Patchiness And Temporal Variation Of Ammonium And Phosphate In Pore Waters Of Intertidal Sediments In A Coastal Lagoon

We generated 4-6 hour time series data by means of in situ samplings to study the variations in the properties of pore water in a mud flat without macrovegetation. The location was the sand bar of a coastal lagoon, haltivay between the mouth and the interna1 extreme, on the northwestem coast of Baja California, Mexico. Salinity, reactive phosphate and ammonium displayed great temporal and spatial variation and irregular variations with depth. Patchiness, mostly created by biota, and percolation with tides are mainly responsible for these changes. Most So/,, values were between 35 and 360/oo. There were events of low So/,, values, as low as 300/00,very localized in time and depth. These low S”/oovalues were attributed to underground freshwater input. Our limited data indicate that most of the horizontal patchiness is in the scale of a few centimeters. With only 7cm of horizontal distance, the maximum:minimum concentration ratios, for the same collection time, were as large as 3.5 for PO4 and 2 for NH4; and the maximum So/,, difference was 3o/,,. PO4 and NH4 vertical profies consistently showed maximum values at S-2Ocm, with minimum values at the sediment surface and at 4Ocm (our maximum sampling depth). PO4 was as high as UrM, and NH4 was as high as 31SuM. In a previous and similar study conducted for sediments in a salt marsh with abundant Spartina foliosa, at the interna1 extreme of the same lagoon, the maximum PO4 and NH4 values reported were up to five and six times higher, respectively, than our maximum values. Our sampling site is more energetic, with much stronger tidal currents than the interna1 extreme.