<p>There is renewed interest in land-based cultivation of green and red seaweeds for production of food ingredients and novel bioproducts under controlled conditions. A common practice is tank cultivation with continuous seawater exchange. A dynamic process model was developed to predict seaweed biomass productivity as a function of seawater exchange rate in land-based aquaculture systems, where dissolved inorganic carbon (DIC) and macronutrients for photosynthetic biomass production are supplied principally by the seawater inflow. The dynamic model predicted DIC concentration, limiting nutrient concentration, biomass density, areal biomass productivity, and pH vs time profiles during the light and dark phases of the photoperiod. The differential material balance equations posed by the model were coupled to a multiplicative Monod equation that quantified the effect of DIC and limiting macronutrient (nitrate) concentrations on specific growth rate. Model predictions were compared to biomass productivity vs. seawater exchange rate data for tank cultivation of green macroalgae (<i>Ulva</i> species) and red macroalgae (<i>Palmaria, Gracilaria</i> species) available in the literature. This model predicted the asymptotic effect of seawater exchange rate and biomass productivity. Furthermore, model predictions revealed that diminished biomass productivity at seawater exchange rates of less than 5 day⁻¹ are due to macronutrient limitations on growth, not DIC limitation, motivating the use of integrated multitrophic aquaculture systems to supply nitrate and phosphate to inlet flow seawater above ambient levels. Overall, the model can be used to determine the optimal seawater exchange rate for a desired biomass productivity and provide predictive capability for future engineering design and economic analysis.</p>