Native Macrophytes Research Articles

Feeding of fish inhabiting native and non-native macrophyte stands in a Neotropical reservoir

Owning to dissimilarity with native macrophytes, invasive macrophytes may negatively affect the diet and foraging efficiency of fishes. We evaluated the invertebrate availability, diet composition, and foraging efficiency of four fish species that use macrophytes as habitat in a native and a highly invasive macrophyte. The samples were taken in a reservoir in Brazil (22° 33′ S, 52°54′ W). We collected invertebrates and fishes simultaneously in macrophyte stands dominated by the invasive Poaceae Urochloa arrecta and the native Eichhornia azurea. We evaluated the foraging efficiency by the total number of prey consumed and by the degree of stomach fullness. We used an approach based on resampling procedures to compare differences in composition of invertebrates, composition of fish diet, and the similarity between invertebrates and fish diet. The abundance of invertebrates and the diet composition differed significantly between plants. On the other hand, fish foraging efficiency did not differ between macrophytes. Despite this lack of difference of foraging efficiency, the lower abundance of food items in the invasive macrophyte and the significant changes in fish diet suggest that U. arrecta is not a favorable feeding habitat for fishes, compared to the native E. azurea.

On the move: New insights on the ecology and management of native and alien macrophytes

Globally, freshwater ecosystems are under threat. The main threats come from catchment land-use changes, altered water regimes, eutrophication, invasive species, climate change and combinations of these factors. We need scientific research to respond to these challenges by providing solutions to halt the deterioration and improve the condition of our valuable freshwaters. This requires a good understanding of aquatic ecosystems, and the nature and scale of changes occurring. Macrophytes play a fundamental role in aquatic systems. They are sensitive indicators of ecosystem health, as they are affected by run-off from agricultural, industrial or urban areas. On the other hand, alien macrophytes are increasingly invading aquatic systems all over the world. Improving our knowledge on the ecology and management of both native and alien plants is indispensable to address threats to freshwaters in order to protect and restore aquatic habitats. The International Aquatic Plants Group (IAPG) brings together scientists and practitioners based at universities, research and environmental organisations around the world. The main themes of the 15th symposium 2018 in New Zealand were biodiversity and conservation, management, invasive species, and ecosystem response and restoration. This Virtual Special Issue provides a comprehensive review from the symposium, addressing the ecology of native macrophytes, including those of conservation concern, and highly invasive alien macrophytes, and the implications of management interventions. In this editorial paper, we highlight insights and paradigms on the ecology and management of native and alien macrophytes gathered during the meeting.

Negative correlations between native macrophyte diversity and water hyacinth abundance are stronger in its introduced than in its native range

AbstractAimWe tested the hypothesis that the diversity and abundance of aquatic macrophytes are negatively related with Eichhornia crassipes abundance in its introduced range, but not in its native range.LocationUpper Paraná River Floodplain, Brazil and Southeast China.MethodsWe sampled aquatic macrophytes patches in Brazil (native range) and China (introduced range) along a biomass gradient of E. crassipes. For each patch, we obtained values of species richness and aquatic macrophytes percentage cover, as response variables in regression models. We also used species accumulation curves to quantify the total plot diversity in dominated and non‐dominated plots for both countries. Finally, we compared the influence of E. crassipes dominance on community composition and beta diversity with Permanova and Permdisp, respectively.ResultsThe regression analyses revealed a negative correlation between macrophyte richness and cover and E. crassipes biomass only in the introduced range. The cumulative number of species decreased at a higher extent in plots dominated by E. crassipes in China, compared with Brazil. Also, species composition changed and beta diversity decreased in the dominated plots in China, but not in Brazil.Main ConclusionsThe reduction of all diversity attributes related to E. crassipes probably results from its engineer species role, which decreases littoral region habitat heterogeneity and affects rare species in the introduced range. Differences between countries may be associated with impacts of water hyacinth on native macrophytes as this plant grows very fast and is highly competitive. Although less probable, biotic resistance at the establishment phase of water hyacinth in sites with higher number of native species is also a possibility. Regardless of the main mechanism explaining our patterns, it is suggested that invasion by water hyacinth is a cause for concern for its higher impacts in the introduced ranges than the native ranges.

