SUMMARY.1. The effects of nutrient addition upon algal periphyton standing crop and benthic community metabolism were investigated in Lake Lacawac, Pennsylvania. Nutrient‐diffusing flower pot substrates were filled with combinations of P (0.015 mol Na2HPO4), N (0.15molNaNO3) and carbon as glucose (0.015mol (LG) or 0.15mol (HG)) or 0.58 mol NaHCO3(C). Fifty‐two pots representing ten treatments were placed in the lake on 5 May 1985 and sampled after 32‐38 days.2. Maximum chlorophyll‐a values of 23.82μgcm−2and 29.72μgcm−2were obtained on NPC and NPHG pots respectively, compared to 0.82μgcm−2on control pots. Algal growth was not enhanced unless N and either organic or inorganic carbon were supplied. Ash Free Dry Mass was especially high on NPHG pots.3. Gross production, net production and community respiration were assessed with light‐dark chambers affixed to the pots using microwinkler measurements of dissolved oxygen. Highest production rates were obtained on NC and NPC pots. Pots with NPLG and NPHG also showed significant increases in gross production compared to control levels (P<0.01). Specific production (=gross production/chlorophyll‐a) was significantly related to standing crop as chlorophylls (P<0.001), alkalinity in the chambers (P<0.00l) and light (P<0. 05), but not to N or P supplied by the pots.4. The periphyton community was dominated by Chlorophyta, which averaged 86.1% of total algal biovolume. Enrichment with NC and NPC significantly enhanced the growth of three Chlorophyceae(Stigeoclonium, Scenedesmusand an unidentified coccoid) and two Cryptophyceae(Cryptomonas. Rhodomonas)(allP<0.05). In contrast, the filamentous green algaMougeotiadominated other treatments, but declined significantly with NPC enrichment (P<0.05). Addition of NPHG stimulated an 8‐fold increase in total biovolume and significant increases in growth ofCryptomonas, Rhodomona