109

Hydrobiologia 331 : 109-120, 1996 . © 1996 Kluwer Academic Publishers . Printed in Belgium .

Phytoplankton community succession in a lake subjected to artificial circulation Richard P. Barbiero l *, Barbara J . Speziale2

&

Steven L . Ashby 3

1 AScI Corporation, 1720 Clay Street, Vicksburg MS 39180, USA 2 Clemson University, 400 Tillman Hall, Clemson SC 29634 USA 3 US Army Corps of Engineers, Waterways Experiment Station, 3909 Halls Ferry Road, Vicksburg MS 39180, USA (* Current address: Department of Botany, University College, Galway, Ireland) Received 7 September 1995 ; in revised form 28 February 1996 ; accepted 16 April 1996

Key words: Phytoplankton succession, artificial circulation, intermediate disturbance

Abstract East Sidney Lake, a small, eutrophic bottom release impoundment in NY, has undergone artificial circulation for three seasons . The artificial circulation system resulted in an overall reduction in the physical stability of the water column, making the lake subject to alternating periods of weak chemical stratification and mixing . Phytoplankton community succession exhibited a high degree of regularity from year to year, culminating in mid summer dominance by heterocystous cyanophytes in all years . Changes in the physical structure of the water column, with attendant changes in Z eu :Z mjx , were not important determinants of phytoplankton community makeup in East Sidney Lake . Seasonal patterns and community characteristics were not affected by artificially induced alterations in stability, but instead were most sensitive to surface temperatures, flushing rate and TN :TP. The timing of cyanophyte blooms was not affected by artificial circulation, nor was maximum seasonal phytoplankton biomass reduced . Introduction Artificial circulation is a widely used water quality control technique (Cooke et al ., 1993) that involves the prevention or disruption of stratification of a lake or reservoir, usually through either mechanical pumping or by diffused air pumping . The technique was originally developed as a means to increase productivity in Michigan trout lakes (Hooper et al ., 1953), but has since been most often employed to improve water quality in impoundments . Artificial circulation can potentially reduce algal standing crops through a number of mechanisms . In the simplest case, the dilution of algal biomass through an enlarged water column could reduce phytoplankton concentrations without an actual reduction in biomass . An increase in mixing depth could bring about light limitation (Lorenzen & Mitchell, 1975) if lake depth is greater than critical depth (sensu Sverdrup, 1953), while the maintenance of oxygenated conditions at the

sediment-water interface could reduce nutrient supplies in cases where internal loading of phosphorus (P) is controlled by iron redox reactions . Shifts in algal dominance from nuisance cyanophytes to less objectionable populations of green algae or diatoms could also occur, through increased CO2 and lowered pH (King, 1970 ; Forsberg & Shapiro, 1980 ; Shapiro, 1984), loss of effectiveness of buoyancy-controlling mechanisms, particularly in the case of subsurface populations, e .g . Oscillatoria (Bernhardt, 1967 ; Steinberg, 1983), and reduced sinking of heavier species, e .g ., diatoms (Lund, 1971 ; Knoechel & Kalff, 1975) . Recently, attention has focused on the effects of intermittent alterations in mixing regime on phytoplankton successional sequences (Reynolds et al ., 1983 ; Trimbee & Harris, 1984 ; Reynolds et al ., 1984 ; Steinberg & Zimmermann, 1988) . Periodic increases in mixing depth could mediate phytoplankton community shifts by altering the light climate (i .e. decreasing Ze11, :Zmj x ), lowering epilimnetic temperature, and pos-



