To produce the results reported here, we further reduce our species list to 13 taxa by retaining the species individually responsible for > 1 o/0 of the total estimated holoplanktonic biomass and by merging the remainder into coarser taxonomic groups. This approach preserves the major shifts in species dominance hierarchy but sacrifices information on abundance and occurrence shifts among the rarer taxa. We again ran separate analyses for log transformed and normalized data. Both methods showed similar significance levels for the separation of spatial groups. We have reported the results from the log transformed data. This version allows the abundance of the various taxa to be judged independently, whereas the normalization used in the classification analyses looked at relative dominance at each site. The autocorrelation analysis consisted of a comparison of the squared chord length

Zooplankton community pattern in a fiord

125

dissimilarity with spatial separation along the midlines of the inlet branches. The d2 estimates resulting from single pair wise comparisons of samples are highly variable. To reduce this scatter and obtain a more stable estimate of the average change in community resemblance with spatial separation, we have followed Mackas (1984) and averaged the dissimilarity estimates from a reduced number of spatial separation intervals ranging from 0 to 50 km in 2-km steps. We assigned weighting coefficients proportional to the number of sample pairs in each interval, and fit the weighted dissimilarity US. separation data to a negative exponential function of the form: d2 = d’, + S( I-exp(R/L). This function gave the best fit to community dissimilarity data from the southern B.C. continental shelf (Mackas, 1984) and also appeared to describe the fjord data reasonably well (Figure 11). The term d2, is the average dissimilarity of spatial replicates; d2, + S is the estimated asymptote for the correlogram curve (the expected dissimilarity at separations large enough that there is no covariance between the communities). R is the spatial separation (measured along the midlines of the branching inlet). The fitted coefficient L describes how rapidly the curve approaches an asymptote. Results

and Discussion

Average dominance hierarchy within the inlet system Average abundance, rate of occurrence, and the estimated average contributions to the total zooplankton biomass are summarized in Table 2. With respect to biomass, the l-10mm zooplankton were dominated by the copepod Metridia okhotensis. Other important taxa were the copepods Metridia paci$ca (the numerical dominant in June 1982), Pseudocalanus sp. (the numerical dominant in August 1981), Calanus marshallae, Oithona spinirostris, Euchaeta elongata, and Neoclanus plumchrus, the amphipods Parathemisto and Cyphocaris, the ostracod Conchoecia, and juveniles of the euphausiid Euphausia pacifica. All are common members of the B.C. coastal fauna (Gardner & Szabo, 1982; Fulton, 1972). The relative importance (as a fraction of the total fauna) of Metridia, Conchoecia, and the two amphipod genera was substantially higher than observed by Mackas & Sefton (1982) in open continental shelf waters off southern Vancouver Island, but the remainder of the species hierarchy was quite similar. Variability about this dominance hierarchy was on average small (Figure 2). Betweensample community similarity was greater than observed in samples from the open coast. Averaged d2 was 0~21 for the August 1981 series and 0.28 for the June 1982 data (versus typical values of about 0.7 for samples collected off Vancouver Island; Mackas, 1984). The greater uniformity of species dominance hierarchy in the fjord system suggest that the system is not being severely stressed by the tailings input. Localized severe pollutant stress should produce strong spatial gradients in species success and therefore greater average between-sample community heterogeneity. A second expected result of pollutant stress is lowered within-sample species diversity. Comparative studies of the California Current (temporally variable, relatively low diversity) and North Pacific Central Gyre (temporally stable, very high diversity) show that spatial variability of community dominance hierarchy is greater in the low diversity system (McGowan, 1977; Hayward & McGowan, 1979; Star & Mullin, 1981). We believe that the observed low community heterogeneity in the fjord system results both from the fact that the fjord environment is enclosed (restricting advective intrusion of water carrying strongly

126

D. L. Mackas Q E. P. Anderson

GO 70 60 50 40 30 20 IO 0 20 nug I 100

23 iug

25 Aug

27Aug

28 Bug

I.. .

90 80 70 60 50 40

E

V66

f g

100

Q

go

k 2

GO

M53

M40

N32

P25

019

013

K7G75 M9

J

70 60 50 40 30 20 IO 0 HAI

HAl.5

HAI-

HA2.5

100 90 80 70 60 50 40 30 20 IO 0 V66

MS3

M40

N32

P25

013 019

K7 G5 C3 M9

Zooplankton

pattern in a fjord

community

127

100 :

90

,E E

80

0”

70

e

60

zit 0 i 2

50 40 30 20 IO HAI

0

HA2

Other Non

(misc.

copepods)

HA3.4

HA3

n

Oifhor~~

SPP.

