- Email: [email protected]
Comparison of Lake Ontario Zooplankton Communities Between 1967 and 1985: Before and After Implementation of Salmonid Stocking and Phosphorus Control
J. Great Lakes Res. 13(3):328-339 Internat. Assoc. Great Lakes Res., 1987
COMPARISON OF LAKE ONTARIO ZOOPLANKTON COMMUNITIES BETWEEN 1967 AND 1985: BEFORE AND AFTER IMPLEMENTATION OF SALMONID STOCKING AND PHOSPHORUS CONTROL
Ora E. Johannsson Great Lakes Laboratory for Fisheries and Aquatic Sciences Canada Centre for Inland Waters 867 Lakeshore Road, P. O. Box 5050 Burlington, Ontario L7R 4A6 ABSTRACT. Two strong, contrasting management strategies were applied to Lake Ontario during the 1970s. There has been a 40 percent reduction in phosphorus loadings to the lake and an exponential increase in salmonid stocking. A comparison of zooplankton community structure and abundancefrom 1981 to 1985 with that of earlier studies (1967 to 1972) found no detectable change in the range of abundances or community composition between the two periods. Clearly, neither management strategy has had a discernible impact on the zooplankton community to date; however, the potential for change in the system remains high. ADDITIONAL INDEX WORDS: Eutrophication, plankton, ecological distribution, fish stocking, phosphorus.
introduced salmonids feed heavily on alewife (Olson 1984, Brandt 1986, Savoie 1986), the principal planktivore in the lake (Lackey 1969, Hutchinson 1971, Christie 1973 and 1974, Janssen 1978). The present study compares community structure and abundance of zooplankton from 1981 to 1985 with that in earlier years to see whether there have been changes in the abundance or structure (and therefore functioning) of this trophic level that might be attributable to either or both of the two lake management strategies.
INTRODUCTION In 1981 open-water zooplankton monitoring commenced on Lake Ontario as part of a more comprehensive biological monitoring program, the Bioindex Program. Prior to this, the most recent comprehensive zooplankton surveys were conducted between 1967 and 1972 (Patalas 1969, Czaika 1974, Watson and Carpenter 1974, McNaught et al. 1975). During the intervening years two important management strategies have been applied to the lake: phosphorus loadings were reduced by 40 percent (Stevens and Neilson 1987), and salmonid stocking was increased exponentially until in 1981 approximately 4.5 million fish were added to the lake annually (Kolenosky and LeTendre 1984). By 1984/1985, 7 to 8 million were stocked annually (Daniels and LeTendre 1985). Both of these management strategies could potentially affect the zooplankton community. Chan:>;es in phosphorus levels in the lake could alter the abundance and type of phytoplankton available as food to zooplankton (Dillon et al. 1978, Nicholls et al. 1986). Increases in the abundance of the top predators in the lake could alter predation pressure on zooplankton through cascading trophic effects (Carpenter et al. 1985). All
METHODS Weekly samples were collected between March and November in 1981 to 1985 at four stations representative of different open-water regions within Lake Ontario. The stations correspond to long-term water quality stations, hence the unique station numbers (Fig. 1). Station 12 is located south of Toronto in an area of frequent summer upwellings; station 41 is in the more stable midlake region; station 81 is in the productive eastern basin; and station 93 is near the south shore, within the influence of the Niagara River. In an effort to decrease the cost of the monitoring pro-
328
LAKE ONTARIO ZOOPLANKTON: 1967-1982
329
SAMPLING LOCATIONS IN LAKE ONTARIO
-41
FIG. 1.
Map of Lake Ontario showing the locations of the four Bioindex monitoring stations.
gram, stations 12 and 93 were dropped in 1985 (cf. Johannsson et al. 1985a). Zooplankton samples were taken with a 30-cm diameter, 70-p. mesh conical net pulled vertically through the water column at 0.8 m.s- 1 from either 20-m depth or from 1 m above the top of the thermocline, whichever was less. The location of the thermocline was defined by rates of change of water temperature, as read from electronic bathythermograph traces. In September 1982 the net was lost and replaced with a 25-cm diameter, 64-p. mesh net for the remainder of the year. As of 1983 a 50-cm diameter, 64-p. mesh net was employed. The zooplankton were preserved in 4070 sugared formaldehyde and later identified and counted under a dissecting microscope using a stratified counting regime (Cooley et al. 1986). The adult Copepoda and Cladocera were identified to species, the copepodids and nauplii to Order. Rotifers and ciliates were not enumerated, as many specimens pass through a 64-p. mesh. In addition, the relative importance of the species was estimated by multiplying abundance by species weight. The weights were taken from the unpublished data of B. Wilson and N.H.F. Watson (Dept. of Fisheries and Oceans, Canada, pers.
comm.). Since mean zooplankton weights can vary between lakes and through the season (e.g., Vijverberg and Richter 1982, Geller and Muller 1985) these biomasses are only rough estimates. To compare zooplankton abundance and species importance between years and among stations independent of seasonal timing, the data were summarized over the season: 30 March to 17 November. Epilimnetic, seasonally-weighted mean (SWM) abundances and total sample biomass were calculated per m 2 • Sampling Methodology
Since the sampling methodologies of earlier zooplankton studies (Patalas 1969, Czaika 1974, Watson and Carpenter 1974, McNaught et al. 1975) and the present program were not identical, several methodological comparisons were performed by reanalyzing and comparing parts of existing data sets. All studies had used fine meshed nets, but the depth of tow and station locations differed. Patalas (1969) found over 90% of most species in the top 50 m, therefore bottom to surface (1972: Czaika 1974) and 0 to 50 m (1967: Patalas 1969, 1970: Watson and Carpenter 1974) tows are comparable. The epilimnetic and 0
O. E. JOHANNSSON
330
TABLE 1. A comparison of peak population estimates for the dominant zooplankton in epilimnetic (E) and whole-column (We) water samples collected in 1982 at mid-lake stations 41 (Bioindex) and 403 (Ficker and Abbott 1984). Numbers'M-2 x 102 Species
Bosmina longirostris Eubosmina coregoni Ceriodaphnia lacustris Daphnia retrocurva Tropocyc/ops prasinus mexicanus Diacyc/ops thomasi
STN-41 STN-403 Percent (epilimnetic) (whole-column) (E/WC) 18,313
23,040
79
1,344
1,920
70
5,375
3,350
160
2,914
3,280
89
565
880
64
1,599
5,520
29
to 20 m tows of the present study, and the 0 to 5 m hauls of the 1972 whole lake study (McNaught et al. 1975), however, are unlikely to sample all species well. A series of vertical profiles taken by Fricker and Abbott (1984) in 1982 in Lake Ontario show that the 0 to 5 m samples do not adequately represent the zooplankton community. Therefore the 1972 whole lake study was not included in the comparison. Fricker and Abbott (1984) also took bottom to surface tows at a station (403) only 10 km east of monitoring station 41, providing the opportunity to compare epilimnetic and total column sampling. Since zooplankton population development is closely associated with the seasonal east-west temperature gradient in Lake Ontario (Patalas 1969, 1972), peak population densities were compared to assess the efficiency of epilimnetic sampling. The comparison of peak populations between Fricker and Abbott's (1984) whole column samples and the present study's epilimnetic samples indicated that the estimates of the T. p. mexicanus and all cladoceran populations were roughly equivalent with the two methodologies (Table 1). However, epilimnetic sampling greatly underestimated D. thomasi and these data were corrected by a factor of 3.1 for comparisons with earlier studies. The effects of spatial sampling patterns were examined using N.H.F. Watson and G.F. Carpen-
ter's raw data for 1970 (Dept. of Fisheries and Oceans, Canada, 1985, pers. comm.). Zooplankton abundance and composition were compared among three data subsets: whole lake (all stations - comparable with the 1967 and 1970 studies), south shore (three stations - the 1972 study), and Bioindex monitoring sites (four stations - present study). When samples were selected from the 1970 data base to represent Bioindex and south shore sampling patterns, the resultant seasonal zooplankton community structures and abundances did not differ significantly from the whole lake estimates. Percent Similarity among the communities produced by the three sampling patterns ranged from 87.6 percent to 93.7 percent for cladocerans and 66.9 percent to 91.9 percent for copepods. Total abundances ranged from 993.5 x 103 m- 2 to 1,230.4 x 103 m- 2 • One minor exception occurred. Tropocyc!ops p. mexicanus is relatively more abundant in the eastern basin than in the main body of the lake and, consequently, the 1972 south shore overemphasize the importance of D. thomasi. Since T. p. mexicanus is generally not an abundant species, total copepod abundances are still comparable among the different station subsets and, therefore, among the different studies. Historical Comparisons
Patalas (1969) summarized the 1967 data as June to October average abundances for each species. In order to compare zooplankton composition and abundances over the years, the data for each survey were recalculated on this basis. Hypolimnetic species were excluded from the analyses since they were poorly sampled in 1981 to 1985 and constituted only a small proportion of zooplankton abundance during this period of the year (Patalas 1969). The 1970 composite figures were derived from the raw data (N.H.F. Watson, Dept. of Fisheries and Oceans, Canada, 1985, pers. comm.) and not from published numbers (Watson and Carpenter 1974) because mean sampling depth was not given in the published work. The 1970 copepod estimates were corrected for nauplii which were not enumerated in that study. The average ratio of nauplii to copepodids between 1981 and 1985 was 0.47; therefore nauplii were estimated as 0.5 x copepodid abundance. In 1972 only the 8 km offshore data (Czaika 1974) were used in order to avoid nearshore effects. These data were converted from density (Czaika 1974,
LAKE ONTARIO ZOOPLANKTON: 1967-1982
Table 1) to numbers per m 2 by multiplying by the mean station depth, 106 m. The 1981 to 1984 D. thomasi abundances were corrected by a factor of 3.1 to overcome the effects of epilimnetic sampling (see above). The 1982 data were split into western and eastern sections as discussed below. The 1985 data were not included since all stations were not sampled. The total abundances of the zooplankton communities were compared across years. In 1967, 1970, and 1972 sampling was conducted monthly while in 1981 to 1985 samples were collected weekly. Since zooplankton abundance can change rapidly (Johannsson et af. 1985a), monthly sampling may miss the seasonal peaks and/or troughs, and provide a rougher estimate of average seasonal abundance than weekly sampling. In order to evaluate the significance of differences in abundance between the earlier studies and the present Bioindex data, the seasonal abundance of zooplankton in each year from 1981 to 1984 was recalculated assuming monthly sampling patterns starting on weeks one, two, three, and four. The mean monthly estimate and 95 percent confidence intervals were then calculated for each year. Significant differences were determined by whether the abundances of earlier studies fell within or outside the 95 percent confidence limits. Comparisons of copepod and cladoceran community structure were made using an index of community association: Percent Similarity, otherwise known as Whittaker's Index of Association (Whittaker 1952). All data were converted to proportions to eliminate the effect of differences in abundance. Percent Similarity was then calculated between all year combinations. Principal component factor scores were determined from the proportionate data in order to present the relationships graphically.
