Dominant Macrophyte Research Articles

Changes in biodiversity and environmental stressors influence community structure of an experimental eelgrass Zostera marina system

Changes in biodiversity can result in decreased ecosystem functioning and loss of ecosystem services, but altered biodiversity is only one of many stressors impacting ecosystems. In many estuaries, environmental stressors such as warming water temperatures and eutrophication are increasing and negatively impacting biological communities, particularly seagrasses such as the important habitat-forming species Zostera marina (eelgrass). These negative impacts may change the diversity, composition, and functioning of this important ecosystem, but the interac- tions of stressors with changes in biodiversity are poorly understood. We manipulated eelgrass communities in a factorial experiment to test how changes in crustacean grazer diversity, warmer water temperatures, and nutrient enrichment interact to affect ecosystem biomass, stability, and community composition. We found that the presence and richness of crustacean grazers had a larger effect on grazer, algal, and sessile invertebrate biomass than experimental warming or nutrient enrichment. Diverse grazer assemblages stabilized epiphytic algal biomass in the face of stressors, and counteracted the promotion of epiphytic microalgae by stressors. Nutrient enrich- ment and warming both promoted epiphytic microalgae, while reducing macroalgae and eel- grass. A more diverse grazer assemblage stabilized epiphytic algal biomass, but we did not detect interactions among environmental stressors and grazer diversity. These results emphasize that loss of herbivore diversity can exacerbate the impacts of environmental stressors on grazing, rel- ative dominance of microalgae versus macrophytes, producer biomass, and stability.

Aquatic macrophytes in a tropical African reservoir: diversity, communities and the impact of reservoir-level fluctuations

ABSTRACT Variation in species composition, distribution and abundance of macrophytes in the Malilangwe reservoir, a tropical African reservoir, was investigated at four sites in three seasons; March (hot-wet season), June (cool-dry season) and September (hot-dry season) over a nine-month period. Thirteen macrophyte species, representing nine families, were recorded during the study period. Submerged macrophytes had a patchy distribution and the mean transect cover per species was below 3% with the exception of Ceratophyllum demersum and Potamogeton pusillus. Canonical Correspondence Analysis (CCA) identified four distinct macrophyte groups closely associated with the three seasons; hot-wet, cool-dry and hot-dry and environmental factors; pH, dissolved oxygen, phosphorus and water level. Change of seasons and subsequent fluctuations in water levels resulted in changes in macrophyte community structure from the dominant emergent macrophytes (Cyperus involucratus, Panicum repens, Ludwigia stolonifera, Phragmites mauritianus and Schoenoplectus corymbosus) in the hot-wet season to submerged macrophytes (Najas horrida, Potamogeton crispus, Potamogeton pusillus, Potamogeton tricarinatus and Persicaria decipiens) in the hot-dry season. Although changes in water level variations seemed to influence macrophyte structure and composition changes, further research is needed to evaluate to what extent water level fluctuations interact with other seasonal factors acting independently.

Marsh macrophyte responses to inundation anticipate impacts of sea-level rise and indicate ongoing drowning of North Carolina marshes

In situ persistence of coastal marsh habitat as sea level rises depends on whether macrophytes induce compensatory accretion of the marsh surface. Experimental planters in two North Carolina marshes served to expose two dominant macrophyte species to six different elevations spanning 0.75 m (inundation durations 0.4–99 %). Spartina alterniflora and Juncus roemerianus exhibited similar responses—with production in planters suggesting initial increases and then demonstrating subsequent steep declines with increasing inundation, conforming to a segment of the ecophysiological parabola. Projecting inundation levels experienced by macrophytes in the planters onto adjacent marsh platforms revealed that neither species occupied elevations associated with increasing production. Declining macrophyte production with rising seas reduces both bioaccumulation of roots below-ground and baffle-induced sedimentation above-ground. By occupying only descending portions of the parabola, macrophytes in central North Carolina marshes are responding to rising water levels by progressive declines in production, ultimately leading to marsh drowning.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-012-2076-5) contains supplementary material, which is available to authorized users.

