Effects of Aeration and Temperature on Nutrient Regeneration from Selected Aquatic Macrophytes 1

A batch incubation study was conducted to investigate the effect of aeration and temperature on the N and P release by decomposing aquatic macrophytes in eutrophic lake water. Nutrient release by waterhyacinth (Eichhornia crassipes [Mart] Spires), pennywort (Hydrocotyle umbellata L.), hydrilla (Hydrilla verticillata L.), and cattails (Typha latifolia L.) was evaluated under aerobic conditions. Waterhyacinth plants tagged with lSN were used to determine the release at varying temperature levels and under anaerobic (oxygen free) and low-dissolved O2 (open system where the only O2 source was diffusion through the overlying water) conditions. Nutrient release was found to be rapid initially due to solubilization, followed by slow nutrient release as a result of microbial decomposition. Under aerobic conditions, NO3formation in the water was found to be significantly related to C/N ratio of the plants. About 48 to 76°70 of the plant N and 67 to 900/0 of the plant P were released at the end of 105 d of aerobic decomposition. After 94 d, about 86 and 88°70 of ~N was released from the plant tissue under anaerobic and low-dissolved O2 conditions, respectively. Nitrogen and phosphorus release were significantly increased with increase in incubation temperature. Additional Index Words: aquatic system, decomposition, anaerobic conditions, nitrogen, phosphorus, waterhyacinth, pennywort, hydrilla, cattails. Ogwada, R. A., K. R. Reddy, and D. A. Graetz. 1984. Effects of aeration and temperature on nutrient regeneration from selected aquatic macrophytes. J. Environ. Qual. 13:239-243. In an aquatic system, macrophyte die-off may occur as a result of plant maturity (older plant parts), freeze damage, or herbicide application. Some of the detritus produced falls beneath the plant canopy and accumulates at the sediment-water interface (Gosz et al., 1973; Godshalk &Wetzel, 1977). By both solubilization and microbial processes (Boyd, 1970; Hargrave, 1972; Hunter, 1976; Puriveth, 1980), the nutrients potentially become available to the adjacent living plants (Fenchel & Jorgensen, 1977; Jewell, 1971). Gaudet (1976) also observed that the retention and decomposition f aerial waterhyacinth (Eichhornia crassipes [Mart] Solms) tissues may serve as an efficient means of nutrient cycling from dead to living tissues. In Florida, about $20 million are spent annually to control aquatic weeds. In spite of the effort to control or eradicate aquatic plants, the problem still persists. This is thought o be partially due to rapid regrowth of new plants as a result of nutrient regeneration during the decomposition of detritus (Reddy & Sacco, 1981; DeBusk et al., 1983). Temperature and dissolved O2 content of the water are probably one of the most important factors in’ Florida Agric. Exp. Stn. Journal Series no. 4869. Received 24 June 1983. 2 Graduate Assistant, Associate Professor, Agric. Res. & Education Center, Inst. of Food & Agric. Sci., Univ. of Florida, Sanford, FL 32771; and Associate Professor, Soil Sci. Dep., IFAS, Univ. of Florida, Gainesville, FL 32611. fluencing the extent and rate of nutrient release from detritus plant tissue (Godshalk & Wetzel, 1978a, 1978b). However, very little is known on the effect of these factors on nutrient release from decomposing aquatic plants in aquatic systems. The objectives of this study were: (i) to evaluate the effect of aeration on the nature and extent of N and P release by selected aquatic plants, and (ii) to determine the effect of temperature on N and P release by decomposing waterhyacinth plant tissue under low-dissolved O2 and O,-free conditions. MATERIALS AND METHODS Nutrient Release Under Aerated Conditions A batch incubation study was conducted to investigate the effect of aerobic conditions on the nature and extent of N and P release by decomposing aquatic macrophytes. Aquatic plants evaluated were waterhyacinth, pennywort (Hydrocotyle umbellata L.), hydrilla (Hydrilla verticellata L.), and cattails (Typha latifolia L.), Each treatment was replicated three times. The plants and eutrophic lake water used in this study were collected from Lake Alice located on the University of Florida campus in Gainesville. The plants were uniformly chopped (= 1 cm), and fresh plant tissue of 3.1 g dry wt equivalence was transferred to triplicate I-L mason jars containing 750 mL of lake water. The jars were capped air-tight and equipped with an inlet and outlet. The inlet was used for continuously bubbling NH3-free air (30 mL/min) and the outlet led to a test tube containing 0.05M H~SO~ to trap NH~ volatilized during decomposition of the plant tissue. The jars were then placed in the dark in an incubation chamber maintained at a constant emperature of 28°C. After 0, 7, 14, 21, 35, 70, and 105 d of incubation, a 50-mL water sample was taken from each system. A portion of the water sample was filtered through 0.45-#m filter paper and analyzed for NH,÷-N, NO~--N, and ortho-P. Unfiltered water samples were analyzed for total Kjeldahl N (TKN) and total P (TP). The initial plant samples and the final residue were analyzed for TKN, TP, and total organic C. Effect of Temperature on Nutrient Release This experiment was conducted in an attempt o simulate the effect of seasonal temperature and dissolved O~ changes on nutrient release from decaying waterhyacinth. Waterhyacinth plants were cultured for 3 weeks in lake water enriched with 5.0 atom 070 ~N as NH,CI, and this has resulted in about 1.0 atom 07o ’~N in the plant tissue. Plants were washed thoroughly before using in the experiments to evaluate ~N release from waterhyacinth at three temperatures (10, 20, and 30° C) under completely anaerobic and low-dissolved O2 conditions. Plants were uniformly chopped and fresh weight of 9.5 g dry wt equivalence was added to 3-L bottles containing 2000 mL of eutrophic Lake Apopka water. Each treatment was replicated three times. The experiment was conducted in the dark in temperature-controlled growth chambers. To create low-dissolved O~ conditions, the bottles were left open to the air where the only O2 source was diffusion through the overlying water. Anaerobic onditions were created by sealing the bottles with air-tight caps containing septums for purging with N~ at the beginning and at the end of each sampling period. Nitrogen and phosphorus in the water were monitored at the end of 0, 7, 20, 42, 67, and 94 d. About 150 mL of the water was removed at each sampling. A portion of the water sample was filtered and analyzed for ortho-P. Unfiltered water samples were analyzed for NH,÷-N, NO3--N, TKN, and TP. At the end of the study, water and residue were passed through 0.45-zm filter paper and residue retained by the filter paper was analyzed for N and P. Initial plant samples and the final residue were analyzed for TKN and TP. All plant and water samples were also analyzed for ~3N. J. Environ. Qual., Vol. 13, no. 2, 1984 239 301 -~-o-Water hyacinth z 0~’~-:~ ’" ’ ’ ’ ’ -o-o-Cattail