An expanded weighted-averaging model for inferring

49

Journal of Paleolimnology 14: 49--67. 1995. © 1995 Kluwer Academic Publisher.f. Printed in Belgium.

An expanded weighted-averaging model for inferring past total phosphorus concentrations from diatom assemblages in eutrophic British Columbia (Canada) lakes Euan D. Reavie', Roland I. Hall',2 & John P. Smol' 1 Paleoecological Environmental Assessment & Research lLJb (PEARL), Department of Biology, Queen j. University, Kingston, Ontario, Canada, K7L 3N6 2present address: Ontario Ministry of Environment & Energy, Science & Technology Branch, Aquatic Science Section, Bellwood Acres Rd., P. 0. Box 39, Dorset, Ontario, Canada, POA JEO

Received 12 August 1994; accepted 19 December 1994

Key words: diatoms, eutrophication, lake management. paleolimnology, British Columbia, lakes. phosphorus, training sets

Abstract Eighteen lakes were added to a published training set of 46 British Columbia (BC) lakes in order to expand the original range of total phosphorus (TP) concentrations. Canonical correspondence analysis (CCA) was used to analyze the relationship between diatom assemblages and environmental variables. Specific conductivity and {TP] eac h explained significant (PSO.05) directions of variance in the distribution of the diatoms. The relationship between diatom assemblages and [TP] was sufficiently strong to warrant the development of a wei g hted ~averag ing (WA) regression and calibration model that can be used to infer past trophic status from fossil diatom assemblages. The relationship between observed and inferred [TP] was not improved by the addition of more e utrophic lakes, however the [TP] range and the number of taxa used in the transfer function are now superior to the original model. Diatom species assemblages changed very little in lakes with TP concentrations greater than 85 j.Lg 1- 1, so we docu me nt the development of a model containing lakes with TP~ 85 p,g 1- 1. The updated model uses 59 training lakes and covers a ra nge of species optima from 6 to 41.9 J.Lg 1- 1 TP, and a total of 150 diatom taxa. The updated infere nce model provided a more realistic reconstruction of the anthropogenic history of a highly eutrophic BC lake. The model can now be used to infer past nutrient conditions in other Be lakes in order to assess changes in trophic status.

Introduction Water quality has declined in many western Canadian lakes as a result of anthropogenic nutrient loading. These lakes have often been subject to perturbations since European settlement began in the mid ~ 1800s, and in some areas, Indi an settlements may have affected water quality well before this time (Woolliams, 1979). However, water quality information measured before 1950 is scarce or lacking for most systems, and realistic estimates of previous ecological conditions are often on ly available in the form of proxy data from the sediment record. Paleolimnological approaches can pro~ vide information on the causes, timing and extent of

ecological disturbances. Such data are often necessary for proper management decisions (Smol, 1992). Moreover, with knowledge of the pre-disturbance state, real~ istic restoration goals can be established. The siliceous remains (valves) of diatoms have been widely used as biomarkers to elucidate environmental conditions in lakes (e.g. Dixit et aI., 1992). Recently, ecologicall y relevant statistical methods have been developed for inferring environmental con~ ditions from diatom assemb lages. These methods are based on multivariate ordination and weighted avcr~ aging rNA) regression (that effectively esti mates the optima and tolerances of several species) and calibration (Birks et at., 1990a). Ecological parameters of the

50 d iatom species are determined by relating modern limnolog ical variabl es to surface sed iment d iatom assemblages. The spec ies-environment relationships are then used to infer envi ronmental conditions from fossi l diatom asse mbl ages (i.e. in a sediment core). Th ese techniqu es are stati stically and ecolog icall y robust. Transfer functi ons ha ve been devel oped for inferrin g trop hic variables in Florida (Whitm ore, 1989). eastern Ca nada (Agbeti , 1992 ; Christie & Smol, 1993), north east United States (Fritz et al., 1993; Di xit & Smol, 1994), Northern Ireland (Anderson et al., 1993) and so utheast England (Benni on, 1994). A strong rela-

bootstrapped error-est im atio n methods using our data. Fi nally, we also describe the effec t of transformi ng spec ies data (Cumming & Smol, I993a) on the old and new in ference models. Our primary goal was to deve lop a more robust quantitati ve model for in fe rring past lake trophi c status in BC lakes. Throughout thi s stud y, com pari sons are made between th e ori ginal and updated mode ls in order to outline changes incurred by ex panding the lake set.

