Showing papers in "Freshwater Biology in 2009"

Microbial biodiversity in groundwater ecosystems

Abstract: Summary 1. Groundwater ecosystems offer vast and complex habitats for diverse microbial communities. Here we review the current status of groundwater microbial biodiversity research with a focus on Bacteria and Archaea and on the prospects of modern techniques for enhancing our understanding of microbial biodiversity patterns and their relation to environmental conditions. 2. The enormous volume of the saturated terrestrial underground forms the largest habitat for microorganisms on earth. Up to 40% of prokaryotic biomass on earth is hidden within this terrestrial subsurface. Besides representing a globally important pool of carbon and nutrients in organisms, these communities harbour a degree of microbial diversity only marginally explored to date. 3. Although first observations of groundwater microbiota date back to Antonie van Leeuwenhoek in 1677, the systematic investigation of groundwater microbial biodiversity has gained momentum only within the last few decades. These investigations were initiated by an increasing awareness of the importance of aquifer microbiota for ecosystem services and functioning, including the provision of drinking water and the degradation of contaminants. 4. The development of sampling techniques suitable for microbiological investigations as well as the application of both cultivation-based and molecular methods has yielded substantial insights into microbial communities in contaminated aquifers, whereas knowledge of microbial biodiversity in pristine habitats is still poor at present. 5. Several novel phylogenetic lineages have been described from groundwater habitats, but to date no clearly ‘endemic’ subsurface microbial phyla have been identified. The future will show if the rather low diversity generally found in pristine oligotrophic aquifers is a fact or just a result of low abundances and insufficient resolution of today’s methods. Refined approaches complemented by statistically rigorous applications of biodiversity estimates are urgently needed. 6. Factors identified to control microbial diversity in aquifers include spatial heterogeneity, temporal variability and disturbances such as pollution with chemical anthropogenic contaminants. Although first insights into the importance of individual biogeochemical processes may be obtained from surveys of microbial diversity within functional groups, direct links to groundwater ecosystem functioning have rarely been established so far.

Transformation of the offshore benthic community in Lake Michigan: recent shift from the native amphipod Diporeia spp. to the invasive mussel Dreissena rostriformis bugensis

Abstract: SUMMARY 1. The native amphipod Diporeia spp. was once the dominant benthic organism in Lake Michigan and served as an important pathway of energy flow from lower to upper trophic levels. Lake-wide surveys were conducted in 1994 ⁄1995, 2000 and 2005, and abundances of Diporeia and the invasive bivalves Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel) were assessed. In addition, more frequent surveys were conducted in the southern region of the lake between 1980 and 2007 to augment trend interpretation. 2. Between 1994 ⁄1995 and 2005, lake-wide density of Diporeia declined from 5365 to 329 m )2 , and biomass (dry weight, DW) declined from 3.9 to 0.4 g DW m )2 . The percentage of all sites with no Diporeia increased over time: 1.1% in 1994 ⁄1995, 21.7% in 2000 and 66.9% in 2005. On the other hand, total dreissenid density increased from 173 to 8816 m )2 , and total biomass increased from 0.4 to 28.6 g DW m )2 . Over this 10-year time period, D. r. bugensis displaced D. polymorpha as the dominant dreissenid, comprising 97.7% of the total population in 2005. In 2007, Diporeia was rarely found at depths shallower than 90 m and continued to decline at greater depths, whereas densities of D. r. bugensis continued to increase at depths greater than 50 m. 3. The decline in Diporeia occurred progressively from shallow to deep regions, and was temporally coincident with the expansion of D. polymorpha in nearshore waters followed by the expansion of D. r. bugensis in offshore waters. In addition, Diporeia density was negatively related to dreissenid density within and across depth intervals; the latter result indicated that dreissenids in shallow waters remotely influenced Diporeia in deep waters. 4. With the loss of Diporeia and increase in D. r. bugensis, the benthic community has become a major energy sink rather that a pathway to upper trophic levels. With this replacement of dominant taxa, we estimate that the relative benthic energy pool increased from 17 to 109 kcal m )2 between 1994 ⁄1995 and 2005, and to 342 kcal m )2

Relationships between structure and function in streams contrasting in temperature

Abstract: Summary 1. We studied 10 first-order Icelandic streams differing in geothermal influence in separate catchments. Summer temperature (August–September) ranged between 6 and 23 °C. 2. Macroinvertebrate evenness and species overlap decreased significantly with temperature whereas taxon richness showed no response. In total, 35 macroinvertebrate species were found with Chironomidae the dominant taxonomic group. Macroinvertebrate density increased significantly with temperature. Dominant species in the warm streams were Lymnaea peregra and Simulium vittatum. Algal biomass, macrophyte cover and richness were unrelated to temperature. Densities of trout (Salmo trutta), the only fish species present, reflected habitat conditions and to a lesser degree temperature. 3. Density of filter-feeders increased significantly with temperature whereas scraper density, the other dominant functional feeding group, was unrelated to temperature. Stable isotope analysis revealed a positive relationship between δ15N and temperature across several trophic levels. No pattern was found with regard to δ13C and temperature. 4. Leaf litter decomposition in both fine and coarse mesh leaf bags were significantly correlated to temperature. In coarse mesh leaf packs breakdown rates were almost doubled compared with fine mesh, ranging between 0.5 and 1.3 g DW 28 days−1. Nutrient diffusion substrates showed that the streams were primarily nitrogen limited across the temperature gradient while a significant additional effect of phosphorous was found with increasing temperature. 5. Structural and functional attributes gave complementary information which all indicated a change with temperature similar to what is found in moderately polluted streams. Our results therefore suggest that lotic ecosystems could be degraded by global warming.

A molecular test for cryptic diversity in ground water: how large are the ranges of macro-stygobionts?

