Hydrobiologia 517: 15–24, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 15 Phosphorus accumulation in the littoral zone of a subtropical lake Karl E. Havens, Bruce Sharfstein, Andrew J. Rodusky & Therese L. East Ecosystem Restoration Department, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL 33406, U.S.A. E-mail: khavens@sfwmd.gov Received 11 June 2003; in revised form 10 September 2003; accepted 14 September 2003 Key words: phosphorus accumulation, littoral zone, wetlands, periphyton, subtropical Abstract In situ mesocosm experiments were performed under summer (1997) and winter (1999) conditions in the littoral zone of a subtropical lake in Florida, USA. The objective was to quantify phosphorus (P) accumulation by various components of the community after adding pulsed doses of dissolved inorganic P. A short-term experiment also was done to quantify the rate of P loss from the water column, with simultaneous use of an inert tracer to confirm that P depletion was not due to leakage of the tanks. In the experiments, added P was rapidly removed from the water; samples collected 3–4 days after adding spikes of near 100 μgl −1 P contained little or no soluble reactive P. In the short-term experiment, we documented that the half-life of added P was approximately 6–8 h in the water column, and that the tanks were not exchanging water with the surrounding lake. Little of the added P ended up in plankton, rooted vascular plants, or sediments. The main sink for P was periphyton, including surface algal mats, benthic algal mats and detritus, and epiphyton. In the summer 1997 experiment, the periphyton was intimately associated with a non-rooted plant (Utricularia), which also may have sequestered P from the water. Structure of the littoral community varied between summer and winter, and this influenced which periphyton component accounted for most of the P removal. In regard to P mass balances, we accounted for 54% of the added P in 1997, when coarse sampling was done. In 1999, when there was more detailed sampling of the community, 92% of the added P was located in various community components. Subtropical littoral periphyton can be a large sink for P, as long as depth and underwater irradiance conditions favor its growth. Introduction The littoral wetland, with its vascular plants, epiphytic algae, benthic algal mats, and heterotrophic compon- ents, can be an important sink for P that enters a lake ecosystem (Rigler, 1956; Wetzel, 1989). When these wetlands occur in the proximity of tributaries, they can sequester soluble P from the inflowing water, redu- cing the total load of P to the lake, or transforming P into forms that are not readily available for phyto- plankton accumulation (Wetzel, 1979; Heath, 1987). At the whole-lake scale, a littoral zone can assimilate a large portion of the total dissolved P from the wa- ter column (Howard-Williams, 1981), in some cases without substantive returns of P to the pelagic zone (Wetzel, 1996). Although this situation is well recog- nized, there exists only limited information regarding the fate of P that enters littoral wetland communities. In fact, only the studies of prairie wetlands in Canada (McDougal et al., 1997), some unpublished work with 32 P accumulation in the Florida Everglades (Davis, 1982) and preliminary calculations in the littoral zone of Lake Okeechobee (Havens et al., 1999a) have at- tempted complete P mass-balances during nutrient addition experiments in natural communities. In this paper we present the results of controlled in situ experiments, carried out specifically to provide this information for a subtropical system. Additions of dissolved inorganic P were made to enclosed nat- ural communities and the fate of the added nutrient was quantified by analysis of P in plankton, floating periphyton mats, epiphyton on plant stems, benthic