The effects of catchment liming on the chemistry and biology of upland Welsh streams: testing model predictions

1. The catchments of three acidified streams in mid Wales were limed in 1987/88. Here we assess their chemical and biological response in comparison with unmanipulated reference streams over a period of 5 years post-liming. 2. Stream chemistry was measured weekly/fortnightly between 1985 and 1992, while macroinvertebrates were sampled annually. Colonization by acid-sensitive taxa was assessed and trends in community structure were monitored by TWINSPAN. Real biological responses were compared with those predicted by an empirical model constructed using chemical data. 3. There were marked changes in stream chemistry following liming: calcium concentrations and pH values increased, while aluminium concentrations decreased to levels similar to those in naturally circumneutral streams. These conditions have persisted since liming. 4. Empirical models predicted that stream invertebrates would respond to the altered stream chemistry, with the establishment of communities typical of circumneutral conditions. 5. Following liming, the taxon richness and abundance of acid-sensitive taxa was significantly higher in limed compared with reference streams. Colonization by, and persistence of acid-sensitive taxa was patchy, however, and richness was still substantially lower than in naturally circumneutral streams. Moreover, contrary to the model predictions, there were no wholesale changes in the structure of macroinvertebrate communities. 6. We conclude that liming has created and maintained chemical conditions suitable for macroinvertebrate communities typical of circumneutral streams, but these chemical changes have not been matched by sustained responses among the biota. The views expressed in this article are those of the authors and do not necessarily reflect those of the NRA.     

The effects of low pH and palliative liming on beech litter decomposition in acid-sensitive streams

The decomposition of allochthonous leaf litter is retarded by stream acidification, but few studies have evaluated whether this effect can be offset by liming – the palliative addition of calcium carbonate either to streams or their catchments. We assessed the response of litter decomposition to pH and experimental liming in Welsh upland streams. Small-mesh (<335 μm) litter-bags containing common beech (Fagus sylvatica L.) were submerged in main river sites along the River Wye, and in replicate acid, circumneutral and experimentally limed tributaries (all n = 3) for 20 days. Beech decomposition was inhibited in acid tributaries and main river sites compared to circumneutral tributaries. Despite having only moderately increased pH relative to acid streams, limed sites had increased decomposition rates that were indistinguishable from naturally circumneutral streams. Decomposition rates increased highly significantly with pH across all 12 sites studied, and values were near identical to those in more prolonged experiments elsewhere. There were no significant variations in shredder numbers with decomposition rate, and no evidence that sites with faster decomposition had smaller shredder proportions. Although based on short-term observations and leaves from just one tree species, these results are consistent with the well-known retardation at low pH of some aspect microbial decomposition (e.g. by hyphomycete fungi). They are among the first to suggest that stream liming to combat acidification might reverse such impacts of low pH. Further data are required on the microbiological causes and ecological consequences of altered detrital processing in acid-sensitive and limed streams.     

Restructuring of Fish Assemblages in Swedish Lakes Following Amelioration of Acid Stress through Liming

A large-scale liming program was initiated in Sweden in the mid-1970s to ameliorate the effects of acidification and to restore acidified lakes and rivers to conditions present before acidification. I compared the fish fauna composition and biomass among limed, acidified, and consistently circumneutral lakes. The study focused on (1) recolonization and fish assemblage development and (2) fish biomass and assemblage composition. Time-series data from 14 limed, 5 acid, and 10 circumneutral lakes, and a data set of 323 limed and 346 nonlimed circumneutral lakes, are analyzed. Long-term monitoring revealed that fish species richness increased after liming and was of the same magnitude in limed lakes as in nonlimed circumneutral lakes after 10–20 years. Species proportions differed between limed and nonlimed lakes, and the relative biomass of Perca fluviatilis (Eurasian perch) was in general higher, whereas cyprinid and pelagic species were less important in the limed lakes than in the nonlimed circumneutral lakes. Recolonization of fish species was found to be a crucial factor for the development of the fish assemblages after liming, and in several limed lakes the reintroduction of fish species has accelerated the recovery of pre-acidification fish assemblages.     