Litter decomposition of the invasive Potentilla anserina in an invaded and non-invaded freshwater environment of North Patagonia

Potentilla anserina is a common invasive species of Patagonian wetlands. Although this species is commonly found in temporary habitats, where it establishes during the dry phase, the littoral zones of lakes are also susceptible, as water fluctuations make this habitat extremely variable in terms of water availability. Decay rates and nutrient release of P. anserina were compared with two native macrophyte species, Eleocharis pachycarpa and Schoenoplectus californicus, in two different freshwater environments: a temporary wetland, where P. anserina develops important stands, and the littoral zone of a lake not yet colonized by the invasive plant. Our results showed that both environments differed in nutrient concentrations, being higher in the temporary wetland. However, the invasive species decomposed faster in the littoral zone of the lake, due to the presence of the amphipod Hyallela patagonica in the litter bags. The invasive plant litter decomposed twice faster than the native species. The species with highest N content, E. pachycarpa, released more N in leachates. After 71 days of decomposition, P. anserina loss more C and gained more N than the native species, while the P content remained constant across the three species. Our results demonstrated that the introduction of an invasive species as P. anserina into new habitats would contribute with dissolved and particulate organic matter impacting nutrient cycling and invertebrate assemblages with consequences to the entire ecosystem.

Does Invasion of Eurasian Milfoil Myriophyllum spicatum Lead to a “Trophic Dead End” and Reduced Food Web Complexity in Gulf of Mexico Estuarine Food Webs?

Estuaries of the northern Gulf of Mexico contain an abundance of habitat-forming submerged aquatic vegetation (SAV) that provide refuge and protection for a variety of freshwater, estuarine, and marine organisms. However, many of these estuaries now contain numerous exotic species, the ultimate impacts of which are unclear. In the Mobile-Tensaw Delta, located in the upper portion of Mobile Bay, Alabama (USA), Eurasian milfoil (Myriophyllum spicatum, hereafter referred to as Myriophyllum) is now the most dominant submerged macrophyte. Myriophyllum is a structurally-complex macrophyte with the potential to dramatically alter estuarine food webs through reduced encounter rates between predators and their prey and other mechanisms. Previously, we surveyed faunal communities using throw traps, trawls, cores, and suction sampling to compare milfoil assemblages with other native macrophytes to explore the interactive role of hydrology, diel periodicity, and macrophyte presence in influencing community structure. Here, we use this previously collected data to generate a preliminary food web analyses to determine if milfoil, due to its high complexity, creates a "trophic dead end" and limits higher trophic level production. We found the number of nodes, links, linkage density, and connectance to all be greater in milfoil than Vallisneria americana (hereafter referred to as Vallisneria), indicating that a diverse, productive, and highly connected food web exists in this invasive habitat.

An invasive and a native macrophyte species provide similar feeding habitat for fish

AbstractInvasive macrophytes usually increase the homogenisation of habitats and can affect local communities with negative effects on the food chain. We evaluated experimentally the composition, density, richness and diversity of the invertebrates in the invasive macrophyte Hydrilla verticillata and assessed the potential effects of this macrophyte on the diet and foraging efficiency of small native fishes. The native macrophyte Egeria najas was used as control, since it has very similar physical structure and physical complexity to H. verticillata. Macrophytes of both species were maintained separately in aquaria with the fishes, and the invertebrates richness and abundance associated with plants and within fish stomach were evaluated. No significant difference was found in the richness and diversity of invertebrates attached to macrophytes; however, the two macrophytes differed in regard to invertebrate composition, and the density was significantly higher in H. verticillata. The fish foraging efficiency was high in both macrophytes, and the difference between them was not significant. The fish diet did not differ significantly between the native and the invasive macrophyte in respect to invertebrate composition, abundance and richness, and trophic niche breadth. We found no evidence that H. verticillata negatively affects the density of associated invertebrates, and as a consequence, fish diet and foraging efficiency. Our results indicate that the replacement of E. najas by a similar biomass of H. verticillata does not affect the feeding behaviour of associated fish species.

Mutual Facilitation Among Invading Nuttall's Waterweed and Quagga Mussels.

