Transport of carbon, nitrogen and phosphorus in a Brittany river, France

Abstract

Concentrations of NO 3 −, NO 2 −, NH 4 +, PO 4 3−, Si(OH) 4 −, DOC, DON, DOP, POC, PON, POP, chlorophyl a, phaeopigments, carbohydrates and proteins were measured for one year in the Morlaix river waters (Brittany coast, France). By comparison with the averages known for the unpolluted rivers of the world, concentrations of NO 3 −, NH 4 +, PO 4 3−, NO 2 − and DON are found to be much higher, and those of nitrate and ammonium exceed even those known for highly polluted rivers. The selective pollution by N compounds is caused by an excessive use of fertilizers in the hinterland although with NH 4 +, however, urban pollution can also be an important factor. Allochthonous sources supply a major fraction of particulate organic compounds to the river waters. Dissolved fractions of N and P in river waters are very large compared to the particulate fractions, whereas carbon is more or less equally distributed between DOC and POC. Transport of these nutrients to the estuary show three different patterns in relation to river discharge changes. Areal loading of the estuary with NO 3 and NH 4 + is greater than that known for other temperate estuaries. The deleterious effects of N pollution are, however, offset by the low freshwater flow, high tidal prism volume, estuarine basin geomorphology and uptake by benthic microalgae.