Effects of acidification on the breeding ecology of a stream‐dependent songbird,the Louisiana waterthrush (Seiurus motacilla)

1. We compared breeding ecology of the Louisiana waterthrush (Seiurus motacilla) on acidified and circumneutral streams in the Appalachian Highlands of Southwestern Pennsylvania from 1996 to 2005. 2. Headwater streams impacted by acid mine drainage and/or acidic precipitation showed reduced pH (range 4.5–5.5) compared to four circumneutral streams (pH c. 7). Acid‐sensitive taxa, including most mayflies (Ephemeroptera), were almost completely absent from acidified streams, whereas several acid‐tolerant taxa, especially stonefly (Plecoptera) genera Leuctra and Amphinemura, were abundant. 3. Louisiana waterthrush breeding density (c. 1 territory km^?1) was significantly reduced on acidified streams compared to circumneutral streams (>2 territories km^?1). Territories on acidified streams were almost twice as long as on circumneutral streams. Territories usually were contiguous on circumneutral streams, but they were often disjunct on acidified streams. Breeding density declined on one acidified stream that we studied over a 10‐year period. 4. Clutch initiation was significantly delayed on acidified streams, on average by 9 days in comparison to circumneutral streams, and first‐egg dates were inversely related to breeding density. Birds nesting along acidified streams laid smaller clutches, and nestlings had shorter age‐adjusted wing lengths. Stream acidity had no effect on nest success or annual fecundity (fledglings/female). However, the number of young fledged km^?1 was nearly twice as high on circumneutral streams as on acidified streams. 5. Acidified streams were characterized by a younger, less site‐faithful breeding population. Individuals were less likely to return multiple years to breed, allowing inexperienced breeders to settle on acidified streams. Pairing success was lower on acidified streams, and we observed four cases of waterthrushes emigrating from territories on acidified streams to nearby circumneutral streams in the following year. 6. We conclude that acidified headwaters constitute lower quality habitat for breeding Louisiana waterthrush. However, breeding birds can apparently compensate for reduced prey resources to fledge young on acidified streams by increasing territory size, foraging in peripheral non‐acidified areas, and by provisioning young with novel prey.     

Episodic acidification affects the breakdown and invertebrate colonisation of oak litter

1. Although European streams are now recovering chemically from acidification, biological recovery is limited. One hypothesis is that continuing acid episodes restrict acid‐sensitive species in recovering locations either through direct toxicity or by affecting ecological processes. Here, we test this hypothesis by assessing the effects of episodic acid exposure on the breakdown and macroinvertebrate colonisation of oak (Quercus robur) litter. 2. Over 83 days, acid episodes of 4 days’ duration were simulated by repeatedly transplanting litter bags of contrasting mesh size between replicate acidic and circumneutral streams around Llyn Brianne (Wales, U.K.). Results were compared against controls from circumneutral streams and circumneutral transplants, while invertebrates colonising litter were compared with adjacent assemblages. 3. Breakdown was retarded significantly by repeated acid exposure in comparison with circumneutral transplants, but only in litter to which invertebrates had access. Overall breakdown was also significantly slower in fine‐mesh than in coarse‐mesh bags. 4. Plecopteran shredders were the major invertebrate colonists of litter, along with smaller numbers of grazers and predators. However, acid exposure eliminated or suppressed acid‐sensitive families, resulting in an overall composition converging on that in acid streams. 5. The rapid loss of sensitive invertebrates from acid‐exposed litter supports the hypothesis that acid episodes suppress biological recovery from acidification through direct physiological effects. However, our litter breakdown data indicate that (i) some effects of acid episodes could be mediated through litter processing; and (ii) episodic acidification could disrupt litter breakdown through effects on invertebrate composition or activity. These data suggest that delayed biological recovery from acidification can reflect a combination of direct toxic and indirect ecological effects.     

