Snail Biomass Research Articles

Temperature effects on periphyton, epiphyton and epipelon under a nitrogen pulse in low-nutrient experimental freshwater lakes

The ongoing global climate change involves not only increased temperatures but may also produce more frequent extreme events, such as severe rainfall that could trigger a pulse of nutrients to lakes. In shallow lakes, this may affect primary producers through a number of direct and indirect mechanisms. We conducted a six-month mesocosm experiment to elucidate how periphyton (on inert substrata), epiphyton and epipelon biomass responded to a nitrogen (N) pulse, an approximately tenfold enrichment of the NO3-pool, under three contrasting warming scenarios: ambient temperature and ca. +3°C and ca. +4.5°C elevated temperatures (hereafter T1, T2 and T3). After the N pulse, we found a higher periphyton biomass at elevated than at ambient temperatures but no change in epiphyton biomass. Epipelon biomass was lower in T3 than in T1. Both periphyton and epiphyton biomasses correlated negatively with snail biomass, while epiphyton biomass correlated positively with light. Different responses to higher temperatures under short-term extreme nutrient loading conditions may be attributed to differences in the access to nutrient sources and light. Our data suggest that the biomass of periphyton in oligotrophic clear-water lakes will increase significantly under conditions exhibiting short-term extreme nutrient loading in a warmer climate.

Nitrogen processing by grazers in a headwater stream: riparian connections

Summary Primary consumers play important roles in the cycling of nutrients in headwater streams, storing assimilated nutrients in growing tissue and recycling them through excretion. Although environmental conditions in most headwater streams and their surrounding terrestrial ecosystems vary considerably over the course of a year, relatively little is known about the effects of seasonality on consumer nutrient recycling these streams. In this study, we measured nitrogen accumulated through growth and excreted by the grazing snail Elimia clavaeformis (Pleuroceridae) over the course of 12 months in Walker Branch, identifying close connections between in‐stream nitrogen processing and seasonal changes in the surrounding forest. Nitrogen processing rates were positively correlated with ecosystem respiration, which was driven by leaf phenology on streamside trees. Snail nitrogen assimilation, growth and excretion were relatively high in spring before leaf emergence, low in summer when canopy shade was extensive and high again in autumn after leaf‐fall. During the time that snails grazed primarily on epilithon (winter, spring and summer), growth and excretion rates followed changes in light and epilithon biomass. In autumn, when snails primarily grazed fallen leaves, leaf‐associated microbes provided large subsidies of nitrogen for the snails. Nitrogen accumulation in snail biomass was greater in the 2 months following leaf‐fall than at any other time of the year. Snails were less important as nitrogen sinks than as sources of recycled nitrogen in Walker Branch. Over the course of the year, snails excreted approximately 12 times more nitrogen than they accumulated in biomass. Nitrogen accrued during growth in spring was subsequently lost in summer when primary production declined and snails underwent tissue loss. Catabolic losses represented >40% of the nitrogen excreted by the snails in summer. Net nitrogen growth efficiency (growth/assimilation), which varied with food availability, was only 8% for the entire year. Neither growth nor excretion was positively correlated with nitrogen concentrations in grazing substrata. Snails achieved high standing crops and were significant contributors to nitrogen spiralling in Walker Branch. On an areal basis, nitrogen in snail biomass (mgN m−2) was two to five times greater than that in epilithon biomass, depending on the season. Snails assimilated and excreted up to 50% of the nitrogen initially taken up by autotrophs and leaf microbes, and they were likely to have additional effects on nitrogen spiraling through egestion and the cropping of assimilative biomass. Primary consumers like Elimia are important catalysts of nutrient movement through headwater streams, decreasing residence times and facilitating fluxes to downstream waters.

Land spreading of sewage sludge in forest plantations: effects on the growth of the duckweed Lemna minor and trace metal bioaccumulation in the snail Cantareus aspersus.

