Seasonal Variation in Nitrate:Phosphate Ratios in the English Channel 1923–1987

Nutrient concentrations measured at station E1 in the English Channel from 1930 to 1987 show a wide range in nitrate:phosphate ratio. A large number of samples, the majority of which were taken in the summer months, have low nitrate:phosphate ratios. Detailed analysis of the time series revealed that, in a significant number of years, mid-summer values of phosphate increased for short periods of time whilst nitrate concentrations remained low. These transient increases in phosphate concentration occurred both in the surface mixed water and in water below the seasonal thermocline; equivalent transients in nitrate concentration were less common. The changes in nitrate:phosphate ratio are discussed in relation to phytoplankton assimilation and nutrient regeneration.