Litter Decomposition of Six Macrophytes in a Eutrophic Shallow Lake (Baiyangdian Lake, China)

AbstractThe decomposition of aquatic macrophytes has important consequences for wetlands because it is closely related to organic matter accumulation and nutrient cycling. A field litter bag experiment was undertaken to investigate the decomposition rates of six dominant macrophytes. And the level of nutrient transfer from plant residues to lake water was also researched in Baiyangdian Lake, a typical macrophyte‐dominated lake in the Northern China Plain. Intact standing plants of Phragmites australis, Typha angustifolia, Nelumbo nucifera, Potamogeton pectinatus, Ceratophyllum demersum, and Chara sp. were collected and different plant tissues incubated near sediment traps. Litter bags were collected at days 6, 26, 60, 104, and 140 to follow biomass loss and nutrient release. Decomposition rates of different macrophytes calculated from an exponential model ranged from 0.0028 to 0.0110 day−1. For the same external environmental conditions, decomposition rates were significantly affected by initial litter chemistry. The remaining biomass was positively related to the C/N and C/P ratios and negatively related to the N and P concentrations and the N/P ratio. Both stems and roots showed a lower decomposition rate than the leaves of the same species due to the higher C/N ratio. The nutrient release from plant litter to the lake water was substantial. Over the incubation period, the release of N from the litter represented 18.10–88.79% of the initial content, and the release of P from the litter represented 24.26–92.83% of the initial content.

Carbon availability limits potential denitrification in watercress farm sediment

Wetlands are widely used to improve water quality, in particular for removal of nutrients such as nitrates. Watercress farms are artificial wetlands that could potentially reduce transfers of nitrate to rivers; however, little is known about denitrification occurring in these particular wetlands. Watercress farming is a declining activity, which could have effects on nitrate removal in these wetlands. Therefore, we investigated the potential for denitrification in sediments from cultivated and abandoned watercress farms using flow-through reactors in the laboratory. Nitrate reduction rates ranged from 1.03 to 7.51μmol NO3−cm−3day−1 (mean±SD: 3.2±2.1μmol NO3−cm−3day−1). In general, sediments downstream of watercress beds exhibited higher nitrate reduction rates and contained more organic carbon than upstream sediments. The highest potential nitrate reduction rates were reached by sediments from cultivated watercress beds. As carbon was hypothesized to be a key factor in controlling nitrate reduction rates, the effects of different carbon sources on these rates were investigated by adding carbon to the sediment. Carbon was added either as labile organic acids (acetate or a mixture of lactate, fumarate, and succinate) or in the form of site-specific macrophytes (watercress [Nasturtium officinale] and reed [Phragmites australis]). Nitrate reduction rates increased significantly upon supply of organic acids or macrophytes. The addition of watercress resulted in the highest potential nitrate reduction rates. This study shows that denitrification in watercress farms is carbon-limited and suggests that the abandonment of watercress farms, with declines in macrophyte production and shifting macrophyte dominance, is likely to decrease the ability of these wetlands to remove nitrate by denitrification. Nitrate removal in watercress farms could be improved by supplying easily available organic C or by management of the vegetation, in particular in the upstream part of the beds.

Effects of substrate and shading on the growth of two submerged macrophytes

Excessive nutrient loading may cause a shift from submerged macrophyte dominance to free-floating macrophyte dominance. Tolerance and persistence of submerged plants in response to shade may be key characteristics in determining when/if such a shift occurs in shallow eutrophic lakes. This study examines how the cover of floating macrophyte (Lemna minor) and shade of dark mesh affect the growth and photosynthetic efficiency of two submerged plants (Vallisneria natans and Myriophyllum spicatum) on different nutrient substrates. We found that low- and mid-cover intensities generally enhanced the leaf/shoot growth of both submerged plants under all cover and substrate types. The relative growth rates (RGR) were slightly enhanced under the treatment of Lemna with low- and mid-intensity cover on both nutrient-rich substrates. The leaf/shoot growth and RGR of both submerged macrophytes generally increased more under Lemna cover than mesh cover. The photosynthetic efficiency (Fv/Fm value) typically increased with the duration of treatment and the cover densities. In addition, these two macrophytes with contrasting growth forms have markedly different growth and survival strategies in response to covers. These results strengthen the hypothesis that submerged plants can successfully develop under a low-intensity cover of floating vegetation on nutrient-rich substrate.