Abstract: SUMMARY 1. Various groundwater habitats have exceptionally high levels of endemism caused by strong hydrographical isolation and low dispersal abilities of their inhabitants. More than 10% of macro-stygobiotic species nevertheless occupy relatively large ranges, measuring from some hundred to over 2000 km in length. These species represent a challenge because their distributions disregard hydrographical boundaries, and their means to disperse and maintain long-term gene flow are unknown. 2. Based on mitochondrial and nuclear gene sequences, we examined the phylogeographic structure of six formally recognised stygobiotic species (Niphargus virei, N. rhenorhodanensis, Troglocaris anophthalmus, T. hercegovinensis, Spelaeocaris pretneri, Proteus anguinus) and searched for cryptic lineage diversity in a genus-wide phylogeny of Niphargus. Using treebased criteria as well as comparative divergence measures, we identified cryptic lineages, which may tentatively be equated with cryptic species. 3. Fourteen analysed nominal stygobiotic species with large ranges emerged as highly diversified, splitting into 51 tentative cryptic lineages. The degree of divergence was within the range or larger than the divergence of other related pairs of sister species. A substantial part (94%) of the cryptic lineages had ranges <200 km in length. One half of them were recorded at single sites only. The largest range recorded was that of a cryptic N. virei lineage (700 km), while none of the very large traditional ranges (e.g. Niphargus aquilex – 2300 km, N. tauri – 1900 km) could be corroborated. 4. These data suggest that small ranges of macro-stygobionts are the rule, and ranges over 200 km are extremely rare. 5. The implications of this result for groundwater biodiversity assessment and conservation include a considerable increase in overall diversity at the regional and continental scale and a strong decrease in faunal similarities among regions, coupled with greater endemism.

Resource quality and stoichiometric constraints on stream ecosystem functioning

Abstract: SUMMARY 1. Resource quality and stoichiometric imbalances in carbon : nutrient ratios between consumers and resources can influence key ecosystem processes. In many streams, this has important implications for food webs that are based largely upon the utilization of terrestrial leaf-litter, which varies widely among litter types in its value as a food source for detritivores and as a substrate for microbial decomposers. 2. We measured breakdown rates and macroinvertebrate colonization of leaf-litter from a range of native and exotic plants of differing resource quality and palatability to consumers [e.g. carbon : nitrogen : phosphorus (C : N : P) ratios, lignin and cellulose content], in a field experiment. We also measured C : N : P ratios of the principal leafshredding invertebrates, which revealed strong stoichiometric imbalances across trophic levels: C : N and C : P ratios typically differed by at least one order of magnitude between consumers and resources, whereas N : P imbalances were less marked. Application of the threshold elemental ratio approach, which integrates animal bioenergetics and body elemental composition in examining nutrient deficiency between consumers and resources, revealed less marked C : P imbalances than those based on the simpler arithmetic differences described above. 3. Litter breakdown rates declined as nutrient imbalances widened and resource quality fell, but they were independent of whether resources were exotic or native. The principal drivers of total, microbial and invertebrate-mediated breakdown rates were lignin : N, lignin : P and fungal biomass, respectively. However, multiple regression using orthogonal predictors yielded even more efficient models of litter breakdown, as consumers responded to more than one aspect of resource quality. For example, fungal biomass and litter C : N both influenced invertebrate-mediated breakdown. 4. Large stoichiometric imbalances and changes in resource quality are likely to have serious consequences for stream ecosystem functioning, especially when riparian zones have been invaded by exotic plant species whose chemical composition differs markedly from that of the native flora. Consequently, the magnitude and direction of change in breakdown rates and, thus, resource depletion, will be driven to a large extent by the

Biodiversity, ecosystem functioning and food webs in fresh waters: assembling the jigsaw puzzle

Abstract: SUMMARY 1. Dramatic advances have been made recently in the study of biodiversity–ecosystem functioning (B-EF) relations and food web ecology. These fields are now starting to converge, and this fusion has the potential to improve our understanding of how environmental stressors modulate ecosystem processes and the supply of ‘goods and services’. 2. Food web structure and dynamics can exert particularly strong influences on B-EF relations in fresh waters, as consumer–resource interactions (e.g. trophic cascades) are often more important than horizontal interactions within trophic levels. For instance, many freshwater food webs are size structured, with large organisms tending to occupy the higher trophic levels and often exerting powerful effects on ecosystem processes. However, because they are also vulnerable to perturbations, non-random losses of these large taxa can alter both food web structure and ecosystem functioning profoundly. 3. Recently, the focus of food web research has shifted away from exploring patterns, towards developing an understanding of processes (e.g. quantifying fluxes of individuals, biomass, energy, nutrients) and how the two interact. Many of the bestcharacterized food webs are from fresh waters, and these ecosystems are now being used to address some of the shortcomings of earlier B-EF studies. I have identified several key gaps in our current knowledge and highlighted potentially fruitful avenues of future B-EF and food web research. 4. A major challenge for this newly emerging research is to place it within a unified theoretical framework. The application of metabolic theory and ecological stoichiometry may help to achieve this goal by considering biological systems within the constraints imposed upon them by physical and chemical laws.

Invasions and niche width: does niche width of an introduced crayfish differ from a native crayfish?

Abstract: 1. Human activities have promoted the spread of species worldwide. Several crayfish species have been introduced into new areas, posing a threat to native crayfish and other biota. Invader success may depend on the ability to utilise a wide variety of habitats and resources. Successful invaders are generally expected to have broader niches and to be more plastic than non-invasive species. 2. Using stable isotope ratios of carbon and nitrogen we compared the niche widths of native noble crayfish and introduced signal crayfish, a successful invader of Swedish streams. The calculation of niche width took account of between-site differences in basal resource isotope signature ranges. We also assessed whether population density, prey biomass or prey diversity affected niche width. 3. At the species level, signal crayfish had twice the niche width of noble crayfish. However, individual populations of noble crayfish and signal crayfish in Swedish streams had similar niche widths. This suggests that signal crayfish has greater plasticity with respect to habitat utilisation and feeding than noble crayfish. Niche width in both species correlated positively with benthic invertebrate biomass and diversity, indicating that animal food sources are important for crayfish. 4. We find that assessing niche width in relation to invader success can be a useful tool trying to predict the impact of invasions on different scales. The findings in this study suggest that invaders and natives will have a similar impact on the stream scale whereas the invader will have a larger impact on the regional scale due to the ability to utilise a wider range of streams. (Less)

