Article 2

through distribution analysis »

Aurélie Rey-Boissezon & Dominique Auderset Joye This manuscript is published in Aquatic Botany

(Charophyte Special Issue 2014) DOI: 10.1016/j.aquabot.2014.05.007

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Highlights

► Waterbody size, conductivity and altitude are important for charophytes species habitat segregation.

► Seven of twelve charophyte species show significant individual habitat marginality.

► Nitellopsis obtusa is a “lake” species, Chara intermedia is a “pond” species.

► Chara strigosa is a specialist species of cold, oligotrophic and hard water.

► Chara contraria and C. vulgaris colonise a wide variety of habitats.

Abstract

Charophytes play an important role in aquatic ecosystems but there is a lack of knowledge on the ecology of these macroalgae. Our aim was to characterise the habitat of 12 charophyte species from 78 sites in Switzerland characterised by a set of environmental variables considered critical to the physiology and survival of these plants (climate, land-use, morphometry and chemistry). We searched (i) to evaluate how environmental gradients explain the distribution of species; and (ii) to identify if some species have narrower habitat range than others (“specialist” vs “generalist” species).

The Outlying Mean Index (OMI) measures the distance between the mean habitat conditions used by a given species and the mean habitat conditions of all studied waterbodies and was used to analyse the distribution of species. Waterbody size, conductivity and altitude were the most discriminating habitat variables of 7 of the 12 species, followed by calcium and total phosphorus. Chara strigosa is considered a specialist of cold oligotrophic hard waters, and three as generalists (Chara contraria, Chara globularis, Chara vulgaris) with respect to our set of charophyte sites. We demonstrate that charophyte species occur together in a narrow environmental envelope but do not occupy exactly similar habitats. Despite a discrepancy among absolute optima of species in different countries, the relative position of species is likely to be transferable to other regions. Improving our understanding about the ecology of charophyte species by considering a broader scale dataset and more fine-scale biotic and abiotic factors is proposed as an important perspective.

Keywords:

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1. Introduction

Charophytes can be major components of some freshwater ecosystems. As primary producers, they provide habitat, food and refuges for periphyton, invertebrates, fish, amphibians and birds. Charophytes also assist in maintaining a clear water state in shallow waterbodies due to their role in biogeochemical cycles (e.g. organic carbon production, phosphorus immobilisation), and on sediment deposition (Stelzer et al., 2005; Bornette and Puijalon, 2011). Many charophyte species are listed on the Red Lists of several European territories (eg. Stewart and Church, 1992; Hamann and Garniel, 2002; Mirek et al., 2006; Blaženčić et al., 2006b; Langangen, 2007; Johansson et al., 2010). Similarly in Switzerland, the majority of the 25 known species declined over the last 200 years (Auderset Joye and Schwarzer, 2012) mainly because of habitat loss and degradation due to agriculture and urban development, eutrophication of water, loss of natural dynamics of river channels and water regulation. Like most endangered species, the conservation of charophytes is dependent on their habitat preservation or rehabilitation. However, they have been poorly studied and we lack information on their ecological requirements. As a group, charophytes are currently strictly associated to clear alkaline oligo-mesotrophic waters (Janse et al., 2008). However, in this relatively narrow environmental envelope, different species may tolerate distinct trophic and alkalinity conditions (Lambert and Guerlesquin, 2002; Lambert, 2002). Temperature requirements could also discriminate charophyte species. Regarding their relation to temperature, spring species that are able to complete their life cycle in cold conditions can be distinguished from summer species, which require more heat energy to germinate, grow and fructify (Bonis et al., 1996; Fernandez-Alaez et al., 2002).

The present study aims to improve scientific knowledge about the ecology of charophyte species by relating their occurrence with environmental information, hence examining their habitat. Our data were a set of 78 charophyte-bearing waterbodies situated in Switzerland and close to the French border. Based upon previous knowledge about charophyte ecology we selected potential explanatory environmental variables.

They take into account the size, the temperature, the surrounding land-use and the water chemistry of the 78 study sites.

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An appropriate way of examining how species are distributed along environmental gradients, while keeping an overall view of the species community, is to perform a multivariate analysis. We used the Outlying Mean Index (OMI) analysis that makes no prior assumptions about the shape of the response curve (Dolédec et al., 2000). OMI separates species based on their marginality, i.e. the distance between the mean habitat conditions used by a given species and the mean habitat conditions of the set of studied sites. It also provides for each species a tolerance index, which is a measure of its habitat amplitude. This method has been successfully applied to identify how environmental gradients influence the distribution of invertebrate or plant species (Besacier-Monbertrand et al., 2009; Kleyer et al., 2012). Although the authors of OMI analysis called it “niche analysis”, we consider that “habitat analysis” is the appropriate term to be used in our study. The niche is the evolutionary result of a species’ morphological, physiological and behavioural adaptations to its surroundings. The physical location where the species actually survives and reproduces is called the habitat. Hence, a species has one niche but may occupy several habitats. In agreement with Kearney (2006), when examining the abiotic and biotic nature of species’ locations on a particular scale of space and time with descriptive/correlative analyses, “habitat modelling” is under consideration. It is an appropriate first step to identify the coarse environmental gradients which influence species distribution when there is a lack of basic ecological information. The determination of habitat characteristics using correlative methods such as the OMI analysis helps to understand the niche but does not describe it entirely.

That implies that habitat models of species are transferable to other geographic regions which are environmentally similar.

The objective of our study was to respond to several questions about charophytes’

habitats in Switzerland: (i) Do selected environmental factors explain the distribution of species? (ii) How are species optimal habitats positioned along environmental gradients? (iii) Can we separate species that colonise all type of habitats of the study area (“generalist” charophytes) from those that are associated with a particular and narrow range of environmental conditions (“specialist” charophytes)?

79 Meteorology and Climatology MeteoSwiss). About one third of the territory is used for farming (37 %), one third is covered by forests and woodlands (31%), one quarter, categorised as “unproductive” either mountains, lakes and rivers (25%), the rest is urbanised.

A previous study dedicated to the extinction risk of charophyte species growing in Switzerland was performed on 1402 sites (amphibian sites, ponds and segments of lake shores) across the whole national territory over 4 years (2006-2009) (Auderset Joye and Schwarzer, 2012; Auderset Joye and Rey-Boissezon, 2014, this issue). 387 observations of charophyte species were listed in those 1402 sites. From this pre-existing presence/absence dataset, we selected 78 waterbodies harbouring charophytes with the objective to ensure a sufficient prevalence of target species, especially of the most threatened ones. This non-random selection of sites would not assist the construction of a predictive species distribution model. It needs unbiased population samples to infer correctly the probability of occurrence of species along environmental gradients (Vaughan and Ormerod, 2003; Araújo and Guisan, 2006). But we assume that this non-random data collection would suffice for showing the difference in abiotic characteristics between sites harbouring charophytes. The 78 sites were finally composed of 59 ponds (maximum depth < 8 m; Oertli et al., 2002) and 19 lakes, all sheltering one or several charophyte species. All study sites are situated in Switzerland, except for three ponds located in France, close to the Swiss border (Fig. 1). Twelve of 21 species listed in Switzerland were present in our set of studied sites. Amongst these species, 9 species belong to the genus Chara, one to Nitella, one to Tolypella and the only current member of the genus Nitellopsis was represented (Table 3). Chara tomentosa, Chara virgata, Tolypella glomerata and Nitellopsis obtusa were present in less than 10%