The sensitivity of surface waters of Great Britain to acidification predicted from catchment characteristics

Paleoreconstructions of ciliate communities reveal long-term ecological changes in temperate lakes

Ciliates are unicellular heterotrophic organisms that play a key role in aquatic planktonic and benthic food webs. Advances in sedimentary DNA (sed-DNA) analysis offer the possibility to integrate these bioindicators in paleoenvironmental reconstructions. In this study, we used the top–bottom paleolimnological approach and metabarcoding techniques applied to sed-DNA to compare the recent and past (i.e. prior to major anthropogenic impacts) ciliate communities of 48 lakes located along an elevation gradient. Our results show an overall decline in the β-diversity in recent time, especially in lowland lakes, which are more strongly exposed to local human pressures. Analyses of the functional groups indicate important restructuration of the food web, including the recent increase in mixotrophs. Moreover, changes in the benthic ciliates were consistent with the widespread increase in deep water anoxia. Our results provided evidence that sed-DNA can uncover information about past ciliate communities on a wide variety of lakes. Overall, our study demonstrates the potential of using ciliates as new paleoindicators, integrating information from the pelagic to the benthic zones, and providing valuable insights into ecosystem functioning through a trait-based functional community approach. As paleoindicator, they thus offer a more holistic view on the long-term changes of aquatic ecosystems.

Conservation challenges and action for the Critically Endangered Singapore freshwater crab Johora singaporensis

The Singapore freshwater crab Johora singaporensis is known only from three streams in Singapore and is among the top 100 most threatened species in the world. It is the only member of the genus Johora, endemic to the Malay Peninsula, to be categorized as Critically Endangered on the IUCN Red List. Extensive surveys conducted over 1 year to determine the extent of the species’ distribution revealed that the population has declined significantly at its type locality in Bukit Timah Nature Reserve, Singapore Island's oldest protected forest area. The species remains extant in two unprotected streams in Bukit Batok and Bukit Gombak, which are located outside the nature reserve. Acidification of streams as a result of acid precipitation may be a factor in the decline of the species. If so, this will be the first documented instance of a species being affected negatively by anthropogenic acidification of a tropical freshwater system. The occurrence of this threatened species in a protected area has not, therefore, guaranteed its survival. There is a need for long-term and continuous monitoring of species of high conservation value, together with other measures that focus on habitat protection. Conservation efforts by the National Parks Board of Singapore in collaboration with other government agencies are ongoing in an effort to ensure the survival of this important species.

Manganese in the upper Severn mid-Wales

The concentrations of manganese (Mn) in the Upper River Severn (the Plynlimon catchments) are examined in relation to rainfall, cloud water, throughfall, stemflow and stream water concentrations where there is over 20 years of monitoring data available. Manganese concentrations are particularly low in rainfall and cloud water, with maximum concentrations occurring under low volumes of catch due to atmospheric "washout" of contaminants and dry deposition. There is strong Mn enrichment in throughfall and stemflow and this is probably linked to cycling through the vegetation. Manganese in the streams and groundwaters are primarily supplied from within-catchment sources. The highest concentrations occur within the tree canopy probably due to element cycling and in groundwaters due to mobilisation from the rock. Manganese concentrations in streams are at their lowest during spring and summer following long dry spells, with rapid increases following subsequent rain. There is no clear long-term trend in Mn concentration in the streams although there are increases in Mn concentrations for years when there is extensive felling of spruce plantation forest and in 1995 following a more extensive dry period. New high resolution monitoring picks up the effects of the rising limb of the hydrograph when concentrations rapidly increase, diurnal patterns during summer low-flow periods and contrasting dynamics between moorland and forested catchments.

Relationship between ammonia stomatal compensation point and nitrogen metabolism in arable crops: current status of knowledge and potential modelling approaches

The ammonia stomatal compensation point of plants is determined by leaf temperature, ammonium concentration ([NH4+]apo) and pH of the apoplastic solution. The later two depend on the adjacent cells metabolism and on leaf inputs and outputs through the xylem and phloem. Until now only empirical models have been designed to model the ammonia stomatal compensation point, except the model of Riedo et al. (2002. Coupling soil-plant-atmosphere exchange of ammonia with ecosystem functioning in grasslands. Ecological Modelling 158, 83-110), which represents the exchanges between the plant's nitrogen pools. The first step to model the ammonia stomatal compensation point is to adequately model [NH4+]apo. This [NH4+]apo has been studied experimentally, but there are currently no process-based quantitative models describing its relation to plant metabolism and environmental conditions. This study summarizes the processes involved in determining the ammonia stomatal compensation point at the leaf scale and qualitatively evaluates the ability of existing whole plant N and C models to include a model for [NH4+]apo.

