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Self-regulation of population dynamics in nutrient-rich (eutrophic) ecosystems has been a fascinating topic for decades in ecological literature. Simple theoretical models predict population oscillations of large amplitudes in such systems, those predictions often being at odds with reality. Plankton communities possess a particular combination of two important properties, making them unique among ecosystems with eutrophication. These are: (i) the existence of a pronounced spatial gradient of the prey growth rate (through light attenuation with depth) and (ii) the presence of fast-moving predator (zooplankton) capable of quick adjustment of grazing load in vertical direction throughout the whole habitat. Surprisingly, the interplay of those factors is rarely taken into account while analysing stability of nutrient-rich plankton communities. In this paper, we construct generic plankton models (based on integro-differential equations) incorporating the light attenuation in the water column as well as food-searching behaviour of zooplankton. We found that the interplay between the two factors would stabilize a system at low species densities even for an 'unlimited' nutrient stock (infinite system's carrying capacity). Different possible scenarios of stabilization have been found. Since both the vertical gradient of light and the active food search by zooplankton in the column are common characteristics of real plankton communities, we suggest that the obtained mechanism of self-regulation is rather generic in nature. We argue that taking into account this mechanism would be important for understanding the dynamics of nutrient-rich low-chlorophyll ocean systems as well as major causes of non-seasonal plankton blooms.

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We employ partial integro-differential equations to model trophic interaction in a spatially extended heterogeneous environment. Compared to classical reaction-diffusion models, this framework allows us to more realistically describe the situation where movement of individuals occurs on a faster time scale than on the demographic (population) time scale, and we cannot determine population growth based on local density. However, most of the results reported so far for such systems have only been verified numerically and for a particular choice of model functions, which obviously casts doubts about these findings. In this paper, we analyse a class of integro-differential predator-prey models with a highly mobile predator in a heterogeneous environment, and we reveal the main factors stabilizing such systems. In particular, we explore an ecologically relevant case of interactions in a highly eutrophic environment, where the prey carrying capacity can be formally set to 'infinity'. We investigate two main scenarios: (1) the spatial gradient of the growth rate is due to abiotic factors only, and (2) the local growth rate depends on the global density distribution across the environment (e.g. due to non-local self-shading). For an arbitrary spatial gradient of the prey growth rate, we analytically investigate the possibility of the predator-prey equilibrium in such systems and we explore the conditions of stability of this equilibrium. In particular, we demonstrate that for a Holling type I (linear) functional response, the predator can stabilize the system at low prey density even for an 'unlimited' carrying capacity. We conclude that the interplay between spatial heterogeneity in the prey growth and fast displacement of the predator across the habitat works as an efficient stabilizing mechanism. These results highlight the generality of the stabilization mechanisms we find in spatially structured predator-prey ecological systems in a heterogeneous environment.

Climate change is particularly rapid in the Arctic, where water temperatures are predicted to increase substantially with implications for Arctic marine organisms, especially ectotherms such as the calanoid copepod Calanus glacialis, a key herbivore in the Arctic marine ecosystem. Feeding depends on temperature, and recent studies indicate different thermal responses in ingestion and respiration implying a possible metabolic mismatch with increasing temperatures. We investigated the thermal response of ingestion and faecal pellet production as an indicator of egestion of the Arctic copepod C. glacialis in incubation experiments at five temperatures ranging from 0 to 10°C and compared the obtained data with published results on temperature dependence of respiration. Copepods were fed ad libitum with the diatom Thalassiosira gravida, and algae concentration was assessed prior and after 4 h feeding experiments. Egested faecal pellets were collected and counted. Ingestion and faecal pellet production rates increased linearly (Q 10 coefficient *1.4-1.7 and *1.8-4.1, respectively). No pronounced effect of feeding history (fed vs. starved for 3 days prior to experiment) was found, but responses in both rates were generally less dependent on temperature in the pre-starved experiment. Q 10 values for ingestion rates were lower than Q 10 values for published respiration rates (*1.8-4.6), indicating that metabolic losses increase stronger with increasing temperature than metabolic gains by ingestion. A persistent imbalance between metabolic losses and energy uptake could lead to reduced fitness for C. glacialis, thereby affecting the population dynamics and distribution of this important species in the Arctic.