Response of the waterbird community to floating pennywort (Hydrocotyle ranunculoides) cover at Ngamo dam,Antelope Park,Zimbabwe.

Little is known on the influence of invasive aquatic weeds on afro‐tropical waterbird communities. We used bird counts in sites of varying floating pennywort Hydrocotyle ranunculoides cover to explore the relationship between the weed and the waterbird community dynamics at Ngamo dam, Antelope Park, Zimbabwe. Waterbird communities in low‐to‐medium weed cover sites were more diverse and abundant compared to sites of high weed cover. In addition to supporting birds such as African Jacana which are able to forage within dense aquatic plants, high weed cover sites were associated with birds whose diets include invertebrates and fish which are likely more abundant and diverse in these sites. In contrast, low‐to‐medium weed cover sites were associated with bird species such as Common Moorhen, Great Egret, Pied Kingfisher and African Fish Eagle which require accessible open water and forage for diving, swimming or wading. The increased bird abundance in low‐to‐medium weed cover sites also likely increases prey for predatory birds. Thus, the negative changes in the waterbird community composition, abundance and diversity in response to increasing floating pennywort cover reflects species‐specific tolerances to floating pennywort and its influence on accessible open water, foraging sites and prey availability.     

Facilitation and Competition among Invasive Plants: A Field Experiment with Alligatorweed and Water Hyacinth

Ecosystems that are heavily invaded by an exotic species often contain abundant populations of other invasive species. This may reflect shared responses to a common factor, but may also reflect positive interactions among these exotic species. Armand Bayou (Pasadena, TX) is one such ecosystem where multiple species of invasive aquatic plants are common. We used this system to investigate whether presence of one exotic species made subsequent invasions by other exotic species more likely, less likely, or if it had no effect. We performed an experiment in which we selectively removed exotic rooted and/or floating aquatic plant species and tracked subsequent colonization and growth of native and invasive species. This allowed us to quantify how presence or absence of one plant functional group influenced the likelihood of successful invasion by members of the other functional group. We found that presence of alligatorweed (rooted plant) decreased establishment of new water hyacinth (free-floating plant) patches but increased growth of hyacinth in established patches, with an overall net positive effect on success of water hyacinth. Water hyacinth presence had no effect on establishment of alligatorweed but decreased growth of existing alligatorweed patches, with an overall net negative effect on success of alligatorweed. Moreover, observational data showed positive correlations between hyacinth and alligatorweed with hyacinth, on average, more abundant. The negative effect of hyacinth on alligatorweed growth implies competition, not strong mutual facilitation (invasional meltdown), is occurring in this system. Removal of hyacinth may increase alligatorweed invasion through release from competition. However, removal of alligatorweed may have more complex effects on hyacinth patch dynamics because there were strong opposing effects on establishment versus growth. The mix of positive and negative interactions between floating and rooted aquatic plants may influence local population dynamics of each group and thus overall invasion pressure in this watershed.     

Growth characteristics of aquatic macrophytes cultured in nutrient-enriched water: I. Water hyacinth,water lettuce,and pennywort

Seasonal growth characteristics and biomass yield potential of 3 floating aquatic macrophytes cultured in nutrient nonlimiting conditions were evaluated in central Florida’s climatic conditions. Growth cycle (growth curve) of the plants was found to be complete when maximum plant density was reached and no additional increase in growth was recorded. Biomass yield per unit area and time was found to be maximum in the linear phase of the growth curve; plant density in this phase was defined as “operational plant density,” a density range in which a biomass production system is operated to obtain the highest possible yields. Biomass yields were found to be 106, 72, and41 t(drywt)ha^-1yr^-1, respectively, for water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), and pennywort (Hydrocotyle umbellata). Operational plant density was found to be in the range of 500–2,000 g dry wt m^-2 for water hyacinth, 200–700 g dry wt m^-2 for water lettuce, and 250–650 g dry wt m^-2 for pennywort. Seasonality was observed in growth rates but not in operational plant density. Specific growth rate (% increase per day) was found to maximum at low plant densities and decreased as the plant density increased. Results show that water hyacinth and water lettuce can be successfully grown for a period of about 10 mo, while pennywort, a cool season plant, can be integrated into water hyacinth/water lettuce biomass production system to obtain high yields in the winter.     

