ALGAL PERIPHYTON GROWTH ON NUTRIENT-DIFFUSING SUBSTRATES - AN INSITU BIOASSAY

Differences in nutrient limitation for dominant species within an algal periphyton community were determined using additions of N and P supplied by nutrient-diffusing artificial substrates. Sealed clay flowerpots were filled with 2% agar and 1 of 9 nutrient treatments (all combinations of K2HPO4 at 0.0, 0.05 and 0.5 mol/l with NaNO3 at 0.0, 0.05, and 0.5 mol/l. The pots were submerged at 0.5 m depth in Douglas Lake, Michigan [USA], and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 days the pots were scraped and analyzed for attached algae. Total algal biomass as chlorophyll a on the pots ranged from 0.17 .+-. 0.02 (SE) .mu.g/cm2 for pots without added nutrients to 15.7 .+-. 2.0 .mu.g/cm2 for pots with K2 HPO4 at 0.05 mol/l and NaNO3 at 0.5 mol/l. Chlorophyll a on pots containing just P (0.05, 0.5 mol/l) increased 6- to 10-fold over controls. The diatoms Epithemia adnata and Rhopalodia gibba and the blue-green alga Anabaena increased significantly on the P-only pots; these species are suspected of N2-fixing capability. Chlorophyll a on pots containing just N (0.05, 0.5 mol/l) increased 1.5- to 2-fold, though this increase was nonsignificant; Achnanthes minutissima, Gomphonema tenellum and Cocconeis placentula showed enhanced growth on these pots. Combinations of N and P caused heavy growths of the filamentous alga Stigeoclonium tenue. Naviculoid diatoms were also most abundant on the N + P pots. Average nutrient levels in Douglas Lake during the study were: NH3, 2.02 .mu.mol/l; NO3, 0.44 .mu.mol/l; and PO4, 0.06 .mu.mol/l. The low ambient concentrations of both N and P, together with results of the periphyton bioassay, indicate that the 2 nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.