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Abstract

We investigated the impact of intertidal groundwater seepage on benthic microalgae
and macrofauna in 4 study sites located in 2 large tidal flat ecosystems along the western coast of
Korea by comparing the chemical, physical, and biological characteristics of ‘glossy’ seepage sites
with those of nearby areas without visually distinct groundwater discharge (dry sediment surface). At
3 of the 4 sites, sediment properties as well as pore water chemistry were similar in groundwater
seepage and dry areas. At the 4th study site, the groundwater seepage areas were more coarsegrained
compared to the dry areas. Here, the groundwater seepage also had lower salinity and
higher nutrient concentrations than the pore water of the dry area and the seawater in a nearby tide
pool. Although diatoms were the dominant algal class in seepage and dry areas alike, the seepage
areas in 3 of the sites had elevated contributions of other marker pigments such as chlorophyll (chl) b
compared to the dry areas. Chl a concentrations were higher in all seepage areas compared to dry
areas, and all dry areas had high pheophytina:chla ratios, indicating a substantial amount of
degraded algal material. In the seepage areas of 3 of the sites, we found large numbers of the snail
Batillaria cumingi, while crab burrows of Scopimera sp. were only present in the neighboring dry
areas. Correlations of sediment chl a concentrations with physicochemical properties of the ambient
pore water indicated that microphytobenthos responded specifically to groundwater seepage, which
may provide shelter from desiccation and salt stress during emersion of the tidal flat. Our results suggest
that globally common groundwater seepage significantly impacts the ecosystem structures and
microphytobenthos production of the tidal flats.