Ammonium release from resuspended sediments in the Laguna Madre estuary

Sediments can play an important role in estuarine ecosystems by buffering environmental nutrient concentrations in overlying waters. This study examined the role of surface chemistry in controlling the release of NH4+ from resuspended sediments in the large, shallow, negative estuary Laguna Madre, TX. Two techniques were used: a generally accepted elutriate method in which sediments were added at a 1:5 ratio to overlying water and a more dilute procedure (closed) where sediments were added at three ratios ranging from 1:10,000 to 1:100. In general, the more dilute procedure resulted in greater release of NH4+ from sediments. This is the result of the relative positions that the final solution concentrations place the systems on for an experimentally determined Langmuir-type adsorption isotherm. The closed system technique also indicated that approximately equal amounts of the released NH4+ came from porewater, loosely-bound quick-release, and tightly-bound slow-release fractions. Experimental results were supported by observations during dredging activities. Model calculations indicate that resuspension events may cause NH4+ releases comparable to NH4+ fluxes from bottom sediments. Resuspension could potentially contribute to such environmental problems in this estuary as the ‘brown tideʹ.