Nuttall’s waterweed (Elodea nuttallii) is the most abundant invasive aquatic plant species in several European countries. Elodea populations often follow a boom-bust cycle, but the causes and consequences of this dynamics are yet unknown. We hypothesize that both boom and bust periods can be affected by dreissenid mussel invasions. While mutual facilitations between these invaders could explain their rapid parallel expansion, subsequent competition for space might occur. To test this hypothesis, we use data on temporal changes in the water quality and the abundance of E. nuttallii and the quagga mussel Dreissena rostriformis bugensis in a temperate shallow lake. Lake Müggelsee (Germany) was turbid and devoid of submerged macrophytes for 20 years (1970–1989), but re-colonization with macrophytes started in 1990 upon reductions in nutrient loading. We mapped macrophyte abundance from 1999 and mussel abundance from 2011 onwards. E. nuttallii was first detected in 2011, spread rapidly, and was the most abundant macrophyte species by 2017. Native macrophyte species were not replaced, but spread more slowly, resulting in an overall increase in macrophyte coverage to 25% of the lake surface. The increased abundance of E. nuttallii was paralleled by increasing water clarity and decreasing total phosphorus concentrations in the water. These changes were attributed to a rapid invasion by quagga mussels in 2012. In 2017, they covered about one-third of the lake area, with mean abundances of 3,600 mussels m−2, filtering up to twice the lake’s volume every day. The increasing light availability in deeper littoral areas supported the rapid spread of waterweed, while in turn waterweed provided surface for mussel colonization. Quantities of dreissenid mussels and E. nuttallii measured at 24 locations were significantly correlated in 2016, and yearly means of E. nuttallii quantities increased with increasing mean dreissenid mussel quantities between 2011 and 2018. In 2018, both E. nuttallii and dreissenid abundances declined. These data imply that invasive waterweed and quagga mussels initially facilitated their establishment, supporting the invasional meltdown hypothesis, while subsequently competition for space may have occurred. Such temporal changes in invasive species interaction might contribute to the boom-bust dynamics that have been observed in Elodea populations.

Liberación de oxígeno radial por las raíces de las plantas nativas de humedales tropicales costeros de Veracruz en respuesta a diferentes condiciones de inundación

Antecedentes: La liberación de oxígeno por las plantas de humedales es un proceso fisiológico que oxigena el sedimento favoreciendo la actividad de los microorganismos aeróbicos, dicha actividad impacta los servicios ambientales de los humedales. Poco se sabe sobre la liberación de oxígeno por las plantas nativas de los humedales de México.Preguntas: ¿Cuál de las macrófitas más abundantes de los humedales de Veracruz, presenta la mayor liberación de oxígeno radial? y ¿Las condiciones de inundación tienen algún efecto en la liberación de oxígeno radial de las macrófitas nativas de los humedales costeros?Área de estudio: Veracruz, México.Métodos: Se mido la producción y porosidad de la raíz, y la liberación de oxígeno por el método de citrato de titanio en 7 de las especies nativas de los humedales de Veracruz, crecidas bajo tres condiciones hidrológicas: capilaridad, saturación e inundación.Resultados: Las especies con mayor peso y volumen de raíz ( Pontederia sagittata, Sagitaria lancifolia y Thalia geniculata ) mostraron bajas liberaciones de oxígeno radial en base a peso seco. La liberación de oxígeno por planta bajo condiciones de inundación mostró diferencias significativas entre las especies, Typha domingensis mostró mayor liberación de oxígeno (148 ±46 µmol O2 d-1) y Leersia ligularis la menor (22 ± 46 µmol O2 d-1). Conclusión: La inundación ocasionó una disminución en la producción de biomasa de las raíces de las plantas nativas de los humedales de Veracruz, un aumento en la porosidad y estimuló la libración de oxígeno por planta, encontrándose diferencias significativas entre las plantas estudiadas.

Aspects of Invasiveness of Ludwigia and Nelumbo in Shallow Temperate Fluvial Lakes.

