Abstract
BackgroundClassically, estuarine planktonic research has focussed largely on the physico-chemical drivers of community assemblages leaving a paucity of information on important biological interactions.Methodology/Principal FindingsWithin the context of trophic cascades, various treatments using in situ mesocosms were established in a closed estuary to highlight the importance of predation in stabilizing estuarine plankton abundances. Through either the removal (filtration) or addition of certain planktonic groups, five different trophic systems were established. These treatments contained varied numbers of trophic levels and thus different “predators” at the top of the food chain. The abundances of zooplankton (copepod and polychaete), ciliate, micro-flagellate, nano-flagellate and bacteria were investigated in each treatment, over time. The reference treatment containing apex zooplanktivores (early juvenile mullet) and plankton at natural densities mimicked a natural, stable state of an estuary. Proportional variability (PV) and coefficient of variation (CV) of temporal abundances were calculated for each taxon and showed that apex predators in this experimental ecosystem, when compared to the other systems, induced stability. The presence of these predators therefore had consequences for multiple trophic levels, consistent with trophic cascade theory.Conclusions/SignificancePV and CV proved useful indices for comparing stability. Apex predators exerted a stabilizing pressure through feeding on copepods and polychaetes which cascaded through the ciliates, micro-flagellates, nano-flagellates and bacteria. When compared with treatments without apex predators, the role of predation in structuring planktonic communities in closed estuaries was highlighted.
Highlights
Trophic interactions play an essential organisational role in community and ecosystem ecology [1]
Predatory top-down control is observed through trophic cascades, across multiple lower trophic levels, underscoring the importance of top predators in food webs and underlying community structure [10,12]
Of zooplankton sampled with the WP-2 type net, the calanoid copepod, Pseudodiaptomus hessei numerically dominated the adult copepod abundance (99.5%) while Prionospio sp. dominated polychaete numbers (99.9%) with the vast majority at an early life-history stage
Summary
Trophic interactions play an essential organisational role in community and ecosystem ecology [1]. Biological communities are comprised of numerous species interacting through complex relationships, yet coexisting in equilibrium [2,3]. Of the many kinds of organisms comprising food webs, top predators are often the most vulnerable to extinction, an aspect of various intrinsic biological traits, including lower population densities and slower reproductive rates [4]. Predatory top-down control is observed through trophic cascades, across multiple lower trophic levels, underscoring the importance of top predators in food webs and underlying community structure [10,12]. Estuarine planktonic research has focussed largely on the physico-chemical drivers of community assemblages leaving a paucity of information on important biological interactions
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