Submerged Macrophyte Communities Research Articles

Ecological regime shifts in salinised wetland systems. II. Factors affecting the dominance of benthic microbial communities

This paper is the second in a pair investigating potential mechanisms for ecological regime change in salinising wetlands. The first paper in this series focused on the responses of the salt-tolerant submerged macrophyte community to salinity. In this second paper, we investigated some of the environmental conditions required for initiation and dominance of benthic microbial communities using a combination of experimental and observational data. Two experiments were carried out. One investigated the importance of prior establishment of benthic microbial communities on their ability to maintain prevalence over macrophyte colonisation (‘persistence’ experiment), while the other investigated hydrology and its effect on sediment perturbation, potential nutrient release and subsequent benthic microbial community establishment (‘flooding’ experiment). The ‘persistence’ experiment measured the biomass of benthic microbial communities and emergence of macrophytes from sediments kept either wet or dry for 4 weeks then flooded at a range of salinities. Benthic microbial biomass was similar across all of the salinities tested (15, 45 and 70 ppt), with a slight increase at higher salinities, suggesting that none of these limited benthic microbial community development. Pre-wetting of sediments usually increased benthic microbial community biomass and reduced macrophyte germination, but the latter was attributed to the presence of anoxic sediments rather than the increased benthic microbial community biomass. Germinating macrophytes emerged through benthic microbial communities or dense heterotrophic bacterial blooms, demonstrating that they could become dominant even when another community was already established. Field data supported these results, suggesting that the development of benthic microbial communities is not limited by salinity alone, but includes other factors, such as the water regime. In the ‘flooding’ experiment, the largest differences in nutrient concentrations ultimately lay between the pre-wet and pre-dry treatments (due to the greater release of nutrients and development of anoxia in the latter) rather than those subjected to fast versus slow flooding. In response to this, highest benthic microbial community biomass was in treatments with pre-wet sediment, corresponding with lower phytoplankton biomass.

Expansion of Myriophyllum spicatum (Eurasian water milfoil) into Lake Nasser, Egypt: Invasive capacity and habitat stability

The invasion of Myriophyllum spicatum into Lake Nasser, and its impact on submerged macrophyte communities are quantitatively documented. Samples of macrophytes, water and hydrosoil were collected from 17 sites, in October and November 2002. The average dry weight standing crop of each species per grapnel haul was determined at each depth zone (sampling site). Twenty-one environmental variables were measured (12 water and 9 hydrosoil variables). Canonical correspondence analysis (CCA) was used to determine species–environment relationships. Comparing these relationships of the present study with those detected in 1988–1990 indicated significant changes in water and hydrosoil characteristics. These changes are also implicated in the submerged macrophyte communities. M. spicatum has replaced the originally dominant submerged macrophyte Najas marina subsp. armata. The study indicated that the invasion of M. spicatum depends not only on its attributes, but also on the physico-chemical characteristics of Lake Nasser.

CONTROLS OF ALGAL ABUNDANCE AND COMMUNITY COMPOSITION DURING ECOSYSTEM STATE CHANGE

Shallow lake ecosystems can shift between clear-water, macrophyte-rich conditions and turbid states with abundant phytoplankton. However, little is known about the controls of algal community composition and primary production before, during, and after ecosystem state change, because long time series that monitor biological change through the transition are scarce. Using proxy data sets derived from sediment cores from two shallow hypertrophic lakes in Denmark, variance-partitioning analysis (VPA) was used to determine the relative importance of changes in total phosphorus (diatom inferred), planktivorous fish density (zooplankton inferred), and submerged macrophyte communities (as macrofossil abundance) as determinants of algal abundance and community composition (as sedimentary pigments) over ecosystem state transitions since 1750 (CE) for Lake Lading and 1900 for Lake Søbygaard. Past variation in densities of planktivorous fish explained 12.3% and 18.2% of historical algal community change in lakes Lading and Søbygaard, respectively, while a further 22.3% and 6% of algal variability was explained by variation in macrophyte abundance. Total phosphorus (TP) alone explained nonsignificant amounts of variance (1.5%, 3.6%) but had a significant effect in combination with macrophytes and fish (27%, 13.4%). State transitions occurred ca. 1940 but were preceded by increases in benthic diatoms and macrophytes, suggesting that transitions were gradual rather than instantaneous. In contrast, green, colonial blue-green, and cryptophyte algae were abundant only during turbid states after ca. 1960 and were correlated to changes in planktivorous fish or fish–TP interactions. Contrary to expectations, the shift from predominantly benthic to pelagic algal production represented only a change in habitat and did not result in an increase in total abundance of primary producers.