Multiscale drivers of water chemistry of boreal lakes and streams

The variability in surface water chemistry within and between aquatic ecosystems is regulated by many factors operating at several spatial and temporal scales. The importance of geographic, regional-, and local-scale factors as drivers of the natural variability of three water chemistry variables representing buffering capacity and the importance of weathering (acid neutralizing capacity, ANC), nutrient concentration (total phosphorus, TP), and importance of allochthonous inputs (total organic carbon, TOC) were studied in boreal streams and lakes using a method of variance decomposition. Partial redundancy analysis (pRDA) of ANC, TP, and TOC and 38 environmental variables in 361 lakes and 390 streams showed the importance of the interaction between geographic position and regional-scale variables. Geographic position and regional-scale factors combined explained 15.3% (streams) and 10.6% (lakes) of the variation in ANC, TP, and TOC. The unique variance explained by geographic, regional, and local-scale variables alone was <10%. The largest amount of variance was explained by the pure effect of regional-scale variables (9.9% for streams and 7.8% for lakes), followed by local-scale variables (2.9% and 5.8%) and geographic position (1.8% and 3.7%). The combined effect of geographic position, regional-, and local-scale variables accounted for between 30.3% (lakes) and 39.9% (streams) of the variance in surface water chemistry. These findings lend support to the conjecture that lakes and streams are intimately linked to their catchments and have important implications regarding conservation and restoration (management) endeavors.

A linked spatial and temporal model of the chemical and biological status of a large, acid-sensitive river network

Freshwater sensitivity to acidification varies according to geology, soils and land-use, and consequently it remains difficult to quantify the current extent of acidification, or its biological impacts, based on limited spot samples. The problem is particularly acute for river systems, where the transition from acid to circum-neutral conditions can occur within short distances. This paper links an established point-based long-term acidification model (MAGIC) with a landscape-based mixing model (PEARLS) to simulate spatial and temporal variations in acidification for a 256 km(2) catchment in North Wales. Empirical relationships are used to predict changes in the probability of occurrence of an indicator invertebrate species, Baetis rhodani, across the catchment as a function of changing chemical status. Results suggest that, at present, 27% of the river network has a mean acid neutralising capacity (ANC) below a biologically-relevant threshold of 20 microeq l(-1). At high flows, this proportion increases to 45%. The model suggests that only around 16% of the stream network had a mean ANC < 20 microeq l(-1) in 1850, but that this increased to 42% at the sulphur deposition peak around 1970. By 2050 recovery is predicted, but with some persistence of acid conditions in the most sensitive, peaty headwaters. Stream chemical suitability for Baetis rhodani is also expected to increase in formerly acidified areas, but for overall abundance to remain below that simulated in 1850. The approach of linking plot-scale process-based models to catchment mixing models provides a potential means of predicting the past and future spatial extent of acidification within large, heterogeneous river networks and regions. Further development of ecological response models to include other chemical predictor variables and the effects of acid episodes would allow more realistic simulation of the temporal and spatial dynamics of ecosystem recovery from acidification.

Response of acid lakes in the UK to reductions in atmospheric deposition of sulfur

United Kingdom emissions of sulfur dioxide declined by 82% between 1970 and 2000. This decline has been accompanied by reductions in the atmospheric deposition of anthropogenic sulfur. A further expected response, possibly delayed, is a reduction in sulfur concentrations, as sulphate, in upland surface waters. Associated water quality changes, including an increase in alkalinity and reduction in acidity are also anticipated. We examine annual data from 1988 to the present from 10 lakes in the UK acid waters monitoring network for evidence of changes in water chemistry as a response to reductions in the atmospheric deposition of sulfur. We show that there is a close association between reduction in sulfur deposition and reduction in lake water concentrations of sulphate and this provides some supporting evidence for the efficacy of the international agreements to reduce S emissions. However, only 3 of the 10 lakes show an upward trend in alkalinity, and this is not related in a simple fashion to reductions in sulphate concentrations. This suggests that recovery from acidification, measured as an increase in pH and alkalinity, while requiring a reduction in sulphate concentration, may not occur over the same time scale.