Emergent Macrophytes Act Selectively on Ammonia-Oxidizing Bacteria and Archaea

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) were quantified in the sediments and roots of dominant macrophytes in eight neutral to alkaline coastal wetlands. The AOA dominated in most samples, but the bacterial-to-archaeal amoA gene ratios increased with increasing ammonium levels and pH in the sediments. For all plant species, the ratios increased on the root surface relative to the adjacent bulk sediment. This suggests that root surfaces in these environments provide conditions favoring enrichment of AOB.

Factors involved in spatiotemporal dynamics of submerged macrophytes in a Portuguese coastal lagoon under Mediterranean climate

Submerged macrophytes are key in coastal ecosystems, with important structural and functional roles. Thus, the characterization of the submerged aquatic vegetation dynamics is of prime importance for assessing the ecological status of coastal ecosystems. The main aim of this study was to assess the spatial and temporal dynamics of submerged macrophytes biomass in a temporarily open coastal lagoon in Melides, Portugal, and to evaluate the physiological performance of the dominant macrophyte in the system. This lagoon is subject to several disturbances such as the dramatic changes in water physicochemical parameters over time, since temporarily opens to the sea occur renewing the lagoon water content. Moreover it is under a Mediterranean climate with the inherently high temporal variability of precipitation and high temperatures during the summer. Our study shows that the submerged macrophyte community was dominated initially by Ruppia cirrhosa and then replaced by Potamogeton pectinatus, which showed a marked temporal and spatial pattern with extremely high values of biomass in August and in the inner part of the lagoon. The spatial and temporal biomass patterns in submerged macrophytes were mainly related with water level, salinity, pH and transparency. The physiological performance of R. cirrhosa was lower when there was greater biomass accumulation and in extremely shallow waters, whereas physiological vigor seemed to be greater at moderate temperatures and in areas with higher salinity and dissolved oxygen. The data from this study can be used to predict submerged biomass macrophyte responses to extreme water quality changes and to feed a decision support system for the best period to promote the seasonal artificial breaching of the sandy barrier.

Multi‐scale patterns of moss and lichen richness on the Antarctic Peninsula

Mosses and lichens are the dominant macrophytes of the Antarctic terrestrial ecosystem. Using occurrence data from existing databases and additional published records, we analyzed patterns of moss and lichen species diversity on the Antarctic Peninsula at both a regional scale (1°latitudinal bands) and a local scale (52 and 56 individual snow‐ and ice‐free coastal areas for mosses and lichens, respectively) to test hypothesized relationships between species diversity and environmental factors, and to identify locations whose diversity may be particularly poorly represented by existing collections and online databases. We found significant heterogeneity in sampling frequency, number of records collected, and number of species found among analysis units at the two spatial scales, and estimated species richness using projected species accumulation curves to account for potential biases stemming from sample heterogeneity. Our estimates of moss and lichen richness for the entire Antarctic Peninsula region were within 20% of the total number of known species. Area, latitude, spatial isolation, mean summer temperature, and penguin colony size were considered as potential covariates of estimated species richness. Moss richness was correlated with isolation and latitude at the local scale, while lichen richness was correlated with summer mean temperature and, for 17 sites where penguins where present with <20 000 breeding pairs, penguin colony size. At the regional scale, moss richness was correlated with temperature and latitude. Lichen richness, by contrast, was not significantly correlated with any of the variables considered at the regional scale. With the exception of temperature, which explained 91% of the variation in regional moss diversity, explained variance was very low. Our results show that patterns of moss and lichen biodiversity are highly scale‐dependent and largely unexplained by the biogeographic variables found important in other systems.

