Chara rudis developed compact meadows in the study Lake Jasne and was a dominant component of the vegetation compared with other macrophyte species, particularly vascular species. The water chemistry parameters place the lake in the group of ecosystems with moderate nutrient availability and high transparency, resulting in characterisation of the lake as mesotrophic (Carlson, 1977, Pełechaty et al., 2007). Good light conditions, along with calcium and bicarbonate availability, promote charophytes in submerged vegetation (Dąmbska, 1964; Hutchinson, 1975; Krause, 1981, 1997; Blindow, 1992a, b). Thus, in the study lake, dense charophyte beds formed a significant cover on the CaCO3-rich substratum. This suggests that the lake offers good conditions for the charophyte-related carbonate precipitation. This is corroborated by alkalinity values that were lower in the charophyte meadows than in the macrophyte-free pelagic zone (Fig. 4). Photosynthetic activity of charophytes results not only in calcium and bicarbonate depletion in the surrounding waters, but also in the phosphorus co-precipitation with carbonates, and nutrients incorporation in biomass (Kufel & Kufel, 2002 and references therein). Although charophytes are considered very effective in modifying water properties (van den Berg et al., 1998), the parameters determined in this study above the chosen patches of C. rudis reflected lake characteristics and so, were lake- rather than site-specific, which might have resulted from morphometric features of Lake Jasne. The maximum and mean depths place the lake in a transitional type between typical shallow and stratified lakes, which suggests a primary role of wind-induced water movement in surface disturbance. Frequent water mixing may make the water chemistry uniform within the lake, similar at each study site and temporarily, rather than spatially variable (Pełechaty, 2006).
The studied charophyte beds were floristically poor and overgrown by C. rudis, whose coverage did not differ among the sites and sampling events. Since, then the constant coverage might have been more important for PVI values, obviously dependent on the depth (Fig. 2), rather than the charophyte morphology, not linked directly with the site characteristics. Since the content of carbonate incrustation was determined in the apices of C. rudis, no relation was found with the site depth, C. rudis morphology or PVI. By contrast, carbonates varied during the study period, following similar pattern of changes at each study site (Fig. 3). This emphasizes the importance of photosynthetic intensity for carbonate precipitation, rapidly increasing at the beginning of summer season and declining in the autumn. If charophyte growth, most intense photosynthesis and precipitation of calcium carbonate are restricted to the apical parts of thalli (Andrews et al., 1984; Coletta et al., 2001), our results suggest that the community structure may be of minor importance for carbonate formation. Light availability, mineral content of water and climate conditions, influencing photosynthetic activity, appear to be the crucial factors. However, the density of plants and area covered by charophyte meadows, apart from their obvious importance for the total carbonate deposition in the sediments, may become significant at the peak growing season, when high photosynthetic rates may affect the chemistry of water, depleting calcium and bicarbonate concentration. In the region where the study reported was performed (mid-Western Poland), we observed lower summer calcium concentrations in lakes with diverse and abundant charophyte vegetation as compared to those with minor contribution of charophytes (Pełechaty et al., 2007).
The influence of photosynthesis can also explain why water above C. rudis beds was 13C-enriched, comparing to the pelagic zone. Preferential uptake of 12CO2 for photosynthetic purposes could have led to a higher rate of heavier isotope in the water. However, the discrepancy in δ13CDIC was not pronounced. Differences in δ18OWATER above the studied charophyte patches and in the pelagic zone were also statistically insignificant showing evenly distributed values of δ18O within surface waters. Under the environmental conditions observed, progressive photosynthetic removal of isotopically light carbon by extensive charophyte meadows reaching up to 60% of the lake area (Pełechaty et al., 2007), causes strong 13C-enrichment of DIC, reflected in the measured δ13CDIC values (Fig. 5). During intense photosynthesis, the water becomes depleted in dissolved CO2 (Herczeg & Fairbanks 1987). When this occurs, CO2 enters water from the atmosphere. In conditions of an isotopic equilibrium with atmospheric CO2 (δ13C ca. −7.8‰, Leng & Marshall, 2004) δ13CDIC amounts to about 1.3 and 0.7‰, at 15 and 20°C, respectively (Mook et al. 1974). δ13CDIC measured in the waters of Lake Jasne is isotopically lighter than this (between −0.8 and −2.5‰), and this shows that DIC is not in an isotopic equilibrium with atmospheric CO2.
