Silicon isotope enrichment in diatoms during nutrient-limited blooms in a eutrophied river system

• We examined the Si isotope fractionation by following a massive nutrient limited diatom bloom in a eutrophied natural system. The Oder River, which is a eutrophied river draining the western half of Poland and entering the southern Baltic Sea, exhibits diatom blooms that cause extreme Si isotope fractionation. The rapid nutrient depletion and fast BSi increase observed during the spring bloom suggest a closed system Rayleigh behavior for DSi and BSi in the river at certain time scales. A Si isotope fractionation factor ((30)epsilon(Dsi-Bsi)) of -1.6 +/- 0.31%. (2 sigma) is found based on observations between April and June, 2004. A very high delta Si-30 value of up to +3.05 parts per thousand. is measured in BSi derived from diatoms. This is about 2 times higher than previously recorded delta Si-30 in freshwater diatoms. The Rayleigh model used to predict the delta Si-30 values of DSi suggests that the initial value before the start of the diatom bloom is close to +2 parts per thousand, which is relatively higher than the previously reported values in other river water. This indicates that there is a biological control of the Si isotope compositions entering the river, probably caused by Si isotope fractionation during uptake of Si in phytoliths. Clearly, eutrophied rivers with enhanced diatom blooms deliver Si-30-enriched DSi and BSi to the coastal ocean, which can be used to trace the biogeochemistry of DSi/BSi in estuaries.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geofysik (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geophysics (hsv//eng)

Nyckelord

Biogenic silica

Si isotopes

A eutrophied river system

Nutrient limitation

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