Litter quality and interactions of macrophytes tissues decomposition in a eutrophic shallow lake

Abstract Macrophyte decomposition is critical for eutrophic shallow lakes because it can influence the accumulation of the sediment layer and lake terrestrialization processes. As three dominant macrophytes in Lake Baiyangdian, N. nucifera ( Nelumbo nucifera ), P. australis ( Phragmites australis ), and T. latifolia L ( Typha latifolia L ) have spread quickly and covered a large proportion of the lake as a result of water nutrient enrichment over the past 30 years. In this study, we investigated the decomposition processes of these three plants using the litterbag method to demonstrate their roles in lake degradation. Decomposition rates of plant tissues (leaves, stems, and roots) and whole plants were determined at three sampling sites that had different nutrient levels. The litter samples displayed a wide range of decomposition rates. Of all samples, the leaves of N. nucifera decomposed the most rapidly, while the stems of P. australis had the lowest decomposition rates. The decomposition correlated significantly with the initial litter phosphorus (P) and nitrogen (N) content, and the ratios of carbon (C) to N (C:N) and C to P (C:P), especially the P-related indicators, implying a relative shortage of P in the sampled species. Although not as strong as litter quality, site chemistry effects also can be observed during decomposition. The site with a high nutrient level in the water body exhibited rapid decomposition and high decomposition rates. Interactions of plant tissues (expressed as the differences between observed and expected decomposition rates) were tested when we investigated the decomposition processes of the whole plants. Observed decomposition rates of the whole plant individuals significantly deviated from the expected rates that were calculated based on the mass losses of plant tissues of the corresponding species. The strengths of the interactions varied greatly with plant species. T. latifolia L displayed significantly positive interactions among plant tissues, while significantly negative interactions of plant tissues were observed for the other two species. Moreover, our results suggest that the interactions of plant tissues greatly vary among the three sampling sites, indicating that both the biotic and abiotic characteristics of an ecosystem can be important drivers controlling litter decomposition.

Regime shift from phytoplankton to macrophyte dominance in a large river: Top-down versus bottom-up effects

The lower Ebro River (Catalonia, Spain) has recently undergone a regime shift from a phytoplankton-dominated to a macrophyte-dominated system. This shift is well known in shallow lakes but apparently it has never been documented in rivers. Two initial hypotheses to explain the collapse of the phytoplankton were considered: a) the diminution of nutrients (bottom-up); b) the filtering effect due to the colonization of the zebra mussel (top-down). Data on water quality, hydrology and biological communities (phytoplankton, macrophytes and zebra mussel) was obtained both from existing data sets and new surveys. Results clearly indicate that the decrease in phosphorus is the main cause of a dramatic decrease in chlorophyll and large increase in water transparency, triggering the subsequent colonization of macrophytes in the river bed. A Generalized Linear Model analysis showed that the decrease in dissolved phosphorus had a relative importance 14 times higher than the increase in zebra mussel density to explain the variation of total chlorophyll. We suggest that the described changes in the lower Ebro River can be considered a novel ecosystem shift. This shift is triggering remarkable changes in the biological communities beyond the decrease of phytoplankton and the proliferation of macrophytes, such as massive colonization of Simulidae (black fly) and other changes in the benthic invertebrate communities that are currently investigated.

Eutrophication, agriculture and water level control shift aquatic plant communities from floating-leaved to submerged macrophytes in Lake Chini, Malaysia

In this study we: (1) present a quantitative spatial analysis of the macrophyte communities in Lake Chini with a focus on the biogeographical distributions of the native Nelumbo nucifera and the invasive Cabomba furcata; (2) examine the environmental changes that affect plant community composition; and (3) outline a conceptual model of the variation of ecological processes that shape the macrophyte communities. Plant species cover, biomass of C. furcata and N. nucifera, and water quality and environmental variables were measured before and after monsoonal floods in September 2009 and April 2010. Permutational multivariate analysis was used to examine the significance of the invasion of C. furcata at different spatial scales. Relationships between plant species cover and environmental variables before and after flooding were examined using principal coordinates analysis and non-parametric multivariate multiple regressions. Our findings suggest that (1) Variation in plant communities was significant at the lake scale and the distribution of plant species changed after annual floods. (2) Invasion by C. furcata significantly affected the overall plant community composition. (3) C. furcata biomass increased after the monsoonal season, which indicates that C. furcata is adapted to flooding events and that it is becoming increasingly abundant. (4) In addition to the strong monsoonal effect, total depth, nutrient concentration, and sediment type were important environmental variables that significantly affected plant community composition. The macrophyte community in Lake Chini is highly dynamic. The spatial and temporal plant community dynamics are associated with flood regime, water quality, and substrate. Human-induced changes in these parameters are likely shifting the macrophyte dominance from floating-leaved to submerged species.

