In general, the nutrient most limiting to phytoplankton growth is phosphorus in freshwaters and nitrogen in the sea. We investigated the effect of N and P enrichment to waters from seven brackish, coastal ponds ranging in salinity from 0 to 31?ppt and to nearby coastal seawater (32?ppt). Phytoplankton from the least saline ponds (0–6.5?ppt) responded to P additions, and biomass increased 2–6 times over unenriched controls. Nitrogen additions stimulated phytoplankton growth only in waters from the most saline coastal pond (31?ppt) and seawater. Phytoplankton from ponds of intermediate salinity did not respond when either N or P was added alone but, as in all ponds, increased greatly in biomass when N and P were added in combination. The high salinity at which a switch from P to N limitation occurred could be due to the high N:P loading ratios to these ponds.