Possible factors limiting the growth of an aquatic weed, hydrilla verticillata (L.F.) Royle under tropical conditions

Abstract

The aim of this study was to help understand the possible factors lirniting the growth 'of an aquatic weed Hydrilla verticil/ata in the littoral waters of Lake Victoria. The study was confined to investigation of inorganic' carbon as source of nutrient and a possible biocontrol of the weed by fresh water snails, Lymnea natale.nsis and Bulinus spp found in the lake. A series of experiments were conducted for the purpose of evaluatiilg these factors. From the studies it was observed that Hydrilla is able to photosynthesize in a medium where HCO-) ions are predominant. The weed would thus be at an advantage where carbon dioxide i~ reduced or lacking as a result of local ecological effects. Hydrilla has a maximum growth in a mcdiu:il of 220 mg per litre (predominantly HCO-) iam) optimum total alkalinity. This would mean that Hydrilla can do weB in calcareous waters, an observation weB supported by the fact that the weed flourishes well in many hard water habitats. Consl1mption of Hydrilla by snails was 34.30/0 of the totul available biomac;s. These results show that while fresh water snails may be playing relatively little role in the control of Hydrilla, it is mo;.t likely that the low inorganic carbon source in Lake Victoria of total alkalinity 55.3 mg per litre ~C02 + HCO-]) may be contributing to the low growth of the aquatic weed. Further studies on nutrient require,ments by Hydrilla in relation to the nutrient status in Lake Victoria littoral zone may contribute profitably to the,Present conclusions.