Trends in water quality and discharge confound long-term warming effects on river macroinvertebrates

Abstract: 1. Climate-change effects on rivers and streams might interact with other pressures, such as pollution, but long-term investigations are scarce. We assessed trends among macroinvertebrates in 50 southern English streams in relation to temperature, discharge and water quality over 18 years (1989–2007). 2. Long-term records, coupled with estimates from inter-site calibrations of 3–4 years, showed that mean stream temperatures in the study area had increased by 2.1–2.9 °C in winter and 1.1–1.5 °C in summer over the 26 year period from 1980 to 2006, with trends in winter strongest. 3. While invertebrate assemblages in surface-fed streams were constant, those in chalk-streams changed significantly during 1989–2007. Invertebrate trends correlated significantly with temperature, but effects were spurious because (i) assemblages gained taxa typical of faster flow or well-oxygenated conditions, contrary to expectations from warming; (ii) more invertebrate families increased in abundance than declined and (iii) concomitant changes in water quality (e.g. declining orthophosphate, ammonia and biochemical oxygen demand), or at some sites changes in discharge, explained more variation in invertebrate abundance and composition than did temperature. 4. These patterns were reconfirmed in both group- and site-specific analyses. 5. We conclude that recent winter-biased warming in southern English chalk-streams has been insufficient to affect invertebrates negatively over a period of improving water quality. This implies that positive management can minimize some climate-change impacts on stream ecosystems. Chalk-stream invertebrates are sensitive, nevertheless, to variations in discharge, and detectable changes could occur if climate change alters flow pattern. 6. Because climatic trends now characterize many inter-annual time-series, we caution other investigators to examine whether putative effects on ecological systems are real or linked spuriously to other causes of change.

Diversity and activity of aquatic fungi under low oxygen conditions

Abstract: Summary 1. The objective was to test whether a decrease in oxygen concentration in streams affects the diversity and activity of aquatic hyphomycetes and consequently leaf litter decomposition. 2. Senescent leaves of Alnus glutinosa were immersed for 7 days in a reference stream, for fungal colonization, and then incubated for 18 days in microcosms at five oxygen concentrations (4%, 26%, 54%, 76% and 94% saturation). Leaf decomposition (as loss of leaf toughness), fungal diversity, reproduction (as spore production) and biomass (ergosterol content) were determined. 3. Leaf toughness decreased by 70% in leaves exposed to the highest O2 concentration, whereas the decrease was substantially less (from 25% to 45%) in treatments with lower O2. Fungal biomass decreased from 99 to 12 mg fungi g−1 ash-free dry mass on exposure to 94% and 4% O2 respectively. Sporulation was strongly inhibited by reduction of dissolved O2 in water (3.1 × 104 versus 1.3 × 103 spores per microcosms) for 94% and 4% saturation respectively. 4. A total of 20 species of aquatic hyphomycetes were identified on leaves exposed to 94% O2, whereas only 12 species were found in the treatment with 4% O2 saturation. Multidimensional scaling revealed that fungal assemblages exposed to 4% O2 were separated from all the others. Articulospora tetracladia, Cylindrocarpon sp. and Flagellospora curta were the dominant species in microcosms with 4% O2, while Flagellospora curvula and Anguillospora filiformis were dominant at higher O2 concentrations. 5. Overall results suggest that the functional role of aquatic hyphomycetes as decomposers of leaf litter is limited when the concentration of dissolved oxygen in streams is low.

Relationships between deforestation, riparian forest buffers and benthic macroinvertebrates in neotropical headwater streams

Abstract: SUMMARY 1. Few studies have evaluated the effectiveness of riparian buffers in the tropics, despite their potential to reduce the impacts of deforestation on stream communities. We examined macroinvertebrate assemblages and stream habitat characteristics in small lowland streams in southeastern Costa Rica to assess the impacts of deforestation on benthic communities and the influence of riparian forest buffers on these effects. Three different stream reach types were compared in the study: (i) forested reference reaches, (ii) stream reaches adjacent to pasture with a riparian forest buffer at least 15 m in width on both banks and (iii) stream reaches adjacent to pasture without a riparian forest buffer. 2. Comparisons between forest and pasture reaches suggest that deforestation, even at a very local scale, can alter the taxonomic composition of benthic macroinvertebrate assemblages, reduce macroinvertebrate diversity and eliminate the most sensitive taxa. The presence of a riparian forest buffer appeared to significantly reduce the effects of deforestation on benthic communities, as macroinvertebrate diversity and assemblage structure in forest buffer reaches were generally very similar to those in forested reference reaches. One forest buffer reach was clearly an exception to this pattern, despite the presence of a wide riparian buffer. 3. The taxonomic structure of macroinvertebrate assemblages differed between pool and riffle habitats, but contrasts among the three reach types in our study were consistent across the two habitats. Differences among reach types also persisted across three sampling periods during our 15-month study. 4. Among the environmental variables we measured, only stream water temperature varied significantly among reach types, but trends in periphyton abundance and stream sedimentation may have contributed to observed differences in macroinvertebrate assemblage structure. 5. Forest cover was high in all of our study catchments, and more research is needed to determine whether riparian forest buffers will sustain similar functions in more extensively deforested landscapes. Nevertheless, our results provide support for Costa Rican regulations protecting riparian forests and suggest that proper riparian management could significantly reduce the impacts of deforestation on benthic communities in tropical streams.

