Zooplankton populations were monitored monthly for a three year period in four Florida lakes. bach lake received an aquatic vegetation control program using grass carp (Ctenophar.~mgodon idella Val.) and herbicide, either alone or in combination. All aquatic vegetation was eliminated in the three lakes with grass carp; it was severely reduced and exhibited qualitative shifts in the lake treated with herbicide alone. Total number of recorded zooplankton species was reduced in each subsequent year as vegetation was eliminated, but mean density 1-I increased greatly. The overall trend in all lakes was toward increasing numbers of small filter-feeding herbivores, with a shift from copepod and copepod-cladoceran dominated to rotifer and small cladoceran dominated systems. It is suggested that these changes reflect primary plankton response to fluctuations in habitat, food base, and planktivore pressure resulting from gradual macrophyte alteration. Changes observed indicate apparent increases in eutrophy.
1. Four lakes in central Florida each received a different weed control treatment consisting of herbicide and/or different stocking ratios of grass carp {Clenopharyrigodo/i idella Val.)-2. Submersed vegetation was eliminated by grass carp within 6 months In Lake Orienta and within 14 months in Clear Lake. Because Little Lake Fairview. treated with herbicide and a smaller stocking ratio of grass carp, retained abundant vegetation levels over the first two study years, grass carp which had been introduced initially were replaced with a greater number of fish at the beginning of the third year. Submersed vegetation was subsequently eliminated in this lake within 10 tnonths. Aquatic vegetation was reduced but not eliminated in Lake Mann which was treated with herbicide only.3. Where macrophytes were removed there tended to be increased bottom dissolved oxygen, higher and more stable total alkalinity values, increased turbidity, lowered transparency,, higher specific conductivity and increased chlorophyll levels. Significant increases in pH were probably due lo low water levels resulting from a severe drought rather than weed control. Low water also affected specific conductivity. Annual means for nutrients did not differ significantly between years in the four lakes.
Knowledge gaps have been identified for legumes cultivation, especially in terms of weed control, while soybean production is still at its initial stages in Luxembourg. The project LeguTec tests different mechanical weed control methods, in organic soybean cultivation. The impact of weeding are observed from an ecological point of view in term of
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