Spatial distribution of freshwater Ca concentrations

Analyzing all available Ca data, sampled during different seasons and across lakes and running waters distributed over 57 countries (altogether 440 599 measurements), we found that concentrations varied between 0.4 and 74.4 mg L−1 (2.5 and 97.5 percentiles, respectively) with a median of 4.0 mg L−1 (median of 279 940 running water samples: 3.76 mg L−1 and median of 160 659 lake samples: 5.50 mg L−1). The lowest Ca concentrations occurred mainly in the boreal region, i.e. Fennoscandia and eastern Canada, but also in the United Kingdom, some regions in eastern North America and in some subtropical and tropical regions of South America (Fig. 2). Freshwater Ca concentrations on a global scale showed a clear right-skewed distribution with only a few freshwater samples having Ca concentrations > 450 mg L−1 (Fig. 3a). These high-Ca freshwater samples had a large influence on the global mean Ca concentration of 12.5 mg L−1 (mean of 279 940 running water samples: 11.12 mg L−1 and mean of 160 659 lake samples: 14.95 mg L−1), making it comparable to the previously reported mean Ca concentration of the world river input to oceans of about 14 mg L−1 15.

Figure 3 Calcium (Ca), pH and carbonate alkalinity of 43 184 lake and running water sites. Panel a shows all available Ca concentration data and panel b shows all available carbonate alkalinity (carbonate alk) data in relation to pH except data from Lake Neusiedl in central Europe, which is shown separately (panel c) because this lake is an endorheic basin with disproportionally high carbonate alkalinity in relation to Ca concentrations (panel d). Panels a and b show the median Ca concentration and carbonate alkalinity for each 0.1 pH unit, based on site-specific long-term median values for running waters (dark blue dots) and lakes (light blue dots). The median values as well as other percentiles are available in Supplementary Table 2. Panel d shows the relationship between Ca2+ and carbonate alkalinity, based on site-specific long-term median values for all freshwater sites. On a global scale, Ca2+ and carbonate alkalinity generally follow a 1:1 relation (dotted line). However, systematic deviations from a 1:1 relation occur, displayed as percentage for each pH unit in panel e. Full size image

The large discrepancy between the global mean and median Ca concentrations in freshwaters reflects the uneven distribution of Ca concentrations in the world where freshwaters with Ca concentrations ≤ 4.0 mg L−1 predominate, with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L−1, a threshold considered critical for the reproduction and survival of a variety of aquatic organisms4. Ca concentrations are often naturally low because of regional geology (e.g. gneissic or granitic bedrocks), highly weathered soils (as in much of the tropics), and/or limited weathering potential in the catchment (mainly due to a cold climate)36,37,38. Low Ca concentrations can also be exacerbated by anthropogenic drivers, such as timber harvesting e.g.32,39 and recovery from historical acidification e.g.2,27. In our global dataset we found critically low Ca concentrations ≤ 1.5 mg L−1 in nutrient-poor, acidic boreal waters with a pH ≤ 5.5 (Fig. 3a and Supplementary Information). These waters were mainly located in regions that commonly have a limited weathering potential in the catchment due to seasonal ice cover and generally rather low annual mean air temperatures37 (Fig. 2). In addition, these waters have historically been exposed to anthropogenic acid deposition, and their catchments have been depleted in base cations40,41. Some low concentrations of Ca were also observed in lowland tropical and subtropical regions (Fig. 2), where soils tend to be much older compared to other geographical regions and therefore often are depleted of readily weatherable carbonate minerals11,13.