Seasonal changes of submerged macrophyte community with different depth profile in a calm bay of Lake Biwa, Japan

The structure of a submerged macrophyte community (vegetation cover, species richness and species composition) and the seasonal changes were investigated at the littoral zone (< 6 m deep) of a calm bay in Northern Basin of Lake Biwa, Japan, where the transparency is high and no waterfowl grazers exist. To evaluate the effect of a small difference in depth profile on the community structure, the vegetation map of 35 m-belt transect (4 m wide) was made monthly at two sites (30 m apart) between March and November 1997. In total, the same 11 species were observed in the two sites, including two alien species, Egeria densa and Elodea nuttallii, and an endemic species, Vallisneria biwaensis. There was no significant difference in species richness in a census between the two sites. The species zonation was unclear in both sites: various mixed species patches were found throughout the study period. There were differences in the growth patterns of five species between the two sites. In a calm and clear-water littoral zone, where there is no disturbance by wave action, a small difference in depth profile can affect the growth patterns of several macrophyte species and then affect the three-dimensional structure of the macrophyte community, which can be a crucial determinant of the habitat structure of small aquatic animals.

Interspecies relationships, community structure, and factors influencing abundance of submerged macrophytes in prairie wetlands

Submerged macrophytes are important components of wetland ecosystems in the Prairie Pothole Region of North America, but factors influencing species abundance and community structure are poorly known. We sampled submerged macrophyte communities and habitat characteristics in 18 prairie wetlands for five years in order to 1) assess interspecific relationships between macrophyte species, 2) test for presence of distinct species assemblages at the whole-basin level, and 3) identify relationships between community composition and limnological characteristics of the wetland basins. Macrophyte communities did not occur as distinct assemblages of species; most species showed eurytopic distributions, and communities varied along gradients of individual species abundances. Redundancy analysis showed that turbidity, presence/ absence of fish, drainage history (restored versus non-drained), average depth, and surface area explained 20% of the variance in species abundance. However, turbidity and drainage history alone accounted for 15% of this variance. Most species showed a negative relationship with turbidity, while drainage history influenced community composition but had little effect on overall abundance at the community level. Our results indicate that restored and non-drained wetlands have similar species richness but differ in species composition. Species richness and total plant abundance in both restored and non-drained wetlands will be greatest in wetlands with low turbidity, moderate depth, and small surface area. However, substantial residual variation indicates that additional, unstudied factors are important determinants of community structure.

Present status of submerged macrophyte growth in Lake Biwa: Recent recovery following a summer decline in the water level

AbstractWhole‐lake areal distribution of submerged macrophyte growth in Lake Biwa was surveyed using aerial photographs in 1994 and 2000, when unusual declines in the water level of approximately 1 m occurred in the dry summers. The estimated total areas of the submerged macrophyte communities were 1441 ha for 1994 and 2825 ha for 2000. In the shallow and eutrophic southern basin, the bottom area covered by vegetation gradually increased year by year after 1994, and the ratio of this area to the total surface area of the basin in 2000 amounted to 52%. The dominant species were two natives,Hydrilla verticillataandMyriophyllum spicatum, and one exotic,Egeria densa. Changes in physical conditions and water quality associated with the decline in the water level, as well as the reactions of submerged plants to aquatic environments and ecosystem dynamics, are discussed.

Benthic macrophyte communities in the Ghar El Melh lagoon (North Tunisia)

This work deals with the biodiversity and distribu- tion of benthic macrophytes in the Ghar El Melh lagoon, a Mediterranean coastal lagoon located in the North of Tunisia. An inventory was made of the benthic flora and submerged macrophyte communities were mapped during two successive campaigns (the summer of 1999 and the winter of 2000). The following 24 macrophyte species were identified: seven red algae, two brown algae, 11 green algae, and four marine angiosperms. The results were compared with available data from the literature. Ruppia cirrhosa is the most dominant species. It is found in all lagoon parts, except in the west sector. Ruppia beds are usually associated with Cladophora forming heterogeneous communities. During summer Ruppia cirrhosa shows a large distribution, covering an area of ca. 21.4 km2, with dense, extensive beds covering 80-100%. In winter, several Cladophora species have a very large distribution as well, covering nearly an area of 28.5 km 2 with an average cover of 46%. The green algae Caulerpa prolifera is confined to the eastern part of the lagoon which is mainly affected by seawater. In comparison with previous situations, many transforma- tions were observed in biodiversity and spatial distribution of the dominant communities. Thus, Cymodocea nodosa and Zostera beds, which dominated in the 1970s, were replaced by Zostera and Caulerpa prolifera in the 1980s and are currently succeeded by Ruppia cirrhosa and Cladophora. Restoration of the Ghar El Melh lagoon will enable an increase in the exchange with the open sea and the circulation of water, in particular in the confined zones. This should considerably improve the water quality and would positively influence the phytobenthic communities.