Scale-dependent effects of fine sediments on temperate headwater invertebrates

Abstract: 1. Anthropogenic activities can increase fine sediment supply to streams over multiple spatial and temporal extents. Identifying the processes responsible, and the scale at which any effects on stream organisms become evident, are key management needs, but appropriately scaled surveys are surprisingly few. 2. We surveyed macroinvertebrates and superficial fine sediments at two spatial resolutions (reach- and patch-scale) in tributaries of the River Usk, a temperate, montane catchment in rural Wales (U.K.). Land use, habitat and geomorphological character were measured on-site or derived from an existing database (=Fluvial Audit). We aimed to identify: (i) how in-stream sediments varied with land use and associated geomorphology; (ii) likely consequences for macroinvertebrates and (iii) any scale-dependence in relationships between macroinvertebrates and sediment character. 3. At both the reach- and patch-scales, bed cover by fine sediment was related directly to the extent of eroding banks 500 m upstream. In turn, sedimentation and bank erosion were negatively correlated with catchment or riparian woodland extent. 4. At the reach scale, macroinvertebrate composition varied with catchment land use and stream chemistry, with richness declining as rough grazing or woodland was replaced by improved grassland. There was no response to deposited sediment except for weak increase in the relative abundance of oligochaetes. 5. By contrast, at the patch scale, fine sediments were accompanied by pronounced changes in invertebrate composition, and we ranked the 27 most common taxa according to their apparent sediment tolerance. General estimating equations showed that total and Ephemeroptera, Plecoptera and Trichoptera richness decreased significantly by 20% and 25% at the most sediment impacted sites (30% cover) by comparison with sediment-free sites. 6. We conclude that sediment deposition in the upper Usk system mostly reflects local bank erosion, with riparian woodland likely to mediate this process through bank stability. Fine sediment release had marked ecological effects, but these were detectable only at patch-to-patch scales. We suggest that investigation of localized sediment release in streams will benefit from scale-dependent or scale-specific sampling, and some effects could go undetected unless sample resolution is selected carefully.

Body Size: The Structure and Function of Aquatic Ecosystems

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Availability of and access to critical habitats in regulated rivers: effects of low‐head barriers on threatened lampreys

Abstract: Summary 1. Conservation of freshwater animal populations requires their access to, as well as sufficient availability of, critical habitats, such as those for reproduction. Abundant small-scale barriers may cause extensive fragmentation of freshwater habitat but, by comparison to larger structures their effects are rarely considered by catchment managers. The relationship between the distribution of, and access to, spawning habitat in a regulated river, characterized by abundant small barriers, was examined for river lamprey Lampetra fluviatilis, a threatened migratory fish. 2. Telemetry of adult lamprey in the River Derwent, North East England was used to quantify upriver migration and access to spawning habitat, together with surveys of spawning habitat availability and spawning activity between 2002 and 2007. 3. Access in to the Derwent appeared severely restricted by a tidal barrage, beyond which lamprey migrated rapidly in unobstructed reaches. Of all lamprey tagged in the lower 4 km of river, or ascending the barrage, 64% and 17% passed the first and second weirs respectively, with high flows crucial for this. Although over 98% of lamprey spawning habitat occurred more than 51 km upstream, on average just 1.8% of river lamprey spawners were recorded there. 4. In order to protect or rehabilitate species or species assemblages, greater attention needs to be paid to the relative spatial distribution of low-head barriers and the resultant availability of key habitats within individual catchments. This is particularly important given the renewed emphasis internationally on low-head hydropower solutions as a source of renewable energy, and the rapid growth in numbers of low-head barriers in many catchments.

Substantial differences in littoral fish community structure and dynamics in subtropical and temperate shallow lakes

Abstract: Summary 1. Fish play a key role in the functioning of temperate shallow lakes by affecting nutrient exchange among habitats as well as lake trophic structure and dynamics. These processes are, in turn, strongly influenced by the abundance of submerged macrophytes, because piscivorous fish are often abundant at high macrophyte density. Whether this applies to warmer climates as well is virtually unknown. 2. To compare fish community structure and dynamics in plant beds between subtropical and temperate shallow lakes we conducted experiments with artificial submerged and free-floating plant beds in a set of 10 shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N), paired along a gradient of limnological characteristics. 3. The differences between regions were more pronounced than differences attributable to trophic state. The subtropical littoral fish communities were characterised by higher species richness, higher densities, higher biomass, higher trophic diversity (with predominance of omnivores and lack of true piscivores) and smaller body size than in the comparable temperate lakes. On average, fish densities were 93 ind. m−2 (±10 SE) in the subtropical and 10 ind. m−2 (±2 SE) in the temperate lakes. We found a twofold higher total fish biomass per unit of total phosphorus in the subtropical than in the temperate lakes, and as fish size is smaller in the former, the implication is that more energy reaches the littoral zone fish community of the warmer lakes. 4. Plant architecture affected the spatial distribution of fish within each climate zone. Thus, in the temperate zone fish exhibited higher densities among the artificial free-floating plants while subtropical fish were denser in the artificial submerged plant beds. These patterns appeared in most lakes, regardless of water turbidity or trophic state. 5. The subtropical littoral fish communities resembled the fish communities typically occurring in temperate eutrophic and hypertrophic lakes. Our results add to the growing evidence that climate warming may lead to more complex and omnivory-dominated food webs and higher density and dominance of smaller-sized fish. This type of community structure may lead to a weakening of the trophic cascading effects commonly observed in temperate shallow lakes and a higher risk of eutrophication.

Groundwater biodiversity in Europe.

Abstract: Summary 1. The spatial patterns of groundwater biodiversity in Europe remain poorly known, yet their knowledge is essential to understand local variation in groundwater assemblages and to develop sound conservation policies. We explore here the broad-scale distribution of groundwater biodiversity across Europe, focussing on obligate subterranean species. 2. We compiled published distributional data of obligate subterranean aquatic taxa for six European countries (Belgium, France, Italy, Portugal, Slovenia and Spain), and conducted a detailed biological survey of six regions (one in Belgium, two in France, one in Italy, one in Slovenia and one in Spain). Based on this data set, we mapped spatial patterns of biodiversity in Europe on a cell grid with 0.2 × 0.2 ° resolution. 3. As of mid-2006, the total number of described stygobiotic species in the six countries was 930 and the total number of genera with at least one described stygobiotic species was 191. The total number of sampling sites where at least one stygobiont had been collected was 4709, distributed in 1228 of the 4668 grid cells covering the study area. 4. Groundwater stygobiotic biodiversity was dominated by Crustacea with 757 species in 122 genera. Insects were represented by only two species of a single genus of dytiscid beetles restricted to south-eastern France. 5. The geographic distribution of stygobionts was extremely heterogeneous. Stygobionts were recorded in 26% of the 4668 grid cells and only 33 cells had more than 20 stygobiotic species. These 33 ‘hot-cells’ of groundwater species richness clustered in seven hotspots. 6. Endemicity was very high, with 43% of the total number of stygobiotic species restricted to a single cell, i.e. <500 km2. 7. Hotspots defined by rarity, number of genera, number of genera with only one species known in Europe, or number of monospecific genera differed markedly in ranking from those based on species richness. However, a core of four hotspots emerged in all cases: one stretching across Slovenia and northeastern Italy, one in the French Pyrenees, one in the Cevennes in southern France and one in the Rhine River valley in northeastern France. 8. Unevenness in stygobiont distribution cannot be explained solely by unevenness in sampling effort. This is indicated in particular by the fact that our comprehensive sampling survey roughly matched the level of taxonomic richness of the studied regions based on previously published information. 9. With sampling effort continuing, a twofold or higher increase in species richness can be expected in several Mediterranean areas, with a potential to discover up to 50% more new species than are currently known in the region.