Metridm Metridio

ooc/f/co okhotensis

HA3.7

HA4

HA5

crustaceans

Ostracods

Euphousiid

juveniles

(top) (bottom)

Amphipods

Figure 2. Changes in dominance hierarchy of the major taxa with time and along the two arms of the inlet. (a) Time series of measurements at site M9 obtained during the August 1981 cruise. The SS and CF%S classification analyses (see text and Figures 3 and 5) put all of these in a single cluster; the ALS algorithm grouped the 23 and 28 August samples with neighbouring samples at sites 013 and Q19. The remaining graphs show the average dominance pattern along the inlet branches: (b) August 1981 in Alice Arm, (c) August 1981 in Hastings Arm, (d) June 1982 in Alice Arm, and (e) June 1982 in Hasting Arm. All regions and time periods are very similar compared to the compositional range found in open continental shelf environments (note, e.g. the consistent dominance of Merridiu spp.).However, consistent spatial trends in the dominance hierarchy can also be seen in the plots.

different community types) and bathymetrically relatively homogenous (reducing the potential for habit differentiation), and (artificially) from the close spacing of samplesin the fjord system (nearby samplestend to be similar, therefore an increase in the fraction of samplesclose enough to show autocorrelation will result in an increase in the average resemblance). The effect of sample spacing can be eliminated by measuring the change of resemblance with separation and comparing ‘ plateau ’ levels at relatively large separation (seeresults of the autocorrelation analysis below; these also showed greater spatial homogeneity within the fjord). The dissimilarity of spatial replicates was also reduced in the fjord [average d’, 0.08 and 0.10 for August 1981 and June 1982 respectively, VS. 0.24 for spatial replicates from the open coast; see also Figure 2(a)], although less than proportionately to the reduction in overall variability. Again, it is probable that the restricted advection imposed by the fjord boundaries reduces the potential for short-term community change at a given location. Regional results

deviations

from

the average

hierarchy

- cluster and ordination

Despite the high overall resemblanceof sampleswithin each data set, there were detectable differences in community composition between different parts of the inlet system.

128

D. L. Mackas & E. P. Anderson

Within-cruise time variability at a single site is illustrated in Figure Z(a), and along-inlet variability (averaged over spatial replicates) is shown in Figure 2(b-e). Dominance by Met&f&r was maintained in both arms of the inlet and in both time periods, but along-inlet trends are also evident. These include higher relative importance of juvenile euphausiids at the heads of the inlets (and generally in the June 1982 sampling period) and a drop in the relative dominance of M. okhotensisfrom the mouth to the head of Alice Arm (due primarily to an increase in the absolute abundance of the other taxa). However, probably the most important feature to be noted in the graphs is the high degree of similarity between replicate samples [Figure 2(a)] and between samples from neighbouring locations. This is supported by the results of the multivariate statistical analyses. Cluster dendrograms for the two cruises are shown in Figures 3 and 4. Most dendrograms had several tight clusters with very high internal homogeneity (mean squared compositional distance to cluster centroid -+ O-05, average within cluster similarity > O-95) plus a scattering of looser clusters and individual outliers. We have used arbitrary threshold resemblance levels of O-05 (SS) and O-95 (ALS and CPS) to define the clusters shown in the dendrograms. In cases where clear breaks in the level of within-cluster resemblance occurred at slightly higher similarity, we have used these breaks to separate additional groups. To a very considerable extent, cluster members group spatially within the fjord system. This can be illustrated using a schematic plotting technique (Figures 5 and 6) adapted from Mullin & Williams (1983). The graphs show transitions in community ‘type ’ along the axis of the inlet system. Samples with bars in the same vertical position belong to the same cluster and have a high degree of compositional similarity (the one exception to this is the bottom bar in each graph, which we used as a catchall for late merging small clusters and individual outliers- see Figures 3 and 4). Replicate and neighbouring samples frequently belonged to the same cluster, and the sequence of cluster types varied in a regular fashion along the inlet axis. In both time periods, samples collected near the tailings discharge (M9, plus the neighbouring sites K7,013 and Q19) tended to be distinct from samples elsewhere in the inlet. In the August 1981 series, much of the compositional contrast between cluster groups (indicated by the length of the dendogram branches in Figure 3) was between these sites at the head of Alice Arm and all other sites within the inlet system. Samples from near the branching point of the inlet system (HA1 and to a lesser extent V66) also had distinctive community composition (and high variability between spatial replicates). This is somewhat surprising since the horizontal exchange of water (driven by tidal currents) is strongest in this region. If the zooplankton were solely passive tracers of their surrounding water, the community composition in this still region would be expected to be a well-mixed blend of the compositional ratios in adjoining regions. In the June 1982 series, the overall range of community composition increased slightly (note change in average dissimilarity cited above). The most distinctive sites in this time period were those at the extreme heads of Alice and Hastings Arms and in the channel between Anyox and Larcom Island; the biological contrast between the outfall sites and outer Alice Arm, Hastings Arm and Observatory Inlet was equal to or less than it had been in the earlier series. The shallow water sites (more intensively sampled in this time period) were often outliers with anomalous dominance hierarchy. Overall, the classification results support some selective shift in the species dominance hierarchy at sites near the outfall. But if the shift is in fact caused by the suspended tailings, this alteration is small, does not extend very far from the discharge point, and is not intensifying drastically with time.

