Nepheloid zone

When P in the nepheloid zone was excluded from the integrations, areal total P concentrations increased from 733 mg/m2 in mid-April to 845 mg/m2 in mid-July, and then declined to 630 mg/m2 by late November. 

Figure 2. Phosphorus concentrations in the bottom nepheloid zone.

In fall the nepheloid zone contains about 20-30 mg/m2 of particulate P, compared with a filtrable pool of 150-250 mg/m2 that is available for advection at turnover. Roughly one-third of the filtrable pool is colloidal. If uniformly resuspended throughout a 160-m water column, nepheloid colloidal P would represent between 0.3 and 0.5 mg of P/m3 (jxg/L). 

Figure 17. Concentrations of particulate P, colloidal P, and dissolved P in the nepheloid zone in August 1989.

A net loss of approximately 230 mg of P/m2 occurred in the water column (nepheloid zone excluded) from the onset of thermal stratification to late October. Measured trap fluxes of 122 mg of P/m2 over this period were almost identical with the measured water-column loss of particulate P (Figure 1). Loadings and fluxes are balanced by regeneration of filtrable P in the nepheloid region. 

Modes of resuspension interactively depend on the concentration profile. The vertical structure of concentration is conveniently subdivided into four zones (Fig. 27.11). In the upper zone the suspension layer (DSL) is dilute, and is characterized by Newtonian flow behavior. The lower zone is occupied by the benthic nepheloid layer (BNL), which contains fluid mud. In the benthic suspension layer (BSL), the concentration is intermediate between DSL and BNL. The suspension in BSL is non-Newtonian but the concentration is not high enough for setthng to be hindered. Finally, at the bottom a consolidating bed (CB) occurs. It possesses an effective stress but is soft enough (i.e., not fully consolidated) for the sediment to be susceptible to resuspension when wave forcing is sufficiently strong. 

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