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AUTHOR’S PROOFS 

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Hydrobiologia : 1–13, 2003. 

© 2003 Kluwer Academic Publishers. Printed in the Netherlands. 

Steady-state assemblages of phytoplankton in four temperate lakes (NE 

U.S.A.) 

Vera Huszar 1 ,CarlaKruk 2 & Nina Caraco 3 

1 Lab. de Ficologia, Museu Nacional/UFRJ, São Cristóvão, Rio de Janeiro (RJ) 20940-040, Brasil 

2 Limnology Section, Universidad de La República, 11400 Montevideo, Uruguay 

3 Institute of Ecosystem Studies, Box AB, 12 545, Milbrook, NY, U.S.A. 

Key words: phytoplankton assemblages, steady-states, driving forces, stratified lakes 

Abstract 

For four temperate lakes (Northeast U.S.A.) we identify periods of persistent phytoplankton assemblages and investigate 

the ecological conditions that correlate to these persistent assemblages. Periods of persistent assemblages, 

here considered as steady-state phases, were defined according to equilibrium criteria (two or three coexisting 

species, contributing to 80% of the standing biomass, for at least 2 weeks) defined by Sommer et al. (1993, Hydrobiologia 

249: 1–7). For all four lakes, samples were taken weekly during the ice-free season and phytoplankton 

attributes (biomass, assemblages, diversity, species richness, change rates) and abiotic variables (temperature, I ∗ – 

as light mean in the mixing zone – zmix, and nutrients) were analysed. Chodikee (CH), an eutrophic and rapidly 

flushed lake, did not show any persistent phase. The remaining three lakes showed single steady-state phases that 

occurred at varying times during the ice-free season. Steady-state phases occurred during early stratification in 

late spring in the stably stratified oligotrophic Mohonk Lake (MO), in the late summer stratification in the mesoeutrophic 

Stissing Lake (ST), and during spring mixing in Wononscopomuc Lake (WO). MO showed a 3-week 

period with dominance of F assemblage (Botryococcus braunii, Willea wilhelmii and Eutetramorus planctonicus), 

characteristic for clear epilimnia, tolerant to low nutrient and sensitive to high turbidity. For three weeks, ST had 

a stable assemblage with dominance of Lo (Woronichinia sp.), common assemblage in summer epilimnion of 

mesotrophic lakes and sensitive to prolonged or deep mixing; and P, assemblage able to live in eutrophic epilimnia 

with mild light and sensitive to stratification and silica depletion. In contrast, the mesotrophic Wononscopomuc 

Lake (WO) showed persistent assemblages during a 4-week period of spring circulation, when a dinoflagellate (Lo) 

was co-dominant with Nitzschia acicularis (C). The latter species is characteristic for mesotrophic lakes, tolerant 

to low light and sensitive to stratification and silica depletion. Both Lo and P assemblages, among seven others, 

had before been quoted, in literature, as dominant in maturing stages. We could not find consistent statistical 

differences between the periods classified as steady-state and non-steady-state. However, the data demonstrated 

that prolonged period of both mixing and stratification can maintain dominant assemblages. Although, historically 

sensed as opposite mechanisms, both mixing and stratification, if persistent, were observed maintaining dominant 

assemblages because both scenario are characterized by environmental constancy. 

Introduction 

Equilibrium and non-equilibrium hypothesis have often 

been used to explain community ecology. In the 

non-equilibrium hypothesis, forces driving the equilibrium 

are weak and the competitive interactions are 

minimal. Under such circumstances, environmental 

disturbances occur frequently enough to disrupt the 

course of competitive exclusion (Harris, 1986). Higher 

diversity is reached when environmental and internal 

disturbances can prevent the establishment of equilibrium 

conditions (Sommer et al., 1993). In nonequilibrium, 

each dominant species occupies a different 

niche which results from and reduces direct 

competition. Then, species composition is more or 

less stable according to environmental predictabil- 

YB: ICPC: DISK: CP: PIPS NO.: 5143981 (hydrkap:bio2fam) v.1.1 

NF9.tex; 28/07/2003; 12:08; p.1 

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ity (Whittaker, 1975). In this sense, both temporal 

and spatial niche segregation are essential to maintain 

communities in equilibrium. In aquatic environments, 

this segregation is uncommon (Harris, 1986) and can 

be reached only in absence of disturbance, when diversity 

is reduced to minimal levels by competitive 

exclusion. 

