Modulation of dissolved oxygen levels in a hypertidal estuary by sediment resuspension

Abstract

Hyperconcentrated benthic layers, which form during neap tides, recruit much of the fine sediment population of the turbidity maximum of a hypertidal estuary. Measurements of tidal amplitude and suspended solids concentration reveal that resuspension of the hyperconcentrated layers occurs between three and eight tides after neap tides rather than during spring tides (12 to 15 tides after neaps). During these resuspension events, dissolved oxygen levels are reduced but recover by spring tides. Peak solids concentrations and critically depressed dissolved oxygen levels are out of phase with tidal current amplitude. Thus observations close to neap and spring tides do not necessaraly capture the extremes of the envelope of water quality conditions.

Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability

“…Moreover, the rapid oxygen depletion of the water in the summer is particularly marked in the Loire Estuary (Thouvenin et al, 1994), although it occurs in all three studied estuaries; resuspension of anoxic ''fluid mud'' at spring tides, i.e. benthic layers with a high TSS concentration (one to several hundred g l -1 ), appears to be the main reason for the hypoxia (Abril et al, 2003(Abril et al, , 2004), as reported for several other macrotidal estuaries (Parker et al, 1994;Uncles et al, 1998).…”

Oxic/anoxic oscillations and organic carbon mineralization in an estuarine maximum turbidity zone (The Gironde, France)

“…Organic carbon budget calculations showed that one-half of the POC entering the MTZ is mineralized within the estuary, the rest being exported to the Atlantic Ocean or sedimented in the estuary (Etcheber 1983;Lin 1988). Oxygen concentrations in surface waters are relatively high, compared to other macrotidal turbid estuaries (Morris et al 1982;Wollast 1983;Parker et al 1994;Thouvenin et al 1994), with concentrations close to saturation levels in riverine and marine parts and about 40-60% of saturation in the MTZ (IFREMER 1994, unpubl. data).…”

“…Two superimposed cycles of sedimentation and resuspension coexist: neap tides are sedimentation periods, whereas spring tides are dominated by erosion (15-d lunar cycle). Moreover, high-and low-water slacks are sedimentation periods, whereas midebb and midflood are erosion periods (semidiurnal cycle) (Inglis and Allen 1957;Allen et al 1980;Le Hir and Karlikow 1991;Parker et al 1994;Reed and Donovan 1994;Grabemann et al 1997). During neap-tide minimum energy periods, surface turbidities are considerably lower, and hyperconcentrated benthic layers (named ''fluid mud'' by Inglis and Allen 1957) accumulate on the bottom.…”

“…Recent works have shown that oxygen concentrations in surface water can be considerably lower a few days after neap tide, during resuspension (Parker et al 1994;Thouvenin et al 1994). It is thus obvious that particulate material is submitted to oxic conditions during spring tides and to anoxic conditions in fluid mud during neap tides.…”

“…Severn Estuaries, Parker et al 1994;Thouvenin et al 1994), the latter component is important because oxygen deficits in surface waters were reported during several days after resuspension events. This is not the case in the Gironde MTZ, where reaeration is sufficient to maintain relatively high oxygen concentrations in the MTZ after resuspensions (Fig.…”