Discussion

5.4.1 Relationship between richness and eutrophication

In this study, four metrics based on richness (species or family level) were effective indicators of excessive nutrient concentration for the colline belt. All richness metrics demonstrated a decrease between the eutrophic and hypertrophic states (Figure 5.2). Assuming that eutrophication is a surrogate of productivity, the resultant curves conform with the second part of the “hump-shaped” pattern most often described in the literature for the

productivity–biodiversity relationship (Rosenzweig, 1995; Chase and Leibold, 2002).

At altitudes above 800m (montane-subalpine and alpine belts), another set of richness metrics were shown to be effective, which integrated only a part of the sensitive metrics from lower altitudes. Furthermore, the relationships with an increase in nutrients, in seven of the eight cases, were positively indicative of an increase in the species number.

This difference with the observed patterns for ponds in the colline belt could result from the lower trophic state of the ponds at higher altitudes. It is possible that for many taxonomic groups, the relationship is described by the first part of the hump-shaped curve, where richness increases with productivity.

The Coleoptera was the only group whose species richness responded to an increase in eutrophication for both the colline and the montane-subalpine belts. This accords with previous studies which have already demonstrated that water beetles (particularly the families Hydraenidae and Elmidae) are considered to be sensitive to eutrophication.

For example, Guignot (1931–1933) demonstrated that, in general, aquatic Adephaga prefer good water quality. Garcia-Criado et al. (1999) and Foster et al. (1992) showed that beetle distribution was correlated with eutrophication. Cooper et al. (2005) showed that trophic status was a key predictor of plant species composition and that the vegetation community structure significantly influences the composition of water beetle assemblages. For the Odonata, Gastropoda and Amphibia, the relationship between eutrophication and species richness was not

significant for the colline belt. In contrast, the relationship was significant for species richness when the four groups were pooled together (COGA). The Gastropoda were found to be insensitive to eutrophication, a surprising result considering their great dependency on water (adult and reproductive stages in water, low mobility and low dispersal). This was particularly unexpected in the case of the Prosobranch gastropods which are gill breathers. It has been confirmed in lake studies that Prosobranch species are particularly sensitive to oxygen depletion (Clarke, 1979;

Mouthon, 1992), which is one of the indirect consequences of nutrient enrichment.

Nevertheless, in our study, the Prosobranch species (Bythinia tentaculata, Valvata cristata, Valvata piscinalis, Viviparus contectus) were present in all trophic states, although they were more commonly found in mesotrophic or eutrophic ponds. Most probably, another chemical variable, calcium concentration, which is known to be the major chemical factor governing the distribution of aquatic molluscs (Boycott, 1936; Briers, 2003), had a more influential role than eutrophication in this study. As the species richness of Coleoptera, and of the combined COGA group, both clearly responded negatively to a nutrient increase (in the colline belt), this underlines that rather than using the entire invertebrate community, a selection of taxonomic groups would be sufficient to assess the effect of eutrophication. At altitudes above 800m, testing metrics derived from the Chironomidae could also be relevant, since chironomids often make up a large proportion of the fauna at these higher altitudes (Lang and Reymond, 1996; Kownacki et al., 2000; Hinden et al., 2005 (Annexe 13)) and are already known to be good indicators of eutrophication (Lods-Crozet & Lachavanne, 1994).

Two other metrics already shown to be sensitive to water quality both in rivers and in English pond studies (Biggs et al., 2000) were also confirmed as indicators of eutrophication in this study: the number of macroinvertebrate and MO families (Table 5.5 and Figure 5.2 (c) and (d)). Therefore, the use of family-level metrics would seem to be as efficient as those at the species level, as often described in the literature (Biggs et al., 2000). However, this was only true between the eutrophic and the hypertrophic states, and it is worth noting that this relationship did not apply to ponds situated

above altitudes of 800m.

For EPT family richness, known as an efficient indicator of water quality in running waters (Gaufin and Tarzwell, 1952; Quinn and Hickey, 1990; MACS, 1996), there was an absence of a relationship with eutrophication in the colline belt. This could be explained by the summer sampling, an inappropriate period for this group, since most larvae would either have emerged as adults or be in a very early stage of development. As a result, some species were probably missed in the sampling process.

Nevertheless, a clear negative relationship was observed for the montane-subalpine and alpine belts.

5.4.2 Relationship between

biological/ecological traits and eutrophication

Biological traits have until now proved to be useful assessment tools for studies on running water. Here, in spite of many significant relationships with pond eutrophication, there were no coherent relationship patterns. The relationships that were significant have probably an indirect connection with eutrophication which could not be identified in this study. Furthermore, no trait categories were particularly responsive for all three indicator groups. It appears that there is a clear need for further investigations before using traits as indicators of eutrophication. For example, trait categories were tested here separately for each invertebrate group, while for further investigations it would be reasonable to test them as a ratio of the whole ecosystem, as sometimes suggested in the literature (Rawer-Jost et al., 2000).

Furthermore, instead of looking at each trait category separately for each invertebrate group, it would be interesting to study, for example, the metric “percentage of scrapers”

for the total invertebrate community of the pond. This, however, would be difficult and time-consuming in practice, as all the invertebrates would have to be determined to the genus level. Another point worth mentioning is that the traits taken from Tachet et al. (2000) are assigned both to adults and larvae of the groups considered, and only to

the genus level. The effectiveness of using biological and ecological traits in scientific studies is still under investigation; thus, it would be worthwhile exploring the tables more precisely and making the necessary modifications.