331
was contributed by B. fongirostris (25 to 50010), D. retrocurva (10 to 24%), and cyclopoid copepodids, principally D. thomasi (25 to 40%) (Table 2). The seasonal pattern of community development is described in detail in Johannsson et af. (1985a) and is similar to patterns reported previously by Patalas (1969), Watson and Carpenter (1974), and Czaika (1974). Zooplankton abundance and biomass showed a wide range of fluctuation between 1981 and 1985 with annual means varying two fold. Lake-wide biomass was low in 1981 (1.15 g per m2) and 1985 (1.13 g per m2), and higher in 1982, 1983, and 1984 (1.90, 1.82, and 1.53 g per mZ, respectively) (Table 2), although the tends were not consistent at all stations (Fig. 2). By far the greatest increases in biomass were in the western end of the lake in 1982 and in the mid-lake in 1984. However, whereas community composition did not change in 1984 with the increase in biomass, there were marked differences between the western end of the lake in 1982 and all other observed zooplankton associations. In the western region in 1982 B. fongirostris contributed between 65 and 77 percent to SWM biomass: the normal range in other years and at other stations was 26 to .48 percent (Table 2). The SWM biomass and percent biomass of all other cladocerans species were low. A comparison of cladoceran community structure within the 1981 to 1984 data base, using the Percent Similarity association index, indicated that the 1982 west lake community was atypical. Percent Similarity of the west lake 1982 cladoceran association with all others ranged from 57.1 percent to 82.8 percent. Therefore, for all historical comparisons the lake was divided in 1982 into two regions: west-stations 12 and 93, and eaststations 41 and 81. Historical Comparisons
RESULTS Zooplankton Communities: 1981 to 1985
The zooplankton community was dominated by the cladocerans Bosmina fongirostris, Daphnia retrocurva, Ceriodaphnia facustris, and Eubosmina coregoni, and the cyclopoids Diacyc/ops thomasi and Tropocyc/ops prasinus mexicanus. All other species contributed less than 1 percent to mean seasonal biomass: information on these other species is available in Johannsson et af. (1985b). The majority of the seasonal biomass
Average summer zooplankton abundance varied greatly between 1967 and 1985 with minimum levels of 12 x 105 per m 2 observed in 1970 and 1972 and maximum levels of 30 x 105 per m 2 in 1967 and 1982 (Fig. 3). Abundances in 1981, 1983, and 1984 were intermediate. Although significant differences in total abundance occurred between years (Table 3), all earlier studies fell somewhere within the range of zooplankton abundances observed between 1981 and 1984. Therefore one must conclude that there has been no significant change in zooplankton abundance between 1967 and 1984.
O. E. JOHANNSSON
332
TABLE 2. Seasonally weighted mean (SWM) zooplankton community biomass (g'm- 1) and species densities (No. x 1()3'm-1) are presented for the principal species in Lake Ontario. The percentage of the total biomass contributed by each species is given in italics. Total biomasses were calculated only from these species and will underestimate total sample biomass by 0.6 to 2.4 percent. Year
Cyclopoid Cyclopoid SWM B. D. E. D. T. p. C. Nauplii Stn Biomass longirostris retrocurva lacustris coregoni thomasi mexicanus Copepodids
1981
12
1982
1983
1984
1985
0.868
119.9
32.0
8.2
4.4
26.2
17.1
181.5
26.3
15.5
1.8
1.1
9.7
3.4
39.7
2.6
24.9
8.0
5.8
52.1
16.3
191.8
129.8
112.4
41
1.001
149.2
28.3
10.5
1.5
1.3
16.7
2.8
36.4
2.6
81
1. 711
184.1
87.9
22.3
33.9
57.1
45.7
301.5
209.5
20.4
21.6
2.5
4.4
10.7
4.5
33.5
2.4
93
1.023
260.9
32.1
14.5
3.3
15.4
10.4
136.5
161.5
48.1
13.2
2.7
0.7
4.8
1.7
25.3
3.2
12
2.120
727.4
16.1
1.8
3.0
33.9
1.9
281.5
66.9
65.2
3.2
0.2
0.3
5.1
0.1
25.2
0.6
41
1.427
281.3
52.2
25.9
6.9
36.6
6.7
243.4
88.9
37.5
15.4
3.4
1.1
8.2
0.8
32.4
1.2
81
2.226
189.1
126.1
30.8
43.8
119.7
16.0
393.9
118.3
16.1
23.8
2.6
4.3
17.2
1.2
33.6
1.1
93
1.867
757.9
11.6
16.6
3.0
18.5
2.2
138.9
65.7
77.1
2.6
1.7
0.4
3.2
0.2
14.1
0.7
12
1.498
267.2
63.4
27.4
18.7
19.8
22.7
266.8
109.5
33.8
17.7
3.4
2.7
4.2
2.6
33.8
1.5
41
1.758
179.0
67.3
29.7
31.8
39.6
51.3
402.6
150.9
19.3
16.0
3.2
4.0
7.2
5.0
43.5
1.7
81
2.306
267.2
119.3
21.6
56.0
33.3
110.5
417.6
226.6
22.0
21.7
1.8
5.3
4.6
8.1
34.4
2.0
93
1.757
163.4
116.8
16.0
11.6
16.0
73.3
377.3
83.5
17.7
27.9
1.7
1.5
2.9
7.1
40.3
1.0
12
1.380
333.4
47.6
9.2
5.7
23.0
8.9
218.7
59.3
45.9
14.5
1.3
0.9
5.3
1.1
30.1
0.9
41
2.019
402.8
57.3
10.0
1.9
55.7
20.5
396.8
114.3
37.9
11.9
0.9
0.2
8.8
1.7
37.3
1.1
81
1.440
214.8
78.1
13.5
23.6
32.2
26.2
242.5
88.4
28.3
22.7
1.8
3.6
7.2
3.1
32.0
1.2
93
1.267
314.9
24.7
20.8
2.1
17.8
5.3
223.3
151.2
47.2
8.1
3.1
0.4
4.5
0.7
33.5
2.4
41
1.012
130.2
37.4
10.0
1.8
37.4
17.6
221.8
65.6
24.5
15.5
1.9
0.4
11.8
3.0
41.7
1.3
81
1.247
209.9
48.0
16.3
26.6
24.5
32.8
215.8
66.9
32.0
16.2
2.5
4.7
6.3
4.5
32.9
1.1
1.9
4.2
1.9
2.2
3.2
1.7
1.9
0.2
Dry Weight per individual (ftg)*
·Weights are estimates of adult body size, taken from the unpublished data of B. Wilson and N.H.F. Watson (pers. comm.) and are presented solely to illustrate the relative size of the species.
A comparison of the structure of the cladoceran and copepod communities between 1967 and 1984, using Percent Similarity, shows that there is a continuum of community structure (Table 4, Fig. 4). In Figure 4 all years having associations greater than 90 percent are circled. The circles overlap
considerably and within each circle there are members from both the 1980s and the earlier studies. Only two communities stand away from the main body of points: the 1982 west cladoceran community and the 1983 copepod community. The former is characterized by a very high percentage of B.
LAKE ONTARIO ZOOPLANKTON: 1967-1982 3.0
STN12 20
1 0
0 3.0 N~
'E
I
~ I
I
I
I
STN41
2.0
~
''""
10
0
0 3.0
'"
E lD
:::;; ~
2.0
Vl
'"
, 0
0
tO
3 0
STN81
I =:,
STN93
2 .0
10
0
-
'"'"
'" '"'"
'"'" '"
.
