The effects of shading on dioecious, pistillate Hydrilla verticillata (L.f.) Royle transplants from Lake Okeechobee, Florida, U.S.A.

Abstract

The effect of shading on morphometric and meristic characteristics of dioecious, pistillate Hydrilla verticillata (L.f.) Royle was investigated in a large outdoor tank using water, sediment, and plants from Lake Okeechobee, Florida. These data were needed for a hydrologically integrated submersed aquatic vegetation model being developed to assess the effects of restoration activities on Lake Okeechobee. Plants were grown in peat sediment and lake water, under ambient temperature (27.0-30.8 °C) and photoperiod (13 L: 11D to 14 L: 10 D). Treatments were established by differentially shading plants with varying numbers of layers of fiberglass window screen. Photon flux density (PFD) measured 0.27 m above the sediment surface ranged from 8 to 154 μmole photons m -2 s -1 , or 1.1 to 21.6 % of average incident photosynthetically active solar radiation (PAR), based on percent transmittance in the tank and averaged continuous daytime measurements from a mid-lake PAR sensor. Plant characteristics examined included ash-free dry mass (AFDM), leaf, node, shoot, and branch numbers, leaf area, cumulative shoot length, and the number of leaves on the upper third of all shoots; all of which decreased linearly with decreasing PAR, and had statistically significant treatment effects. No statistically significant treatment effects were found for maximum shoot and mean and maximum internodal lengths, or when the number of leaves or branches on the upper third of all shoots were expressed as a percentage of their totals, indicating that significant treatment-induced internodal elongation and canopy formation had not occurred. Mean absolute and relative growth rates ofH. verticillata were estimated to be 11.6 ± 1.9 mg AFDM plant -1 d -1 (mean ± SE) and 0.04 ± 0.003 d -1 , respectively. Estimated photosynthetic PFDs for no net growth (NNG) was negative, with an upper 95 % confidence limit of 13 μmole photons m -2 s -1 , or 1.8 % of mean incident PAR for absolute, and 7 μmole photons m -2 s -1 , or 0.9 % of mean incident PAR for the mean relative growth rate. These results demonstrate that H. verticillata has a high growth rate and can grow in very low light, which may impart an important adaptive advantage given the poor light climate typical of much of Lake Okeechobee and many other disturbed, culturally hypereutrophic waterbodies.