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10 PDF OUTPUT 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. PDF OUTPUT 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
Page 1 and 2: AUTHOR’S PROOFS PDF OUTPUT Hydrob
Page 3 and 4: PDF OUTPUT Figure 1. Depth-time dia
Page 5 and 6: Figure 2. Seasonal variation of env
Page 7 and 8: Figure 3. Seasonal variation of phy
Page 9: Figure 5. Seasonal variation of env
Page 13: References APHA, 1992. Standard Met

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