Inorganic nutrients in precipitation over the Magdalen Islands area (Quebec, Canada) and their impact on the primary productivity of the lagoons

Concentrations of nitrate, ammonium and phosphate have been monitored from June to October 1989 in rain water collected at the Magdalen Islands (Gulf of St. Lawrence, Québec, Canada). Nitrate was the main dissolved organic nitrogen (DIN) compound with concentrations ranging from 2.2 to 95 μM. Ammonium was occasionally dominant and varied between 0.7 and 41 μM. Phosphate concentrations were low and extremely variable with values ranging from 0.2 μM to 2 μM. All three inorganic nutrients were positively correlated and the relationships best described by a non-linear regression model. NH₄⁺:NO₃₋ atomic ratios fell within the range of those previously measured, i.e. the northeast part of North America, and suggest a continental origin for both DIN and phosphate. Measured pH values failed to show high levels of acidity (pH=4.8 ±0.4).For the lagoonal system of the Magdalen Islands, atmospheric deposition is the major source of nitrate during the summer period we surveyed. In such an ecosystem the atmospheric inputs of DIN are greater than those from the sediment and may at times contribute up to 70% of the phytoplankton primary production requirements. In contrast, phosphate of rain origin was only of marginal importance relative to sediment inputs.