Impacts of agricultural land use on ecosystem structure and whole-stream metabolism of tropical Cerrado streams

Abstract: Summary 1. The tropical Cerrado savannah, the second-largest South American biome, is undergoing the most rapid land-use change in South America, including a wholesale shift from native vegetation to pasture and cropland. However, the consequences of this development for aquatic ecosystem functioning remain unclear. In this study, we investigated how agriculture affects chemical, physical and biological characteristics of headwater streams in the Brazilian Cerrado, and how these changes relate to an important ecosystem function, i.e. ecosystem metabolism. 2. Three paired samplings of pristine and agricultural streams were conducted considering the most abundant morphotypes of Cerrado streams. Nutrient concentrations, stream morphometry and hydrodynamics, sediment transport, as well as benthic microbial biomass (BMB), and its chlorophyll a (Chl-a) content were measured. In parallel, whole-stream gross primary production (GPP) and community respiration (CR) were estimated using a diel open-channel dissolved O2 change technique. 3. Agricultural streams had consistently higher nutrient concentrations, less variability in channel morphology, smaller channel cross-sectional areas, smaller transient storage zones, higher current velocities and higher boundary shear stresses than pristine streams. At base flow, agricultural streams exhibited a midstream band of shifting sediments, while pristine streams had stable sediments. Both agricultural and pristine streams were dominated by thick microbial mats. Due to differences in hydrodynamics and sediment stability, these microbial mats covered the entire stream bed in pristine streams, but were restricted to the stream margins in agricultural streams. 4. As a result, BMB of agricultural streams was diminished by a factor of 7.5 compared to pristine streams. Interestingly, CR of agricultural streams was diminished only by a factor of 2.0, because higher BMB-specific respiration efficiencies, potentially due to increased nutrient availability, compensated for lower BMB due to physical stress. Similarly, the 3.3-fold elevated GPP in agricultural compared to pristine streams was only to a minor degree due to increased benthic Chl-a, but mainly a result of 2.4-fold higher Chl-a-specific primary production efficiencies, potentially due to increased nutrient availability. 5. In conclusion, agriculture impacted the studied Cerrado streams through two antagonistic mechanisms: physical stress resulted in decreased BMB and whole-stream CR by preventing BMB accrual in the central streambed, but higher nutrient availability led to increased primary production and respiration efficiencies in marginal zones, as well as higher whole-stream GPP. Finally, measures of whole-stream metabolism were useful indicators of stream ecosystem health and allowed for the differential assessment of the effects of physical stress and eutrophication.

Contrasting responses to catchment modification among a range of functional and structural indicators of river ecosystem health

Abstract: Summary 1. The value of measuring ecosystem functions in regular monitoring programs is increasingly being recognised as a potent tool for assessing river health. We measured the response of ecosystem metabolism, organic matter decomposition and strength loss, and invertebrate community composition across a gradient of catchment impairment defined by upstream landuse stress in two New Zealand streams. This was performed to determine if there were consistent responses among contrasting functional and structural indicators. 2. Rates of gross primary production (GPP) and ecosystem respiration (ER) ranged from 0.1 to 7.0 gO2 m−2 day−1 and from 0.34 to 16.5 gO2 m−2 day−1 respectively. Rates of GPP were variable across the landuse stress gradient, whereas ER increased linearly with the highest rates at the most impacted sites. Production/respiration (P/R) and net ecosystem metabolism (NEM) indicated that sites at the low and high ends of the stress gradient were heterotrophic with respiration rates presumably relying on organic matter from upstream sources, adjacent land or point sources. Sites with moderate impairment were predominantly autotrophic. 3. Declines in the tensile strength of the cotton strips showed no response across part of the gradient, but a strong response among the most impaired sites. The rate of mass loss of wooden sticks (Betula platyphylla Sukaczev) changed from a linear response to a U-shaped response across the impairment gradient after water temperature compensation, whereas leaf breakdown at a subset of sites suggested a linear loss in mass per degree-day. Three macroinvertebrate metrics describing the composition of the invertebrate community and its sensitivity to pollution showed similar linear inverse responses to the landuse stress gradient. 4. The first axis of a redundancy analysis indicated an association between landuse stress and various measures of water quality, and wooden stick mass loss, the invertebrate metric % EPT [percentage of macroinvertebrate taxa belonging to the Ephemeroptera, Plecoptera and Trichoptera (excluding Hydroptilidae] taxa, P/R and NEM, supporting the utility of these structural and functional metrics for assessing degree of landuse stress. The second axis was more strongly associated with catchment size, ER and GPP which suggests that these indicators were responding to differences in stream size. 5. Our results suggest that nonlinear responses to catchment impairment need to be considered when interpreting measurements of ecosystem function. Functional indicators could be useful for detecting relatively subtle changes where the slope of the response curve is maximised and measurements at the low and high ends of the impairment gradient are roughly equivalent. Such responses may be particularly valuable for detecting early signs of degradation at high quality sites, allowing management responses to be initiated before the degradation becomes too advanced, or for detecting initial moves away from degraded states during the early stages of restoration. Close links between structural and functional indices of river health across an impairment gradient are not necessarily expected or desirable if the aim is to minimise redundancy among ecological indicators.