Selection of dominant phytoplankton species in 

lakes depends upon a complex and mainly unpredictable 

combination of factors. The physical structure 

of the system (hydrology, temperature and light), the 

availability of nutrients (phosphorus, nitrogen, silicate 

and carbon) and the zooplankton community are 

the most important phytoplankton conditioning factors 

(Reynolds, 1980). Phytoplankton grow very fast and 

most species are cosmopolitan implying that colonisation 

of new habitats is hardly a limiting factor 

(Reynolds, 1997). We can assume that everything 

can growth everywhere and filter the spectrum of 

possible assemblages using information about their 

environmental tolerances and sensitivities. The longer 

certain conditions persist, the poorer is the survival 

of the sensitive groups and the better the survival 

of the tolerant ones. In less severe environments, 

more species will potentially operate successfully, but 

as their growth depletes nutrients and reduces light 

availability, conditions become more restrictive and 

the identity of survivors becomes more predictable. 

After constant environmental conditions over 12–16 

generations, the phytoplankton community reaches 

a climax or climactic equilibrium (Reynolds, 1993). 

When disturbances conditions prevail, plagioclimatic 

assemblages can develop (Reynolds, 1988). In these 

situations, we expect to find dominant species or particular 

species assemblages with different levels of stability 

and diversity according to characteristics of the 

system. In this sense, Sommer et al. (1993) ensure that 

a equilibrium phase development is attained when two 

or three species, contributing to more than 80% of the 

standing biomass, coexist with no changes for, at least, 

2 weeks. Considering that the phytoplankton community 

is usually far from thermodynamic equilibrium 

and most pelagic systems are neither well-organised 

nor successionally well advanced (Reynolds, 1997), 

hereinafter we will use de word steady-state since it 

can be held by nutrients, or light, or flushing, or stratification, 

or spatial transport, or grazers, but always it 

is dominated by few species for which the conditions 

are the best. 

A number of models have been developed to understand 

the regulatory influence of environmental factors 


acting upon phytoplankton assemblages (e.g. Moss 

et al., 1996). Most of the predictive models of spatial 

and seasonal variation treat phytoplankton either 

as a single entity or by major taxonomic divisions. 

Far from being a uniform group, however, freshwater 

phytoplankton is composed of organisms drawn from 

11 or so algal phyla, with highly diverse characteristics, 

which can be grouping in assemblages, based 

on the physiological, morphological and ecological attributes 

of the species that potentially and alternatively 

may dominate or co-dominate the system. 

Opposing the traditional view, which concentrates 

on taxonomic dominance at division level, 

the morphological-functional framework (Reynolds, 

1997; Reynolds et al., 2002) has performed better 

than the approach (Huszar & Caraco, 1998; Fabbro & 

Duivenvorden, 2000; Kruk et al., 2002) in simulating 

compositional changes. The last version of the scheme 

(Reynolds et al., 2002) outlined 31 assemblages (labelled 

as an alpha-numeric code) with the basic pattern 

of their distinctive ecologies. According to Reynolds 

(1997) dominant, near monospecific populations of 

some assemblages appear strong candidates to qualify 

and, under specific cases, to achieve the maturing 

stage sensu Odum’ rules (Odum, 1969). We hypothesized 

that, provided steady-state phases in a lake, 

phytoplankton will present a higher and less variable 

biomass, lower diversity and change rates, and will be 

represented by particular assemblages conditioned by 

the environmental attributes. 

In this paper, we apply data from the phytoplankton 

community of four temperate lakes with different 

mixing regimes and trophic states, located in northeast 

U.S.A., in order to identify periods of low variability 

in biomass and composition of the assemblages 

(steady-states) and to understand the ecological conditions 

which lead, maintain (tolerances) and disrupt 

(sensitivities) such persistence. 

Materials and methods 

Study sites 

The four lakes (Chodikee, Stissing, Wonoskopomuc 

and Mohonk) are located in northern U.S.A. and have 

different residence times, mixing patterns and trophic 

states (Table 1). A detailed discussion about the relationship 

between phytoplankton composition and 

physical and chemical variables of these same lakes 

can be found in Huszar & Caraco (1998). 

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Figure 1. Depth-time diagrams of water temperature ( ◦ C) recorded for the four lakes. SS=steady-states. 