1 10 sibly entraining nutrient-rich bottom water, creating conditions more characteristic of winter/spring . Theoretically, such alterations in mixing depth could disrupt the natural seasonal progression by shifting the competitive advantage too quickly between individual species for one to attain dominance (Reynolds et al ., 1983) . This would lead to an alternation between, or mixture of, winter/spring assemblages and early summer r-selected species ; a delay in the development of late summer K-selected species (e .g . cyanophytes, Ceratium) ; and a population maximum below the nutrient determined carrying capacity (Reynolds et al ., 1984) . These considerations have lead to the suggestion that artificial circulation systems be operated at intensities sufficient to facilitate wind-driven mixing, while still permitting increases in stability during periods of calm (Reynolds et al ., 1984) . Another potential consequence of intermittent perturbations of stratification is an increase in phytoplankton community diversity, as predicted by Connell's (1978) Intermediate Disturbance Hypothesis (IDH) (Sommer et al ., 1993) . In the absence of disturbance, competitive exclusion (Hardin, 1960) should result in low diversity in the relatively homogeneous environment of the mixed zone of a lake, where phytoplankton compete for a relatively small number of limiting resources (Sommer, 1991) . In this study, the phytoplankton community of a small reservoir (East Sidney Lake) was observed for six seasons, during four of which changes in the mixing regime of the lake were effected in differing degrees by an artificial circulation unit . Here we examine the factors determining phytoplankton community development, with particular attention to the potential effects of artificial circulation . A fuller treatment of the water quality effects of artificial circulation will be presented elsewhere (Barbiero et al ., 1996) .

Materials and methods Site description

East Sidney Lake is a small (A = 85 ha, z w = 15 .7 m, z = 4 .9 m) bottom withdrawal impoundment located in the north branch of the Susquehanna River basin in south-central New York state, USA . Enriched run-off from the predominantly agricultural watershed (Ashby & Kennedy, 1990) results in high phosphorus concentrations, hypolimnetic anoxia, excessive phytoplankton populations and decreases in water clarity during the late summer (Kennedy et al ., 1988) . In an attempt

to improve water quality in the reservoir and tailwater, an artificial circulation system was installed at the lake in 1989 . This consisted of a compressor delivering 1 .8 m3 of air per minute at a pressure of 345 kPa through a diffuser placed along the 9 .1 m contour of the lake . Operation began on 27 July 1989, but was discontinued after two weeks due to a timer malfunction . In the three subsequent years, the system was operated from approximately late May to early October . Further details on system design and operation are provided elsewhere (Barbiero et al ., 1996) . Sample collection and analysis

Data reported in this study were collected from a representative site near the deepest point in the lake . Water samples were collected approximately weekly (bimonthly in 1988) during the growing season (MayOctober) from 1988-1993 .' Temperature and dissolved oxygen were measured at 1 m intervals with YSI model 57 and 50b meters . Stability, defined as the minimum theoretical energy required to mix the reservoir from an initially stratified state to an isothermal state (Symons, 1969), was calculated from the following equations : Stability = PEM - PES (J) Potential energy of mixed system (PEM)= gEg 1 Potential energy of stratified system (PES)=

g>

ph

m Vihi

1 psVjhti

where : Am and pi, = mean water densities of each layer for the mixed and stratified cases, respectively (kg m -3 ) ; Vi = volume of each layer (m 3 ) ; hi = height to the centroid of each layer above the bed of the reservoir (m) ; g = acceleration due to gravity (m s-2 ) ; n = total number of layers. The mixed depth (Z wx ) was defined as the depth of water above a density gradient >0 .08 kg m -3 m -1 , with obvious instances of surficial heating excluded . The euphotic zone was estimated as (Reynolds et al., 1983) : Zeu = 3 .7/(0 .05 + 222/Zsd) where : Zsd = Secchi depth . Discrete water samples for chemical analyses were collected at 2 m intervals . Integrated samples for phytoplankton identification and enumeration were collected from the euphotic zone (estimated in the field as twice the Secchi depth) with a 3 .8 cm ID tube sampler. Samples for total phosphorus (TP) and total nitrogen (TN)

111 were digested with persulfate prior to analysis. TP was analyzed with the ascorbic acid method (APHA, 1985) on a Technicon AAII auto analyzer. TN was analyzed after 24 hour reduction to ammonium with DeVarda's alloy (Raveh & Avnimelech, 1979) and read with the phenate method at 630 nm (APHA, 1985) . Phytoplankton samples were preserved with Lugol's solution (APHA, 1985) and identified and enumerated using an Zeiss compound microscope equipped with phase optics . Biovolumes were estimated by approximating the volume of appropriate geometrical solids .

data . In contrast, CCA explicitly incorporates environmental data by constraining ordination axes to linear combinations of measured environmental variables, and is thus called a direct gradient analysis technique . Additionally, CCA assumes a unimodal response of species to environmental gradients, rather than a linear response, as is typical of other ordination techniques .

