Subhendu Chakraborty

NUTRIENT-PHYTOPLANKTON-ZOOPLANKTON DYNAMICS IN THE PRESENCE OF ADDITIONAL FOOD SOURCE — A MATHEMATICAL STUDY

In the present paper we propose four nutrient-phytoplankton-zooplankton models with and without additional food source. Each of the models is incorporated with the influence of toxin released by phytoplankton population. Existence and local stability analysis are performed for each of the models. Extensive numerical experiments are performed to observe the global dynamics of the model system. Our observations indicate that the chance of extinction of population decreases due to additional food source even if in the presence of toxin released by phytoplankton.

Spatial dynamics of a nutrient–phytoplankton system with toxic effect on phytoplankton

The production of toxins by some species of phytoplankton is known to have several economic, ecological, and human health impacts. However, the role of toxins on the spatial distribution of phytoplankton is not well understood. In the present study, the spatial dynamics of a nutrient-phytoplankton system with toxic effect on phytoplankton is investigated. We analyze the linear stability of the system and obtain the condition for Turing instability. In the presence of toxic effect, we find that the distribution of nutrient and phytoplankton becomes inhomogeneous in space and results in different patterns, like stripes, spots, and the mixture of them depending on the toxicity level. We also observe that the distribution of nutrient and phytoplankton shows spatiotemporal oscillation for certain toxicity level.

Harmful algal blooms: combining excitability and competition

Harmful algal blooms (HABs) characterized by a large concentration of toxic species appear rather rarely, but have a severe impact on the whole ecosystem. To study on possible trigger mechanisms for the emergence of HABs, we consider a nutrient-phytoplankton-zooplankton model to find the conditions under which a toxic phytoplankton species is able to form a bloom by winning the competition against its nontoxic competitor. The basic mechanism is related to the excitability of the system, i.e., the ability to develop a large response on certain perturbations. In a large class of models, a HAB results from a combined effect of nutrient enrichment and selective predation on different phytoplankton populations by zooplankton. We show that the severity of HAB is controlled by nutrient enrichment and zooplankton abundance, while the frequency of its occurrence depends on the strength of selectivity of predation. Thereby the intricate interplay between excitability, competition, and selective grazing pressure builds the backbone of the mechanism of the emergence of HABs.

Modeling succession of key resource-harvesting traits of mixotrophic plankton.

Unicellular eukaryotes make up the base of the ocean food web and exist as a continuum in trophic strategy from pure heterotrophy (phagotrophic zooplankton) to pure photoautotrophy (‘phytoplankton’), with a dominance of mixotrophic organisms combining both strategies. Here we formulate a trait-based model for mixotrophy with three key resource-harvesting traits: photosynthesis, phagotrophy and inorganic nutrient uptake, which predicts the trophic strategy of species throughout the seasonal cycle. Assuming that simple carbohydrates from photosynthesis fuel respiration, and feeding primarily provides building blocks for growth, the model reproduces the observed light-dependent ingestion rates and species-specific growth rates with and without prey from the laboratory. The combination of traits yielding the highest growth rate suggests high investments in photosynthesis, and inorganic nutrient uptake in the spring and increased phagotrophy during the summer, reflecting general seasonal succession patterns of temperate waters. Our trait-based model presents a simple and general approach for the inclusion of mixotrophy, succession and evolution in ecosystem models.

Trophic Strategies of Unicellular Plankton

Unicellular plankton employ trophic strategies ranging from pure photoautotrophs over mixotrophy to obligate heterotrophs (phagotrophs), with cell sizes from 10−8 to 1 μg C. A full understanding of how trophic strategy and cell size depend on resource environment and predation is lacking. To this end, we develop and calibrate a trait-based model for unicellular planktonic organisms characterized by four traits: cell size and investments in phototrophy, nutrient uptake, and phagotrophy. We use the model to predict how optimal trophic strategies depend on cell size under various environmental conditions, including seasonal succession. We identify two mixotrophic strategies: generalist mixotrophs investing in all three investment traits and obligate mixotrophs investing only in phototrophy and phagotrophy. We formulate two conjectures: (1) most cells are limited by organic carbon; however, small unicellulars are colimited by organic carbon and nutrients, and only large photoautotrophs and smaller mixotrophs are nutrient limited; (2) trophic strategy is bottom-up selected by the environment, while optimal size is top-down selected by predation. The focus on cell size and trophic strategies facilitates general insights into the strategies of a broad class of organisms in the size range from micrometers to millimeters that dominate the primary and secondary production of the world’s oceans.

The cost of toxin production in phytoplankton: the case of PST producing dinoflagellates

Many species of phytoplankton produce toxins that may provide protection from grazing. In that case one would expect toxin production to be costly; else all species would evolve toxicity. However, experiments have consistently failed to show any costs. Here, we show that costs of toxin production are environment dependent but can be high. We develop a fitness optimization model to estimate rate, costs, and benefits of toxin production, using PST (paralytic shellfish toxin) producing dinoflagellates as an example. Costs include energy and material (nitrogen) costs estimated from well-established biochemistry of PSTs, and benefits are estimated from relationship between toxin content and grazing mortality. The model reproduces all known features of PST production: inducibility in the presence of grazer cues, low toxicity of nitrogen-starved cells, but high toxicity of P-limited and light-limited cells. The model predicts negligible reduction in cell division rate in nitrogen replete cells, consistent with observations, but >20% reduction when nitrogen is limiting and abundance of grazers high. Such situation is characteristic of coastal and oceanic waters during summer when blooms of toxic algae typically develop. The investment in defense is warranted, since the net growth rate is always higher in defended than in undefended cells.