Mean δ18O of waters in Lake Jasne equals about −1.8‰ and is strongly enriched in 18O compared to the average δ18O of local shallow groundwaters, amounting to ca. −9.2‰ (d’Obryn et al., 1997). A shift to heavier oxygen isotope composition of waters may have resulted from the lake morphology, as it is a small and shallow water body with no surface inflow and temporal outflow, inactive during the study. However, mean yearly δ18OWATER is expected to be isotopically lighter than mean δ18OWATER measured (−1.8‰), since the period investigated in this study is restricted to 5 months, including three with the highest mean air temperature during the year, that is commonly characterized by 18O-enriched waters.
Stable carbon isotope composition of the C. rudis carbonate incrustation followed the pattern of month-to-month changes similar to that of carbonate content. An increase in δ13C observed during the early summer is a result of an intense photosynthetic activity of C. rudis. Due to higher proportion of 12CO2 incorporated preferentially during photosynthesis, the remaining DIC becomes 13C-enriched, and results in isotopically heavier calcite precipitated as an incrustation (cf. Pentecost & Spiro, 1990; Andrews et al., 1997). A subsequent decline in δ13C in Chara carbonates and DIC (Fig. 5) may be explained in two ways: (a) by August the primary production diminished, and hence 12CO2 uptake decreased. The inflowing waters supplying the lake, not influenced by photosynthesis, had a diluting effect on carbon isotope composition of DIC and resulted in δ13CDIC decrease; (b) it is also possible that isotopically light carbon might have been partially recycled by decomposition of organic matter produced during the spring and early summer, e.g. decomposition of phytoplankton sinking through the water column. Decomposition of C. rudis thalli did not play a role as no sign of atrophy at the stem base was observed until late September when 12C enrichment in DIC was noted.
It is important that the differences between carbonates and water above the studied beds exceeded 2‰. That was particularly evident in the case of the carbon isotope record. In all cases but one the difference in δ13C between DIC and incrustation exceeded 2.2‰ during the whole study period (Fig. 5), with δ13CCHARA usually from 2.3 to 3.1‰ higher than carbon isotope value of the DIC. Experimental studies show that δ13CCALCITE is around 1–1.5‰ less negative than the DIC value (Emrich et al., 1970; Romanek et al., 1992), which decreases the difference in carbon isotope values measured in Chara incrustations and DIC. The data indicate a state of disequilibrium during calcite precipitation on C. rudis stems leading to 13C-enrichment in carbonate. However, it is important to note that site-specific isotope composition of DIC is created by charophyte photosynthetic activity closest to Chara thalli (within Chara beds, in particular). It is possible that DIC in surface waters (0.5 m deep) is influenced less than DIC of the Chara immediate environment and thus, is 13C-depleted in comparison to incrustations. Still, most authors agree that δ13C in Chara incrustations is subject to significant photosynthetically driven metabolic effects (Andrews et al., 2004, and references therein).
Table 2 presents oxygen stable isotope data of water and incrustation as well as water temperature measured and calculated using Kim & O’Neil (1997) equation re-expressed by Leng & Marshall (2004):
$$ T\,\left( {^\circ {\text{C}}} \right) \, = { 13}. 8- 4. 5
8\left( {\delta_{\text{CARB}} -\delta_{\text{WATER}} } \right) +
0.0 8\left( {\delta_{\text{CARB}} -\delta_{\text{WATER}} }
\right)^{ 2} $$
Table 2 Difference between water temperature calculated (T
C) on the basis of δ18OWATER and δ18OCHARA (Kim & O’Neil equation re-expressed by Leng & Marshall, 2004) and measured (T
M) in C. rudis beds studied in Lake Jasne between June and October, 2008
Significant discrepancies can by seen between the temperature measured (T
M) and calculated (T
C) that can be interpreted as resulting from oxygen isotope disequilibrium during carbonate precipitation. However, it is important to note that temperature as well as δ18OWATER were measured in waters at 0.5 m depth, while Chara beds studied occurred at 1.0, 1.5 and 2.0 m. Oxygen stable isotope signatures of surface waters may be, and probably are, 18O-enriched (due to preferential evaporation of H
162
O molecules, e.g. Li & Ku, 1997) in comparison to waters within Chara patches. Decreasing δ18OWATER used in calculations only by 0.8 and 1.2–1.5‰ for July and August, respectively, brings T
C very close to T
M. Differences between T
C and T
M increase from July to September and decrease in October. This, according to our suggestions, indicates the strongest influence of evaporation on δ18O surface waters in summer months.