Results Physical and chemical response to artificial circulation

Data analysis A number of quantitative methods were used for the interpretation of the phytoplankton community data . The diversity of the phytoplankton community was calculated with the Shannon-Weaver index (Shannon, 1948) : k

H' _ - PilnPi i-1

where: K = number of species ; Pi = proportion of total numbers in species i . Diversity is comprised of both the number of species, and the evenness with which individuals are distributed among those species . The evenness component of diversity (Pielou, 1966) was calculated as :

J' = H' H' where H.] = In species number . Phytoplankton assemblages were identified with a hierarchical fusion clustering procedure . Prior to analysis, biovolumes were converted to natural logarithms to reduce the undue influence of a few dominant species, and rare species (<3%) were excluded from analysis to reduce noise (Gauch, 1982) . An a priori number (14) of clusters were produced using a weighted average linkage algorithm (WPGMA ; Sokal & Michener, 1958) . The relationship between community composition and environmental factors was explored with canonical correspondence analysis (CCA) using the program CANOCO (ter Braak, 1989) . CCA is an ordination technique in which a species-sample matrix is organized along (e .g . 2) axes by reciprocal weighted averaging of both sites and species . Typically, ordination analysis seeks to discern implied environmental gradients through the analysis of community composition

The artificial circulation unit was operated briefly in 1989, and then continuously over the growing seasons of 1990-1992, with 1988 and 1993 serving as control years . Since both hours of operation per day and estimated airflow varied over the course of the study, a daily rate of energy input to the system was estimated by multiplying airflow by hours of operation (Figure 1 a) . During the two control years the lake stratified by mid June, in spite of relatively small temperature differences between the bottom and surface waters (Figure 2a) . In 1988 stability remained high during the summer (Figure lb), while in 1993, cooling of the epilimnion in late July permitted a mixing event. Stratification persisted in both years until September . In 1989, stratification was disrupted several times, both by the brief operation of the artificial circulation system and by numerous summer storms (Figure lc) . However, mean stability over the season was only slightly lower than that during the control years . In the three years in which the artificial circulation system was operated continuously, mean seasonal stability was lower than in either control year (Figure lb), although stratification was not entirely prevented . Stratification during these years was weak, and temperature differences through the water column were generally less than 2 ° C (Figure 2a) . The relatively warm temperatures in the bottom waters promoted rapid oxygen consumption at the sediment surface during quiescent periods, and the alternation of oxygen depletion and reaeration, evident in oxygen isopleths, indicates periods of stratification interspersed with mixing events during these years (Figure 2b). This was particularly apparent in 1991 and 1992 . The estimated euphotic zone deepened during early summer in all years, and then contracted in response to the development of summer blooms . Based on



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Figure 1 . A . Daily airflow from compressor ; B . Stability, with growing season means ; C . Theoretical annual flushing rate (yr 1 ), calculated

daily ; D . Estimated Zm ix (---), calculated Zeu (-) and Zeu :Zmix (0); E . Epilimnetic total phosphorus (-) and total nitrogen (--- ) ; and F. TN:TP ratio and abundance of heterocysts (as percent of total cells) for : A . planctonica (CI) and Aph. flos-aquae (0) . Data only shown when heterocysts present ; no data was collected in 1988. Dashed line indicates a TN :TP ratio of 30.

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Figure 2 . A . Temperature (° C) ; and B . Dissolved oxygen (mg 1 - i) isopleths . Bars indicate mixing events, determined from oxygen profiles .

