Suspended Clays Alter Trophic Interactions in the Plankton
This paper explores how suspended clays alter the trophic interactions that govern composition and density of the zooplankton community in a small eutrophic North Carolina (USA) lake. Enclosures (12 m^3) were used in a complete, triplicated, cross-classified design. The four treatments were; BASS (1 piscivorous fish/enclosure), BASS + CLAY, SUNFISH (16 planktivorous fish/enclosure), SUNFISH + CLAY. The +CLAY treatments received 100 g@?m^-^2@?d^-^1 of montmorillonitic clay. It as anticipated that turbidity from clay would release crustacean zooplankton from visual planktivores. Contrary to this expectation, crustacean zooplankton populations in the SUNFISH + CLAY were fourfold less than in the SUNFISH treatment. The reduction was not linked to deleterious effects of clay, since zooplankton populations were higher in the BASS + CLAY than the BASS treatment. Instead, crustacean zooplankton appeared to be governed primarily by Chaoborus predation. Suspended clay weakened the link between visually predaceous fish and Chaoborus, which in turn strengthened the effect of Chaoborus predation on crustacean zooplankton. Since fish selectively cropped larger instars of Chaoborus, rotifers (primarily Keratella cochlearis), which are mostly eaten by smaller instars of Chaoborus, were unaffected by clay.
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