TYPE OF SUSPENDED CLAY INFLUENCES ZOOPLANKTON RESPONSE TO PHOSPHORUS LOADING

The effects on zooplankton community structure of P loading and two different kinds of suspended sediments were tested in a small lake. Limnocorrals were used in a complete, triplicated, six-treatment-block design. Treatments were loading with P, kaolinitic clay (K), K + P, montmorillonitic clay (M), and M + P plus a control without additions. P fertilization caused blooms of blue-green bacteria and resulted in reductions of the dominant copepod (Diaptomus mississippiensis) and cladocerans (Diaphanosoma brachyurum, Bosmina longirostris, and Ceriodaphnia reticulata), while causing a 5-fold increase in rotifers (mostly Keratella cochlearis). Zooplankton in the K and K + P treatments were similar to the control, but cladoceran species other than Diaphanosoma were reduced. M was more detrimental to zooplankton than K, causing a 10-fold decrease for all crustaceans. We attribute this in part to lower algal densities in the M treatment. Simultaneous P loading mitigated the effects of M on Diaphanosoma, rotifers, and copepods and may involve a transfer loop in which dissolved organic C adsorbed on clay becomes available to filter-feeding zooplankton. The effects of mineral turbidity apparently cascaded up the food chain, with both kinds of clays reducing densities of Chaoborus larvae.