ti onship has generall y been found between diatom

Methods

species com pos iti on in the surface sedi ments of the ca libratio n lakes and the nutrient co ncentrations of the lakewater. Hall & Smo l ( 1992) described the relati onship between several measured environmental variabl es and diatom distributi ons in 46 (37 fo llowi ng data screening) British Col umbia (BC) lakes, and generated a WA model for infe rring lakewater [TP] from sedimentary d iatom assem bl ages. Th eir model was developed to infer hi stor ical [TP] within the range of 5 to 28 j.Lg I- I from foss il diatom assemblages, a rather limited range of trophic states. Because many lake management problems tend LO devel op above th e upper limit o f their model (i.e. above 30 fLg I- I), we attempted to ex pand the range of thi s tran sfer fun cti on. The present stud y sought to ex pand the Hall & Smol (1992) training set to include more eutrophic lakes. This expa nsion was considered necessary for fo ur reaso ns. First, only two lakes in the old set contai n TP in ambient concentrati ons greater tha n 30 fLg I- I, howeve r many BC lakes wi th water quality problems have much higher T P concentrations. By incl ud ing additional lakes in the eutroph ic range, we hoped to provide more realisti c inferences for highl y eutrophi c systems. Second , many of th e small benthi c di atoms (primaril y Frag ilaria p iIJllata) are common in BC lakes and ex hibit a wide tolerance to trophic statu s, hence th ey are diffi cult to class ify ecolog ically. As a result, they are co nsid ered to be relatively poor indicators. Expa nsion of the lake set should provide more calibrati on data to defin e more accuratel y the environmental optima of species already described by Hall & Smol ( 1992). In th is way, we hoped that the obfuscating effect of small be nthic diatoms can be reduced. Third , a superior error-estimating procedure (bootstrapping; descri bed by Line et at., 1994) has rece ntly beco me widely accessi bl e. In th is paper, we compare old errorestimatio n meth ods (used by Hall & Smol, 1992) to

Six ty- four study lakes were chosen fro m throughout BC (Fig. I). Considering the heterogenei ty of BC so ils, geology, climate, vegetati on and topography, it was inevi table that a broad range of physical, chemi cal and biolog ical characteristi cs would be represented by the lake set (Table I). We selected lakes to maximi ze the [TP] grad ient, and th e lakes range from 5 to 138 fLg I- I . So me water chemi stry valu es are averages of several measurements, whereas others are based on single measurements taken at the time o f surface sedim ent samplin g. Hall & Smol (1 992) prov ide co mplete detail s concernin g sampling. Other components whi ch can strongly influence diatom di stributio ns were selected more conservati vely. We selected lakes with pH between 6.2 and 8.5, co nducti vity less th an 460 ttS cm- I , and maximum lake depth greater than 4 m. Six biogeoclimatic zones (defined by Bei l et ai., 1976) were included in the study area: the Interior Douglas Fi r, Coastal Douglas Fir, Cari bou Aspen - Lodgepole Pine, Interi or Western Heml ock, Boreal White and Black Spruce, and Sub-Boreal Spruce reg ions. All samplin g was · performed by th e British Columbia Ministry of th e Environment (MOE) from the deepest porti on of each lake. Water for chemical anal yses was taken from a I m depth usin g a Van Dorn sampler. The top I cm of surface sediment was carefull y collected with an Ekman grab. The water chemistry data for the original 46 lakes have been updated an d include additi onal data that have been made available sin ce Hall & Slnol 's ( 1992) study was completed (BC Ministry of Enviro nment, unpu blished data). The new data were primaril y trophic status measurements from archi ved doc uments. Newlyacqu ired data were combined wi th Hall & Smol's original measurements, and average TP concentratio ns, fo r example, were recalc ul ated for some lakes.

