A Lakewide Comparison Study of Phytoplankton Biomass and its Species Composition in Lake Huron, 1971 to 1985

J. Great Lakes Res. 17(4):553-564 Internat. Assoc. Great Lakes Res., 1991

A LAKEWIDE COMPARISON STUDY OF PHYTOPLANKTON BIOMASS AND ITS SPECIES COMPOSITION IN LAKE HURON, 1971 TO 1985

Joseph C. Makarewicz

Department of Biological Sciences State University of New York at Brockport Brockport, New York 14420 Paul Bertram

Great Lake National Program Office U.S. Environmental Protection Agency Chicago, Illinois 60604 ABSTRA CT. Phytoplankton were collected at offshore sites during 23 cruises in the spring, summer and autumn of 1983, 1984, and 1985. Forty common species and varieties from a total of 448 taxa accounted for 89.8% of the total abundance and 88.1% of the total biomass. The average phytoplankton density (mean ± S.£.) from April through December was as follows: 19852,020± 113 cells/mL, 1984-2,918± 196 cells/mL, 1983 -2,567± 178 cells/mL. Average biomass for the same period was: 1985-0.34±0.021 g/m 3, 1984-0.41 ± 0.44 g/m 3, 1983-0.37±0.040 g/m 3 • The following predominant diatoms were observed in 1974, 1983, 1984, and 1985: Asterionella formosa, Cyclotella comensis, C. comta, C. ocellata, Fragilaria crotonensis, Melosira islandica, Tabellaria flocculosa, and Rhizosolenia spp. The historical consistency of the mesotrophic-eutrophic diatom ratio, the similarity of dominant species, the predominance of oligotrophic and mesotrophic diatom indicator species, and the similarity in phytoplankton biomass suggest little change in the trophic status of the offshore waters of Lake Huron from 1971 to 1982-85. INDEX WORDS: Lake Huron, phytoplankton, oligotrophic.

INTRODUCTION

historical trend evaluation (Munawar and Munawar 1982), and has resulted in a call for standardized techniques for surveillance of the Great Lakes (Munawar and Munawar 1980). In this study the 1983-85 phytoplankton data assemblage presented makes it possible to examine the geographic and seasonal relationships prevailing in Lake Huron and to compare them to the 1971 and 1980 studies that utilized similar enumeration techniques.

Nutrient loading of lakes, navigation, fish management policies, shoreline alteration, contaminant production and, in general, economic development, ultimately affect lake ecosystems. Ecosystems respond to stress with compensatory changes in community structure and function mediated at the population level (Boesch and Rosenberg 1981). Because phytoplankton have short carbon turnover rates, are sensitive to water quality conditions, and represent an integrative response to perturbation of the lake ecosystem, the determination of phytoplankton abundance and species composition have become established as methods to trace long-term changes in the lakes (Munawar and Munawar 1982). Phytoplankton analyses in the Great Lakes have been carried out since the end of the last century. However, because of the diversity of techniques used, the data base is inconsistent, cannot provide

METHODS Phytoplankton were collected at offshore sites during 22 cruises in the spring, summer, and autumn of 1983(7), 1984(8), and 1985(7) (Table 1, Fig. 1). For analysis of spatial and temporal distributions of phytoplankton assemblages, eastern and western stations of similar latitude were considered to be representative of the same water mass. Geographic references are reported as western station locations (Table 1). An 8-liter PVC Niskin bottle 553

554

MAKAREWICZ and BERTRAM

TABLE 1. Dates and stations sampled in Lake Huron, 1983-1985. Stations sampled by helicopter in January and February were similar, but not identical to Group 2 stations. Sampling Dates-1983

Sampling Dates-1984

Group

21-24 Apr 6-8, 12 May 2-4 Jul 4-6 Aug 19-21 Aug 16-18 Oct 24-26 Oct 10-11 Feb 1984

12-15 Apr 4-5 May 5-7 Jul 3-4 Aug 10-12 Aug 17-18 Aug 30 Nov-2 Dec 10-12 Dec 15-16 Jan 1985 9-10 Feb 1985

