Nitrate:phosphate ratios and Emiliania huxleyi blooms

It has been hypothesized that phosphate limitation, classically indicated by NO3:PO4 ratios > 16, is one of the critical factors allowing the coccolithophorid Emiliania huxleyi to bloom. This hypothesis is based on physiological studies showing that E. huxleyi has an exceptionally high affinity for orthophosphate and is able to use organic phosphate. Indeed, E. huxleyi has been found to bloom at high NO3: PO4 ratios in some mesocosm studies and in the oceanic northeast North Atlantic. Recent E. huxleyi blooms on the southeastern Bering Sea shelf, however, occurred under low NO3: PO4 conditions, which is indicative of nitrogen rather than phosphorus stress. A review of field studies of blooms where nitrate and phosphate were measured indicates that NO3: PO4 was in fact frequently low. A survey of most of the areas of the world ocean where satellite-detected E. huxleyi blooms occur also shows that NO3:PO4 ratios are generally low. These observations suggest that E. huxleyi is able to exploit situations where either phosphorus or nitrogen is limiting to competing species. They also indicate that attention should be directed to examining organic nitrogen, organic phosphorus, and ammonium during E. huxleyi blooms to better understand the role macronutrients play in these blooms.