In eight hard-bottom tidal littoral mesocosms oxygen concentrations and temperature were measured every 30 s and registered as 15 min-averages. The mesocosms were fed with water from the Oslofjord (residence time about 2 h) and the measurements were also performed in the inflow. In addition, salinity in one mesocosm, light intensity in air and atmospheric pressure were registered. Based on the diurnal variation of the difference in oxygen concentration in the mesocosms and in the inflow, gross primary production and respiration were estimated. In the inflow of six of the mesocosms, nitrogen and phosphorus were dosed in concentrations from 1 to 32 ~S^~M N with a N/P ratio of 16. Two mesocosms served as controls. The data of the first year of the experiment, which are presented and discussed here, did not show a clear increase of the gross primary production, in dependence of the nutrient addition. However, for the months April to June 1999 there was a slight indication for an increase of gross production in relation to the nutrient dosing. The role of the background nutrient concentrations is discussed and cannot be completely responsible for the lack of response.