Variation of stable carbon and nitrogen isotopes composition of plants and sediments along pH gradient of soft-water lakes in Poland

The soft-water lake vegetation is sensitive to changes in water quality, especially pH and nutrient concentration. Furthermore, little is known about the biogeochemistry of those types of water bodies. Therefore, to recognize the relationship between the aquatic plants and the co-created sediments, we applied in our study the analysis of stable carbon and nitrogen isotopic composition (δ¹³C and δ¹⁵N) of organic matter of ten characteristic plants for soft-water lakes and sediments on which they have grown. We investigated physicochemical parameters of two types of water: one from the immediate surroundings of plants and the second type collected just above or directly from sediment (if they were more organic and looser). In the middle of the vegetation season (June 2020), the studies were performed on 14 soft-water lakes along a pH gradient (from 4.78 to 9.21). We found a high positive relationship between δ¹³C values of plants and sediments (Spearman rank correlations r= 0.69; N=85) and moderate positive relationships between δ¹⁵N values of plants and sediments (r= 0.31; N=85). Both for δ¹³C and δ¹⁵N, the variability of plants isotopic values was higher in plants organic matter than in sediments (for plants; δ¹³C from -33.76‰ to -9.93‰ and δ¹⁵N from -5.49‰ to 5.95‰; for sediments δ¹³C from -30.13‰ to -13.60‰ and δ¹⁵N from -2.92‰ to 4.82‰). In the case of Lobelia dortmanna, Fontinalis antipyretica, Luronium natans and Isoëtes lacustris δ¹³C values were higher in organic matter of the sediments than in investigated aquatic plants. On the other hand, especially samples for Elodea canadensis and Myriophyllum alterniflorum had opposite patterns, where values of δ¹³C were much higher in plants. The δ¹⁵N values of plants were lower than those reported for the deposits, and this pattern was more constant, with two exceptions recorded for Luronium natans and Chara globularis. Comparing the physicochemical parameters of surrounding and sediments waters, we found only high differences in total nitrogen concentration (TN) where higher concentration was reported in sediment water. In addition, the distribution of environmental variables for both water from anong plants and sedimentary water (Principal Components Analyzes - PCA's) indicates a higher relationship between the values of δ¹³C and δ¹⁵N of plant and sediments organic matter and the TN concentration in the sediment water. Moreover, the results of PCA for both waters types showed some relationship of δ¹³C of plants and sediments with pH, conductivity and Ca²⁺ concentration, which were more evident for sediment water. Founded here, strong relationships between plants and sediments δ¹³C values might confirm that in the cases of most investigated plants, they highly participate in sediment creation in those low-productive soft-water lakes. However, this assumption is less established when we focus on δ¹⁵N results. Moreover, both δ¹³C and δ¹⁵N of plants organic matter varied more than sediments, suggesting that allochthonous materials are also engaged in sediments creations. The further species-specific analysis is needed to better explain the present trends and relationships.

The studies were financed by Polish National Science Centre, under project No 2019/32/C/NZ8/00147.