Aquatic invertebrate herbivores in association with Myriophyllum spicatum in Otsego Lake, summer 1999

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1 80 Aquatic invertebrate herbivores in association with Myriophyllum spicatum in Otsego Lake, summer 1999 Robert L. Johnson, Jason A. Toner, Alison C. C. Colbyl NTRODUCTON Two invertebrate herbivores, a small aquatic moth Acentria ephemerella and an aquatic weevil uhrychiopsis lecontei occur in Otsego Lake, New York. Present in water bodies throughout New York State, these herbivores appear to influence submersed aquatic macrophyte diversity and abundance. Acentia ephemerella and. lecontei feed on the invasive aquatic macrophyte Myriophyllum spicatum L. (urasian watermilfoil), and have the potential to act as possible biological control agents ofm. spicatum (Sheldon and Creed, 1995; Johnson et al., 1998; Johnson et a., in press). Another herbivore, the midge Cricotopus myriophylli, associated with M spicatum (Oliver, 1984; Macrae et al., 1990) is present in Otsego Lake (Fagnani and Harman, 1987). First documented in Otsego Lake in 1986 (Dayton and Swift, 1987), M spicatum is an introduced species that often dominates an aquatic plant community and can cause lake management problems. However, previous macrophyte surveys in Otsego Lake indicate a diverse plant community prior to and following the invasion ofm spicatum (Muenscher, 1936; Harman and Doane, 1970; Brady and Lamb, 1977; Harman et al., 1980; Dayton and Swift, 1987; Harman, 1994). Harman et al. (1997) reports 22 species of aquatic macrophytes identified in 1969 and 1976, and 23 species identified in Although at this time M spicatum is a dominant species in the aquatic plant community, the consensus ofmost lake users is that this plant is not at problematic levels in Otsego Lake (Harman, 1998). One possible reason for this agreement is the failure ofm spicatum to form a dense canopy at the surface of Otsego Lake as it does in many other New York lakes. Creed and Sheldon (1994) and Johnson et al. (1998) suggest that the feeding on the apical meristem ofm spicatum by invertebrate herbivores limit elongation and canopy formation by this plant. Merrifield and Harman (1988) collected M spicatum from Otsego Lake in After two weeks in an incubator it was observed that many apical meristems were missing and that A. nivea (= ephemerella) was present. Miner (1997) recorded the presence ofa. ephemerella and other possible herbivores in Otsego Lake from her survey ofm spicatum for meroplankton. Therefore, in 1998 and 1999 we surveyed apical meristems ofm spicatum to evaluate the spatial and temporal distribution, along with the abundance ofa. ephemerella and. lecontei in Otsego Lake. n addition, we sampled submersed aquatic macrophyte biomass in 1998 to quantify plant spatial diversity and species abundance to help investigate the association between the plant community and herbivore density. 1 Department of cology and volutionary Biology, 1999 BFS visiting researcher. Cornell University.

2 81 MTHODS Herbivore Surveys We sampled four sites chosen by Miner (1997), including Rat Cove (A), South Marina (B), NW Comer (C), and Hyde Bay (D) in Otsego Lake, by boat for herbivores on 2 July and 11 August 98, and 11 August 99. The herbivore sampling at Rat Cove occurred at three different locations (quadrants 1, 2, 3) with quadrants 1 and 2 deeper and further from shore than quadrant 3 (Map 1). We sampled within quadrant 2 on 2 July 98, quadrant 3 on 11 August 98 and 11 August 99, and quadrant 1 on 11 August 99. At each site, we haphazardly collected M spicatum stems with a grapple hook, and randomly selected 25 apical meristems (growing tips) from those stems. ach apical meristem was 25 cm in length, and placed in an individual plastic-zipper bag. We transported the bags containing the apical meristems in a cooler chest back to our laboratory, and stored samples in a refrigerator pending examination under a stereoscopic microscope. We recorded the presence of all herbivores on each 25 cm apical meristem, and from these herbivore counts we estimated the number of individuals per apical meristem. We report the number of. lecontei using all life stages (eggs, larvae, pupae, and adults), and the number ofa. ephemerella using larvae,. pupae, and adults. n addition, we recorded any signs of herbivores (e.g. pupa chambers, eggs, and shelters) and M spicatum tissue damage. Biomass On 11 August 98, we sampled submersed aquatic plant biomass in Rat Cove (quadrant 3), South Marina, NW Comer, and Hyde Bay. To ensure random sampling we tossed a 0.25 m 2 quadrat frame twenty times from a boat at each site. A diver using an air buoy underwater breathing apparatus hand-harvested all above sediment plant biomass from each tossed quadrat frame, and placed each harvested sample in a separate plastic bag. n the laboratory, we washed each sample to remove any non-plant material, and separated each sample to individual plant species. Determination of dry weight for these separated plant species occurred after oven drying at 105 C for 48 hours. We placed plant biomass samples not analyzed immediately in a freezer for later separation and dry weight detennination. We calculated from these individual dry weights the mean biomass for each plant species at each site. We report aquatic plant biomass as grams of dry weight per square meter, and as percentages of the total harvested biomass. RSULTS AND DSCUSSON We found the aquatic weevil uhrychiopsis lecontei and the aquatic moth Acentria ephemerella in the 1998 and 1999 samples ofm. spicatum apical meristems. n July 1998, we observed both herbivores present at low densities in the NW Comer and Hyde Bay, and absent