'"'"
'"'"'"
'" '"'"
Year
FIG. 2. Trends in seasonally weighted mean zooplankton biomass between 1981 and 1985 at the individual stations.
longirostris, and the latter by a much higher percentage of T. p. mexicanus. DISCUSSION Two strong, contrasting management strategies have been applied to Lake Ontario. One (phosphorus reduction) potentially reduced the food base of the ecosystem while the other (salmonid stocking) increased the forage demand by the top predators. However, the present examination of total zooplankton abundance and community composition indicates that there has been no consistent change in either parameter. Neither reductions in phosphorus loadings to the lake nor the introduction of large numbers of top predators has exerted a detectable effect on the zooplankton community to date. The zooplankton community is buffered from these perturbations by trophic levels both above and below. These trophic levels should show more pronounced responses to the perturbations than the intermediate zooplankton
333
level (McQueen et al. 1986). Yet there is little evidence of marked changes even in the adjacent trophic levels. Stevens and Neilson (1987) found no change in mid-lake summer chlorophyll a levels between 1974 and 1982. Johannsson et al. (1985a) found some evidence of a decrease in algal biomass and a change in dominant species composition between the early 1970s and 1981 and 1982, but variability between the early data bases weakened the conclusions. On the other side,salmonid predation has had little effect on the alewife population. There were no alewife censi before 1976 (O'Gorman and Sneider 1986). However, by the 1960s there were few large predatory fish in the lake (Christie 1973) and in the 1972 lake-wide assessment of fish communities alewife were found to be by far the most abundant species in the lake (Christie and Thomas 1981). The lack of large piscivore populations in the early 1970s implies that the alewife population was likely resource limited at this time. In the 1980s the alewife population still shows signs of being resource, not predator, limited. Condition factor has been falling steadily from 1979 until 1984 (O'Gorman et al. 1987). With the continuing application of perturbations to the Lake Ontario ecosystem, changes in community structure may well occur in the near future. The Lake Michigan ecosystem has many similarities to that in Lake Ontario. Between 1976 and 1984 it passed from an alewife dominated system characterized by small zooplankton species to a piscivore dominated system characterized by large zooplankton species (Evans and Jude 1986). The cause of the change is not clear. One theory states that a series of cold winters initially reduced the alewife population and then decreased the survival of juveniles in succeeding years (Eck and Brown 1985). Salmonid predation would then exert an additional control. The other theory maintains that salmonid predation pressure alone was sufficient to reduce the alewife population (Stewart et al. 1981, Jude and Tesar 1985). Either or both of these scenarios could occur in Lake Ontario. As in Lake Michigan, major winter dieoffs are associated with severe winters in Lake Ontario (O'Gorman and Sneider 1986), and presently more salmonids are stocked per unit area in Lake Ontario than in Lake Michigan (Table 5). However, the actual number of salmonids required to control the alewife population will differ to some extent between the lakes due to differences in productivity, in salmonid survival, and in the ratio
334
O. E. JOHANNSSON
3000
~
\
~
A
\
If \
I \
\
I \ \
I I
\ \
o
\
2000
\
\\
'"E
\
I
\
o z
----
,\
6\
~
CD
o
1000
.,c: "c:
\
I
,
C?
\
6.
\.----
_-6.- ----
----
.-_ -
--
\
I I
\ \
-~
\ \
~ .... ..p
6.
:J
.Q
~
n ... '"
. ~
....... . . ., 0
00 ......
~~
~
w~
.-;;; ...."''''"''''
....'" ~
., .. '"~ '"
.-
Year
FIG. 3. Average June to October Lake Ontario zooplankton abundances from 1967 to 1984. Corrections have been made for nauplii in the 1970 data and to the Diacyclops thomasi estimates from 1981 to 1984, see text. S = south shore subset, B = Bioindex subset, E = eastern region, W = western region. ~--~ = Total abundance, solid bar = Copepoda, open bar = Cladocera
of species stocked. Nevertheless, the potential for marked changes in the Lake Ontario ecosystem presently exist. Although zooplankton community structure showed no significant change between 1967 and
TABLE 3. Estimates of average zooplankton abundance between June and October from 1967 to 1984, assuming various monthly sampling strategies in 1981 to 1984. Diacyclops thomasi 1981 to 1984 estimates have been corrected by a factor of 3.1 (see text). No·M-2
X
103
Year
Mean
S.D.
1967 1970 1972 1981 1982 1983 1984
3,186 1,230 1,245 1,995 2,792 1,682 1,914
+484 +702 + 317 + 351
95070 Confidence Interval
1,027 1,388 1,048 1,212
-
2,963 4,196 2,316 2,616
1984, variations in structure occurred. In 1982 an atypical cladoceran community developed in the western end of the lake and in 1983 the copepod community was unusual. Both of these communities appear to be correlated with altered thermal conditions. The 1982 west lake cladoceran community was characterized by a marked increase in B. longirostris which extended into October, long past the time when the population normally declined (early August), and by a concomitant decrease in D. retrocurva. The summer of 1982 was cool and upwelling events were common in the western end of the lake. The slow growth of D. retrocurva was undoubtedly temperature related (Patalas 1969, Fig. 5). The superabundance of B. longirostris cannot be explained by temperature directly. The lack of a mid-summer decline in abundance suggests a release from predation pressure. Temperature may be indirectly responsible, altering the overlap of the zooplankton and alewife populations. Alewife distribution is skewed toward higher temperatures
335
LAKE ONTARIO ZOOPLANKTON: 1967-1982
TABLE 4. Percent Similarity of cladoceran and copepod average June to October communities. The 1981 to 1984 Diacyclops thomasi estimates were corrected by a factor of 3.1 prior to analysis. All calculations were based on proportionate data. BS = Bioindex sites, SS = south shore sites. Cladocera Year
1967
1970
1970BS
1970SS
1972
1981
1982E
1982W
1983
1967 1970 1970BS 1970SS 1972 1981 1982E 1982W 1983 1984
92.5 88.1 80.0 89.2 84.6 92.2 62.8 95.2 77.5
93.7 87.6 82.5 92.8 93.7 70.4 91.8 85.2
91.6 81.6 96.3 92.2 74.7 90.0 87.9
87.6 94.4 84.1 82.8 81.9 96.3
78.3 88.6 57.1 90.7 71.2
89.4 77.3 87.3 92.2
66.9 97.1 81.7
64.8 85.3
79.5
1984
Copepoda Year
1967
1970
1970BS
1970SS
1972
1981
1982E
1982W
1983
1967 1970 1970BS 1970SS 1972 1981 1982E 1982W 1983 1984
84.8 92.9 82.1 86.0 97.3 87.6 84.5 73.7 96.5
91.9 66.9 98.8 87.5 72.4 69.3 88.9 81.3
75.0 93.1 95.6 80.5 77.4 80.1 89.4
68.1 92.2 98.5 99.8 55.8 85.6
88.7 73.6 70.5 87.7 92.5
84.9 81.8 63.6 93.4
96.9 57.3 87.1
56.0 85.8
70.2
TABLE 5.
A comparison of the total number of salmonids stocked annually in Lakes Ontario and Michigan. Lake Ontario
Year 1970 1972 1981 1982 1983 1984
No. Stocked 2,592 1,651 4,477 5,330 6,471 8,084
a)
No. KM-2C) 132 84 227 271 328 410
a) Daniels and LeTendre (1985) b) Annual Report of the Great Lakes Fishery Commission (1985) c) Schelske and Roth (1973) surface area of Lake Ontario 19,684 km 2 surface area of Lake Michigan 58,016 km 2 d) not reported
Lake Michigan No. Stockedb ) 7,470 7,708 13,210 16,231 16,093 (d)
1984
No. KM- 2 127 133 228 280 278
O. E. JOHANNSSON
336
Factor 2 2.0
CLADOCERA
Factor 1 - 2.0
1.0
-1.0
2.0
.72 8.4
-1.0
'82W
- 2.0
COPEPODA (/J
o
I~
.... ....
CD
....
co
w:i: CIlCll
COCiO
Fa c t or 1 _ _---'-----''--_ _~....'_---!..--'.'--_--U."---'-._----''--_..JlI!U' - 2.0
- 1.0
o
1.0
---'-
_
2.0
FIG. 4. Spatial representation of community similarity using principal component factor scores to separate the years (communities). All communities with Percent Similarities greater than 90% are circled. Top) Cladocera, Bottom) Copepoda. Year designations as per Figure 3.
in summer (Olson 1984), thus alewife may have moved across the lake with the warm epilimnetic water during upwelling events. In 1983 T. p. mexicanus was more abundant that usual (Table 2) and composed 45 percent of the copepod assemblage. That year the SWM Jun-Oct water temperature was 2-3C o higher than in other years. T. p. mexicanus reproduces rapidly in the warm epilimnetic waters producing two to five generations in a normal year (Johannsson et al. 1985a). D. thomasi, on the other hand, spends more time in the cooler metalimnion and hypolimnion (Table 1 and reproduces only two to three times a year: once in the spring, once in the summer, and perhaps again in the fall (Johannsson et
al. 1985a). It does not respond as rapidly to increased temperature as does T. p. mexicanus. Hence the ratio of the two copepods may be related in part to temperature conditions. In conclusion, there was no significant change in the range of abundance or structure of the zooplankton community between 1967 and 1984 in spite of significant reductions in phosphorus loadings and increases in salmonid stocking to the lake. However, the continuous application of stresses to the system in the form of severe weather conditions, improved lamprey control, and continued high salmonid stocking levels may at some point switch the system from one dominated by planktivores to one dominated by piscivores. At
337
LAKE ONTARIO ZOOPLANKTON: 1967-1982
~
I,
/1
I \
I I I
I I /0
L 10
I
E
,
I
~
\
I
0
I1l
I
L.
