Nutrients (organic C, P, N, Si) in the eutrophic River Loire (France) and its estuary

The Loire estuary has been surveyed from 1982 to 1985 by 13 isochronous longitudinal profiles realized at low tide. Nutrient (SiO2, NO3−, NH4+, PO3−4, particulate organic carbon or POC) patterns are very variable depending on the season, the estuarine section [river, upper-inner estuary, upstream of the fresh-water-saline-water interphase FSI, the lower-inner estuary characterized by the high turbidity zone (HTZ), the outer estuary] and the river discharge. Biological processes are dominant. In the eutrophied River Loire (summer pigment > 100 μg l−1), the high algal productivity (algal POC > 3 mg l−1) results in severe depletion of SiO2, PO43−, NO3−. The enormous biomass (55 000 ton algal POC/year) is degraded in the HTZ where bacterial activity is intense. As a result, there is generally a regeneration of dissolved SiO2 and PO43−, a marked NH4+ maximum, while NO3− is conservative or depleted when the HTZ is nearly anoxic. Other processes can be considered including pollution from fertilizer plans (PO43−, NH4+) and from a hydrothermal power plant (NH4+). In the less turbid outer estuary, nutrients are generally conservative. Major variations of concentrations are observed in the lowest chlorinity section (Cl− < 1 g kg−) and also upstream the FSI, defined here as a 100% increase in Cl−. Nutrient inputs to the ocean are not significantly modified for SiO2 and NO2−, but are increased by 70% and 180% for PO43− and NH4+ and depleted by 60% for POC. Odd hydrological events, especially some floods, may perturbate or even mask the usual seasonal pattern observed in profiles.

[1]  S. Wofsy,et al.  The chemical control of soluble phosphorus in the Amazon estuary , 1986 .

[2]  E. Woodward,et al.  Conservative behaviour of riverine dissolved organic carbon in the Severn Estuary: chemical and geochemical implications , 1983 .

[3]  R. Delmas,et al.  Evolution saisonnière des nutriments dans un écosystème eutrophe d'Europe occidentale (la rade de Brest). Interactions marines et terrestres , 1983 .

[4]  E. Rehm.  The distribution of phosphorus in the Weser River estuary , 1985 .

[5]  C. Lorenzen,et al.  DETERMINATION OF CHLOROPHYLL AND PHEO‐PIGMENTS: SPECTROPHOTOMETRIC EQUATIONS1 , 1967 .

[6]  F. C. Tan,et al.  Sources, sinks and distribution of organic carbon in the St. Lawrence Estuary, Canada , 1983 .

[7]  G. Cauwet.  Automatic determination of dissolved organic carbon in seawater in the sub-ppm range , 1984 .

[8]  M. Meybeck.  Carbon, nitrogen, and phosphorus transport by world rivers , 1982 .

[9]  D. M. Nelson,et al.  Chesapeake Bay nutrient and plankton dynamics: III. The annual cycle of dissolved silicon , 1983 .

[10]  A. J. Bennekom,et al.  The seasonal cycles of reactive silicate and suspended diatoms in the Dutch Wadden Sea , 1974 .

[11]  G. Cadée.  Tidal and seasonal variation in particulate and dissolved organic carbon in the western dutch Wadden Sea and Marsdiep tidal inlet , 1982 .

[12]  H. Etcheber,et al.  Characterization of the particulate organic matter in the Loire Estuary (France) using ETS activity measurements , 1986 .

[13]  A. W. Morris,et al.  Very low salinity regions of estuaries: important sites for chemical and biological reactions , 1978, Nature.

[14]  P. Froelich,et al.  Modeling estuarine nutrient geochemistry in a simple system , 1984 .

[15]  W. Broenkow,et al.  Processes controlling the dissolved silica distribution in San Francisco Bay , 1975 .

[16]  R. Wollast,et al.  Biogeochemical properties of an estuarine system: the River Scheldt , 1978 .

[17]  D. Hammond,et al.  Nutrient exchange across the sediment-water interface in the Potomac River estuary , 1982 .

[18]  R. Létolle,et al.  The application of isotopic and biogeochemical markers to the study of the biochemistry of organic matter in a macrotidal estuary, the Loire, France , 1988 .

[19]  M. Lucotte,et al.  Forms of phosphorus and phosphorus–iron relationships in the suspended matter of the St. Lawrence Estuary , 1983 .

[20]  R. Wollast,et al.  Study of the behavior of dissolved silica in the estuary of the Scheldt , 1971 .

[21]  J. Sharp,et al.  Optical Properties, Suspended Sediments, and Chemistry Associated with the Turbidity Maxima of the Delaware Estuary , 1983 .

[22]  A. W. Morris,et al.  Particle Dynamics Particulate Carbon And The Oxygen Minimum In An Estuary , 1982 .

[23]  M. Meybeck,et al.  Origines et comportement du carbone organique dans l'estuaire de la Loire , 1986 .

[24]  S. Manickam,et al.  Composition and mineralogy of suspended sediment in the fluvio-estuarine zone of the Loire river, France , 1985 .

[25]  E. Boyle,et al.  Biological Uptake of Dissolved Silica in the Amazon River Estuary , 1975, Science.

[26]  G. Billen,et al.  Algal and microbial processes involved in particulate organic matter dynamics in the Loire estuary , 1988 .

[27]  J. Salomon,et al.  Effects of tides on mixing and suspended sediment transport in macrotidal estuaries , 1980 .

[28]  C. Culberson,et al.  The chemistry of the Delaware estuary. General considerations1 , 1982 .

[29]  A. W. Morris,et al.  Nutrient distributions in an estuary: Evidence of chemical precipitation of dissolved silicate and phosphate , 1981 .

[30]  Edward A. Boyle,et al.  The chemical mass balance in the Amazon plume I: The nutrients , 1981 .

[31]  M. Ewald,et al.  Biogéochimie de la matière organique en milieu estuarien: stratégies d'échantillonnage et de recherche élaborées en Loire (France) , 1983 .

[32]  G. Berger,et al.  Nutrient distribution in the Zaire estuary and river plume , 1978 .

[33]  D. Hammond,et al.  Nutrient Budgets in the Hudson River Estuary , 1975 .

[34]  S. Knox,et al.  Statistical analysis of estuarine profiles: III. Application to nitrate, nitrite and ammonium in the Tamar estuary , 1986 .