Macroinvertebrate delta C-13 variability analysis for the assessment of lake trophic functioning

The aim of this study was to better understand the relationships of within- and among-lake variability of macroinvertebrate carbon stable-isotope ratios with both lake trophic potential and lake trophic functioning. Two approaches based on the macroinvertebrate communities were undertaken on 11 French lakes: the Lake Biotic Index (LBI), a lake biological quality assessment method (Verneaux et at. 2004 a), and the carbon isotopic signatures of macroinvertebrates expressed by metrics of dispersion (standard deviation of the littoral macroinvertebrate mean delta C-13 values and differences between littoral and deep mean delta C-13 values (Delta delta C-13)). The littoral macroinvertebrate mean delta C-13 values obtained ranged from -11.7 parts per thousand to -38.6 parts per thousand. This great among-lake variability can be partially explained by different geological substrata on which the lakes are located and, potentially, by the lake organic matter recycling activity. Moreover, the depth-related differences in macroinvertebrate community delta C-13 (Delta delta C-13 values) or macro invertebrate taxa (taxon C-13 depletion: T Delta delta C-13) were highly correlated to the values of a LBI sub-index. which was itself correlated to hypolimnion oxygenation and deep sediment organic matter contents. This result Suggests that the studied lakes showed various degrees of deep recycled carbon assimilation. The depth-related macroinvertebrate C-13 depletion may then be considered as describing the result of lake trophic functioning. However, because no correlation was observed between littoral macroinvertebrate delta C-13 variability and lake trophic potential, it seemed difficult to conclude as to the influence of different carbon sources on this potential.