Diverging effects of anthropogenic acidification and natural acidity on community structure in Swedish streams

High environmental temperature and low pH stress alter the gill phenotypic plasticity of Hoven's carp Leptobarbus hoevenii

Climate warming and low pH environment are known to negatively impact all levels of aquatic organism from cellular to organism and population levels. For ammonotelic freshwater species, any abiotic factor fluctuation will cause disturbance to the fish, specifically at the gills which act as a multifunctional organ to support all biological processes. Therefore, this study was designed to investigate the effect of temperature (28 vs. 32°C) and pH (7.0 vs. 5.0) stress on the gill plasticity of Hoven's carp after 20 days of continuous exposure. The results demonstrated that high temperature and low pH caused severe changes on the primary and secondary lamellae as well as the cells within lamellae. An increasing trend of the proportion available for gas exchange was noticed at high temperature in both pH exposures, which resulted from a reduction of the primary lamellae width with elongated and thinner secondary lamellae compared to fishes at ambient temperature. Following exposure to high temperature and acidic pH, Hoven's carp experienced gill modifications including aneurysm, oedema, hypertrophy, curling of secondary lamellae, epithelial lifting, hyperplasia and lamellae fusion. These modifications are indicators of the coping mechanism of Hoven's carp to the changing environment in order to survive.

Multi-stressor impacts on fungal diversity and ecosystem functions in streams: natural vs. anthropogenic stress

Biological assemblages are often subjected to multiple stressors emerging from both anthropogenic activities and naturally stressful conditions, and species' responses to simultaneous stressors may differ from those predicted based on the individual effects of each stressor alone. We studied the influence of land-use disturbance (forest drainage) on fungal decomposer assemblages and leaf decomposition rates in naturally harsh (low pH caused by black-shale dominated geology) vs. circumneutral streams. We used pyrosequencing to determine fungal richness and assemblage structure. Decomposition rates did not differ between circumneutral and naturally acidic reference sites. However, the effect of forest drainage on microbial decomposition was more pronounced in the naturally acidic streams than in circumneutral streams. Single-effect responses of fungal assemblages were mainly related to geology. Community similarity was significantly higher in the naturally acidic disturbed sites than in corresponding reference sites, suggesting that land-use disturbance simplifies fungal assemblages in naturally stressful conditions. Naturally acidic streams supported distinct fungal assemblages with many OTUs (operational taxonomic unit) unique to these streams. Our results indicate that fungal assemblages in streams are sensitive to both structural and functional impairment in response to multiple stressors. Anthropogenic degradation of naturally acidic streams may decrease regional fungal diversity and impair ecosystem functions, and these globally occurring environments therefore deserve special attention in conservation planning.

The double-edged sword of humic substances: contrasting their effect on respiratory stress in eastern rainbow fish exposed to low pH

High amounts of humic substances (HS) are commonly found in natural acidic waterways and have been suggested to offer some protection against low pH. This study investigated the ability of HS to decrease respiratory stress in eastern rainbowfish (Melanotaenia splendida splendida) exposed to decreases in pH (range of 7-3.5) in soft and hard water. Repeated measures ANOVA revealed a significant difference in respiration (time taken for ten operculum movements) between pH and HS treatments, with a significant interaction between pH and HS present in 5/6 trials. Respiratory stress was shown to increase with increasing acidity, but significantly decreased in treatments with HS (10 and 20 mg/L) compared to those without. The fish exposed to pH treatments without HS also displayed increased hyperactivity, larger operculum movements and increased mucous production. Increased morbidity was shown in HS treatments at pH 3.5 (soft water) and at pH 4 (hard water) compared to treatment without HS. This indicates that HS is helpful in ameliorating the effects of decreased pH on respiration at sublethal pH levels; however, as pH decreases further, it seems that HS increases the toxicity (morbidity) of the low pH.

Wetlands serve as natural sources for improvement of stream ecosystem health in regions affected by acid deposition

For over 40 years, acid deposition has been recognized as a serious international environmental problem, but efforts to restore acidified streams and biota have had limited success. The need to better understand the effects of different sources of acidity on streams has become more pressing with the recent increases in surface water organic acids, or 'brownification,' associated with climate change and decreased inorganic acid deposition. Here, we carried out a large scale multi-seasonal investigation in the Adirondacks, one of the most acid-impacted regions in the United States, to assess how acid stream producers respond to local and watershed influences and whether these influences can be used in acidification remediation. We explored the pathways of wetland control on aluminum chemistry and diatom taxonomic and functional composition. We demonstrate that streams with larger watershed wetlands have higher organic content, lower concentrations of acidic anions, and lower ratios of inorganic to organic monomeric aluminum, all beneficial for diatom biodiversity and guilds producing high biomass. Although brownification has been viewed as a form of pollution, our results indicate that it may be a stimulating force for biofilm producers with potentially positive consequences for higher trophic levels. Our research also reveals that the mechanism of watershed control of local stream diatom biodiversity through wetland export of organic matter is universal in running waters, operating not only in hard streams, as previously reported, but also in acid streams. Our findings that the negative impacts of acid deposition on Adirondack stream chemistry and biota can be mitigated by wetlands have important implications for biodiversity conservation and stream ecosystem management. Future acidification research should focus on the potential for wetlands to improve stream ecosystem health in acid-impacted regions and their direct use in stream restoration, for example, through stream rechanneling or wetland construction in appropriate hydrologic settings.