1 13 Table 1 . Principal membership (average biomass, mm 3 1 - '), by major taxonomic group, of the 14 clusters identified with WPGMA. For ease of presentation, clusters containing similar dominant taxa are combined into groups, with different clusters within a group designated by small case letters . Group la Cryptophytes Dinoflagellates' Diatoms Green flagellates Mallomonas

3 .60 1 .28 1 .05 0.64 0.29

Group 3b Ceratium Chrysophytes 2 Diatoms Cryptophytes Mallonumas

3 .56 2.99

Cryptophytes Gomphosphaeria Other cyanophytes

7 .36

Group 2 Cryptophytes

0.96

Mallomonas Green flagellates

4.59 2 .61 2.18

Diatoms Anabaena Desmids

0.65 0.64 0.40

Diatoms

1 .93

Aphanizomenon

0.33

Group 4a Anabaena

2 .33 1 .57

Aphanizomenon Cryptophytes Gomphosphaeria

0 .56

Green flagellates

Group 5b Aphanizomenon Green flagellates

Group lb Dinoflagellates Cryptophytes

6.96 0 .63 0.60 0.53 0 .43

Group 7a

0 .72 0.58

Group 4b Anabaena Gomphosphaeria Aphanizomenon

97 .60 7 .13 1 .46

0.63

0 .26

Cryptophytes Green flagellates

Desmids Diatoms

6.17 4 .68 2 .69

Aphanizomenon Gomphosphaeria Dinoflagellates'

17 .20 3 .56 0.69

Green flagellates Cryptophytes

0 .87 0 .52

Other cyanophytes Oscillatoria

0.13 0.09

Group 5c Aphanizomenon

Desmids

21 .60

Group 7b Desmids

Aphanizomenon Gomphosphaeria

11 .23 1 .86 1 .41

Cryptophytes Diatoms Green flagellates

0 .54

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Oscillatoria Anabaena

11 .94 1 .59

0.52

Group 5d

Group 3a Ceratium Aphanizomenon Cryptophytes

8 .51 2 .82 0 .66

Chrysophytes 2 Anabaena

0 .49 0 .43

Group 5a Aphanizomenon Anabaena Oscillatoria Gomphosphaeria Cryptophytes

18 .94 3 .80 1 .24 0 .91 0 .82

Group 6 Oscillatoria Aphanizomerum Anabaena

4 .06

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0 .39 0 .26

1 .44

1 .25

40 .92 1 .06 0 .71 0 .65 0 .61

' excluding Ceratium 2 excluding Mallomonas

Zeu : Z,,,;x , artificial circulation produced a deterioration in light conditions, although inter-annual differences were most apparent in early to mid summer (Figure Id) . Later in the season, low transparency kept Zeu :Zmix below 0 .2 in all years except 1988 . The variability in Zeu :Zmix is undoubtedly underestimated in Figure Id, since calculation of Z n" did not take into account mixing events occurring between sample dates . Epilimnetic TP tended to increase over the course of the summer, starting in about June or early July (Figure 1 e) . Extended periods of hypolimnetic anoxia in the control years 1988 and 1993 permitted the build-up of TP to maximum hypolimnetic concentrations of 170 and 260 µg P 1 -1 , respectively (Barbiero et al ., 1996), and epilimnetic increases over the course of the season suggested entrainment of bottom water (Figure le) . In treatment years, alternations between weak stratification, with consequent low dissolved oxygen, and mixing events probably permitted phosphorus release and

subsequent entrainment . Except in 1990, mean inflow TP concentrations during August and September were low (<22 µg P 1 -1 ), and combined with relatively low flows, suggest that increases in epilimnetic TP during these periods were derived from internal sources . Epilimnetic TN exhibited similar seasonal trends as TP, generally decreasing in early summer, and increasing later in the season (Figure le) . Epilimnetic TN :TP ratios typically increased throughout the early summer to a maximum in late June/early July, and then declined throughout the rest of the season, usually dropping below 30 by mid to late July (Figure If) . Phytoplankton community composition The phytoplankton community usually experienced an initial decline in biovolume at the beginning of the sampling period, after which a mid-season dinoflagellate community often formed a short-lived maximum



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taxonomic group .

