Vallisneria Americana Michx Research Articles

Spatial-temporal shifts in submersed aquatic vegetation community structure resulting from a selective herbicide treatment in Lake Sampson, Florida, USA

Thayer J, Enloe S, Prince C, MacDonald G, Leary J. 2024. Spatial-temporal shifts in submersed aquatic vegetation community structure resulting from a selective herbicide treatment in Lake Sampson, Florida, USA. Lake Reserv Manage. 40:248–263. Submersed aquatic vegetation (SAV) is a major ecological component of Florida’s shallow lake systems. Hydrilla (H; Hydrilla verticillata [L.F.] Royle) is a nonnative, invasive SAV dominating many of these lakes and a high management priority in conservation. Lake Sampson is a shallow 776-ha lake in north Florida dominated by hydrilla since the 1980s, with native Illinois pondweed (P; Potamogeton illinoensis Morong) and American eelgrass (V; Vallisneria americana Michx.) historically represented. In March 2021, an in-water treatment of the herbicide florpyrauxifen-benzyl was administered to the littoral zone covering 46% of the lake surface. Five months after treatment (MAT), H infestations were reduced by 99%, recruitment by 69%, and invasions to cohabitating native populations by 90%. Conversely, total combined P populations increased occupancy from 15% to 23%, while V populations exhibited only a slight decrease in occupancy, going from 19% to 18%. By 17 MAT, hydrilla infestations, recruits, and invasions had all increased, while native populations started to retract from earlier gains. Moreover, surveys revealed a significant shift in SAV communities to shallower depths that were immediate 1 month after treatment and sustained 17 months later with the posttreatment reduction of hydrilla exclusively occupying deeper water. Posttreatment changes in water quality were apparent and possibly influential to SAV community dynamics. Although this in-water herbicide treatment exhibited good selectivity and reduced hydrilla competition, unintended side effects in water quality may have impeded the long-term succession of resilient native SAV communities that could compete with the recovery of hydrilla.

Calm after the storm? Similar patterns of genetic variation in a riverine foundation species before and after severe disturbance.

In summer 2011, Tropical storms Lee and Irene caused an estimated 90% decline of the submersed aquatic plant Vallisneria americana Michx. (Hydrocharitaceae) in the Hudson River of New York (USA). To understand the genetic impact of such large-scale demographic losses, we compared diversity at 10 microsatellite loci in 135 samples collected from five sites just before the storms with 239 shoots collected from nine sites 4 years after. Although 80% of beds sampled in 2011 lacked V. americana in 2015, we found similar genotypic and genetic diversity and effective population sizes in pre-storm versus post-storm sites. These similarities suggest that despite local extirpations concentrated at the upstream end of the sampling area, V. americana was regionally resistant to genetic losses. Similar geographically based structure among sites in both sampling periods suggested that cryptic local refugia at previously occupied sites facilitated re-expansion after the storms. However, this apparent resistance to disturbance may lead to a false sense of security. Low effective population sizes and high clonality in both time periods suggest that V. americana beds were already small and had high frequency of asexual reproduction before the storms. Dispersal was not sufficient to recolonize more isolated sites that had been extirpated. Chronic low diversity and reliance on asexual reproduction for persistence can be risky when more frequent and intense storms are paired with ongoing anthropogenic stressors. Monitoring genetic diversity along with extent and abundance of V. americana will give a more complete picture of long-term potential for resilience.

Aquatic vegetation types identified during early and late phases of vegetation recovery in the Upper Mississippi River