51 Table I. Selected limnological data from the 64 study lakes in British Columbia. Only the important variables identified by canonical correspondence analysis are shown here . Biogeoclimalic zones: CDF, Coastal Dougla. 40 p.g 1- ' ) lakes. Table 5 presents estimates of [TP] optima for the 150 diatom taxa used in our model , calculated using WACALIB , and Fig. 7 illustrates how species optima chan ged following expansion of the lake set. The [TP] optima represented by the taxa increased from a range of 16.5 (6.8 to 23.3) p.g 1- ' (Hall & Smol, 1992) to 35.9 (6.0 to 41.9) p.g 1- '. Many spec ies, such as Cyclotella michiganiana, Navicula minima and Fragilaria pinnata, now have strikingly higher estimated optima, whereas the [TP] optima of most other taxa increased only slightly or were unaffected . The optima

" = 0.75 RMSE = 0.25 n = 37

5.

" = 0.61 RMSE = 0.47 n = 59

5.

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.,...

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·:

5

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e 30

Observed TP

30

5.

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20

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r' = 0.46 RMSE boo' = 0.48 n = 59

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Observed TP 1 :1

" = 0.30 RMSE boot = 0.40 n = 37

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Observed TP

5.

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Observed TP

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50

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Observed TP

FiR. 5. The relationship between observed and WA dialOm-inferred lTP] values (J1.g I - I) using classical regression and (a) simple WA and the original 37-1ake data set (Hall & Smol, 1992), (b) simple WA and the updated 59-lake data set, (e) simple WA with sq uare roOi transformed species dOli;) and the original data set, (d) simple WA with square root transfo rmed species data and the updated calibrnlion SCI, (e) boolslrnp WA with square root transfonned species data and the original datu sel, and (0 bootstrap WA with square root trnnsfonncd species data and the updnled calibration set. RMSE = root mean squared clTOr of the relationship between observed and inferred [TP) values .

59

Table 5. Weighted averages of diatom taxa to [TPJ estimated using weighted averaging calibration and regression. Included are values indicating the number of lakes in which each species occurred. Taxa labelled with 'BC' have a previously unknown taxonomic description, but were observed by Hall and Smol (1992) Number of

Number

Diatom taxon

1 2 3 4 5 6 7 8 9 10

FragiLan'Q brevisln"ata vaL capilata Hcrib.

11

Cyclotella kuetzingiana var. radiosa Fricke

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

CymbelLa diluviana (Krasske) M. B. Florin Fragilaria sp. 3 PIRLA

Auiacoseira sp. 12 PIRLA Aclmamhes levanderi Hust.

Navicula explallata Hust. Demicula kuetzingjj Grun.

Fragilaria virescens vaT. exiguo. Grull. Cyclotella kueltingiana vaT. pianefophora Fricke

Fragililn"o sp. 4 PIRLA

Anomoeoneis vitrea (Gron.) R. Ross Ac1l1umthes bioreli Benn.

Amphora parallelism'aro Manguin Cymbella cesarii (&abenh.) Grun. Srephanodiscus sp. 5 Be Slauroneis phoenicemeron (Nitzsch) Ehrenb. Synedra radians Kfitz. Fragilan'a brevislriala var. injlala (pant.) Hust. Diaroma hiema{e var. mesodon (Ehrenb.) Grun. Navicula cryprocephala Kiltz. Amphora rhumensis (Mayer) Krieger Gomphonema angustum (Agardh) Cymbella minuta Hilse ex Rabenh. Achnamhes biasoletliana Grun. Cyc/orella bodanica var. affinis Gron. Achnanthes microcephala (K1I.tz.) Cleve Navicula laevissima Kiltz. Navicula utermoehlii Hust. Mastogioia smil~il' Thwaites ex W. Smith Achnanthes conspicua A. Mayer Achnanthes linearis CN. Smith) Grun. Achnanthes pimraro Hust. Cyclotella meneghiniana Kutz. Tobellaria /enestrara (Lyngb.) Kiltz. Cymbella microcephala Grun. Navicula subminuscula Manguin Gomphonema subtile Ehrenb. Fragilaria sp. 2 PIRLA Aclmanthes suchlandJii Rust. Amphora libyca Ehrenb. ex Ktltz. Epirhemia argus (Ehrenb.) Ki1tz. Amphora pediculus (Klitz.) Grun. ex A Schmidt Synedra delfcarissima Gron. Cyclolella cf. radiosa (Grun.) Lemm. Fragilan'a pi111iata var. intercedens (Grun.) Hust. Cyclotella bodanica Grun. Navicula radiosa KOtz.

