Recent invasion of a cordgrass, Spartina alterniflora, into Korean tidal flat and its positive and negative effects on native benthic community

A exotic cordgrass, Spartina alterniflora firstly invaded the west coast of South Korea in 2009 and expanded rapidly to be about ~300 ha during the previous decade. Korean government prepared the management plan of the species, listed as a harmful marine organism, to hamper further expansion. It is controversy whether introduction of S. alterniflora affects positively or negatively on native intertidal ecosystem of Korea. This study aims to investigate which type of physical treatments effectively inhibits growth and expansion of S. alterniflora and how macrobenthic faunal community was affected by the corresponding treatment. Four physical treatments were applied, i.e., mowing only (MO), sediment turnover (ST), sediment turnover with biological substitution (STS) of native plant, Suaeda japonica, and fully taking out sediment (TO). Monthly observations and samplings were conducted from Nov 2017 to Mar 2019. STS method reduced the growth by 50%, 95%, and 93% reduction in length of stem, density, and biomass. Microphytobenthic Chl-a was 76.5±13.6 mg L^-1 (mean±SE) in unvegetated mudflat, significantly higher than that in MO treatment, 56.6±8.0 mg L^-1. Mud content and loss on ignition in unvegetated mudflat was 95.3±1.0 % and 6.4±0.3 %, lower than those in MO (97.9±0.4 % and 7.0±0.2%). A total number of faunal species was found to be 23 (unvegetated), 21 (ST), 20 (STS), but reduced to be 10 in MO. Macrophthalmus japonicus (crab), Assiminea sp. (gastropod), and Moerella jedoensis (bivalve) did not occur in MO, but density of Perinereis linea (nereid polychaete) increased in MO. ST and STS was effective and efficient to inhibit the growth of cordgrass. S. alterniflora changed sediments to be muddier and organic-rich, and affected positively polychaetes and negatively crabs and mollusks. Physical control of S. alterniflora by sediment turnover would be caused by exposure of grass root to seawater and cold weather during the winter, which will be our next research topic.