Periphyton biomass was measured bimonthly at FCE LTER sites in Shark Slough. Fifteen 9.8 cm2 cores were removed from the floating periphyton mat and composited into a single sample. Percent aerial cover of floating periphyton was estimated visually in the field. Data are presented in terms of grams dry and ash-free dry mass per area. Total phosphorus, nitrogen and carbon concentration in the periphyton mat is also presented on a dry weight basis. This is part of a continuous data collection to test the hypothesis that phosphorus and nitrogen limit productivity at the upstream and downstream ends, respectively, of the FCE transects. Conclusions await longer-term data collection.