2 2 2 2 2 2 1

Group 1 2 1 1 2 1 1 2

Sampling Dates-1985 22-23 Apr 29-30 Apr 15-16 Aug 23-25 Aug 29-30 Aug 18-19 Nov 27-28 Nov

Group 1 2 1 2 1 1

2

The vertical pairings below indicates east-west station grouping of similar latitude. Group 1 Stations: 57, 53, 48, 43, 38,29,93, ,92, 91, 90 Group 2 Stations: 61, 54, ,45, 37, 32, 27, 15, 12,09,06

mounted on a General Oceanics Rossette sampler with a Guild-line electrobathythermograph (EBT) was used to collect phytoplankton. One-liter composite phytoplankton samples were obtained by composition equal aliquots from samples collected at depths of 1, 5, 10, and 20 m. Phytoplankton samples were immediately preserved with 10 mL of Lugol's solution, while formaldehyde was added upon arrival in the laboratory. The settling chamber procedure (Utermohl 1958) was used to identify (except for diatoms) and to enumerate phytoplankton at a magnification of 500 x. A second

.•

27

RESULTS AND DISCUSSION

~

15

92

12

• • 9

.91

Lake Huron Main Lake Sampling Locations

• 6 90 •

•

FIG. 1.

identification and enumeration of diatoms at 1,250 x was performed after the organic portion was oxidized with 30% HzO z and HN0 3 • The cleaned diatom concentrate was air dried on a # 1 cover slip and mounted on a slide (75 x 25 mm) with HYRAX™ mounting medium. The cell volume of each species was computed by applying average dimensions from each sampling station and date to the geometrical shape such as sphere, cylinder, prolate spheroid, etc., that most closely resembled the species form. At least 10 specimens of each species of each sample were measured for the cell volume calculation. When fewer than 10 specimens were present, those present were measured as they occurred. For most organisms, the measurements were taken from the outside wall to outside wall. The protoplast was measured with loricated forms, while the individual cells of filaments and colonial forms were measured. For comparative purposes, biovolume (j.tm 3 /L) was converted to biomass (g/m 3 ) assuming the specific gravity of phytoplankton to be 1.0 (Willen 1959, Nauwerck 1963).

Lake Huron sampling stations.

Annual Abundance of Major Algal Groups From 1983 to 1985, 449 species representing 110 genera from eight divisions comprised the offshore phytoplankton community of Lake Huron. Forty common species and varieties accounted for 89.8010 of the total abundance and 88.1 % of the total bio-

LAKE HURON PLYTOPLANKTON 1971-1985

555

TABLE 2. Summary of common phytoplankton species occurrence in Lake Huron during the period 1983 to 1985. Common species were arbitrarily defined as having an abundance of 2: 0.5% of the total cells or 2: 0.5% of the total biomass.

Taxon BACILLARIOPHYTA

Asterionella formosa Cyclotella comensis v. 1 CYe/otella comensis v. 2 Cye/otella comta Cye/otella kiitzingiana v. phanetophora Cye/otella ocellata Cye/otella sp. Cye/otella stelligera Cymatopleura solea v. apiculata Fragilaria crotonensis Fragilaria intermedia v. fallax Melosira islandica Rhizosolenia eriensis Rhizosolenia longiseta Rhizosolenia sp. Stephanodicscus alpinus Stephanodiscus niagarae Stephanodiscus transilvanicus Tabellaria flocculosa

Average Cells/mL

010 of Total Cells

168 1,367 101 72 446 1,000 148 267 3 375 60 122 131 295 143 19 5 8 181

20.7 63.4 15.7 4.0 18.7 63.6 14.1 16.2 0.1 34.3 3.9 12.0 10.3 8.3 4.7 0.9 0.2 0.5 19.0

0.83 2.55 0.63 0.16 0.75 2.56 0.57 0.65 0.00 1.38 0.16 0.48 0.41 0.33 0.19 0.04 0.01 0.02 0.76 12.49

6.8 2.2 0.5 12.3 3.5 4.9 0.6 0.3 4.5 27.9 2.9 15.5 45.6 6.3 32.8 2.5 4.8 5.7 51.4

1.82 0.58 0.13 3.28 0.93 1.32 0.16 0.09 1.21 7.47 0.76 4.17 12.23 1.68 8.78 0.66 1.29 1.52 13.77 61.86

