Phytoplankton responses to reduction and elimination of submerged vegetation by herbicides and grass carp in four Florida lakes

Four central Florida lakes were monitored for 3 years to determine the effects of macrophyte reduction and elimination by grass carp and herbicide on phytoplankton populations. Clear Lake, in which grass carp were stocked after a year of baseline studies, had all macrophytes eliminated within 14 months. The density of phytoplankton increased significantly from an initial 24-month mean of 165 000 cells 1^?1 to a mean level of 787 900 cells 1^?1 in the third year. In Little Lake Fairview, stocked with grass carp in both the first and third years of study, vegetation was not eliminated until the 34th month. Phytoplankton density increased significantly from a 24-month mean of 64 200 cells 1^?1 to a mean of 370 200 cells 1^?1 in the third year. Lake Orienta, stocked with grass carp in the first year, had all vegetation eliminated within 6 months. Phytoplankton abundance did not change significantly over the course of the study (mean: 2 700 000 cells 1^?1). Lake Mann was treated with herbicide alone after an initial year of baseline data collection. Submerged vegetation was reduced in the second year but increased to near initial biomass levels in Year 3, with Nitella sp. replacing Hydrilla verticillata (L.f.) Royle as the dominant macrophyte. No significant yearly changes were noted in density of phytoplankton in this lake. Lake Orienta was the only lake in which the number of phytoplankton species collected differed significantly between years. Cyanophytes (notably species of Anacystis, Microcystis and Anabaena) and the diatom, Fragilaria sp., predominated in all lakes as macrophytes were removed. Most changes observed were in the direction of apparent increased trophic state. Major shifts in the phytoplankton assemblage (e.g., Shannon diversity indices and abundance) coincided with periods of maximal fluctuation in vegetation. Reversals of these tendencies were observed during prolonged stable periods of both high and low macrophute biomass.