J.L. Birrien

Nutrients and primary production in permanently well-mixed temperate coastal waters

Abstract Seasonal changes of nutrients and primary production were studied in Morlaix Bay (western English Channel) waters in 1979–1980. This region is characterized by strong tides and consequently the water column remains permanently well-mixed. Owing to this unique hydrological regime, primary production and nutrients follow seasonal cycles different from the classic temperate water cycles. Because of the instability of the water mass, phytoplankton development in spring is slow and attains a maximum only in summer. A minor autumnal primary production peak is absent. The slow development of phytoplankton results in a slow and prolonged utilization of nitrate and phosphate which attain their lowest values only at the end of summer. Predominance of diatoms in the spring phytoplankton results in a rapid utilization of silicon which reaches its lowest concentration as early as May. Ammonium shows a unique cycle in that it accumulates in the water column in spring-summer and is utilized by phytoplankton only when nitrate concentration is low. Nitrite changes show two peaks: a major peak in autumn associated with nitrogen remineralization and a minor peak in spring due to nitrite excretion by phytoplankton. Seasonal development of primary production in these waters is successively limited by silicon, nitrate and total inorganic nitrogen depletion in spring-summer before being eventually limited by light. This is substantiated by the seasonal changes of absolute 14 C assimilation values, solar energy utilization efficiency and assimilation number. Based on these results, it is hypothesized that the ratio between the depth of the euphotic zone and the depth of the water column will indicate whether light or nutrients limit primary productivity in well-mixed temperate waters.

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

Concentrations of NO3−, NO2−, NH4+, PO43−, 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 NO3−, NH4+, PO43−, NO2− 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 NH4+, 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 NO3 and NH4+ 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.