Chemical and biological recovery of Lake Saudlandsvatn, a formerly highly acidified lake in southernmost Norway, in response to decreased acid deposition

We studied acid-sensitive organisms in Lake Saudlandsvatn in southernmost Norway in relation to acidification: brown trout (Salmo trutta), the caddisfly Hydropsyche siltalai and the zooplankter Daphnia longispina. The study lake was highly acidified with episodic pH depressions <5.0 in the 1970s and 1980s, and sulphur (S) deposition five times greater than the critical load. Chemical recovery following reduced deposition of S became evident in the late 1990s, when the pH increased to 5.5-6.0. By 2000, S deposition had decreased to the critical load. The lake sustained a good brown trout population until the early 1980s, but then it started to decline and nearly went extinct ten years later. Severe recruitment failures were found in most years prior to 1995, both in the inlet and outlet stream. However, since 2003 a marked recovery of the brown trout population has occurred in the lake. During the 1980s, the H. siltalai disappeared from the lake tributaries. In 1996, the species reappeared, and increased highly in abundance from 2000 and onwards. The first post-acidification record of D. longispina from net hauls in Lake Saudlandsvatn was in 2002. Palaeolimnological data confirmed their presence prior to acidification. Any significant recovery in all three organism groups coincided with an acid-neutralizing capacity (ANC) of >20 μeq L⁻¹ and toxic inorganic aluminium of <30 μg L⁻¹. Projections made with the MAGIC model indicate that unless further reductions in deposition of S are made, the ANC will fluctuate around the ANC survival threshold for the biological elements described. Thus, full biological recovery will not occur in the near future.

Impact of calcium and TOC on biological acidification assessment in Norwegian rivers

Acidification continues to be a major impact in freshwaters of northern Europe, and the biotic response to chemical recovery from acidification is often not a straightforward process. The focus on biological recovery is relevant within the context of the EU Water Framework Directive, where a biological monitoring system is needed that detects differences in fauna and flora compared to undisturbed reference conditions. In order to verify true reference sites for biological analyses, expected river pH is modeled based on Ca and TOC, and 94% of variability in pH at reference sites is explained by Ca alone, while 98% is explained by a combination of Ca and TOC. Based on 59 samples from 28 reference sites, compared to 547 samples from 285 non-reference sites, the impact of calcium and total organic carbon (TOC) on benthic algae species composition, expressed as acidification index periphyton (AIP), is analyzed. Rivers with a high Ca concentration have a naturally higher AIP, and TOC affects reference AIP only at low Ca concentrations. Four biological river types are needed for assessment of river acidification in Norway based on benthic algae: very calcium-poor, humic rivers (Ca<1 mg/l and TOC>2 mg/l); very calcium-poor, clear rivers (Ca<1 mg/l and TOC<2 mg/l); calcium-poor rivers (Ca between 1 and 4 mg/l); moderately calcium rich rivers (Ca>4 mg/l). A biological assessment system for river acidification in Norway based on benthic algae is presented, following the demands of the Water Framework Directive.

Spatiotemporal patterns of drivers of episodic acidification in Swedish streams and their relationships to hydrometeorological factors

This study examined the spatiotemporal patterns of episodic acidification in 87 weakly buffered streams in Sweden at a monthly sampling frequency during a ten-year study period (1998-2007). Time series of pre-industrial pH (pH(0)) were reconstructed from the acidification model Meta(MAGIC), and the acidification impact was defined by the difference between the pH(0) and the contemporary pH (i.e., DeltapH=pH(0)-pH(t)). Acidification episodes were defined as observations for which the pH(t) was at least 0.4 units lower than average, in combination with a pH at least 0.2 units higher than average. Thus, only occasions in which the stream water was both more acidic and more acidified than average were characterized as acidification episodes. For each observed episode, the primary cause was identified from one of the following five possible drivers: dilution, increase in sulfate, nitrate or organic acids, or sea salt deposition. In total, 258 episodes were observed during the study period. The study showed that streams that were acidified during baseflow (DeltapH>0.4), but not chronically acidic (pH>5.2), were subjected to regular episodic acidification. Dilution was the single most important cause and the main driver for 58% of the identified episodes. Increases in sulfate concentrations were also relatively common (26% of episodes), whereas increases in nitrate and organic acids as well as sea salt deposition were of minor importance. The total number of dilution-related acidification episodes within a year had a significant (p=0.005) positive correlation (r=+0.83) with the average annual precipitation. Occurrences of sulfate episodes were related to droughts during the preceding summers. While the number of streams that are susceptible to episodic acidification will decrease as a consequence of recovery from acidification, the hydrological and meteorological consequences of future climate change may make episodic acidification more common.