Periphyton nutrient limitation and other potential growth-controlling factors in Lake Okeechobee, USA

Periphyton nutrient limitation was assessed in Lake Okeechobee, a large, sh allow, eutrophic lake in the southeastern U.S.A. Nutrient assays were perfo rmed to determine if the same nutrients that limit phytoplankton also limit periphyton growth in the lake. Nutrient diffusing clay substrates containi ng agar spiked with nitrogen, phosphorus, or both, along with nutrient-free controls, were incubated at four sites in the lake. Three sites were locat ed in a pelagic-littoral interface (ecotone) and one site was located in th e interior littoral region. Incubations lasted for 20-26 days, and were rep eated on a quarterly basis between 1996 and 1997, to incorporate seasonal v ariability into the experimental design. The physical and chemical conditio ns at each site also were measured. Periphyton biomass (chlorophyll a and a sh-free dry mass) was highest at the littoral and northern ecotone sites. A t the littoral site, nitrogen limited biomass in four of five incubations, although the largest biomass differences between the treatments and control s (less than or equal to3 mug cm(-2) as chl) were probably not ecologically significant. Periphyton biomass at the western and southern ecotone sites was low compared to the other two sites. Increases in water column depth an d associated declines in light penetration strongly correlated with periphy ton growth and suggested that they may have limited growth most often at al l three ecotone sites. Nitrogen also was found to limit periphyton growth a pproximately 20% of the time at the ecotone sites and phosphorus was found to limit growth once at the west site.