5.4.3 Conclusion

This study has demonstrated that metrics derived from species or family richness are relevant for describing the level of eutrophication, and can be integrated into a future pond assessment method. However, the set of metrics to be used for the assessment of excessive nutrient levels should differ between high-altitude and lowland zones. The metrics related to biological/ecological traits demonstrated many relationships with eutrophication, but for the moment they should not be used for an assessment method as further investigations are needed to identify and select the appropriate metrics. A future challenge would be the selection, integration and calibration of the relevant metrics, within a combined multi-metric index to assess the effect of eutrophication.

The present study focused on the evaluation of a single effect, eutrophication (either the consequence of human disturbance or the natural evolution of the ecosystem). Further investigation of the bio-assessment of ponds would enable the identification of metrics sensitive to other important aspects of impairment, such as land use in the buffer zone surrounding the water body, or in the wider catchment. Additional metrics that account for the presence of introduced and invasive species and/or fish would also probably help to improve the assessment, because of their known effects on the biological equilibria in ecosystems. This set of metrics would assess the quality of the pond (water, environment).

Coupled with an assessment of biodiversity (i.e. identified using the PLOCH method (Oertli et al., 2005b)) this would allow the development of a general biological assessment method for ponds with the purpose of re-establishing the conditions for good ecological status, as recommended by the WFD.

CHAPITRE 6

Eutrophication: are mayflies (Ephemeroptera) good bioindicators for ponds ?

Nathalie Menetrey, Beat Oertli, Michel Sartori, Jean-Bernard Lachavanne. 2008.

Hydrobiologia. 597: 125-135.

Abstract

Ephemeroptera larvae are recognized worldwide for their sensitivity to oxygen depletion in running waters, and are therefore commonly used as bioindicators in many monitoring programmes. Mayflies inhabiting lentic waters, like lakes and ponds, in contrary have been poorly prospected in biomonitoring. For this purpose, a better understanding of their distribution in lentic habitats and of the relations of species presence with environmental conditions are needed.

Within this framework, 104 ponds were sampled in Switzerland. The Ephemeroptera are found to be an insect order particularly well represented in the ponds studied here (93% of the lowland ponds).

Nevertheless, in terms of diversity, they are relatively poorly represented (mean species number = 1.9). Two species dominated: Cloeon dipterum (Baetidae) and Caenis horaria (Caenidae).

The investigations contributed to the updating of the geographical distribution of the species in Switzerland, as many of the observations appear to be from new localities.

The trophic state of ponds appears here to be important for Ephemeroptera communities. First, there is a negative relationship between total phosphorus (TP) concentrations and species richness. Second, the presence of Caenis horaria or Cloeon dipterum is dependent on the trophic state. Caenis horaria is most closely associated with low levels of TP concentrations, while Cloeon dipterum appears to be less sensitive, and is most frequently found in hypertrophic conditions.

A probable consequence of these relations, is that Baetidae are always present when Caenidae are also present. Contrastingly, Baetidae is observed as the only mayflies family present in several ponds.

6.1 Introduction

Mayflies are considered as ‘‘keystone’’ species and their presence is believed to be an important environmental indicator of oligotrophic to mesotrophic (i.e. low to moderately productive) conditions in running waters (Barbour et al., 1999; Bauernfeind &

Moog, 2000). A high sensitivity of mayfly taxa to oxygen depletion, acidification, and various contaminants including metals, ammonia and other chemicals was demonstrated in both observational and experimental studies (Hubbard & Peters, 1978; Resh & Jackson, 1993; Moog et al., 1997; Hickey & Clements, 1998). Various Biological Indices including mayflies to assess water quality have been developed over the years (Lenat, 1988;

Metcalfe, 1989; Kerans & Karr, 1994).

Subsequently, many of the biological water quality assessment methods for streams include Ephemeroptera, as for example the EPT (Ephemeroptera + Plecoptera + Trichoptera) taxa richness (Lenat & Penrose, 1996) which has been incorporated into studies in the United States and in many other countries. Other examples include the River InVertebrate Prediction and Classification System (RIVPACS) for the UK (Wright et al., 1998) and the « Indice Biologique Global Normalise » (IBGN) for France (AFNOR, 1992). A major EU project with 14 participating member states entitled STAn-dardisation of River Classifications (STAR) has now been established, which will calibrate different biological survey results against ecological quality classifications that have to be developed for the WFD of 2000 (Furse et al., 2006).

On the contrary, mayflies inhabiting lentic waters (e.g. lakes and ponds), have been poorly used in biomonitoring programmes (see however Madenjian et al., 1998). Nevertheless, in such environments, we could expect that mayflies also adequately integrate some aspects of water quality. Ephemeroptera have also other advantages for monitoring: they are highly visible, relatively easy to sample and are represented by only a few species in such habitats, which makes identification easier. In Lake Erie, Ephemeroptera are successfully used in biomonitoring, following the example of a recent study that showed burrowing mayfly nymphs (Hexagenia spp.) to be

associated with an improvement of the ecosystem health (Schloesser & Nalepa, 2002).

In smaller waterbodies like ponds, the water quality is rarely assessed. Nevertheless, with the implementation of the WFD, such procedures will be developed. This is already the case in some European states (UK, see Biggs et al., 2000; Catalonia, see Boix et al., 2005; Switzerland, see Menetrey et al., 2005).

For the purpose of better understanding the importance of Ephemeroptera in the assessment of water quality in lentic habitats and especially in ponds, a better understanding is needed of: (i) the distribution of mayflies in such habitats; (ii) the relations of species presence with environmental conditions. In this study, the distribution of mayflies is investigated for 104 ponds from Switzerland.

In a second step, their presence is assessed in relation to environmental variables, particularly the trophic state indicators (total phosphorus (TP), total nitrogen (TN) and conductivity). Finally, we will examine whether a new metric using Ephemeroptera can be proposed for inclusion in rapid bioassessments methods for swiss ponds.