Table 1. Morphometric data for the studied lakes. Loc=localization; zmax=maximum depth; Ret.time=retention 

time 

Lakes Loc. Area (km2 ) zmax (m) Ret.time Mixing Trophic State1 (years) 

Chodikee NY 0.24 6 0.3 Discontinuous Cold Eutrophic 

Polymictic 

Stissing NY 0.28 8 0.7 Dimictic Meso- eutrophic 

Wononscopomuc CT 1.43 32 3 Dimictic Mesotrophic 

Mohonk NY 0.07 19 3 Dimictic Oligotrophic 

1 Vollenweider & Kerekes (1982); Nürnberg (1996). 

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Field sampling 

Samples were taken weekly during the ice-free season 

(April–October). Limnological samples were taken 

from the surface (0.5 m) at a fixed point in the deepest 

area of each system, with exception of temperature 

which was measured biweekly in profile (four or 

six depths). Phytoplankton and nutrients were taken 

in polyethylene bottles; dissolved inorganic carbon 

(DIC) from gas-tight, 60 ml containers. pH was measured 

in situ (Fisher accumet 1001-meter) and temperature 

with YSI temperature oxygen meter. Water 

transparency by Secchi depth and conductivity with 

YSI 3000 T-L-C meter. 

Before samples were analysed or preserved, they 

were kept in the dark on ice. Samples for dissolved 

nutrients were filtered within 8 h using Whatman GF/F 

filters. Nutrients and DIC samples were preserved with 

clean H2SO4 to pH 2.0 and 1.0, respectively (Caraco 

et al., 1993). Phytoplankton was preserved in Lugol 

solution. 

Sample analysis 

Nutrient concentrations were estimated by colorimetric 

analysis using a Technicon or ALPKEM autoanalyser 

(APHA, 1992). DIC was measured on a Shimadzu 

ASI-5050 analyser. Alkalinity was calculated 

from DIC, pH, temperature and conductivity (Cole et 

al., 1994). 

Phytoplankton was concentrated prior to counting 

from 250 ml to 20 ml. Concentration was accomplished 

by settling and using peristaltic pump 

to remove overlying water. Following concentration, 

phytoplankton, except picoplankton, were enumerated 

in random fields (Uhelinger, 1964), using the settling 

technique (Utermöhl, 1958). The units (cells, 

colonies and filaments) were enumerated, at least to 

100 specimens of the most frequent species (counting 

error5% of relative abundance) with similar 

morphological and ecological features. 

Periods of persistent assemblages, considered as 

steady-state phases, were defined according to Sommer 

et al. (1993), where a maximum of three species 

dominate (70–80%) the community for, at least, three 

weeks without significant change in total biomass. 

Non-parametric Kruskall-Wallis H-tests (H statistic) 

were used to test for differences among different conditions 

in the phytoplankton community according 

to total biomass and differences among the steadystate 

indicators and assemblages between steady and 

non-steady-state phases. Significant differences were 

considered with p values lower than 0.05 and reported 

with the H statistic and p number values. The 

results about lake chemical, physical regimes and 

phytoplankton seasonal cycles are reported for each 

lake and by periods, which were identified based on 

the variation in phytoplankton biomass and species 

composition. 

Results 

Chodikee Lake (CH) 

This was stratified only in June and July and was intermittently 

mixed during the rest of the ice-free season 

(Fig. 1). Despite it was the most productive (1.1 µM 

annual mean of total-P) among the four lakes, this 

slightly eutrophic system was relatively poor in dissolved 

inorganic N and P (annual mean SRP=0,1 µM 

and DIN 4.5 µM); SRSi occurred in high levels (annual 

mean=104 µM), with increasing concentrations 

from spring to late summer (Fig. 2). 

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Figure 2. Seasonal variation of environmental variables in Chodikee 

Lake (SRP=soluble reactive phosphorus; SRSi=soluble reactive 

silicon; SS=steady-states). 

According to the established criteria, the seasonal 

cycle of phytoplankton in Chodikee lake was divided 

to five periods (Fig. 3). Total biomass was significantly 

different among these periods (H=19,35; p < 

0.001). Period I (April 15–May 27) showed low biomass 

and co-dominance of N-fixing cyanobacteria, 

H1 and H2 assemblage (Anabaena spiroides Klebahn 

and A. solitaria Klebahn, respectively) and C and P 

diatom assemblages [Aulacoseira ambigua (Grunow) 

Simonsen and Fragilaria crotonensis Kitton, respectively] 

(Table 2). During period II (June 08–28), high 

biomass and dominance of H1 assemblage (Aphanizomenon 

gracile Lemmermann) were registered. Period 


III (July 6–27), with low biomass and a diversified 

community, showed similar contribution of A. gracile 

(H1) andWoronochinia sp. (Lo), Pandorina morum 

O. Müller (G)andStrombomonas sp. (W1). Period IV 

(August 2–30) had the highest biomass and dominance 

of cyanobacteria, mainly Woronichinia sp. (Lo) and 

A. gracile (H1). Finally, during period V (September 

7–October 12), low biomass and co-dominance of A. 

gracile (H1)andA. ambigua (C) were observed. 