Intensive sampling and transplantation experiments reveal continued effects of episodic acidification on sensitive stream invertebrates

1. Although mean pH is increasing in acidified streams throughout Europe, benthic invertebrates are recovering only slowly. One developing hypothesis is that acid episodes continue to restrict recolonisation and recovery. Here, we used intra‐annual sampling in replicate circumneutral, acid and episodic streams at the Llyn Brianne experimental catchments to assess possible mayfly exposure to episodic effects quantified from a risk index based on long‐term discharge. Episodic effects were then tested using short‐term transplantations of animals in enclosures. 2. No mayflies occurred in acid streams while Baetis rhodani occurred in episodic streams but declined to zero density following low pH in autumn. Ephemerella ignita, Baetis vernus, B. muticus, Rhithrogena semicolorata, Ecdyonurus spp. and Heptagenia lateralis occurred only in the circumneutral streams. The first two species were present as nymphs only during July and August so that episodic exposure risk was minimal, but the remaining species occurred as nymphs in all months. 3. Baetis rhodani transplanted during base‐flow (September 2003) and high‐flow (April 2004) were exposed to either; (i) continual (chronic) exposure in the acid streams over 16 days or (ii) repeated short‐term (episodic) exposure to acid conditions for 2 × 4 day periods interspersed with 4‐day recovery periods in the circumneutral streams. Baetis survival in the circumneutral streams (always pH > 5.7) remained high during both low‐ and high‐flow. By comparison, mortality increased (P < 0.01) during chronic and episodic exposure, but only during high‐flow (mean pH 3.8–3.9, cf. 5.5–5.8 at low flow) when mortality varied significantly in the order chronic (>80%) > episodic (>40%) >circumneutral (<10%). 4. We conclude that, despite Europe‐wide trends towards chemical recovery from acidification, even short exposures to high‐flow events at Llyn Brianne are still sufficiently acid to reduce the survival of B. rhodani. Most mayfly species absent from acid and episodic streams have life cycles that would increase acid exposure risks during autumn and winter, and this may be sufficient to explain their current distribution.     

Effects of spring acid episodes on macroinvertebrates revealed by population data and in situ toxicity tests

1. While streams in Europe and North America are now recovering chemically from chronic acidification, severe episodic acidification continues to threaten sensitive biota. To appraise further the biological importance of episodic acidification, we surveyed the distribution of the mayfly Baetis alpinus in streams in the Southern Alps (Canton Ticino, Switzerland) in relation to runoff acidity during spring floods. Moreover, to improve mechanistic understanding, in situ toxicity assays were carried out on nymphal B. alpinus during low flows and spring floods, both in streams prone to acid episodes and in well‐buffered controls. 2. Streams surveyed for invertebrates represented three groups which contrasted in susceptibility to episodic acidity. Group one included streams that were acid (alkalinity <0) in spring; group two streams were susceptible to acid episodes because of low base‐flow alkalinity (<200 μeq L^?1); and group three streams were well‐buffered and unlikely ever to be acid. The abundance of B. alpinus was similar among groups during stable flows in winter and summer, but was significantly lower in groups one and two following spring snowmelt. 3. During the bioassays, control streams remained circumneutral to alkaline (pH >6.4). By contrast, episodic streams were circumneutral at low flows, but became acid (pH 4.5–5.6 and total dissolved aluminium to 276 μg L^?1) during intense spring snowmelt. After 15‐day exposures, nymphal B. alpinus survival in the circumneutral control streams exceeded 92% irrespective of flow. In the episodic streams, survival matched the controls during low flows, but declined significantly to 10–20% during acid episodes in spring. Shorter exposure (2–4 days) to pH 5.5–5.7 did not cause significant mortality either during the exposure or over the following 7 days, indicating that B. alpinus might recover from short acid episodes. 4. Our data suggest that the spring distribution of B. alpinus in acid sensitive parts of the Alps directly reflects the toxicity of acid runoff during snowmelt. Our study illustrates that even mild episodic acidification can have significant consequences in Alpine streams for one of the most important invertebrate indicators of acidity.     

