Do stable carbon and nitrogen isotope values of Nitella flexilis differ between softwater and hardwater lakes?

Abstract

The composition of stable carbon and nitrogen isotopes (δ13C and δ15N) is widely used for tracking the origin of organic matter (OM) present in lacustrine sediments. These isotopes also define the evolution of OM in food web loops. Charophyte species Nitella flexilis C. Agardh, 1824 can be found in different aquatic environments where it contributes significantly to sediment formation and influences biota function. Therefore, it is crucial to study more about δ13C and δ15N in different lake types. Here, we present the results of the first comprehensive study of N. flexilis δ13C and δ15N, which add to the knowledge of the C and N isotope records of charophytes. We obtained the δ13C and δ15N records of N. flexilis OM from hardwater and softwater lakes and checked for differences between these records. We also analyzed the differences in physical and chemical parameters. Finally, we compared the δ13C and δ15N records with physical and chemical parameters to identify the variables that have the highest influence on N. flexilis δ13C and δ15N values. Our study showed that both δ13C and δ15N did not differ significantly in the two types of lakes, although the lakes had significant differences in several physical and chemical parameters (pH, Ca2+, dissolved inorganic carbon, total phosphorus, conductivity). However, we observed that δ13C values were influenced by light conditions (photosynthetic active radiation, depth, dissolved OM), while δ15N values were influenced by the total nitrogen concentration in water.

The Possible Use of Stable Carbon and Nitrogen Isotope Signal and Spectral Analysis to Identify Habitat Condition of Aquatic Plants

“…The δ 13 CORG and δ 15 NORG values of aquatic vegetation, especially submerged macrophytes, are characterized by very high variability dependent on the environmental conditions in which they thrive [5,[13][14][15][16]43]. However, for certain species, habitat differences are not always clearly reflected in their isotopic values [14].…”

“…The δ 13 CORG and δ 15 NORG values of aquatic vegetation, especially submerged macrophytes, are characterized by very high variability dependent on the environmental conditions in which they thrive [5,[13][14][15][16]43]. However, for certain species, habitat differences are not always clearly reflected in their isotopic values [14]. The ecological characteristics of a given species significantly influence both δ 13 CORG and δ 15 NORG values, including whether they grow in loose clusters or form dense underwater mats [16].…”

“…Due to the nonreactivity with HCL during tests, the material from softwater lakes remained nondecarbonated. The final amount obtained after the abovementioned treatments was about 500 mg. For the single analyses, the necessary weight of the samples was about 2 mg. Further details of the method used are outlined in Pronin et al [14]. Samples prepared using this approach were analyzed on a Flash Smart EA elemental analyzer (Thermo Scientific, Waltham, MA, USA) connected to a mass spectrometer for Delta V stable isotope ratios (Thermo Scientific, USA).…”

“…Analyses of carbon and nitrogen isotope compositions in derived matter (δ 13 C of organic matter δ 13 C ORG and δ 15 N of organic matter δ 15 N ORG ) are often used in studies of freshwater ecosystems [12], where scientists focus on submerged macrophytes [13][14][15][16][17][18][19]. The isotopic information about aquatic plants recorded as δ 13 C ORG and δ 15 N ORG serves various purposes, including characterizing and tracking food webs in aquatic ecosystems [20,21] and determining the source of organic matter deposited in the sediments [22][23][24][25], etc.…”

“…Within complex lake ecosystems, numerous factors contribute to shaping the δ 13 C DIC and δ 15 N DIN . These enclose a comprehensive range of factors, including but not limited to water exchange over time, temperature fluctuations, biological activities of organisms inhabiting the investigated water ecosystem, and various other interrelated elements [13][14][15][16][17][18][19]. In the presented study, the focus on shaping the δ 13 C ORG and δ 15 N ORG will be minor due to the need to stress the usefulness of the obtained results for identifying the origin of the analyzed plant material.…”