Comparison of Ca concentrations between lakes and running waters

On a global scale, we observed that Ca concentrations in lakes and running waters gradually increased along a pH gradient, up to a pH of around 8.0 (Fig. 3a). The Ca increase along the pH gradient was similar between lakes and running waters (Fig. 3a and Supplementary Table 2). However, despite similarities between lentic and lotic waters, we found significantly higher Ca concentrations in our running waters than lakes along the entire pH gradient (non-parametric Wilcoxon test for 42 different pH categories within the range 4.5–8.6: p < 0.05 for 40 out of 42 pH categories; Supplementary Table 2). Lower Ca concentrations in lakes than in running waters might simply reflect that the majority of the lake sites considered in this study were located in relatively cold geographical regions with a limited weathering potential, while our running water sites were more equally distributed across latitudes. There are, however, other reasons why Ca concentrations in lakes can be lower than in running waters. In the more precipitation-fed lakes, Ca originating from the catchments and entering via groundwater and stream inputs is often significantly diluted by the direct capture of precipitation onto the lake surface. Lakes also have a substantial internal loss of Ca caused by various biotic and abiotic processes, such as phyto- and zooplankton uptake and subsequent sedimentation42 and CaCO 3 precipitation and sedimentation in alkaline lakes e.g.43,44. CaCO 3 supersaturation and precipitation commonly occur at pH > 8.0, which likely explains why we found no further Ca concentration increases above this pH (Fig. 3a).

Ca concentrations and carbonate alkalinity

Ca and carbonate alkalinity concentrations showed a similar pattern along the pH gradient, with maximum concentrations in the range of pH 8.0–9.0 (Fig. 3b). The observed pattern along the pH gradient followed the theoretical dissolved inorganic carbon speciation in natural waters45. Taking the long-term surface-water median value for each site, we found that Ca and carbonate alkalinity strongly co-varied (Kendall’s tau: p < 0.0001), approximately following a 1:1 relation (Fig. 3d). Thus, we found that Ca and carbonate alkalinity are generally in balance, suggesting that their concentrations are controlled by similar drivers, particularly carbonate mineral weathering, which supports our conceptual model (Fig. 1). At high pH, i.e. in hardwater lakes, we suggest that Ca and carbonate alkalinity remain in balance due to CaCO 3 precipitation as a sink.

Despite the overall Ca and carbonate alkalinity co-variation across the range of freshwater concentrations, we observed hydrology- and pH-related deviations from this general pattern. We found that isolated, saline waters, e.g. the endorheic Lake Neusiedl in central Europe, can substantially deviate from a Ca:alkalinity ratio of 1:1. In Lake Neusiedl, where carbonate alkalinity is disproportionally high in relation to Ca (Fig. 3d), the excess alkalinity is largely balanced by sodium.

In addition to endorheic saline lakes, waters with pH < 7.0 also could show deviations from a Ca:alkalinity ratio of 1:1. In these waters we observed a Ca excess in relation to carbonate alkalinity that gradually increased with decreasing pH (Fig. 3e), suggesting that acid anions other than bicarbonate become relevant in the ion balance. One important ion associated with acidification is sulfate, which is characteristic of acid deposition from the atmosphere, but can also originate from fertilizers46 and acid mine drainage47, as well as natural mineral deposits (e.g., oxidation of iron sulfides). Sulfate can cause an accelerated leaching of Ca from soils into freshwaters by decreasing the soil pH and thereby triggering Ca displacement by the hydrogen ion in the soil sorption complex46,48,49. Since we found the Ca excess in relation to carbonate alkalinity mainly in waters located in regions that historically have been exposed to acid deposition40 (i.e. eastern North America, eastern Europe and Fennoscandia), we infer that acid deposition was the ultimate driver decoupling the relationship between Ca and carbonate alkalinity, consistent with our conceptual model (Fig. 1). Accordingly, Ca concentrations in regions that are presently recovering from acidification are expected to return towards pre-industrial conditions where Ca and carbonate alkalinity are strongly coupled and in balance, and Ca concentrations are lower than they were at the peak of catchment acidification (Fig. 1). Post-acidification recovery might even drive Ca concentrations below pre-acidification concentrations due to a depletion of base cation stores in the catchment soils of acid-sensitive regions31.