Carnivory and resource‐based niche differentiation in anuran larvae: implications for food web and experimental ecology

Abstract: SUMMARY 1. Food webs represent the paths of material and energy flow through organisms in an ecosystem. Anuran larvae are important components of pond food webs: they are abundant, consume large quantities of food and serve as prey for many organisms. However, there are very basic uncertainties about the feeding ecology of anuran larvae; for instance, as to which trophic level they belong and whether species differ in resource use. Because anuran larvae have been employed in model systems in experimental ecology for decades, these uncertainties could lead to misinterpretation of published experiments, or inadequate designs of experiments directed at general, conceptual issues in ecology. 2. Using 13 Ca nd 15 N stable isotope and gut content analyses of free-ranging and enclosed tadpoles of four ranid species (Lithobates sylvaticus, L. pipiens, L. clamitans, L. catesbeianus) in the food webs of six wetlands, we tested the following null hypotheses: (i) that anuran larvae are strict primary consumers; (ii) that they are non-selective feeders and therefore exhibit little feeding niche differentiation; (iii) that they are opportunistic consumers and (iv) that their diet remains unchanged through ontogeny. 3. All four species consumed and assimilated substantial amounts of animal food; bullfrog larvae, in particular, appear to be predatory. Significant feeding niche differentiation among species occurred with respect to the sources of carbon, consumption of animal matter and nutritional quality of food ingested. We further documented opportunistic feeding habits and ontogenetic shifts in diet. 4. Collectively, these studies revealed complex trophic relationships that might require a reconsideration of the role of anuran larvae in pond food webs, as well as a reinterpretation of results of previous studies employing anuran larvae in model experimental systems.

Diversity, ecology and evolution of groundwater copepods

Abstract: Summary 1. With few exceptions, copepods dominate over other crustacean and non-crustacean invertebrate groups in ground water. They have colonised a vast array of habitats in continental ground waters, where they are represented by over 1000 species in six orders: Platycopioida, Misophrioida, Calanoida, Cyclopoida, Harpacticoida, Gelyelloida. However, members of only the last four orders entered genuine fresh ground water. 2. Stygobiotic copepods show a wide range of morphological and physiological adaptations to different groundwater habitats. They frequently exhibit simplifications in body plans, including reductions in appendage morphology, which is regarded as a result of paedomorphic heterochronic events. 3. Copepod distributions at small spatial scales are most strongly affected by habitat type and heterogeneity, with sediment grain size and availability of organic matter being important habitat characteristics. Large-scale spatial distributions (biogeographical) are mainly related to past geological, climatic and geographical processes which occurred over medium to long time scales. 4. Such processes have affected colonisation patterns and diversification of copepods in ground water, leading to a number of phylogenetic and distributional relicts and a high degree of endemism at different taxonomic levels. This is reflected in the composition of groundwater copepod communities characterised by distantly related species in the phylogenetic tree. 5. Copepods dominate the species richness of groundwater fauna in all regions and on all continents where more than cursory surveys have been carried out, i.e. in Europe, North and Latin America as well as in Australia. 6. Species-specific microhabitat preferences, high proportions of local endemics, high proportions of phylogenetic and distributional relicts, and higher-level taxonomic diversity are all factors suggesting that copepods are a useful indicator group of overall species richness for defining conservation priorities in ground water.

Ecological thresholds in European alpine lakes

Abstract: SUMMARY 1. Species assemblages of diatoms, rotifers, chydorids, planktonic crustaceans and chironomids were studied in 235 alpine lakes in the Alps, Pyrenees, Tatras (Western Carpathians), Retezat (Southern Carpathians) and Rila Mountains (Balkans). 2. For all taxonomic groups we found a hierarchical structure in the community assemblage using distinct scales of lake clustering (number of k-means groups) based on species composition similarity (Hellinger distance). We determined the optimal partition in assemblage types (i.e. number of lake clusters) for each taxonomic group by maximising the sum of the taxon indicative value (IndVal) and performed discriminant analyses, using environmental variables not conditioned by geographical patterns. Relevant environmental variables differed among and within taxonomic groups. Therefore the assemblages respond to a complex environmental mosaic, with the exception of diatom assemblages, which followed an acid–base gradient. 3. The significant environmental variables could be grouped into four general factors: lake size, tropho-dynamic status, acid–base balance and ice-cover duration (i.e., altitudinal gradient). Lake size was significant for the highest number of assemblage types; however, the most significant factor differed among taxonomic groups: acid–base balance for diatoms, lake size for rotifers, ice-cover duration for chydorids and planktonic crustaceans and tropho-dynamic status for chironomids. No single environmental typology accounted for the assemblage structure of all taxonomic groups. 4. However, defining ecological thresholds as values within environmental gradients at which the rate of change in assemblages is accelerated relative to points distant from that

Effects of N : P loading ratios on phytoplankton community composition, primary production and N fixation in a eutrophic lake

Abstract: 1. The aim of this study was to assess the effects of different nitrogen (N) to phosphorus (P) loading ratios on phytoplankton community composition and primary production in a naturally eutrophic ...