(Figure 3) . In all years, late summer communities were overwhelmingly dominated by the heterocystous cyanophytesAnabaena planctonica Brunn . and/or Aphanizomenon flos-aquae (L.) Ralfs . Neither annual maximum biovolumes nor mean biovolumes during periods of cyanophyte dominance exhibited reductions during mixing years (Figure 3) . Heterocysts were usually seen on cyanophyte filaments during periods when TN:TP was below 30 (Figure if), and were more abundant on A . planctonica than on Aph. flos-aquae filaments . Heterocysts were particularly abundant in 1993, a year of sustained low TN :TP, and increased notably throughout the season on A . planctonica, which was sub-dominant that year. Diversity typically decreased throughout the season to a minimum coincident with the development of the cyanophyte community, and often recovered somewhat after the collapse of the bloom (Figure 5) . Notable exceptions to this occurred in 1991, when diversity values remained high all season, and in 1993, when diversity remained high during the period of cyanophyte dominance . Declines earlier in the season corresponded to the early summer biomass minimum, and to periods of dominance by Ceratium hirundinella 0 . F. Mull .

Table 2 . Results of canonical correlation analysis .

Axis l Axis II Total Inertia Eigenvalue

0 .405

0 .219 4.72

Species-environment correlation Cumulative percentage variance

0 .835

0 .664

of species data 8 .6 of species-environment relations 43 .5

13 .2 67 .1

Interset correlation of environmental variables with axes name AXI Surftemp -336 Total-P -671 Total-N TN :TP Stability Flushing

AXII -470

-448 539

230 5 -391

-68 547

-390 302

These declines were often the result of decreases in the evenness, rather than richness, component of diversity . Interannual differences in mean diversity showed no apparent relationship with mixing .

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Group 1=0 ; Group 2=0 ; Group 3= 0 ; Group 4= A ; Group 5=0 ; Group 6=0 ; Group 7=*.

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Figure 5. A . Species richness ; B . Diversity (H'), with seasonal means ; and C . Evenness (J') .

Cluster analysis From the fourteen clusters produced by WPGMA, seven main phytoplankton assemblages were apparent (Table 2) . Group 1 was dominated by cryptophytes, dinoflagellates (excluding C . hirundinella) and diatoms ; group 2 had no clear dominants and appeared to be a transitional stage, containing taxa from previ-

ous and subsequent groups . Group 3 was dominated by C. hirundinella . There were three main cyanophyte dominated assemblages ; group 4 in which A . planctonica was dominant, group 5 dominated by Aph . flosaquae, and group six, dominated by Oscillatoria geminata Menegh . Several additional clusters were composed of a limited number of dates ; three of these had Aph. flos-aquae as a dominant, one was an A. plancton-