Acute effects on perch (Perca fluviatilis) and long-term effects on whitefish (Coregonus lavaretus pallasi) of liming of an acidified lake

The acute effects on perch and long‐term effects on whitefish were studied after liming one side of a lake [initial pH: 4.6–5.5; aluminium (Al): 29–54 μg L^?1; Ca²⁺: 0.02–0.07 mmol L^?1] that was divided into two parts with a plastic curtain. Some Al precipitation was observed on the gill surface of perch 1 day after liming; the concentration of plasma sodium of perch females in the limed side was also higher than in the acidic side. Nearly 6 months after liming, at the spawning time of whitefish, the plasma chloride concentration of whitefish in the limed side was higher and blood glucose concentration lower than in the acidic side. Four of five whitefish males from the limed side, but only one of five males and none of the five females from the acidic side, were ready to spawn. The growth of whitefish in the limed side was also more rapid. These changes illustrate that liming had no acute harmful effects on perch, and allowed whitefish to recover from acidity‐related physiological stress.     

Lake responses to long‐term disturbances and management practices

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Assessing the short-term response of stream diatoms to acidity using inter-basin transplantations and chemical diffusing substrates

1. Acid‐base status has major effects on diatoms, but there is little information on their short‐term response to changing acidity. This is despite the use of diatoms as bioindicators in streams where acid episodes are important during rainstorms (hours to days) or snowmelt (days to weeks). In the Llyn Brianne experimental catchments (Wales, UK), we attempted to mimic the effects of short‐term acidification by (i) reciprocally transplanting diatoms between two streams of contrasting acidity and (ii) using acid‐diffusing substrates. 2. Diatom diversity decreased rapidly on substrata transplanted from the circumneutral into the acidic stream, and increased in the reciprocal transplantation. Changes in dominant taxa occurred within three days in the acidic stream because of the rapid growth of Eunotia exigua, and by nine days in the circumneutral stream because of the proliferation of Achnanthidium minutissimum. Transplants were near indistinguishable from ambient assemblages by day 12. 3. There were no effects of enclosures on assemblage composition, but diatoms responded more rapidly to altered chemistry in enclosures with coarse mesh (26 × 50 mm) than finer mesh (320 μm). 4. Chemical diffusing substrates comprised terracotta tiles attached to dosing reservoirs that created locally acid (using H₂SO₄) or metal‐rich conditions (using MnSO₄) in the circumneutral stream over 26 days. Diatom responses were compared with reference substrates dosed with deionised or circumneutral stream water, and we also assessed whether effects were moderated by macroinvertebrate grazers. 5. Surface pH was lower by 1–2 pH units on acid‐dosed substrates than on reference tiles or in surrounding streamwater. Grazed assemblages on acid‐dosed substrates differed significantly from ungrazed reference assemblages, acquiring significantly greater relative abundance of Eunotia spp. However, the magnitude of response was less than in the between‐stream transplantations either because (i) metal exposure and base cation concentrations differed between the transplants and dosing substrates or (ii) diatom response to reduced pH on the diffusing substrates was restricted by the scarcity of acidobiontic diatoms in the circumneutral stream. Similar filter, founder or dominance effects might also affect diatom responses to real acid episodes. 6. These data show that diatom assemblages can respond rapidly and directly to changes in acid‐base status, but short‐term acidification might affect diatoms more rapidly than subsequent recovery. Because the experimental methods used were imperfect representations of episodic effects, diatom response to real acid events requires further field evaluation.     

Determinants of community organization of a South African mesic grassland

Abstract. Question: What is the long‐term influence of nutrient availability, productivity and soil pH on grassland community organization? Location: Ukulinga research farm, KwaZulu‐Natal, South Africa. Methods: The influence of fertilization on soil pH, nitrogen (N) and phosphorus (P) on variation in plant traits, community composition and species richness were examined in a 50‐year grassland fertilization experiment. Results: Averaged over 30 years, above‐ground net primary production (ANPP) was 337, 428 and 518 g.m^‐2 in sites not fertilized, fertilized with N, and fertilized with N plus P respectively. ANPP depended directly on N‐fertilization but not on P‐fertilization or liming, and responded positively to the interaction of N (first limiting nutrient) and P (second limiting nutrient). Short narrow‐leaved grass species —Themeda triandra, Tristachya leucothrix and Setaria nigrirostris— dominated sites of lowest ANPP where N was limiting (unfertilized, P‐fertilized or limed sites). A tall narrow‐leaved species, Eragrostis curvula, dominated sites of intermediate ANPP where P was limiting (N‐fertilized sites). By contrast, a tall broad‐leaved species, Panicum maximum, dominated the most productive sites where neither N nor P were limiting (N‐ and P‐fertilized sites). Certain species responded to liming and type of N‐fertilizer apparently because of their effects on soil pH. N‐fertilization reduced the density of herbaceous dicots (forbs) from 14 (unfertilized) to two (high N, no P, no lime) and five species per m² (high N, no P, limed). This effect was attributed to increased ANPP and a decrease in soil pH from 4.6 (KCl) in unfertilized sites to 3.49 (high N, no lime) and 4.65 (high N and lime). Soil acidification had no effect on grass species richness but influenced the abundance of certain species. Conclusions: Grassland community organization is determined not only by the influence of N availability, but also by the hierarchical interaction of N and P availability, in part through their compounded effect on ANPP, and by individualistic species responses to soil pH.     