I
>
::::I
I
I
I I I I
(.)
~
i
~I
I I I I
I I I
5
N
I I
\ I
I I I
I
I
I
10
0,
I
I I I I I 0 I I
4
0
•
,I 0
•
•
1d+---~--~-·-,..---,.----,.---180
200
220
240
260
280
•
300 500
Julian Date
•
1000
1500
2000
Cumulative Epilimnetic Temperature (Degree Days)
FIG. 5. Relationships between Daphnia retrocurva abundance, calendar date, and cumulative epilimnetic temperature at station 12. Only points with similar water chloride concentrations are joined. Temperature < l5°C = solid symbols, > 15° = open symbols, 1981 = circles, 1982 = squares.
such a point, the zooplankton community will change dramatically.
ACKNOWLEDGMENTS I would like to thank U. Borgmann, C.K. Minns, K. Patalas, CM. Wood, and an unknown reviewer for their comments and helpful discussions of this manuscript, W. Geiling for counting the samples, and Captain Berchem, the crew of the CSS Bayfield, and Technical Operations personnel for collecting the samples.
REFERENCES Brandt, S. B. 1986. Food of trout and salmon in Lake Ontario. J. Great Lakes Res. 12: 200-205. Carpenter, S. R., Kitchell, J. F., and Hodgson, J. R. 1985. Cascading trophic interactions and lake productivity. Biosci. 35: 634-639. Christie, W. J. 1973. A review of the changes in the fish species composition of Lake Ontario. Great Lakes Fish. Comm. Tech. Rep. No. 23. _ _ _ _ . 1974. Changes in the fish species composition of the Great Lakes. J. Fish. Res. Board Can. 31: 827-854. _ _ _ _ , and Thomas, N. A. 1981. Biology. In IFYGL-the International Field Year for the Great
338
O. E. JOHANNSSON
Lakes. eds. E. J. Aubert and T. L. Richards, pp. 329340. Ann Arbor, Mich: National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory. Cooley, J. M., Moore, J. E., and Geiling, W. T. 1986. Population dynamics, biomass and production of the macro-zooplankton in the Bay of Quinte during changes in phosphorus loadings. In Project Quinte: point-source phosphorus control and ecosystem response in the Bay of Quinte, Lake Ontario, eds. C. K. Minns, D. A. Hurley, and K. H. Nicholls, pp. 166-176. Can. Spec. Pub!. Fish. Aquat. Sci. No. 86. Czaika, S. C. 1974. Crustacean zooplankton of southwestern Lake Ontario in 1972 during the International Field Year for the Great Lakes. In Proc. 17th Conj. Great Lakes Res., pp. 1-16. Internat. Assoc. Great Lakes Res. Daniels, M. E., and LeTendre, G. C. 1985. Lake Ontario salmonid stocking and marketing program. In Minutes of the 1985 Annual Meeting of the Great Lakes Fisheries Commission, pp. 45-51. Dillon, P. J., Nicholls, K. H., and Robinson, G. W. 1978. Phosphorous removal at Gravenhurst Bay, Ontario: an 8 year study on water quality changes. Verh. Internat. Verr. Limnol. 20: 263-271. Eck, G. W., and Brown, E. H., Jr. 1985. Lake Michigan's capacity to support lake trout (Salvelinus namaycush) and other salmonines: an estimate based on the status of prey populations in the 1970s. Can. J. Fish. Aquat. Sci. 42: 449-454. Evans, M. S., and Jude, D. J. 1986. Recent shifts in Daphnia community structure in southeastern Lake Michigan: a comparison of the inshore and offshore regions. Limnol. Oceanogr. 31: 56-67. Fricker, H. J., and Abbott, A. 1984. Zooplankton abundance in a north-south cross section of Lake Ontario (1982). Unpublished Data Report, Aquatic Ecology Division, National Water Research Institute. Canada Centre for Inland Waters, Burlington, Ontario. Geller, W., and Muller, H. 1985. Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species. J. Plankton Res. 7: 1-18. Great Lakes Fishery Commission. 1985. Annual Report for the year 1983. 1451 Green Rd., Ann Arbor, Michigan, U.S.A. Hutchinson, B. P. 1971. The effect of fish predation on the zooplankton of ten Adirondack lakes, with particular reference to the alewife, Alosa pseudoharengus. Trans. A mer. Fish. Soc. 100: 325-335. Janssen, J. 1978. Feeding-behaviour repertoire of the alewife, Alosa pseudoharengus, and the ciscoes Coregonus hoyi and artedii. J. Fish. Res. Board Can. 35: 249-253.
Johannsson, O. E., Dermott, R. M., Feldkamp, R., and Moore, J. E. 1985a. Lake Ontario Long Term Biological Monitoring Program. Report for 1981 and 1982. Can. Tech. Rep. Fish. Aquat. Sci. No. 1414. _ _ _ _ , 1985b. Lake Ontario Long Term Biological Monitoring Program. 1981, 1982 Data Base. Can. Data Rept. Fish. Aquat. Sci. No. 552. Jude, D. J., and Tesar, F. J. 1985. Recent changes in the inshore forage fish of Lake Michigan. Can. J. Fish. Aquat. Sci. 42: 1154-1157. Kolenosky, D. P., and LeTendre, G. C. 1984. Lake Ontario salmonid stocking and marketing program. Great Lakes Fish. Comm. 1984 Lake Ontario Comm. Rept.: 181-212. Lackey R. T. 1969. Food interrelationships of salmon, trout, alewives, and smelt in a Maine lake. Trans. A mer. Fish. Soc. 98: 641-646. McNaught, D. c., Buzzard, M., and Levine, S. 1975. Zooplankton production in Lake Ontario as influenced by environmental perturbations. U.S. Environm. Protect. Agency, Off. Res. Developm., Environm. Res. Center, Corvallis, Oregon. EPA660/3-75-021 McQueen, D. J., Post, J. R., and Mills, E. L. 1986. Trophic relationships in freshwater pelagic ecosystems. Can. J. Fish. Aquat. Sci. 43: 1571-1581. Nicholls, K. H., Heintsch, L., Carney, E., Beaver, J., and Middleton, D. 1986. Some effects of phosphorous loading reductions on phytoplankton in the Bay of Quinte, Lake Ontario. In Project Quinte: pointsource phosphorus control and ecosystem response in the Bay of Quinte, Lake Ontario, eds. C. K. Minns, D. A. Hurley, and K. H. Nicholls, pp. 145-158. Can. Spec. Pub!. Fish. Aquat. Sci. No. 86. O'Gorman, R., and Sneider, C. 1986. Dynamics if alewives n Lake Ontario following a mass mortality. Trans. A mer. Fish. Soc. 115: 1-14. _ _ _ _ , Bergstedt, R., and Eckert, T. H. 1987. Prey fish dynamics and salmonine predator growth in Lake Ontario, 1978-84. Can. J. Fish. Aquat. Sci., in press. Olson, R. A. 1984. Summer resource utilization by salmonids and their prey in Lake Ontario. M.Sc. thesis, State University College, Fredonia, N.Y. Patalas, K. 1969. Composition and horizontal distribution of crustacean plankton in Lake Ontario. J. Fish. Res. Board Can. 26: 2135-2164. _ _ _ _ . 1972. Crustacean plankton and the eutrophication of St. Lawrence Great Lakes. J. Fish. Res. Board Can. 29: 1451-1462. Savoie, P. 1986. Salmonid diet survey. In 1985 Annual Report. Lake Ontario Fisheries Assessment Unit, Ontario Ministry of Natural Resources. Schelske, C. L., and Roth, J. C. 1973. Limnological survey oflakes Michigan, Superior, Huron, and Erie.
LAKE ONTARIO ZOOPLANKTON: 1967-1982 University of Michigan, Great Lakes Research Division, Pub!. No. 17. Stevens, R. J. J., and Neilson, M. A. T. 1987. Response of Lake Ontario to reductions in phosphorous load: 1967-1982. J. Fish. Aquat. Sci. (in press). Stewart, D. J., Kitchell, J. F., and Crowder, L. B. 1981. Forage fishes and their salmonid predators in Lake Michigan. Trans. Amer. Fish. Soc. 110: 751-763. Vijverberg, J., and Richter, A. F. 1982. Population dynamics and production of Daphnia hyalina Leydig
339
and Daphnia cucullata Sars in Tjeukemeer. HydrobioI. 95: 235-259. Watson, N. H. F., and Carpenter, G. F. 1974. Seasonal abundance of crustacean zooplankton and net plankton biomass of Lakes Huron, Erie, and Ontario. J. Fish. Res. Board Can. 31: 309-317. Whittaker, R. H. 1952. A study of summer foliage insect communities in the Great Smoky Mountains. Ecol. Monogr. 22: 1-44.