According to the selected criteria we did not 

identify any steady-state in this rapidly flushed shallow 

lake. 

Stissing Lake (ST) 

This was stratified from May to September (Fig. 1). 

Light in the mixing zone was highest at the beginning 

of the stratification. This meso-eutrophic lake (0.6 µM 

annual mean of total-P) showed low annual mean and 

similar changes of the DIN and SRP concentrations 

(14.1 and 0.07 µM, respectively), with peak nutrient 

concentration in the spring, decreasing from April to 

June (Fig. 4). As the other three lakes, Stissing had 

relatively high levels of SRSi (75 µM). 

Phytoplankton biomass (H=10.45; p

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Table 2. Percentages of biomass (means) of phytoplankton assemblages (Ass.) as dominant groups of species, by period in each lake according to Reynolds (1997) and Reynolds et al. (2002) 

Period I % Ass. Period II % Ass. Period III % Ass. Period IV % Ass. % Ass. 


Chodikee 15/04–27/05 08/06–28/06 06/07–27/07 02/08–30/08 07/09–12/10 

Anabaena solitaria 6 H2 Aphanizomenon gracile 52 H1 A. gracile 14 H1 A. solitaria 5 H2A. solitaria 8 H 

Anabaena spiroides 15 H1 C. hirundinella 17 Lm Woronichinia sp. 19 Lo A. spiroides 9 H1A. gracile 22 H 

Cryptomonas 5 Y F. crotonensis 18 P Synechocystis 5 X1 A. gracile 20 H1 Woronichinia 8 L 

curvata aquatilis 

Ceratium 5 Lm Pandorina morum 14 G Woronichinia sp. 38 Lo A. ambigua 23 C 

hirundinella 

Aulacoseira ambigua 13 C Strombomonas sp. 15 W1 P. morum 10 G 

Fragilaria 9 P 

crotonensis 

Chlorophyceae 3 7 

Stissing 20/04–02/06 08/06–10/08 17/08–12/10 

C. curvata 12 Y A. solitaria 9 H2 Woronichinia sp. 26 Lo 

C. marsonii 6 Y C. hirundinella 14 Lm C. curvata 10 Y 

Asterionella sp. 20 C F. crotonensis 11 P C. hirundinella 4 Lm 

F. crotonensis 19 P Centrales 10 A F. crotonensis 27 P 

Centrales 19 A B. braunii 14 F B. braunii 7 F 

Wononscopomuc 20/04–18/05 24/05–22/06 28/06–02/08 10/08–15/10 

Dinoflagelado 3 53 Lo Anabaena flos-aquae 21 H1 A. flos-aquae 21 H1 A. flos-aquae 6.7 H1 

Nitzschia acicularis 27 C Aphanothece nidulans 8 K A. solitaria 27 H2 A. nidulans 8.5 K 

C. hirundinella 21 Lm A. nidulans 27 K Aphanocapsa elachista 6.9 K 

C. bodanica 16 A F. crotonensis 8 P Oscillatoria sp. 7.2 S 

F. crotonensis 12 P C. hirundinella 10 Lm C. hirundinella 19 Lm 

Dinobryon divergens 5 E C. bodanica 12.4 A 

Chrysamoeba radians 7.5 X2 

Mohonk 15/04–18/05 02/06–13/09 27/09–12/10 

Tabellaria fenestrata 58 N Eutetramorus 19 F B. braunii 34 F 

planctonicus 

Dinobryon 18 E B. braunii 25 F W. wilhelmii 52 F 

cylindricum 

Botryococcus braunii 4 F Willea wilhelmii 37 F Oocystis borgei 8 F 

NF9.tex; 28/07/2003; 12:08; p.6

Figure 3. Seasonal variation of phytoplankton groups biomass in 

the four lakes, by periods (SS=steady-states; I. II. III. IV=periods). 

low and less variable concentrations (Table 3). Biomass 

and change rates were significantly higher during 

the steady-state, but non significant differences were 

found for diversity, evenness and species richness 


Figure 4. Seasonal variation of environmental variables in Stissing 

Lake (abbreviations as in Fig. 2). 