Problems with the application of diatom‐total phosphorus transfer functions: examples from a shallow English lake

1. A diatom‐total phosphorus transfer function has been applied to a sedimentary diatom sequence from Groby Pool, a small shallow lake in Leicestershire, U.K.2. Extensive aquatic plant records exist for Groby Pool dating back over two centuries. These records, in conjunction with selected aquatic pollen and herbarium diatom data, provide independent, qualitative evidence for the progression of eutrophication and its effects on aquatic plant communities and habitat structure.3. Before 1800, Groby Pool was probably mesotrophic with clear water and a diverse submerged macrophyte community, but subsequently it experienced considerable nutrient enrichment. Key evidence for this includes: (i) historical plant records indicating the loss of species associated with mesotrophic waters and their replacement by others typical of eutrophic conditions, (ii) a significant increase in the percentage of planktonic diatoms in the lake sediment record (particularly Cyclostephanos dubius) after 1890, and (iii) increases in percentages of Stephanodiscus parvus and Cocconeis placentula in the second half of the twentieth century.4. Diatom‐inferred total phosphorus (DI‐TP) estimates were inconsistent with the qualitative evidence for eutrophication at Groby Pool. In particular the DI‐TP profile was thought to overestimate phosphorus during the period of dominance by small Fragilaria spp. before 1890, and to misjudge the timing and direction of subsequent changes in nutrient loading.5. This study highlights some of the problems associated with the application of diatom‐TP transfer functions to sedimentary diatom sequences from shallow lakes. The major problem relates to the frequent dominance of non‐planktonic diatoms in the sediments of these systems, many species of which (particularly small Fragilaria spp.) appear to be more sensitive to changes in habitat availability than to phosphorus. Potential ways of improving diatom‐TP models via altered approaches to sampling are suggested.

Inter-annual variation in submerged macrophyte community biomass and distribution: the influence of temperature and lake morphometry

We monitored the biomass and distribution of submerged macrophyte communities in five lakes in the Eastern Townships of Quebec, Canada, in two climatically different growing seasons. Early season warm temperatures allowed for much deeper macrophyte colonization (25–170%), an average of 300 g m −2 greater wet weight biomass (g m −2) and a 45–1160% increase in whole lake biomass in warm 1998 compared to a cooler 1997. In contrast, the inter-year variation in underwater light climate had no effect on macrophyte colonization as water column turbidity was generally higher in the warmer year, although irradiance did affect the distribution and biomass within seasons. The positive relationship between phytoplankton and whole system macrophyte biomass observed in this study cautions that the negative relationship in highly eutrophic shallow systems does not appear applicable to deeper systems characterized by lower macrophyte biomass to lake volume ratios. Earlier growing seasons, as predicted by climate change models, would result in greater biomass and distribution of submerged macrophyte communities, thereby modifying the structure and functioning of north temperate lakes. The effect will be a function of lake morphometry and most pronounced in shallow systems.