3 82 Site A: Rat Cove Site B: South Marina Map 1. Sample sites used for collection ofmilfoil stems, Otsego Lake, New York.

4 83 in the South Marina (Figure 1). By August 1998, we found A. ephemerella densities low or absent in all four sites and. lecontei densities considerably higher in the NW Comer (Figure 2). n our July 1998 sampling ofquadrant 2 at Rat Cove, we failed to find. lecontei and we detected a high density ofa. ephemerella (Figure 1). However, in August 1998, we observed the opposite trend in the shallow Rat Cove site (quadrant 3). n the shallow quadrant 3,our analysis shows A. ephemerella absent, but. lecontei present at a high density (Figure 2). n August 1999, we sampled two Rat Cove sites to compare a deep and further from shore site (quadrant 1) to the near shore quadrant 3 (Figure 3). n 1998 and 1999, A. ephemerella were the dominant herbivore in the deep Rat Cove quadrants 1 and 2, while. lecontei dominated the shallow quadrant 3 (Figure 1,2,3). This observation suggests distance from the shoreline or water depth may be important to the abundance and life histories ofthese herbivores. n August 1999, we found significantly higher densities ofa. ephemerella than. lecontei at all sites except the shallow Rat Cove site (quadrant 3). Hyde Bay sustained the highest density of 0.68 A. ephemerella per apical meristem (Figure 3). We associate this high density ofa. ephemerella with the limitation ofm spicatum growth. xperiments in the laboratory and field, and the M spicatum declines reported in Cayuga Lake, New York and Kawartha Lakes, Canada (Van Dusen et al., 1998; Johnson et a., 1998; Painter and McCabe, 1988) report this approximate density associated with limited M spicatum growth. We identified thirteen species of aquatic plants in the 1998 Otsego Lake biomass survey (Table 1). We found considerable variation in total plant biomass across the four sites, ranging from 61.0 at Hyde Bay to g/m 2 at the NW Comer site (Figure 5). M. spicatum comprises 48,3, 15, and 50 % of the total plant biomass at Rat Cove, South Marina, NW Comer, and Hyde Bay, respectively (Appendix). The dry biomass of M spicatum ranged from 2.9 in South Marina to 55.7 g/m 2 in Rat Cove (Figure 6), and is the dominant plant species in Rat Cove and Hyde Bay. n contrast, we found lodea and Chara the dominant plant species at the South Marina and NW Comer sites (Figure 7, Appendix). The density ofm. spicatum stems correspond with the biomass reported at all four sites (Figure 6, 8). Our comparison of herbivore densities between August 1998 and 1999 found distinct variation, with A. ephemerella increasing from 0.06 to 0.37 and. lecontei decreasing from 0.29 to However, these densities represent only one point in time, and this variation is most likely influenced by several factors. One possible reason for this difference is that we sampled two years with differing climatic conditions that could influence the life histories of these herbivores. Another consideration for these density differences is the location of the sampling sites within the lake. The specific locations we sampled in 1998 in Rat Cove and in the NW comer may directly bias our findings. Although we only have three sampling dates over two years it appears that there is a difference in herbivore species density, possibly related to the distance from shore. Our perception is that the Rat Cove and NW comer sites that we sampled in on August 1998 were closer to shore, because the diver sampling plant biomass could easily stand in the chest-high water depth in those areas. The. lecontei densities reported for these two locations on 11 August 98 are high (Table 3). n August 1999, our