COMPARISON OF LAKE ONTARIO ZOOPLANKTON COMMUNITIES BETWEEN 1967 AND 1985: BEFORE AND AFTER IMPLEMENTATION OF SALMONID STOCKING AND PHOSPHORUS CONTROL
Ora E. Johannsson Great Lakes Laboratory for Fisheries and Aquatic Sciences Canada Centre for Inland Waters 867 Lakeshore Road, P. O. Box 5050 Burlington, Ontario L7R 4A6 ABSTRACT. Two strong, contrasting management strategies were applied to Lake Ontario during the 1970s. There has been a 40 percent reduction in phosphorus loadings to the lake and an exponential increase in salmonid stocking. A comparison of zooplankton community structure and abundancefrom 1981 to 1985 with that of earlier studies (1967 to 1972) found no detectable change in the range of abundances or community composition between the two periods. Clearly, neither management strategy has had a discernible impact on the zooplankton community to date; however, the potential for change in the system remains high. ADDITIONAL INDEX WORDS: Eutrophication, plankton, ecological distribution, fish stocking, phosphorus.
introduced salmonids feed heavily on alewife (Olson 1984, Brandt 1986, Savoie 1986), the principal planktivore in the lake (Lackey 1969, Hutchinson 1971, Christie 1973 and 1974, Janssen 1978). The present study compares community structure and abundance of zooplankton from 1981 to 1985 with that in earlier years to see whether there have been changes in the abundance or structure (and therefore functioning) of this trophic level that might be attributable to either or both of the two lake management strategies.
INTRODUCTION In 1981 open-water zooplankton monitoring commenced on Lake Ontario as part of a more comprehensive biological monitoring program, the Bioindex Program. Prior to this, the most recent comprehensive zooplankton surveys were conducted between 1967 and 1972 (Patalas 1969, Czaika 1974, Watson and Carpenter 1974, McNaught et al. 1975). During the intervening years two important management strategies have been applied to the lake: phosphorus loadings were reduced by 40 percent (Stevens and Neilson 1987), and salmonid stocking was increased exponentially until in 1981 approximately 4.5 million fish were added to the lake annually (Kolenosky and LeTendre 1984). By 1984/1985, 7 to 8 million were stocked annually (Daniels and LeTendre 1985). Both of these management strategies could potentially affect the zooplankton community. Chan:>;es in phosphorus levels in the lake could alter the abundance and type of phytoplankton available as food to zooplankton (Dillon et al. 1978, Nicholls et al. 1986). Increases in the abundance of the top predators in the lake could alter predation pressure on zooplankton through cascading trophic effects (Carpenter et al. 1985). All
METHODS Weekly samples were collected between March and November in 1981 to 1985 at four stations representative of different open-water regions within Lake Ontario. The stations correspond to long-term water quality stations, hence the unique station numbers (Fig. 1). Station 12 is located south of Toronto in an area of frequent summer upwellings; station 41 is in the more stable midlake region; station 81 is in the productive eastern basin; and station 93 is near the south shore, within the influence of the Niagara River. In an effort to decrease the cost of the monitoring pro-
328
LAKE ONTARIO ZOOPLANKTON: 1967-1982
329
SAMPLING LOCATIONS IN LAKE ONTARIO
-41
FIG. 1.
Map of Lake Ontario showing the locations of the four Bioindex monitoring stations.
gram, stations 12 and 93 were dropped in 1985 (cf. Johannsson et al. 1985a). Zooplankton samples were taken with a 30-cm diameter, 70-p. mesh conical net pulled vertically through the water column at 0.8 m.s- 1 from either 20-m depth or from 1 m above the top of the thermocline, whichever was less. The location of the thermocline was defined by rates of change of water temperature, as read from electronic bathythermograph traces. In September 1982 the net was lost and replaced with a 25-cm diameter, 64-p. mesh net for the remainder of the year. As of 1983 a 50-cm diameter, 64-p. mesh net was employed. The zooplankton were preserved in 4070 sugared formaldehyde and later identified and counted under a dissecting microscope using a stratified counting regime (Cooley et al. 1986). The adult Copepoda and Cladocera were identified to species, the copepodids and nauplii to Order. Rotifers and ciliates were not enumerated, as many specimens pass through a 64-p. mesh. In addition, the relative importance of the species was estimated by multiplying abundance by species weight. The weights were taken from the unpublished data of B. Wilson and N.H.F. Watson (Dept. of Fisheries and Oceans, Canada, pers.
comm.). Since mean zooplankton weights can vary between lakes and through the season (e.g., Vijverberg and Richter 1982, Geller and Muller 1985) these biomasses are only rough estimates. To compare zooplankton abundance and species importance between years and among stations independent of seasonal timing, the data were summarized over the season: 30 March to 17 November. Epilimnetic, seasonally-weighted mean (SWM) abundances and total sample biomass were calculated per m 2 • Sampling Methodology
Since the sampling methodologies of earlier zooplankton studies (Patalas 1969, Czaika 1974, Watson and Carpenter 1974, McNaught et al. 1975) and the present program were not identical, several methodological comparisons were performed by reanalyzing and comparing parts of existing data sets. All studies had used fine meshed nets, but the depth of tow and station locations differed. Patalas (1969) found over 90% of most species in the top 50 m, therefore bottom to surface (1972: Czaika 1974) and 0 to 50 m (1967: Patalas 1969, 1970: Watson and Carpenter 1974) tows are comparable. The epilimnetic and 0
O. E. JOHANNSSON
330
TABLE 1. A comparison of peak population estimates for the dominant zooplankton in epilimnetic (E) and whole-column (We) water samples collected in 1982 at mid-lake stations 41 (Bioindex) and 403 (Ficker and Abbott 1984). Numbers'M-2 x 102 Species
Bosmina longirostris Eubosmina coregoni Ceriodaphnia lacustris Daphnia retrocurva Tropocyc/ops prasinus mexicanus Diacyc/ops thomasi
STN-41 STN-403 Percent (epilimnetic) (whole-column) (E/WC) 18,313
23,040
79
1,344
1,920
70
5,375
3,350
160
2,914
3,280
89
565
880
64
1,599
5,520
29
to 20 m tows of the present study, and the 0 to 5 m hauls of the 1972 whole lake study (McNaught et al. 1975), however, are unlikely to sample all species well. A series of vertical profiles taken by Fricker and Abbott (1984) in 1982 in Lake Ontario show that the 0 to 5 m samples do not adequately represent the zooplankton community. Therefore the 1972 whole lake study was not included in the comparison. Fricker and Abbott (1984) also took bottom to surface tows at a station (403) only 10 km east of monitoring station 41, providing the opportunity to compare epilimnetic and total column sampling. Since zooplankton population development is closely associated with the seasonal east-west temperature gradient in Lake Ontario (Patalas 1969, 1972), peak population densities were compared to assess the efficiency of epilimnetic sampling. The comparison of peak populations between Fricker and Abbott's (1984) whole column samples and the present study's epilimnetic samples indicated that the estimates of the T. p. mexicanus and all cladoceran populations were roughly equivalent with the two methodologies (Table 1). However, epilimnetic sampling greatly underestimated D. thomasi and these data were corrected by a factor of 3.1 for comparisons with earlier studies. The effects of spatial sampling patterns were examined using N.H.F. Watson and G.F. Carpen-
ter's raw data for 1970 (Dept. of Fisheries and Oceans, Canada, 1985, pers. comm.). Zooplankton abundance and composition were compared among three data subsets: whole lake (all stations - comparable with the 1967 and 1970 studies), south shore (three stations - the 1972 study), and Bioindex monitoring sites (four stations - present study). When samples were selected from the 1970 data base to represent Bioindex and south shore sampling patterns, the resultant seasonal zooplankton community structures and abundances did not differ significantly from the whole lake estimates. Percent Similarity among the communities produced by the three sampling patterns ranged from 87.6 percent to 93.7 percent for cladocerans and 66.9 percent to 91.9 percent for copepods. Total abundances ranged from 993.5 x 103 m- 2 to 1,230.4 x 103 m- 2 • One minor exception occurred. Tropocyc!ops p. mexicanus is relatively more abundant in the eastern basin than in the main body of the lake and, consequently, the 1972 south shore overemphasize the importance of D. thomasi. Since T. p. mexicanus is generally not an abundant species, total copepod abundances are still comparable among the different station subsets and, therefore, among the different studies. Historical Comparisons
Patalas (1969) summarized the 1967 data as June to October average abundances for each species. In order to compare zooplankton composition and abundances over the years, the data for each survey were recalculated on this basis. Hypolimnetic species were excluded from the analyses since they were poorly sampled in 1981 to 1985 and constituted only a small proportion of zooplankton abundance during this period of the year (Patalas 1969). The 1970 composite figures were derived from the raw data (N.H.F. Watson, Dept. of Fisheries and Oceans, Canada, 1985, pers. comm.) and not from published numbers (Watson and Carpenter 1974) because mean sampling depth was not given in the published work. The 1970 copepod estimates were corrected for nauplii which were not enumerated in that study. The average ratio of nauplii to copepodids between 1981 and 1985 was 0.47; therefore nauplii were estimated as 0.5 x copepodid abundance. In 1972 only the 8 km offshore data (Czaika 1974) were used in order to avoid nearshore effects. These data were converted from density (Czaika 1974,
LAKE ONTARIO ZOOPLANKTON: 1967-1982
Table 1) to numbers per m 2 by multiplying by the mean station depth, 106 m. The 1981 to 1984 D. thomasi abundances were corrected by a factor of 3.1 to overcome the effects of epilimnetic sampling (see above). The 1982 data were split into western and eastern sections as discussed below. The 1985 data were not included since all stations were not sampled. The total abundances of the zooplankton communities were compared across years. In 1967, 1970, and 1972 sampling was conducted monthly while in 1981 to 1985 samples were collected weekly. Since zooplankton abundance can change rapidly (Johannsson et af. 1985a), monthly sampling may miss the seasonal peaks and/or troughs, and provide a rougher estimate of average seasonal abundance than weekly sampling. In order to evaluate the significance of differences in abundance between the earlier studies and the present Bioindex data, the seasonal abundance of zooplankton in each year from 1981 to 1984 was recalculated assuming monthly sampling patterns starting on weeks one, two, three, and four. The mean monthly estimate and 95 percent confidence intervals were then calculated for each year. Significant differences were determined by whether the abundances of earlier studies fell within or outside the 95 percent confidence limits. Comparisons of copepod and cladoceran community structure were made using an index of community association: Percent Similarity, otherwise known as Whittaker's Index of Association (Whittaker 1952). All data were converted to proportions to eliminate the effect of differences in abundance. Percent Similarity was then calculated between all year combinations. Principal component factor scores were determined from the proportionate data in order to present the relationships graphically.
