2. Materials and Methods
4.7 Article 5
phenology of a fertile population of Nitellopsis obtusa (Desv.) J. Groves »
Aurélie Rey-Boissezon, Dominique Auderset Joye, Tamara Garcia & Jean-Bernard Lachavanne
This manuscript has been submitted to Hydrobiologia in November 2013
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Abstract
Nitellopsis obtusa inhabits high depth in lakes where it survives vegetatively. The present study asked in which temperature and water depth conditions fertile N. obtusa occur.
During 2009 and 2010, we monitored water temperature and water depth in a shallow gravel pit sheltering a fertile population of N. obtusa whose distribution, morphology and phenology was studied. We analysed the biological attributes variability using Hill-and-Smith ordination. Generalized Additive Models were implemented to model the relationship between the developmental stages of N. obtusa and accumulated heat energy (“growth degree-day” or GDD) and water depth (irradiance conditions).
N. obtusa proved to be summer annual, growing slowly in areas of 1 to 3 m deep with a highest probability to fructify in maximal depth (2-3 m). The ripe antheridia required less heat energy to develop (1500 GDD) than ripe oospores (2500 GDD). Signs of decay occurred from 2500 GDD in shallowest areas, signaling the end of N. obtusa life cycle.
Oospores germination was not observed suggesting that particular conditions necessary for the dormancy breakage were not gathered during our study.
We proved that N. obtusa would be able to survive in a wide range of ecological conditions thanks to physiological and phenological adaptations.
Keywords : charophyte; phenology; plasticity, resilience; degree-days; shallow water.
1. Introduction
Nitellopsis obtusa is an eurasiatic charophyte species generally found deep (2 - 14m) in permanent lakes and in still waters of secondary river channels (Corillion 1975;
Krause 1985; Korsch, Raabe & de Meyer 2008). Calcified oospores, known as gyrogonites, of N. obtusa constitute sexual microfossils which are frequently found in Quaternary sediment records from African localities and are used as biomarker of deep and cold lakes (Soulié-Märsche 1993). Nevertheless, under such conditions, current populations of N. obtusa maintain themselves by means of vegetative propagules, and
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leave no fossil records (Soulié-Märsche, Benammi & Gemayel 2002). In extant populations, ripe gametangia are observed in rare cases. Male specimens are observed more often than female ones and ripe oospores are extremely scarce. This phenomenon has been known for a long time. Migula (1897) and Willèn (1960) struggled to find any fertile specimens or cases with male and female occurring in same localities. Olsen (1944) Corillion (1975) and more recently Langangen (2007) made the same observations about N. obtusa in Denmark, France, Sweden and Finland.
Nevertheless, some evidence of recent changes in the distribution area and the reproductive mode of N. obtusa were reported during the last three decades in the Northern hemisphere. Krause (1985) mentioned that N. obtusa was expanding its colonisation area in Europe. This phenomenon was confirmed by several very recent studies. In France the distribution area is shifting from West to East across the territory (Bailly & Schaefer 2010). The species has been recorded in new localities since 2006 in the Wielpolska region of Poland (Gabka 2009) and in new ponds created by the digging in floodplains in Germany (Korsch et al. 2008). In Switzerland, N. obtusa colonizes slightly eutrophicated large lakes situated in the lowlands (Auderset Joye & Rey-Boissezon, in prep.; Rey-Boissezon & Auderset Joye, in prep.; Auderset Joye & Schwarzer 2012).
In addition, N. obtusa was reported outside its known eurasiatic distribution area in North America (St Laurence River) in 1978 by Geis et al. (1981), where it has been spread by ballast waters of commercial ships during the 1600-1700 era (Mann, Proctor
& Taylor 1999). Soon after, Schloesser, Hudson & Nichols (1986) recorded it at shallow depths (mean of 2.1 m) in the St-Clair-Detroit river system in the Great Lakes region, suggesting that it was expanding its colonization range.
Indeed, few observations suggest that N. obtusa is abandoning the deepest parts of lakes to colonize shallower waters. This phenomenon was first mentioned by Krause (1985) who associated this shift in the apparent habitat of N. obtusa with a shift towards sexual reproduction. This suggests that the optimal conditions for sexual reproduction of N. obtusa are those prevailing in relatively shallow waters where temperatures and light availability are relatively high.
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The influence of water temperature on the growth of N. obtusa was studied only by Willèn (1960). He observed well developed antheridia and oogonia in two distinct lakes of Central Sweden in 1959, a year characterized by the highest temperature and number of hours of sunshine compared to the four preceding years. He suggested that the development of gametangia by N. obtusa could be a response to such conditions. Mann et al. (1999) mentioned that only male specimens were found in populations reported by Geis et al. (1981) and then by Schloesser et al. (1986) in the Great Lakes region.
Moreover, in both cases, N. obtusa was found at depth below 3 m only. More recently, in Michigan lakes, the species was recorded both in deeper and in very shallow waters and showed extremely active growth during the last hot summer months between 2006 and 2009. No attention was paid to the reproduction mechanisms but the picture used to describe the species growing in shallow waters shows well developed antheridia (Pullman & Crawford 2010). In this region, N. obtusa is expanding so rapidly that it is now considered as an invasive species.
In the French Alps, we recently discovered a population of N. obtusa which is remarkable and corroborates these anecdotal changes. First of all, it colonises an
"unusual" habitat, a shallow gravel pit (Rey-Boissezon & Auderset Joye 2012). Secondly, it is highly fertile, producing a large amount of gyrogonites, a phenomenon extremely uncommon and thus poorly understood. Over four years (2009-2012), we monitored water level and temperature fluctuations of this shallow gravel pit, together with macrophyte species distribution (Rey-Boissezon and Auderset Joye, in prep). In this previous study we demonstrated that charophytes species succeed each other along a gradient of drought-disturbance, from permanently flooded habitats to those that dried 3 months in autumn. Particularly N. obtusa proved to be associated to permanently inundated conditions and was a dominant species during 2009 after several years of high water level. Because of the successive dry phases that occurred from 2009 to 2012, the species has almost disappeared from the study site since 2011 and its re-cover has not been observed yet (summer 2013, unpublished results). The germination and establishment of N. obtusa in low disturbance conditions could be related to its life-history patterns.
