Response of suspension-feeding clams to natural removal of bioturbating shrimp on a large estuarine intertidal sandflat in western Kyushu, Japan

On a 3.4-km2 intertidal sandflat in Ariake Sound, Japan, four phytoplankton-feeders were dominant in the benthic community in terms of abundance and biomass in spring-summer, 2004: ghost shrimp, Nihonotrypaea japonica, mud shrimp, Upogebia major, and clams, Ruditapes philippinarum and Mactra veneriformis, each occupying a specific one-fourth area of the sandflat. N. japonica is a deposit feeder, while the others are suspension feeders. N. japonica and U. major dwell in 0.7- and 2-m deep burrows, respectively. Subsequent monitoring conducted every spring during 2005 to 2008 revealed that (1) probably due to typhoon- and stingray-induced substratum disturbances for mud and ghost shrimp populations, respectively, these had greatly shrunk from their original mid- to low-tide habitat in 2005 and high- to mid-tide one in 2006, respectively, (2) vacated space was colonized by juveniles of both clams and a formerly subdominant, suspension-feeding clam, Meretrix lusoria, (3) only Ma. veneriformis and Me. lusoria were found as adults there, with R. philippinarum adults remaining limited to the low-tide zone, and (4) loss in sum total population biomasses due to that in the shrimp' was compensated by the gain in the clam species'. These findings suggest that the five species compete for (1) space, with superiorities exerted by shrimps to clams through the former's intense bioturbating activities and by Ma. veneriformis and/or Me. lusoria to R. philippinarum in the high- to mid-tide zone and vice versa in the low-tide zone, and (2) food to the limit of the carrying capacity provided by the sound. Furthermore, estimates were made of the benthic community biomass in the past based on a long-term record (or estimate) of chlorophyll-a concentrations (proxy measure for food) for the sound water column and of the R. philippinarum population biomass based on its fishery-landing record. The latter estimate would account for 60% of the former one in the late 1970s, when the first eutrophication is believed to have been in progress. The subsequent decrease in benthic community biomass was suggested in response to that in phytoplankton abundance, reaching the one-fourth level in 2004-2008. In the long run, three alternative states would have occurred in the benthic community, from (1) R. philippinarum population boom in the 1970s, through (2) remarkable increases in the populations of the shrimps and Ma. veneriformis in place of R. philippinarum in the early to mid-1980s, followed by a plateau until 2004, to (3) the one described here for the 2004-2008 period.