Gurbir Perhar

Modelling the role of highly unsaturated fatty acids in planktonic food web processes: Sensitivity analysis and examination of contemporary hypotheses

Abstract Aquatic food web models typically treat the constituent trophic levels as static elements interacting with one another and the environment. Dynamic biological stoichiometry has relaxed this assumption and considers evolutionary responses in said elements. The incorporation of organismal response in food web models holds promise for a more realistic portrayal of ecosystem dynamics. Recent advances in aquatic ecology pinpoint the importance of highly unsaturated fatty acids (HUFAs) on food web interactions and ecosystem resilience. In this study, we utilized a HUFA explicit submodel in conjunction with a limiting nutrient–phytoplankton–zooplankton–detritus (NPZD) mathematical system to incorporate elements of the physiology of individual animals into the context of plankton dynamics. Our HUFA-augmented plankton model provided a realistic platform to examine functional properties and physiological strategies that modulate resource procurement in different trophic environments and to effectively link variability at the organismal level with ecosystem-scale patterns. First, we were able to illustrate the implications of the filling-cup hypothesis, in which species’ fitness stems from dynamic HUFA turnover rates in response to bottom-up stresses. We then examined an evolutionary hypothesis of consumer fitness dependence on HUFA quota management strategies, whereby adaptive individuals with low HUFA minimum and optimum requirements gain competitive advantage. Several studies have reported higher HUFA concentrations in consumers than producers, and our results suggest that this pattern could be driven by a combination of conservative turnover and elevated bioconversion rates. Oligotrophic settings showed strong reliance upon exogenous phosphorus subsidies and frequently yielded inverted food web biomass distributions. With the prevalence of eutrophic conditions, consumer growth is primarily controlled by HUFA availability, and the associated biochemical limitation can ultimately result in patterns of algal accumulation. Finally, our study discusses directions to improve the representation of the producer–grazer interactions and thus advance our understanding of the factors that determine the flow of nutrients and energy to the higher trophic levels.

Using Daphnia physiology to drive food web dynamics: A theoretical revisit of Lotka-Volterra models

Abstract The Lotka–Volterra model is the most commonly used framework to describe the dynamics of ecological systems in which two species interact, one as a predator and the other as prey. Theoretical ecologists have since built on variants of these equations, frequently applying them to model the dynamics of algal-herbivore interactions in aquatic systems. In this study, we augment a Lotka–Volterra system by introducing a bioenergetically-explicit, ecophysiological model to examine how variations in resource allocation affect zooplankton growth and subsequently phytoplankton dynamics. Ingested material within a zooplankters gut is separated into distinct internal congener pools that are used to support physiological processes occurring in a hierarchical direction: neurological functions, energetics, osmoregulatory maintenance, waste management, and finally growth. Consistent with the predictions of the “stoichiometric knife edge” theory, our analysis suggests that a balanced algal congener composition is required to optimize zooplankton internal congener saturations, resulting in a maximal allocation of energy to growth. In examining the advantages rendered by different strategies of minimum and optimum somatic quotas when experiencing phosphorus-enrichment conditions, we show that herbivores with narrow homeostatic bounds and animals with low minimum quotas (or depletion specialists) achieve optimal performance first. Our analysis also predicts patterns of multiple stable equilibria in which the same environmental conditions can be characterized by dramatically different prey-to-predator ratios. Importantly, abrupt shifts from one state to another can be induced not only by short-term variations in food abundance but also by variations in the nutritional quality of the prey. Our predictions have profound implications for connecting microscopic processes with macroscopic patterns and offer new insights into the multitude of factors that modulate food web dynamics.

