Spatial and temporal variability in modelled nutrient fluxes from the unpolluted Shannon estuary, Ireland, and the implications for microphytobenthic productivity

Nutrient (NOx, NH4, PO4) flux rates were calculated from core profiles for three regions in the intertidal area of the Shannon estuary and compared to measured fluxes in laboratory incubations. Mean annual flux rates at the three sites ranged from 1.75 mumol N m(-2) h(-1) to -82.3 mumol N m(-2) h(-1) (sediment release and uptake, respectively as NOx), 2.82 mumol N m(-2) h(-1) to 26.3 mumol N m(-2) h(-1) (sediment release as NH4) and 0.13 mumol P m(-2) h(-1) to 2.2 mumol P m(-2) h(-1) (sediment release as PO4). Spatial variability was estimated within one station (5 m(2)), along a transect perpendicular to the shore and among the three regions, while temporal variability was estimated from monthly samples. NOx (nitrate + nitrite) fluxes showed the greatest spatial and temporal variability. While fluxes of all nutrients at the three regions were significantly different, there was much greater homogepeity within the one station and along the transect. The results indicated considerable N consumption in the topmost sediment layers, and this is suggested as a major contribution to microphytobenthic productivity, although the sediment flux contribution to the total estuary nutrient input was negligible. (C) 2004 Elsevier Ltd. All rights reserved.