Influence of porewater exchange on nutrient dynamics in two New Zealand estuarine intertidal flats

We use concomitant radon (²²²Rn, a natural groundwater tracer) and nutrient time series observations upstream and downstream of two New Zealand estuarine intertidal flats to assess porewater exchange rates and/or submarine groundwater discharge (SGD) and their influence on surface water nutrient dynamics. A detailed radon mass balance model and an uncertainty analysis revealed porewater exchange rates of 27 ± 7 and 14 ± 6 cm d^{− 1} (or cm³ cm^{− 2} d^{− 1}) in Waikareao and Te Puna, respectively. The upscaled porewater exchange rates were slightly higher than the creek input upstream of both intertidal flats. A water and salt balance compared to the radon balance indicated that about 16% (Waikareao) and 49% (Te Puna) of the total volume of porewater exchange consisted of fresh SGD and the remaining was related to seawater recirculation in intertidal sediments. Porewater exchange in Waikareao released about 1.9- and 1.6-fold more total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) than creek inputs at the upstream end of the intertidal flat even though observations followed a 140 mm rain event. Dissolved organic nitrogen (DON) accounted for about 25% of TDN in shallow porewater. Nitrate dominated the nitrogen pool in Waikareao and ammonium was the main form of nitrogen in Te Puna porewaters. These dominant porewater N species were reflected in the surface waters that showed a relative enrichment of nitrate in Waikareao and ammonium in Te Puna as upstream waters travel to the downstream station and collect seeping porewater along the way. We suggest that porewater exchange may act as a buffered nutrient source to the estuary, continually releasing nutrients to surface waters and potentially sustaining primary production when other nutrient inputs cease. This study illustrates that combining bottom up (i.e., porewater exchange flux estimates) and top down (i.e., nutrient response and transformations in the water column) evidence may provide deeper insight when assessing the contribution of porewater to nutrient dynamics in estuaries.