Effects of salinity and light on biomass and growth of Vallisneria americana from Lower St. Johns River,FL, USA

A mesocosm study was conducted to determine the effects of variable salinity and light on Vallisneria americana Michx. (wild celery) and associated algal community components in the lower St. Johns River, Florida. Fifteen centimeter diameter intact plant plugs were collected from the LSJR in March 2001 and transported to mesocosm facilities in Lafayette, Louisiana. A factorial experimental design was used consisting of three salinity levels (1, 8, and 18 ppt), three light levels (0, 50, and 90% shading), and three replicate mesocosms of each for a total of 27 mesocosms. The experiment consisted of a 4-week acclimation period followed by a 5-month treatment period. V. americana responded negatively to increased salinity. Although V. americana survived 8 ppt salinity, growth was limited. At 18 ppt, almost all V. americana aboveground biomass had perished within 10 weeks, but when salinity was lowered back to 1 ppt, approximately 20% of the aboveground biomass recovered within the following 10 weeks. At midtreatment harvest, light did not affect V. americana biomass directly (P = 0.8240), but by final harvest (20 weeks) light affected belowground biomass (P < 0.0014). Both salinity and light affected algal growth. Macroalgae dominated 1 ppt salinity treatments in ambient light, but phytoplankton dominated 8 and 18 ppt salinity treatments in ambient light. Algal communities were greatly inhibited by 90% shading. While salinity directly impacted V. americana growth and survival, light effects were less direct and involved algal community associations.