5 84 reported herbivore densities show a high. lecontei count only in the shallow near-shore Rat Cove site (quadrant 3). At the same time, we record A. ephemerella abundant at all sites including the most distant from shore Rat Cove site (quadrant 1) and the NW comer site, which we believe is further from shore than the NW August 1998 sampling site. n addition, we observed a similarly in the relationship of Otsego's surface area and mean depth, its densities ofa. ephemerella and. lecontei, and its abundance of M spicatum to the relationships ofthe same parameters from other sampled lakes. These three parameters fall in line with the same parameters reported from a survey conducted in 1997 on five New York Lakes (Johnson et al. in press). The 1997 five-lake survey reveals A. ephemerella densities correlating positively with lake surface area and mean depth, while. lecontei densities correlate negatively.. lecontei densities correlate positively with percent composition and dry mass of M spicatum, while A. ephemerella correlates negatively. These results suggest an association between M spicatum abundance, lake size, and these two herbivore species found on M spicatum. The invertebrate herbivores in Otsego Lake cause damage to M spicatum by feeding on the plant and more than likely limit its elongation. However, it is unclear if this herbivory is the sole reason for lack of excessive growth which keeps it from forming surface mats which also might explain why this plant is not perceived as an esthetic problem in the lake.

6 85 Otsego Lake: July 98 Herbivore Densities (])... A. A. ephemerella (J) 0.3 'C. lecontei (]) :2: ro 0.2 o 0 «0.1 -(J) (]) ~ _iJii-_ _... ~ 1i (]) Rat Cove South Marina NN Corner Hyde Cove (quadrant 2) Sample Site Figure 1. Density ofm spicatum herbivores in July 1998, measured as the mean number of individuals (±1 S) per apical meristem (n = 25), collected from four sites in Otsego Lake, New York.. lecontei numbers include eggs, larvae, pupae, and adults. A. ephemerella numbers include larvae, pupae, and adults. (])... (J) 1.0 'C (]) :2: co.q 0 «Otsego Lake: August 98 Herbivore Densities (J) (]) 0.25 ~. 2.0 ~ 0.0 (]) A. A. ephemerella 'r. lecontei u i Rat Cove South Marina NN Corner Hyde Cove (quadrant 3) Sample Site Figure 2. Density ofm. spicatum herbivores in August 1998, measured as the mean number of individuals (±1 S) per apical meristem (n = 25), collected from four sites in Otsego Lake, New York.. lecontei numbers include eggs, larvae, pupae, and adults. A. ephemerella numbers include larvae, pupae, and adults.

7 86 (l)... CJ) 'C (l) ~ CO o Q. «-CJ) (l) L..~.n L. (l) Otsego Lake: August 99 Herbivore Densities A. ephemerella. lecontei Rat Cove twv Corner Rat Cove (quadrant 3) South Marina Hyde Cove (quadrant 1) Sample Site Figure 3. Density ofm spicatum herbivores in August 1999, measured as the mean number of individuals (±1 S) per apical meristem (n = 25), collected from five sites in Otsego Lake, New York.. lecontei numbers include eggs, larvae, pupae, and adults. A. ephemerella numbers include larvae, pupae, and adults. ~.~ _ Annual A. ephemerella Density Variation (l)... -'" CJ) 'C 0.8, (l) ~ CO Q. « ~. Rate Cove South -Marina twv-corner Hyde Cove (quadrant 3) Sample Site Figure 4. Density ofa. ephemerella in August 1998 and 1999, measured as the mean number of individuals (±1 S) per apical meristem (n = 25), collected from four sites in Otsego Lake, New York. Numbers include larvae, pupae, and adults.