Stream macroinvertebrate occurrence along gradients in organic pollution and eutrophication

Abstract: SUMMARY 1. We analysed a large number of concurrent samples of macroinvertebrate communities and chemical indicators of eutrophication and organic pollution [total-P, total-N, NH4– N, biological oxygen demand (BOD5)] from 594 Danish stream sites. Samples were taken over an 11-year time span as part of the Danish monitoring programme on the aquatic environment. Macroinvertebrate communities were sampled in spring using a standardised kick-sampling procedure whereas chemical variables were sampled six to 24 times per year per site. Habitat variables were assessed once when macroinvertebrates were sampled. 2. The plecopteran Leuctra showed a significant negative exponential relationship (r 2 = 0.90) with BOD5 and occurred at only 16% of the sites with BOD5 above 1.6 mg L )1 . Sharp declines with increasing BOD5 levels were found for the trichopteran families Sericostomatidae and Glossosomatidae although they appeared to be slightly less sensitive than Leuctra. Other plecopterans such as Isoperla showed a similar type of response curve to Leuctra (negative exponential) but occurred at sites with relatively high concentrations of BOD5 up to 3–4 mg L )1 . In contrast, the response curve of the isopod Asellus aquaticus followed a saturation function reaching a plateau above 3–4 mg L )1 BOD5 and the dipteran Chironomus showed an exponential increase in occurrence with increasing BOD5 concentration. 3. Macroinvertebrate occurrence appeared to be related primarily to concentrations of BOD5 ,N H 4–N and total-P whereas there were almost no relationships to total-N. Occurrence of a number of taxa showed a stronger relationship to habitat conditions (width and substrate) than chemical variables. 4. Important macroinvertebrate taxa are reduced at concentrations of BOD5 that are normally perceived as indicating unimpacted stream site conditions. Our results confirmed sensitivity/tolerance patterns used by existing bioassessment systems only to some degree.

Assessing ecosystem functioning in streams affected by forest management: increased leaf decomposition occurs without changes to the composition of benthic assemblages

Abstract: 1. Landscape management practices that alter energetic linkages between aquatic and terrestrial habitats can affect associated ecosystem processes, and ultimately the provision of ecosystem services of importance to humanity. Such effects cannot always be inferred from current biomonitoring schemes, which are typically based on assessment of community structural parameters rather than functional attributes related to important ecosystem-level processes. 2. We investigated effects of forest clearcutting, a major landscape-level disturbance known to alter the energetic basis of aquatic food webs, on headwater streams in northern Sweden. The key ecosystem process of leaf decomposition was measured as an index of ecosystem functioning. The biomass of detritivorous shredders was also quantified, along with various community structural parameters associated with the diversity, composition and functional guild organisation of benthic macroinvertebrate assemblages. 3. No differences in macroinvertebrate abundance, diversity or assemblage composition were detected between forested and clearcut streams, and most functional guilds were similarly unaffected, though species density of scrapers was higher in forested than clearcut channels. 4. In contrast, mass loss of two leaf species was elevated in all clearcut streams, with evidence for increases in the efficiency per degree-day of both the microbial and detritivore mediated fractions of decomposition. 5. Increased rates of leaf mass loss in the clearcut streams were associated with greater phosphate concentrations and shredder biomass, and with an increased relative abundance of broadleaves in standing stocks of benthic litter. Together, these findings indicate a more rapid transfer of energy and nutrients through the detrital pathways of our clearcut streams. 6. These results demonstrate the utility of litter decomposition assays for monitoring effects of forest management on stream ecosystem functioning, and have implications for nutrient cycles in landscapes extensively influenced by forest management. The markedly different responses of our functional and structural measures to clearcutting highlight the value of incorporating methods for the functional assessment of ecosystems into biomonitoring schemes.

Community structure in temporary freshwater pools: disentangling the effects of habitat size and hydroregime

Abstract: Summary 1. Hydroregime (duration, frequency and predictability of the aquatic phase) is a key feature of temporary aquatic habitats that not only moulds community structure and diversity (species sorting) but also life history characteristics of the inhabitants (natural selection). However, since hydroregime is a complex multidimensional entity that cannot be estimated from short term observations, morphometric variables are commonly used as proxies for hydroregime, making it impossible to separate effects of habitat size and hydroregime on biota. 2. We have used a simple hydrological model, validated with recent (average r2 = 96%) and historic water level observations (average r2 = 81%), to accurately reconstruct hydroregime based on historical rainfall and evaporation data in a cluster of 36 temporary rock pools in central South Africa. 3. Using the model output, we demonstrated that both hydroregime and habitat size had unique and shared effects on temporary pool biota and that these effects depended on the dispersal modes of the taxa. Model-generated hydrological data explained more variation in community patterns than short-term observations of hydroperiod. Hydroregime was more important for passive dispersers than for active dispersers that can migrate when pools dry up. Basin morphometry was a relatively poor predictor of pool hydroregime. We concluded that simple modelling may greatly improve the resolution of studies linking hydroregime to biological variables. 4. An accurate account of hydrological variation provides a firm foundation to understand community and population structure and dynamics in temporary aquatic habitats. Since many of these habitats have been destroyed or degraded in recent decades, our findings and tools may contribute to the development of reliable conservation guidelines.

The non-indigenous diatom Didymosphenia geminata alters benthic communities in New Zealand rivers

Abstract: SUMMARY 1. Blooms of the benthic, stalked diatom Didymosphenia geminata were first observed in New Zealand in 2004. Since then, D. geminata has spread to numerous catchments in the South Island and is also spreading in its native range. The species is a rare example of an invasive alga in lotic systems. 2. Ecosystem effects may be expected as D. geminata attains unusually high biomass in rivers. We examined data from three independent studies in three South Island, New Zealand, rivers for evidence of effects on periphyton biomass and benthic invertebrate communities. 3. The combined results confirmed that the presence of D. geminata was associated with greatly increased periphyton biomass and, in most cases, increased invertebrate densities. We also recorded shifts in community composition, dominated by increased densities of Oligochaeta. Chironomidae, Cladocera and Nematoda also generally increased in density with D. geminata. Significant increases or declines in other invertebrate taxa were inconsistent among rivers. 4. In all three studies, increased spatial invertebrate community homogeneity was associated with high D. geminata biomass at the within-river scale. However, no declines in taxon richness or diversity were detected. 5. Although ecosystem effects of D. geminata on existing periphyton biomass and invertebrate communities are measurable, no inferences can be made from the present data about effects on higher trophic levels (fish).