Wastewater plants generated annually millions of tons of sewage sludge (SS). Large amounts of this organic residue are spread on agricultural lands as a fertilizer, although it is viewed as a major potential source of contamination, presenting a danger to the terrestrial and aquatic environments. This study was undertaken to evaluate the impact of this practice on the duckweed Lemna minor and the snail Cantareus aspersus. Sludge was applied to soil either at six different loading rates equal to 0, 0.4, 3, 10, 30, and 60tons dry matter (DM) ha(-1) for L. minor test or at three rates equal to 0, 30, and 60tons DMha(-1) for C. aspersus test. At the highest rate of SS application (60tons DMha(-1)), the eluates showed that an increase in pH (6.1) resulted in a decrease in Al levels. Thus, the high stimulation of L. minor growth observed after this high rate of SS application can be explained by (i) a reduction in Al toxicity after precipitation and (ii) macro- and micronutrient enrichment. At a rate of SS application of only 30tons DMha(-1), growth appeared to be slightly significant (p < 0.05), in spite of the significant increase in essential mineral elements. However, it is very difficult to discriminate between Al toxicity and pH effects. For the test with C. aspersus, the snail biomass was not affected by sludge application over the exposure period. Mortality was extremely low, with a rate of less than 4% at the last sampling date. Yet, Cu, Pb, and Cd accumulated significantly in the soft body of snails exposed to SS application, suggesting that the amount of metals excreted is lower than that absorbed. In contrast, Zn levels remain constant, inferring that absorption and elimination of Zn are balanced at the beginning of the experiment.

Nghiên cứu ảnh hưởng của mật độ ốc bươu vàng (Pomacea canaliculata) đến sinh trưởng và tỷ lệ sống của ốc bươu đồng (Pila polita)

This study was carried out in 60 days to evaluate the effects of density of golden apple snails (GAS) on the growth and survival rate of black apple snails (BAS). The experiment was designed with 5 treatments such as 1).100% BAS, 2). 75% BAS:25% GAS, 3).50% BAS:50% GAS, 4).25% BAS:75% GAS and 5).100% GAS (by quantity) in triplicates. The snails’ initial weights were from 0.70 - 1.00 g and stocked in cultured tanks at the density of 200 snails/m2. Diet for snails included vegetable and pellet with ratio of 1:1(dry weight) and daily quantity feeding equivalent to 4% of snail biomass. Results showed that growth rate of BAS was lower than GAS in all treatments (p

The response of two submerged macrophytes and periphyton to elevated temperatures in the presence and absence of snails: a microcosm approach

Global warming may affect snail–periphyton–macrophyte relationships in lakes with implications also for water clarity. We conducted a 40-day aquaria experiment to elucidate the response of submerged macrophytes and periphyton on real and artificial plants to elevated temperatures (3°C) under eutrophic conditions, with and without snails present. With snails, the biomass and length of Vallisneria spinulosa leaves increased more at the high temperature, and at both temperatures growth was higher than in absence of snails. The biomass of periphyton on V. spinulosa as well as on artificial plants was higher at the highest temperature in the absence but not in the presence of snails. The biomass of Potamogeton crispus (in a decaying state) declined in all treatments and was not affected by temperature or snails. While total snail biomass did not differ between temperatures, lower abundance of adults (size >1 cm) was observed at the high temperatures. We conclude that the effect of elevated temperature on the snail–periphyton–macrophyte relationship in summer differs among macrophyte species in active growth or senescent species in subtropical lakes and that snails, when abundant, improve the chances of maintaining actively growing macrophytes under eutrophic conditions, and more so in a warmer future with potentially denser growth of periphyton.

Scientific Opinion on the environmental risk assessment of the apple snail for the EU

At EFSA's request, the Plant Health Panel (PLH) performed an environmental risk assessment (ERA) of the apple snail for the EU and validated the ERA approach presented in the PLH ERA guidance document. Improvements and suggestions for simplification of the ERA are provided. One service-providing unit-shallow fresh water areas-was identified for this ERA. The effects of resistance, resilience and management on snail population dynamics in the short (5 years) and the long term (30 years) were estimated. In line with the PLH ERA guidance rating system, expert judgement was used to evaluate separately the impacts on (i) ecosystem traits, (ii) ecosystem services and (iii) biodiversity components. Snail biomass values were predicted to be higher in the short term than in the long term. For ecosystem services, moderate risk was estimated for genetic resources, climate regulation, pest and disease regulation and pollination in both the short and long term; for food, risk was assessed as moderate in the short term and major in the long term; for water and erosion regulation, risk was assessed as major both in the short term and in the long term; for fresh water, risk was assessed as massive both in the short term and in the long term; and for nutrient cycling and photosynthesis and primary production of macrophytes, risk was assessed as massive in the short term and as major in the long term. For biodiversity components, risk for genetic diversity and native species diversity was estimated as major in both the short and the long term; risk for native habitats was assessed massive in the short term and major in the long term; and for threatened species and habitats of high conservation value, risk was determined as massive in both the short and the long term.

