Stoichiometric constraints on secondary production in a nutrient rich headwater stream food web

Abstract

Most consumers maintain body tissue Carbon:Nitrogen:Phosphorus within narrow constraints, despite ratios often varying markedly among basal resources. Elemental quality may be a major factor governing resource selection by, and production of, consumers. Elemental imbalances between resources and consumers could constrain the flux of matter through food webs. Here the aim was to characterise and quantify the distribution and flux of C, N and P in a headwater stream food web. The stream water was nutrient rich (TP=208 μg L-1; TON=7 mg L-1), resulting in low C:N and C:P of basal resources. Nevertheless, elemental imbalances were evident between consumers and basal resources, particularly for organisms feeding on detritus. Although taxonomic differences existed, detritivores were typically depleted in N and P compared to taxonomically related species. Detritus comprised 97% of the biomass entering the food web, resulting in excess consumption of C and, therefore, stoichiometric imbalances. 75% of the tree canopy was removed from the heavily shaded experimental reach, resulting in a significant increase in benthic algae and a subsequent reduction in elemental imbalance and an increase in secondary production. It appears that stoichiometric constraints on secondary production were relaxed by increasing the availability of high quality resources in this detritus based food web. Most previous studies have calculated imbalances based on diet assumed from trophic level or functional feeding groups; however, the results presented here from gut contents analysis highlight the potentially erroneous interpretation of assumed measures of imbalance. Furthermore, static measures of elemental imbalances, i.e. the difference in elemental quality between consumers and their resources, do not give an adequate measure of these constraints. The dynamic measures explored here, give an assessment of relative rates of supply (rate of consumption) and demand (from metabolism). Such dynamic measures are crucial to understanding the role of ecological stoichiometry in determining key ecosystem processes.