Recovery of Submerged Vegetation and Its Fishery Benefit in Bao'an Lake

The recovery of submerged macrophyte community is very important to the purification of water quality and the restoration of beautiful landscape, and also to the sustentation of the lake fishery with high quality and high economic benefit because the fishery species with high marketprice, such as Chinese mitten crab Eriocheir sinensis H. Mime-Edwards and mandarin fish Sini- perca chuatsi (Basilewsky) , do not grow well without the densely submerged macrophyte com- munity. By catching the herbivorous fishes (grass carp Ctenopharyngodon idellus, Megalobrama amblycephala Yih, Cyprinidae) and the omnivorous fishes (common carp Cyprinus carpio L., crucian carp Chrassius auratus L. ) which are harmful to the submerged macrophyte growth, and transplanting the aquatic plants (Vallisneria natans, Potamogeton macckianus, Myriophyllum spicatum, Ceratophyllum demersum) on the calm days in early spring when the water depth was 0.1 - 1. 0 m, the submerged macrophyte community was recovered in Xiao-si-hai Lake (area is 133. 3 hm~2 ) near to the middle reach of Yangtze River, China in 1995. Because of the recovery of submerged macrophyte community, the traditional fishery type which applying fertilizer to culture the plankton feeder (silver carp Hypophthalmichthys molitrix and bighead Aristichthys nobilis ) with low market price was given up, and began with a new fishery by stocking the crab and the mandarin fish to gain a great benefit in Xiaosihai Lake. The biomass, cover rate of submerged macrophyte is 2730gm~(-2), 100 %, respectively, in autumn in 1997, and the crab production is 4739. 5kg in 1997.

The species composition and distribution of submerged macrophyte communities in Lake Ogawara, Aomori Prefecture.

The species composition and distribution of submerged macrophyte communities in Lake Ogawara, Aomori Prefecture.

Changes in Distribution and Abundance of Submerged Macrophytes in the Inner Bay at Long Point, Lake Erie: Implications for Foraging Waterfowl

Submerged aquatic macrophytes in Long Point Bay provide food for several species of migrating waterfowl, particularly diving ducks. The percent occurrence of several plant species in the diets of six species (207 birds) of diving ducks were studied between 1992 and 1994. Of 29 plant species consumed, Vallisneria americana was the most commonly occurring in the diet. Chara vulgaris, Potamogeton spp., Polygonum spp., and Najas flexilis/quadalupensis were also important dietary items. The seeds of a number of emergent macrophyte species were also consumed. Submerged macrophytes were sampled at 312 locations in the Inner Bay of Long Point, Lake Erie, in 1991, 1992, and 1995. Distribution and percent abundance were compared with a similar study in 1976 and with observations of macrophytes in 1962. In comparison with the 1976 study, Chara vulgaris is still dominant over much of the Bay. V. americana and N. flexilis/N. guadalupensis have expanded their ranges, probably a result of increasing light penetration associated with the filtering of suspended material by introduced zebra mussels Dreissena polymorpha. The continued widespread distribution of C. vulgaris, and the expansion of V. americana and N. flexilis/N. guadalupensis, has probably benefitted waterfowl, as these species have a high nutritive value and are readily consumed by diving ducks at Long Point. Myriophyllum spicatum, an exotic species introduced in the early 1970s, increased in distribution between 1976 and 1991 but decreased in distribution and percent abundance by 1995. Increased light transparency may have contributed to this decline, as M. spicatum typically grows best in turbid, eutrophic environments. The decline of this species has probably been beneficial to waterfowl because it commonly out-competes native plants and it is underutilized by ducks at Long Point. Ceratophyllum demersum appears to be a more prominent member of the submerged macrophyte communities in the 1990s, as it was not reported previously. In comparison with 1962, major changes have occurred in species composition and relative abundance: M. spicatum, Najas spp., and Elodea canadensis have increased, Potamogeton spp. and Nitella spp. have declined. Macrophyte distribution and abundance on the Inner Bay is influenced by environmental, biological, geomorphological and geological variables. However, the anthropogenic introduction of two exotic species, M. Spicatum and the zebra mussel, appears to have had a pronounced influence on the community structure of submerged macrophytes in Long Point Bay.

Regeneration of Submerged Macrophyte Populations in a Disturbed Lake Ontario Coastal Marsh

Previous studies in disturbed Great Lakes coastal marshes have determined that seed banks for submerged macrophytes tend to be depauperate if not absent. This was thought to be a major factor that would prevent the regeneration of macrophyte populations under improved conditions, so the transfer of seed or adult plants from healthy wetlands within the same region has been recommended as a restoration strategy. Cootes Paradise is a large, disturbed coastal marsh with a poor seed bank for submerged plants. In this report a large increase in submerged macrophyte population densities following the reduction of carp ( Cyprinus carpio) densities from approximately 700 kg/ha in 1996 to 50 kg/ha in 1997 is documented. Much of this regeneration occurred in areas devoid of aquatic vegetation in 1996. It was determined that these plants developed from vegetative structures buried in the sediment. It is recommended that detailed surveys of both seed and vegetative propagule banks be undertaken before assessing the likelihood of the recovery of submerged macrophyte communities in disturbed coastal marshes.

