Sengupta, M. and Dalwani, R. (Editors). 2008 Proceedings of Taal2007: The 12th World Lake Conference: 1058-1066 Management of Aquatic Exotic Plants: The Case of Elodea Species. G. Thiébaut, F. Di Nino, M-C. Peltre and P. Wagner Laboratoire Interactions Ecotoxicité, Biodiversité Ecosystèmes (L.I.E.B.E.), Université Paul Verlaine de Metz, Umrcnrs 7146, Avenue Général Delestraint, 57070 Metz, France Email : thiebaut@univ-metz.fr ABSTRACT Elodea canadensis Michaux, coming from North America has been introduced in Europe in 1836 and soon became a widely distributed and troublesome species in Europe, after which it often became integrated in the biocoenosis. Elodea nuttallii (Planch.) St John was first introduced into Europe in 1939. Only female plants were observed. Apparently, E. nuttallii is actively spreading in many parts of Europe and seems to be replacing E. canadensis in many localities. The impact of several frequencies of hand-pulling on the biomass and on the architecture of E. nuttallii and of E. canadensis was evaluated by assessment of morphological traits in stream and in a reservoir respectively. The stream is invaded by E. nuttallii and the reservoir by E. canadensis. Handpulling caused a temporary drastic reduction of biomass of E. nuttallii and of E. canadensis. After handpulling, the spread of E. nuttallii and of E. canadensis was area restricted. The development of native aquatic plants was favoured two years after the last hand-pulling. Furthermore, no significant difference was observed in the architecture of Elodea sp. between the reference and experimental areas. Elodea appeared as disturbance–tolerant species. Keywords: non indigenous species, freshwaters, France, biological traits, hand-pulling. INTRODUCTION In France, biological invasion research in freshwater systems has focused on a few plants such as Ludwigia species (Dutartre and Oyarzabal 1993, Dutartre et al. 1997) or Elodea species (Thiébaut et al. 1997, Barrat-Segretain 2001, 2004, 2005, Greulich and Trémolières 2002, Barrat-Segretain et al. 2002). An indigenous of North America, Elodea canadensis Michaux, first recorded in the early 19th century in the British Isles (Simpson 1984, 1990), is now naturalised and widespread in Europe. Elodea canadensis became a persistent weed following its naturalisation, choking waterways before declining to its present, less-abundant (but still common) level (Cook and Urmi-König 1985, Thiébaut et al. 1997, Barrat-Segretain 2001). Another species from North America, E. nuttallii (Planch.) St John was first found in Belgium in 1939 and had spread into northern France by the end of the 1950s. For the past 30 years it has been colonising numerous ponds and streams in metropolitan France, except in the southeast. Elodea nuttallii is replacing E. canadensis at many sites (Mériaux and Gehu 1979, Thiébaut et al. 1997, Barrat-Segretain 2001). Although E. canadensis and E. nuttallii have been spreading for several years in eastern France, this species is relatively more problematic in other European countries (for example in Germany, Switzerland, Brittany, Belgium, Sweden). In many bodies of water, it has been necessary to control overabundant aquatic plants. Large infestations of plants which cause considerable environmental impact, such as Ludwigia spp. or Hydrocharitaceae have the highest priority for control. There are several methods of managing aquatic plants: mechanical or manual harvests, handpulling, biological control, changing the aquatic environment and chemical control. The management techniques chosen must be appropriate both to the type of weed problem and to the uses and function of the body water. To our knowledge, no research on the impact of hand-pulling of Elodea species (E. nuttallii and E. canadensis) has been done. To evaluate the impact of hand-pulling on the two Elodea species (E. nuttallii, E. canadensis), an approach based on measurements of morphological traits was used. In this paper, we aim to determine the impact of the hand-pulling on E. nuttallii and on E. canadensis biomass, on Elodea population and architecture, through measurement of morphological traits. MATERIAL AND METHODS Study area The study was performed in the Vosges mountains ((NE France). The landscape pattern consists of sandstone mountains. The regional climate is sub continental. Two sites were selected: a river and a reservoir. The first one, located in the Northern Vosges, is the Falkensteinbach stream. This stream is characterized by overabundance of E. nuttallii (mean cover percentage visually estimated between 70 and 90 % in July). The invasive species E. nuttallii colonized the streams of this area since the end of 1970’s (Muller 1990, Thiébaut and Muller 1999). A 100m length of stream was chosen. Two spot-checks (Reference site, experimental site E divided in two sub-units: E1 and E2) located at 45 m intervals (buffer zone) were selected (Figure 1). The second one, located in the Donon mountains, is a reservoir used as a source of supply for the retaining water (0.5 million cubic metre water lake). It is also a leisure and nautical centre. The lake area is 39 ha. This artificial lake is characterized by overabundance of E. canadensis (mean cover percentage visually estimated between 70 and 90 % in July) since 2003. Two spot-checks (Reference site, experimental site E) located within the littoral zone were selected. Each area (reference, experimental) measured 20 x 20 m. Chemical survey of the water 500 ml of water were collected monthly from the end of February to the end of October in 2003 and in 2005 in the Falkensteinbach stream. It was collected in July, August and October 2006 in the lake “La Plaine”. Analyses were performed immediately upon returning to the laboratory (less than 24 hours after sample collection). Alkalinity was determined by titration (AFNOR 1990). Conductivity and pH were measured using a combined glass electrode and corrected for temperature (25°C). Reactive soluble phosphorus and ammonia were analysed using spectrophotometer (single reagent ascorbic acid technique for phosphorus, and indophenol’s technique for ammonia, AFNOR 1990). Samples for nitrate, sulfate and chloride analyses were determined in the laboratory with ion chromatography. Experimental protocol Hand-pulling Hand-pulling involves removing entire plants (leaves, stems, and roots) from the area of concern and disposing of them in an area away from the shoreline. During hand pulling, Elodea plants are manually removed from the bottom, with care taken to remove the entire root crown and to not create fragments. During hand pulling, we dig around and beneath the plant roots with a tool and gently lift the entire plant out of the sediment. Elodea plants can be readily removed from sediments. Once plants are removed, we place them into bags for transportation to the surface. Site 1: Falkensteinbach river The factor tested was the impact of diffrent frequencies of hand-pulling in the stream: - one hand-pulling was practised on E1 and E2 on February 2003, - a second hand-pulling was realised on E2 on May 2003, Reference site 36 m Water flow direction Buffer site 45m E1 15 m E2 15 m 1 hand-pulling 3 hand-pulling February 2003 February & May 2003 Experimental area: 30m March 2005 Figure 1. Description of the experimental site 1: The Falkensteinbach river. 1059 - a third hand-pulling was realised on E2 on March 2005. The impact of management on the architecture of plant and on the biomass was assessed each month until the end of the study. Impact of the management on the morphology of Elodea species Thirty plants were collected each month in site 1 (reference, E1 and E2) from March to October 2003 and from March 2005 to July 2005. They were removed in July and October 2006 in site 2 (reference, experimental area). Apical shoots of 3cm length, corresponding to the optimal growth area, was cut off. Lateral shoots included the initial lateral which developed from the nodes on the apical original shoot and the others lateral which was a development either from the same nodes, or from the nodes on the lateral shoots (Kunii 1981). Nine morphological traits were measured of each plant by a ruler (see for example E. nuttallii: Figure 2): • trait 1: main shoot length • 2: lateral shoot length (initial +secondary shoots) • 3: number of lateral shoots (initial shoot) • 4: length of total shoot (main + lateral) • 5: bulk. It corresponded to the ratio between dried biomass of the plant and length of total shoot. • 6: length of ten internodes after the cut of three cm apex. • 7: length of a leaf located at the sixth whorl • 8: width of a leaf located at the sixth whorl • 9: surface of a leaf located at the sixth whorl. One leaf on this sixth whorl is cut, fixed on a paper with sailor tape. Surface was calculated by using logician Scion image V. 1.63. Site 2 The lake “La Plaine”, where manual methods are being used for Elodea canadensis eradication, typically have Elodea canadensis in small patches within the littoral zone. Two hand-pulling (one in July 2006, and a second one in October 2006) were tested in the litteral zone close to the beach. Biomass production: Ten plots (0.2 m2 area/plot) were randomly placed each month in site 1 (reference, E1 and E2). The biomass was studied in July, in October 2006 and in June 2007 in site 2 (reference, experimental area). The vegetation was dug out manually by species. Elodea sp. were weighed. Three replicates of E. nuttallii or E. canadensis were dried at 65° C for three days. Results were expressed in Dry Mass/m2. The experiments were realised from March 2003 to October 2003 in the three areas (Reference, E1, E2) and from March 2005 to July 2005 in the Falkensteinbach river (reference site 1, E2). The biomass was measured in July 2006, in October 2006 and in June 2007 in the Lake La Plaine in the reference and in the experimental areas. Sixth whorl leaf 3 cm apex cut Lateral shoot Ten apical internodes Main shoot Roots Figure 2. Diagram illustrating E. nuttallii and its morphological traits. 