Impacts of an Invasive Snail (Tarebia granifera) on Nutrient Cycling in Tropical Streams: The Role of Riparian Deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N.

Herbiciding invasive reed: indirect effects on habitat conditions and snail–algal assemblages one year post-application

Invasive common reed (Phragmites australis) can rapidly form expansive, near-monotypic stands, and thereby lower plant diversity and change marsh habitat structure. Consequently, North American wetland managers often use herbicides, such as glyphosate-based AquaNeat® and imazypr-based Habitat®, to control its establishment and spread. However, herbiciding might indirectly affect benthic community structure by directly altering habitat structure, and habitat alterations may vary with herbicide and concentration. These effects may be particularly pronounced ≥1 year post-herbiciding when dead above-ground biomass collapses and submerges. To evaluate how herbicide-caused alterations in habitat affect key trophic linkages, we compared snail and epiphytic algal assemblages, and habitat conditions, among 20- × 20-m replicated plots of reed treated with either AquaNeat® (30 % solution), Habitat® (5 % solution), or left herbicide-free (i.e., controls) in an eutrophic Lake Erie coastal marsh 1-y post-herbiciding. Both herbicides equally reduced reed above-ground growth by >90 % relative to controls. Fossaria spp. and Gyraulus parvus snails were more abundant in herbicide-treated plots than in controls, but Shannon-Wiener diversity was similar (H′ ≈ 1.0) across treatments. All snails collected were pulmonates, suggesting habitat drying might be driving assemblage structure. Snails were denser in plots with metaphyton (mostly Spirogyra) than without, and metaphyton was more abundant in herbicide-treated plots with higher incident light levels and warmer water temperatures than in controls. Snail biomass was positively related to amount of benthic macro-organic matter but not epiphytic algal biomass, which was similar among treatments. Diatoms dominated algal communities in all treatments. In June, Navicula spp. was dominant in controls, whereas Nitzschia palea and Aulacoseira italic, and Nitzschia spp., were dominant in AquaNeat® and Habitat® treatments, respectively. However, algal and diatom assemblages were similar in treatments by early-July when marsh water levels significantly decreased and nitrate levels were <1 μg/L. Marsh hydrologic patterns may mediate herbiciding’s indirect effects on trophic structure.

Atrazine does not affect algal biomass or snail populations in microcosm communities at environmentally relevant concentrations

The herbicide atrazine is a photosynthetic inhibitor used around the world in agricultural applications. Contamination of surface waters adjacent to treated areas can directly reduce growth of nontarget aquatic autotrophs, but the severity of impacts is highly dependent on species sensitivity and exposure concentration. Secondary effects resulting from macrophyte or phytoplankton decline may include an expansion of the more tolerant periphyton community. Recently, this shift in the autotrophic community has been proposed as a mechanism for increased rates of parasite infections in amphibians via augmented populations of aquatic snails which act as intermediate hosts to larval trematodes. To further clarify this relationship, an outdoor microcosm study was conducted to examine the effects of atrazine on primary production and snail populations over a range of environmentally relevant concentrations. In July 2009, 15 experimental ponds were treated to achieve initial concentrations of 0, 1, 10, 30, and 100 µg/L atrazine. Over a period of 73 d, measures were taken of macrophyte, phytoplankton, and periphyton biomass, growth, and fecundity of caged snails (Physella spp. and Stagnicola elodes) and free-living snails (Physella spp.). Except for declines in macrophyte biomass at the highest treatment level, no consistent relationships were found between atrazine concentration and any measured parameter. Comparison of these results with previous findings highlights the variability of responses to atrazine exposure between similarly constructed freshwater communities, even at concentrations up to 20 times higher than sustained environmental levels.

