In this study, we defined the tolerance limits of various aquatic invertebrates in acidic and alkaline waters in order to predict how acidification alters their distributions. Research revealed that impoverished water quality due to acidification is followed by a decrease in diversity and a general shift in the community structure from acid-sensitive taxa to more acid-tolerant taxa in affected lakes [39]. However, some investigations conducted in the 1980s, have found no effect of pH on invertebrates (e.g., Simpson et al. [40]; Winterbourn and Collier [41]). Research on the structure of benthos communities along a pH gradient is very scarce. Moreover, there is a lack of such research in the forest mining lakes. Because fewer work have been carried out analysing the effects of acid stress on invertebrates, to the best of our knowledge, our paper presents the first quantitative data from a field study in forest mining lakes along a gradient of pH.
In the global range, forest lakes provide habitats for rare, protected, and endemic species. In industrial areas, their role is even more significant because they are valuable water habitats for birds, amphibians, and invertebrate fauna. The reservoirs that have been created as a result of human activity usually have a poor fauna [42]; however, because of location in forests and isolation from other water environments, mining lakes are habitats of diverse snail fauna, including species that only occur sporadically in other types of reservoirs [43]. Our study revealed diverse invertebrate communities and a total of 56 taxa ranging from 28 in the EAML to 44 in the NML. However, the diversity when measured by the diversity indices, the diversity of invertebrates was not found to be significantly different between the four groups of mining lakes studied.
We found significant differences in the density of invertebrates. We identified taxa whose densities tended to decrease with a decreasing pH values (Glossiphonidae, Gastropods: Lymnaeidae and Physidae, Chironomidae and Oligochaeta). However, we also found taxa that were more tolerant to low pH and their density increased along with decreasing pH; therefore, it can be concluded that there is a straightforward relationship between the acid conditions and the presence/absence of specific invertebrates in relation to acidification [44]. Considering the interaction of acidifying pollutants with water environments, it seemed appropriate to assess how significant these interactions are in the context of invertebrates. Benthos belong to a generally abundant group seen in waters affected by human activity. However, there are many invertebrates that are sensitive to acidification, and some even disappear at pH values as high as 6.0 [45-47].
In the Extremely acidic lakes most abundantly by Dytiscidae, Chironomidae and Oligochaeta. The same was also generally true for ACML in the case of Oligochaeta and Chironomidae; however, gastropods also occurred abundantly in these lakes (ACML). Generally, the relative abundance of taxa that are highly sensitive to low pH decreases at pH 6.5 with only a few present below pH 6.0 [48], whereas the occurrence of taxa that are less sensitive to low pH is relatively high at pH 6.0 and such taxa are only occasionally found below pH 5.5. While acid-sensitive species may disappear at moderate levels of acidification, acid-tolerant taxa may appear or even increase in abundance, thereby resulting in a small or no overall decrease in biodiversity in aquatic habitats [49]. Although Gammarus sp., mosquito larvae, Leptophlebidae, and Pisidium can tolerate very low pH [50], we did not found them in EAML.
Although we found numerous occurrences of some taxa with increasing acidity, this was not the case for snails. They were the most abundant in lakes having a pH range of 6.0–6.7, and were also found in EAML. Mollusks need for calcium to build their shell, and thus, pH is an essential factor in molluscan ecology [51]. Changes seen in periphyton composition and abundance along with decreasing pH are also significant because it is the main source food or mollusks. This could be the reason for the low abundance of some invertebrates, including snails. On the other hand, other invertebrates such as crayfish are also absent in acidic waters, and their absence is not always linked with the calcium concentration in water as a limiting factor but rather with the biology of biocalcification [52, 53].