331
was contributed by B. fongirostris (25 to 50010), D. retrocurva (10 to 24%), and cyclopoid copepodids, principally D. thomasi (25 to 40%) (Table 2). The seasonal pattern of community development is described in detail in Johannsson et af. (1985a) and is similar to patterns reported previously by Patalas (1969), Watson and Carpenter (1974), and Czaika (1974). Zooplankton abundance and biomass showed a wide range of fluctuation between 1981 and 1985 with annual means varying two fold. Lake-wide biomass was low in 1981 (1.15 g per m2) and 1985 (1.13 g per m2), and higher in 1982, 1983, and 1984 (1.90, 1.82, and 1.53 g per mZ, respectively) (Table 2), although the tends were not consistent at all stations (Fig. 2). By far the greatest increases in biomass were in the western end of the lake in 1982 and in the mid-lake in 1984. However, whereas community composition did not change in 1984 with the increase in biomass, there were marked differences between the western end of the lake in 1982 and all other observed zooplankton associations. In the western region in 1982 B. fongirostris contributed between 65 and 77 percent to SWM biomass: the normal range in other years and at other stations was 26 to .48 percent (Table 2). The SWM biomass and percent biomass of all other cladocerans species were low. A comparison of cladoceran community structure within the 1981 to 1984 data base, using the Percent Similarity association index, indicated that the 1982 west lake community was atypical. Percent Similarity of the west lake 1982 cladoceran association with all others ranged from 57.1 percent to 82.8 percent. Therefore, for all historical comparisons the lake was divided in 1982 into two regions: west-stations 12 and 93, and eaststations 41 and 81. Historical Comparisons
RESULTS Zooplankton Communities: 1981 to 1985
The zooplankton community was dominated by the cladocerans Bosmina fongirostris, Daphnia retrocurva, Ceriodaphnia facustris, and Eubosmina coregoni, and the cyclopoids Diacyc/ops thomasi and Tropocyc/ops prasinus mexicanus. All other species contributed less than 1 percent to mean seasonal biomass: information on these other species is available in Johannsson et af. (1985b). The majority of the seasonal biomass
Average summer zooplankton abundance varied greatly between 1967 and 1985 with minimum levels of 12 x 105 per m 2 observed in 1970 and 1972 and maximum levels of 30 x 105 per m 2 in 1967 and 1982 (Fig. 3). Abundances in 1981, 1983, and 1984 were intermediate. Although significant differences in total abundance occurred between years (Table 3), all earlier studies fell somewhere within the range of zooplankton abundances observed between 1981 and 1984. Therefore one must conclude that there has been no significant change in zooplankton abundance between 1967 and 1984.
O. E. JOHANNSSON
332
TABLE 2. Seasonally weighted mean (SWM) zooplankton community biomass (g'm- 1) and species densities (No. x 1()3'm-1) are presented for the principal species in Lake Ontario. The percentage of the total biomass contributed by each species is given in italics. Total biomasses were calculated only from these species and will underestimate total sample biomass by 0.6 to 2.4 percent. Year
Cyclopoid Cyclopoid SWM B. D. E. D. T. p. C. Nauplii Stn Biomass longirostris retrocurva lacustris coregoni thomasi mexicanus Copepodids
1981
12
1982
1983
1984
1985
0.868
119.9
32.0
8.2
4.4
26.2
17.1
181.5
26.3
15.5
1.8
1.1
9.7
3.4
39.7
2.6
24.9
8.0
5.8
52.1
16.3
191.8
129.8
112.4
41
1.001
149.2
28.3
10.5
1.5
1.3
16.7
2.8
36.4
2.6
81
1. 711
184.1
87.9
22.3
33.9
57.1
45.7
301.5
209.5
20.4
21.6
2.5
4.4
10.7
4.5
33.5
2.4
93
1.023
260.9
32.1
14.5
3.3
15.4
10.4
136.5
161.5
48.1
13.2
2.7
0.7
4.8
1.7
25.3
3.2
12
2.120
727.4
16.1
1.8
3.0
33.9
1.9
281.5
66.9
65.2
3.2
0.2
0.3
5.1
0.1
25.2
0.6
41
1.427
281.3
52.2
25.9
6.9
36.6
6.7
243.4
88.9
37.5
15.4
3.4
1.1
8.2
0.8
32.4
1.2
81
2.226
189.1
126.1
30.8
43.8
119.7
16.0
393.9
118.3
16.1
23.8
2.6
4.3
17.2
1.2
33.6
1.1
93
1.867
757.9
11.6
16.6
3.0
18.5
2.2
138.9
65.7
77.1
2.6
1.7
0.4
3.2
0.2
14.1
0.7
12
1.498
267.2
63.4
27.4
18.7
19.8
22.7
266.8
109.5
33.8
17.7
3.4
2.7
4.2
2.6
33.8
1.5
41
1.758
179.0
67.3
29.7
31.8
39.6
51.3
402.6
150.9
19.3
16.0
3.2
4.0
7.2
5.0
43.5
1.7
81
2.306
267.2
119.3
21.6
56.0
33.3
110.5
417.6
226.6
22.0
21.7
1.8
5.3
4.6
8.1
34.4
2.0
93
1.757
163.4
116.8
16.0
11.6
16.0
73.3
377.3
83.5
17.7
27.9
1.7
1.5
2.9
7.1
40.3
1.0
12
1.380
333.4
47.6
9.2
5.7
23.0
8.9
218.7
59.3
45.9
14.5
1.3
0.9
5.3
1.1
30.1
0.9
41
2.019
402.8
57.3
10.0
1.9
55.7
20.5
396.8
114.3
37.9
11.9
0.9
0.2
8.8
1.7
37.3
1.1
81
1.440
214.8
78.1
13.5
23.6
32.2
26.2
242.5
88.4
28.3
22.7
1.8
3.6
7.2
3.1
32.0
1.2
93
1.267
314.9
24.7
20.8
2.1
17.8
5.3
223.3
151.2
47.2
8.1
3.1
0.4
4.5
0.7
33.5
2.4
41
1.012
130.2
37.4
10.0
1.8
37.4
17.6
221.8
65.6
24.5
15.5
1.9
0.4
11.8
3.0
41.7
1.3
81
1.247
209.9
48.0
16.3
26.6
24.5
32.8
215.8
66.9
32.0
16.2
2.5
4.7
6.3
4.5
32.9
1.1
1.9
4.2
1.9
2.2
3.2
1.7
1.9
0.2
Dry Weight per individual (ftg)*
·Weights are estimates of adult body size, taken from the unpublished data of B. Wilson and N.H.F. Watson (pers. comm.) and are presented solely to illustrate the relative size of the species.
A comparison of the structure of the cladoceran and copepod communities between 1967 and 1984, using Percent Similarity, shows that there is a continuum of community structure (Table 4, Fig. 4). In Figure 4 all years having associations greater than 90 percent are circled. The circles overlap
considerably and within each circle there are members from both the 1980s and the earlier studies. Only two communities stand away from the main body of points: the 1982 west cladoceran community and the 1983 copepod community. The former is characterized by a very high percentage of B.
LAKE ONTARIO ZOOPLANKTON: 1967-1982 3.0
STN12 20
1 0
0 3.0 N~
'E
I
~ I
I
I
I
STN41
2.0
~
''""
10
0
0 3.0
'"
E lD
:::;; ~
2.0
Vl
'"
, 0
0
tO
3 0
STN81
I =:,
STN93
2 .0
10
0
-
'"'"
'" '"'"
'"'" '"
.