16

0.5

0.02

2.6

0.70

160 1,325 196 254 589 540 859

19.0 36.0 10.4 20.5 36.8 26.5 128.7

0.77 1.45 0.42 0.83 1.48 1.07 5.17 lLl8

0.1 10.1 3.3 4.5 6.0 3.7 1.8

0.03 2.71 0.89 1.20 1.60 0.98 0.48 7.89

387

36.6

1.47

0.8

0.20

115 31 33 360

20.2 5.4 3.4 164.0

0.81 0.22 0.14 6.59 7.76

0.6 10.3 1.9 12.6

0.15 2.77 0.51 3.37 6.81

1,145 4,606 434 1,047 1,554 1,080 548 644

14.1 318.4 14.6 50.2 65.8 59.6 17.9 13.1

0.57 12.80 0.58 2.02 2.65 2.40 0.72 0.53 22.26

Total CHLOROPHYTA Cosmarium sp. CHRYSOPHYTA

Chrysophycean coccoids Chrysosphaerella longispina Dinobryon cylindricum Dinobryon divergens Dinobryon sociale Dinobryon sociale v. americanum Haptophyceae Total COLORLESS FLAGELLATES Colorless flagellates CRYPTOPHYTA

Chroomonas norstedtii Cryptomonas erosa Cryptomonas pyrenoidifera Rhodomonas minuta v. nannoplanktica

Mean Biomass

Maximum Cells/mL

Total

mg/m 3

% of Total Biomass

CYANOPHYTA

Agmenellum quadruplicatum Anacystis montana v. minor Coccochloris elabans Coelosphaerium naegelianum Gomphosphaeria acustris Oscillatoria limnetica Oscillatoria minima Oscillatoria subbrevis Total

0.01 1.5 0.2 0.2 0.3 0.4 0.3 0.9

0.00 0.40 0.05 0.06 0.08 0.12 0.08 0.24 1.02

Continued

556 TABLE 2.

MAKAREWICZ and BERTRAM Continued

Maximum Cells/mL

Taxon PYRROPHYTA Ceratium hirundinella Gymnodinium helveticum f. achroum Gymnodinium sp. Gymnodinium sp. #2 Total UNIDENTIFIED Unidentified flagellate-ovoid Unidentified flagellate-spherical Total

070 of Total Cells

8 8 8 8

0.1 0.2 0.3 0.2

0.00 0.01 0.01 0.01 0.03

7.0 2.1 1.9 3.0

1.89 0.58 0.52 0.81 3.80

1,587 5,211

493.2 366.0

19.83 14.72 34.55

14.0 7.8

3.75 2.08 5.83

Total

89.76

mass (Table 2). The average phytoplankton density (mean ± S.E.) from April through December was as follows: 1985 -2,020± 113 cells/mL, 19842,918 ± 196 cells/mL, 1983 -2,567± 178 cells/mL. Average biomass for the same period was: 19850.34±0.021 g/m3, 1984-0.41 ±0.44 g/m 3 , 19830.37±0.040 g/m 3 • From 1983 to 1985, the Bacillariophyta contained the largest number of species (range = 120 to 158) (Table 3) and the highest relative biomass (range = 62.70/0 to 70.1 % of the total, Table 4). The Chlorophyta had the second highest number of species (Table 3), while the Chrysophyta and Cryptophyta accounted for the

TABLE 3. Number of species and genera observed in each algal division or grouping, Lake Huron, 1983, 1984, and 1985. Division BAC CHL CHR CRY CYA COL PYR EUG UNI CAT Total

Species 1983 1984 1985 158 73 36 22 13

156 64 35 17

13

13 13

10 4 3 1 333

9 1 4 0 312

120 32 25 14 9 3 3 0 3 1 210

Mean Biomass mg/m 3

Average Cells/mL

Genera 1983 1984 1985 29 28 10 3 6 4 4 3

28 28 12 4 7 5 4 1

26 16 10 3 6 2 3 0

1 88

0 89

1 67

% of Total

Biomass

88.10

second highest biomass. Cyanophyta and Chlorophyta relative biomass was higher in 1985 than 1984 and 1983, while Pyrrophyta biomass was considerably lower in 1985 than in 1983 and 1984 (Table 4). Highest overall densities were attained by the Cyanophyta and the Bacillariophyta (Table 4). Unidentified organisms represented 33.8% of the total cells but only 5.9% of the total biomass for the 3-year period.

