Dynamics of seston constituants in the Ari�ge and Garonne rivers (France)

Abstract

Water contents of suspended matter, algal pigments, particulate organic carbon and particulate phosphorus were measured in the rivers Garonne (2 sites) and Ariege (1 site) throughout an annual cycle . The general trend of the parameters was similar at the three sites . Depending on the sites, the period of algal growth (chlorophyll a + phaeopigments > 25 ug 1 -1), lasted from two to six weeks in August-September . The algal peaks reached 50 to 90 yg 1 -' of total pigments. High contents of particulate organic carbon (> 2 mg 1 -') occurred at the end of summer (coinciding with algal growth), and during the November and May floods . In summer 50-75 % of the suspended matter was organic, in spring this was 10 times less . The high linear correlation between particulate organic carbon and pigment contents (r = 0 .87 ; P = 0 .0001) suggested an algal origin of at least part of the particulate carbon. Algal carbon was minor in the annual fluxes of particulate carbon (25 to 39% depending on the sites), but relatively high in comparison with other rivers . The mean particulate phosphorus content calculated over the year was 24 pg 1 -' ; it varied from 15 µg 1 -' during the high water period to 28 µg 1 -' during the low water period . Likewise the percentage of particulate phosphorus in the suspended matter varied from 0 .17 to 0 .40 . A negative linear correlation existed between particulate phosphorus content and specific discharge (r = -0.46 ; P = 0 .0001) .The very marked seasonal trend of the parameters and the interactions led us to differentiate two modes of the rivers' functioning : a `hydrologic' phase and a `biological' phase . The hydrologic phase (high water) was dominated by the processes of erosion and transfer over the whole catchment area and the flood plain, while the biological phase was characterized by a high primary production in the river bed .

Hydrological connectivity, and the exchange of organic matter and nutrients in a dynamic river–floodplain system (Danube, Austria)

“…During the long stagnant periods of phase I, nutrient levels and primary productivity are low in the floodplain, as has also been shown by other investigators (Ertl, 1985;Furch & Junk, 1993;Vegas-Villarrubia & Herrera, 1993;Hein et al, 1996). Phytoplankton compete for the limiting nutrients at the time when grazing rates by zooplankton are high (Chauvet & Fabre, 1990;Heiler et al, 1995). The present authors define phase I, with autogenic processes dominating (e.g.…”

Control of C,N,P distribution in soils of riparian forests

“…The lack of correlation between TP content and soil grain size (SC) in E-type forest soils can be related to the fact that an important part of phosphorus (mainly Pi) in floodplains comes from suspended matter deposits (Walling & Webb 1987). Since E-type forests are submitted to high current velocity during flood, only coarse suspended matter, poor and relatively constant in phosphorus (Chauvet & Fabre 1990), are uniformly deposited and hide any correlation with grain size distribution.…”

Inorganic phosphate in exposed sediments of the River Garonne

“…These conditions permit settling of finer particles from the river water. At this period of the hydrological cycle, the concentration of suspended matter is low (mean value based on 33 measures: 7.42 mg 1-1), but the phosphate content of the particles is high (0.40%, Chauvet & Fabre, 1990). Moreover, the low velocity and shallowness of the water permit growth of attached algae which can physically trap suspended particulate matter (Hart, 1982).…”