1 16 ica/Gomphosphaeria aponina var. deliculata Virieux association distinguished on the basis of high biomass, two were dominated by desmids, and two clusters, each composed of a single sample date, had dominant taxa which were similar to groups I and 3, respectively. To facilitate interpretation in later analyses, clusters dominated by A . planctonica and Aph . flos-aquae were assigned to groups 4 and 5, respectively ; the two desmid clusters were combined to form group 7, and the remaining two clusters were added to groups 1 and 3, respectively. These groups allow a general sequence to be discerned from the six years of phytoplankton community data (Figure 4) . Community composition typically proceeded from the early summer group 1 to the mixed assemblage, and then entered a short period of dominance by C. hirundinella . Community development then diverged, leading to dominance by eitherA . planctonica (1988, 1989, 1991), or Aph . flos-aquae (1990, 1993) . An intermediate situation existed in 1992, in which an early Aph. flos-aquae population gave way to A . planctonica later in the summer, while in 1993 O . geminata was briefly dominant in late summer . The mixed assemblage sometimes recurred at the end of the season, and in 1993 group 1 returned at the end of the season . Differences in successional sequence and community composition between mixing and non-mixing years were not pronounced . In all years, mid-summer communities were dominated by either A . planctonica orAph flos-aquae,, with mixing having apparently little to do with determining which predominated . The two most similar years, in terms of successional sequence, summer dominance, and number of clusters were 1990 (7 clusters) and 1993 (8 clusters), mixing and nonmixing years, respectively . The onset of cyanophyte dominance was remarkably consistent, occurring in mid July in all years except 1989 and 1990, when it was delayed to mid and early August, respectively . Both of these years experienced episodes of unusually high flushing due to summer storms . Canonical correspondence analysis The eigenvalues of a CCA ordination axis provide a measure of the variation in species data that is explained by that axis . The first two ordination axes had eigenvalues of 0.405 and 0 .219, respectively (Table 3) . Monte Carlo simulations indicated that both the first axis and the trace were significant at p = 0 .01 . In interpreting the biplots of CCA, the length of the envi-

ronmental line indicates the strength of its correlation with the ordination axes, and its proximity to the axis indicates the degree of correlation . The biplot of sites and environmental variables resulting from canonical correspondence analysis indicated the former to be organized primarily along two gradients : one of decreasing surface temperature and increasing flushing rate extending through the upper right hand quadrant (quadrant I) ; and the other of increasing TN :TP passing through the lower right hand quadrant (quadrant II) (Figure 6) . The proximity of the flushing rate and surface temperature lines, and to a lesser extent stability, indicate that these variables were correlated with each other. Similarly, decreasing TN :TP was correlated with increasing TP concentrations . Plots of samples in which symbol size is scaled on the basis of surface temperature (Figure 6b) and TN :TP (Figure 6c) confirm these as important environmental factors . Distinct patterns were apparent in the distribution of different phytoplankton groups . Group 1 occurred at high flushing rate and low surface temperature, but widely varying TN:TP Group 2 was the most variable, occurring at many points along the two main gradients, and occupying extreme positions with respect to both surface temperature and TN :TP Group 3 was found mainly at low flushing rate and high temperature, and varying, though generally high, TN :TP. All three bloom groups were centered at low TN :TP and low flushing rate/high surface temperature, with group 4, dominated by A. planctonica, the most variable of these . A plot of species in ordination space (Figure 7) indicated that most early summer unicellular species, e.g . Cyclotella, Ankistrodesmus, Rhodomonas, Cryptomonas and Pediastrum, were concentrated in quadrant I . Mallomonas and Peridinium/Gymnodinium, dominant components of group 1, were at the extreme of ordination axis 1 . The colonial chlorophytes, e.g . Sphaerocystis, Oocystis, were found in quadrant II, as was the dinoflagellate C. hirundinella . Cyanophytes (A . planctonica, Aph . flos-aquae, Chroococcus, Gomphosphaeria) formed an extremely compact group to the left of the second axis in an area of low TN :TP and high surface temperature . Surprisingly, the nonheterocystous cyanophyte O . geminata occupied an extreme position with respect to TN :TP Trajectories of seasonal community development were plotted on the ordination axes for the six years of the study (Figure 8) . A generalized seasonal trajectory can probably best be seen in 1993, a year with no manipulation and the highest data density . The early

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summer community (group 1) started in quadrant I, and passed to the transitional group 2 as temperature increased and TN :TP increased . The C. hirundinelladominated group 3 was located at the extreme point of TN :TP in quadrant II . As TN:TP decreased, and temperature increased slightly, dominance shifted to heterocystous cyanophytes (Aph. flos-aquae in this case) and the line moved to a region to the left of the 2nd ordination axis . Cooler temperatures and slightly increased TN :TP moved the community back into quadrant I, passing through group 6 and 2, back to group 1 . Marked divergence from the beginning and end of this pattern in 1991 and 1988, respectively, was probably due in part to truncated sampling seasons . Otherwise, this sequence, at least in its broad outlines, was observed in all years . Differences between mixing and non-mixing years were not seen, and in fact the trajectory of 1990 offered the closest parallel to that of 1993 .