Interactions Between the Invasive New Zealand Mudsnail (Potamopyrgus Antipodarum) and a Native Snail (Fossaria Bakerilymnaea Bulimoides Group) in the Greater Yeelowstone

Understanding invasive species impacts is critical to determining how an ecosystem may function after an introduction. Invasive species can alter the structure and function of ecosystems, reduce biological diversity, and alter communities through predation, facilitation and competition. In the past 30 years, the invasive New Zealand mud snail (Potamopyrgus antipodarum) has established in areas of conservation concern in the American West including Yellowstone National Park. To develop a greater understanding of the impact of P. antipodarum on the native co-occurring snail, Fossaria (Bakerilymnaea) bulimoides group, we conducted two experiments to assess the interactions occurring between these snails. We found that F. bulimoides growth was reduced by all interactors, but especially by P. antipodarum. In addition, growth of F. bulimoides was much more affected by high biomass of snails than P. antipodarum. P. antipodarum grew more in the presence of interactors and their growth was facilitated by the presence of the native snail F. bulimoides.

Interactions among invaders: community and ecosystem effects of multiple invasive species in an experimental aquatic system

With ecosystems increasingly supporting multiple invasive species, interactions among invaders could magnify or ameliorate the undesired consequences for native communities and ecosystems. We evaluated the individual and combined effects of rusty crayfish (Orconectes rusticus) and Chinese mystery snails [Bellamya (=Cipangopaludina) chinensis] on native snail communities (Physa, Helisoma and Lymnaea sp.) and ecosystem attributes (algal chlorophyll a and nutrient concentrations). Both invaders are widespread in the USA and commonly co-occur within northern temperate lakes, underscoring the importance of understanding their singular and joint effects. An outdoor mesocosm experiment revealed that while the two invaders had only weakly negative effects upon one another, both negatively affected the abundance and biomass of native snails, and their combined presence drove one native species to extinction and reduced a second by >95%. Owing to its larger size and thicker shell, adult Bellamya were protected from crayfish attack relative to native species (especially Physa and Lymnaea), suggesting the co-occurrence of these invaders in nature could have elevated consequences for native communities. The per capita impacts of Orconectes (a snail predator) on native snails were substantially greater than those of Bellamya (a snail competitor). Crayfish predation also had a cascading effect by reducing native snail biomass, leading to increased periphyton growth. Bellamya, in contrast, reduced periphyton biomass, likely causing a reduction in growth by native lymnaeid snails. Bellamya also increased water column N:P ratio, possibly because of a low P excretion rate relative to native snail species. Together, these findings highlight the importance of understanding interactions among invasive species, which can have significant community- and ecosystem-level effects.

Parasite-Induced Changes in Nitrogen Isotope Signatures of Host Tissues

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the delta13C and delta15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, delta15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.

CONSEQUENCES OF NICHE OVERLAP FOR ECOSYSTEM FUNCTIONING: AN EXPERIMENTAL TEST WITH POND GRAZERS

While the number of studies investigating the effects of species diversity on ecosystem properties continues to expand, few have explicitly examined how ecosystem functioning depends quantitatively on the degree of niche complementarity among species. We report the results of a microcosm experiment where similarity in habitat use among aquatic snail species was evaluated as a predictor of changes in community and ecosystem properties due to increasing species richness. Replicate microcosms with all possible one- and two-species combinations of a guild of six snail species were stocked with identical initial snail biomass. Microcosms with two species of snails had greater final snail biomass, lower attached algae biomass, and less total organic matter than monocultures. Snail species differed in their use of five distinct habitat types in the microcosms. Similarity in habitat use between a species pair was negatively related to the magnitude of change (e.g., deltaEF [change in ecosystem function]) in dissolved oxygen. periphyton biomass, and accrual of organic matter with a change in diversity. However, using the most stringent criterion for complementarity effects (e.g., Dmax [proportional deviation of the total polyculture yield from the highest yielding monoculture]), a relationship between species' niche similarity and changes in function with increasing species richness was only observed for dissolved oxygen. The identity of snail species present in the microcosms had strong effects on total organic matter, snail biomass, dissolved oxygen, periphyton biomass, and sedimentation rate. In this study, herbivore identity, sampling effects, and niche complementarity all appear to contribute to species richness effects on pond ecosystem properties and community structure. The analytical approach employed here may profitably be used in other systems to quantify the role of niche complementarity in species richness-ecosystem function relationships.