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using ¹³CO₂ pulse labelling, in an upland grassland ecosystem in Scotland. The use of ¹³C enabled detection, in the field, of the effect of a 4‐year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher δ¹³C‰ values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4‐year period of liming indicated that the standing biomass was higher in plots that received lime. Lower δ¹³C‰ values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990–1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.     

The influence of introduced trout on the benthic communities of paired headwater streams in the Sierra Nevada of California

1. Non‐native trout have been stocked in streams and lakes worldwide largely without knowledge of the consequences for native ecosystems. Although trout have been introduced widely throughout the Sierra Nevada of California, U.S.A., fishless streams and their communities of native invertebrates persist in some high elevation areas, providing an opportunity to study the effects of trout introductions on natural fishless stream communities. 2. We compared algal biomass and cover, organic matter levels and invertebrate assemblages in 21 natural fishless headwater streams with 21 paired nearby streams containing stocked trout in Yosemite National Park. 3. Although environmental conditions and particulate organic matter levels did not differ between the fishless and trout streams, algal biomass (as chlorophyll a concentration) and macroalgal cover were, on average, approximately two times and five times higher, respectively, in streams containing trout. 4. There were no differences in the overall densities of invertebrates in fishless versus paired trout streams; however, invertebrate richness (after rarefaction), evenness, and Simpson and Shannon diversities were 10–20% higher in fishless than in trout streams. 5. The densities of invertebrates belonging to the scraper‐algivore and predator functional feeding guilds were higher, and those for the collector‐gatherer guild lower, in fishless than trout streams, but there was considerable variation in the effects of trout on specific taxa within functional feeding groups. 6. We found that the densities of 10 of 50 common native invertebrate taxa (found in more than half of the stream pairs) were reduced in trout compared to fishless streams. A similar number of rarer taxa also were absent or less abundant in the presence of trout. Many of the taxa that declined with trout were conspicuous forms (by size and behaviour) whose native habitats are primarily high elevation montane streams above the original range of trout. 7. Only a few taxa increased in the presence of trout, possibly benefiting from reductions in their competitors and predators by trout predation. 8. These field studies provide catchment‐scale evidence showing the selective influence of introduced trout on stream invertebrate and algal communities. Removal of trout from targeted headwater streams may promote the recovery of native taxa, community structure and trophic organisation.     

Effects of episodic acidification on macroinvertebrate assemblages in Swiss Alpine streams