8 Otsego Lake Total Plant Biomass rj) U) co 100 o co... co 50 o l e:: co <D o ~ Rat Cove South Marina NN Corner Hyde Cove (quadrant 3) Sample Site Figure 5. Mean total plant biomass, measured as dry weight g/m 2, for 1998 in Otsego Lake, New York. Biomass is the mean oftwenty, O.25m 2 samples for each ofthe four sites. rror bars indicate one standard error Otsego Lake Biomass: M. spicatum N ) U) 40 U) co e:: co 10 <D ~ o Rat Cove South Marina NN Corner Hyde Cove (quadrant 3) Sample Site Figure 6. Abundance ofm. spicatum, measured as dry weight g/m 2, for 1998 in Otsego Lake, New York. Biomass is the mean oftwenty O.25m 2 samples for each ofthe four sites. rror bars indicate one standard error.

9 Otsego Lake Biomass:. canadensis --. N 60 0> 50 '-'" (J) (J) 40 co 30.0 (0 20 c co 10 a> ~ o Rat Cove South Marina twv Corner (quadrant 3) Sample Site Hyde Cove Figure 7. Abundance of. canadensis, measured as dry weight g/m 2, for 1998 in Otsego Lake, New York. Biomass is the mean of twenty O.25m 2 samples for each of the four sites. rror bars indicate one standard error. --. N (J) a>... (J) '-'" L.. 70 a>.0 :::J z c co 20 a> ~ M. spicatum Stem Density. ::: Rat Cove South Marina twv Corner Hyde Cove (quadrant 3) Sample Site Figure 8. Mean number (±1S) ofm spicatum stems per m 2 in 1998 from four Otsego Lake, New York sites. Note that quadrats analyzed for stem numbers were n=10 for Rat Cove, and n=20 for South Marina, NW Comer, and Hyde Cove. Table one. Submersed aquatic plants identified by 1998 biomass survey of Otsego Lake, NY.

10 89 Table 1. Submersed aquatic plants identified by 1998 biomass survey of Otsego Lake, NY. Scientific name lvfegalodonta beckii Ceratophyllum demersum Chara vulgaris lodea canadensis Heteranthera dubia l\1yriophyllum spicatum Nitella spp. Potamogeton amplifolius Potamogeton crispus Potamogeton pectinatus Potamogeton pusillus Potamogeton zosteriformis Vallisneria americana Common name Water Marigold Coontail Muskgrass lodea, Canadian waterweed Water stargrass urasian watennilfoil Stonewort Large-eafPondweed Curlyleaf pondweed Sago pondweed Slender pondweed Flat-stemmed pondweed elgrass, Wild celery Table 2. Summary of July 2, 1998 urasian watennilfoil apical meristem herbivore survey in Otsego Lake, New York. Location: A B C D TOTAL # of apical meristems: A. ephemerella Larvae 4 a A. ephemerella Pupae 2 a a a 2 A. ephemerella Adults a a a a a A. ephemerella ggs a a a a a Total 6 a AcentriaJapical meristem 0.24 a mean=0.15. lecontei Larvae a a a a a. lecontei Pupae a a a a a. lecontei Adults a a a 1 1. lecontei ggs. a a Total a a uhrychiopsislapical meristem a a mean=0.07 Locations: A - Rat Cove (quadrant 2 B - South Marina C-NWComer D - Hyde Cove

11 90 Table 3. Summary ofaugust 11, 1998 urasian watennilfoil apical meristem herbivore survey in Otsego Lake, New Yark. Location: A B C D TOTAL # of apical meristems: A. ephemerella Larvae A. ephemerella Pupae A. ephemerella Adults A. ephemerella ggs Total Acentria/apical meristem mean=0.06. lecontei Larvae lecontei Pupae lecontei Adults lecontei ggs Total uhrychiopsis/apical meristem mean=0.29 Locations: A - Rat Cove (quadrant 3) B, C, D - Same as Table 1 Table 4. Summary ofaugust 11, 1999 urasian watennilfoil apical meristem herbivore survey in Otsego Lake, New York. Location: A B C D TOTAL # ofapical meristems: ~ A. ephemerella Larvae A. ephemerella Pupae A. ephemerella Adults ( A. ephemerella ggs Total f Acentria/apical meristem mean=0.37. lecontei Larvae l. lecontei Pupae lecontei Adults lecontei ggs Total uhrychiopsis/apical meristem mean=0.06 Locations: A - Rat Cove (quadrant 3) B - South Marina C - NWComer D - Hyde Cove - Rat Cove (quadrant 1) ~ "