(Table 5), neither for abiotic factors between steady 

and non-steady-states. 

Wononscopomuc Lake (WO) 

A mesotrophic system (0.34 µM annual mean of 

total-P) was consistently stratified during summer 

with full circulation early spring (Fig. 1). Very low 

concentrations of dissolved inorganic N and P (annual 

mean=1.81 and 0.08 µM, respectively) were 

registered. Nitrate decreased from the spring toward 

summer going undetectable through its end. SRSi was 

lower than in CH and ST, but still relatively high considering 

algal requirements. Light was lower during 

spring time (Fig. 5). 

NF9.tex; 28/07/2003; 12:08; p.7 

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Table 3. Water variables weekly, during steady-state phases in Stissing (ST), Wonoscopomuc (WO) and Mohonk (MO) lakes. zeuf = 

euphotic zone; zmix = mixing zone; zmax = maximum depth; I ∗ = mean light in the zmix; DIN = dissolved inorganic nitrogen; SRSi = 

soluble reactive silicon 


ST WO MO 

Aug-17 Aug-25 Aug-30 Sep-7 Apr-20 Apr-27 May-5 May-12 May-18 Jun-8 Jun-13 Jun-22 Jun-28 

Temperature ( ◦ C) 23.5 23.2 23 20.8 7.4 13.4 16 13.5 19.4 21.1 22.9 21.4 

zeuf (m) 5.9 6.8 7.6 6.5 7.3 8.6 7.3 7.3 7.3 12.2 10.8 9.7 17.6 

zmix (m) 4.0 4.0 5.0 6.0 17.0 12.0 9.0 9.0 8.0 4.0 5.0 7.0 7.0 

zmax (m) 8.0 8.0 7.9 8.0 31 31 31 31 31 19 19 19 19 

I ∗ (mol/m 2 /day) 23.2 17.2 29.0 23.8 2.5 8.1 13.6 8.0 9.4 70.3 41.4 26.9 43.9 

pH 8.1 8.1 8.2 8.2 8.2 8.4 8.6 8.8 8.8 8.7 7.7 7.4 7.5 

Alkalinity (µEq l −1 )1928 1988 1945 2009 2183 2258 2241 2390 2599 215 172 216 181 

NNO3 − (µM) 0.0 0.0 0.0 0.0 1.1 1.1 0.0 0.3 0.1 0.8 0.8 1.1 1.7 

NNH4 + (µM) 1.9 1.9 1.7 1.7 2.5 2.6 2.9 3.5 3.4 1.1 1.2 1.3 1.6 

DIN (µM) 1.9 1.9 1.7 1.7 3.5 3.6 2.9 3.9 3.5 1.1 1.3 1.3 1.7 

PPO 3− 4 (µM) 0.04 0.06 0.06 0.14 0.07 0.06 0.06 0.07 0.06 0.01 0.03 0.01 0.03 

SRSi (µM) 104 101 32 17 14 13 13 

CO2 (µM) 32 36 31 31 43 16 10 9 1 9 19 15 

According to the changes in phytoplankton biomass 

(H=25.85; p < 0.001) and composition, a 

unimodal pattern of biomass was observed and three 

periods were recognised (Fig. 3). Period I (April 20– 

May 18) had intermediate biomass levels and a high 

dominance of a non-identified dinoflagellate (Lo) and 

Nitzschia acicularis (Kützing) W. Smith (C). Period 

II (May 24–June 22) presented, in general, low 

biomass and a mixed population of Anabaena flosaquae 

Brébisson ex. Bornet & Flahault (H1) andC. 

hirundinella (Lm), among others. Period III (June 28– 

August 2) showed a highly variable biomass with a 

peak in July 6, dominated mainly by C. hirundinella 

(Lm) and Cyclotella bodanica Grunow (A). This 

period had also a high contribution of H1 and H2 

(A. flos-aquae and A. solitaria, respectively) and K 

(Aphanothece nidulans Richter) assemblages. During 

period IV (August 10–October 15) a diversified community 

was registered with similar contribution of 

cyanobacteria (H1 and K), dinoflagellates (Lm) and 

diatoms (A). 