Discussion The artificial circulation system did not completely prevent stratification in East Sidney Lake . In spite of reductions in stability and near isothermal conditions throughout the water column, periodic stratification of dissolved oxygen was still apparent in all three years of system operation . Artificial circulation functioned mainly to increase the susceptibility of the water column to mixing, rather than to maintain completely mixed conditions, and treatment years can probably best be categorized as having experienced alternating periods of stability and mixing, in contrast to the control years in which more stable stratification was maintained . These changes in stability did not result in the expected changes to the phytoplankton community, i .e . longer periods of dominance by the transitional group 2, delays in the appearance of, and reductions in the size of, cyanophyte assemblages, and lower over-



118

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Figure 7. Selected species plotted in ordination space from CCA (Ana = A . planctonica; Anki = Ankistrodesmus convoluta ; Aph = Aph. flos-aquae ; Ast = Asterionella formosa ; Botr = Botrydiopsis ; Cart = Carteria ; Cera = Ceratium hirundinella; Chlm = Chlamydomonas; Chlor = Chlorococcum; Chro = Chroomonas ; chry of = chrysophycean microflagellates ; Coela = Coelastrum; Coelo = Coelosphaerium; Cruc = Crucigenia tetrapedia; Cryp = Cryptomonas; Cycl = Cyclotella; Desm = Desmids ; Dict = Dictyosphaerium; Dino = Dinobryon; Eugl = Euglenoids ; Gom = Gomphosphaeria ; Mall = Mallomonas ; Melo = Melosira ; Ochr = Ochromonas ; Oocy = Oocystis ; Osc = Oscillatoria geminata ; Pedi = Pediastrum ; Peri = Peridinium ; Rho = Rhodomonas ; Scen = Scenedesmus ; Schr = Schroderia setigeria ; Sph = Sphaerocystis ; Trac = Trachelomonas.

all summer biomass . Rather than being determined by mixing regime, the timing of the cyanophyte populations depended upon low flushing, high temperatures and low TN :TP, with the latter apparently the predominant factor. Without exception, communities dominated by cyanophytes first occurred after summer TN :TP ratios remained below 30, while surface temperatures at the onset of cyanophyte blooms varied from 25 .5 °C (1991) to 22 .5 °C (1989) . Smith (1983), in a survey of lake data, found that TN:TP ratios under 30 promoted cyanophyte dominance, and the same ratio has been found to indicate nitrogen limitation in Florida lakes (Kratzer & Brezonik, 1981) . TN :TP ratios usually had dropped below 30 by mid July, a notable exception being 1989, when ratios were erratically above 30 until August . In that year the bloom was delayed

Figure 8. Seasonal trajectories of phytoplankton assemblages plotted in ordination space. Symbols as in Figure 4 .

until September, and occurred when temperatures had already fallen to 22 .5 °C . Additionally, both 1989 and 1990, another year in which the bloom was somewhat delayed, were characterized by episodic high flushing during spring and early summer . These storm events could have had a direct effect on delaying the blooms, e .g ., by increasing cellular washout, or an indirect effect by promoting higher epilimnetic TP through the disruption of stratification (1989) or increased loading (1990) . In both cases, smaller, faster growing species would be favored (Dickman, 1969) . Negative correlations between cyanophytes and flushing rate have been noted by Perry et al . (1990) and Tundisi et al . (1992). In East Sidney Lake, it is tempting to conclude that the shift to heterocystous cyanophytes was a result of their N-fixing capabilities under increasing N limitation. While available N was not measured in this study, the appearance of heterocysts during periods of low TN :TP offers indirect evidence for this, although summer communities also occasionally contained sub-