12 91 RFRNCS Brady, P., and S. Lamb Changes in the aquatic flora of Otsego Lake between 1935 and n 9 th Ann. Rept., pp SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Creed, R.P., Jr. and S.P. Sheldon The effect oftwo herbivorous insect larvae on urasian watermilfoi. 1 Aquat. Plant Manage. 32: Dayton, B.R., and D.R. Swift Composition of aquatic macrophyte communities in Otsego Lake: a preliminary study. n 19 th Ann. Rept., pp SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Fagnani, lp., and W.N. Harman The Chironomidae of Otsego Lake with keys to the immature stages ofthe subfamilies Tanypodinae and Diamesinae (Diptera). Occas. Pap. No. 20. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Harman, W.N., L.P. Sohacki, M.F. Albright, and D.L. Rosen The state ofotsego Lake Occas. Pap. No. 30. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Harman, W.N A comparison ofthe Otsego Lake macrobenthos communities between 1935 and n 26 th Ann. Rept., pp SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Harman, W.N., and T.R. Doane Changes in the aquatic flora of Otsego Lake, New York, between 1935 and N.Y. Fish Game 1 17: Johnson, R.L., PJ. Van Dusen, J.A. Toner, and N.G. Hairston, Jr. n press. urasian watermilfoil biomass associated with insect herbivores in New York. 1 Aquat. Plant Manage. Johnson, R.L,.M. Gross, and N.G. Hairston, Jr Decline ofthe invasive submersed macrophyte Myriophyllum spicatum (Haloragaceae) associated with herbivory by larvae ofacentria ephemerella (Lepidoptera). Aquat. col. 31: MacRae, LV., N.N. Winchester, and R.A. Ring Feeding activity and host preference of the milfoil midge Cricotopus myriophylli Oliver (Diptera: Chronomidae). J. Aquat. Plant Manage. 28: Merrifield, M.l and W.N. Harman The taxonomy, distribution and abundance of Myriophyllum spicatum (Myrtiflorae: Haloragacea) in Otsego Lake, NY. pp ll.. 20 th Ann. Rept. (1987). SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Miner, M An inventory ofmeroplankton associated with Myriophyllum spicatum, focusing on Acentria ephemerella, in Otsego Lake, summer n 30 th Ann. Rept pp SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta.

13 92 Muenscher, W.e Aquatic vegetation of the Susquehanna and Delaware areas. n A biological survey of the Delaware and Susquehanna watersheds. pp N.Y. State Dep. nviron. Conserv., Albany. Oliver, D.R Description of a new species of Cricotopus van del' Wulp (Diptera: Chironomidae) associated with Myriophyllum spicatum. Can. nt. 116: Painter, D.S. and KJ. McCabe nvestigation into the disappearance of urasian watermilfoil from the Kawartha Lakes, Canada. 1. Aquat. Plant Manage. 26: Sheldon, S.P. and R.P. Creed, Jr Use ofa native insect as a biological for an introduced weed. col. Appl. 5: Appendix 1A. August 11, 1998 biomass data collected in Rat Cove (quadrant 3). ach dry weight (g) represents the species composition in one 0.25m 2 quadrat frame. We collected biomass from twenty quadrat frames. Site Species DW98-1 DW98-2 DW98-3 DW98-4 DW98-5 DW98-6 DW98-7 DW98-8 A Chara vulgaris A lodea canadensis A Heteranthera dubia A Myriophyllum spicatum A Potomogeton amplifolius A Potomogeton pectinatus A Potomogeton zosteriformis A Vallisneria americana Total DW98-9 DW98-10 DW98-11 DW98-12 DW98-13 DW98-14 DW98-15 DW98- A Chara vulgaris A lodeacanadensis A Heteranthera dubia A Myriophyllum spicatum A Potomogeton amplifolius A Potomogeton pectinatus A Potomogeton zosteriformis A Vallisneria americana Total DW98-17 DW98-18 DW98-19 DW98-20 tot98dw %98DW g/.25m2 g DW/m2 A Chara vulgaris A lodeacanadensis

