Submerged aquatic vegetation (SAV) such as seagrass meadows affects the transport and burial of organic matter (OM) because SAV alters hydrodynamics as a drag force. The quantification of OM burial rate in vegetated areas considering the drag effect is critical in terms of quantification of the role of coastal ecosystems in climate change mitigation. Although a number of studies have investigated on the drag force of SAV, the spatial scale of the previous studies was small, e.g., indoor flume experiments and in situ experiments using enclosures with a scale of several meters. Here we performed field observations and developed a new 3-D physical model to investigate the drag force of SAV on current fields. The study site, the Furen Lagoon, is a semi-enclosed brackish lagoon (area: 57.4 km2, mean depth: 1~2 m, about 68% is covered with seagrass meadows). Two scenario cases, i.e., “with-SAV” and “without-SAV” were calculated and compared with the observed data.
We found that SAV operated as a drag force interrupted the inflow of saline water and fresh water and reduced mixing of saline water with fresh water. A particle tracking simulation showed that the rate of the whole lagoon water exchange was reduced by ca. 34% in the presence of vegetation. Although the with-SAV case showed a vertically unidirectional flow, which is one of the flow characteristics in the Furen lagoon, the without-SAV case showed an opposite current component direction between the surface and bottom layer, therefore the model showed that SAV influenced the vertical circulation of tidal estuary. Furthermore, the model simulated that SAV affected the distribution of current velocity indicating a decrease (70~89%) in the vegetated area and, in turn, an increase in the unvegetated area.
The 3-D model developed in this study can be used for better quantification of OM burial rate considering the drag effects of SAV on hydrodynamics and waves. Furthermore, the new model is applicable to the restoration projects of seagrass meadows and seaweeds in terms of selection of better sites as an accurate prediction of currents and to the estimation of waves and current energy decreased by vegetation.