TP optimum (p.g/L) 6.0 6.8 6.9 7.3 7.8 7.8 8.0 8.0 8.2 8.2 8.3

8A 8.6 8.7 9.0 9.0 9.1 9.2 9.3

9A 9.5 9.6 9.6 9.8 10.0 10.0 10.1 10.2 10.4 10.4

10.6 to.6 10.7 10.7 10.8 10.8 10.8 10.9 10.9 11.0 11.0 11.2 11.3 11.4 11.4 11.5

11.6 11.8

occurrences

7 2 3 3 6 2 2 8 12 4 5 9 4 3 4 2 8 2 25 5 16 13 5 15 20 18 12 15 7 8 27 15 4 5 2 21 14 4 16 7 27 13 42 5 3 25 28 13

60 Table 5. Continued Number

Diatom taxon

49 50 51 52 53

Rhizosolenia eriensis W. Smith Srephanodiscus alpinus Hust. Rhopalodia gibba (Ehrenb.) O. MOil. Cyclorella sp. 2 BC Achnamhes clevei Grun. Fragilaria pinnara var. Ian cellula (Schum.) Hust. Nirzschia fomicola Grun. Fragilaria brevisrriara Grun. Asrerionella ralfsii W. Smith Acll/ll1mhes derha Hohn & Hellerman Cymbella delicarula Kotz. Fragilaria consrruens var. venter (Ehrenb.) Grun. Nirzschia demicula Grun. Synedra ulna var. chaseana Thomas Synedra parasirica eN. Smith) Hust. Aulacoseira iralica var. renuissima (Grun.) Simonsen Fragilaria sp. 1 BC Achnanrhes exigua Grun. Cyclorella kuerzingiana Thwaites Navicula cari Ehrenb. Srephanodiscus medius Hakansson Fragilaria lapponica Grun. Srephanodiscus sp. 2 BC Synedra cyclopum Brutschy Navicula scll11dei Krasske Achnanrhes minurissima Kiitz. Cyclorella srel/igera (Cleve & Grun.) Van Heurck Navicula subaromoides (Hust.) Lange.Bertalot & Archibald Diaroma renue var. eiongarum Lyngb. Srephanodiscus minurulus (Kiltz.) Cleve & Moller Navicula pupula Kiltz. Cye/orella comensis Grun. Srephanodiscus hanrzschii Grun. Cocconeis diminura Pant. Achnanrhes lanceolata var. dubia Grun. Navicula pseudovemralis Hust. Cocconeis placemula var. lineara (Ehrenb.) Van Heurck Asrerionella formosa Hasall Srephanodiscus hanrzschii fo . renuis HAkansson & Stoermer Cocconeis placenrula var. euglypra (Ehrenb.) Grun. Fragilaria consrruens var. binodis (Ehrenb.) Grun. Achnanrhes lanceolata (Breb. ex Kiitz.) Grun. Aulacoseira disrans (Ehrenb.) Simonsen Cymbella sinuara Gregory Srephanodiscus cf. vesribulis Hakansson, Theriot, & Stoermer Fragilaria capucina var. mesolepta (Rabenh.) Rabenh. Achnamhes exigua var. hererovalva Krasske Fragilaria brevisrriara var. elliprica H6rib. Achnanrhes lanceolara var. elliprica Cleve Srephanodiscus cf. pseudoexcemricus HAkansson & Stoermer Aulacoseira subarcrica (0. Mull.) Simonsen Navicula virabunda Hust. Srephanodiscus parvus Stoermer & Hakansson

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101

TP optimum ()lg/L)

Number of occurrences

11 .8 11.9 11.9 12.0 12. 1 12.2 12.2 12.2 12.2 12.3 12.5 12.5 12.6 12.6 12.7 12.7 12.8 12.8 13.0 13 .0 13.0 13 .1 13.2 13.3 13.4 13.5 13.5 13.8 13.8 13.9 14.0 14.0 14. 1 14.2 14:3 14.5 14.6 14.6 14.7 14.8 14.8 15.0 15 .1 15.3 15 .5 15.7 15.8

10 15 7 2 20 32 27 52 5 17 10 47 15 6 19 13 20 26 13 15 13 35 2 12 9 42 37 26 8 29 26

