Littoral periphyton responses to nitrogen and phosphorus: an experimental study in a subtropical lake

portions of the natural littoral community in a subtropical lake were enclo sed in mesocosms and subjected to high rates of nitrogen (N) and/or phospho rus (P) loading over a 28-day period. Changes in periphyton structure and f unction were compared with untreated controls to evaluate limiting resource effects. Mesocosms that received only P displayed minor increases in whole -community metabolism, measured by in situ oxygen evolution during mid-day hours, and small increases in the chlorophyll a (CHLA) content of surface p eriphyton mats. Mesocosms that received only N displayed significant increa ses in the CHLA content of epiphyton growing on submerged stems of Eleochar is. The most striking treatment effects were observed when both P and N wer e added. In that case, there were highly significant increases in the CHLA content of surface periphyton mats, epiphyton associated with submerged Utr icularia, and periphyton growing on artificial substrata. There also were i ncreases in the rates of phytoplankton carbon uptake and whole-community me tabolism. These results indicate that periphyton and phytoplankton were co- limited by P and N, in contrast to previous findings of strong P limitation in more pristine regions of this marsh, and in the nearby Florida Everglad es. The community responses in the present study may reflect early eutrophi cation effects, because the study site was located in close proximity to th e lake's eutrophic pelagic region. (C) 1999 Elsevier Science B.V. All right s reserved.