A steady-state phase was found (Table 3) during 

early mixing in period I (April 20–May 18), with 

dominance (79–84%) of N. acicularis (C assemblage; 

H=16.60; p < 0.001) and a non-identified dinoflagellate 

(Lm assemblage; =16.60; p < 0.001). The 

biomass of these assemblages (Table 5) was not significantly 

different from the rest of the study, and, unlike 

the results in ST, the dominant species in each as- 

semblage was different (N. acicularis: H=16.60; p< 

0.001; dinoflagellate: H=16.60; p < 0.001). Total 

mixing, low light conditions, relatively high concentrations 

of DIN and SRP were observed in this phase 

(Table 3). Differently from ST, many abiotic variables 

were significantly different during the steady-state 

period. The tendency of fluctuations in abiotic conditions 

was increasing light (H=9.273; p < 0.01) 

and temperature (H=9.273; p

Figure 5. Seasonal variation of environmental variables in 

Wononscopomuc Lake (abbreviations as in Fig. 2). 

mass; H=6.39; p

10 


Table 4. Weekly phytoplankton attributes during steady-state phases in stissing (ST). Wonoscopomuc (WO) and Mohonk (MO) 

lakes (Div = diversity; Ch. Rates = change-rates) 

ST ST ST ST WO WO WO WO WO MO MO MO MO 

Aug 17 Aug 25 Aug 30 Sep 07 Apr 20 Apr 27 May 05 May 12 May 18 Jun 08 Jun 13 Jun 22 Jun 28 

Biomass (mg l −1 ) 2.6 3.2 3.2 2.2 0.6 0.6 0.5 0.7 0.7 0.3 0.1 0.3 

Div. (bits mg −1 ) 2.7 2.9 2.4 2.3 1.6 1.9 1.9 1.9 2.5 2.1 1.9 2.3 

Evenness (%) 61 63 56 55 49 50 54 60 69 54 53 58 

Ch. rates (day −1 ) 0.20 0.32 0.12 0.35 0.05 0.09 0.04 0.12 0.15 0.07 0.28 0.16 

Species richness 22 25 21 19 10 13 11 8.9 12 16 11 15 

(taxa per sample) 

Assemblages (%) 64 63 77 76 80 84 82 79 71 89 94 85 

Lo -Woronichinia sp. Lm- Dinoflagellate F-Botryococcus braunii 

P – Fragilaria crotonensis C -Nitzschia acicularis F- Willea wilhelmii 

F– Botryococcus braunii F-Eutetramorus planctonicus 

Table 5. Mean of phytoplankton attributes during the total study, and the 

steady and non-steady states in Chodikee (CH), Stissing (ST), Wononscopomuc 

(WO) and Mohonk (MO) lakes. Results from Kruskall-Wallis median 

test are shown for difference between steady and non-steady states for each 

lake, including H and p values. ns. refers to non significant results. Sp. rich 

= species richness. The significantly different mean values are marked with 

∗ and ∗∗ . ∗ Refers to results concordant with the expected and ∗∗ refers to 

significant results opposed to that expected by Sommer’s criteria (Sommer et 

al., 1993) 

Period Biomass Diversity Eveness Sp. rich. Change 

mg l −1 Bits mg −1 % Taxa/sample day −1 

TOTAL 

CH 5.1 2.3 56 18 0.2 

ST 1.8 2.6 63 18 0.1 

WO 0.5 2.2 56 17 0.1 

MO 0.9 1.6 42 15 0.2 

STEADY-STATE 

ST 2.8 ∗ 2.6 59 22 0.24 ∗∗ 

WO 0.6 1.8 ∗ 53 11 ∗ 0.08 ∗ 

MO 0.3 2.2 ∗∗ 59 ∗∗ 13 0.17 

NON STEADY-STATE 

ST 1.6 ∗ 2.6 64 17 0.11 ∗∗ 

WO 0.5 2.5 ∗ 57 21 ∗ 0.16 ∗ 

MO 0.7 1.6 ∗∗ 43 ∗∗ 15 0.18 

KRUSKALL-WALLIS MEDIAN TEST 

ST 5.283 ns ns ns 5.689 

0.022 0.017 

WO ns 4.246 ns 8.001 4.827 

0.039 0.005 0.028 

MO ns 4.523 4.331 ns ns 

0.033 0.037 

NF9.tex; 28/07/2003; 12:08; p.10

It is worthwhile to mention that, considering all 

the three lakes together where steady-states were identified, 

we could not find any statistical difference 

between steady and non-steady-state, using biotic and 

abiotic variables. 