(SS)

Wlthin-cluster

Figure 3. Dendrogmms and references). The give the top-to-bottom

vor~~nce

Simdority

S

I-

r

(CPS) Slmllarlty

S’

-

resulting from classification of the August 1981 samples by three different clustering algorithms (see Methods section, technical bars at the right margins of the dendrograms indicate our interpretation of the best classification categories (see text). Alphabetical sequence plotted in Figure 5. Unlabelled ’ outlier ’ samples were grouped to form the final (bottom) category in Figure 5.

r

(AL3

details labels

HRP.5

I a

(5s)

With-cluster

L

(ALS) Similarity

S

r

Figure 4. Dendrograms resulting from classification of the June 1982 samples. The bars at the right margins cluster membership and alphabetical labels give the top-to-bottom sequence plotted in Figure 6. Unlabelled (bottom) category in Figure 6.

vorioncr

S’

indicate our interpretation of were grouped to form the final

Similarity

of the dendrograms ‘ outlier ’ samples

(CPS)

Zooplankton

(“)

community pattern in afjord

A-),

yi

131

Granby-Larcom

Obsf;r(ve,?ory

Figure 5. Sequence of cluster membership along the branching inlet axis, August 1981 series. The three graphs give results from three different classification algorithms. Samples were collected over a two-week time span, and the number of independent samples at each site ranged from one to five (see Table 1). Except for the bottom line in each bar (which is a composite of late-joining outliers) samples plotted in the same relative vertical position share cluster membership and have a high degree of compositional Similarity.

The time variability of the zooplankton community (late summer of one year versus early summer of the following year) is substantially greater than the spatial variability within a single time period. Clustering of merged normalized data from the two cruises gave nearly complete separation of cluster membership for samples from different cruises. For example, when we apply the sameresemblance levels for cluster definitions chosen above, of the five multiple-member clusters formed by CPS algorithm (totalling 67 out of 76 samples),only one cluster showed sharedmembership between time periods and this was due to the crossover of a single sample. Results of the principal components analysis give a good representation of the overall relationship among samples. Figure 7 shows the position of sampleson the first three principal component axes, using combined log-transformed data from both time periods (unlike the clustering methods, this treatment responds to proportional variation in the total biomass of all taxa). For simplicity, the figures show the data from each cruise on separateplots. If superimposed and labelled to show sampling date, they would show the results of the complete ordination. Symbol used to label the samplescorrespond to the site classification shown in Figure 8. Like the cluster analysis results, the plots of principal component scores demonstrate the separation of time periods and the tendency for compositional similarity of neighbouring sites within each of the deep water regions. However, the PCA results show more clearly that within the inlet system (and unlike the offshore samplesanalysed by

132

D. L. Mackas & E. P. Anderson

Observatory Inlet

Hastings Arm AllCl?

Arm

Figure 6. Sequence of cluster membership along the inlet, June 1982 series. The three graphs show results from the three different classification algorithms. Most sites had two replicate samples (Table 1). Except for the bottom line in each bar (which is a composite of late-joining outliers) samples plotted in the same relative vertical position share cluster membership and have a high degree of compositional similarity.

