Temporal and spatial changes in population structure of the freshwater macroalga Nitellopsis obtusa (Desv.) J.Groves

Figures & data

Figure 1. Bathymetric map of Lake Bois d’Avaz (46.070°N/6.445° E).

Table 1. Physical and chemical characteristics of the water of Lake Bois d’Avaz based on 11 sampling dates between October 2009 and October 2011. n = total number of analyses (see Rey-Boissezon and Auderset Joye 2012 for details).

	Total Phosphorus	NO3^–	NH4^+	SO4^2–	Cl^+	Ca^2+	Mg^2+	Conductivity	Turbidity	Dissolved oxygen	Phytoplankton biomass
	μg.l^−1 l	mg.l^−1	mg.l^−1	mg.l^−1	mg.l^−1	mg.l^−1	mg.l^−1	μS.cm^−1	NTU	%	μg.l^−1
mean ± sd	21 ± 30	1.3 ± 1.8	0.16 ± 0.15	30 ± 11	19 ± 6	94 ± 29	7 ± 2	529 ± 138	6 ± 3	85 ± 36	131 ± 174
n	43	35	24	33	24	33	33	58	58	51	24

Table 2. Phytosociological units of submerged aquatic plants according to the Swiss reference (Prunier et al., 2017).

Phytosociological alliance	Description	Characteristic species present in Bois d’Avaz
Potamion pectinati Koch ex Görs in Oberd. 1977	Units characterized by slow-growing species such as tall pondweeds, milfoil and naias in moderately deep waters (1 to 5 m) or by horned pondweed in shallow water (0.3 to 3 m).	Potamogeton pectinatus, Myriophyllum spicatum, Myriophyllum verticillatum, Najas marina, Zannichellia palustris
Parvopotamion Vollmar 1947	Units characterized by fast-growing vascular species such as coontails, waterweeds, and small leaves pondweeds in disturbed areas or areas likely to dry out (water level fluctuations), getting rapidly warm.	Potamogeton berchtoldii, Potamogeton crispus
Charion globularis Krausch 1964	Units composed of large stoneworts, in permanent, deep or shallow (1 to 15 m) waters.	Chara globularis, Chara contraria, Chara hispida, Chara strigosa
Charion vulgaris Krause et Lang ex Krause 1981	Units composed of small to moderately large stoneworts, in shallow neutral to alkaline water (less than 1.5 m), more or less temporary.	Chara vulgaris, Chara aspera, Tolypella glomerata

Figure 2. Annual duration of low level phases. L50 = low water level at which 50% of the surface area dried out (−0.68 m). L75 = low water level at which 75% of the surface area dried out (−1 m).

Figure 3. Spatial and temporal variation in the abundance of N. obtusa. The scale is expressed as the percentage of cover in each quadrat. Small points represent the quadrats without N. obtusa.

Figure 4. Indval value of N. obtusa and four alliances at each summer. All indval values are statistically significant (p value < 0.001).

Figure 5. N. obtusa from the lake Bois d’Avaz. (a) Female specimen with ripe calcified oospores (22 September 2010, ≈ 2700 G.D.D.). (b) Male specimen with ripe antheridia (27 July 2010, ≈ 1850 G.D.D.). (c–e) Lateral, apical and basal view of a gyrogonite. (f) Star-shaped bulbil. (photos: Boissezon A).

Table 3. Morphological and sexual features of N. obtusa from lake Bois d’Avaz.

	Mean ± sd (min–max)	n
Height (cm)	29 ± 17 (1–100)	110
Axis diameter (μm)	911 ± 176 (400–1300)	110
Internode lenght (cm)	4.2 ± 3.9 (0.3–16)	52
Branchlets lenght (cm)	3.6 ± 2.4 (0.3–9.3)	52
Antheridia diameter (μm)	607 ± 354 (70–1500)	669
Not-calcified oospore lenght (μm)	787 ± 72 (700–890)	6
Not-calcified oospore width (μm)	592 ± 53 (500–650)	6
Gyrogonite lenght (L.P.A., μm)	1072 ± 137 (660–1580)	751
Gyrogonite width (L.E.D., μm)	873 ± 138 (520–1180)	751
Isopolarity Index (ISI)	123 ± 11 (90–219)	751
Spire width (μm)	183 ± 31 (88–260)	751
Spiral index (Cs)	6 ± 0.8 (4–10)	751

Note: L.P.A. = Longest Polar Axis; L.E.D. = Largest Equatorial Diameter.

Table 4. Characteristics of sampling periods and relative frequency (%) of phenological phases of N. obtusa (maximal value for each phenophase is in bold).

Period number	G.D.D. (16 March, 4°)	Mean day duration (hours)	Mean depth (cm)	Water level decrease (cm)*	Corresponding dates	Number of samples	Relative frequency of phenophases
							Sterile young	Male	Female	Senescent
1	<300	12.8	136	7	24 March to 14 April	6	100	17	0	0
2	301–700	15	128	19	12 to 21 May	10	70	30	0	0
3	701–1200	16	97	28	9 to 24 June	24	17	71	63	0
4	1201–1600	16	79	47	2nd to 14 July	12	0	50	83	0
5	1601–1900	15	52	59	15 to 28 July	20	0	65	80	20
6	1901–2200	14.6	81	62	6 to 16 August	9	0	33	89	33
7	2201–2500	14	64	67	17 August to 1st September	11	0	55	82	55
8	2501–2900	13	49	89	15 to 22 September	12	0	25	75	67
9	2901–3500	11	58	103	1st October to 3rd December	6	0	67	67	100

*In reference to the maximum water level of the year.

Figure 6. Proportion of (a) ripe antheridia, (b) oospores and (c) abortive oogonia observed on N. obtusa at different periods. See Table 4 for more details about the sampling periods. P-values from Kruskal–Wallis comparison tests are all significant.

Rey-Boissezon, A., and D. Auderset Joye. 2012. “A Temporary Gravel Pit as A Biodiversity Hotspot for Aquatic Plants in the Alps.” Archives des Sciences 65 (1–2): 177–190.

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