Discussion 

We consider that the functional approach does capture 

much of the ecology of the phytoplankton and 

can be used as a verifiable quantitative method of 

describing community structure and changes (Fabbro 

& Duivenvorden, 2000; Reynolds et al., 2002; 

Kruk et al., 2002). In this context, we analysed 

the identified steady-state periods in the lakes using 

functional groups, with assemblages as descriptors. 

Assemblages were useful for describing community 

conditions during phases with different stability. Important 

differences were found between steady and 

non-steady states. In these situations, dominant near 

monospecific community with different levels of stability 

and diversity, according to characteristics of the 

conditions, were recorded. 

Lo and Lm assemblages (dinoflagellates and cyanobacteria 

of summer epilimnion of mesotrophic and 

eutrophic lakes), M (colonial cyanobacteria of diel 

mixed eutrophic waters of low latitudes), S (cyanobacteria 

of turbid and mixed layers of enriched temperate 

systems), R (cyanobacteria forming maximum 

metalimnetic), H (N-fixing cyanobacteria), N (diatoms 

and desmids of mesotrophic epilimnion) and P 

(of eutrophic epilimnion) had been quoted as possible 

to achieve maturing stages (Reynolds, 1997) sensu 

Odum’ rules (Odum, 1969). Among these seven assemblages, 

two were found in this study (Lo and P). 

Now, we add other two (F and C) as candidates to 

integrate the list. 

According to criteria by Sommer et al. (1993), we 

found one steady-state in three of the four studied 

lakes. Chodikee lake (CH), a rapidly flushed and shallow 

system did not show any steady-state as expected 

by Sommer’s indicators of stability. However, clear 

periods of constant community structure were identified 

in the other lakes. Stissing (ST), Wononscopomuc 

(WO) and Mohonk (MO) lakes, showed a relative 

constancy of composition during, at least, 3 weeks 

without significant change in biomass. Steady-state 

phases occurred in the very-well stratified ST during 

the summer and in MO during end-spring, but also 

during mixing times in WO. 


The main factor of persistence that caused and 

maintained the steady-state in ST was stratification. 

As a consequence, low nutrient levels were registered 

during this state. The disruption was a consequence 

of decreasing temperature and mixing, leading to decreasing 

light and increasing CO2. During the steadystate 

period in ST, P and Lo assemblages were dominant 

and occurred until the end of summer stratification, 

when the mixing zone was deepening (4–6 m). Lo 

refers to stratified mesotrophic lakes, which tolerate 

segregated nutrients through the water column and 

are sensitive to deep mixing. P assemblage includes 

diatoms, which are depending on physical mixing, requiring 

a continuous or semi-continuous mixed layer 

of, at least, 2–3 m in thickness (Reynolds et al., 2002). 

As expected, decreasing temperature and deepening 

of mixing zone favoured the disruption of the Lo 

dominance. However, P-assemblage, which should be 

favoured by those conditions, also declined. Among 

the stability indicators only total biomass and change 

rates were significantly different between states. The 

observed total biomass, as expected, was higher during 

the stable state, but a non-expected higher change 

rate was found. The coexistence of representatives 

of both assemblages with 22 other species did not 

make possible to find the expected lower diversity, 

resulting of the decreasing environmental and internal 

disturbances, which prevents the establishment of 

equilibrium conditions (Sommer et al., 1993). 

The dominant F assemblage during the steadystate 

period in MO was represented by non-motile 

but near-neutrally buoyant colonial green-algae, which 

perform better in clear waters and are tolerant of 

deep mixing and low nutrient concentrations. Indeed, 

phytoplankton growth in MO was considered, during 

a whole ice-free season, strongly limited by both 

phosphorus and nitrogen (Huszar & Caraco, 1998). In 

addition, during the steady-state phase, the lake had 

a deep and very clear mixing zone. F assemblage, 

composed by almost the same species, was dominant 

during the rest of the stratification period. However, 

the absolute and relative biomass were much more 

variable during non-steady-state. The main factor of 

constancy was stratification with low nutrient concentrations. 

The factors of disruption were associated with 

deepening of the mixing zone. None of the stability 

indicators observed showed the expected pattern 

according to Sommer et al. (1993). Moreover and unexpectedly, 

diversity and evenness were higher during 

the steady-state. 

11 

NF9.tex; 28/07/2003; 12:08; p.11

12 

According to the literature, a non expected situation 

during equilibrium conditions, differently from 

the other, was registered in WO. In this system, the 

factors of constancy were associated with persistent 

mixing, low light and high nutrient concentrations. 