16.3 16.5 16.5 16.7 17.1 17.1

17 17 3 21 25 20

12 19 26 19 19 13 37 9 10 14 26 14 8 3 10 27

61

,Ie 5. Continued

Number

DialOm taxon

102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 11 8 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150

Calone;s bacillum (Gron.) Cleve Navicula ventralis Krasske

TP optimum (p.g/l)

Number or occurrences

17.1 17.4

4 6 11 16 5 31 7 6 11 4 3

Fragilaria vauchen'ae (Kiltz.) 1. B. Petersen

17.4

Navicula cryptotenella Lange-Bertalot Aulacoseira undulata (Ehrenb.) Klitz.

Coccolleis placentula Ehrenb. Neidium ampliarum (Ehrenb.) Krammer

17.5 17.6 17.6 17.9 17.9 18.3 18.8

Pinllularia brau1lii (Gron.) Cleve Navicula absoluta Hust.

1B.9

Gomphonemo parvulum (KOtz.) Klitz.

19.2

Fragilaria crotonensis Kilton Tubellaria jlocculosa Slr. lip sensu Koppen Nitzschia palea (Kiltz.) W. Smith

Szephallodiscus "iagarae Ehrenb . Cyclotella oce/lata Pant. Admonthes delicatula (KUtz.) Grun. Achnanthu petersenii' Hust. Navicula kuelbsii Lange-Benalol Fragilaria 1Ianana Lange-Bertalol Fragi/aria conslruellS (Ehrenb.) Grun. Gomphonemo angustatum (Kutz.) Rabenh . Slauroneis anceps Ehrenb. Cyclotefla bodanico var. lemanico (0. MUller ex Schroter) Nitzschia gracilis Hant. Cymbella silesiaca Bleisch Pinnulan'a viridis (Nitzsch) Ehrenb . Pinnuum'o nodosa Ehrenb .

I B,8

19 ,3

19.5 19.5 19,8

19.9 20.2 20.5 20.6 20.7 21.0 21.0 21.1 21.2 21.4

Navicula scutteloides W. Smith ex: Greg. Fragilan'a pinnato Ehrenb .

21.7

Navicula seminuloides Hust. Gy rosigma accunu'natum (KOtz.) Rabenh.

21.9 22.0 22. 1 22.2 22.6 22.7 23.1 23.1 23.2 23 .3 24.2 24.7 26.0 26.3 26.9 27.4 27.5 28.8 36.0 41.9

Navicula/arca Bust. Cycloszep}ulnOS tholiformis Stoermer, Hakansson, and Theriot Au/acouira amhigua (Grun.) Simonsen Navicula millima Grun. Tabe/lan'ajloccuJosa str.

m sensu Koppen

Aulacoseira lirala var. lacustris (Grun.) R. Ross Fragilaria exigua Grun. Fragilaria capucina Desm. AcJmanthes sp. 1 BC Aulacoseira icalica (Ehrenb.) Simonsen Aulacoseira perglabra var. jlorinae Camburn Cyclozella michiganiana Skvortzow Aulacoseira granulato var. angustissima (0 . MUll. ) Simonsen Frogilaria pinnara var. acunu'nata A. Mayer Aulacouira distans var, nivalis C:W. Smith) Kirchner Aufacoseira firma (Ebrenb.) Ross Aufacoseira granulata (Ehrenb.) Simonsen Synedra milluscula Grun.

2 1. 8

16 6 20 11 6 3 11 13 40 10 9 7 9 12 7 3 10 53 26 4 3 11 27 28 11 7 2 7 9 3 722 4 11 5 4 2 2

62 Table 6. Summary o f the available [TPJ inference models and (heir geographical locations and (TPJ ranges. Whitmore's ( 1989) model used a broad trophic classification scheme. so TP concentrations were not available

I.a ke TP rallge C,lg/ L) Author(s)

SllIdy location

Main trophic variable(s)

Whitmore (1989)

Florida

-trophic Siale index ·

Number of lakes

30

lower

uppe r

Agueti (1992)

Sou thern O lllario

TP, Secchi

28

2

63

lIall and Smal ( 1992)

British Columbia

37

5

28

Anderson et a!. ( 1993)

Northern Ireland

TP TP

43

25

800

Christie and Smal (1993)

Southeastern Ontario

TP. lotal nitrogen

36

6

75.6

Fritz el al. (1993)

Michigan

41

Bennion ( 1994)

Soulheaslern England

TP TP

30

25

646

Dixit and Smal (1994)

Northeastern U.S.A .