Periphyton responds differentially to nutrients recycled in dissolved or faecal pellet form by the snail grazer Theodoxus fluviatilis

Summary1. We aimed to separate the effects of grazers on periphyton via grazing from that of nutrient recycling from their faecal pellets.2. We set up three different experimental treatments (snails/no snails/faecal pellets) and sampled over 16 days. The ‘snail’ treatment contained a low density (snail biomass c. 14 g−2) of the gastropod grazer Theodoxus fluviatilis and the ‘faecal pellet’ treatment received the same amount of faecal pellets as were produced in the ‘snail’ treatment. Whereas the ‘faecal pellet’ treatment provided extra nutrients to periphyton from the faeces, the ‘snail’ treatment provided nutrients in the form of both faeces and in excreta. There was also direct grazing on periphyton in the ‘snail’ treatment. The ‘no snail’ was not grazed and received no nutrients in faeces or excreta.3. We measured periphyton C, N and P content, chlorophyll‐a (chl‐a), primary production, bacterial biomass, bacterial production and bacterial respiratory activity. In the water column we measured dissolved inorganic N and soluble reactive P.4. Snails increased the amount of dissolved inorganic N in the water. On day 16, the periphyton N : P ratio in the ‘faecal pellet’ treatment was lower, and periphyton P content was higher, than in the other two treatments. N : P ratios decreased over time in the ‘faecal pellet’ treatment. Primary and bacterial production were positively correlated in all treatments.5. Algal chl‐a and primary production of periphyton per unit area and periphyton chl‐a : C ratios increased over the 16 day in the ‘snail’ treatment, and thus excretion of dissolved N by snails had a stronger positive effect on the periphyton community than N and P in faecal pellets.6. Our data show that excretion and egestion can have different effects on periphyton, probably because of the higher proportion of dissolved N in excreta and the higher proportion of P recycled in faecal pellets. The relative effect of nutrients recycled in egesta or in excretions, probably depends on the form of nutrient limitation of the periphyton. Further, the different components of the periphyton matrix could react differently to the different forms of nutrient recycling.7. We conclude that direct grazing effects are less important than nutrient effects when nutrients are limiting and grazing pressure is low. Further, the spatial separation of different grazing effects can lead to differences in periphyton production and nutrient stoichiometry. This might be an explanation for the patchiness of periphyton in nature.

Dragonfly predators influence biomass and density of pond snails

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.

Spatio-temporal variations in intertidal mudflat erodability: Marennes-Oléron Bay, western France

A portable erosion device (ERIS) was deployed in November 2000 and May 2001 to investigate the spatio-temporal variation of fine-grained sediment erosion over a transect on the intertidal mudflats of Marennes-Oléron Bay, western France. The objective was to relate changes in bed erodability over diurnal emersion periods to bioturbation, rapid changes in sediment characteristics, and microphytobenthos biomass. Macrofaunal assemblages were dominated by the bivalve, Scrobicularia plana, on the upper mudflat, and by the gastropod, Hydrobia ulvae, on the middle mudflat. A power relationship was obtained where τ cr was plotted as a function of sediment density. Results from the upper part of the mudflat deviated from this relationship due to bioturbation by S. plana. No influence of microphytobenthos (either chlorophyll a or extracellular polymeric substances (EPS) content) was found on τ cr. The bivalve feeding activities altered consolidated sediments (sediment dry mass concentration of ∼700 g l −1) so that they became resuspended easily ( τ cr<1 Pa). Fluff layer erosion was observed, with an erosion rate dependent upon the biomass of H. ulvae. It was also found that bioturbation by H. ulvae was implicated (combined with grazing activities) in chl a resuspension during the erosion of the fluff layer. Snail bioturbation stimulated the chl a enrichment of eroded material, whilst the high snail biomass produced a decrease in suspended chl a due to higher grazing rates.

