Aquatic microcosm studies often increase either chamber height or base diameter (to increase water volume) to test spatial ecology theories such as “scale” effects on ecological processes, but it is unclear whether the increase of chamber height or base diameter have the same effect on the processes, i.e., whether the effect of the shape of three-dimensional spaces is significant. We orthogonally manipulated chamber height and base diameter and determined swimming activity, average swimming velocity and grazing rates of the cladocerans Daphnia magna and Moina micrura (on two algae Scenedesmus quadricauda and Chlorella vulgaris; leading to four aquatic algae-cladoceran systems in total) under different microcosm conditions. Across all the four aquatic systems, increasing chamber height at a given base diameter significantly decreased the duration and velocity of horizontal swimming, and it tended to increase the duration but decrease the velocity of vertical swimming. These collectively led to decreases in both average swimming velocity and grazing rate of the cladocerans in the tall chambers (at a given base diameter), in accordance with the positive relationship between average swimming velocity and grazing rate. In contrast, an increase of base diameter at a given chamber height showed contrasting effects on the above parameters. Consistently, at a given chamber volume increasing ratio of chamber height to base diameter decreased the average swimming velocity and grazing rate across all the aquatic systems. In general, increasing chamber depth and base diameter may exert contrasting effects on zooplankton behavior and thus phytoplankton-zooplankton interactions. We suggest that spatial shape plays an important role in determining ecological process and thus should be considered in a theoretical framework of spatial ecology and also the physical setting of aquatic microcosm experiments.

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Although spatial ecology has achieved a great success in the passing decades, the importance of habitat orientation has not been well studied, especially for its effects on prey-predator dynamics. Here, we examined the responses of zooplankton activity and grazing rate to habitat orientation and their consequences on the stability of phytoplankton-zooplankton system in a two-factor factorial experiment involving habitat orientation (three levels; small, medium, and large base area, respectively) and habitat size (64 ml and 512 ml) using two algal-grazer systems (Chlorella pyrenoidosaDaphnia magna and C. pyrenoidosa-Moina micrura). In both systems, grazer density increased with increasing base area for a given chamber volume and with increasing chamber volume for a given orientation in the first 6 days, followed by a dramatic decrease, which corresponded to increasing the amplitude of density fluctuations in both zooplankton and phytoplankton species. Such an algal-grazer dynamics could be accounted for by the greater average swimming ability and grazing rate observed in large-based and large-volumed chambers. Our results demonstrate that habitat orientation affects the zooplankton behavior and population dynamics of both zooplankton and phytoplankton species, which further influences the stability of phytoplankton-zooplankton systems.In natural ecosystems, species habitats may be placed in space with different orientations. For example, either a two-dimensional (e.g. forest patches to bird species; but not a circular habitat) or three-dimensional (e.g. waters for aquatic species; but not a spherical habitat) habitat may be placed in space at different orientations (e.g. angles relative to light direction for two-dimensional habitats and different sides as chamber base for three-dimensional habitats, see Supplementary Fig. S1), even if the habitat size and shape hold constant. Habitat orientations are diverse in natural ecosystems, particularly under human-induced disturbances that often break up a single continuous habitat into several pieces of fragments.Changes in habitat orientation can affect many environmental conditions (e.g. light, water current, heat, wind, nutrients, Earth's gravity and magnetic field) 1, 2 and thus might have consequences on various ecological processes. For example, van Kleunen & Fischer 3 found that habitat orientation (relative to the light direction) plays an important role in determining the morphology of stoloniferous rosette species by varying light intensity in different directions. Community composition of migrating organisms can also be affected by habitat orientation (relative to their moving directions). Generally, habitats oriented perpendicular to the dispersal paths of migrating organisms intercept more species (birds or fishes) and individuals than comparable habitats oriented parallel to the line of travel in both terrestrial 2, 4 and aquatic ecosystems 5 . However, the mechanisms underlying the habitat orientation effects have scarcely been explored, particularly in t...

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Habitat orientation has recently been demonstrated to affect the foraging behavior, growth, and production of plankton grazers. Because the orientation effect may vary with species, we hypothesize that habitat orientation may alter interspecific interactions between animal species. We experimentally investigated how habitat orientation (placing cuboid chambers in three orientations with long, medium, and small side as the chamber height) affected the interaction between two common cladoceran species, Daphnia magna and Moina micrura, which competitively exploited green algae of Chlorella pyrenoidosa at two volume scales (64 and 512 ml). Results show that chamber orientation and volume additively affected the behavior and species performance of the grazers. Specifically, both grazer species generally decreased their average swimming velocity, grazing rate (on algal cells), body size, and survival and reproduction rates with increasing chamber height for both chamber volumes and with decreasing chamber volume regardless of chamber orientation. Nevertheless, the decrease magnitude was greater for M. micrura with increasing chamber height but was greater for D. magna with decreasing chamber volume. Correspondingly, when cocultured, the density ratio of D. magna to M. micrura increased with increasing chamber height but decreased with decreasing chamber volume. At the end of the experiment, none of D. magna individuals survived in the small and short (large‐based) chambers, and few M. micrura individuals survived in large and tall (small‐based) chambers. These results indicate that both habitat orientation and size affect the outcome of interspecific competition between grazer species. We suggest that variation in habitat orientation may improve community coexistence and species diversity in nature.