Algal Periphyton Growth on Nutrient‐Diffusing Substrates: An in situ Bioassay

Abstract

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 one of nine nutrient treatments (all combinations of K 2 HPO 4 at 0.0, 0.05, and 0.5 mol/L with NaNO 3 at 0.0, 0.05, and 0.5 mol/L). The pots were submerged at 0.5 m depth in Douglas Lake, Michigan, and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 d 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) mg/cm 2 for pots without added nutrients to 15.7 ± 2.0 mg/cm 2 for pots with K 2 HPO 4 at 0.05 mol/L and NaNO 3 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 N—fixing capability. Chlorophyll a on pots containing just N (0.05 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 growth 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: NH 3 , 2.02 μmol/L; NO 3 , 0.44 μmol/L; and PO 4 , 0.06 μmol/L. The low ambient concentrations of both N and P, together with results of the periphyton bioassay, indicate that the two nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.