The use of Fick's First Law for predicting porewater nutrient fluxes under diffusive conditions

Porewater fluxes into or out of the sediments in aquatic systems are frequently estimated using Fick's First Law. This assumes that diffusive processes control the transport of nutrients. As a corollary, advection, bioturbation and chemical processes are assumed not to be significant. Through a series of sediment core incubations, this paper seeks to quantify the uncertainties involved in such assumptions. Duplicate sediment cores were collected from 28 sites along the Swan–Canning Estuary, returned to the laboratory and incubated under oxic or anoxic conditions. Porewater nutrient concentrations, sediment porosity and initial nutrient concentrations in the overlying water were measured. These parameters were used to estimate the expected nutrient fluxes via Fick's First Law. The estimated fluxes were then compared with measured fluxes of nutrients out of the sediments over the incubation period. Severe bioturbation occurred in several of the anoxic treatments, resulting in large releases of nutrients into the overlying water. Aside from these bioturbated cores, phosphate and ammonium fluxes under anoxic conditions are well predicted by Fick's First Law. Nitrate fluxes are predicted well under oxic conditions. For coarse sediments (D10 averaging 0·3 mm) and under redox conditions favourable for nutrient release, Fick's First Law predicted close to 100% of the observed fluxes. For finer grain sediments (D10 < 0·01 mm), Fick's First Law overestimated the observed flux by up to 40%. Under unfavourable oxygen conditions, chemical retardation processes are likely to dominate fluxes, and the error associated with using Fick's First Law is increased. These results confirm the usefulness of using Fick's First Law for a baseline estimate of nutrient fluxes under favourable redox conditions. Much greater care must be taken when using Fick's First Law to estimate nutrient fluxes under unfavourable redox conditions, or under conditions when bioturbation is likely to be severe. Copyright © 2001 John Wiley & Sons, Ltd.

[1]  J. Turner,et al.  Interaction between shallow groundwater, saline surface water and nutrient discharge in a seasonal estuary: the Swan–Canning system , 2001 .

[2]  G. Masselink.  The effect of sea breeze on beach morphology, surf zone hydrodynamics and sediment resuspension , 1998 .

[3]  A. E. Greenberg,et al.  Standard methods for the examination of water and wastewater : supplement to the sixteenth edition , 1988 .

[4]  J. Cornwell,et al.  Denitrification in estuarine sediments determined by membrane inlet mass spectrometry , 1998 .

[5]  N. Peters,et al.  Trends in total phosphorus and total nitrogen concentrations of tributaries to the Swan–Canning Estuary, 1987 to 1998 , 2001 .

[6]  E. Kristensen,et al.  Direct measurement of dissolved inorganic nitrogen exchange and denitrification in individual polychaete (Nereis virens) burrows , 1991 .

[7]  Jenny Davis,et al.  Macrofauna and nutrient cycling in the Swan River Estuary, Western Australia: experimental results , 2001 .

[8]  B. Jørgensen,et al.  Diffusive boundary layers and the oxygen uptake of sediments and detritus1 , 1985 .

[9]  C. Hopkinson,et al.  Benthic metabolism and nutrient cycling along an estuarine salinity gradient , 1999 .

[10]  R. J. Clifton,et al.  The Effect of Macrofauna on Porewater Profiles and Nutrient Fluxes in the Intertidal Zone of the Humber Estuary , 1999 .

[11]  G. Billen.  An idealized model of nitrogen recycling in marine sediments , 1982 .

[12]  Bernard P. Boudreau,et al.  A method-of-lines code for carbon and nutrient diagenesis in aquatic sediments , 1996 .

[13]  M. Krom,et al.  The diffusion coefficients of sulfate, ammonium, and phosphate ions in anoxic marine sediments1 , 1980 .

[14]  Martin W. Marsden,et al.  Lake restoration by reducing external phosphorus loading: the influence of sediment phosphorus release , 1989 .

[16]  William J. Snodgrass,et al.  Model for Sediment Oxygen Demand in Lakes , 1986 .

[17]  Porewater nutrient fluxes in a shallow fetch-limited estuary , 1999 .

[18]  F. Neudörfer,et al.  A microbial biosensor for the microscale measurement of bioavailable organic carbon in oxic sediments , 1997 .

[19]  Harold F. Hemond,et al.  Chemical fate and transport in the environment , 1994 .