The relationship between invasive plant functional traits and their invasiveness is still the subject of scientific investigation, and the backgrounds of transition from non-native to invasive species in ecosystems are therefore poorly understood. Furthermore, our current knowledge on species invasiveness is heavily biased toward terrestrial species; we know much less about the influence of allochthonous plant traits on their invasiveness in aquatic ecosystems. In this paper, we present the results of a study on physiological and ecological traits of two introduced and three native macrophyte species in the Mantua lakes system (northern Italy). We compared their photophysiology, pigment content, leaf reflectance, and phenology in order to assess how the invasive Nelumbo nucifera and Ludwigia hexapetala perform compared to native species, Nuphar lutea, Nymphaea alba, and Trapa natans. We found L. hexapetala to have higher photosynthetic efficiency and to tolerate higher light intensities than N. nucifera and the native species especially at extreme weather conditions (prolonged exposure to high light and higher temperatures). Chlorophyll a and b, and carotenoids content of both allochthonous species were substantially higher than those of native plants, suggesting adaptive response to the ecosystem of Mantua lakes system. Higher variability of recorded data in invasive species was also observed. These observations suggest advanced photosynthetic efficiency of the invasive species, especially L. hexapetala, resulting in faster growth rates and higher productivity. This was supported by the evaluation of seasonal dynamics mapped from satellite remote sensing data. This study provides empirical evidence for the relationship between specific plant physiological traits and invasiveness of aquatic plant species, highlighting the importance of trait studies in predicting ecosystem-level impacts of invasive plant species.

Association Between the Success of an Invasive Macrophyte, Environmental Variables and Abundance of a Competing Native Macrophyte.

The success of invasive species depends on the overcoming of abiotic and biotic filters. Abiotic variables likely have greater relative importance over invasion at broad spatial scales, while biotic interactions are more important at fine spatial scales. In this study, we tested the hypotheses that (i) the abundance of the invasive Hydrilla verticillata is more correlated with abiotic factors than with competing native species at broad spatial grain; and that (ii) H. verticillata abundance is more correlated with competing native species than with abiotic factors at fine spatial grain. Here, we considered spatial scale as the grain size (i.e., the extent of sampling unit) assuming broad spatial scales as a large area encompassing the entire patches of macrophytes, and fine spatial scales as a small area inside one macrophyte patch. We collected the abundance of hydrilla and the competing native species along with environmental variables in a large subtropical reservoir. To evaluate how the relative importance of the abiotic factors and the competing native species vary between spatial grains we used Bayesian Generalized Linear Models. At broad grain, the abundance of the competing native species, maximum fetch (positive correlation), turbidity and conductivity (negative correlation) were the most important factors to explain the hydrilla abundance. At fine grain, alkalinity, total organic matter of the sediment and the abundance of a competitive native species (all negative correlations) were the most important variables. Our results indicate a greater importance of abiotic factors at broader grains while competitive interactions seem to be important only in the finer spatial grains. Environmental heterogeneity may explain the positive correlation between native and invasive abundances at broad grain, while the negative correlation at fine grain suggests the effect of competition. In synthesis, we show that the abiotic factors that explain the invasion success of a submerged invasive macrophyte are the same in two spatial grains, but the importance of biotic interactions changed with grain. Thus, our data suggest that models that attempt to explain the success of invasive plants, should consider spatial scales.

Native snails choose an invasive macrophyte over a native macrophyte as a food resource

Invasive species cause ecological and economic impacts on invaded ecosystems, although the presence of native species hampers the propagation of invasive species due to biotic resistance. We tested the effects of grazing by the native channeled applesnail (Pomacea canaliculata (Lamarck, 1828)) over the invasive macrophyte Hydrilla verticillata (L. f.) Royle (water thyme) and the native macrophyte Egeria najas Planch to evaluate the potential of herbivory as a mechanism to resist invasion. Both macrophyte species were offered, individually and combined, as food resources to the snail to evaluate its feeding preference. The macrophyte energy content and stiffness were also measured. The results indicate a higher H. verticillata biomass consumption by P. canaliculata when this macrophyte was available both individually and combined with E. najas, suggesting that H. verticillata is more palatable to the snail, despite this macrophyte being stiffer than the native one. Therefore, P. canaliculata may offer resistance to H. verticillata invasion. The feeding preference and high rates of herbivory by this snail on H. verticillata likely are associated with the higher energy content of the invasive, compared with the native, macrophyte. Experiments combining different food availabilities and snail densities are necessary to evaluate the biological control capacity of P. canaliculata under different scenarios.

