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Abstract. I investigated the effect of CO2-enrichment on productivity of two aquatic plant species [Zostera marina L., Nereocystis luetkeana (Mert.) P. & R.] that form significant components of coastal ecosystems in the Pacific Northwest. Short-term (i.e., 2-hr) experiments showed that doubling CO2 resulted in up to a 2.5-fold increase in Zostera net apparent productivity (NAP). Nereocystis NAP was increased 2.2 – 2.8 fold. In experiments involving seven enrichment treatments, NAP increased with increasing CO2 between ambient (1.0×) and 2.5× CO2 in both Zostera and Nereocystis. Nereocystis and Zostera NAP was lowest at highest (i.e., 5×) CO2 concentrations. In growth experiments, mean growth rate of Zostera increased with increasing CO2 during one of the two trials. I conclude that increasing CO2 in the surface waters of the coastal ocean would predictably result in increased NAP of these two species. These results supplement limited published data showing that shallow estuarine and marine systems are vulnerable to increased carbon dioxide.