Seasonal variability in phytoplankton stable carbon isotope ratios and bacterial carbon sources in a shallow Dutch lake

Abstract

Ecosystem metabolism of lakes strongly depends on the relative importance of local vs. allochthonous carbon sources and on microbial food-web functioning and structure. Over the year ecosystem metabolism varies as a result of seasonal changes in environmental parameters such as nutrient levels, light, temperature, and variability in the food web. This is reflected in isotopic compositions of phytoplankton and bacteria. Here, we present the results of a 17-month study on carbon dynamics in two basins of Lake Naarden, The Netherlands. One basin was restored after anthropogenic eutrophication, whereas the other basin remained eutrophic. We analyzed natural stable carbon isotope abundances (d 13 C) of dissolved inorganic carbon, dissolved organic carbon and macrophytes, and combined these data with compound-specific d 13 C analyses of phospholipid-derived fatty acids, produced by phytoplankton and bacteria. Isotopic fractionation (e) between phytoplankton biomass and CO 2(aq) was similar for diatoms and other eukaryotic phytoplankton, and differences between sampling sites were small. Highest e values were observed in winter with values of 23.5 6 0.6& for eukaryotic phytoplankton and 13.6 6 0.3& for cyanobacteria. Lowest e values were observed in summer: 10.5 6 0.3& for eukaryotic phytoplankton and 2.7 6 0.1& for cyanobacteria. The annual range in d 13 C bact was between 6.9& and 8.2& for the restored and eutrophic basin, respectively, while this range was between 11.6& and 13.1& for phytoplankton in the restored and eutrophic basin, respectively. Correlations between d 13 C phyto and d 13 C bact were strong at both sites. During summer and fall, bacterial biomass derives mainly from locally produced organic matter, with minor allochthonous contributions. Conversely, during winter, bacterial dependence on allochthonous carbon was 39-77% at the restored site, and 17-46% at the eutrophic site.

CO2 limited conditions favor cyanobacteria in a hypereutrophic lake: An empirical and theoretical stable isotope study

“…Figure a shows that the modeled δ 13 C phyto values were in general agreement with δ 13 C Fixed , indicating that the model captured natural variations in δ 13 C reasonably well. Both δ 13 C phyto and δ 13 C Fixed exhibited the same inverse and logarithmic relationship with pCO 2 , which is consistent with previous observations of a strong dependence of δ 13 C phyto on ambient [CO 2 ] aq concentration (Rau et al ; Gu et al ; Lammers et al ). However, the observed relationship between δ 13 C Fixed (and δ 13 C phyto ) and c e cannot be directly attributed to changes in phytoplankton fractionation, because the pool of DIC utilized by phytoplankton is not isotopically constant, but instead varies with a variety of factors, including air–water exchange, pH, and lateral DIC inputs (Fig.…”

“…). These relatively low fractionation factors are consistent with previously measured values for cyanobacteria of 2.7–13.6‰ (Popp et al ; Lammers et al ), and are far below the maximum photosynthetic fractionation factor typical of carbon‐replete conditions (∼ 30‰, Keller and Morel ). That both ε p‐DIC and ε p‐POC decreased during the bloom period (Fig.…”

“…The input pigment ratio matrix for the four classes was adapted from a study of lacustrine phytoplankton species (Schl€ uter et al 2006;Paerl et al 2014). Input pigment matricies of Mackey et al (1996) and Lewitus et al (2005) were used to test the sensitivity of Chemtax output to different algal class-specific pigment ratios.…”

“…Relationship between pCO 2 and the fraction of total Chl a represented by cyanobacteria. Individual data points and error bars represent the average and standard deviation of chemtax output using a range of previously published group-specific pigment ratios (Mackey et al 1996;Lewitus et al 2005;Schl€ uter et al 2006;Paerl et al 2014). Van Dam et al…”

Hydrological and Ecological Controls on Autochthonous Carbonate Deposition in Lake Systems: A Case Study From Lake Wuliangsu and the Global Perspective

“…Due to large differences in geographic characteristics among lakes, the potential uncertainties in the %DIC ORG calculation from temperature variation and the inconsistent values between sedimentary δ 13 C org and the δ 13 C DIC-ORG should be accounted for. Furthermore, in deep lakes, δ 13 C org in sediments is usually 0-7‰ lower than at the surface, due to the occurrence of lake stratification and microbial degradation (Hollander et al, 1992;Kufel & Kufel, 1997;Lammers et al, 2017;Leng & Marshall, 2004;Schwalb et al, 2013). However, temperature only has a limited impact on the absolute %DIC ORG values (~2%; Figure 3); thus, it would not alter the general pattern, that is, that %DIC ORG values in large and deep lakes are lower than in small and deep lakes and in shallow lakes.…”