The role of eutrophication in the biological control of water hyacinth, <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>, in South Africa

South Africa has some of the most eutrophic aquatic systems in the world, as a result of the adoption of an unnecessarily high 1 mg l⁻¹ phosphorus (P) standard for all water treatment works in the 1970 s. The floating aquatic macrophyte, water hyacinth (Eichhornia crassipes (Mart.) Solms (Pontederiaceae)), has taken advantage of these nutrient rich systems, becoming highly invasive and damaging. Despite the implementation of a biological control programme in South Africa, water hyacinth remains the worst aquatic weed. A meta-analysis of published and unpublished laboratory studies that investigated the combined effect of P and nitrogen (N) water nutrient concentration and control agent herbivory showed that water nutrient status was more important than herbivory in water hyacinth growth. Analysis of long-term field data collected monthly from 14 sites around South Africa between 2004 and 2005 supported these findings. Therefore the first step in any water hyacinth control programme should be to reduce the nutrient status of the water body.     

The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Megamelus scutellaris (Hemiptera: Delphacidae), a biocontrol agent of water hyacinth,is not sufficiently specific for release in Australia

Water hyacinth Eichhornia crassipes (Pontederiaceae) is one of the world's worst invasive species, responsible for damaging aquatic systems in many warmer parts of the globe including north America, Africa, Asia and Australia. The planthopper Megamelus scutellaris Berg (Delphacidae) has been released in USA and approved for release in South Africa for biocontrol of water hyacinth. We assessed this agent for suitability for release in Australia and found that a related native aquatic plant, Monochoria cyanea (Pontederiaceae) is within the fundamental host range of this insect. Adult survival, oviposition and development of nymphs to adult was equally high on M. cyanea as on the target species, although the quality of these next generation adults was lower than those reared on the target species. This demonstrates that M. scutellaris is not sufficiently specific for release in Australia. Nymphal development to adults occurred only in very low numbers on the three other Australian species of Monochoria. M. cyanea only occurs in Australia so M. scutellaris is still a possible water hyacinth biocontrol candidate for other regions depending on the results of assessment of the risk to local species of Monochoria. This study demonstrates the effectiveness of modern biocontrol agent assessment and reinforces the importance of testing of local non-target species.     

Ecological and socio‐economic impacts of invasive water hyacinth (Eichhornia crassipes): a review

1. Water hyacinth (Eichhornia crassipes) is one of the world’s most invasive aquatic plants and is known to cause significant ecological and socio‐economic effects. 2. Water hyacinth can alter water clarity and decrease phytoplankton production, dissolved oxygen, nitrogen, phosphorous, heavy metals and concentrations of other contaminants. 3. The effects of water hyacinth on ecological communities appear to be largely nonlinear. Abundance and diversity of aquatic invertebrates generally increase in response to increased habitat heterogeneity and structural complexity provided by water hyacinth but decrease due to decreased phytoplankton (food) availability. 4. Effects of water hyacinth on fish are largely dependent on original community composition and food‐web structure. A more diverse and abundant epiphytic invertebrate community may increase fish abundance and diversity, but a decrease in phytoplankton may decrease dissolved oxygen concentrations and planktivorous fish abundance, subsequently affecting higher trophic levels. 5. Little is known about the effects of water hyacinth on waterbird communities; however, increases in macroinvertebrate and fish abundance and diversity suggest a potentially positive interaction with waterbirds when water hyacinth is at moderate density. 6. The socio‐economic effects of water hyacinth are dependent on the extent of the invasion, the uses of the impacted waterbody, control methods and the response to control efforts. Ecosystem‐level research programmes that simultaneously monitor the effects of water hyacinth on multiple trophic‐levels are needed to further our understanding of invasive species.     