Ma&as & Sefton, 1982), the spatial variability of the community composition tends toward a continuum rather than a series of abrupt shifts between disjoint clusters. At least in the three-dimensional space provided by the first three principal components (which account for 54.1 YOof the variance of the log-transformed data) there appears to be a relatively smooth intergradation of composition (i.e. the cluster types adjoin or overlap). The largest (and the only clearly disjoint) variability is between the two time periods and appears as a strong separation along the frrst principal component axis [compare Figures 7(a) and 7(c)]. This can be interpreted largely as a variation in total biomass.Most taxa were more abundant in the June 1982 samples.However, PCA of the merged relative composition data (not shown) also gave strong separation between the two time periods based on shifts in the speciesbalance. The compositional variability responsible for the within-time period (i.e. spatial) differences between regions is distributed among all three component axes, aswere the weightings for individual species. Despite the swing in amount and composition between the two time periods (primarily a translation along component one), the within-time period ordination of the spatial groups remained fairly stable, suggesting some between-cruise persistence of the community dominance pattern [note Figures 7(b) VS.7(d)]. When compared with samplesfrom the sametime period, the inner Alice Arm (outfall) sites scored consistently low on component three and relatively high on components

Zooplankton

community pattern in a fjord

I (a)

133

b)

component

I

(349%)

component

3

COmpOnen+

3

(87%)

:d)

Component

I

( 34

9%)

( 8 7%)

Figure 7 (a-d). Ordination of zooplankton samples by principal components. The analysis shown here uses log-transformed zooplankton biomass estimates, and combines the data from both time periods; however, the two time periods are plotted separately to clarify the presentation of within-time period spatial zonation. The axes of the figures are sample scores on the first three principal components. Percentage values in parentheses give the fraction of the total variance accounted for by each component. August 1981 samples are plotted in 7(a) (components 1 and 2) and 7(b) (components 3 and 2). June 1982 samples are plotted in 7(c) and 7(d). Note the strong separation of the two time periods along component 1, and the within-time period tendency for neighbouring sites to be grouped together in each component space.

one and two. Outer Alice Arm sites scored low on components one and two and high on component three, while Hastings Arm sampleshad high scoreson components two and three. The shallow water sites were more variable (weak grouping in component space) and tended to be scattered around the periphery of the cloud of deep water samplepoints from each cruise. The ordination results also suggest a compositional anomaly near the outfall sites (note the separation in component space of the spatially adjoining sample groups from ‘ inner ’ vs. ‘ outer ’ Alice Arm), but indicate that the effect, if present, is small relative to the between-time periods variability of the entire inlet system and is not progressively intensifying with time (i.e. these two regions are not more strongly separated in the later sampleseries). Our sampling programme was designed to examine spatial variability within the inlet and does not resolve the pattern of temporal variability (other than to show that its amount is large compared to the intensity of the within-time-period spatial variability). We believe that most of the difference between sample periods can be attributed to the normal seasonalcycle. Our observed changesare consistent with but lessextreme than

134

D. L. Mackas t3 E. P. Anderson

129.50'

129.40'

129.30'

55.30'

55.20'

Figure 8. h4ap showing the location of regional groups plotted in Figure 7. The ‘ shallow ’ sites (labelled with squares) were combined to give the simplified spatial/ depth classification used in this discriminant function analyses (see text). Stippled arrow shows location of tailings outfall.

those reported (in greater detail) by Stone (1977) from Knight Inlet (approximately 50”4O’N, 126”OO’W). He found peak concentrations of most copepods in May-June. A mid- to late summer decline in abundance tracked the decline in phytoplankton productivity caused by high surface turbidity associatedwith the summer freshet. Surface

Zooplankton community pattern in a fiord

135

3. Estimated statistical significance levels for pairwise discrixrdnation between regional groups based on their zooplankton community structure. Values in the upper left half of the table are from the August 1981 series, those in the lower right half are from the June 1982 series. TABLE