TABLE 4. Relative abundance of major phytoplankton divisions in Lake Huron, 1983, 1984, and 1985. BA C = Bacillariophyta, CAT = Chloromanophyta, CHL = Chlorophyta, CHR = Chrysophyta, COL = Colorless Flagellates, CRY = Cryptophyta, EUG = Euglenophyta, CYA = Cyanophyta, PYR = Pyrrophyta, UN] = Unidentified. 070 Biomass 1984 1985

Division

1983

BAC CAT CHL CHR COL CRY CYA EUG PYR UNI

70.05 .02 3.55 7.30 .15 8.51 1.72 .12 3.33 5.24

62.75 0.00 2.74 10.93 .11 7.92 1.47 .06 7.88 6.12

1983

67.37 8.65 .02 <0.01 3.70 3.11 10.03 11.92 .74 .42 8.41 9.26 2.02 21.92 .00 .01 1.27 .11 5.89 45.43

% Cells 1984 1985 17.18 0.00 3.58 14.27 .68 7.20 26.95 .01 .15 29.99

18.18 < .01 4.56 14.31 6.02 11.24 19.48 .00 .11 26.08

LAKE HURON PLYTOPLANKTON 1971-1985 1.6

557

A

1983 1984 v 1985 0

•

1.2

•

I"")

E

...........

0.8

O'l

0.4

0.0

A

M

J

J

A

a

S

N

D

F

J

MONTH

B

0.6

0.5

./.~

0 I"")

E

...........

•

0.4-

O'l

0.3

0.2

61

54

45

37

32

27

12

9

6

STATION FIG. 2. Seasonal (A) and geographical (B) biomass trends in 1983, 1984, and 1985, Lake Huron. In panel B, the x-axis represents station numbers. East and West stations were combined to give a single North-South axis.

Seasonal Abundance and Distribution of Major Algal Groups Seasonally, biomass peaked in late spring or early summer (Fig. 2a). The Bacillariophyta were dominant throughout the study period accounting for as much as 78.2070 but never less than 27.4070 of the phytoplankton biomass (Fig. 3). The large drop in the relative importance of diatoms in August of 1983 (to - 30070 of the total biomass) was not

observed in 1984 and 1985. The lack of a bloom of Rhizosolenia eriensis in August of 1983, observed in 1984 and 1985, appeared to be the cause of the decrease in relative importance of the diatoms in 1983 (Table 5). The Chrysophyta succeeded the diatoms in 1984 and 1985, when there was a small decrease in the relative importance of the diatoms seasonally. In 1983, the year of the large decrease in relative importance of the diatoms, the Cyanophyta and

MAKAREWICZ and BERTRAM

558 100 80

60

40 20 0 90 110130150170190210230250270290310330350370390410

Julian Day from 1983 100 U) U)

tU

E

80

0

m

60

c(1)

...0

(1)

40

n.

.~

.!1 ::J E ::J

0

20

0 90 110 130 150 170 190 210230250270290310330350370390410

Julian Day from 1984 100 80

60 40

20

0 90 110 130 150 170 190 210230250270290310330350370390410

Julian Day from 1985

o

SAC

~ CHR

I::::j CYA

~ CRY

FIG. 3. Seasonal trends in relative biomass ofselected divisions ofphytoplankton, 1983-1985. Data points are the average for all stations.