1060 Statistical analysis After the verification of the normal distribution of the values and homogeneity of variance, one way ANOVAs were used to test the management and the temporal effect. For each significant difference of P < 0.05, a HDS Tuckey’s posthoc test was performed. were present in the end of 2003. However at the beginning of 2005, E. nuttallii have again colonized the experimental area. A higher biomass was measured in the reference site 1 in 2005 than in 2003. The hand-pulling had also a significant impact on the production of biomass of E. canadensis (Figure 3b). Table 1. Water chemical composition. Mean annual values. RESULTS Chemical characteristics of the water The water of the two studied sites was neutral, with low mineral content (Table 1). The site 1 was characterized by a high nitrogen concentration (NNH4+ = 109 ± 40µg/l) and a moderate concentration of phosphate (PPO43- = 37 ± 10 µg/l), whereas the site 2 was defined by a moderate nutrient level (Table 1). Biomass production There was a significant difference in the biomass produced among one, two and three hand-pulling in site 1. No stands of E. nuttallii were found after July in site E2 in 2003 (Figure 3a). Only some individuals La Plaine pH conductivity µScm-1 -1 Faklensteinbach 7.67 ± 0.5 6.95 ± 0.12 92 ± 6.7 71 ± 6 alkalinity µeq L 520 ± 7 291 ± 29 phosphate µgPL-1 30 ± 10 37 ± 10 ammonia µgNL-1 40 ± 10 109 ± 40 0.66 ± 0.05 0.55 ± 0.10 -1 nitrate mgNL sulphate mg L-1 6.50 ± 0.26 9.37 ± 0.75 chloride mg L-1 5.73 ± 0.78 5.7 ± 0.21 1200 Biomass g/m² 1000 800 600 400 reference E1 E2 200 M ar ch 2 Ap 003 ril 20 0 M ay 3 2 Ju 003 ne 20 Ju 03 ly Au 20 0 Se gus 3 t pt em 200 be 3 r2 O ct ob 003 er M 200 ar ch 3 2 Ap 005 ril 20 0 M ay 5 2 Ju 005 ne 20 05 0 Figure 3. Evolution of the biomass of Elodea species in reference site and in hand pulling areas. 1061 Reference Experimental area Biomass g/m² 90 80 70 60 50 40 30 20 10 0 July 2006 October 2006 June 2007 Figure 3a. Biomass of E. nuttallii in the Fakensteinbach river; Main shoot length (cm) 160,00 140,00 120,00 100,00 reference 80,00 E1 60,00 E2 40,00 20,00 M ar ch 03 Ap ril 03 M ay 03 Ju ne 03 Ju ly Au 03 gu st Se 0 pt em 3 be r0 O 3 ct ob er 03 M ar ch 05 Ap ril 05 M ay 05 Ju ne 05 0,00 Figure 3b. Biomass of E. canadensis in the Lake La Plaine. Impact of management on the architecture of Elodea species and on the biodiversity No significant difference was observed in the architecture of E. nuttallii (Figure 4a) or E. canadensis (Figure 4b) between the plants collected in reference and in experimental areas. At the 1062 beginning of the study, the biodiversity was low in the river Falkensteinbach. E. nuttallii was the dominant species. The decrease of E. nuttallii stands allow the development of native aquatic plants such as Ranunculus peltatus or Callitriche platycarpa two years after the end of the management in 2007. Length of 10 internodes(cm) 12,00 10,00 8,00 reference 6,00 E1 E2 4,00 2,00 M ar ch 03 Ap ril 03 M ay 03 Ju ne 03 Ju ly 03 Au gu st Se 03 pt em be r0 O 3 ct ob er 03 M ar ch 05 Ap ril 05 M ay 05 Ju ne 05 0,00 Leaf width (cm) 0,45 0,40 0,35 0,30 reference 0,25 E1 0,20 E2 0,15 0,10 0,05 M ar ch 03 Ap ril 03 M ay 03 Ju ne 03 Ju ly 03 Au gu st Se 03 pt em be r0 O 3 ct ob er 03 M ar ch 05 Ap ril 05 M ay 05 Ju ne 05 0,00 Figure 4a : Impact on the hand-pulling on the architecture of E. nuttallii. Example of 3 traits. : trait 1: main shoot length; trait 6: length of ten internodes; trait 8: width of a leaf located of the sixth whorl. Main shoot length (cm) 70,00 60,00 50,00 40,00 30,00 20,00 10,00 0,00 Hand-pulling reference 1063 Leaf width (cm) 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 Hand-pulling reference 10 internode length (cm) 10,00 9,00 8,00 7,00 6,00 5,00 4,00 3,00 2,00 1,00 0,00 Hand-pulling reference Figure 4b. Impact on the hand-pulling on the architecture of E. canadensis. trait 1: main shoot length; trait 6: length of ten internodes; trait 8: width of a leaf located of the sixth whorl. DISCUSSION Impact of hand-pulling on the E. nuttallii and on E. canadensis No significant difference was established for morphological traits of E. nuttallii /E. canadensis between plants collected in the experimental and in the reference areas. Hand-pulling have no impact on the architecture of Elodea species. Our results