Diel variations of certain physico-chemical parameters of water in selected aquatic systems

A field study was conducted during the months of October, January, May, and July (1979–80) to examine the diel variations in dissolved O₂ (DO), pH, dissolved CO₂, bicarbonate and carbonate alkalinity, NH₄-N, NO₃-N, and PO₄-P concentration, and conductivity (EC) of the water in six aquatic systems. Water in hyacinth (Eichhornia crassipes) ponds showed very little or no diel or seasonal variations in DO, pH, dissolved CO₂, and bicarbonate alkalinity. Dissolved O₂ concentration of the water under floating hyacinth cover was in the range of 0.2–3.0 µg/ml, while dissolved CO₂ levels were in the range of 10–35 µg/ml. In the aquatic systems with no floating vegetation, i.e., elodea (Egeria densa) pond, cattail (Typha sp.) pond, control pond (filamentous algae and Chara spp.), and eutrophic lake (algae in Lake Apopka), DO and pH of the water increased during mid-day and decreased during the night. Dissolved O₂ levels in these ponds were in the range of 5–20 µg/ml during mid-day and 2–8 µg/ml during the night, while pH of the water was in the range of 8–9.5 during mid-day and decreased to 7–8 during the night. An inverse relationship was observed between bicarbonate and carbonate alkalinity of the water in the aquatic systems with no floating vegetation while no carbonates were detected in the water with floating hyacinth plants. Ammonium N, NO₃-N and PO₄-P concentration of the water in these aquatic systems showed very little or no diel variations.Florida Agricultural Experiment Stations Journal Series No. 2788.     

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.     

Catchment context and the bottom‐up regulation of the abundance of specialist semi‐aquatic weevils on water hyacinth

1. Bottom‐up regulation is prevalent in plant–herbivore interactions and is thought to be particularly important in the case of aquatic plants and their specialist insect herbivores. 2. Recently published mesocosm studies have shown that the abundance of specialist Neochetina weevils, N. bruchi and N. eichhorniae, on water hyacinth (Eichhornia crassipes) are principally under the influence of nutrients in plant tissues. 3. We examined historical patterns of the abundance of these species of semi‐aquatic weevils in two water bodies from catchments with significantly different nutrient loads in subtropical Australia to test the validity of the published conceptual model of bottom‐up regulation. 4. Our results revealed that these weevils are indeed under bottom‐up regulation under field conditions. However, the nature of this regulation appears to be influenced by the broader catchment context of the water hyacinth‐infested water body, with plant tissue nutrients influencing weevil abundance more in the catchment with lower nutrient run‐offs. 5. Our findings reaffirm the importance of bottom‐up regulation in plant–insect interactions, add to the growing evidence of indirect effects spanning terrestrial and aquatic ecosystems, and inform management of water hyacinth using these weevils as biocontrol agents.     

Herbivory on aquatic vascular plants by the introduced golden apple snail (<Emphasis Type="Italic">Pomacea canaliculata</Emphasis>) in Lao PDR

The effect of naturally found densities of the exotic and herbivorous golden apple snail (Pomacea canaliculata) on three dominant aquatic plants – duckweed (Lemna minor), water hyacinth (Eichhornia crassipes) and morning glory (Ipomoea aquatica) – was assessed in a wetland survey and quantified in a field experiment in Laos in southeast Asia. Snail grazing reduced plant biomass, but plant species were differently affected by grazing. Duckweed had almost disappeared after 6 and water hyacinth after 21 days, whereas morning glory remained at 80% of initial biomass after 32 days. Snail growth was lowest on morning glory and, when all plant species were simultaneously presented to snails, this plant was not preferred. We suggest that the negative effect the golden apple snail had on the growth of these plant species in field enclosures is present in the natural environment as well. This new and intense herbivory could have serious negative effects on invaded freshwater ecosystems in this region.     