Aquaculture, non‐native salmonid invasions and associated declines of native fishes in Northern Patagonian lakes

Abstract: SUMMARY 1. Even though intensive aquaculture production of salmonids in lakes occurs in many locations around the world published studies on the survival and reproductive success of escaped cultured salmonids in freshwater ecosystems are not common. A recent expansion of aquaculture in Chile has led it to become the world’s second largest producer of cultured salmonids. 2. We document the recent history of escaped and self-sustaining salmonid populations over a wide spatial scale and a long temporal scale in Chilean Patagonian lakes. Our hypotheses are that salmonid density in lakes will be higher where there is intensive aquaculture, due to greater numbers of potential escapees. Secondly, if non-native salmonids have adverse impacts on native fishes, increases in the abundance of non-native species should be associated with decreases in relative abundance of native species. Finally, if the first two hypotheses are correct we anticipate that diets of salmonids may show evidence of predation on native fishes, diet overlap with native species, and evidence of the influence of feed from aquaculture operations in the diets of salmonids and native fishes. 3. We sampled six lakes with gill nets from 1992 to 2001. Our results show that the relative abundance of free-living salmonids is closely related to the level of fish farming production. Salmonids are the top predators and in lakes with fish farming the main prey item is native fishes. The relative abundance of native fishes has decreased, most likely due to predation by salmonids. 4. Our study contributes to the understanding of the effects of non-native salmonids in oligotrophic lakes, and it provides a starting point to judge the establishment of new fish farming sites in lakes around the world.

Measuring stream macroinvertebrate responses to gradients of vegetation cover: when is enough enough?

Abstract: Summary 1. Management of stream biodiversity is often tightly linked with the restoration and protection of riparian and catchment vegetation. Despite that, there are no established guidelines on how much forest should be retained or replanted in riparian zones and surrounding catchments to maintain or re-establish instream ecological integrity. In this study, we assess relationships between vegetation cover at multiple spatial scales (reach, segment and catchment) and macroinvertebrate metrics that reflect community structure, ecological condition and biodiversity at 138 Waikato, New Zealand, stream sites sampled in 2006. 2. Percentage of catchment vegetation in native forest had stronger relationships with measured diversity and condition metrics than segment or reach scale measures of riparian vegetation. Functional feeding group metrics were weakly associated with upstream catchment vegetation cover. 3. Of the macroinvertebrate metrics tested, the RIVPAC O/E and an organic pollution tolerance metric based on species presence–absence (Macroinvertebrate Community Index; MCI) had the strongest relationships with percentage native riparian vegetation, followed by the quantitative MCI and measures of the richness and relative abundance of Ephemeroptera, Plecoptera and Trichoptera faunas. The O/E and MCI indicated that catchments with 80–90% in native forest or scrub (low-growing trees) were associated with faunas indicative of “clean” water quality. 4. Of the biodiversity indices considered Fisher’s α Index of species richness had the strongest relationship with percent native riparian vegetation in the upstream catchment. There are no established thresholds for measuring biodiversity loss in New Zealand streams, but this analysis indicates that on average streams draining catchments with 40–60% upstream native vegetation cover retain 80% of the mean biodiversity present in pristine forest streams. 5. This research indicates that riparian management aimed at enhancing macroinvertebrate biodiversity and the ecological condition of streams is likely to be more successful when focused on protecting and/or restoring headwater catchments rather than short stretches of stream.

Does mobility explain variation in colonisation and population recovery among stream fishes

Abstract: Summary 1. Colonisation and population recovery are crucial to species persistence in environmentally variable ecosystems, but are poorly understood processes. After documenting movement rates for several species of stream fish, we predicted that this variable would influence colonisation rates more strongly than local abundance, per cent occupancy, body size and taxonomic family. We also predicted that populations of species with higher movement rates would recover more rapidly than species with lower movement rates and that assemblage structure would change accordingly. 2. To test these predictions, we removed fishes from a headwater and a mainstem creek in southwest Virginia and monitored colonisation over a 2-year period. Using an information–theoretic approach, we evaluated the relative plausibility of 15 alternative models containing different combinations of our predictor variables. Our best-supported model contained movement rate and abundance and was 41 times more likely to account for observed patterns in colonisation rates than the next-best model. Movement rate and abundance were both positively related to colonisation rates and explained 88% of the variation in colonisation rates among species. 3. Population recovery, measured as the per cent of initial abundance restored, was also positively associated with movement rate. One species recovered within 3 months, most recovered within 2 years, but two species still had not recovered after 2 years. Despite high variation in recovery, the removal had only a slight impact on assemblage structure because species that were abundant in pre-removal samples were also abundant in post-removal samples. 4. The significance of interspecific variation in colonisation and recovery rates has been underappreciated because of the widely documented recovery of stream fish assemblages following fish kills and small-scale experimental defaunations. Our results indicate that recovery of the overall assemblage does not imply recovery of each component species. Populations of species that are rare and less mobile will recover more slowly and will be more vulnerable to extinction in systems where chemical spills, hydrological alteration, extreme droughts and other impacts are frequent.

Assessing and conserving groundwater biodiversity: synthesis and perspectives

Abstract: Summary 1. This paper is a synthesis of a special issue on groundwater biodiversity with a focus on obligate subterranean species, the stygobionts. The series of papers constitutes a great leap forward in assessing and understanding biodiversity patterns because of the use of large quantitative data sets obtained over a broad geographic scale. They also represent a conceptual shift, away from a purely taxonomic and phylogenetic focus to the analysis of whole groundwater assemblages. 2. The general patterns emerging for groundwater fauna are: very high levels of endemism, low local diversity relative to regional diversity, a limited number of lineages, occurrence of many relicts, and truncated food webs with very few predators. 3. β-Diversity is at least as important as α-diversity in determining total richness at different scales (aquifer, basin and region) and overall taxa richness increases across spatial scales. 4. Advances in understanding groundwater biodiversity patterns further include identification of several important factors related to geology and hydrology that determine the composition of European stygobiotic assemblages. 5. Important challenges for future research include improving sampling strategies, filling gaps in sampling coverage, intensifying research on theoretical and statistical models, and including functional and genetic diversity components in biodiversity assessments. 6. Strategies are proposed for protecting groundwater biodiversity and an argument is made to integrate biodiversity in groundwater management. Applying principles such as complementarity and flexibility for groundwater biodiversity conservation is a major step toward delineating a reserve network that maximise species representation at the European scale.