TP e l, pH , Secchi

64

0.8

154.5

Our model

Britis h Columbia

TP

59

5

85

I

of a few taxa, such as Denticula kuetlillgii (Denticula elegans in Hall & Smol ( 1992)), decreased. OUf [TP] inference model was developed from lakes rang ing between 5 and 85 jJ.g ,- I, and ca n now be used to infer past lakewater [TP] with in this range fro m fossil diatom assembl ages. Th e model can be used to infer past [TP] in lakes that also fa ll with in the range of values for the important environmental variables (conducti vity 0--410 J-l,S em - I ; lake depth > 4 m ; pH 6.2-8.5). Lakewater [TP] can be estimated using the classical deshrinking regress ion equation y=O. 18x+ 2.30 where x and y represent, respectively, uncorrected and corrected [TP] inferences (Birks et al. . 1990a, b). Thi s eq uation was deri ved from the model illustrated in Fig. 5d. Applying the model. Re markable differences were not-

ed between the origi nal and updated models when in ferring [TP] from the fossi l assembl ages of Wood Lake (Fig. 8). The reconstruction using the 37 lake model (Hall & Smol, 1992) described a mild eutrophi catio n event whi ch coi ncided with th e relative increase in Stephanodiscus minutulus; an increase from 21 to 29 {Lg I- I TP from the bottom of the core to the mid1960s. From Ihe I 960s to 1992, [TP] apparently dipped slightly but generall y maintained a mildly eutrophic level. The 59 lake model provided a [TP) profile more consistent with known historical changes at Wood Lake (Walker et aI., 1994). The approximately 85-year period of eutrophi cati on (-I 876- 1960s) described above

51

40

r2 = 0_09

a. "

"..i!

n

I-

= 59

20

.. " • ........

.. ....

.,/>., =0.42). The diatom assembl ages from these Mi ch igan lakes are und oubtedl y hi ghly regulated by trophic vari ab les, as dem onstrated by their relationship to total nitrogen, Secchi depth and chlorophyll a (as described in a CCA). On the other hand, BC lakes ex hibit vast extremes of environmental conditions (main ly because we sampled a wider range of biogeoclimatic zones), and the [TP] signal has probably been ' blurred' by the cumulative influence of these condi ti ons. The avai lab ility of environmental data is a chal lenge when compi lin g any training set (A nderson etal., 1993; Christi e & Smo i, 1993). Ideally, the chemical information for each lake represents an average of several measurements, taken over a few years prior to the sampling of surficial sedi ment. In this way there is little error in assu ming that the diatom communities represe nted in the sediments are a product of the measured environ mental conditi ons. Often it is not logistically possi ble to obtain a ri gorous assessment of environmental variability for all sites, primarily because of climate, access, and resource constraints. These sources of error were inev itable in thi s calibration; however, by co nsidering numerous sites, red undancies in environmental and biolog ical data may red uce inaccuracies. The WA calibration calc ulati ons were performed usi ng classical regression (Fig. 5; inverse regression data not show n), because this resulted in the lowest RM SE and th e least noticeable trend in the residuals from the observed versus in fe rred [TP] analyses. Hall & Smol ( 1992) used similar methods. WA calibration