1. Despite long‐standing ecotoxicological evidence that episodes of acidification in streams are important biologically, there is still uncertainty about their effects on invertebrate communities. We surveyed 20 streams in an acid sensitive Alpine area (Canton Ticino, Switzerland), where episodes are driven by snowmelt in spring and by rainstorms at other times of the year. Samples of water and macroinvertebrates were collected in pre‐event conditions (winter and summer) and during periods of high flow (spring and autumn). 2. Using pH, [Ca²⁺] and [Alⁿ⁺], streams were clustered into six acid–base groups that were either well buffered (groups 4–6), soft‐water with stable pH (group 3), or poorly buffered with low pH at high flow (groups 1 and 2). 3. Severe episodes occurred during snowmelt, when the group 1 streams became acidic with pH down to 5.0 and [Alⁿ⁺] up to 140 μg L^?1. pH declined to 6.2 in streams of group 2, but remained > 6.6 in groups 3–6. 4. Detrended canonical correspondence analysis showed that the streams sensitive to episodes (groups 1 and 2) had different invertebrate assemblages from well‐buffered sites (groups 4 and 5) or soft‐water stable streams (group 3), with faunal differences largest following spring snowmelt. Empididae, Isoperla rivulorum, Rhithrogena spp. and Baetis spp. were scarce in streams sensitive to episodes (groups 1 and 2). By contrast, Amphinemura sulcicollis was scarcer in hard‐water streams (groups 4–6). Taxonomic richness was lower in the episodic streams of group 1 than in other streams. 5. Together, these results indicate clear biological differences between acid‐sensitive streams with similar low‐flow chemistry but contrasting episode chemistry. Severe episodes of acidification appear to affect macroinvertebrate assemblages in streams in the southern Swiss Alps.     

Phytoplankton in 63 limed lakes in comparison with the distribution in 500 untreated lakes with varying pH

As a result of the different weathering of lime in soils and varying acid deposition, the pH of Swedish oligotrophic lake waters has a broad range from ca. 4 to 8.5. Phytoplankton species occur in all lakes, but the composition and species richness varies in relation to pH. The maximum number of taxa was recorded within the pH-range 7.0–7.6, while the number decreased at higher pH and especially at pH below 5.6. During 1975–1985, thousands of lakes in Sweden were limed with the intention to restore biotic diversity and production. In order to find out whether there are differences in quality between limed and unacidified waters, the distribution of 57 selected phytoplankton taxa in 63 limed lakes was compared with that in 500 unlimed lakes with varying pH. The study shows that most common species in unacidified lakes also occur in the limed ones. However, several species of Cyanophyceae and Conjugatophyceae were missing especially in lakes that were strongly acidified before liming. On the other hand, some taxa were more frequent in the limed lakes.     

Naturally acid freshwater ecosystems are diverse and functional: evidence from boreal streams

Acidification has strongly affected natural ecosystems due to the deposition of acidifying pollutants in geographical areas with low buffering capacity. Here we show both that taxonomic richness of benthic invertebrates (as a measure of stream health) and breakdown rate of leaf litter (a major process in stream ecosystems) in three naturally acid streams were not significantly different from richness and breakdown rates of three circumneutral control streams in the same area in northern Sweden. In contrast, a comparative data-set representing a gradient from circumneutral to acidified streams in France showed decreasing richness and litter breakdown rates (by 60% and 70% between pH 6.5 and 4.5, respectively) typical of acidified streams. The strong negative effects found in many freshwater systems may be a consequence of low capacity to adjust to rapidly changed conditions. In contrast, organisms in naturally acidic systems, such as streams of the boreal region in catchments with accumulating organic material and a geology poor in buffering substances, seem better adapted as a consequence of protracted exposure to low pH over evolutionary time. These results have implications for the management of streams and rivers in northern Sweden, where considerable efforts are spent on remediation without consideration of the fact that the natural state of many of these systems is acidic.     

Long‐term effects of liming in Norwegian softwater lakes: the rise and fall of bulbous rush (Juncus bulbosus) and decline of isoetid vegetation

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Recent land‐use changes affect stream ecosystem processes in a subtropical island in Brazil

Land‐use changes such as conversion of natural forest to rural and urban areas have been considered as main drivers of ecosystem functions decline, and a large variety of indicators has been used to investigate these effects. Here, we used a replicated litter‐bag experiment to investigate the effects of land‐use changes on the leaf‐litter breakdown process and leaf‐associated invertebrates along the forest–pasture–urban gradient located in a subtropical island (Florianópolis, SC, Brazil). We identified the invertebrates and measured the litter breakdown rates using the litter bags approach. Litter bags containing 3 g of dry leaf of Alchornea triplinervia were deployed on forest rural and urban streams. Principal component analysis, based on physico‐chemical variables which, confirmed a gradient of degradation from forest to urban streams with intermediate values in rural areas. In accordance, shredder richness and abundance were lower in rural and urban than in forest streams. The land‐use changes led also to the dominance of tolerant generalist taxa (Chironomidae and Oligochaeta) reducing the taxonomic and functional diversity in these sites. Leaf‐litter breakdown rates decreased from forest to rural and finally to urban areas and were associated with changes in pH, water velocity, dissolved oxygen and abundance of leaf‐shredding invertebrates, although global decomposition rates did not differ between rural and urban streams. Overall, this study showed that land‐use changes, namely to rural and urban areas, have a strong impact on tropical streams ecosystems, in both processes and communities composition and structure. Despite of being apparently a smaller transformation of landscape, rural land use is comparable to urbanisation in terms of impact in stream functioning. It is thus critical to carefully plan urban development and maintain forest areas in the island of Florianópolis in order to preserve its natural biodiversity and aquatic ecosystems functioning.     