On the decline and restoration of submerged vegetation in eutrophic shallow lakes

对国外近年来有关富营养化浅水湖泊的退化与恢复方面的研究进展以及生物操纵作用作了述评.对有关富营养化湖泊沉水植被衰退的有代表性的3种假说也作了介绍.根据八五期间武汉东湖水生植被重建的研究工作和多年积累的湖泊水生植被调查资料,提出了长江中下游浅湖富营养化过程中沉水植技演替的模式.;Both the advance in the study of the degradation and rcsioration of eutrophic shallow lakes and the role of biomanipalation in the recovery of damaged lake ecosystems have been reviewed in the present paper. Three models have also been introduced on the decline of submerged macrophytes to interpret the succession of submerged macrophyte communities in the process of eu-trophicatoa of the shallow lakes in the middle and lower reaches of the Changjiang River. Based on the well-dcxrumented references and the researches on the decline and restoration of aquatic vegetation conducted in Donghu Lake of Wuhan during the 8th Five Year Plan period (1991-1995), the ant hors put forward a model.

Fall Food Habits of American Wigeon at Long Point, Lake Erie, Ontario

We studied food habits of an abundant migrant, the American wigeon ( Anas americana), staging during the 1991 fall migration at Long Point, a World Biosphere Reserve and a Ramsar site on the north shore of Lake Erie. Food samples from 149 wigeons were analyzed; stems and leaves of aquatic plants (predominantly submerged macrophytes) comprised over 92% aggregate dry mass of the diet, seeds made up 7.8%, and animal matter 0.6%. A diverse array of plant species was identified; however, results indicate that wigeons were showing selectivity in plant species consumed. Stems and leaves of muskgrass ( Chara spp.), elodea ( Elodea canadensis) and bushy pondweeds ( Najas flexilus and N. guadalupensis) (aggregate percent dry mass 37%, 22%, and 18% for proventriculi, and 15%, 12%, and 38% for gizzards, respectively) comprised the bulk of the diet, although neither bushy pondweed nor elodea wholly dominated submerged macrophyte communities. Several submerged macrophytes, such as coontail ( Ceratophyllum demersum) and pondweeds ( Potamogeton spp.) which are common and widespread at Long Point and reported frequently as important in the diet of American wigeons elsewhere, were not found at all or only in trace amounts. Some sex, age, and seasonal differences in diet were detected; Chara spp. and tubers were eaten more frequently by adults than by immatures, Elodea canadensis was eaten in late fall by females but not by males, immatures ate Myriophyllum spicatum seeds more frequently than did adults, and Najas spp. was consumed more in late fall than in early fall.

The effect of salinity on the reproduction of coastal submerged macrophytes in experimental communities

AbstractThe effects of salinity on the reproduction of coastal submerged macrophyte species were studied on samples of communities from six seasonal marshes in two outdoor experiments performed in autumn and in spring. The submerged macrophyte communities were submitted to five different salinity levels (0, 1, 2, 4 and 6 g/1 Cl−1). In a companion paper (Grillas, van Wijck &amp; Bonis 1993) three groups of species were distinguished on the basis of their biomass production over the salinity range 0 to 6 g/1 Cl−1: (1) glycophytes (non‐salt‐tolerant species), (2) salt‐tolerant species and (3) halo‐phytes. This part of the study describes the impact of salinity on the reproduction of the individual species during the two experiments.The species differ in their capacity to reproduce in the autumn; only Zannichelliapedunculata and Tolypella hispánica were able to produce fruits in that season. For all species reproduction was greater in spring and strongly correlated with biomass, except for Chara canescens. Differences in reproductive effort over the salinity range amplified the halophytic nature of Ruppia marítima and Chara canescens and the intolerance of Callitriche truncata and Chara contraria. For the other species, reproductive effort did not differ significantly over the salinity range.Regarding the effect of salinity on biomass and reproductive effort of individual species, there were large differences in the total weight of propagules produced at the community level and in the relative contribution of individual species. The resulting quantitative changes in the species composition of the seed bank could affect the structure of the communities by their effects on the establishment and survival of species populations.

