Our paper presents daily fluctuations in the concentration of oxygen in two small moderately eutrophied lakes in the Bytowskie Lake District (northern Poland), measured with the aid of a manually operated oxygen and temperature probe. Based on the results of these measurements, gross primary production, respiration, and net production of the ecosystem were calculated. It was observed that rhythmic changes in oxygen concentrations occurred in the epilimnion and – under favorable conditions – also in the upper part of the metalimnion. However, changes in the concentration of oxygen due to biological processes in the metalimnion were compounded by fluctuations caused by internal seiches, especially in shallower sites closer to the lake shores. In May and July the reach of the production layer coincided with the isopleth of 100% saturation with oxygen and, apart from the epilimnion, also encompassed the upper part of the metalimnion. In May, when the thermocline was particularly shallow, much of the primary production and respiration took place in the metalimnion.