An analysis of allochthonous and autochthonous contributions to brook trout diet using hydrogen, nitrogen, and carbon stable isotopes

Background/Question/Methods

A dynamic interdependence exists between a river and its riparian zone.  Studies have shown that reciprocal prey subsidies can make up a large proportion of consumer diet, and that these subsidies fluctuate seasonally.  This explains, in part, why human induced landscape changes such as deforestation, urbanization or agriculture can significantly affect the survival and health of in-stream consumers like fish.  Brook trout, a sensitive species that requires cool, clean and well-oxygenated water, have been particularly devastated by human land use.  Rice Creek is a unique stream in southeastern Minnesota in that it has maintained a naturally reproducing population of native brook trout.  Stable isotopes of nitrogen, carbon and hydrogen, as well as fish gut contents, were analyzed to characterize the food web of Rice Creek and to quantify the spatial and temporal variation in fish diet, including the contribution of terrestrial subsidies.  We hypothesized that brook trout feeding would shift with the seasonal change from a diet which relies heavily on aquatic sources in the winter, to one that relies more on terrestrial inputs in the summer.  As anthropogenic activities threaten the health of the population, characterizing brook trout diet will help prioritize resources that need protection.

Results/Conclusions

Contrary to our predictions, δ¹³C and δD values remained relatively constant over time, suggesting that on a large scale brook trout rely on the same subsidies throughout the year.  Furthermore, based on the isotope values, fish appear to be feeding on terrestrial prey more than on aquatic prey: δD and δ¹³C values are closer to resources of terrestrial origin than aquatic resources (-125.19‰ and -25.40‰ respectively).  Although, on average, fish diet does not exhibit temporal variation, there may be small scale differences in feeding that are not represented in the time-integrated isotope values.  Identification of gut contents revealed slight changes in food utilization on a week-by-week basis.  It is likely that fish rely on the most abundant species in the stream, but that a single source may only subsidize the fish for a short period of time.  These small shifts would not appear in the isotope values, but are nevertheless important in terms of understanding how fish obtain energy.  The inclusion of δD facilitated a more extensive analysis of the food web and terrestrial inputs.  Future research on the turnover time of δD in fish tissue and contribution of environmental water to δD values will help refine this analysis further.