Plant invasions in the rivers of the Iberian Peninsula,south-western Europe: A review

Aquatic and riparian ecosystems are known to be highly vulnerable to invasive alien species (IAS), especially when subjected to human-induced disturbances. In the last three decades, we have witnessed a growing increase in plant invasions in Portugal and Spain (Iberian Peninsula, south-western Europe), with very detrimental economic, social and ecological effects. Some of these species, such as the giant reed (Arundo donax L.) and the water hyacinth (Eichhornia crassipes (Mart.) Solms-Laub.), number among the world's worst weeds. We present an appraisal of this invasive alien river flora and the most problematic aquatic weeds. We review various aspects of invasion ecology, including spatial and temporal patterns of invasion, species invasiveness, species traits of invasive weeds, and relationships between human disturbance in rivers and surrounding areas and invasibility, and contextualize them in overall state-of-the-art terms. We also acknowledge the use of IAS as bioindicators of the ecological quality of rivers, wetlands and riparian zones. Remote-sensing tools and Geographic Information Systems for detecting and monitoring IAS in Iberian rivers are presented.     

The dynamics and ecology of exotic tropical species in floating plant mats: Lake Naivasha,Kenya

The floating water fern Salvinia molesta has occurred in Lake Naivasha since the early 1960s and during this period has obstructed fishing activities and navigation. In recent years the extent of Salvinia has declined markedly. Since 1988, a second floating weed, Eichhornia crassipes (water hyacinth), has colonised the lake. Salvinia formed large mats around the edges, bays and lagoons and had very few plant or invertebrate species associated with it. Mats of E. crassipes, however, support a greater abundance and variety of animal and plant species, which shows a relationship with mat size. This seems to have facilitated plant succession but without a zonation typical of the classic hydroseral sequence for the lake.     

Turning a Problem Into Profit: Using Water Hyacinth (<Emphasis Type="Italic">Eichhornia crassipes</Emphasis>) for Making Handicrafts at Lake Alaotra,Madagascar

In diverse ecosystems, invasive plant species represent a serious threat for nature conservation by leading to loss of native species as well as environmental degradation. The water hyacinth (Eichhornia crassipes) belongs to the top 10 worldwide most troublesome aquatic weeds. This study assesses the use of this invasive plant as a new source of raw material for handicrafts to substitute for the traditionally used papyrus (Cyperus madagascariensis) in the poor and remote region of Lake Alaotra in Madagascar. It has been recently observed that the current exploitation of papyrus in this region adds pressure on the receding Alaotra wetland and notably decreases habitat and food resources for the locally endemic and critically endangered lemur species Hapalemur alaotrensis. Within our research, water hyacinth handicrafts were produced by local artisans and compared with papyrus handicrafts regarding production path (collection, transportation, and processing of raw material) with a focus on financial costs, workload, and selling price. In addition, structured interviews were conducted with town-based handicraft retailers and potential customers to specify market expectations (quality, design, and marketing strategies). Our study revealed that despite the requirement for a longer time regarding raw material selection and drying (7 days vs 3 days) as well as additional financial costs (23% in addition) for ornaments, water hyacinth handicrafts displayed many advantages in comparison to papyrus handicrafts: (a) assembling water hyacinth handicrafts was easier and faster (33% less time investment), (b) led to robust products that were rated as being of acceptable to very good quality by 91% of the interviewees (c) and sold at three times (mean 2.25 US$) the sale prices of the traditional papyrus handicrafts (mean 0.75 US$). The use of water hyacinth has therefore the potential to increase local household incomes, open up new markets, and attenuate the pressure on the Alaotra wetland biodiversity by reducing the use of papyrus. A SWOT (strengths, weaknesses, opportunities and threats) analysis combined with research findings on rural handicrafts was drafted to identify four main steps for promoting the market establishment of water hyacinth handicrafts at Lake Alaotra: creation of artisans’ clusters and networks, improvement of managerial and marketing competence, access to finance and exploration of markets possibilities. This research can provide insights for other small rural handicraft enterprises in developing countries facing problem with water hyacinth throughout the world.     