AbstractAssemblage patterns and processes of aquatic vegetation in most large floodplain rivers are not well understood, particularly after plant recovery. Identifying vegetation types, which are recurring plant groupings based on species composition, diversity, and abundances, can describe plant assembly patterns and environmental drivers that aid conservation planning and management. We used a 22‐year dataset (n = 18,000 sampling plots) to identify aquatic vegetation types during an “early phase” and “late phase” of plant recovery at multiple spatial scales nested within a 500‐km river reach of the Upper Mississippi River, USA. We hypothesized that vegetation types varied according to scale because of the stark environmental differences among riverine habitats and differing regional species pools along the river's latitudinal gradient, and that the late phase of recovery had developed several new vegetation types. We first used cluster analyses at multiple spatiotemporal scales to identify the number of vegetation types and their characteristics, such as indicator species, species compositions and abundances, and diversity index. Then we applied a multivariate regression to pinpoint environmental factors (such as hydrodynamics, system productivity, local habitat, and water quality) that structured those vegetation types. Clustering revealed that ~90% of plots irrespective of recovery phase were not classified into vegetation types, which indicated that most aquatic sampling plots are unique in species composition and unpredictable. However, impounded areas upriver from dams had matured five vegetation types: lotus (Nelumbo lutea Willd.), submersed (a mix of 11 common submersed species), watercelery (Vallisneria americana Michx.), arrowheads (Sagittaria rigida Pursh and Sagittaria latifolia Willd.), and a diverse community (with high diversity indices and multiple life forms). The vegetation types were associated with three environmental gradients related to inundation depth and duration, system productivity, and water clarity. These five vegetation types are known to be of high ecological value to fish and wildlife and thus targets for restoration, for example, the watercelery community is principal forage for migrating canvasback ducks (Aythya valisineria) along the Mississippi River flyway. Our results provide insights on vegetation assembly during recovery and aid habitat conservation by providing quantitative, environmental targets for restoration.

Assessing the Potential to Extrapolate Genetic-Based Restoration Strategies Between Ecologically Similar but Geographically Separate Locations of the Foundation Species Vallisneria americana Michx

Assessing the Potential to Extrapolate Genetic-Based Restoration Strategies Between Ecologically Similar but Geographically Separate Locations of the Foundation Species Vallisneria americana Michx

Does salt source matter? Comparing growth of Hydrilla verticillata (L.f.) Royle and Vallisneria americana Michx. under elevated salinity levels induced by different salt sources

Does salt source matter? Comparing growth of Hydrilla verticillata (L.f.) Royle and Vallisneria americana Michx. under elevated salinity levels induced by different salt sources

Predicting submerged aquatic vegetation cover and occurrence in a Lake Superior estuary

Submerged aquatic vegetation (SAV) provides the biophysical basis for multiple ecosystem services in Great Lakes estuaries. Understanding sources of variation in SAV is necessary for sustainable management of SAV habitat. From data collected using hydroacoustic survey methods, we created predictive models for SAV in the St. Louis River Estuary (SLRE) of western Lake Superior. The dominant SAV species in most areas of the estuary was American wild celery (Vallisneria americana Michx.). Maximum depth of SAV in 2011 was approximately 2.1m. In regression tree models, most of the variation in SAV cover was explained by an autoregression (lag) term, depth, and a measure of exposure based on fetch. Logistic SAV occurrence models including water depth, exposure, bed slope, substrate fractal dimension, lag term, and interactions predicted the occurrence of SAV in three areas of the St. Louis River with 78–86% accuracy based on cross validation of a holdout dataset. Reduced models, excluding fractal dimension and the lag term, predicted SAV occurrence with 75–82% accuracy based on cross validation and with 68–85% accuracy for an independent SAV dataset collected using a different sampling method. In one area of the estuary, the probability of SAV occurrence was related to the interaction of depth and exposure. At more exposed sites, SAV was more likely to occur in shallow areas than at less exposed sites. Our predictive models show the range of depth, exposure, and bed slope favorable for SAV in the SLRE; information useful for planning shallow-water habitat restoration projects.

Changes in submersed aquatic vegetation affect predation risk of a common prey fish Lucania parva (Cyprinodontiformes: Fundulidae) in a spring‐fed coastal river

Abstract Predation risk can impact population dynamics and ecosystem structure. Changes in key habitats, such as shifts between types of submersed aquatic vegetation, may affect predation risks experienced by certain organisms that use vegetation as a predation refuge. Relative predation risks experienced by Lucania parva (Baird & Girard) were assessed among three habitats in a Florida coastal stream using tethering experiments. Relative predation risks were highest in bare substrate, intermediate in a rooted macrophyte, Vallisneria americana Michx, and lowest in filamentous macroalgae. These results suggest that L. parva predation risk may decline following shifts from macrophytes to filamentous macroalgae, potentially leading to increased L. parva population size. These findings counter a widely held perception that such habitat shifts yield exclusively negative faunal effects, and are useful for assessing implications of habitat change and guiding restoration activities.

