A 14 d perlod of dense but stable phytoplankton stock in a brackish water earthen pond (0 2 ha area 0 7 m depth) was charactenzed to provide a baseline for study of instablllty Results ~llus-trate the potential of ponds to serve as microcosms of natural systems Pnmary production and community respiration were assessed by diel curve analysls of oxygen and inorganic carbon sampled every 30 min at 3 depths Neither stocks nor dlel oxygen regimes were destablllzed by 2 Isolated days of low light, the flrst accompanied by heavy rainfall Among nutrlent elements, only Inorganic nitrogen exhiblted marginally lim~tlng values Daytime net production (dNPP) of oxygen ranged from 0 to 0 26 m01 m-2 d-l , carbon uptake from 0 01 to 0 22 m01 m-'d. ' Nlghttime respiration (nR) approximately matched dNPP, resulting in low mean diel net production (NPP) Minimal estimates of daytime respiration (dR) were substantially greater than nR and dNPP, minimal gross production (GPP = dR + dNPP) averaged 2 5 times dNPP Est~mated dR vaned wlth dNPP In a stabilizing negative feedback, poss~bly mediated by photosynthet~c products Both dNPP and NPP vaned with diel irradiance but nR d~d not Both dNPP and nR decreased with depth, posltlve NPP was concentrated in the upper layer Stocks and oxygen cycles were more resistant to disturbance by low light than predicted by models assuming 1 0 m pond depth We suggest for further examination that stability was related to the shallow depth of thls pond, which permitted sufficient light penetration to the bottom layer for positlve dNPP on most dates