Vegetation cyclic shift in eutrophic lagoon. Assessment of dystrophic risk indices based on standing crop evaluations

Orbetello lagoon (Tuscany, Italy) is a meso-eutrophic, shallow-water ecosystem which has undergone cyclic shifts in macrophyte dominance since 1970. Field data on the total standing crops of Chlorophyceae and Rhodophyceae (from 1983 to 2011) and Angiospermae (seagrass; from 1998 to 2007), produced each year by the ecosystem, was acquired. A general lagoon environment quality score (decay level DL, categories 0–4) was attributed (a posteriori) for each annual dataset examined. Univariate and multivariate statistical analyses were used to relate total macrophyte standing crops of 29 years of monitoring to the general quality score. Two indices were proposed (Abundance of Macroalgae Index, AMI, and Abundance of Seagrass Index, ASI, the latter in two versions, ASI-1 and ASI-2) and tested against DL. The results showed that biomass data did not accurately describe general environmental quality of the lagoon ecosystem, whereas the indices gave a better fit. AMI showed the best performance, demonstrating that macroalga data was much more informative than seagrass data. Values of AMI over 4.0 were significantly associated with critical general quality scores (DL > 2).

Growth of the dominant macrophyte Carex aquatilis is inhibited in oil sands affected wetlands in Northern Alberta, Canada

Carex aquatilis could be a suitable species for wetland reclamation in mined boreal landscapes as those of the oil sands industry (Northern Alberta). We compared the performance of C. aquatilis plants established in oil sands industrial wetlands (directly affected by processed materials), on-site indirectly affected wetlands, and off-site natural wetlands. We studied leaf chemistry, fluorescence, gas exchange rates (net photosynthesis and transpiration rates), and morphological features of plants. Despite higher photochemistry efficiency in oil sands populations, plants did not channelize surplus energy into increased carbon assimilation rates. Oil sands populations registered lower culm heights and leaf lengths than natural populations. Plants growing in industrial wetlands were shorter and accumulated more sodium in leaves than plants from indirectly affected wetlands. Evidence indicated that C. aquatilis was a promising species for reclamation as it was tolerant to pollution. Though it survived, C. aquatilis, showed a restricted growth in the oil sands wetlands thereby possibly limiting carbon assimilation at the stand level. Oil sands wetlands amended with freshwater and established over non-industrial materials provided better growing conditions for C. aquatilis and demonstrated management tools for local reclamation efforts.

Ecophysiological studies of the non-indigenous species Gracilaria vermiculophylla (Rhodophyta) and its abundance patterns in Ria de Aveiro lagoon, Portugal

Gracilaria vermiculophylla is the dominant macrophyte in the Ria de Aveiro lagoon, Portugal (40°38′N, 8°43′W), which is a highly urbanized estuary impacted by oyster cultivation and finfish aquaculture. This study aimed to understand the success of G. vermiculophylla in estuaries by monitoring its abundance and phenology throughout an annual cycle. This species thrives in soft-bottom communities, mostly entangled in the substrate amongst dead bivalve shells and tubes of the polychaete Diopatra neopolitana but it may be also found free-floating; it is present year-round. Carposporophytes were found throughout the year and at all sampling locations. The life history of this taxon was completed in the laboratory. Spore germination and growth success were investigated in a fully factorial designed array of temperatures (5, 10, 15, 20 and 25°C), photoperiods (8, 12 and 16 h day-lengths) and photon flux densities (40 and 100 µmol photons m−2 s−1). Germination occurred under all conditions, being highest at 20°C under the long day treatment. Germination was lowest at 5°C, with no spore survival. The growth rates of the sporelings were affected by the interaction of the three factors tested. These were minimal at 10°C (2.21% ± 1.14 in 8-h days for tetrasporophytes and 2.60% ± 0.57 in 12-h days for gametophytes) and maximal at 20°C and long day treatments for both gametophyte and tetrasporophyte phases (9.96% ± 0.62 and 16.86% ± 0.73, respectively). No differences were found in the growth rates of the mature haploid or diploid phases. Gracilaria vermiculophylla is reproductive throughout the year. It attains high rates of germination and growth success under a wide range of environmental conditions. With such broad tolerances to environmental factors, G. vermiculophylla has a high invasive potential, which helps to explain its current dominance in the Ria de Aveiro lagoon.