It is also crucial to note that δ18OCHARA measured in this study refers to incrustation formed at the apical part of Chara stems and thus reflects δ18OWATER and water temperature during the month preceding the moment of water collection and temperature measuring. This is evident in the data collected in September, where T
M (ca. 14.9°C) is probably lower than the actual temperature in which Chara incrustations precipitated during the preceding month. This is a possible reason why the discrepancy between T
C and T
M increases in September.
Considering the discussion, it can be suggested that incrustations may precipitate in isotopic equilibrium with ambient waters within Chara beds, and the discrepancy between T
C and T
M (Table 2) is caused by 18O-enrichment of surface waters.
Our study pointed at the relationships between the stable isotope composition and the water chemistry during calcite precipitation and the content of mineral incrustation. Although the linkage with the content of carbonate incrustation seems obvious, it was only statistically significant for the δ18O. The negative correlation of δ18O in incrustation and Chara dry weight may be explained as a consequence of kinetic isotope effects that result from discrimination against the heavy isotopes of oxygen during the hydration and hydroxylation of CO2 (McConnaughey, 1989). Rapid precipitation of CaCO3 in conditions of intense photosynthesis, e.g. within dense Chara beds, is associated with strong disequilibrium leading to depletion of 18O in carbonates (McConnaughey, 1989, and references therein). Hence, δ18O values in stem incrustation are not equilibrium values, being offset to isotopically lighter compositions relative to equilibrium. This contradicts the suggestion made above describing a possible state of isotope equilibrium during precipitation of Chara incrustation. Andrews et al. (2004) assess the offset as about 1.5‰. In this study, the discrepancy observed between δ18O in incrustations and water is greatest between July and September with a peak in August (Fig. 5). Such a record is due to coexistence of two opposite processes influencing oxygen isotope values, i.e. evaporation resulting in 18O-enrichment in water and a kinetic effect during rapid precipitation of calcite on stems causing 18O-depletion of carbonates.
It is important to note that kinetic isotope effects during intense photosynthesis also influence carbon isotopes. The kinetic depletion of 13C in carbonates may even mask the photosynthetic 13C-enrichment in carbonate and result in δ13C of the carbonate isotopically lighter than δ13CDIC (McConnaughey, 1989). However, the latter is not the case in C. rudis incrustations in Lake Jasne (Fig. 5). Still, kinetic isotope effects could and probably did result in δ13C in incrustation closer to δ13CDIC in comparison to conditions of slow CaCO3 precipitation during slow or moderately intense photosynthesis.
In agreement with Coletta et al. (2001), our results showed that the δ13C of the carbonate incrustation of C. rudis apices was heavier than the DIC. The opposite was found for the δ18O (Fig. 5). The authors cited provided data for C. hispida L. Both charophyte species are closely related (C. rudis is often considered a variety of C. hispida), and the results can be assumed as comparable. As compared to C. hispida, C. rudis and water above the studied beds appeared to be far more enriched in heavier carbon isotope. It was also the case for oxygen isotope, but the difference was smaller. In our opinion, different times of collection and different types of aquatic environments in particular, combined with varied photosynthesis rate may explain the inconsistencies found. Coletta et al. (2001) provided also the data on the stable isotope composition in other charophyte species, including C. rudis (exactly C. hispida var. rudis, Coletta et al., 2001). Interestingly, the results given for the last species are comparable to our results. As our data are limited to one species, further study is needed to find out if the correlations observed here are of broader significance.