Invertebrate communities found in the EAML and ACML were clearly different from those in the NML and ALML. The lowest density of invertebrates was found in extremely acidic waters (mean 770 ind m2), whereas the highest was found in the alkaline lakes (mean 1682 ind m2). Among the lakes with alkaline waters, the highest density was found with the high abundance of Physidae snails (maximum density: 11 980 ind m2). A similar number of Odonata occurred in mining lakes. Heteroptera was the least numerous in the EAML and ALML. In aquatic habitats with a very acidic water, benthic fauna is dominated by Chironomus sp. and water beetle, as found in the study of Wickham et al. [54] in the acidic reservoir (pH 3.2). This finding was also confirmed for a similar pH by Rodrigues and Scharf [55] in the case of chironomids, which were the most abundant of the insects. The results obtained by Raddum and Sæther [56] showed the lowest abundance of chironomids in Norwegian lakes with pH ranging from 4.4 to 6.2; however, the number of species decreased with decreasing pH, with the lowest number being found in a humic acid lake. According to Mossberg and Nyberg [57], in lakes with different values of pH, the composition of benthos is very much the same; however, they found that chironomids clearly exhibited a tendency to occur in assemblages consisting of only one genus (Chironomus sp.) in acid lakes. This thesis was also confirmed by Økland and Økland [51] in acidic Norwegian lakes. Our results showed that the composition of benthos differed between the four groups of mining lakes.
We found oligochaetes to be tolerant of high pH (maximum density 7240 ind/m2); however, they were also abundant in the EAML (maximum density 1164 ind/m2). As was shown by Crisman et al. [58] in Florida, their occurrence decreased significantly in acidic lakes, while the proportion of chironomids increased. However, we cannot confirm these results because we found the density of chironomids to be the highest in the ALML (mean density 1254 ind m2) and in the EAML (mean density; 682 ind m2). The density of other annelids (Glossiphonidae) decreased with the decreasing values of pH, while Erpobdellidae were the most abundant in the EAML. In Norway, leeches mainly occurred at a pH >5.5; however, they were also found in environments with pH ≥ 4.2 in Sweden [59]. We did not find more recent studies in the literature on the occurrence of leeches in acidic environments.
Some of the relationships between invertebrates and environmental variables, such as the content of nutrients (nitrites, nitrates, and phosphates) and iron, were nonsignificant,. It is likely that this lack of response occurred because most benthic invertebrates are habitat generalists in anthropogenic reservoirs. Benthos can routinely endures pronounced and unpredictable changes in the environment; hence, they probably possess potential durability which makes them resistant to most natural variations in the habitat conditions. The physiological effects of high pH (>9) on aquatic life have not been studied much because such pH of the water is less common in aquatic habitats [60]. High values of pH were not found in leaf-fed forest lakes, but productive agricultural lakes can have quite high pH values however they were not studied to date in the context of the invertebrate community composition. In many forest lakes, we observed high values of pH. In alkaline conditions, we found 40 taxa with high densities of some groups, whereas Wickham et al. [54] recorded a total of 37 insect genera. Increase in pH is usually associated with increasing numbers of taxa and individuals. Acidification is responsible for decreasing phosphorus input, increasing humic inputs and increasing the mobilization of toxic heavy metals. A study by Courtney and Clements [61] showed that chronic metal pollution might have resulted in communities that are tolerant of metal but more sensitive to acidic pH. Although our findings only partly reflected this generalization, we confirm the statement that acidic habitats generally have fewer taxa and lower taxa densities.
Polluting discharges from coal mines and acid mine drainage have a higher acidity, and high concentrations of iron and sulphates [62, 63]. In this study, we found extremely acidic water, high conductivity, and high content of chlorides and iron in some of examined mining lakes which are located in the mining area (Table 2). Acid mine drainage are toxic to benthic invertebrates and affects communities to ones dominated by a few tolerant species, they impair ecosystem processes (e.g., decomposition) [64-66]. Because of the increased pollutants emission, there is a systematic increase in the areas under the influence of acidification. This is a global trend that is visible in many regions of the world [48] and also in Poland. The effect of this trend are manifested by forests that are dying and affect the functioning of environments, thus causing changes in their structure due to the elimination of taxa that are sensitive to low pH, as found in our study: Odonata, Ephemeroptera, Trichoptera, and snails (Supplementary Table S7). According to Nisbet and Evans [67], it seems that, due to their forest locations, water environments are particularly vulnerable to acidification because of the weak capacity of these lakes. It is of particular importance because the impact of acidification manifests itself within a short period of time due to the high degree of sensitivity of different groups of freshwater invertebrates [64, 68].