1 19 stantial quantities of non-N-fixing cyanophytes, in particular O . geminata and G. aponina var. deliculata . Artificial circulation was not effective in reducing the size of summer cyanophyte populations, as might be predicted from theory . With the exception of 1989, large differences in mean biomass during periods of cyanophyte dominance were not seen from year to year . Likewise, increases in diversity, predicted by Connell's IDH, were not seen during mixing years . The mixing years 1990 and 1992, along with the control year 1993, had the lowest mean diversity . Interestingly, these years also contained the greatest number of different phytoplankton assemblages, as identified by cluster analysis, and the largest populations of Aph. flos-aquae. TN:TP ratios showed similar patterns in these years, increasing to high levels in late June/early July, and then falling to very low levels during August and September. It is possible that the greater extremes in TN :TP in July/August contributed to greater variation in phytoplankton assemblages over time, particularly in 1990 and 1993 . These results contrast with other studies which have reported changes in phytoplankton community structure as a result of naturally or artificially induced variability in the physical structure of the water column . Harris & Piccinin (1980) observed reductions in surface chlorophyll peaks during physically variable summers in Hamilton Harbor of Lake Ontario . Seasonal successional trends, however, were largely unaffected . A typical successional progression was not seen in Guelph Lake, Ontario, during a summer marked by intermittent mixing, compared to a more typical year (Trimbee & Harris, 1984) . Diatoms were more prevalent during the mixed summer, and Aph . flosaquae achieved a much greater abundance, in contrast to Microcystis . Possible reasons given for the change in species dominance in late summer included lower TN :TP ratios and increased recruitment of Aphanizomenon from oxic sediments . Steinberg & Zimmermann (1988) have reported using intermittent - mixing to successfully decrease cyanophyte dominance (mainly Oscillatoria), and reduce phytoplankton biomass in general . It was hypothesized that an inability to adapt quickly to changed light and nutrient conditions contributed to the Oscillatoria decline. Disturbance in the form of periodic riverine inflows into a shallow lake in the Middle Parana River floodplain has been shown to result in reversions (sensu Reynolds, 1980) from K-selected to r-selected species (Garcfa de Emiliani, 1993) . Artificial disruption of thermal stratification in a small tropical reservoir resulted in a prolonga-

tion of diatom dominance while silica wasn't limiting, although ultimate dominance by cyanophytes was not averted, and biomass levels were not reduced (Hawkins & Griffiths, 1993) . Increases in phytoplankton diversity with increased mixing have also been reported . Jacobsen & Simonsen (1993) observed an increase in diversity and a shift from Aph. flos-aquae to cryptomonads after a storm event induced complete circulation in the eutrophic Lake Godstrup, Denmark . This shift in community structure from an equilibrium community to an earlier successional stage was attributed to a reduction in light levels, an increase in nutrients and possibly mechanical stress on the Aphanizomenon population . Moustaka-Gouni (1993) also found physical disturbances to increase diversity and set back phytoplankton succession in Lake Volvi, Greece . Chorus & Schlag (1993) found small-scale changes in mixing conditions to affect community composition and diversity in two lakes in Germany. The present results suggest that changes in the physical structure of the water column, in and of themselves, should not be expected to alter phytoplankton community makeup unless they effect changes in the underlying physical or chemical factors that determine successional sequences . Of the factors examined in East Sidney Lake, these apparently included TN :TP, temperature and flushing rate, and they were not sufficiently altered by artificial circulation to affect phytoplankton community size or structure .

Acknowledgments Field sampling in 1988 and 1989 was coordinated by J . Titus, State University of New York, Binghamton, NY and conducted by W. Swears from 1990-1993 . Water quality analyses were conducted at the USAE Trotters Shoal Limnological Research Facility, Calhoun Falls, SC . This work was jointly sponsored by the U .S . Army Engineer District, Baltimore, and the Office of the Chief, U .S . Army Engineers . Permission to publish this material was granted by the Chief of Engineers .

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