Toxicity of the herbicides bromacil and simazine to the aquatic macrophyte, Vallisneria americana Michx

Vallisneria americana Michx. (tapegrass) is an ecologically important submersed, vascular aquatic plant that provides food and shelter for many aquatic and waterfowl species. This plant often occurs close to land areas where herbicides are used. Nontarget exposure of these plants to herbicides may compromise ecological structure and function. The objective of the present study was to evaluate the suitability of several endpoint measurements for determining no-observable-adverse effect concentrations (NOAECs), lowest-observable-adverse effect concentrations (LOAECs), and median effective concentration values (EC50s) for tapegrass exposed to the herbicides bromacil (0-0.092 mg/L) and simazine (0-0.592 mg/L) following a 13-d single-pulse exposure and 15-d (bromacil) or 14-d (simazine) postexposure periods. The NOAEC/LOAEC/EC50 for fresh weight gains, new leaf production, and total leaf growth after 13-d exposure to bromacil were 0.020/0.036/0.032, 0.036/0.054/0.036, and 0.036/0.054/0.043 mg/L, respectively. The same respective NOAEC/LOAEC/EC50s for simazine were <0.058/0.058/0.067, 0.229/0.344/0.154, and 0.058/0.116/0.081 mg/L. Reductions in quantity and fresh weight of daughter plants produced and stolon fresh weights occurred at bromacil concentrations > or = 77, 0.020, and 0.036 mg/L, respectively; and simazine concentrations > or = 0.344, >0.592, and > or = 0.116 mg/L, respectively. Neither herbicide affected leaf greenness, total chlorophyll concentrations, or carbohydrate allocation. Although toxicity was shown for many endpoints, most EC50 values were greater than aquatic life benchmark values for algae used by the U.S. Environmental Protection Agency (U.S. EPA), but less than for aquatic plants, indicating that V. americana would likely be protected by use of the algal benchmark criteria.

Effects of Nutrient Pre-Exposure on Atrazine Toxicity to Vallisneria americana Michx. (Wild Celery)

Accelerated eutrophication is common to many freshwater and marine environments and often co-occurs with the presence of anthropogenic chemicals. However, the toxic effects of common chemical stressors such as herbicides in the presence of elevated nutrients are not well understood for most aquatic flora, particularly vascular species. To provide insight, field-collected Vallisneria americana Michx. (wild celery) were sequentially exposed to three nutrient concentrations for 3 months and then to nominal 11 and 110 microg L(-1) atrazine for 96 h. Nutrient concentrations (combined NH(4)(+), NO(2)(-), NO(3)(-), PO(4)(-)) were based on ambient concentrations in the St. Johns River (FL) and ranged from 0.013 to 0.668 mg L(-1). Nutrient pretreatment potentiated the toxicity of atrazine as determined by chlorophyll fluorescence activity. Electron transport rates (ETR) were significantly less (48-59%) for plants pretreated with low and ambient nutrient levels in the presence of an average of 107.5-128.1 microg L(-1) atrazine. Significant ETR reductions were also observed for plants exposed to an average of 11.4 microg L(-1) atrazine after exposure to nutrients three times the ambient concentration in the St. Johns River. The results indicate the importance of considering the presence of nutrients in chemical hazard assessments, particularly for phytotoxicants and nontarget vascular plants.

Effects of salinity and light on biomass and growth of Vallisneria americana from Lower St. Johns River, FL, USA

A mesocosm study was conducted to determine the effects of variable salinity and light on Vallisneria americana Michx. (wild celery) and associated algal community components in the lower St. Johns River, Florida. Fifteen centimeter diameter intact plant plugs were collected from the LSJR in March 2001 and transported to mesocosm facilities in Lafayette, Louisiana. A factorial experimental design was used consisting of three salinity levels (1, 8, and 18 ppt), three light levels (0, 50, and 90% shading), and three replicate mesocosms of each for a total of 27 mesocosms. The experiment consisted of a 4-week acclimation period followed by a 5-month treatment period. V. americana responded negatively to increased salinity. Although V. americana survived 8 ppt salinity, growth was limited. At 18 ppt, almost all V. americana aboveground biomass had perished within 10 weeks, but when salinity was lowered back to 1 ppt, approximately 20% of the aboveground biomass recovered within the following 10 weeks. At midtreatment harvest, light did not affect V. americana biomass directly (P = 0.8240), but by final harvest (20 weeks) light affected belowground biomass (P < 0.0014). Both salinity and light affected algal growth. Macroalgae dominated 1 ppt salinity treatments in ambient light, but phytoplankton dominated 8 and 18 ppt salinity treatments in ambient light. Algal communities were greatly inhibited by 90% shading. While salinity directly impacted V. americana growth and survival, light effects were less direct and involved algal community associations.

