PHYSICAL REVIEW E 99, 012406 (2019) Zooplankton selectivity and nutritional value of phytoplankton influences a rich variety of dynamics in a plankton population model Nandadulal Bairagi, * Suman Saha, Sanjay Chaudhuri, and Syamal Kumar Dana Centre for Mathematical Biology and Ecology, Department of Mathematics, Jadavpur University, Kolkata 700032, India (Received 1 August 2018; revised manuscript received 5 November 2018; published 8 January 2019) Mathematical modeling may be an excellent tool to analyze and explain complex biological phenomena. In this paper, we use a mathematical model to reveal various interesting dynamical features of phytoplankton- zooplankton interaction and attempt to explain the reason for contrasting dynamics shown by different laboratory and field experiments. Our study shows that the phytoplankton-zooplankton interaction in a pelagic system is very complex and the plankton dynamics, including the bloom phenomenon, strongly depends on the selective predation of zooplankton and the nutritional value of phytoplankton. The study supports the existing hypothesis that decoupling at the plant-animal interface may occur due to strong fish predation on zooplankton. In addition, we argue that decoupling of the food chain may also occur under low to intermediate nutrient inflow if zooplankton feeds on phytoplankton having lower nutritional value. It is also shown that nutrient enrichment can destabilize an otherwise stable system if zooplankton feeds on highly nutritious prey, but unable to destabilize the system if zooplankton feeds on low-nutritious prey. This may be one possible explanation to the longstanding question: Why do some experiments show the paradox of enrichment and others do not? DOI: 10.1103/PhysRevE.99.012406 I. INTRODUCTION The planktonic food web is a very complex process that is still partially understood in spite of tremendous efforts given by researchers. Different factors have been identified that may affect the planktonic food web, namely, selective predation [1,2], nutrient inflow [3,4], grazing by higher preda- tors [5], etc. Food selection is a natural characteristic of zooplankton [6], and its food selectivity criteria vary to a large extent [7]. When zooplankton is exposed to a variety of phytoplanktons, it selects a prey item depending on its size [8], digestibility [1], toxicity [9], availability [10–12], and nutrition value [1,13]. Several studies confirm that Calanoid copepods (zooplankton) can discriminate between toxic and nontoxic dinoflagellates [14], noxious and innoxious blue- greens [15], and live and dead algae of the same species [16]. Prey or food selectivity has significant impact on population stability [17]. Stoichiometric modulation of predation also shows contrasting dynamics in a pelagic system [18]. Different experiments confirm that a number of phy- toplankton species have the ability to produce toxic sub- stances (toxin-producing phytoplankton) and others do not (nontoxic phytoplankton). The toxin-producing phytoplank- tons are sometimes less preferred or avoided by herbivo- rous zooplankton. In a recent work, Pal et al. [9] showed that the zooplankter (Artemia salina) can discriminate toxic phytoplankton (Microcystis aeruginosa) from the nontoxic phytoplankton (Chaetocerous gracilis). They observed that zooplankton is less inclined to opt for food based on size than the preferential selectivity for the safe nontoxic food species. It has also been demonstrated that the toxin produced * nbairagi.math@jadavpuruniversity.in by phytoplankton has widely differing effects on its grazers. Some zooplankton can digest or ingest toxin-producing phy- toplankton without any harm, whereas some others experience deleterious effects [19]. Studies suggest that phytoplankton food quality for herbiv- orous zooplankton may play an important role in planktonic food web interactions in pelagic systems [1,2,13,44]. An opti- mal filter feeder selects foods based on their nutritional value. They first capture algae and then reject or ingest them after assaying their nutritional value [2]. In laboratory experiments, Demott [1] demonstrated that diet selection of Eudiaptomus spp. was strongly influenced by both algae quality and the availability of other foods. In particular, it was shown that discrimination against low-quality algae was strong enough in an abundance of high-quality algae, and weak when high- quality algae were scarce. Experiments also confirm that it can distinguish between the digestible and the digestion-resistant algae. In a recent work, Danielsdottir et al. [13] showed that phytoplankton biomass was suppressed and zooplankton can withstand strong predation pressure of zooplanktivory if the algal food quality is high. On the other hand, trophic decoupling at the plant-animal interface may occur if the food quality is low. The planktonic copepod Acartia tonsa has been demonstrated to exhibit prey-switching behavior depending on the availability of alternative preys [12]. These biological observations motivated us to study the dynamical behavior of the planktonic food web under the dual effect of zooplankton’s selectivity and the nutritional value of phytoplankton. We use here a mechanistic predator-prey model that makes a balance between simplicity and realism so as to explain some of the observed behaviors, as cited above, from the dynamical system viewpoint. We incorporate in the model the effects of phytoplankton food quality as well as zooplankton’s selectivity in an explicit way and then study 2470-0045/2019/99(1)/012406(12) 012406-1 ©2019 American Physical Society