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Here we attempt to bring new insights into this rarely observed phenology pattern of N. obtusa and its relationship with water temperature and water depth. Our three main assumptions are (1) that N. obtusa growth is dependent upon several factors, in particular water temperature and depth, (2) that each developmental stage corresponds to an invariable amount of heat. For this purpose we measured a variety of morphological characteristics on N. obtusa plants, sampled every two or four weeks throughout the growth season in 2009 and 2010 (years of high abundance for this species) and related it to the corresponding water depth in reference to period of maximum water level and heat energy accumulated at the sampling date.
2. Materials and Methods
2.1 Study site
Lake Bois d'Avaz is situated in the French pre-Alps (Haute-Savoie) in the intermediate basin of the Arve River (06°26'35.0'' E / 46°04'17.6'' N) at an altitude of 452 m. It has a surface area of approximately 47'000m2 and a perimeter of 1600 m. It is a former gravel pit that was partially filled by silt originating from an exploited gravel pit. The site is a shallow water-body characterized by a depth below 1 m for 75% of its surface area during our 4 years survey. The deepest parts are the western (up to 2 m) and the eastern (up to 3m) sides of the lakes (Rey-Boissezon & Auderset Joye 2012). The lake receives water from rain, a hillslope aquifer and a small temporary tributary.
According to our results, Lake Bois d’Avaz has oligo to mesotrophic waters, clear to slightly turbid and strongly mineralized (Rey-Boissezon & Auderset-Joye, 2012). Strong inter and intra annual variations of water regime were observed before and during our study about N. obtusa life cycle (Rey-Boissezon & Auderset Joye, in prep.). One year before the start of our study, in 2008, most of the total surface area stayed inundated during the whole year whereas in 2009 and 2010 a water level decrease occurred in summer, leading to the drying of the half of the lake from the end of summer to the end of autumn. As a consequence, the shallower parts were submitted to wet/dry cycles whereas the deepest parts (>133 cm) remained inundated during the whole monitoring (Rey-Boissezon & Auderset Joye, in prep.) The thermal regime of lake bois d’Avaz shows strong seasonal variations. According to our measurements between 2009 and 2012, the mean daily water temperature was 14.5°C but it reached 27.8 °C during the hottest
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months (July 2010) and fall below 4°C during coldest winter (December 2011) when an ice layer covered the entire lake (Rey-Boissezon & Auderset Joye 2012). A spatial heterogeneity exists: the eastern extremity displays more buffered water temperature than the rest of the lake, indicating that it receives more groundwater input.
Nevertheless we did not measure any thermal stratification of the lake, even in the deepest parts, indicating that water is well mixing (Rey-Boissezon & Auderset Joye 2012).
2.2 Nitellopsis obtusa
N. obtusa cover (%) and depth was recorded in July 2009 and in May and July 2010 in 0.25 m2 square plots equally spaced along transects 20 metres apart covering the whole lake (Oertli et al., 2005). The frequency and the percentage cover of the species were calculated using data each sampling date. The frequency of N. obtusa was defined as the ratio between the number of plots where the species has been recorded and the total number of plots. The mean cover was calculated as the ratio between the sum of N.
obtusa cover values and the number of plots where the species has been recorded.
Strong water level decrease occurred from summer to autumn in 2009 and 2010 so that water depths recorded in May and July were often lower than those under which N.
obtusa start to grow. Hence, we defined the depth of each plot containing N. obtusa in reference to the maximum water level measured during our study (16th April 2009). To all plots depth measured the day of field sampling we added the difference in water level decrease recorded by the logger since the 16th April 2009. This reference depth was used in statistical analyses.
To describe its phenology, we collected samples of N. obtusa from March to December 2009 and from March to October 2010 every two to four weeks. In order to collect a maximum of plants, samples were allocated according to the distribution of the species on the day of the visit. Sampling was therefore not random because of the unpredictable spatial and temporal variability of N. obtusa occurrence. When possible, plants from a maximum of ten locations per date were sampled to highlight a possible spatial heterogeneity of growth. But, since N. obtusa regressed in response to summer water level decrease (Rey-Boissezon & Auderset Joye, in. prep.), the resulting number of samples were not similar between 2009 and 2010 (n=65 and n=45 respectively).
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Samples were carefully collected by hand or using a heavy hook in order to get entire plants and to determine if they originated from sexual (oospore germination) or vegetative propagules (bulbils or thallus fragments). N. obtusa samples were fixed in ethanol (70%). The laboratory observations and biometric measurement were made using a Leica stereomicroscope M205C with a maximum magnification of 160x. To assess the plant phenology, the following development stages were described: new sprouts, sterile plant, male or female fructification (proportion of ripe and abortive organs), calcification of oospores, signs of decay. Total height, axis diameter, internode length, branchlet length, antheridia diameter, oospore and "gyrogonite" (calcified oospore) morphology (length, width, spiral number, spiral width, apical and basal morphology) were recorded. Depending on the phenological stage of plants (early or late stage of fructification, decay), the number of male or female gametangia that could be measured to calculate minimal, maximal and mean values varied between samples (from 4 to 31 for antheridia, from 6 to 56 for gyrogonites). Several qualitative and quantitative attributes of the vegetative and reproductive organs of collected plants were recorded for further multivariate analysis (tab. 1).