Carbon turnover rate of Gammarus tissue

Background/Question/Methods

Stable isotope ratios (δ13C) are widely used as ecological tracers to determine the habitat of primary producers supporting consumers in food webs.  Consumers usually have a δ13C value very similar to their food, and carbon isotope ratios are conserved as organic matter and transferred through food webs.  One advantage of carbon isotope ratios over traditional gut content analysis is that the δ13C of an animal’s tissue integrates information about diet over time.  One disadvantage, so far, is that for many animals the C turnover time in their tissues is unknown, so it is not clear over what time period the diet has been integrated.  This is particularly true for freshwater invertebrates. Using natural abundance 13C analysis, we have determined carbon isotopic turnover rates for the exoskeleton, internal viscera (without fore, mid and hind gut), whole body without gut contents and the whole body of the freshwater amphipod Gammarus lacustris, a widespread and abundant aquatic macroinvertebrate.  In three laboratory controlled water temperatures (15 ^oC, 18 ^oC, 23 ^oC), G. lacustris were fed a diet of C4 plant material, corn (Zea mays, δ13C  = -13.11‰, SE 0.11).  Background carbon isotope ratios of G. lacustris (δ13C  =  -25.10‰, SE 0.11) are similar to those of a C3 plant abundant in their habitat (Phalaris arundinacea, δ13C  = -27.05‰, SE 0.04).  Over a series of sampling dates, G. lacustris were removed from each of the three temperatures and analyzed for stable carbon isotope ratios.

Results/Conclusions

Over time, the δ13C of G. lacustris tissue showed a strong exponential relationship with δ13C increasing to values close to the introduced C4 forage.  Linear regression of natural log-transformed δ13C indicated that carbon turnover was more rapid at 15 and 18 °C than at 21 °C.  Abrupt shifts in δ13C of 4‰ were observed in as little as 10 days within whole body analyzes.  Our results are also consistant with other studies suggesting that the clearing of gut contents in primary consumers before stable isotope analysis is unnecessary.  In a separate experiment examining specific tissues, we found significantly more rapid δ13C turnover for exoskeleton tissue compared to visceral tissue at 12 ^oC.  These findings suggest that the assimilation of carbon within organisms can be monitored more quickly and accurately with stable isotope analysis than previously suggested.