Carnivore identity mediates the effects of light and nutrients on aquatic food‐chain efficiency

Abstract

Summary Food chain efficiency (FCE), the proportion of primary production converted to production of the top trophic level, can influence several ecosystem services as well as the biodiversity and productivity of each trophic level. Aquatic FCE is affected by light and nutrient supply, largely via effects on primary producer stoichiometry that propagate to herbivores and then carnivores. Here, we test the hypothesis that the identity of the top carnivore mediates FCE responses to changes in light and nutrient supply. We conducted a large‐scale, 6‐week mesocosm experiment in which we manipulated light and nutrient (nitrogen and phosphorus) supply and the identity of the carnivore in a 2 × 2 × 2 factorial design. We quantified the response of FCE and the biomass and productivity of each trophic level (phytoplankton, zooplankton, and carnivore). We used an invertebrate, Chaoborus americanus, and a vertebrate, bluegill sunfish (Lepomis macrochirus), as the two carnivores in this study because of the large difference in phosphorus requirements between these taxa. We predicted that bluegill would be more likely to experience P‐limitation due to higher P requirements, and hence that FCE would be lower in the bluegill treatments than in the Chaoborus treatments. We also expected the interactive effect of light and nutrients to be stronger in the bluegill treatments. Within a carnivore treatment, we predicted highest FCE under low light and high nutrient supply, as these conditions would produce high‐quality (low C:nutrient) algal resources. In contrast, if food quantity had a stronger effect on carnivore production than food quality, carnivore production would increase proportionally with primary production, thus FCE would be similar across light and nutrient treatments. Carnivore identity mediated the effects of light and nutrients on FCE, and as predicted FCE was higher in food chains with Chaoborus than with bluegill. Also as predicted, FCE in Chaoborus treatments was higher under low light. However, FCE in bluegill treatments was higher at high light supply, opposite to our predictions. In addition, bluegill production increased proportionally with primary production, while Chaoborus production was not correlated with primary production, suggesting that bluegill responded more strongly to food quantity than to food quality. These carnivore taxa differ in traits other than body stoichiometry, for example, feeding selectivity, which may have contributed to the observed differences in FCE between carnivores. Comparison of our results with those from previous experiments showed that FCE responds similarly to light and nutrients in food chains with Chaoborus and larval fish (gizzard shad: Clupeidae), but very differently in food chains with bluegill. These findings warrant further investigation into the mechanisms related to carnivore identity (e.g., developmental stage, feeding selectivity) underlying these responses, and highlight the importance of considering both top‐down and bottom‐up effects when evaluating food c...

Urbanization decreased the multitrophic fish richness and ecosystem functioning in neotropical streams

“…Our results indicate that this occurred because of increased urbanization in the streams, the richness carnivorous, omnivorous, and detritivorous shes decreased, and particularly of carnivores, which was the trophic group that most positively affected the standing stock biomass of the streams. Carnivorous shes are key components in aquatic ecosystems and may affect biomass production through direct and indirect pathways by structuring the food web and controlling interactions between the lowest trophic groups and consequently their biomass (Rock et al 2016). Moreover, most carnivorous shes in our study are generalists (i.e., they are able to feed on benthos and pelagic sh), which is similar to those found in the Europe (Maureaud et al 2019).…”

Light and nutrient supply mediate intraspecific variation in the nutrient stoichiometry of juvenile fish

“…Finally, a previous experiment, conducted in these same mesocosms, showed that juvenile bluegill production is highly correlated with phytoplankton production and relatively insensitive to phytoplankton quality (Rock et al. ). Thus, experimental results support the hypothesis that bluegill growth and body stoichiometry are strongly influenced by primary production as driven by light and nutrients.…”

“…However, some carnivores are relatively insensitive to the variation in herbivore quality induced by the variation in algal quality, and in these cases, algal quality effects do not travel up the food chain (Malzahn et al. ; Rock et al., in press).…”

Predominant Non-additive Effects of Multiple Stressors on Autotroph C:N:P Ratios Propagate in Freshwater and Marine Food Webs

“…In a similar study, Plum et al ( 2015 ) found that the nature of the interactive effects of light and nutrients on algal C:P and N:P ratios were mostly synergistic (four out of five algal bioassays; see reference 20 in Figure S1 ), and it was under high light and N reduced conditions when the highest copepod biomass was attained. Further, in a recent study, Rock et al ( 2016 ) identified that carnivores can affect the mediated effects of light and nutrients on aquatic food chain efficiency. Interestingly, if we examine their carnivore-bluegill treatment (see reference 25 in Figure S1 ), the effects of light and nutrient on seston C:P were synergistic, and under these conditions herbivore efficiency was at its highest.…”