The factors of disruption were increasing stratification 

and light, as well as decreasing nutrient lakes. 

In this lake, the steady-state phase occurred during 

the vernal development, with dominance of a nonexpected 

large dinoflagellate. Lo was not expected, 

because the lake was in early mixing with relatively 

high nutrient concentrations and Lo performed better 

in stratified columns, with segregated nutrients, and 

is sensitive to prolonged or deep mixing (Reynolds et 

al., 2002). The spatial transport within a system can 

complicate the interpretation, because dinoflagellates 

can be transported from the benthos or littoral area to 

the pelagic region during the mixing. A similar situation 

was reported to lake Kinneret (Israel), where, 

for many years, the lake supported a periodic spring 

sequence, reaching a regular dominance of Peridinium 

gatunense (Lo), which does not resist the nutrient deficiency 

through summer time and enters in benthic 

resting stages (Berman et al., 1992). The dominance 

was shared with the diatom Nitzschia acicularis (C), 

whose habitat template in WO was different to other 

species of Nitzschia, which have been included in 

D assemblage of shallow mixed and enriched lakes 

(Reynolds et al., 2002). It can be ascribed to a deep 

mixed zone, but with low light, and to a soft, but 

not enriched waters. These features are close to C assemblage, 

which can develop in mixed conditions and 

is tolerant to low light and carbon deficiency, but sensitive 

to stratification and SRSi depletion. The probable 

spatial transport of the dinoflagellate, during the mixing 

in WO, made possible the co-existence with the 

diatom, both with different requirements. During the 

steady-state, diversity, species richness and change 

rate were lower and concordant with the expected. 

According to the observations presented in this paper, 

the steady-state periods occurred during strong 

stratification, but also in mixed water columns. What 

it can tell us is that long periods of spring mixing 

can also set up and maintain dominant assemblages. 

Assemblages persistence can be hold as long as the 

physical conditions are unchanged and the current 

crop of organisms can fill (or partially fill) the carrying 

capacity of the system. Steady-states were ended only 

by major hydrographic changes, stratification or mixing. 

In the shallow and rapidly flushed Chodikee Lake, 

constant hydrological conditions maintained a highly 


variable environment, preventing the appearance of a 

differentiable steady-state, concordant at least to some 

extent to definition by Sommer et al. (1993), but also 

resembling the plagioclimaxic conditions described by 

Reynolds (1993). 

In synthesis, constancy of hidrological conditions, 

characterised either by mixing or stratification, during 

3 or 4 weeks, leaded to a dominance of particular 

assemblages composed by few species. However, the 

expected equilibrium with consistent reduction of diversity, 

species richness and change rates were not 

observed in the community. Considering the habitat 

template and phytoplankton attributes, not consistent 

statistical differences between steady-state and nonsteady-state 

phases were found in the phytoplankton 

stability indicators. Some lakes, for example, did 

not show the expected lower diversity and species 

richness during steady-states (ST) and other showed 

even higher values than in non-steady-states (MO). 

The comparison of steady-states among systems, as 

a whole, was not possible, probably because criteria 

by Sommer et al. (1993) are not useful for defining 

periods in any situation. Its application should be narrowed 

to those cases when plankton is not changing 

rapidly (it is not in ascendancy, collapse or rapid succession), 

or when physical conditions coincide with 

what Reynolds (1993) called plagioclimax, or any 

other condition which can be maintained along with 

high diversity or low biomass. Steady-states can be 

promoted and ended by major hydrographic changes 

(either it stratifies or it mixes). What matters to promote 

and maintain persistent assemblages is the environmental 

constancy, independently of its relation with 

a particular physical condition. 

Acknowledgements 

The authors would like to thank Dr Luigi Naselli- 

Flores for financial support to attend the 13th Workshop 

of the International Association of Phytoplankton 

Taxonomy and Ecology; David Fisher for technical 

support; Rich Miller and Peter Raymond for collection 

of samples and nutrient analysis. This research 

was financially supported by Conselho Nacional de 

Desenvolvimento Científico e Tecnológico, Brasil, 

Fundação de Amparo à Pesquisa do Estado do Rio 

de Janeiro, Brasil and National Science Foundation, 

U.S.A. This is a contribution of Institute of Ecosystem 

Studies. 

NF9.tex; 28/07/2003; 12:08; p.12

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