Inner

Alice Arm

Outer

Alice

Arm

Observatory/Hastings

Shallow

sites

Shallow water sites

Observatory and Hastings AlTIE

Outer Alice Arm

Inner Alice Arm

InsufTicient data to test Insufficient data to test Insufficient data to test -

l’=O.O25

P=O-01

-

Insuflicient data to test

-

P=O.25

Insufficient

data to test Insufficient data to test P=O.lO

P=O.Ol

Insufficient data to test

turbidity levels are somewhat higher in Knight Inlet than in the northerly reaches of the Observatory Inlet system (Farrow et al., 1983; Burling et al., 1983) and chlorophyll concentrations are lower in Knight Inlet (Stone, 1977; J. R. Forbes, personal communication) so the summer reduction in zooplankton stocks is likely to be less extreme in the Observatory Inlet system. Other possible causes for the between-time period differences in zooplankton biomass and community composition include spatially diffuse pollutant stress from the tailing discharge which (unlike the actual suspended sediment plume) affects the entire inlet system more or less homogeneously, large-scale climatically forced inter-annual variability (trends or multi-year cycles of the sort identified in the North Sea by Colebrook, 1978), or slow successional shifts driven by local physical and biological conditions within the inlet system. Compositional basis and statistical significance of station groups discriminunt function results We wished to identify the species groups which (by their increased or decreased abundance) appeared to do well or poorly in each of the segments of the inlet system, both to aid interpretation of the overall spatial pattern and to guide a parallel laboratory investigation of the effect of the suspended tailings on survival and physiological activity of the zooplankton. The spatial groups used in the discriminant function analysis are shown in Figure 7, and the results are summarized in Tables 3 and 4. The comparisons between the two time periods are not shown, but were highly significant in a discriminant function classification by sampling period (see also Table 2). Samples from the June 1982 series had higher concentrations of most of the dominant taxa (e.g. the two Mettidiu species, Chnus, euphausiids, and Sugitta) while amphipods, ostracods and some of the rarer copepod genera were more abundant in the August samples. There was also a reversal in the relative abundance of the two Acartia species. A. longiremis was more abundant and A. clausi much less abundant in the June 1982 samples. Most of the differences were again consistent with the pattern of seasonal shifts reported by Stone (1977).

D. L. Mackas &YE. P. Anderson

136

TABLE 4. List of taxa which were distinctively regional subdivisions. Between-region differences responsible for much of the community zonation August Region

Abundant

abundant or rare within each of the in the concentration of these taxa are pattern in the inlet.

1981

June 1981 Rare

Inner

Alice Arm

Euphausiids Conchoecia

Neoculanus Calanus

Outer

Alice Arm

Neocalanus

Euphausiids Conchoecia

Abundant Euphausiids SUgi%2 Mecridia pacifica Larvaceans Neocalanus

Rare Conchoecia Larvaceans Neocahmu Acattia spp.

Metridia pacijica Observatory/Hastings

Shallow

sites

Sagiua Calanus Metridia pacifica M. okhotensi~

Euphausiids

Larvaceans

Conchoecib Neocalanus Amphipods Calanus

Euphausiids

Sqgirta Merridia

okhote?lsis Neocalanus

Euphausiids pteropods

Table 3 gives the estimated statistical significance of within-time period discrimination between the various inlet segments (probability that equally large compositional differences between group means would occur by chance). Probability values in the upper left half of the table are from the August 1981 series, those in the lower right half are from June 1982. Significance levels are approximate and are based on the assumption of between-group homogeneity of the within-group variance and covariance of species abundance. This assumption tends to exaggerate the apparent significance of comparisons with internally heterogeneous groups (principally the ‘ shallow water ’ set) and underestimate the separation between internally homogeneous groups (e.g. outer Alice Arm vs. outer Hastings Arm). The discrimination results agree with those from the cluster and ordination analyses. In August 1981 the most significant differences in composition were between the inner Alice Arm group and the remainder of the inlet system. The inner Alice Arm samples were less distinctive in June 1982 (the only significant discrimination is from the Observatory/Hastings group). However, the shallow sites at the periphery of the inlet system (which were less intensively sampled in the earlier series) show significant compositional differences from both the outer Alice Arm and the Observatory/Hastings groups. Taxa which were particularly rare or abundant in each regional group (relative to the within-cruise mean) and which account for much of the difference between regional groups are summarized in Table 4. A few of the differences were stable over the time interval between the two collections. For example, Euphati pacifica was consistently most abundant in the inner portions of Alice and Hastings Arms. Neocalanusplutnchrus showed almost the opposite pattern, with highest abundances in outer Alice Arm, outer Hastings Arm and Observatory Inlet. The heads of Alice and Hastings Arms had relatively high concentrations of Calanus marshallae. Other strong regional differences

Zooplankton

August

Figure sample In both Arms. change juvenile

community

1981

pattern in a fjord

137

June

1982

9. Population density of Euphausia pm&a WS. sample location for the two series. This species showed the strongest and most consistent spatial variability. time periods, highest abundances occur near the heads of Alice and Hastings Overall abundances are substantially higher in the June 1982 series (note scale in plot) and the population in this time period is numerically dombiated by stages.

