P. Le Corre

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.

ƒ-Ratios calculated with and without urea uptake in nitrogen uptake by phytoplankton

Abstract Examination of data from literature showed that non-inclusion of urea uptake in nitrogen uptake by phytoplankton results in statistically significant overestimates off. The extent to which f was overestimated because of non-inclusion of urea uptake depended upon the ecosystem type and was 6, 17, 24, 42 and 55%,, respectively, in upwelling, coastal, polar, oceanic and estuarine waters. Systematic urea uptake measurements thus become necessary, but where such data are lacking, corrections are possible with the ratio between the f values calculated with and without urea uptake for each ecosystem.

Estimation of nitrate flux in a tidal front from satellite‐derived temperature data

Regressional relations between sea surface temperature and NOAA satellite advanced very high resolution radiometer radiometric data, and between in situ temperature and nitrate concentrations in the surface and subsurface waters of the Ushant tidal front were significant. Nitrate images constructed from satellite-derived temperature data agreed well with sea truth data. Nitrate transport into the euphotic zone in a spring-neap tidal cycle, as deduced from satellite images, agreed with nitrate uptake rate and phytoplankton productivity. It is suggested that satellite-derived nitrate images can be useful tools for assessing nitrate-based productivity over large geographical areas in short time intervals.

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.

Assimilation Aand Regeneration of Nutrients off the West Coast of Brittany

A high degree of variation in hydrographic conditions is found in the so-called Iroise Sea, within less than 100 km of the west coast of Brittany. Tidal current maximal velocity, especially, ranges there from about 0·5 knot to more than 8 knots (locally, near the island of Ushant), i.e. practically as wide a range as found over the whole of north-west European shelf seas. Pelagic ecosystems accordingly exhibit a high degree of variety, related not only to classical inshore-offshore gradients, but also to the extent of vertical mixing or stratification. Areas where different physical and biological conditions prevail are generally separated by rather clearcut boundaries. The better-known of these is the Ushant thermal front, which runs in summer across the whole entrance to the English Channel, but also extends into the Iroise. In addition, freshwater runoff results in thermohaline stratification, or at least in the existence of thermohaline vertical gradients, in the two major bays of the west coast of Brittany. The relevant area is limited seawards by a thermohaline front, the Iroise inner front (Grail & Le Fevre, 1967; Le Fevre & Grall, 1970), beyond which are found the well-mixed waters inshore of the Ushant front. Fig. 1 sums up these hydrographic patterns in the area taken here into consideration.

Nitrification rates, nitrite and nitrate distribution in the Almeria-Oran frontal systems (eastern Alboran Sea)

In the western Mediterranean Sea, the Almeria-Oran frontal zone constitutes the eastern boundary of the Alboran Sea. During the “Almofront-1” cruise (April–May 1991) physical, chemical and biological features of the front and surrounding Atlantic and Mediterranean Waters were investigated. In the present study, the nitrite and nitrate concentrations and nitrification rates were measured. The nitracline was close to the surface in the north, close to the Spanish coast and in the south on the left side of the jet. The highest concentration of nitrite (0.25 μM) was located in the incoming Atlantic Waters. The ammonium oxidation rates were from 0 to 16 nmol 1−1 d−1, except in Atlantic Waters in which the rates reached 28–42 nmol 1−1 d−1. For all types of superficial water, i.e. Atlantic, jet core, Mediterranean, the nitrite produced by nitrification could account for the in situ concentrations in less time than the ages of the water masses. In contrast, to reach the ambient concentration, the production of nitrate would need more time than the lifetime of the water masses, except for the Atlantic Waters. In these Atlantic Waters, showing an increased rate of nitrate production, the in situ concentration of nitrate would be reached within its lifetime. The contribution of the nitrate produced by nitrification to the nitrate uptake by primary producers was estimated to range from 7% (Mediterranean Waters) to 40% (Atlantic Waters).

Vertical distribution of cadmium, copper and lead in the eastern Alboran Sea: enrichment of metals in surface waters

The vertical distribution of cadmium, copper and lead was studied in the 0–500 m layer of the Eastern Alboran Sea. The concentrations measured compare favourably with recent data for the neighbouring area (53 pM < Cd < 124 pM; 1.4 nM< Cu < 2.8 nM, and 90 pM < Pb < 310 pM). The analysis of six profiles showed the presence of a sub-surface water mass, the northwestern Mediterranean Water (NWMW, 75-30 m), with high concentration of trace metals. Metal-salinity relationships, typified by cadmium, showed that the Atlantic surface waters (ASW) with low metal concentrations before entering the Mediterranean Sea become enriched with metals from some as yet not clearly understood source near the Straits of Gibraltar. The salinity-Cd relationship, taken together with flux rates of salt and Cd, however, shows that the Modified Atlantic water (MAW) gains its high metal concentrations during its transit in the Alboran Sea principally from a mixing with the underlying metal-rich NWMW.