A Survey of Algal Epiphytes from Vallisneria americana Michx. (Hydrocharitaceae) in the Lower St. Johns River, Florida

Epiphytic algae may be responsible for a signifi cant amount of primary productivity in aquatic ecosystems, but also reduce available light, compete for nu- trients, and increase drag on their living substrates. A survey of the epiphytic algal community of the submersed aquatic macrophyte Vallisneria americana Michx. (Hydrocharitaceae) in the lower St. Johns River, FL was conducted over a 17-month period. A total of 122 infrageneric taxa were collected at four sites along a 93-km stretch of the river, yielding 32 diatoms (Bacillariophyta), 42 green algae (Chlo- rophyta), 45 blue-green algae (Cyanobacteria), 2 euglenoids (Euglenophyta), and 1 dinofl agellate (Pyrrophyta) taxa identifi ed over the study period. However, only diatoms and cyanobacteria were seen at all sampling dates, and only a Cocconeis sp. was identifi ed at all sites on all dates. This study provides baseline survey data of the epiphytic algal community in order to assess future anthropogenic effects.

Fish abundance and community composition in native and non‐native plants following hydrilla colonisation at Lake Izabal, Guatemala

Abstract Fish community composition was assessed among six macrophyte habitats, including hydrilla, Hydrilla verticillata (L.F.) Royle, common native species (bulrush, Scirpus spp., muskgrass, Chara spp., eelgrass, Vallisneria americana Michx. and Illinois pondweed, Potamogeton illinoensis Morong) and no‐plants, to assess potential impacts of recent hydrilla colonisation on the littoral fish community at Lake Izabal, Guatemala. Fish biomass was significantly different among habitats, with hydrilla supporting the highest fish biomass. Fish density did not differ significantly among habitats. Total fish species richness was similar (12‐15 species) among habitats, but community composition changed with macrophyte presence. Biomass of mojarra, Cichlasoma maculicauda Regan, which supported the most important subsistence fishery at the lake, was significantly different among habitats and had the greatest biomass in the hydrilla habitat. Although hydrilla may adversely affect native plants, lake access and other uses, it provided useful fish habitat and likely was not detrimental to the Lake Izabal fish community composition.

Restoration of Wildcelery, Vallisneria americana Michx., in the Lower Detroit River of the Lake Huron-Lake Erie Corridor

American wildcelery ( Vallisneria americana Michx.) is a valuable submersed aquatic plant that was negatively affected by pollution and urban runoff in the lower Detroit River for much of the 20th century. Following 25 years of water-pollution and urban-runoff abatement initiated in the early 1970s, we postulated that water clarity had increased and that this would allow restoration of wildcelery in the lower Detroit River. In addition, water clarity increased in the late 1980s due to water filtration and particulate removal by exotic dreissenid mussels ( Dreissena polymorpha and D. bugensis), which could contribute to potential wildcelery restoration. We sampled wildcelery in 1996–97 and compared these data to wildcelery data from 1950–51 and 1984–85. Over the 48-year period of comparison, areal density of wildcelery tubers decreased 72% (from 51.2 million to 14.4 million tubers) between 1950–51 and 1984–85 then increased 251% (from14.4 million to 50.5 million tubers) between 1984–85 and 1996–97. As a result, overall areal abundance was about the same in 1950–51 as in fall 1996–97. However, tuber densities in spring 1996 were similar to historical low abundances in springs of 1984–85. Then between spring and fall 1996, tuber densities increased 333% and remained relatively abundant through October 1997 indicating the beginning of the restoration of wildcelery in the lower Detroit River. In addition, we believe further reductions of turbidity through continued pollution-abatement programs and water filtration by dreissenid mussels combined with habitat protection and active management of wildcelery will contribute even further to the restoration of wildcelery in the Detroit River in the 21st century.