either disappeared or were reversed between the two time periods. For example, the ostracod Conchoecicrwas extremely abundant in inner Alice Arm in August 1981 but was rare in the sameregion in June 1982. Similarly, Metridiu okhotensiswas abundant at the shallow sites in August 1981 (somewhat surprisingly, since it is usually considered to migrate to considerable depth in daylight hours), but was relatively rare at the shallow sites in June 1982. Euphatia paczjk~ showed particularly strong variability both between regions and between time periods. Most of the other major taxa had high and relatively uniform rates of occurrence (seeTable 2); their variability was confmed to moderate (typically three- to four-fold) changes in abundance. However, E. pacifica was absent from many of the August 1981 samples(regular occurrence was restricted to inner Alice Arm sites). The average abundance (7.8 rne3) and rate of occurrence (95% of samples)were both much higher in June 1982, but the spatial pattern of abundance variation was similar (Figure 9). In both time periods, highest abundances were near the heads of the inlets (where the physical enviromnent provides locally high turbidity due to sediment inputs and correspondingly reduced sub-surface illumination). Several mechanismscould give apparent or real up-inlet increasesin zooplankton abundance. The lower (daytime) depth limit of die1

138

D. L. Mackas & E. P. Anderson

Length 5

3

y&e,&

(mm) 7

9

HA3.7

II

3

Length (mm) 5 7 9

II

in*%)

HA3 In.61 M53(n.W V66 in *w

Hostings

Arm

Alice

Arm

Figure 10. Length-frequency distribution of Eupkausiu pacifica along Hastings and Alice Arms, June 1982 samples. Number of individuals measured for each histogram are shown in parentheses. As well as being more abundant (Figure 8) the animals are larger near the heads of the inlets.

vertical migration might be reduced where light penetration is least, locally increasing the daytime catch. However, we found the same pattern in our June 1982 night samples and therefore believe that spatial variability in either vertical migration range or visual sampler avoidance is insuRicient to explain the observed abundance gradients. Animals which avoid the surface for much of the day can also be trapped at inlet heads by a deep inflow, surface outflow estuarine circulation. However, if this caused the euphausiid concentration, we would also expect concentration of smaller deep living and weak or nonmigratory species such as Scolecithricella minor, Scaphocalanus brevicornis, and Neocalanus plumchrus. The first two show little or no regional pattern (they are also rare), while N. plumchrus is most abundant in the outer parts of the inlets and is quite rare at the inlet heads. This implies that the deep up-inlet estuarine flow is either too weak to affect significantly the distribution of the animals or that these species resist up-inlet transport. High abundance of euphausiids at the heads of the inlets does not necessarily imply locally greater availability as food for higher trophic levels. An additional mechanism which could give high concentrations of euphausiids (and one that is consistent with the distribution of several other taxa) is that the turbid water may provide a partial refuge from visual predation. Most of the larger bodied and more visible taxa (e.g. euphausiids, amphipods, chaetognaths, pteropods, large migratory copepods) are most abundant in the inner section of Alice and Hastings Arms. Average and maximum body size of the euphausiids also appear to be larger (suggesting better survival) near the heads of the inlets (Figure 10). Substantial numbers of chum salmon smolt (c. lo’-lo6 per inlet per year) emerge into the heads of the inlets during late April-May of each year (EL Huber,

Zooplankton

community pattern in a fjord

139

0.61

IO

20

30

Separation

i 513

40

(km)

Figure 11. Correlogram of community dissimilarity ZIS. spatial separation. Averge dissimilarity of spatial replicates is low and increases steadily with spatial separation out to about 15-20 km. The plateau of the curve is lower and reached at shorter separations in the fjord system than in an open contiental shelf ecosystem to the south. Symbol size is proportional to the number of sample pairs averaged from within a 2 km separation interval. Filled circles are from the August 1981 samples, open circles from the June 1982 series. Dashed line shows the best fit to the merged data.

personal communication). Observations in the Strait of Georgia (summarized in Healey, 1980) suggest that salmonid juveniles are likely to be resident in the inlet roughly 1.5-2 months. Their feeding activity would contribute (along with reduced primary productivity which accompanies high surface turbidity) to a summer decline in zooplankton abundance. Spatial

autocorrelation

of community

pattern

along the inlet axis

The increase in community dissimilarity with increasing separation along the inlet axis is shown in Figure 11. In both time periods, dissimilarity increased steadily out to about 20 km separation, followed by a levelling off (June 1982 data) or a subsequent decline (August 1981). The June series is likely to be more representative of ‘ average ’ resemblance shifts, since samplesfrom this period gave much more uniform coverage of the inlet system (Table 1). The best fitting parameters for the equation, d2 = d’, + S( 1-exp(R/L)),