'"'"
'"'"'"
'" '"'"
Year
FIG. 2. Trends in seasonally weighted mean zooplankton biomass between 1981 and 1985 at the individual stations.
longirostris, and the latter by a much higher percentage of T. p. mexicanus. DISCUSSION Two strong, contrasting management strategies have been applied to Lake Ontario. One (phosphorus reduction) potentially reduced the food base of the ecosystem while the other (salmonid stocking) increased the forage demand by the top predators. However, the present examination of total zooplankton abundance and community composition indicates that there has been no consistent change in either parameter. Neither reductions in phosphorus loadings to the lake nor the introduction of large numbers of top predators has exerted a detectable effect on the zooplankton community to date. The zooplankton community is buffered from these perturbations by trophic levels both above and below. These trophic levels should show more pronounced responses to the perturbations than the intermediate zooplankton
333
level (McQueen et al. 1986). Yet there is little evidence of marked changes even in the adjacent trophic levels. Stevens and Neilson (1987) found no change in mid-lake summer chlorophyll a levels between 1974 and 1982. Johannsson et al. (1985a) found some evidence of a decrease in algal biomass and a change in dominant species composition between the early 1970s and 1981 and 1982, but variability between the early data bases weakened the conclusions. On the other side,salmonid predation has had little effect on the alewife population. There were no alewife censi before 1976 (O'Gorman and Sneider 1986). However, by the 1960s there were few large predatory fish in the lake (Christie 1973) and in the 1972 lake-wide assessment of fish communities alewife were found to be by far the most abundant species in the lake (Christie and Thomas 1981). The lack of large piscivore populations in the early 1970s implies that the alewife population was likely resource limited at this time. In the 1980s the alewife population still shows signs of being resource, not predator, limited. Condition factor has been falling steadily from 1979 until 1984 (O'Gorman et al. 1987). With the continuing application of perturbations to the Lake Ontario ecosystem, changes in community structure may well occur in the near future. The Lake Michigan ecosystem has many similarities to that in Lake Ontario. Between 1976 and 1984 it passed from an alewife dominated system characterized by small zooplankton species to a piscivore dominated system characterized by large zooplankton species (Evans and Jude 1986). The cause of the change is not clear. One theory states that a series of cold winters initially reduced the alewife population and then decreased the survival of juveniles in succeeding years (Eck and Brown 1985). Salmonid predation would then exert an additional control. The other theory maintains that salmonid predation pressure alone was sufficient to reduce the alewife population (Stewart et al. 1981, Jude and Tesar 1985). Either or both of these scenarios could occur in Lake Ontario. As in Lake Michigan, major winter dieoffs are associated with severe winters in Lake Ontario (O'Gorman and Sneider 1986), and presently more salmonids are stocked per unit area in Lake Ontario than in Lake Michigan (Table 5). However, the actual number of salmonids required to control the alewife population will differ to some extent between the lakes due to differences in productivity, in salmonid survival, and in the ratio
334
O. E. JOHANNSSON
3000
~
\
~
A
\
If \
I \
\
I \ \
I I
\ \
o
\
2000
\
\\
'"E
\
I
\
o z
----
,\
6\
~
CD
o
1000
.,c: "c:
\
I
,
C?
\
6.
\.----
_-6.- ----
----
.-_ -
--
\
I I
\ \
-~
\ \
~ .... ..p
6.
:J
.Q
~
n ... '"
. ~
....... . . ., 0
00 ......
~~
~
w~
.-;;; ...."''''"''''
....'" ~
., .. '"~ '"
.-
Year
FIG. 3. Average June to October Lake Ontario zooplankton abundances from 1967 to 1984. Corrections have been made for nauplii in the 1970 data and to the Diacyclops thomasi estimates from 1981 to 1984, see text. S = south shore subset, B = Bioindex subset, E = eastern region, W = western region. ~--~ = Total abundance, solid bar = Copepoda, open bar = Cladocera
of species stocked. Nevertheless, the potential for marked changes in the Lake Ontario ecosystem presently exist. Although zooplankton community structure showed no significant change between 1967 and
TABLE 3. Estimates of average zooplankton abundance between June and October from 1967 to 1984, assuming various monthly sampling strategies in 1981 to 1984. Diacyclops thomasi 1981 to 1984 estimates have been corrected by a factor of 3.1 (see text). No·M-2
X
103
Year
Mean
S.D.
1967 1970 1972 1981 1982 1983 1984
3,186 1,230 1,245 1,995 2,792 1,682 1,914
+484 +702 + 317 + 351
95070 Confidence Interval
1,027 1,388 1,048 1,212
-
2,963 4,196 2,316 2,616
1984, variations in structure occurred. In 1982 an atypical cladoceran community developed in the western end of the lake and in 1983 the copepod community was unusual. Both of these communities appear to be correlated with altered thermal conditions. The 1982 west lake cladoceran community was characterized by a marked increase in B. longirostris which extended into October, long past the time when the population normally declined (early August), and by a concomitant decrease in D. retrocurva. The summer of 1982 was cool and upwelling events were common in the western end of the lake. The slow growth of D. retrocurva was undoubtedly temperature related (Patalas 1969, Fig. 5). The superabundance of B. longirostris cannot be explained by temperature directly. The lack of a mid-summer decline in abundance suggests a release from predation pressure. Temperature may be indirectly responsible, altering the overlap of the zooplankton and alewife populations. Alewife distribution is skewed toward higher temperatures
335
LAKE ONTARIO ZOOPLANKTON: 1967-1982
TABLE 4. Percent Similarity of cladoceran and copepod average June to October communities. The 1981 to 1984 Diacyclops thomasi estimates were corrected by a factor of 3.1 prior to analysis. All calculations were based on proportionate data. BS = Bioindex sites, SS = south shore sites. Cladocera Year
1967
1970
1970BS
1970SS
1972
1981
1982E
1982W
1983
1967 1970 1970BS 1970SS 1972 1981 1982E 1982W 1983 1984
92.5 88.1 80.0 89.2 84.6 92.2 62.8 95.2 77.5
93.7 87.6 82.5 92.8 93.7 70.4 91.8 85.2
91.6 81.6 96.3 92.2 74.7 90.0 87.9
87.6 94.4 84.1 82.8 81.9 96.3
78.3 88.6 57.1 90.7 71.2
89.4 77.3 87.3 92.2
66.9 97.1 81.7
64.8 85.3
79.5
1984
Copepoda Year
1967
1970
1970BS
1970SS
1972
1981
1982E
1982W
1983
1967 1970 1970BS 1970SS 1972 1981 1982E 1982W 1983 1984
84.8 92.9 82.1 86.0 97.3 87.6 84.5 73.7 96.5
91.9 66.9 98.8 87.5 72.4 69.3 88.9 81.3
75.0 93.1 95.6 80.5 77.4 80.1 89.4
68.1 92.2 98.5 99.8 55.8 85.6
88.7 73.6 70.5 87.7 92.5
84.9 81.8 63.6 93.4
96.9 57.3 87.1
56.0 85.8
70.2
TABLE 5.
A comparison of the total number of salmonids stocked annually in Lakes Ontario and Michigan. Lake Ontario
Year 1970 1972 1981 1982 1983 1984
No. Stocked 2,592 1,651 4,477 5,330 6,471 8,084
a)
No. KM-2C) 132 84 227 271 328 410
a) Daniels and LeTendre (1985) b) Annual Report of the Great Lakes Fishery Commission (1985) c) Schelske and Roth (1973) surface area of Lake Ontario 19,684 km 2 surface area of Lake Michigan 58,016 km 2 d) not reported
Lake Michigan No. Stockedb ) 7,470 7,708 13,210 16,231 16,093 (d)
1984
No. KM- 2 127 133 228 280 278
O. E. JOHANNSSON
336
Factor 2 2.0
CLADOCERA
Factor 1 - 2.0
1.0
-1.0
2.0
.72 8.4
-1.0
'82W
- 2.0
COPEPODA (/J
o
I~
.... ....
CD
....
co
w:i: CIlCll
COCiO
Fa c t or 1 _ _---'-----''--_ _~....'_---!..--'.'--_--U."---'-._----''--_..JlI!U' - 2.0
- 1.0
o
1.0
---'-
_
2.0
FIG. 4. Spatial representation of community similarity using principal component factor scores to separate the years (communities). All communities with Percent Similarities greater than 90% are circled. Top) Cladocera, Bottom) Copepoda. Year designations as per Figure 3.
in summer (Olson 1984), thus alewife may have moved across the lake with the warm epilimnetic water during upwelling events. In 1983 T. p. mexicanus was more abundant that usual (Table 2) and composed 45 percent of the copepod assemblage. That year the SWM Jun-Oct water temperature was 2-3C o higher than in other years. T. p. mexicanus reproduces rapidly in the warm epilimnetic waters producing two to five generations in a normal year (Johannsson et al. 1985a). D. thomasi, on the other hand, spends more time in the cooler metalimnion and hypolimnion (Table 1 and reproduces only two to three times a year: once in the spring, once in the summer, and perhaps again in the fall (Johannsson et
al. 1985a). It does not respond as rapidly to increased temperature as does T. p. mexicanus. Hence the ratio of the two copepods may be related in part to temperature conditions. In conclusion, there was no significant change in the range of abundance or structure of the zooplankton community between 1967 and 1984 in spite of significant reductions in phosphorus loadings and increases in salmonid stocking to the lake. However, the continuous application of stresses to the system in the form of severe weather conditions, improved lamprey control, and continued high salmonid stocking levels may at some point switch the system from one dominated by planktivores to one dominated by piscivores. At
337
LAKE ONTARIO ZOOPLANKTON: 1967-1982
~
I,
/1
I \
I I I
I I /0
L 10
I
E
,
I
~
\
I
0
I1l
I
L.
