Consumer‐driven nutrient dynamics in freshwater ecosystems: from individuals to ecosystems

Abstract

The role of animals in modulating nutrient cycling [hereafter, consumer-driven nutrient dynamics (CND)] has been accepted as an important influence on both community structure and ecosystem function in aquatic systems. Yet there is great variability in the influence of CND across species and ecosystems, and the causes of this variation are not well understood. Here, we review and synthesize the mechanisms behind CND in fresh waters. We reviewed 131 articles on CND published between 1973 and 1 June 2015. The rate of new publications in CND has increased from 1.4 papers per year during 1973-2002 to 7.3 per year during 2003-2015. The majority of investigations are in North America with many concentrating on fish. More recent studies have focused on animal-mediated nutrient excretion rates relative to nutrient demand and indirect impacts (e.g. decomposition). We identified several mechanisms that influence CND across levels of biological organization. Factors affecting the stoichiometric plasticity of consumers, including body size, feeding history and ontogeny, play an important role in determining the impact of individual consumers on nutrient dynamics and underlie the stoichiometry of CND across time and space. The abiotic characteristics of an ecosystem affect the net impact of consumers on ecosystem processes by influencing consumer metabolic processes (e.g. consumption and excretion/egestion rates), non-CND supply of nutrients and ecosystem nutrient demand. Furthermore, the transformation and transport of elements by populations and communities of consumers also influences the flow of energy and nutrients across ecosystem boundaries. This review highlights that shifts in community composition or biomass of consumers and eco-evolutionary underpinnings can have strong effects on the functional role of consumers in ecosystem processes, yet these are relatively unexplored aspects of CND. Future research should evaluate the value of using species traits and abiotic conditions to predict and understand the effects of consumers on ecosystem-level nutrient dynamics across temporal and spatial scales. Moreover, new work in CND should strive to integrate knowledge from disparate fields of ecology and environmental science, such as physiology and ecosystem ecology, to develop a comprehensive and mechanistic understanding of the functional role of consumers. Comparative and experimental studies that develop testable hypotheses to challenge the current assumptions of CND, including consumer stoichiometric homeostasis, are needed to assess the significance of CND among species and across freshwater ecosystems.

Effects of incubation conditions on nutrient mineralisation rates in fish and shrimp

“…Therefore, we recommend that investigators interested in quantifying CND to model the functional role of organisms in ecosystem processes should experiment with their incubation methods to mitigate the effects of the GSP as much as possible. To echo the sentiments of Atkinson et al (2016), our results suggest that comparative studies employing standardised methods and metrics are needed to quantify the relative importance of CND on ecosystem processes among species and across ecosystems. Therefore, it is important to conduct temporal tests of mineralisation rates for species of interest (Whiles et al, 2009).…”

“…Published work on CND has focused on mineralisation rates and their relation to diet, body size and stoichiometry (Atkinson et al, 2016), but our work highlights the need for more studies on the influence of animal behaviour and physiology on CND. Published work on CND has focused on mineralisation rates and their relation to diet, body size and stoichiometry (Atkinson et al, 2016), but our work highlights the need for more studies on the influence of animal behaviour and physiology on CND.…”

“…Although it is not always the case, studies have demonstrated that the functional contribution of aquatic consumers to nitrogen (N) and phosphorus (P) cycling can be important at the ecosystem level (Atkinson, Capps, Rugenski, & Vanni, 2016). For example, McIntyre et al (2008) documented the fundamental role fish communities can play in N dynamics in N-limited systems.…”

“…Hundreds of studies examining CND have been published over the past few decades using a variety of methods (Atkinson et al, 2016). Although some previous work has documented the impact methodological choices can have on element-specific mineralisation measurements (e.g., Capps & Flecker, 2013a;Whiles, Huryn, Taylor, & Reeve, 2009), it is difficult to compare mineralisation rates between studies, among organisms and across ecosystems.…”

Increased light availability and nutrient cycling by fish provide resilience against reversing eutrophication in an agriculturally impacted reservoir

“…Consumer‐mediated nutrient cycling in lakes can be an important source of limiting nutrients for primary producers (Vanni ; Atkinson et al ), and thus may provide resilience against the reversal of eutrophication. The gizzard shad ( Dorosoma cepedianum ) often dominates fish biomass in productive lakes and reservoirs of the Midwestern and Southeastern United States, especially those in highly agricultural watersheds (Vanni et al ).…”