are d2, = O-094, S = 0.2 1, and L = 7 km. A similar analysis for samplesfrom the open continental shelf off southern Vancouver Island (Mackas, 1984) showed higher dissimilarity of spatial replicates (d’, = 0.24) and a more sustained increase of dissimilarity (S= 0.83, L = 46 and 120 km in the cross-shelf and alongshore directions, respectively). The initia1 (small spatial scale)rise of dissimilarity is similar in both regions. The striking difference is that the correlogram is truncated within the inlet (lower S, shorter L). We believe this is because large-scale horizontal mixing is restricted by the inlet geometry. The open coastal system is frequently affected by large eddies and meanders superimposed on the average circulation. In contrast, exchange of water and populations within the inlet is controlled by a combination of tidal mixing, estuarine circulation, and occasional flushing events. The latter two are likely to introduce large amounts of ‘ new ’ water but normally do so in inlet-wide depth strata rather than in discrete horizontal sectors. Compared to the open coast, spatial structure in the inlet is therefore lesslikely to arise from time variable stochastic fluctuations in the circulation pattern and more likely to

140

D. L. Mackas t3 E. P. Anderson

arisefrom the temporally persistent spatial pattern in features and processessuch asinlet bathymetry and tidal and estuarine circulation. The cluster dendrograms and schematic maps (Figures 3-6) suggest that the community variability is not distributed homogeneously within the inlet. A few locations, principally the inlet heads and the region surrounding the branch point are pockets of high variance where samplestend to be dissimilar even from closely neighbouring sites. Other locations, principally the middle reaches of Alice and Hastings Arms, have low overall variability and gradual gradients of community composition. The number of available samplesis insufficient to fit separatecorrelograms for the different parts of the inlet. However, averages of the small separation (< 5 km) dissimilarities illustrate the greater community variability in these subregions. The inlet heads average 0.253 for the O-5 km separation band-nearly as great asthe large separation asymptote for the entire inlet. Community gradients within the branch region (comparisons involving sites HAL, V66, and OBl) are lesssharp; the near-field dissimilarities average 0.195. The main arms of the inlet system show the most gradual compositional change, with an average d2 of 0.133 for the O-5 km band. The autocorrelation results indicate that locations separated by about 5 km or less have a significantly higher degree of compositional similarity than the inlet average. This meansthat detectable (and significant in a statistical sense)between-region differences in community composition can be due to spatial isolation and normal habitat partitioning as well as to localized perturbation by pollutant inputs. The compositional anomaly needs to be relatively ‘ large ’ and/or persistent and cumulative over time before we can reliably attribute it to the pollutant stress. On the other hand, the overall range of compositional variability within the inlet system (estimated by d2, + S) is small if compared with an unperturbed open coast environment. An anomaly large enough to be clearly greater than the normal range of within inlet spatial variability (we believe that our observations were near but not exceeding this threshold) would still represent a relatively modest shift in community composition. Conclusions From the above results, we conclude the following. First, the spatial component of the zooplankton community variability in the Observatory Inlet fjord system as a whole is small, in comparison both to the range of temporal variability in the system and to the range of spatial variability observed in an open coastal environment a few hundred kilometres to the south. Secondly, despite the fact that all sampleshad a relatively similar dominance hierarchy, the multivariate methods used in this paper allow objective and relatively consistent grouping of samplesinto biologically similar and spatially adjoining subgroups. Thirdly, most of the analysesindicated that samplescollected near the tailings outfall of the AMAX/Kitsault mine form one of these regional subgroups. Fourthly, some features of the speciesdominance hierarchy (most notably the relative successof euphausiids and other large zooplankton) which serve to make this region distinctive are recurrent over a timespan of 10 months. Euphausiids are more abundant at sites with turbid water located near the heads of the inlet. Fifthly, the observed spatial pattern is consistent with (but not conclusive evidence for) pollutant stress of the zooplankton community within a zone of a few kilometres surrounding the tailings outfall; however, they also indicate that any changes to date have been relatively minor (i.e. shifts in speciesdominance hierarchy rather than major addition or deletion of species). Finally,