Role of sediment structuring by detritus on colonization and interspecific competition of one native and one invasive submerged macrophyte

The deposition of plant detritus changes sediment features, but little is known about how the accumulation of detritus affects the colonization of invasive and native submerged macrophytes. We tested the predictions that (i) submerged macrophyte occurrences correlate positively with the presence of detritus over sediment; (ii) the colonization of submerged macrophytes increases in the presence of detritus; (iii) the invasive macrophyte Hydrilla verticillata grows faster than the native macrophyte Egeria najas independent of the presence of detritus and (iv) E. najas is affected by competition with H. verticillata, and competition is mediated by the presence of detritus. We evaluated the co-occurrences of submerged macrophytes and detritus in situ and experimentally tested the effects of detritus on submerged macrophyte growth with and without competition. The presence of submerged macrophytes and detritus was negatively correlated in situ. Our experiments indicate that the detritus of emergent macrophytes, including invasive macrophytes, enhances nutrients after reflooding and increases the early growth of submerged macrophytes but does not influence the outcome of competition. Thus, the facilitation of invasive success at the beginning of detritus decomposition (indicated by our experiments) may be counteracted by negative effects after a long decomposition period (indicated by our field data).

Response of Macrophyte Traits to Herbivory and Neighboring Species: Integration of the Functional Trait Framework in the Context of Ecological Invasions.

With the increase in the number of introduced species each year, biological invasions are considered as one of the most important environmental problems for native biodiversity. In invaded habitats, the establishment of exotic plant species depends on the abiotic and biotic environment. Herbivores and neighboring plants (native or exotic) comprise an important part of the latter. Herbivores cause trophic and non-trophic damage to focal plants, which respond to herbivory by varying their different traits quantitatively (e.g., growth rate and biomass changes) and qualitatively (e.g., variation in morphological and chemical defenses strategies affecting plant palatability). Neighboring plant species also affect functional traits and the fitness of focal plant species, thus herbivore effects on a focal plant could also depend indirectly on the palatability and defensive traits of the neighboring species inside the community. Here, in a first step toward the integration of associational susceptibility/resistance theories in the field of ecological invasion, we performed a microcosm experiment to consider the effects of an exotic crayfish on the growth rate, morphological traits and damage level of three macrophytes (two exotic, one native) growing in pairwise combinations. We found that (i) the response to herbivore presence and to neighboring species identity seemed to be species specific, and (ii) crayfish enhance the fragmentation rate of the two exotic macrophytes Ludwigia grandiflora and Egeria densa in the presence of the native macrophyte Myriophyllum spicatum, which could indirectly facilitate their invasion success. Indeed, fragmentation can increase dispersal abilities of the exotic macrophytes considered in this study as they are able to generate new plants from their fragments. However, our results showed that the interaction herbivore-neighbor species was hardly significant. Our paper presents some first results on associational resistance/susceptibility and lays the foundation for developing a general framework that combines plant community ecology and biological invasion ecology to explain invasive species success.

The relative roles of dispersal and establishment for shaping whole-lake aquatic macrophyte richness, diversity, and community structure in hemiboreal lakes

ABSTRACTAquatic macrophyte diversity and structure play an important role in freshwater communities, yet little research on macrophytes has focused on native communities in the undisturbed hemiboreal lakes of North America. We used data collected from 15 inland lakes on Isle Royale National Park (Michigan, USA) to identify relationships among macrophyte metrics that describe dispersal and introduction potential (richness, diversity, community structure via growth form, and connectivity) and lake metrics that control establishment and growth potential. Results from partial least square regression analyses found that the most important and recurring predictors were those corresponding to establishment and growth. Alkalinity was the major contributor to the variation seen in richness, Shannon evenness, and Simpson inverse diversity metrics. Sediment phosphorus also contributed significantly to richness and diversity, but negatively. For growth form, half of the variation in submersed species related to alkalinity while almost half of variation in emergent species related to alkalinity and water color. Most of the variation in floating-leaved species was negatively related to alkalinity and water color. Our results suggest a high relative importance of establishment and growth metrics compared to introduction and dispersal metrics for understanding native macrophyte communities in North American hemiboreal lakes.

