Microphytobenthos seasonality determines growth and reproduction in intertidal bivalves

The annual cycles of phytoplanktonic and microphytobenthic biomasses as food sources for intertidal bivalves were investigated in a temperate estuarine muddy sandflat of Kwangyang Bay (Korea) from January to December 2002. Seasonal patterns in growth and reproductive activity of a suspension-feeder Laternula marilina and a deposit-feeder Moerella rutila were examined in order to assess their relationship with the annual cycles of pelagic and benthic microalgae, and were combined with analyses of delta C-13 and delta N-15 of bivalve tissues and their potential food resources. Biomasses of phytoplankton and microphytobenthos peaked in late spring-summer, and demonstrated a unimodal pattern of seasonal variation. Photosynthetic pigment composition showed a predominance of diatom marker pigment, fucoxanthin, in the water and the sediments throughout the year. Similar patterns of annual cycles in pelagic and benthic microalgal biomasses and similarities in taxonomic composition indicated that resuspended microphytobenthos is an important contributor to the bay's phytoplankton component. This was supported by the delta C-13 values of suspended particulate organic matter (POM) and physical characteristics of the bay. Synchrony in growth and reproductive activity was observed for both bivalves: their shell and tissue growth and gonadal development were achieved together during late spring and summer when chlorophyll a (chl a) concentrations were highest. This temporal coupling of macrofaunal and microalgal processes indicated that the activities of interticial bivalves might depend largely on microphytobenthos seasonality. The isotopic signatures of the bivalve tissues demonstrated their dependence on organic matter of microphytobenthic source irrespective of season, despite a comparable contribution of phytoplankton to the diet of the suspension-feeding bivalve. These results highlight the importance of seasonal development of microphytobenthos as an available food source during the critical period of gamete production and growth for both suspension- and deposit-feeding bivalves.