I
>
::::I
I
I
I I I I
(.)
~
i
~I
I I I I
I I I
5
N
I I
\ I
I I I
I
I
I
10
0,
I
I I I I I 0 I I
4
0
•
,I 0
•
•
1d+---~--~-·-,..---,.----,.---180
200
220
240
260
280
•
300 500
Julian Date
•
1000
1500
2000
Cumulative Epilimnetic Temperature (Degree Days)
FIG. 5. Relationships between Daphnia retrocurva abundance, calendar date, and cumulative epilimnetic temperature at station 12. Only points with similar water chloride concentrations are joined. Temperature < l5°C = solid symbols, > 15° = open symbols, 1981 = circles, 1982 = squares.
such a point, the zooplankton community will change dramatically.
ACKNOWLEDGMENTS I would like to thank U. Borgmann, C.K. Minns, K. Patalas, CM. Wood, and an unknown reviewer for their comments and helpful discussions of this manuscript, W. Geiling for counting the samples, and Captain Berchem, the crew of the CSS Bayfield, and Technical Operations personnel for collecting the samples.
REFERENCES Brandt, S. B. 1986. Food of trout and salmon in Lake Ontario. J. Great Lakes Res. 12: 200-205. Carpenter, S. R., Kitchell, J. F., and Hodgson, J. R. 1985. Cascading trophic interactions and lake productivity. Biosci. 35: 634-639. Christie, W. J. 1973. A review of the changes in the fish species composition of Lake Ontario. Great Lakes Fish. Comm. Tech. Rep. No. 23. _ _ _ _ . 1974. Changes in the fish species composition of the Great Lakes. J. Fish. Res. Board Can. 31: 827-854. _ _ _ _ , and Thomas, N. A. 1981. Biology. In IFYGL-the International Field Year for the Great
338
O. E. JOHANNSSON
Lakes. eds. E. J. Aubert and T. L. Richards, pp. 329340. Ann Arbor, Mich: National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory. Cooley, J. M., Moore, J. E., and Geiling, W. T. 1986. Population dynamics, biomass and production of the macro-zooplankton in the Bay of Quinte during changes in phosphorus loadings. In Project Quinte: point-source phosphorus control and ecosystem response in the Bay of Quinte, Lake Ontario, eds. C. K. Minns, D. A. Hurley, and K. H. Nicholls, pp. 166-176. Can. Spec. Pub!. Fish. Aquat. Sci. No. 86. Czaika, S. C. 1974. Crustacean zooplankton of southwestern Lake Ontario in 1972 during the International Field Year for the Great Lakes. In Proc. 17th Conj. Great Lakes Res., pp. 1-16. Internat. Assoc. Great Lakes Res. Daniels, M. E., and LeTendre, G. C. 1985. Lake Ontario salmonid stocking and marketing program. In Minutes of the 1985 Annual Meeting of the Great Lakes Fisheries Commission, pp. 45-51. Dillon, P. J., Nicholls, K. H., and Robinson, G. W. 1978. Phosphorous removal at Gravenhurst Bay, Ontario: an 8 year study on water quality changes. Verh. Internat. Verr. Limnol. 20: 263-271. Eck, G. W., and Brown, E. H., Jr. 1985. Lake Michigan's capacity to support lake trout (Salvelinus namaycush) and other salmonines: an estimate based on the status of prey populations in the 1970s. Can. J. Fish. Aquat. Sci. 42: 449-454. Evans, M. S., and Jude, D. J. 1986. Recent shifts in Daphnia community structure in southeastern Lake Michigan: a comparison of the inshore and offshore regions. Limnol. Oceanogr. 31: 56-67. Fricker, H. J., and Abbott, A. 1984. Zooplankton abundance in a north-south cross section of Lake Ontario (1982). Unpublished Data Report, Aquatic Ecology Division, National Water Research Institute. Canada Centre for Inland Waters, Burlington, Ontario. Geller, W., and Muller, H. 1985. Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species. J. Plankton Res. 7: 1-18. Great Lakes Fishery Commission. 1985. Annual Report for the year 1983. 1451 Green Rd., Ann Arbor, Michigan, U.S.A. Hutchinson, B. P. 1971. The effect of fish predation on the zooplankton of ten Adirondack lakes, with particular reference to the alewife, Alosa pseudoharengus. Trans. A mer. Fish. Soc. 100: 325-335. Janssen, J. 1978. Feeding-behaviour repertoire of the alewife, Alosa pseudoharengus, and the ciscoes Coregonus hoyi and artedii. J. Fish. Res. Board Can. 35: 249-253.
Johannsson, O. E., Dermott, R. M., Feldkamp, R., and Moore, J. E. 1985a. Lake Ontario Long Term Biological Monitoring Program. Report for 1981 and 1982. Can. Tech. Rep. Fish. Aquat. Sci. No. 1414. _ _ _ _ , 1985b. Lake Ontario Long Term Biological Monitoring Program. 1981, 1982 Data Base. Can. Data Rept. Fish. Aquat. Sci. No. 552. Jude, D. J., and Tesar, F. J. 1985. Recent changes in the inshore forage fish of Lake Michigan. Can. J. Fish. Aquat. Sci. 42: 1154-1157. Kolenosky, D. P., and LeTendre, G. C. 1984. Lake Ontario salmonid stocking and marketing program. Great Lakes Fish. Comm. 1984 Lake Ontario Comm. Rept.: 181-212. Lackey R. T. 1969. Food interrelationships of salmon, trout, alewives, and smelt in a Maine lake. Trans. A mer. Fish. Soc. 98: 641-646. McNaught, D. c., Buzzard, M., and Levine, S. 1975. Zooplankton production in Lake Ontario as influenced by environmental perturbations. U.S. Environm. Protect. Agency, Off. Res. Developm., Environm. Res. Center, Corvallis, Oregon. EPA660/3-75-021 McQueen, D. J., Post, J. R., and Mills, E. L. 1986. Trophic relationships in freshwater pelagic ecosystems. Can. J. Fish. Aquat. Sci. 43: 1571-1581. Nicholls, K. H., Heintsch, L., Carney, E., Beaver, J., and Middleton, D. 1986. Some effects of phosphorous loading reductions on phytoplankton in the Bay of Quinte, Lake Ontario. In Project Quinte: pointsource phosphorus control and ecosystem response in the Bay of Quinte, Lake Ontario, eds. C. K. Minns, D. A. Hurley, and K. H. Nicholls, pp. 145-158. Can. Spec. Pub!. Fish. Aquat. Sci. No. 86. O'Gorman, R., and Sneider, C. 1986. Dynamics if alewives n Lake Ontario following a mass mortality. Trans. A mer. Fish. Soc. 115: 1-14. _ _ _ _ , Bergstedt, R., and Eckert, T. H. 1987. Prey fish dynamics and salmonine predator growth in Lake Ontario, 1978-84. Can. J. Fish. Aquat. Sci., in press. Olson, R. A. 1984. Summer resource utilization by salmonids and their prey in Lake Ontario. M.Sc. thesis, State University College, Fredonia, N.Y. Patalas, K. 1969. Composition and horizontal distribution of crustacean plankton in Lake Ontario. J. Fish. Res. Board Can. 26: 2135-2164. _ _ _ _ . 1972. Crustacean plankton and the eutrophication of St. Lawrence Great Lakes. J. Fish. Res. Board Can. 29: 1451-1462. Savoie, P. 1986. Salmonid diet survey. In 1985 Annual Report. Lake Ontario Fisheries Assessment Unit, Ontario Ministry of Natural Resources. Schelske, C. L., and Roth, J. C. 1973. Limnological survey oflakes Michigan, Superior, Huron, and Erie.
LAKE ONTARIO ZOOPLANKTON: 1967-1982 University of Michigan, Great Lakes Research Division, Pub!. No. 17. Stevens, R. J. J., and Neilson, M. A. T. 1987. Response of Lake Ontario to reductions in phosphorous load: 1967-1982. J. Fish. Aquat. Sci. (in press). Stewart, D. J., Kitchell, J. F., and Crowder, L. B. 1981. Forage fishes and their salmonid predators in Lake Michigan. Trans. Amer. Fish. Soc. 110: 751-763. Vijverberg, J., and Richter, A. F. 1982. Population dynamics and production of Daphnia hyalina Leydig
339
and Daphnia cucullata Sars in Tjeukemeer. HydrobioI. 95: 235-259. Watson, N. H. F., and Carpenter, G. F. 1974. Seasonal abundance of crustacean zooplankton and net plankton biomass of Lakes Huron, Erie, and Ontario. J. Fish. Res. Board Can. 31: 309-317. Whittaker, R. H. 1952. A study of summer foliage insect communities in the Great Smoky Mountains. Ecol. Monogr. 22: 1-44.