Aquatic vegetation community structure response to hydrilla management with triploid grass carp, herbicide, and native vegetation planting

Schad AN, Dick GO. Aquatic vegetation community structure response to hydrilla management with triploid grass carp, herbicide, and native vegetation planting. Lake Reserve Manage. 34:417–425.To aid conservation managers conducting hydrilla (Hydrilla verticillata (L.f.) Royle) management, we evaluated changes in aquatic vegetation community structure and dominance in dioecious hydrilla-infested ponds by applying several management tools and monitoring vegetation changes on an annual basis over a 4 yr period. The study was conducted in fully vegetated (dioecious hydrilla and native macrophytes) 0.3 ha mesocosm ponds using 4 levels of triploid grass carp (Ctenopharyngodon idella Valenciennes) stocking densities (0–129 per vegetated hectare), a one-time second-year herbicide treatment to reduce standing biomass of hydrilla and other submersed aquatic vegetation (SAV), and native plantings to provide the basis for replacing hydrilla as it was controlled. Grass carp stocking density had an inverse relationship with coverage of total vegetation, hydrilla, volunteer SAV, and volunteer emergents. All vegetation categories recovered from the one-time herbicide application, although this was suppressed by higher densities of grass carp. Grass carp did not significantly affect planted growth forms (SAV, floating-leaved, and emergent), with increases in mean coverage in all treatments through time, although coverage was lowest at the highest grass carp density. Vegetation dominance shifted from hydrilla to native in both control and low grass carp density treatments, with most SAV eliminated from treatments stocked with more than 72 per hectare.

High phenolic content fails to deter mesograzer consumption of Myriophyllum spicatum (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the USA. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of herbivores in determining milfoil invasion success via the potential mechanisms of enemy release and biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured macrophyte phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was more than two times higher than in the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory. Furthermore, amphipod consumption rates in our study were an order of magnitude higher than amphipod consumption rates reported in milfoil’s native range, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis in choice experiments. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed 2.5 times more milfoil than E. canadensis in a choice feeding trial. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance.

Macrophytes are highly sensitive to the herbicide diquat dibromide in test systems of varying complexity

The herbicide diquat dibromide is used in North America to manage nuisance macrophytes. However, its effect on native macrophytes is less clear and it could cause indirect effects on other aquatic biota. This study determined the sensitivity of both native and non-native macrophytes grown in test systems with varying complexity to diquat dibromide applied directly to water following label directions. In an outdoor mesocosm experiment and single species greenhouse concentration-response tests, Elodea canadensis Michx., Myriophyllum spicatum L., Ceratophyllum demersum L. and Hydrocharis morsus-ranae L. were exposed to a range of diquat dibromide concentrations (4.7 – 1153 µg/L), corresponding to 0.4 – 100% of the recommended label rate of the formulated product. The mesocosm experiment contained all four plant taxa in the same system along with caged amphipods (Hyalella azteca Saus.), tadpoles (Lithobates pipiens Schreber), phytoplankton and periphyton; however, this study focuses on the macrophytes only. In both test systems, severe direct effects of diquat dibromide on macrophytes were detected, with almost 100% mortality of all macrophytes in both test systems at 74 µg/L. The most sensitive species in the single species tests, E. canadensis, showed almost 100% mortality at concentrations below the HPLC-based method detection limit of 5 µg/L. Effects occurred very rapidly and showed no difference in severity between native and non-native macrophytes or complexity of test systems. These results suggest that diquat dibromide could be applied at a considerably lower label rate, depending on the characteristics of the waterbody, while still achieving effective control of nuisance macrophytes.

Effective phytoremediation of low-level heavy metals by native macrophytes in a vanadium mining area, China.