Examination of the role of detritus food quality, phytoplankton intracellular storage capacity, and zooplankton stoichiometry on planktonic dynamics

article i nfo Nutrient stoichiometric ratios are primary driving factors of planktonic food web dynamics. Ecological stoichi- ometry theory postulates the elemental ratios of consumer species to be homeostatic, while primary-producer stoichiometry may vary with ambient nutrient availability. The notion of phytoplankton intracellular storage is far from novel, but remains largely unexplored in modeling studies of population dynamics. We constructed a seasonally-unforced, zero-dimensional, nutrient-phytoplankton-zooplankton-detritus (NPZD) model that con- siders dynamic phytoplankton phosphorus reserves and quasi-dynamic zooplankton stoichiometry. A generic food quality term is used to express seston biochemical composition, ingestibility, and digestibility. We exam- ined the sensitivity of the planktonic food web patterns to light and nutrient availability, zooplankton mortality, and detritus food quality as well as to phytoplankton intracellular storage and zooplankton stoichiometry. Our results reinforce earlier findings that high quality seston exerts a stabilizing effect on food web dynamics. How- ever, we also found that the combination of low algal and high detritus food quality with high zooplankton mor- tality yielded limit cycles and multiple steady states, suggesting that the heterogeneity characterizing seston nutritional quality may have more complicated ecological ramifications. Our numerical experiments identify re- source competition strategies related to nutrient transport rates and internal nutrient quotas that may be bene- ficial for phytoplankton to persevere in resource-limiting habitats. We also highlight the importance of the interplay between optimal stoichiometry and the factors controlling homeostatic rigidity in zooplankton. In par- ticular, our predictions show that the predominance of phosphorus-rich and tightly-homeostatic herbivores in nutrient-enrichedenvironmentswithlowseston foodqualitycanpotentiallyresultinhighphytoplanktonabun- dance, high phytoplankton-to-zooplankton ratios, and acceleration of oscillatory dynamics. Generally, our modeling study emphasizes the impact of both intracellular/somatic storage and food quality on prey-predator interactions, pinpointing an important aspect of food web dynamics usually neglected by the contemporary modeling studies.

Development of a metabolite-driven daphnia ecophysiological model

One of the founding principles of modern aquatic ecology is that human-induced perturbations in the autotroph-herbivore interface have the potential to affect ecological processes at higher trophic levels. Thus, zooplankton’s physiological state can be an early warning sign of broader impairments of aquatic ecosystems. Based on this reasoning, the micro-crustacean Daphnia is often identified as a keystone freshwater species, but its bioenergetic motivations and physiological priorities remain only partially understood. Using a bioenergetically explicit ecophysiological approach, we model how trade-offs in resource allocation can shape a daphnids growth. Our multi-faceted hierarchical approach to metabolite utilization challenges the popular paradigm of elemental stoichiometry being the primary regulatory factor of algal food quality. We examine the post-gut bioenergetic ramifications of an unbalanced diet, showing that animal growth can be significantly compromised by the elevated energetic requirements of homeostasis. Our modeling framework offers an excellent stepping-stone to connect zooplankton physiological processes with the signals of external stressors, and subsequently evaluate the patterns of mass and energy flow at an ecosystem scale. The proposed microscopic-to-macroscopic strategy will likely offer a new prospect towards the development of early warning systems for the management of freshwater resources.

Examination of the effects of largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) on the ecosystem attributes of lake Kawahara-oike, Nagasaki, Japan

article i nfo The introduction of largemouth bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus )i nto the freshwater ecosystems of Japan has resulted in the suppression and/or replacement of native species, generating considerable concerns among resource managers. The impacts of largemouth bass and bluegill on native fauna have been examined in aquaria and isolated farm ponds, but there is limited work examining the likelihood to fundamentallymodifyingJapanslakes.Theobjective of thepresentstudy istoexaminethedirectandsynergistic ecological effects of largemouth bass and bluegill on the biotic communities of Lake Kawahara-oike, Nagasaki, Japan, using an ecosystem (Ecopath) modeling approach. Specifically, we examine whether the two fish species have played a critical role in shaping the trophodynamics of the lake. We attempt to shed light on the trophic interactions between largemouth bass and bluegill and subsequently evaluate to what extent these interactions facilitate their establishment at the expense of native species. We also examine how these changes propagate through the Lake Kawahara-oike food web. Our study suggests that the introduction of bluegill has induced a range of changes at multiple trophic levels. The present analysis also provides evidence that largemouth bass was unable to exert significant top-down control on the growth rates of the bluegill population. Largemouth bass and bluegill appear to prevail over the native fish species populations and can apparently coexist in large numbers in invaded lakes. Future management strategies controlling invasive species are urgently required, if the integrity of native Japanese fish communities is to be protected.