Water hyacinth,Eichhornia crassipes (Pontederiaceae), reduces benthic macroinvertebrate diversity in a protected subtropical lake in South Africa

The socio-economic impacts of the free-floating aquatic plant water hyacinth, Eichhornia crassipes (Pontederiaceae), on aquatic systems are well documented, yet the impacts on aquatic biodiversity, particularly invertebrate biodiversity, are less well understood. This study aimed to determine whether the presence of water hyacinth altered the diversity and assemblage structure of benthic macroinvertebrates in a conservation area. The benthic macroinvertebrate assemblage was sampled over 1 year at five sites under water hyacinth mats and at five sites without water hyacinth at Lake Nsezi—Nseleni River in the vicinity of Richards Bay, KwaZulu-Natal, South Africa. Artificial substrates were placed beneath water hyacinth mats or in the open water to allow for colonization by freshwater macroinvertebrates, and left for a period of 6 weeks, repeated on seven occasions. Twenty nine families comprising 18,797 individuals were collected, 817 (13 families) individuals were from under water hyacinth mat sites compared to 17,980 (27 families) individuals from open water sites. Ninety-eight percent of individuals collected were, however, the invasive snail, Tarebia granifera. Open water samples were separated from samples beneath the water hyacinth mat by non-metric Multidimensional Scaling, indicating reduced biodiversity associated with the presence of water hyacinth. Exclusion of the dominant Thiaridae from the analysis did not alter the groupings. Family richness(s) and abundance (N) were significantly higher in open water communities(S: H₃ = 21.09; P = 0.0001; N: H₃ = 22.58; P = 0.00001), while evenness (J’) was higher under water hyacinth mats (H₃ = 20.13; P = 0.0002). The presence of water hyacinth had a significantly negative impact on aquatic macroinvertebrate biodiversity in a conservation area, and therefore the control of this invasive aquatic plant must play a major role in catchment management.     

Assessing the true status of the fish species Labeo cylindricus (Peters 1868) (Teleostei: Cyprinidae) in Lake Baringo,Kenya

Eutrophication contributes to the proliferation of alien invasive weed species such as water hyacinth Eichhornia crassipes. Although the South American moth Niphograpta albiguttalis was released in South Africa in 1990 as a biological control agent against water hyacinth, no post-release evaluations have yet been conducted here. The impact of N. albiguttalis on water hyacinth growth was quantified under low-, medium- and high-nutrient concentrations in a greenhouse experiment. Niphograpta albiguttalis was damaging to water hyacinth in all three nutrient treatments, but significant damage in most plant parameters was found only under high-nutrient treatments. However, E. crassipes plants grown in high-nutrient water were healthier, and presumably had higher fitness, than plants not exposed to herbivory at lower-nutrient levels. Niphograpta albiguttalis is likely to be most damaging to water hyacinth in eutrophic water systems, but the damage will not result in acceptable levels of control because of the plant's high productivity under these conditions. Niphograpta albiguttalis is a suitable agent for controlling water hyacinth infestations in eutrophic water systems, but should be used in combination with other biological control agents and included in an integrated management plan also involving herbicidal control and water quality management.     

Feeding behavior and spatial distribution of two planthoppers,Megamelus scutellaris (Delphacidae) and Taosa longula (Dictyopharidae), on water hyacinth

Megamelus scutellaris Berg (Delphacidae) and Taosa (Cuernavaca) longula Remes Lenicov (Dictyopharidae) are specialist planthoppers that feed and reproduce on the invasive aquatic weed, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae). They overlap geographically in several regions of South America and may, therefore, interact and compete for food and microhabitat. Preliminary observations indicated that both species do not feed on the same part of the plant. We hypothesized that they partition the resource; hence, we studied (1) the feeding mechanism at the tissue level and (2) the spatial distribution of both species on the water hyacinth plant. Salivary sheaths were detected through histological sections of plant tissues using light microscopy. The location of either planthopper species on the plant was recorded when in the presence or absence of the other species. Both species produced true salivary sheaths, mostly branched (M. scutellaris: 82%; T. longula: 84%), ending in phloem (M. scutellaris: 56%; T. longula: 52%), and xylem tissues (M. scutellaris: 24%; T. longula: 28%). They resided on different parts of the water hyacinth plant even when they did not coexist; nymphs of T. longula occurred primarily on the back side of the leaf laminas, while nymphs of M. scutellaris occupied the basal zone of the petioles. This study shows that these planthoppers complement each other and could be used in combination as control agents for water hyacinth. Further experimental studies and field observations are necessary to quantify interactions.     

