Trophic Level Stability-Inducing Effects of Predaceous Early Juvenile Fish in an Estuarine Mesocosm Study

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.

Fish predation regimes modify benthic diatom community structures: Experimental evidence from an in situ mesocosm study

“…Three experimental treatments, of three replicate mesocosm enclosures each were deployed in the main channel in the middle reach of the estuary (channel depth ≈ 1.8 m) during a closed mouth period. Each enclosure was constructed from a translucent 200 μm thick, virgin polyethylene bag and fitted to an 80 × 80 cm frame, with each corner secured to a 5 L buoy, elevating the open top of the bag from the water's surface (Wasserman et al 2013). Each 1.4 m deep mesocosm was completely sealed from the surrounding waters and moored to a concrete block, anchored in the estuarine sediment.…”

“…Mesocosms were filled with 1000 L of water, on site, by transferring water from the estuary into each mesocosm using 10 L buckets. As transferred water was presumed to have natural densities of plankton, all water was strained through a 1 mm sieve during transfer to exclude large predatory zooplankters such as mysids and early-life history stage fish (Hansen et al 1994;Wasserman et al 2013). Treatment 1 was stocked only with the natural densities of plankton that were present in all three treatments, as verified by Wasserman et al (2013).…”

“…As transferred water was presumed to have natural densities of plankton, all water was strained through a 1 mm sieve during transfer to exclude large predatory zooplankters such as mysids and early-life history stage fish (Hansen et al 1994;Wasserman et al 2013). Treatment 1 was stocked only with the natural densities of plankton that were present in all three treatments, as verified by Wasserman et al (2013). Two different zooplanktivorous fish species were, however, stocked into treatments 2 and 3, respectively, at natural early-life history fish densities.…”

The Ecology and Food Web Dynamics of South African Intermittently Open Estuaries

“…Changes in the zooplankton community structure within IOCEs have been linked to amongst others, mouth phase, freshwater inflow, seasonality and over-wash events [14,32]. More recent studies indicate that predation by early life history fish may also play an important role in structuring the zooplankton communities within these systems during the close phase [33,34]. Breaching events are typically associated with a reduction in the abundance and biomass of the zooplankton as estuarine rich waters are exported to the marine environment [14,20].…”

“…Recent studies have demonstrated that the predation impact of juvenile fish, through trophic cascades, contributed to plankton food web stability within IOCE [33]. In the absence of the predators, total water column chlorophyll-a concentrations were reduced reflecting increased grazing activity by smaller heterotrophs [33,44].…”