Abiotic influences on the biomass of Vallisneria americana Michx. in the Upper Mississippi River

AbstractAmerican wildcelery, Vallisneria americana Michx. is an ecologically important component of aquatic communities in the Upper Mississippi River (UMR). We conducted a study in 2002 to determine the association of several abiotic factors on the vegetative growth of Vallisneria in Navigation Pool 8 (Pool 8) of the UMR. We measured turbidity, percent light absorbance, surface water ammonium, surface water nitrate, current velocity, conductivity, pH and water depth throughout one growing season at 56 stratified sites based on where Vallisneria occurred in previous years. Sediment and aboveground biomass samples were collected during peak growth. Sediment was analysed for organic content, particle size, pore water nitrate and pore water ammonium. Vallisneria biomass samples were dried to constant mass. Because some sites were without water for much of the growing season, only data from 52 sites were reported. Biomass was associated with depth, percent light absorbance, turbidity and wind fetch. Vallisneria was abundant in the depth range of 0.55 to 1.03 m, in areas receiving at least 38% of surface light and in areas exposed to greater wind fetch (&gt;2000 m). Our results suggest that the primary abiotic variable associated with Vallisneria americana in the UMR is light, not nutrients. Published in 2007 John Wiley &amp; Sons, Ltd.

The Distribution and Abundance of Aquatic Macrophytes in Swan Lake and Middle Lake, Minnesota

ABSTRACT Point intercept surveys were conducted on Swan Lake and Middle Lake, Minnesota to quantitatively assess aquatic macrophyte distribution and abundance. A sediment particle size analysis was conducted to determine if sediment particle size influenced sago pondweed (Stuckenia pectinata (L.) Börner) and wild celery (Vallisneria americana Michx.) distribution. Swan Lake had more macrophyte species than Middle Lake. Sago pondweed distribution was different between 2001 and 2002 in Middle Lake. Wild celery was only found in Swan Lake and its distribution was also different between sample years. The presence of sago pondweed was positively related to percentage of clay in Swan Lake sediments. The presence of wild celery was negatively related to the percentage of clay in the sediment. No relationship was found between sediment particle size and sago pondweed growing in Middle Lake.

Response of submersed aquatic vegetation (SAV) in Lake Pontchartrain, Louisiana to the 1997–2001 El Niño Southern Oscillation shifts

Lake Pontchartrain is a large, shallow, low salinity estuary north of New Orleans, Louisiana. It is a water quality impaired system with restoration efforts in progress. One restoration goal is the reestablishment of historic submersed aquatic vegetation (SAV;Vallisneria americana Michx. andRuppia maritima L.), which has been in a state of decline since first studied in 1953. Annual SAV surveys and monthly water quality monitoring were conducted at four to five sites from 1996 through 2003 to evaluate trends and determine the causes of SAV change. We found a rapid increase in the distribution and abundance ofR. maritima in 1999 that persisted through 2002. An El Nino Southern Oscillation shift occurred between 1997 and 2001, which produced a drought in southern Louisiana as an ancillary effect of La Nina. This study was conducted to investigate causal links between the El Nino to La Nina climate phase shift and SAV change. We found that salinity and water clarity increased during La Nina. Increased water clarity produced a rapid increase in the euryhaline speciesR. maritima in deeper water and at historic sites where SAV had not been found since 1953. As salinity increased, the freshwater speciesV. americana andMyriophyllum spicatum L. declined, andNajas guadalupensis (Spreng.) Magnus andPotamogeton perfoliatus L. disappeared. In 2003, after the La Nina phase, salinity and water clarity decreased,R. maritima decreased, and the freshwater species increased, butP. perfoliatus was still absent. We found that salinity controlled SAV species composition, and water clarity controlled SAV colonization depth (Zcol=2.3/Kd). Our study demonstrated that climatic shifts cause cyclic changes in Lake Pontchartrain SAV and that restoration could be accomplished by improving water clarity. Due to the sensitivity of SAV to environmental change, similar responses to short-term and long-term climate changes should occur in other estuarine systems.

