There have been several studies where the isotopic composition of organisms has been determined seasonally, but fewer have examined separate organs. In this context, separate organs (e.g. gonad, digestive gland and muscle), of a suspension-feeder, the scallop Pecten maximus, were used to assess seasonal changes of both stable isotopes and biochemical components. Our study used multiple indicators (stable carbon and nitrogen isotopes ratios, biochemical components and seston Chl a) to track nutritive activity and energy allocation in P. maximus from the Bay of Brest (France). In addition to seasonal variation in the isotopic composition of P. maximus tissues, we found strong differences in the mean isotopic signatures of different organs. This has serious implications for interpretation of animal diets and potential use in animal physiology. Furthermore, we present evidence that seasonal variations of metabolism will cause changes in the isotopic composition not related to changes in the diet. Interpretation of isotopic data may require consideration of values from several separate organs. Finally, δ 15 N appears powerful to track metabolite fates in the scallop P. maximus.
We conducted laboratory experiments with 85 assembled phytoplankton communities composed of species from four predefined functional groups (chlorophytes, diatoms, cyanobacteria, chrysophytes) to distinguish the relative importance of species diversity from functional group diversity on carbon uptake. We separated the observed diversity effects on carbon uptake into those caused by species with particularly important traits (selection effect) and those caused by positive interactions among species (e.g., complementary resource use or facilitation [complementarity effect]). Additionally, we measured the composition of photosynthetically active pigments and light absorbance in communities and monocultures, and related them to species and functional diversity effects on carbon accrual. Biodiversity effects were weak or even absent in pure cyanobacterial and diatom communities compared to strong effects in chlorophytes. Complementarity effects and light absorbance increased as functional (i.e., phylogenetic) diversity increased. There was a positive correlation between complementarity effects on carbon accrual and light absorbance. These findings support hypotheses regarding biodiversity-productivity relationships in phytoplankton communities based on niche separation along spectral light gradients.
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