didn’t corroborate the conclusions of Abernethy et al. (1996) that showed a 44% reduction for length response after two cuts. Elodea species appeared as a disturbance-tolerant species. The responses of 1064 Elodea species to the stress (hand-pulling) was eliminated very quickly, less than one month. The plants grow fast and reach the water surface only few weeks after hand-pulling. In our study, the maximum of biomass of E. nuttallii was obtained in June 2005. The three treatments induced a drastic reduction of E. nuttallii in the stream, less than 2% in the end of the study. The biomass response of E. canadensis was more marked in our area (reduction of 63% after one treatment) than in the study of Abernethy et al. (1996), which measure biomass reductions of 41 % after one cut. Management options After several hand-pulling the spread of E. nuttallii and of E. canadensis is area restricted. This method is efficient. However, due to expense and the time intensive nature of manual methods, sites suitable for hand pulling are limited to lakes or ponds only lightly infested with invasive species. This method may also be applicable in waterbodies where no herbicide use can be tolerated such as in a lake used as a municipal drinking water supply and running waters. E. canadensis appeared to be less susceptible to cutting-based weed control measures than Myriophyllum spicatum, a native species (Abernethy et al. 1996). Sabbatini and Murphy (1996) using a multivariate approach, showed that E. canadensis has a strong tolerance of management based on disturbance, such as cutting. Cutting, especially mechanical control, could induce a reduction of the biomass of indigenous plants and allow Elodea to spread to new areas because this management break up the plant. Stem fragmentation is the main dispersal mode for Elodea sp. Both Elodea species showed similar resistance to currents, while fragment regeneration and colonisation were only slightly higher in E. nuttallii than in E. canadensis (BarratSegretain et al. 2002). Management such as handpulling or harvests favor the downstream plant propagation. However harvests can be quite useful in areas where the weed is already established, or when the species will disperse into areas unfavourable to its survival (Bowmer et al. 1995). The growth of E. canadensis is affected by low light intensity, contrary to E. nuttallii (BarratSegretain 2004). E. nuttallii and E. canadensis have wide amplitude in nutrient levels (Robach et al. 1995, Rolland et al. 1999, Thiébaut and Muller 2003, Thiébaut 2005). Changes in management practices especially those that have the potential to directly impact the river will be important to help manage exotic macrophytes. This includes practices such as maintaining and enhancing the condition of riparian vegetation and, stormwater and nutrient management. Reduction of nutrients entering the river is important in the fight against exotic macrophytes species, as these species grow best in high nutrient waters. The biological control of aquatic macrophytes has received considerable interest all over the world. Although Elodea species are often preferred food for waterfowl or crayfish (Lodge 1991), they are avoided by many insect herbivores (Newman 1991). However, a slightly higher palatability was established for E. nuttallii than for E. canadensis by Limnea stagnalis (Barrat-Segretain et al. 2002). E. nuttallii is avoided by herbivorous larvae of Acentria ephemerella (Erhard et al. 2007). No herbivore damage of apical meristems was observed for E. canadensis because Acentria ephemerella larvae also avoid feeding on this species removed leaves below the apical tips (Gross et al. 2001). In the lake La Plaine, the spread of E. canadensis may be also explained by the abundance of this aquatic lepidopteran in dense mats of Elodea. An other solution was to do nothing and to “wait and see”. A noticeable decline of E. canadensis in European freshwaters (Simpson 1990) and of E. nuttallii indeed was also reported after the peak of the outburst in Japan’s lake (Nagasaka et al. 2002). E. nuttallii populations exhibited a genetic uniformity that made them vulnerable to attack by fungi or pathogens. Further work is needed to improve our knowledge to estimate the ecological risk of management on biodiversity and on ecosystem function. The risk of adverse side-effects for users of the water and for the ecosystem health must always be taken into account. Stronger enforcement of existing laws, coupled with an intensive public education campaign, is needed to prevent further non indigenous species introduction. AKNOWLEDGEMENTS The assistance of Muriel Julita, Celine Trochain, P. Rousselle and of the fishermen was greatly appreciated. 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