Bioavailability of sediment-bound metals for Vallisneria americana Michx, a submerged aquatic plant, in the St. Lawrence River

We have assessed the bioavailability of sediment-bound metals (Cd, Cr, Cu, Ni, Pb, and Zn) in the St. Lawrence River using a rooted aquatic plant, Vallisneria americana Michx, as the biomonitor species. The host sediments were subjected to a sequential leaching procedure to determine the partitioning of the metals present in the surficial oxic stratum; previously published equations were then used to estimate the free metal ion concentrations at the root-sediment interface. Suspected metal accumulation in foliage from point-source pollution of the water column obscured sediment-plant relationships at several sampling stations, but in general, plant metal concentrations could be predicted on the basis of sediment geochemistry (including sediment metal concentrations). Metal concentrations in Vallisneria tissues correlated more closely with estimates of bioavailable sediment-bound metal than with total metal concentrations in sediments, particularly for Cd, Pb, and Zn. Roots proved to be better bioindicator organs than shoots for monitoring sediment contamination. Overall, the results demonstrate that Vallisneria would be a useful biomonitor species of metal contamination in the St. Lawrence River.

Bioavailability of sediment-bound metals for &lt;i&gt;Vallisneria americana&lt;/i&gt; Michx, a submerged aquatic plant, in the St. Lawrence River

We have assessed the bioavailability of sediment-bound metals (Cd, Cr, Cu, Ni, Pb, and Zn) in the St. Lawrence River using a rooted aquatic plant, Vallisneria americana Michx, as the biomonitor species. The host sediments were subjected to a sequential leaching procedure to determine the partitioning of the metals present in the surficial oxic stratum; previously published equations were then used to estimate the free metal ion concentrations at the root-sediment interface. Suspected metal accumulation in foliage from point-source pollution of the water column obscured sediment-plant relationships at several sampling stations, but in general, plant metal concentrations could be predicted on the basis of sediment geochemistry (including sediment metal concentrations). Metal concentrations in Vallisneria tissues correlated more closely with estimates of bioavailable sediment-bound metal than with total metal concentrations in sediments, particularly for Cd, Pb, and Zn. Roots proved to be better bioindicator...

Competition between Hydrilla verticillata and Vallisneria americana as influenced by soil fertility

The influence of soil fertility on competitive interactions between dioecious hydrilla [ Hydrilla verticillata (L.f.) Royle] and American eelgrass ( Vallisneria americana Michx) was investigated. Addition series experiments were conducted with mixed plantings of Hydrilla : Vallisneria grown at two levels of soil fertility. Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In monocultures, Hydrilla biomass averaged six times higher at high fertility as compared to biomass at low fertility, whereas only a two-fold increase in biomass was obtained with Vallisneria grown in similar fertility treatments. In mixed cultures at high fertility, Hydrilla was the stronger competitor relative to Vallisneria, with one Hydrilla plant being competitively equivalent to 7.2 Vallisneria plants in terms of their respective abilities to reduce Hydrilla biomass. Under nutrient limiting conditions, however, Hydrilla growth and its competitive advantage over Vallisneria were both depressed, and Vallisneria was the dominant species. These data indicated that differential response to increased nutrient inputs is one major determinant of competitive success in mixtures of dioecious Hydrilla and Vallisneria.

Metal accumulation in American wild celery (Vallisneria americana Michx.) in the St. Lawrence River: effects of water depth and exposure to current

The sensitivity of Vallisneria americana Michx. as an indicator species of metal concentrations was assessed through the examination of its spatial (between- and within-site) and temporal (short- and long-term) variability. Eight macrophyte beds located in the St. Lawrence and Ottawa rivers were selected to contrast metal concentrations found in plants exposed to different types of waters, upstream and downstream of their confluence, in the Greater Montreal urban area. Comparisons among sites revealed higher metal concentrations, both in water and in plant tissues, at sites exposed to Ottawa River "brown" waters than at sites exposed to St. Lawrence River "green" waters. Within each site, samples represented a broad range in terms of water depths, exposure to current, incident light intensity, and total plant biomass. At all sites, metal concentrations in plant tissues were lowest in the shallow water found in sheltered, dense beds of submerged aquatic vegetation and increased in deep, open-water areas beyond the limits of dense vegetation. This persistent gradient may result from local differences in plant growth rates, exposure to currents, and (or) metal bioavailability. The use of a sampling strategy designed to control for within-site (depth-balanced) variability makes it possible to measure spatial and (or) temporal differences on the order of 20% whereas unbalanced sampling designs may lead to erroneous conclusions. Significant reductions in metal concentrations in plant tissue were observed both between 1994 and 1996 (Fe, Mn, Pb, Zn, N) and between 1976 and 1996 (Cd, Cr, Pb, Zn, N). Particular care must be given to sampling design if V. americana is to be used as a biological indicator of further long-term reductions in metal concentrations.