LAKE HURON PLYTOPLANKTON 1971-1985

Cryptophyta succeeded the diatoms. Diatoms regained their spring predominant position by autumn. The Cryptophyta were more prevalent during isothermal winter conditions (Fig. 3). Geographical Abundance and Distribution of Major Algal Groups

Algal biomass in 1983, 1984, and 1985 decreased from northern Lake Huron to Station 37 (northeast of Saginaw Bay), where biomass increased to Station 12 or 9 and then decreased south to Station 6 (Fig. 2b). This pattern was caused predominately by diatoms and somewhat by chrysophytes (Fig. 4). The increase south of Station 37 was particularly evident in the spring of each study year (Fig. 5). With the exception of Station 61, Cyanophyta biomass was generally higher in northern Lake Huron in 1983 and 1985 and decreased precipitously to Stations 27 and 32 before increasing in southern Lake Huron (Fig. 6). The Chrysophyta (Fig. 4), and in some years the Pyrrophyta (Fig. 6), had higher biomasses south of Station 32 than north of Saginaw Bay. No obvious geographical pattern was observed with the Chlorophyta. In 1974, midlake stations in southern Lake Huron were affected by populations of phytoplankton from Saginaw Bay (Stoermer and Kreis 1980). This transport of eutrophication-tolerant algal populations into southern Lake Huron from

559

Saginaw Bay was thought to be mitigated in recent years (Stoermer and Theriot 1985). If true, this suggests that the higher biomass and the species composition observed in the southern Lake Huron are due to factors within southern Lake Huron watershed and not Saginaw Bay. Historical Changes in Species Composition

The literature pertaining to phytoplankton of the offshore waters of Lake Huron is sparse. Fenwick (1962, 1968) published some qualitative data, and Parkos et al. (1969) listed species observed. Quantitative data from offshore sites sampled in 1971 exist (Munawar and Munawar 1979, 1982; Vollenweider et al. 1974). Stoermer and Kreis (1980) reported on an extensive sampling program in southern Lake Huron including Saginaw Bay during 1974. Lin and Schelske (1978) reported on a single offshore station sampled in 1975. An intensive study of the entire lake basin was performed in 1980 (Stevenson 1985), but only a few offshore stations were sampled. Diatoms have been the dominant division since at least 1971. Dominant diatoms in 1971 included species of Asterionella formosa, A. gracillima, Cyclotella comta, C. glomerata, C. ocellata, C. michiganiana, Melosira islandica, and M. granulata. In addition, species such as Fragilaria crotonensis and Tabellaria fenestrata were common,

560

MAKAREWICZ and BERTRAM 0.40

o •

0.30

~V;

1983 1984

BAC

1985

~~

0.20

0.10 0.04 0.03

,.., E

"01

v 0.02 0.01 0.00

0.10

CHR 0.08 0.06 0.04 0.02 0.00

61

54

45

37

32

27

12

9

NORTH

SOUTH

FIG. 4. Geographical biomass trends in Lake Huron, 1985-1985. BAC riophyta, CHL = Chlorophyta, CHR = Chrysophyta.

and cryptomonads, such as Rhodomonas minuta and Cryptomonas erosa, also contributed very heavily during different seasons. The following similar common diatoms were observed in 1974, 1983, 1984, and 1985: Asterionella formosa, Cyclotella comensis, c. comta, C. ocellata, Fragilaria crotonensis, Melosira islandica, Tabellaria flocculosa, and Rhizosolenia spp. Synedra fiJiformis was present in 1983, 1984, and 1985 (2.1 cells/

6

=

Bacilla-

mL) but was not as common as in the 1974 southern Lake Huron plus Saginaw Bay data (52.4 cells/mL). The lower abundance of C. stelligera in 1983 (6.5 cells/mL), 1984 (25.3 cells/mL), and 1985 (13.4 cells/mL) compared to 1971 (> 5070 of the phytoplankton biomass) (Munawar and Munawar 1979), 1974 (54 cells/mL) (Stoermer and Kreis 1980), and 1975 (111 cells/mL) (Lin and Schelske 1978) was probably caused by the lack of sampling

LAKE HURON PLYTOPLANKTON 1971-1985 0.30

1983 ~APR 21 -

0.20

561

_MAY

6 -

24 8

0.10

0.00 0.08 0.06

~APR 12 _MAY 4 -

15 5

1984

~APR 22 _APR 29 -

23 30

1985

r-r)

E

0.04

01

0.02

"--. 0.00

0.20

0.10

0.00

-+----"~

61

54

45

37

32

27

12

9

6

FIG. 5. Spring geographical biomass trends in Lake Huron, 1983-1985.