without species transformation resu lted in a decrease in the strength of the relati onshi p with an increase in size of the lake set (Figs Sa, b), whereas, when the spec ies data were tran sformed, the strength of the relati onsh ip did not decrease as markedl y (Figs Sc, d). The additi on of lakes apparentl y increased th e spread of puinls around the I: I line, thereby decreasing the WA regress ion coefficients and increasing RM SE va lues. However, the bootstrapped rel ationship was stron ger in the aug mented lake set, likely because the in creased sample size and enl arged [TP] gradient (Fig. Se, f) permitted more accurate estim ates of the true [TP] optima of the diatom taxa; the new lakes al so provided add iti onal data for the random selection algorithm used durin g bootstrapp ing. Although our new model (Fig. Sd) had a lower regression coefficient (Figs Sc, d) and a larger error (Figs Se, f) than th e old model, we believe that it can provide more realistic inferences ofTP. We make thi s asserti on because the [TP] gradient has been greatly enlarged, and the diato m optima have likely been more real isticall y defined (di scussed below). The new WA model can be used to infer past lakewater [TP] from diatom assemblages preserved in the sediments of British Columbia lakes, ideall y with in the range of S- 8S j.Lg I- I . Our [TP] reconstructi ons for Wood Lake clearl y illustrate the limitations of the original 37-lake trai nin g set. Although a slight eutrophication trend was inferred by the old model, it appeared unresponsive to the severe changes that occurred as a result of cultural nutrient loading to Wood Lake (Stockner & Northcote, 1974). Furthermore, the new model clearly indicates the improvement in water quality caused by a decrease in the retention tim eofthe lake. A disti llery, which was constructed in 1971 , pumps large vo lumes of cooling water from nearby Okanagan Lake into a creek upstream from Wood Lake, and since then the water replacement time has decreased from 30 to 14 years (Walker et al., 1994). The new model also recogni zes the F. capucina var. mesoiepta bloom (4-S cm) as a signifi cant nutrient peak. It is possible that a sudden nutrient-ri ch in put occurred c. 1986, but unfortunately, historical measurements are unava ilable for confirmati on. The new model also satisfactorily inferred the recent TP concentration in Wood Lake (actual TP 45.4 j.Lg I-I, in fe rred TP 4S.8 j.Lg I- I; Reavi e et al., 1995), whereas th e old model inferred a mu ch lower val ue(27. I I"g l ~ ').

By expanding th e dataset we have developed a more useful too l for inferring nutrient conditi ons in BC lakes. Most importantly, th e range of [TP] optima has been increased, makin g the model more usefu l for

in ferring eutro phi c condi tions. Also, weighted average optima for 19 morc taxa have been added. The poss ibility of encou ntering a lake with incompatible nutrient co ndition s, o r fossil diatom assemblages with poor analogues to the training set, has become less likely. The (TP] range of o ur model is comparable with that of others (Table 6). Agbeti ( 1992), Christie & Smol ( 1993), Fritz ef at. ( 1993 ) and D ixit & Smol ( 1994) e mpl oy a lake (TP] range similar to ours. A clear difference in lake nutrie nt burdens occ urs between North Ameri ca and the British Isles. M odels from Northern Ireland (A nderson er ai., 1993) and E ngland (Bennio n, 1994) reac h TP concentrations almost ten-fold of our maximum [TP]. Moreover, the British Isles training sets are devo id of oligotrophi c lakes «25 J-Lg 1- 1), whi le the North American mode ls suffic iently fill this portion of the [TP] gradient. The North American models generally inferred [TP] more accurately withi n the ol igo- mesotrophic (TP] spectrum , whereas the British mode ls were more useful for inferring ex tremely e utrophic conditions. The high producti vity of British lakes is probably attributable to a very shall ow average depth a nd a greater degree of land use. This compariso n between avai lable trophic infere nce models illustrates that reg io nal differe nces in limnological c haracteristi cs occur. Furthermore, species optima change relative to the size of the rTP] g rad ien t. Benefits of the calibratio n approach are made clear by applications, suc h as o ur Wood Lake example. What begins as an array of complex diatom stratigraphi c data can be transformed into info rmation that is easy to unde rstand and communicate, and is useful for lake management. Quantitative reconstruction s can defin e the timing and extent of cu ltural disturbance, and identify pre-di sturbance condition s. Reconstructions can also reveal geograph ic a reas where lakes tend to be naturall y e utrophic, and hence where remediation is not like ly to improve water quality.

Ac knowledgm ents We th ank Col in McKean and Rick Nordin at the BC Ministry of the Environment, Victoria, for the origi nal sediment and enviro nme ntal data, as we ll as updated water qual ity information. Ian Walker at Okanagan Uni versity College provided th e sediments and 210 Pb dates for Wood Lake. We thank o ur colleagues at PEA RL fo r many useful di scussions of this project.

This research was fund ed by the Natural Sciences and Engineering Research Counci l of Canada via a grant to JPS. R. Battarbee edi ted this contribut io n.

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