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Florida Lake Management Society Annual Conference, Naples, Florida, June 4 – 7, 2007

THE EFFECT OF SHADING ON PISTILLATE HYDRILLA VERTICILLATA (L. F.) ROYLE TRANSPLANTS FROM LAKE OKEECHOBEE, FLORIDA, USA

H. J. Grimshaw South Florida Water Management District West Palm Beach, FL

Introduction

The effect of shading on morphometric and meristic characteristics of pistillate Hydrilla verticillata (L. f.) Royle was investigated in a large outdoor tank using water, sediment, and plants from Lake Okeechobee, Florida.

Methods

Plants were grown in peat sediment and lake water, under ambient temperature (27 – 31 oC) and photoperiod (13L : 11D). Treatments were established by differentially shading plants with varying numbers of layers of fiberglass window screen. Photon flux density (PFD) ranged from 8 to 154 mmole 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.

Results and Discussion

Plant characteristics examined included ash-free dry mass, total leaf, internode, shoot, and branch numbers, leaf area, and cumulative shoot length; all of which decreased linearly with decreasing PAR, and had statistically significant treatment effects. No statistically significant treatment effects were found, however, when leaf and branch numbers on the upper third of each shoot were expressed as a percentage of their totals, indicating canopy formation had not yet occurred.

The apparent photosynthetic PFD for no net growth of H. verticillata, measured approximately a quarter meter above the sediment surface, approximated zero, with an upper 95% confidence limit of 13 mmole photons m-2 s-1, or 1.8% of mean incident PAR. These results suggest that H. verticillata can grow in very low light, which likely is an important adaptation given the poor light climate typical of this and many other culturally eutrophic water bodies.

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NOTES

Session 7A – Page 4

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