in 1983 to 1985 during mid and late July when this species is dominant. Both Cryptomonas erosa and Rhodomonas minuta var. nannoplanktica were dominant in 1971, 1974, 1983, 1984, and 1985. Dominant chrysophytes in 1971 were Dinobryon divergens and Chrysophaerel/a longispina. In 1983, 1984, and 1985, these two species were common along with D. cylindricum and D. sociale (Table 2). D. cylindricum and D. sociale var. americanum appeared to have increased in abundance since 1974 (Table

6). Haptophytes were also numerically abundant. In general, species composition of common offshore algae has changed little since 1971. Indicator Species Dominant species were predominantly indicators of oligotrophy. Dominant diatoms in Lake Huron in 1983, 1984, and 1985 were Rhizosolenia sp., Tabel/aria jlocculosa (biomass), and Cyclotel/a comensis (numerically). Four species of Cyclotella

562

MAKAREWICZ and BERTRAM 0.02

1983 1984 1985

CYA

0.01

0.00 -t----;---+-----.;I----+--t---+--+---+----l

0.05

CRY v

0.04 0.03 0.02 0.01

61

54

45

37

32

27

12

9

NORTH

FIG. 6. Geographical biomass trends in Lake Huron, 1983-1985. CYA nophyta, PYR = Pyrrophyta, CRY = Cryptophyta.

(c. comensis, C. comta, C. kuetzingiana var. planetophora, and C. ocellata) represented 9.4010, 6.6%, and 7.5% of the total biomass in 1983, 1984, and 1985. Rhizosolenia eriensis is often grouped with oligotrophic offshore dominants even though it may occur in greater abundance in areas receiving some degree of nutrient enrichment (Stoermer and Yang 1970). Except for C. comensis, whose ecological affinities are poorly under-

6

SOUTH =

Cya-

stood (Stoermer and Kreis 1980), these species are associated with oligotrophic or mesotrophic conditions. Tabellaria flocculosa is commonly associated with mesotrophic conditions (Tarapchak and Stoermer 1976). Dominant chrysophytes (1983-1985) included Dinobryon sociale var. americanum, D. divergens, and D. cylindricum, which are often associated with several small members of the genus eye/otella

LAKE HURON PLYTOPLANKTON 1971-1985

563

LAKE HURON

TABLE 6. Abundance ofDinobryon cylindricurn and Dinobryon sociale arnericanurn in 1974, 1983-85. 1974 data are from Stoermer and Kreis (1980). cells/mL D. cylindricum

D. sociale americanum

0.1 13.8 18.1 3.1

0.1 42.1 37.6 9.2

1974 1983 1984 1985

1'0

E

...........

E

0.8

~,i

Cl

0.0 1970

1974

1978

1982

1986

(Schelske et al. 1972, 1974) included in the classical oligotrophic diatom plankton association of Hutchinson (1967). Dominant cryptophytes, cyanophytes, and dinoflagellates were Rhodomonas minuta var. nannoplanktica, Cryptomonas erosa, Anacystis montana var. minor (numerically), and Ceratium hirundinella from 1983 to 1985.

FIG. 7. Historical offshore biomass trends in Lake Huron. Values are the mean ± S.E. (wide vertical bar) and the range (narrow vertical bar). The 1971 data are from Munawar and Munawar (1979). The 1980 data are modified from a Great Lakes National Program Office (U.S. Environmental Protection Agency) data base (Makarewicz et al. 1991).

Trophic Status Studies available of the Lake Huron phytoplankton assemblage (Munawar and Munawar 1979; Nicholls et al. 1977; Schelske et al. 1972, 1974) indicated that the waters of northern Lake Huron generally contain phytoplankton assemblages indicative of oligotrophic conditions. The ratio of mesotrophic to eutrophic species in Lake Huron has changed little from 1971 (Table 7), suggesting that the trophic status of the lake has not

changed. The designation of the offshore waters of southern Lake Huron as oligotrophic based on phytoplankton composition in 1983, 1984, and 1985 is not unlike the trophic status suggested by Stoermer and Kreis (1980) for the offshore waters of southern Lake Huron in 1974. The suggestion of an oligotrophic status for the offshore waters from the composition of the indicator species agrees well with the trophic status as determined by the biomass classification scheme of Munawar and Munawar (1982). With a mean biomass of 0.37, 0.41, and 0.34 g/m 3 for 1983, 1984, and 1985, respectively, Lake Huron remains oligotrophic.

