Changes in algal assemblages along observed and experimental phosphorus gradients in a subtropical wetland, USA

1. We wanted to determine if changes in algae in the Everglades were due to increased phosphorus (P) loading. Epiphytic algae, water chemistry, and su rface sediment chemistry were characterized from 32 sloughs along a P gradi ent in the Everglades and changes in the algal assemblages along the P grad ient were compared with those along an experimental P gradient of in situ m esocosms. The sloughs are the wettest open water habitats characterized by floating and submerged aquatic plants in the Everglades. 2. Algal species composition was much more sensitive to P concentration tha n algal biomass. The diatom species variance among sloughs, captured by 1st ordination axis, was more highly correlated with total P (TP) in surface s ediments (r = -0.79), than soluble reactive P (SRP) (r = -0.08) and TP (r = -0.48) in the water column. Algal biomass (mu g chl a cm(-2)) was not sign ificantly correlated with P (SRP: r = 0.22, TP: r = 0.19, sediment TP: r = 0.07) along the P gradient in the Everglades. Cluster analysis classified d iatom species assemblages in 32 doughs into three groups (TWIN I, II, III), which corresponded to three zones along the P gradient. Dominant diatom sp ecies shifted from Mastogloia smithii (40.3%), Cymbella scotica (22.3%), an d Fragilaria synegrotesca (21.8%) in TWIN I to Nitzschia amphibia (22.4%) a nd C. microcephala (12.4%) in TWIN III. TP in surface sediments and TP in e piphyton assemblages increased 4- and 5-fold from TWIN I to TWIN III, respe ctively. 3. Patterns in epiphytic assemblages along the experimental P gradient in t he mesocosms were very similar to those along the Everglades P gradient. Sh annon diversity indices and species richness significantly increased along both P gradients. TN : TP ratio in epiphyton assemblages significantly decr eased as sediment TP increased along both P gradient. Ordination analysis s howed that diatom assemblages in the impacted zone (TWIN III) were ordinate d closely to the assemblages from the highest P treatments in the mesocosms . The assemblages from the less impacted zone (TWIN I) were ordinated close ly to the assemblages from controls in the mesocosms. 4. Concurrence between results of our survey and experiments suggest that c hanges in epiphytic assemblages along the P gradient in the Everglades are caused by increases in P concentrations.