Heavy metal contamination, particularly vanadium contamination in mining and smelting areas, is a worldwide serious problem threatening the ecological system and human health. The contamination level of vanadium, arsenic, cadmium, chromium, mercury, and lead in sediments and waters in a vanadium mining area in China was investigated in the present study. The behavior of heavy metal uptake by 12 native aquatic macrophytes was evaluated, including 5 species of emergent aquatic plants (Acorus calamus, Scirpus tabernaemontani, Typha orientalis, Phragmites australis, and Bermuda grass), 3 species of floating plants (Marsilea quadrifolia, Nymphaea tetragona, and Eleocharis plantagineiformis), and 4 species of submerged plants (Hydrilla verticillata, Ceratophyllum demersum, Myriophyllum verticillatum, and Potamogetom crispus). Different heavy metal accumulation abilities were found across these macrophytes. Generally, they tended to accumulate higher contents of chromium, and C. demersum showed a particularly higher accumulation capacity for vanadium. The heavy metals were preferentially distributed in roots, instead of translocation into leaves and stems, indicating an internal detoxification mechanism for heavy metal tolerance in macrophytes. In 24-day laboratory hydroponic experiments, the macrophytes had a satisfied phytoremediation performance for heavy metals, when their concentrations were at the microgram per liter level. Particularly, vanadium was effectively removed by P. australis and C. demersum, the removal efficiencies of which were approximately 50%. In addition, a combination of terrestrial plant (Bermuda grass) and aquatic macrophytes (P. australis, M. quadrifolia, and C. demersum) exhibited high uptake capacity of all the six heavy metals and their residual concentrations were 95 (vanadium), 39.5 (arsenic), 4.54 (cadmium), 17.2 (chromium), 0.028 (mercury), and 7.9 (lead)μg/L, respectively. This work is of significant importance for introducing native macrophytes to remove low-level heavy metal contamination, particularly vanadium, and suggests phytoremediation as a promising and cost-effective method for in situ remediation at mining sites.

Competitive Effects Hinder the Recolonization of Native Species in Environments Densely Occupied by One Invasive Exotic Species.

The responses of native plants to competition with invasive plants depend mainly on the density of the invasive plants and on the ability of the native plants to compete for resources. In this study, we tested the influence of the invasive exotic Urochloa arrecta (Poaceae) on the early colonization of two native species (Pontederia cordata and Leersia hexandra) of aquatic macrophytes. Our hypotheses were (i) the competitive effects of U. arrecta on the native species P. cordata and L. hexandra are density-dependent and that (ii) these species respond differently to competitive interactions with the invasive species. We conducted the experiments in a greenhouse and in the field, in a tropical reservoir. The biomass of U. arrecta (ranging from 206.2 to 447.1 g) was manipulated in the greenhouse in trays with different densities. After the establishment of the invasive species, we added P. cordata and L. hexandra propagules to each tray. In the field, a propagule of P. cordata was planted in 36 sites with different densities of U. arrecta. The biomass and length of the natives and the biomass of the invasive species were measured in the greenhouse and in the field experiments. The biomass and length of the native plants decreased with increasing biomass of the exotic species in both experiments, showing that the competition between U. arrecta and native species depends on the density of the exotic species. The root:shoot ratio of L. hexandra decreased with increasing U. arrecta biomass, but the opposite occurred for P. cordata. These results indicate that native species exhibit different strategies of biomass allocation when interacting with U. arrecta. The strong competitive effects of U. arrecta and the different responses of the native species help to explain the reduced diversity of native macrophytes observed in sites colonized by U. arrecta. The results also suggest that in a scenario of dominance of exotic species, recolonization by native macrophytes is unlike to occur naturally and without human interventions that reduce the biomass of the exotic species.

Relationships between the spread of Caulerpa filiformis and fish communities on temperate rocky reefs.

The previously sub-dominant native marine macrophyte Caulerpa filiformis is now dominant on many sub-tidal rocky reefs in New South Wales (NSW), Australia and is expanding its distribution. As C. filiformis is highly chemically defended and structurally different to co-occurring habitat-forming macrophytes, two key attributes that govern fish assemblages, we hypothesized that fish assemblages, particularly herbivorous fishes, would be different at sites where C. filiformis occurred from where it was previously absent and within sites, fish community structure would be correlated to the cover of C. filiformis. We investigated these hypotheses by determining reef-associated fish assemblage attributes (assemblage structure, species richness, total abundance, Shannon-Weiner diversity, abundance of herbivorous species) along transects within sites where C. filiformis was present and absent. Surprisingly, despite large patches and very high densities of C. filiformis on the reefs we sampled, at larger spatial scales (i.e., among sites) no fish assemblage metrics differed between sites with large stands of C. filiformis and sites without the alga. Moreover the abundance of one dominant herbivore, the rock cale Aplodactylus lophodon, was greater at sites within large beds of C. filiformis. At smaller spatial scales, however, i.e. within sites where C. filiformis was present, fish assemblages did vary as a function of C. filiformis cover along transects, although this was not consistent across sampling times. Overall, our results suggest that the potential effects of the spread of this alga on faunal communities warrants further investigation.