Increases in soil organic carbon sequestration can reduce the global warming potential of long‐term liming to permanent grassland

The application of calcium‐ and magnesium‐rich materials to soil, known as liming, has long been a foundation of many agro‐ecosystems worldwide because of its role in counteracting soil acidity. Although liming contributes to increased rates of respiration from soil thereby potentially reducing soils ability to act as a CO₂ sink, the long‐term effects of liming on soil organic carbon (C_org) sequestration are largely unknown. Here, using data spanning 129 years of the Park Grass Experiment at Rothamsted (UK), we show net C_org sequestration measured in the 0–23 cm layer at different time intervals since 1876 was 2–20 times greater in limed than in unlimed soils. The main cause of this large C_org accrual was greater biological activity in limed soils, which despite increasing soil respiration rates, led to plant C inputs being processed and incorporated into resistant soil organo‐mineral pools. Limed organo‐mineral soils showed: (1) greater C_org content for similar plant productivity levels (i.e. hay yields); (2) higher ¹⁴C incorporation after 1950s atomic bomb testing and (3) lower C : N ratios than unlimed organo‐mineral soils, which also indicate higher microbial processing of plant C. Our results show that greater C_org sequestration in limed soils strongly reduced the global warming potential of long‐term liming to permanent grassland suggesting the net contribution of agricultural liming to global warming could be lower than previously estimated. Our study demonstrates that liming might prove to be an effective mitigation strategy, especially because liming applications can be associated with a reduced use of nitrogen fertilizer which is a key cause for increased greenhouse gas emissions from agro‐ecosystems.     

Climate change effects on upland stream macroinvertebrates over a 25-year period

Climate change effects on some ecosystems are still poorly known, particularly where they interact with other climatic phenomena or stressors. We used data spanning 25 years (1981–2005) from temperate headwaters at Llyn Brianne (UK) to test three hypotheses: (1) stream macroinvertebrates vary with winter climate; (2) ecological effects attributable to directional climate change and the North Atlantic Oscillation (NAO) are distinguishable and (3) climatic effects on macroinvertebrates depend on whether streams are impacted by acidification. Positive (i.e. warmer, wetter) NAO phases were accompanied by reduced interannual stability (=similarity) in macroinvertebrate assemblage in all streams, but associated variations in composition occurred only in acid moorland. The NAO and directional climate change together explained 70% of interannual variation in temperature, but forest and moorland streams warmed respectively by 1.4 and 1.7°C (P<0.001) between 1981 and 2005 after accounting for NAO effects. Significant responses among macroinvertebrates were confined to circumneutral streams, where future thermal projections (+1, +2, +3°C) suggested considerable change. Spring macroinvertebrate abundance might decline by 21% for every 1°C rise. Although many core species could persist if temperature gain reached 3°C, 4–10 mostly scarce taxa (5–12% of the species pool) would risk local extinction. Temperature increase in Wales approaches this magnitude by the 2050s under the Hadley HadCM3 scenarios. These results support all three hypotheses and illustrate how headwater stream ecosystems are sensitive to climate change. Altered composition and abundance could affect conservation and ecological function, with the NAO compounding climate change effects during positive phases. We suggest that acidification, in impacted streams, overrides climatic effects on macroinvertebrates by simplifying assemblages and reducing richness. Climatic processes might, nevertheless, exacerbate acidification or offset biological recovery.