Nutrient limitation, bioenergetics and stoichiometry: A new model to predict elemental fluxes mediated by fishes

Abstract

Energy flow and nutrient cycling dictate the functional role of organisms in ecosystems. Fishes are key vectors of carbon (C), nitrogen (N) and phosphorus (P) in aquatic systems, and the quantification of elemental fluxes is often achieved by coupling bioenergetics and stoichiometry. While nutrient limitation has been accounted for in several stoichiometric models, there is no current implementation that permits its incorporation into a bioenergetics approach to predict ingestion rates. This may lead to biased estimates of elemental fluxes. Here, we introduce a theoretical framework that combines stoichiometry and bioenergetics with explicit consideration of elemental limitations. We examine varying elemental limitations across different trophic groups and life stages through a case study of three trophically distinct reef fishes. Further, we empirically validate our model using an independent database of measured excretion rates. Our model adequately predicts elemental fluxes in the examined species and reveals species‐ and size‐specific limitations of C, N and P. In line with theoretical predictions, we demonstrate that the herbivore Zebrasoma scopas is limited by N and P, and all three fish species are limited by P in early life stages. Further, we show that failing to account for nutrient limitation can result in a greater than twofold underestimation of ingestion rates, which leads to severely biased excretion rates. Our model improved predictions of ingestion, excretion and egestion rates across all life stages, especially for fishes with diets low in N and/or P. Due to its broad applicability, its reliance on many parameters that are well‐defined and widely accessible, and its straightforward implementation via the accompanying r‐package fishflux, our model provides a user‐friendly path towards a better understanding of ecosystem‐wide nutrient cycling in the aquatic biome. A free Plain Language Summary can be found within the Supporting Information of this article.

Toward a better understanding of fish‐based contribution to ocean carbon flux

“…Experiments, field data, and models would need to carefully consider how to incorporate the influences of these factors to ensure that general and robust relationships are developed. An example of this is the approach taken by Schiettekatte et al (2020), whereby limiting nutrients are used as the minimum parameter for consumption and excretion rates, and factors such trophic level, life stage, and temperature are considered where possible, using well-established open-access databases. Experimental design and laboratory/ field data collection should consider that the results will be applied as model inputs, used for other species, and extrapolated spatially to estimate regional and global fish carbon flux.…”

“…However, Cavan and Hill discuss the link between primary production, fisheries production and carbon flux through fishes. Schiettekatte et al (2020) combine C, N, and P ratios in food with minimum dietary requirements of fishes to estimate nutrient flux through fishes; however, estimates for carbon flux were not reported.…”

Integral functions of marine vertebrates in the ocean carbon cycle and climate change mitigation

“…105 Nutrient pumps Marine vertebrates provide a vector for nutrient transport via excretion, egesta, and movement within and between habitats. 109,110 These processes can result in horizontal nutrient transfer across ecosystems, vertical mixing across the surface layer, or nutrient recycling. 111,112 Nutrients provided by marine vertebrates can be a source of nutrition for other animals, enabling maintenance of healthy populations that store C in biomass.…”

Response and Effect Traits of Coral Reef Fish

“…For example, when this review was conducted, no papers were found linking traits to nutrient cycling, even though the process of nutrient cycling is important to the productivity of the ecosystem (Allgeier et al, 2016). A notable addition to the literature addressing this gap is a paper and companion R package proposing a trait-based approach to model nutrient cycling (Schiettekatte et al, 2020). The authors use traits such as body size, life stage, and diet to model fish ingestion and excretion rates, and accurately predict these rates for three species.…”