Lake Zooplankton Carbon Sources: The Role Of Terrestrial Inputs And The Effects Of Depth And Taxonomic Composition
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The relative contribution of allochthonous and autochthonous production in zooplankton nutrition has been of interest since the net heterotrophy of lakes was recognised to be common. I measured the 13C signature of epilimnetic CO2, particulate organic carbon (POC), and zooplankton in 27 north-temperate lakes in late summer and used the relationships between the POC and zooplankton 13C signatures and the CO2 signature to estimate the autochthonous contribution to these fractions of the plankton. My hypothesis was that POC and zooplankton signature would reflect the 13CO2 signature if they were autochthonous. Conversely, increasing allochthonous C would result in a 13C signature of POC or zooplankton that is increasingly influenced by the allochthonous 13C signature (δ-28‰) and decreasingly dependent on the CO2 signature. The average autochthonous contribution to epilimnetic POC was estimated to be between 62 and 75%. Epilimnetic zooplankton were, on average, between 77 and 91% autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13C. The relationship between 13C POC and 13CO2 allowed us to estimate average photosynthetic fraction as -15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer. I expanded the above analysis by culling the literature for 13C stable isotope data of lake CO2, POC, and zooplankton. I found that, similar to the lakes that I had sampled, POC signature showed a strong influence of allochthonous C, and inferred that it was close to 50% allochthonous on average. I calculated an autochthonous fractionation of -14.1‰ for the metadata, which was similar to that of the lakes I sampled. While POC had a considerable allochthonous contribution, zooplankton signatures were strongly related to the CO2 signatures, suggesting that their carbon was mostly autochthonous. Therefore, while terrestrial inputs form a major portion of POC, zooplankton C, on average, was largely autochthonous. I also examined the differences in 13C/15N among zooplankton taxa, and differences in 13CO2, 13C/15N of POM, and 13C/15N of zooplankton with depth. There were small differences among the 15N of various taxa, and I did not detect differences in 13C amongst taxa. I found vertical heterogeneity was most marked in 13CO2 signatures, which generally depleted appreciably with increasing lake depth. The signatures of 13C POM and 13C zooplankton also generally depleted with depth, but much less so than did 13CO2. I interpret this as indicating that a large portion of POM and zooplankton C in the metalimnia and hypolimnia of these lakes is derived from C fixed in the epilimnia.