TABLE 7. Distribution of indicator diatom species in Lake Huron. The classification scheme of Tarapchak and Stoermer (1976) was utilized. M1 = mesotrophic but intolerant of nutrient enrichment, M2 = mesotrophic and tolerant of moderate nutrient enrichment, E = eutrophic. 1971, 1975-76, and 1983 data are from Munawar and Munawar (1979), Lin and Schelske (1978), and this study. 1971 1975-762 1983 3 19843 1985 3 1

1

2

3

Ml

M2

E

Ml +M2IE

6 2 7 6 5

3 4 2 3 3

3 2 2 3 3

3.0 3.0 4.5 3.0 2.7

Only diatoms contributing > 5070 of the seasonal biomass are classified. Only "abundant" diatom species are classified. Only diatoms contributing> 0.5% of the biomass for the study period are classified.

Historical Changes in Community Abundance and Biomass Quantitative phytoplankton data exist for the offshore waters of Lake Huron from at least 1971. The collections of Stoermer and Kreis (1980) were from 44 stations in southern Lake Huron and Saginaw Bay. Lin and Schelske (1978) collected from one offshore station in 1975. In both studies, phytoplankton were concentrated on Millipore filters rather than by the settling chamber procedure used in the GLNPO Data Base (1980) and this study. Thus, data sets are not strictly comparable. Munawar and Munawar (1979, 1982) collected phytoplankton at nine different offshore sites with a 20-m integrating sampler from April to December of 1971. Because Uterm6hl's (1958) procedure for enumeration of algae was employed, these data

564

MAKAREWICZ and BERTRAM

were directly comparable to the 1980, 1983, 1984, and 1985 data sets. Phytoplankton biomass between 1971, 1980, 1983, 1984, and 1985 was not significantly different (Fig. 7). The consistency of the mesotrophic-eutrophic ratio through time, the similarity of dominant species, the occurrence of oligotrophic and mesotrophic indicator species, and the similarity of the offshore biomass suggest little change in the trophic status (oligotrophic) of the offshore waters of Lake Huron since 1971.

REFERENCES Boesch, D. F., and Rosenberg, R. 1981. Responses to stress in marine benthic communities. In G. W. Barrett and R. Rosenberg (eds), Stress Effects on Natural Ecosystems, pp. 179-200. John Wiley & Sons Ltd. Fenwick, M. G. 1962. Some interesting algae from Lake Huron. Trans. Am. Microsc. Soc. 81:72-76. _ _ _ _ . 1968. Lake Huron distribution of Tabellaria fenestrata var. geniculata A. Cleve and Coelastrum reticulation var. polychordon Korshik. Trans. Am. Microsc. Soc. 87:376-383. GLNPO Data Base. 1980. Phytoplankton data base, Lake Huron. EPA, Great Lake National Program Office, Chicago, Illinois. Hutchinson, G. E. 1967. A Treatise on Limnology. Vol II. Introduction to Lake Biology and Limnoplankton. J. Wiley & Sons, N. Y. Lin, C. K., and Schelske, C. L. 1978. Effects of nutrient enrichment, light intensity and temperature on growth of phytoplankton from Lake Huron. EPA600/3-79-049. Makarewicz, J. c., Lewis, T., and Bertram, P. 1991. Phytoplankton and zooplankton in Lakes Erie, Huron, and Michigan: 1985. U.S.EPA Great Lakes National Program Office, Chicago, Illinois. EPA905/3-90-003. Munawar, M., and Munawar, I. F. 1979. A preliminary account of Lake Huron phytoplankton, AprilDecember 1971. Fish. Mar. Servo Tech. Rep. 917. _ _ _ _ , and Munawar, I. F. 1980. The importance of using standard techniques in the surveillance of phytoplankton indicator species for the establishment of long range trends in the Great Lakes: a preliminary example, Lake Erie. In Can. Tech. Rep. Fish. Aquat. Sci. No. 976, pp. 59-86.

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