Ca concentration changes over time

We tested our expectation of a return to stoichiometrically balanced Ca and carbonate alkalinity in freshwaters by analyzing time series of Ca, carbonate alkalinity, pH and sulfate from 1980 to 2017. Altogether we analyzed complete time series of Ca and pH from 297 freshwaters, including carbonate alkalinity from 211 freshwaters from 1980 to 2017 (see Methods). Almost all freshwaters with available time series were located in regions that have historically been exposed to acid deposition40, and thus all of them could potentially demonstrate declining Ca concentrations (Fig. 1). We found that out of the 297 freshwaters more than half (164 sites) showed significantly decreasing Ca concentrations since 1980 (Table 1). In waters with significantly decreasing Ca concentrations, Ca no longer showed a positive relationship to carbonate alkalinity, implying that Ca concentrations have decreased despite constant or increasing carbonate alkalinity. Instead of being coupled to the temporal change in carbonate alkalinity, Ca concentrations showed a very strong positive relationship to the temporal change in sulfate concentrations (Table 1).

A coherent decline in sulfate and Ca concentrations over time was especially apparent in a river mouth in southern Sweden, which demonstrated the most significant Ca concentration decline in Sweden since 1980 (Fig. 4a,b). The coherent sulfate and Ca decreases corresponded to a return of Ca concentrations to an approximate charge balance with carbonate alkalinity (Fig. 4c). We suggest that acidification associated with anthropogenic sulfate sources is an important driver of the displacement from natural conditions where Ca and carbonate alkalinity are strongly coupled and in balance. Our suggestion is supported by region-specific patterns for freshwaters located in two geographical regions that are well known to have recovered from historical acidification—the US Adirondacks and Sweden30,50. In these two regions, we observed a highly significant positive relationship between sulfate concentrations and Ca deviation from the 1:1 ratio between Ca and carbonate alkalinity, with the vast majority of data showing a Ca excess (Fig. 4d,e). Thus, our results provide evidence that acidification caused the decoupling between Ca and carbonate alkalinity across large regions of the globe via a selective sulfate-driven Ca leaching from catchment soils into freshwaters. In many regions, acid deposition has now been mitigated and the Ca-alkalinity coupling is becoming re-established.

Figure 4 Temporal variation in freshwater chemistry. Shown are temporal variations in median sulfate concentration (SO 4 2−, panel a), calcium concentration (Ca, panel b), and Ca2+ deviation from the 1:1 relationship between Ca2+ and carbonate alkalinity (panel c) at the mouth of the Göta älv at Trollhättan, the Swedish river with the largest Ca decline from 1980 to 2017. The Ca2+ deviation from a 1:1 relation to carbonate alkalinity was further determined for 66 inland waters in Sweden and 44 lakes in the US Adirondacks, all located in regions that have historically been exposed to acid deposition. Panels d and e show the relationship between yearly median SO 4 2− concentrations and the Ca2+ deviation from a 1:1 relation to carbonate alkalinity during 1980 to 2017. The relationships are highly significant (Kendall’s tau correlation: p < 0.001) for both Swedish and US freshwaters. Full size image

Despite being in regions that have been exposed to acid deposition, 45% of the freshwaters in our dataset did not show a statistically significant decline in Ca concentrations over recent decades (Table 1). There are many possible reasons to explain this pattern: (a) liming and other management activities have occurred in the catchment e.g.51,52, (b) the critical load of acidity for surface waters by inputs of acids from acid deposition did not exceed the natural weathering potential53, (c) waters have already recovered from acidification, reaching a balance with carbonate alkalinity according to the conceptual model (Fig. 1), (d) mineral weathering rates have increased due to climate change e.g.54,55, (e) there has been an increased use of Ca in CaCl 2 road salt56 and for soil liming in agricultural areas46, (f) there has been an increase in concentrations of dissolved organic carbon and its associated cations such as Ca, and (g) available intra-annual measurements were too infrequent and time-series too short to detect an existing trend with statistical significance.

In most of the waters in which Ca concentrations did not significantly decline, we found a strong positive relationship between Ca and carbonate alkalinity but not between Ca and sulfate concentrations (Table 1). These results suggest that although drivers for Ca concentrations in lakes and running waters might vary substantially, Ca and carbonate alkalinity generally exist in stoichiometrically balanced proportions. We found, however, that sulfate has the potential to induce strong deviations in the global co-variation between Ca and carbonate alkalinity, presumably reflecting strong mineral acidity associated with sulfate sources. Since anthropogenic sulfate is not only associated with acid precipitation but also with fertilizers and acid mine drainage, the consequences of widespread sulfate pollution need to be carefully evaluated.