Abstract A method for estimating the annual mean mass exchange of dissolved solids (as measured by conductivity) and total phosphorus between coastal and offshore waters is discussed. This estimate is evolved from concentration contours, water volumes and loadings using the assumptions that ground water exchange and atmospheric loadings are negligible compared to other loadings and evaporation and precipitation are approximately equal. Wind effects are averaged out by measuring concentrations when prevailing winds are parallel to the shore in one direction and when the winds are in the opposite direction then averaging the results. Temperature and seasonal effects are averaged by measuring concentrations in late spring, midsummer and early fall, then averaging the results. The annual mean mass exchange for the nearly conservative substance (dissolved solids) is evaluated from the continuity equation using loading figures, and the concentration contours for conductivity. A coastal residence time is then estimated for conductivity. The coastal residence time is defined as the volume of the substance contained in the coastal region divided by the mass exchanges for steady state. Then a similar technique is used for total phosphorus with the mass lost to sedimentation determined by assuming the coastal residence times for the dissolved solids and total phosphorus are similar. These estimates are gross but compatible with presently available loading and survey data. From the residence times, total phosphorus loadings and details of the coastal region, it is possible to estimate the trophic state. The method was applied to surveys in the Thunder Bay region where the coastal region residence time was found to be 40 days and 40% of the discharged total phosphorus being lost to uptake sedimentation. The trophic state at the shallow nearshore region was found to be more eutrophic than the deeper, farther offshore regions. The same method applied to surveys in the Midland-Penetang region of Georgian Bay estimated the coastal residence time at 52 days, however, these surveys were not designed to provide a mean annual value.