Fish effects on benthos and plankton in a Mediterranean salt marsh

Fish predation has an important role structuring benthic and planktonic assemblages in shallow waters. Habitat complexity may mitigate the effect of fish predation, thus reducing prey–predator encounters and prey capture ability. In this respect, benthic organisms may be less affected by predation than planktonic organisms because macrophytes would increase habitat complexity, thus providing a physical refuge. However, certain small fish may move and live within vegetation, resulting in an increase in prey–predator encounters, and so physical refuges provided by macrophytes would be less effective. To test the effects of small fish on aquatic invertebrates, we performed a field experiment using mesocosms in a Mediterranean salt marsh, using Aphanius iberus as the small fish species (< 5 cm) and Ruppia cirrhosa as the dominant macrophyte. Although, A. iberus preys on benthic and planktonic organisms, it has traditionally been described as a benthic species. Hence, a stronger effect on benthos is expected if habitat complexity does not reduce the predation ability of this small fish. We therefore analyzed and compared the strength of the effects of A. iberus on both benthic and planktonic organisms. The fish was found to have stronger effects on planktonic than benthic organisms. The presence of A. iberus was coincident with a decline in the abundance of large water-column invertebrates such as gammarids and jellyfish, and an increase in the abundance of medium-sized plankton. The total biomass of the benthic assemblage increased when A. iberus was present. These results suggest that aquatic invertebrates of Mediterranean salt marshes may use macrophytes as refuge from small fish species.

Integrating stomach content and stable isotope analyses to elucidate the feeding habits of non-native sharptooth catfish Clarias gariepinus

Sharptooth catfish Clarias gariepinus was introduced into the Eastern Cape Province, South Africa, in 1976 and there are concerns about its possible negative impacts on native biota. This study investigated its trophic impact by examining its feeding habits. Stomach content and stable isotope analyses were compared from three localities—the Great Fish River, Sundays River and Glen Melville Dam. Stomach content analysis indicated a catholic diet dominated by fish particularly in all localities. Spatially, however, the diets revealed differences based on the dominance of macrophytes that were only present in the rivers, and aquatic invertebrates that appeared more diverse within the Great Fish River compared to other localities. By contrast, stable isotopes revealed a more generalised feeding pattern with no clear dominance of particular prey. Stable isotopes further showed that the catfish was a complex predator, with large catfish being top predators whereas smaller size groups appeared to feed lower in the food chain. An ontogenetic shift in diet was evident, with small fish predominantly consuming aquatic invertebrates and shifting towards fish with increasing size. High dietary overlap suggests the potential risk associated catfish feeding, especially the potential of piscivory by small catfish that are more likely to persist in shallow and marginal where endangered indigenous minnows occur. The alteration of environmental conditions, especially flow by inter-basin water transfer (IBWT) schemes, was inferred to have had a probable influence its invasion success. Occurrence of other invaders, which was facilitated by the IBWT together with the catfish, posits the risk of invasion meltdown within the study systems.

Identification and quantification of aquatic vegetation with hyperspectral remote sensing in western Nevada rivers, USA

This study used a portable spectrometer to assess the feasibility of using airborne hyperspectral imagery to map the dominant types and amounts of aquatic vegetation in the Carson and Truckee Rivers of western Nevada. Spectral reflectance data were acquired for a number of periphyton and macrophyte types, and corresponding vegetation samples were processed in the laboratory to quantify chlorophyll a (chla) and ash-free dry mass. The dominant periphyton and macrophyte communities encountered in the field could be identified with an overall accuracy of greater than 95%. The lowest individual class accuracy was 82% for one community type, primarily green filamentous algae (GF), which was brown in colour and mixed with diatoms and sediments. Separate stepwise regression models were developed for chla and biomass of each type of vegetation. Regression models had r 2s greater than 0.92, except for the aforementioned brown-coloured community of mixed algae that had r 2s of just over 0.5 for both laboratory measurements. This study suggests good prospects for airborne hyperspectral surveys of aquatic vegetation for water quality studies, assuming a sensor with a high signal-to-noise ratio, high spatial resolution and good environmental conditions at the time of image acquisition.