14 93 A Heteranthera dubia A Myriophyllum spicatum A Potomogeton amplifolius A Potomogeton pectinatus A Potomogeton zosteriforrnis A Vallisneria americana Total Appendix 1B. August 11, 1998 biomass data collected in the South Marina. ach dry weight (g) represents the species composition in one O.25m 2 quadrat frame. We collected biomass from twenty quadrat frames. Site Species DW98-1 DW98-2 DW98-3 DW98-4 DW98-5 DW98-6 DW98-7 DW98-8 B Ceratophyllum demersum B Chara vulgaris B lodea canadensis B Heteranthera dubia B Myriophyllum spicatum B Nitella spp B Potomogeton pectinatus B Potomogeton pusillus B Potomogeton zosteriforrnis B Vallisneria americana Total DW98-9 DW98-1O DW98-11 DW98-12 DW98-13 DW98-14 DW98-15 DW98-16 B Ceratophyllum demersum B Chara vulgaris B lodea canadensis B Heteranthera dubia B Myriophyllum spicatum o B Nitella spp B Potomogeton pectinatus o B Potomogeton pusillus O.o B Potomogeton zosteriforrnis B Vallisneria americana Total DW98-17 DW98-18 DW98-19 DW98-20 tot98dw %98DW g/.25m2 g DW/m2 B Ceratophyllum demersum o

15 94 B Chara vulgaris B lodea canadensis B Heteranthera dubia B Myriophyllum spicatum B Nitella spp B Potomogeton pectinatus B Potomogeton pusillus B Potomogeton zosteriformis B Vallisneria americana O.OlO Total / Appendix 1C. August 11, 1998 biomass data collected in the NW Comer. ach dry weight (g) represents the species composition in one a.25m 2 quadrat frame. We collected biomass from twenty quadrat frames. Site Species DW98-1 DW98-2 DW98-3 DW98-4 DW98-5 DW98-6 DW98-7 DW98-8 C Bidens beckii o C Ceratophyllum demersum o C Chara vulgaris C lodea canadensis C Heteranthera dubia o C Myriophyllum spicatum C Nitella spp. o C Potomogeton crispus o C Potomogeton pectinatus o C Potomogeton pusillus o C Potomogeton zosteriformis o lO45 C Valisneria americana o DW98-9 DW98-lO DW98-11 DW98-12 DW98-13 DW98-14 DW98-15 DW98-16 C Bidens beckii o C Ceratophyllum demersum o C Chara vulgaris C lodea canadensis C Heteranthera dubia C Myriophyllum spicatum C Nitella spp. o C Potomogeton crispus o 0 0 a C Potomogeton pectinatus o C Potomogeton pusillus C Potomogeton zosteriformis o C Valisneria americana o Total DW98-17 DW98-18 DW98-19 DW98-20 tot98dw %98DW g/.25m2 g DW/m2 C Bidens beckii

16 95 C Ceratophyllum demersum o o C Chara vulgaris C lodea canadensis C Heteranthera dubia o o C Myriophyllum spicatum C Nitella spp C Potomogeton crispus 0.05 o o o C Potomogeton pectinatus o C Potomogeton pusillus o C Potomogeton zosteriformis o C Valisneria americana o o Total Appendix 1D. August 11, 1998 biomass data collected in Hyde Cove. ach dry weight (g) represents the species composition in one a.25m 2 quadrat frame. We collected biomass from twenty quadrat frames. Site Species DW98-1 DW98-2 DW98-3 DW98-4 DW98-5 DW98-6 DW98-7 DW98-8 D Ceratophyllum demersum o D lodea canadensis o D Heteranthera dubia o o o o o o o o D Myriophyllum spicatum D Potomogeton pectinatus o o o o o o o o D Potomogeton zosteriformis o o o o o o o o Total DW98-9 DW98-1O DW98-11 DW98-12 DW98-13 DW98-14 DW98-15 DW98-16 D Ceratophyllum demersum D lodea canadensis D Heteranthera dubia o o o D Myriophyllum spicatum D Potomogeton pectinatus o o D Potomogeton zosteriformis o o o o Total DW98-17 DW98-18 DW98-19 DW98-20 tot98dw %98DW g/.25m2 gdw/m2 D Ceratophyllum demersum D lodea canadensis D Heteranthera dubia D Myriophyllum spicatum D Potomogeton pectinatus D Potomogeton zosteriformis Total