Survival of brown trout during spring flood in DOC-rich streams in northern Sweden: the effect of present acid deposition and modelled pre-industrial water quality

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.

Toxicity of inorganic aluminium at spring snowmelt--in-stream bioassays with brown trout (Salmo trutta L.)

Although the acid load has decreased throughout Scandinavia, acidic soils still mobilise aluminium (Al) that is harmful to brown trout. We hypothesise that there are thresholds for Al toxicity and that the toxicity can be traced from the water content to gill accumulation and the consequential physiological effects. During snowmelt, yearlings were exposed to a gradient of pH and inorganic monomeric Al (Al(i)) in humic streams to study the toxic effects and mortality. Gill Al and physiological blood analyses [haemoglobin (Hb), plasma chloride (P-Cl) and glucose (Glu)] were measured. As the water quality deteriorated, Al accumulated on the gills; Hb and Glu increased; P-Cl decreased, and mortality occurred. Moribund fish had significantly increased gill Al and Hb, suggesting that respiratory disturbances contributed to mortality. Decreased P-Cl and plasma availability indicated an ion regulatory disturbance and possibly circulatory collapse. Al(i) should be less than 20 μg/L, and pH higher than 5.0, to sustain healthy brown trout populations. These thresholds can be used to fine-tune lime dose, as both Al(i) and pH levels have to be balanced to prevent harm in the recovering aquatic biota. Although Al is tightly linked to pH, local variation in Al availability in soil and bedrock affects the Al release and subsequent toxic Al(i) episodes in some catchment areas.

Forest land cover continues to exacerbate freshwater acidification despite decline in sulphate emissions

Evidence from a multi-date regional-scale analysis of both high-flow and annual-average water quality data from Galloway, south-west Scotland, demonstrates that forest land cover continues to exacerbate freshwater acidification. This is in spite of significant reductions in airborne pollutants. The relationship between freshwater sulphate and forest cover has decreased from 1996 to 2006 indicating a decrease in pollutant scavenging. The relationship between forest cover and freshwater acidity (pH) is, however, still present over the same period, and does not show conclusive signs of having declined. Furthermore, evidence for forest cover contributing to a chlorine bias in marine ion capture suggests that forest scavenging of sea-salts may mean that the forest acidification effect may continue in the absence of anthropogenic pollutant inputs, particularly in coastal areas.

Spatial and seasonal variations in stream water delta34S-dissolved organic matter in northern Sweden

The discharge of terrestrial dissolved organic matter (DOM) by streams is an important cross-system linkage that strongly influences downstream aquatic ecosystems. Isotopic tracers are important tools that can help to unravel the source of DOM from different terrestrial compartments in the landscape. Here we demonstrate the spatial and seasonal variation of delta34S of DOM in 10 boreal streams to test if the tracer could provide new insights into the origin of DOM. We found large spatial and seasonal variations in stream water delta34S-DOM values ranging from -5.2 per thousand to +9.6 per thousand with an average of +4.0 +/- 0.6 (N = 62; average and 95% confidence interval). Large seasonal variations were found in stream water delta34S-DOM values: for example, a shift of more than 10 per thousand during the spring snowmelt in a wetland-dominated stream. Spatial differences were also observed during the winter base flow with higher delta34S-DOM values in the fourth-order Krycklan stream at the outlet of the 68 km2 catchment compared to the small (< 1 km2) headwater streams. Our data clearly show that the delta34S-DOM values have the potential to be used as a tracer to identify and generate new insights about terrestrial DOM sources in the boreal landscape.

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

Anthropogenic acidification caused by aerial deposition of acidifying substances is known to have detrimental effects on freshwater biota, including reductions in species diversity and ecosystem functioning. However, such impairment is not found in systems acidified to a similar extent by natural processes. A proposed explanation for this difference is that freshwater organisms have had far more time to evolve and adapt to natural than anthropogenic acidification. Thus, where acidity is natural, adaptation may account for diverse and functional communities. Here, we investigated whether adaptations--that were previously implied to occur on small spatial scales--may explain the species richness patterns on a much larger geographical scale, apply to ecological functioning, and are relevant in Sweden, where natural acidity is geologically relatively recent. Therefore, we compared differences in species diversity and ecosystem process rates between 24 acidic and circumneutral streams in northern Sweden, where acidity is natural, and southern Sweden, where acidity is largely anthropogenic. In agreement with our predictions, the difference in macroinvertebrate species richness between acidic and circumneutral streams was threefold larger in the region where acidity was anthropogenic than where it was natural, albeit marginally non-significantly. In contrast, no such trend was found for the rates of decomposition by microbes and leaf-feeding macroinvertebrates, possibly due to functional redundancy. The structure of species assemblages differed between acidic and circumneutral sites and between the regions. Our results agree with the notion that freshwater biota are adapted to natural acidity, but competing explanations including other differences in water chemistry and differences in the biogeographical colonization histories may also account for part of the observed patterns. Since naturally acidic environments similar to those in northern Sweden are widespread, we predict that diverse and functionally efficient freshwater communities that are well adapted to such conditions are more common than currently recognized.

Monitoring the effect of three humic acids on a model membrane system using 31P NMR

The sorption of three humic acids to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine multilamellar vesicle model membrane systems was studied by phosphorus nuclear magnetic resonance (31P NMR). The effects of temperature and pH were investigated. The gel --> bilayer transition did not appear to be affected by any of the humic acids at pH 7; however, all three humic acids induced a perturbation to this transition and to the bilayer structure at pH 4. On the basis of the findings from this and other work, a conceptual adsorption/absorption model for the sorption of humic acid (HA) to biomembranes has been put forward. The model requires an initial adsorption step initiated at an acidic pH by hydrogen bridging and electrostatic interactions between the functional groups of the HAs and the head groups of the phospholipids. Once the HA material is adsorbed, its hydrophobic domains can further seek a more thermodynamically favorable environment within the bilayer using hydrophobic interactions. These interactions lead to the HA being absorbed into the membrane, which subsequently induces the observed perturbation by disturbing the ordered packing of the phospholipid tail groups. This model is also related to other humic substances/biomembrane observations in the literature.