Seasonality of living epiphytic foraminifera from seagrass beds in nearshore zones of the Ryukyu Islands, Japan

Seasonality of epiphytic foraminiferal populations living on leaves of seagrass has been studied in seagrass beds in the nearshore zone of Irabu Island, the Ryukyu Islands. For this study, seagrasses in a 100 cm² sample area were collected monthly between February 1996 and November 1996, and in April 1997. The epiphytic foraminiferal assemblage consists predominantly of calcareous taxa, being dominated by a phytal form species, Calcarina calcar d'Orbigny, throughout the year. The assemblage is also associated with Hauerina pacifica, Vertebralina striata, Lamellodiscorbis sp. and Glabratella millettii during late winter and spring, and with Quinqueloculina seminulum, Q. semireticulosa, Bolivina striatula, and Cymbaloporetta plana during fall. Therefore, crawling forms increased in spring, and free living and attached forms are relatively abundant in fall. The species diversity increases in spring and fall, but drops in summer. The total abundances of foraminifers fluctuate seasonally from 2,258 individuals/g seagrass dried in spring to 128 individuals/g seagrass dried in late summer. Population dynamics of the dominant species C. calcar controls strongly these seasonal changes of total assemblage. The percentage of adult specimens with reproduction chambers and a large increase in the specimens smaller than 500 μm indicate that asexual reproduction typically occurs during spring. The spring is, therefore, most comfortable season for all epiphytic foraminifers, whereas summer season is so severe for survival and most of foraminifers die during late spring through summer. These seasonal variations in the epiphytic foraminifers are probably related to the changing regional environmental conditions within seagrass beds, such as food supply or physical factors, e.g. temperature, salinity and vertical circulation of water column.