Differential facilitative and competitive effects of a dominant macrophyte in grazed subtropical wetlands

Summary 1. Plant–plant interactions fluctuate between competition and facilitation depending upon ecological conditions and species traits. Facilitative interactions are expected to increase in frequency via associational defences with increasing consumer pressure. The ability of species to cope with competition and ⁄ or ecological stressors may alter the outcome of plant–plant interactions. 2. We conducted a transplant experiment to determine if native and non-native grasses and forbs respond similarly to interactions with Juncus effusus L., an unpalatable benefactor species, along a grazing intensity gradient in two contrasting pasture types: intensively managed and semi-natural. We expected competitive taller, erect species (grasses) and non-natives to obtain stronger facilitative effects than shorter, sprawling species (forbs) and natives. 3. We transplanted two species each of grasses and forbs, including one native and one non-native species of each, into 1-m 2 grazed plots or exclosure plots either with or without Juncus in wetlands experiencing a range of grazing intensities across the two pasture types. 4. Interactions with Juncus varied from facilitation in grazed plots to competition in exclosures for the two grasses and the non-native forb. Juncus did not facilitate the native forb in grazed conditions. Grazing negatively influenced all species. For the grasses, facilitation intensity increased as grazing intensity increased, while facilitation intensity for forbs was unrelated to grazing intensity. 5. Synthesis. Facilitation intensity of a dominant macrophyte increased with consumer pressure for tall grasses in productive subtropical wetlands. These findings suggest that facilitation can allow native and non-native species to pass through human-induced ecological filters, such as cattle grazing, with implications for understanding forces governing community composition in disturbed environments.

Higher abundance and diversity in faunal assemblages with the invasion of Codium fragile ssp. fragile in eelgrass meadows

The present study examined how species-specific attributes of the invasive alga Codium fragile ssp. fragile (hereafter Codium) and eelgrass Zostera marina influenced faunal assemblages associated with eelgrass ecosystems in iles de la Madeleine, eastern Canada. Direct association of species with Codium (<1 m) was evaluated and compared to eelgrass faunal assemblages from invaded beds. Potential neighborhood effects (ca. 10 m) were assessed by comparing invertebrates associated with eelgrass in invaded and non-invaded beds. The influence of Codium on more mobile species was evaluated by comparing fish assemblages associated with macrophytes in invaded and non-invaded beds. In addition, species-area relationships for low mobility species associated with Codium were contrasted between beds where Codium or eelgrass was the dominant structuring macrophyte. Codium had distinct invertebrate species assemblages, as there was a greater abun- dance and diversity of invertebrates associated with Codium than with eelgrass, and multivariate assemblage structure differed between macrophyte types. In contrast, no neighborhood patterns were observed as invertebrate assemblages associated with eelgrass did not differ between invaded and non-invaded beds, suggesting that the mechanisms that account for differences between the invertebrate assemblages are a function of the macrophyte's nature and operate at small spatial scales. Fish community structure differed between invaded and non-invaded beds, largely due to a greater abundance of 2 fishes, Apeltes quadracus and Tautogolabrus adspersus, in invaded beds. Epifaunal abundance and species richness were positively correlated with Codium thallus biomass, and the abundance relative to biomass correlation was greatest where Codium was the dominant macrophyte. Experimental manipulation of thallus structure to create straight or branched Codium fronds with similar surface area showed that associated invertebrate and gastropod abundance was not influenced by this factor and indicated that factors other than macrostructure complexity influ- enced the presence of epifauna on the introduced alga. Overall, the present study suggests that the invasion of eelgrass beds by Codium increases faunal density and diversity. However, longer-term effects of Codium invasion in this and other eelgrass ecosystems require further assessment, as the severity of the invasion effects on faunal communities likely depends on the interaction between Codium and eelgrass.