Zooplankton

community

pattern

in a fjord

141

the dominant spatial scale of the mosaic of zooplankton community variability within this fjord system is finer grained than the scale of community patchiness observed on the open continental shelf off southern Vancouver Island. We interpret this as the result of a reduction in large-scale horizontal eddy activity that is imposed by the shorelines of the inlet. Acknowledgements Sample collection was done abroad the charter vessel Bastion City and the CSS Vector with the assistance of G. Jewsbury, K. Denman and J. R. Forbes. The Coastal Zone Oceanography group at I.O.S. gave auxilliary information and logistic support during the June 1982 cruise. M. Galbraith assisted with sample identifications. M. Mullin, C. Davis, R. Brinkhurst and an anonymous referee provided constructive comments. References Anderson, E. P. & Mackas, D. L. (in prep.) Zooplankton samples from Alice Arm, Hastings Arm, and Obse~atory Inlet. Canadian Contractor Report of Hydrography and Ocean Sciences. Anderson, T. W. 1958 An Introduction to Multivariate Statistical Analysis. Wiley, New York. 374 pp. AMAX of Canada Ltd. 1982 Kitsault Mine Environmental Monitoring Program, 1981 Annual Report (2 vols). AMAX of Canada Ltd. 1983 Kitsault Mine Environmental Monitoring Program, 1982 Annual Report (2 vols). Burling, R. W., McInerney, J. E. & Oldham, W. K. 1981 A technical assessment of the Amax/Kitsault molybdenum mine tailings discharge to Alice Arm, British Columbia. 154 pp. Burling, R. W., McInerney, J. E. & Oldham, W. K. 1983 A continuing technical assessment of the Amaxi Kitsault molybdenum mine tailings discharge to Alice Arm, British Columbia. 65 pp. Colebrook, J. M. 1978 Continuous plankton records: zooplankton and environment, North-East Atlantic and North Sea, 1948-1975. Oceanologica Acta, 1,9-23. Farrow, G. E., Syvitski, J. P. M. & Tunnicliffe, V. 1983 Suspended particle loading on the macrobenthos in a highly turbid fjord: Knight Inlet, British Columbia. Canadian Journal of Fisheries and Aquatic Sciences, 4O(Suppl. l), 273-288. Fasham, M. J. R. &Angel, M. V. 1975 The relationship of the zoogeographic distributions of the planktonic ostracods in the northeast Atlantic to the water masses. Journal of the Marine Biological Association of the United Kingdom, 55,739-767. Freeland, H. J., Farmer, D. M. & Levings, C. D. 1980 Fjord Oceanography. Plenum, New York. 715 pp. Fulton, J. D. 1972 Keys and references to the marine Copepoda of British Columbia. Fisheries Research Board of Canada Technical Report, 313,63 pp. Gardner, G. A. 1980 A preliminary examination of zooplankton species groupings and associated oceanographically defined regions along the British Columbia mainland coast. In Fjord Oceanography (Freeland, H. 1.. Farmer, D. M. & Levinps, C. D., eds). Wilev. New York. DD. 407-413. Gardner, G. A. & %abo, I. i982 British Col&bia pelagic marinecopepoda: an-identification manual and annotated bibliography. Canadian Special Publication of Fisheries and Aquatic Sciences, 62,536 pp. Hayward, T. L. & McGowan, J. A. 1979 Pattern and structure in an oceanic zooplankton community. American Zoologist, 19,1045-1055. Healey, M. C. 1980 The ecology of juvenile salmon in Georgia Strait, British Columbia, In Salmonid Ecosystems of the North Pacific (McNeil, W. J. & Hinsworth, D. C., eds). Oregon State University Press, Corvallis. pp. 203-229. Hirota, J. 1981 Potential effects of deep-sea minerals mining on macrozooplankton in the North Equatorial Pacific. Marine Mining, 3, 19-57. Littlepage, J. 1978 Oceanographic and marine biological surveys: Alice Arm and Hastings Arm, British Columbia 1974-1977.78 pp. McGowan, J. A. 1977 What regulates pelagic community structure in the Pacific? In Oceanic Sound Scatterina Prediction (Andersen. N. R. & Zahuranec. B. T., eds). Plenum, New York. DD. 42w44. Mackas, D. L. 1984 Spa&al auto&relation of plank& c&m&ity composition in a-continental shelf ecosystem. Limnology and Oceanography, 29,451-471. Ma&as, D. L. & Sefton, H. A. 1982 Plankton species assemblages off southern Vancouver Island: geographic pattern and temporal variability. Journal of Marine Research, 40,1173-1200.

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142

Menxel,

D. W. 1977 Summary

of experimental

results:

controlled

ecosystem

pollution

experiment.

Bulletin

of Marine Science 27,142-145. Mullin,

M. M.

& Williams,

W. T. 1983 Spatial-temporal

scales of zooplanktonic

assemblages

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