The Influence of Emergent Macrophytes on Porewater PO<sub>4</sub><sup>3-</sup> Distribution and Physic Transport in the Estuary Wetlands

Abstract

Emergent macrophytes growth and development are believed to play an important role in nutrients physical transport processes and biogeochemical cycles. In the present study, a high-resolution in-situ sampling technology (Porewater equilibrators, Peepers) was employed to obtain vertical porewater PO43- profiles from the vegetated and the non-vegetated sediments, and accurately quantify benthic flux across sediment-water interface by Fick's first law applying porewater profiles of peepers, and coupled with core sediments incubation experiments as comparison. In addition, to distinguish benthic organism contribution to measured apparent benthic flux, we used HgCl2 (0.5% by weight) to suppress bottom fauna activity during the experiment. The results showed that porewater PO43- concentrations were far lower in vegetated than in the non-vegetated sediments, and also significantly lower in overlying water than in porewater. Whereas, porewater PO43- distribution fluctuated sharply at the upper sediments and kept an approximative constant below 8cm depth. Additionally, the average molecular diffusion flux applying Fick's first law fluctuated slightly within the range of 0.004 to 0.018 mg m-2 d-1(i.e., PO43- from porewater diffused into overlying water), which was higher in typha latifolia site, but lower in zizania latifolia site. PO43- average apparent diffusion flux based on core sediments incubation experiments varied between 1.03 and 6.78 mg m-2 d-1, and an opposite pattern was observed with respect to emergent macrophyte effects, as the PO43- flux at phragmites australis site was low (only 19% of control), but reached as high as 126% of the control in typha latifolia site. In unsterilized treatments (i.e., benthic organism participation), PO43- average net fluxes were lower up to an order of magnitude and more variable compared with those in sterilized treatments. Our results highlighted emergent macrophytes (e.g., phragmites australis) in estuary wetlands could efficiently relieved release risk from sediments, and reduce dissolved reactive phosphorus diffusion physical barrier crossed sediment-water interface by adding porosity in surface sediment.