Phosphorus transfer from sediments by Myriophyllum spicaturn l

The uptake of phosphorus, the biomass, and the standing P stock were measured over the course of a year in roots and shoots of the Eurasian watermilfoil, Myriophyllum spicatum, from Lake Wingra, Wisconsin. The resulting data were used to estimate the relative contributions of root and shoot uptake to the phosphorus economy of the plant and to examine the role of the plant in moving phosphorus between sediment and water. The total yearly uptake of P by a square meter of Myriophyllum was 3.0 g P m-*. Root uptake accounted for 2.2 g, shoot uptake only 0.8 g. The rate of P release from healthy shoots was insignificant, but about 2.8 g P m-* yr-l was lost due to shoot turnover. Since most of the P uptake is by the roots and much of the plant P is transferred to and lost by the shoots, Myriophyllum is a potentially important vector in the movement ofP from the sediments to the water. The net transfer of P from the sediments to shoots of Myriophyllum in Lake Wingra is about 2.0 g P m-* yr-l. Release of this P during decay makes Myriophyllum an important source of P for pelagic phytoplankton and can explain much of the previously reported P export from the littoral zone of Lake Wingra. Studies of nutrient uptake have shown that roots of submersed aquatic plants are capable of P uptake (Bristow and Whitcombe 197 1; DeMarte and Hartman 1974; Best and Mantai 1978) and that P uptake by roots exceeds that by shoots in nature (Bark0 and Smart 1980; Best and Mantai 1978; Carignan and Kalff 1980), primarily because of the much higher concentrations of SRP in sediment porewaters (Carignan 1982). The predominance of root P uptake makes it unlikely that submersed macrophytes compete with phytoplankton for P, as suggested by Fitzgerald (1969), or act as a filter for allochthonous inputs, as suggested by Wetzel and Allen (1972). Instead, submersed plants represent a potential pathway for the movement of P from the sediments to the water (McRoy et al. 1972), and littoral vegetation may function as a source of P for pelagic phytoplankton (Hutchinson and Bowen 1950; Cooke et al. 1977; Prentki et al. 1979). Despite the evidence for the predominance of root P uptake in many freshwater macrophytes and the evidence of P flux from I This work was supported by NSF grant DEB 7519777 to M. S. Adams. C. Smith was also supported by the NSF Graduate Fellowship Program. * Present address: Dep. Plant Pathol., Univ. Wisconsin-Madison, Madison 5 3 706. vegetated littoral zones, there are few measurements of the magnitude of annual P transfer from the sediments by macrophytes. In contrast with eelgrass, freshwater macrophytes do not appear to leak significant quantities of P from intact, healthy shoots (Carignan and Kalff 1982), so that P release must depend on decay (Bark0 and Smart 1980). Phosphorus release during the decay of submersed macrophytes has been studied several times (Jewel1 197 1; Nichols and Keeney 1973; Carpenter 1980a), yet the measurement of P flux due to decay remains problematic because of the difficulty of measuring the amount of P lost by decaying shoots. The difficulty arises because shoot turnover in some species may be essentially continuous (Carpenter 1980b) and because lost shoot portions are difficult to collect. The Eurasian watermilfoil, Myriophyllum spicatum L., is a particularly interesting freshwater macrophyte. In North America it is an exotic nuisance species that displaces native plant species from lakes and forms dense, nearly monospecific growths (see Grace and Wetzel 1978). In many North American lakes, Myriophyllum rose to a high level of dominance, maintained this dominance for lo-20 years, and then declined (Carpenter 1980~). During the period of abundance, Myriophyllum sometimes reached very high levels of biomass and pro-