Euhydrophyte communities of the River Nile and its impoundments in Egyptian Nubia

The River Nile and its two impoundments, the Aswan Reservoir and Lake Nasser, are important habitats for submerged and floating-leaved freshwater macrophytes (euhydrophytes) in the desert region of Egyptian Nubia. Ordination and classification analysis of survey data collected during the period 1980–1986 suggested that the Aswan High Dam forms a man-made vegetational boundary, delineating two basic macrophyte community types. In Lake Nasser a community dominated by Najas spp. is present. In the Aswan Reservoir and the R. Nile downstream of the old Aswan Dam the euhydrophyte community is dominated by a Potamogeton crispus — Ceratophyllum demersum association. Differences between the two community types appear to be related to differences in physical factors (e.g. water level fluctuation and flow regime), and water chemistry, to phenological factors, and to the differing successional ages of the macrophyte communities of the Nile system upstream and downstream of the Aswan High Dam. There is some evidence for depth zonation of the submerged macrophyte community in both lake and river habitats. Strategy analysis of the euhydrophyte communities present upstream and downstream of the High Dam, over the period 1963–89, indicated that successful established-phase strategy types were similar on both sides of the dam. In the long term there seems little to prevent euhydrophyte species at present confined to below the High Dam from crossing this boundary to colonise Lake Nasser.

Biomass density and the relationship between submerged macrophyte biomass and plant growth form

The utility of biomass density (biomass per unit volume) as a quantitative descriptor of the growth form of submerged macrophytes is evaluated and confirmed. Biomass density (BD) is a species specific characteristic which does not appear to be influenced by plant development. Secondly, BD reflects the growth form of the dominant species in the stand, because stands of species with a similar growth form also have similar BD's. Lastly, the BD of submerged macrophyte stands is closely related to the flowering pattern of the dominant species, and has important implications for light capture and light competition with neighbouring plants. Thus, BD should be a useful tool in the quantitative analysis of submerged macrophyte community structure.

Strategy analysis of submerged lake macrophyte communities: an international example

The plant strategy approach was used to assess relationships between the submerged macrophyte vegetation and environmental (natural and anthropogenic) pressures acting in four contrasting lakes: Lake Nasser (Egypt), a large man-made water body, and three North European natural or semi-natural lakes, Steinsfjord (Norway), and Loch Dee and Laird's Loch (Scotland). These lakes provide a high-variance data set over large physical, geographical and climatic gradients. Two complementary approaches were used to classify the established-phase strategy types of populations of euhydrophytes found in these water bodies. The relative contribution of each strategy element to the “community strategy” of the whole lake euhydrophyte flora was determined using a simple strategy index technique. The overall set of strategies prevailing in the submerged macrophyte community of each lake was visualised by plotting the percentage occurrence of each strategy type into the appropriate compartment of the triangular established-phase strategy model of Grime (1979). Comparisons were drawn both between lakes and, in the case of Steinsfjord, between the lake euhydrophyte communities present on five sampling occasions over a 58-year period. The strategy approach provided insight into the interaction between environmental pressures (such as trophic state and water level regime) and the macrophyte communities. Thus a high incidence of stress-tolerant traits occurred in the plants of habitats stressed by low nutrient availability in conjunction with low pH. Competition- and disturbance-tolerant traits were more frequent in the plant communities of two productive lakes impacted by regular fluctuation of water level.

The Effects of Herbivorous Snails on Submerged Macrophyte Communities in Minnesota Lakes.

The influence of herbivorous snails on freshwater macrophytes was examined both in the laboratory and the field. In the laboratory, 14 macrophyte species grown in the absence of herbivores had growth rates ranging from 1-10%/d. When grown with four densities of herbivorous snails, species that grew fastest in the absence of herbivores were, in general, most negatively influenced by grazing. In food choice tests, snails typically preferred the plant species that grew fastest in the absence of herbivores. Snail densities were manipulated in a small mesotrophic lake, Christmas Lake. In areas where high snail densities were maintained, macrophyte species richness decreased, and the plants remaining were the species least preferred in laboratory choice tests. In surveys of eight additional lakes having a range of snail densities, the relationship between snail density and macrophyte species composition and diversity was consistent with the laboratory and Christmas Lake results. The levels of herbivore damage in the lake were measured for one macrophyte species. For the leaves examined, from 6 to 13% of the total leaf area was lost to herbivores. These results demonstrate that herbivorous snails can strongly influence the distribution, abundance, and diversity of freshwater macrophytes, and suggest that the role of herbivores in freshwater macrophyte communities may be similar to their role in terrestrial and marine communities.