Biological control of three floating water weeds,<Emphasis Type="Italic">Eichhornia crassipes,Pistia stratiotes</Emphasis>, and <Emphasis Type="Italic">Salviniamolesta</Emphasis> in the Republic of Congo

Since 1999, four specific weevils (Coleoptera, Curculionidae) were released in the Republic of Congo against three exotic floating water weeds: Neochetina eichhorniae Warner and N. bruchi Hustache against water hyacinth, Neohydronomus affinis Hustache against water lettuce, and Cyrtobagous salviniae Calder and Sands against water fern. Recoveries of exotic weevils were made from all 24 release sites except one, and all four species have established and spread (up to 800 km for water hyacinth weevils). Within a few years of releases, control of water fern and water lettuce was such that fishing and navigation could be resumed, while reductions of water hyacinth populations were only beginning.     

Phylogenetic taxonomy of the marine mite genus Rhombognathides (Acari: Halacaridae: Rhombognathinae)

This study examined the impacts of the alien waterweed, water hyacinth, on the abundance and diversity of aquatic macroinvertebrates in the littoral areas of northern Lake Victoria in Uganda. The weed had undergone explosive growth on the lake causing serious disruption to people, the economy and the ecosystem. This study was confined to impact of the weed in the littoral zone, not to the large floating mats of vegetation which float across the lake and clog large areas of shoreline.The littoral area studied comprised of fringing mats of Eichhornia crassipes (Mart) Solms (water hyacinth) to the lakeward of Cyperus papyrus; water hyacinth mats undergoing colonisation by Vossia cuspidata (Roxb.) Griff.; and a typical Cyperus papyrus L shore with no outer floating mat of water hyacinth. Numerical abundance (Nos. m^–2) and diversity (No. of taxa) of macroinvertebrates recovered from pure Eichhornia crassipes and the Eichhornia-Vossia succession increased from the fringe of the Cyperus papyrus towards the open water. In the typical Cyperus papyrus fringe, in the absence of water hyacinth, abundance was highest at the papyrus/open water interface and dropped off sharply towards open water. The Shannon–Weaver diversity index (H) of macroinvertebrates decreased progressively from pure Eichhornia crassipes stands, to Vossia/Eichhornia beds and Cyperus papyrus stands (H=0.56, 0.54 and 0.34, respectively) but were not significantly different. Dissolved oxygen decreased from open water into vegetation where it approached anoxia. Water hyacinth appeared to enhance the abundance and diversity of aquatic macroinvertebrates at the interface with the open water. The impoverished abundance and diversity of the macroinvertebrates deeper into the vegetation mats suggested negative environmental impacts of the water hyacinth when the fringe is too wide. Further research is recommended to establish the optimum width of the fringe of stationery water hyacinth that promotes maximum abundance and diversity of aquatic macroinvertebrates and, possibly, of other aquatic life. Since this study in 1997, there has been a dramatic decrease in Eichhornia infestations and by June 2000 it appeared largely to exist only as fringing vegetation in bays and inlets.     

Herbicide treatment alters the effects of water hyacinth on larval mosquito abundance

Invasive aquatic weeds are managed with herbicides to reduce their negative impacts on waterways in many areas, including the California Delta Region. Herbicides create a dynamic environment of living and decomposing plant matter that could affect larval mosquitoes and other invertebrates, such as their predators and competitors. Our objective was to compare the number of larval mosquitoes in water or water hyacinth, before and after an herbicide treatment. We created replicated pond mesocosms with water hyacinth, water hyacinth treated with glyphosate and an oil adjuvant, open water, and water with glyphosate plus adjuvant. We sampled for larval mosquitoes and other aquatic invertebrates. Before herbicide addition, there was a trend for more larval mosquitoes in open water tanks than in tanks with water hyacinth. Herbicide application resulted in an immediate decrease of larval mosquitoes. As decay progressed, larval mosquitoes became most abundant in mesocosms with herbicide‐treated hyacinth and very few larval mosquitoes were found in other habitat treatments. Although the numbers of predatory and competitor insects had some variation between treatments, no clear pattern emerged. This information on how invasive weed management with herbicides affects larval mosquitoes will allow control practices for larval mosquitoes and invasive weeds to be better integrated.