Abstract
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.
Highlights
Small-scale experiments such as microcosm and mesocosm studies are an important approach to generate and test ecological theories [1,2]
To the best of our knowledge, few studies have explicitly distinguished the effects of increasing chamber height and base area, and it is not clear whether increasing height has the same effect on the ecological processes as increasing base area, i.e., whether the effect of the shape of such a three-dimensional space on species interactions should be determined
dissolved oxygen (DO) was unaffected by chamber height, base diameter and chamber shape, and was always higher than 7.9 mg L-1 in all the treatments during the experiment (S1 Table), indicating that oxygen availability was not a potential limitation for the growth of grazer individuals
Summary
Small-scale experiments such as microcosm and mesocosm studies are an important approach to generate and test ecological theories [1,2]. Among the previous studies addressing effects of spatial scale on phytoplankton-zooplankton interactions, some examined the effects of increasing chamber height [6], and some others examined the effect of base area [14] or both [1,5,13]. To the best of our knowledge, few studies have explicitly distinguished the effects of increasing chamber height and base area, and it is not clear whether increasing height has the same effect on the ecological processes as increasing base area, i.e., whether the effect of the shape of such a three-dimensional space on species interactions should be determined
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