Acidification sensitivity and critical loads of acid deposition for surface waters in China

Although decades of severe acid deposition have not resulted in serious surface water acidification in China, at present, the risk of some freshwaters becoming acidified cannot be neglected. To know more clearly about the possible impact acid deposition would have on Chinese surface waters, it is necessary to study the sensitivity of those surface waters to acidification and their critical loads of acid deposition on a national scale. Here we assess the acidification sensitivity of Chinese surface waters using an approach based on geology, soils, land use and hydrological conditions. Critical loads of S, N and acidity were also evaluated by a first-order acidity balance (FAB) model. Results show that most surface waters in China have very high critical loads of S and acidity and are not susceptible to acidification. Surface waters can be divided into three groups according to both sensitivity classes and critical loads. The few most sensitive surface waters are located in the northern part of Daxinganling region, with critical loads of S deposition and acidity lower than 2 keq ha(-1) year(-1). Surface waters in the northeastern region draining dark brown forest soils and in southern China belong to the second class of acidification sensitivity and their critical loads of S and acidity are generally between 2 and 15 keq ha(-1) year(-1), indicating they are not likely to be acidified under any flow conditions. Surface waters in other parts of China will not be acidified to any degree, with critical loads much higher than 15 keq ha(-1) year(-1). The magnitude and spatial distribution pattern of acidification sensitivity have significant similarity to the critical loads of S and acidity for Chinese surface waters. Although most surface waters are not likely to be acidified, attention should still be paid to the possible adverse impact acid deposition would have, especially in northeastern China, where the surface waters are the most sensitive, and the southern region, where acid pollution is among the most severe in China.

Partitioning the variation within the acid neutralizing capacity of surface waters in Scotland in relation to land cover, soil and atmospheric depositional factors

A method of decomposing the variation in the acid neutralizing capacity (ANC) of surface waters in Scotland is described. Using national datasets, a series of variables relating to 703 catchments across Scotland is divided into three components representing (i) land cover, (ii) soil and (iii) atmospheric deposition/altitude. Redundancy analysis (RDA) and (partial) redundancy analysis are used to quantify the amount of variation in ANC uniquely attributable to each of these components, independent of the effects of the others. The variation accounted for by covarying combinations of these components is also determined. Approximately 55% of the total variation in ANC across the 703 sites is explained by the variables representing catchment characteristics and atmospheric deposition. Of this, 8.5%, 2.4% and 6.9% are uniquely attributable to the land cover, soil and deposition/altitude components, respectively. A further 38% of ANC variation is associated with the covariation between components, with 18% accounted for by the combination of all three. Approximately 45% of the variation in ANC remains unexplained. The results reflect the integrated nature of catchment processes and demonstrate, for these data, that it is a combination of land cover, soil and deposition and altitude factors which most explain variation in freshwater ANC level. The approach offers a tool with which to assess the sensitivity of surface waters to acid deposition at a regional scale and provides a way of identifying regional differences in catchment response to acid loading.

Modelling instream nitrogen variability in the Dee catchment, NE Scotland

The Integrated Nitrogen in CAtchments model (INCA) was applied to the River Dee, Aberdeenshire, NE Scotland. To a first approximation the model was able to simulate the annual mean streamwater NO3-N concentrations observed along the length of the main channel. This provided the basis for using INCA to subsequently explore the effects of N deposition and land use management on streamwater NO3-N concentrations and loads. On an annual timescale, the model predictions suggest that NO3-N concentrations will decrease by 5% following a 20% reduction in fertiliser application. Furthermore, model results also suggest that a 50% increase in N deposition will cause a 15% increase in the streamwater NO3-N concentrations. The utility of INCA as a tool for catchment management is discussed, current limitations are highlighted and possible improvements are suggested.