Effects of eutrophication and snails on Eurasian watermilfoil (Myriophyllum spicatum) invasion

Exotic species can invade and establish new habitats both as a result of their own traits, and as a result of the characteristics of the environment they invade. Here, we show that the abundance of the invasive submerged aquatic plant, Myriophyllum spicatum (Eurasian watermilfoil) is highly dependent on the conditions of the environment in a mesocosm experiment. M. spicatum is allelopathic towards epiphytic algae, and in the absence of algivorous snails, we found that the abundance of both algae and M. spicatum significantly increased with experimentally increased nutrient loading, while the abundance of native submerged macrophytes declined. However, when snails were present, snail biomass increased with increasing nutrient loading, and M. spicatum biomass was consistently low while native submerged macrophyte biomass was consistently high. Our results stress the importance of the interaction between species traits and environmental conditions when considering the invasiveness of certain exotic species and the invasibility of certain environments.

URBANIZATION AFFECTS STREAM ECOSYSTEM FUNCTION BY ALTERING HYDROLOGY, CHEMISTRY, AND BIOTIC RICHNESS

Catchment urbanization can alter physical, chemical, and biological attributes of stream ecosystems. In particular, changes in land use may affect the dynamics of organic matter decomposition, a measure of ecosystem function. We examined leaf-litter decomposition in 18 tributaries of the St. Johns River, Florida, USA. Land use in all 18 catchments ranged from 0% to 93% urban which translated to 0% to 66% total impervious area (TIA). Using a litter-bag technique, we measured mass loss, fungal biomass, and macroinvertebrate biomass for two leaf species (red maple [Acer rubrum] and sweetgum [Liquidambar styraciflua]). Rates of litter mass loss, which ranged from 0.01 to 0.05 per day for red maple and 0.006 to 0.018 per day for sweetgum, increased with impervious catchment area to levels of approximately 30-40% TIA and then decreased as impervious catchment area exceeded 40% TIA. Fungal biomass was also highest in streams draining catchments with intermediate levels of TIA. Macroinvertebrate biomass ranged from 17 to 354 mg/bag for red maple and from 15 to 399 mg/bag for sweetgum. Snail biomass and snail and total invertebrate richness were strongly related to breakdown rates among streams regardless of leaf species. Land-use and physical, chemical, and biological variables were highly intercorrelated. Principal-components analysis was therefore used to reduce the variables into several orthogonal axes. Using stepwise regression, we found that flow regime, snail biomass, snail and total invertebrate richness, and metal and nutrient content (which varied in a nonlinear manner with impervious surface area) were likely factors affecting litter breakdown rates in these streams.

RELATIVE STRENGTH OF TOP‐DOWN, BOTTOM‐UP, AND CONSUMER SPECIES RICHNESS EFFECTS ON POND ECOSYSTEMS

Theory and experiments demonstrate that the number of species in a local area can determine rates of ecosystem processes, but we know little about how the strength of that control compares with other influences or how it might vary across ecological gradients. Here I report results of a pond mesocosm experiment where consumer (snail) species richness, predation intensity, and nutrient availability were crossed in a full‐factorial design. This design allowed a direct comparison of the strength of the different factors on food web properties and ecosystem functioning (i.e., system‐level fluxes of energy or matter), and an evaluation of how the factors may interact.Systems with higher snail species richness had greater secondary production, consumer biomass, ecosystem respiration, and macrophyte stem growth, and lower epiphyton and periphyton biomass. However, snail species richness effects on periphyton and epiphyton were context dependent; predators reduced the effect of increasing snail richness on the biomass of attached algae. Species richness effects were most often the result of a biological mechanism (e.g., differential resource use), rather than being solely artifacts of the experimental design (e.g., sampling effects). The effects of nutrient enrichment and predation were mostly predictable from simple food chain models; increases in nutrient availability led to increased algal biomass, primary production, and snail biomass, while predators decreased snail biomass and indirectly augmented algae.Snail species richness effects on the biomass of many functional groups were as strong or stronger than those of a substantial nutrient enrichment or of the addition of a voracious predator (Belostoma flumineum). Nutrient enrichment had the most pronounced effects on several system‐level processes (e.g., primary production, sedimentation). Species richness had weaker effects on ecosystem properties than on particular functional groups, probably because of compensatory responses among different producer functional groups. In summary, this experiment suggests that the number of consumer species in a system can have large and meaningful effects on the distribution of biomass in a food web, and that these effects can depend on ecological context.

Exotic snails dominate nitrogen and carbon cycling in a highly productive stream

Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.