Changes in planktonic microbial components in interaction with juvenile oysters during a mortality episode in the Thau lagoon (France)

Abstract Oysters modify the planktonic microbial community structure by their filtration and NH 4 excretion activities. While many studies have been conducted on this subject with adult oysters, none had been carried out in situ with juveniles. Pacific oyster juveniles ( Magallana gigas, previously Crassostrea gigas ) died massively all over the world since 2008 in relation with OsHV-1 infection. During mortality episodes, sick and dead oysters are not separated from healthy live ones, and left to decay in the surrounding environment, with unknown consequences for the nutrient cycle and planktonic microbial components (PMC). The present study aimed to elucidate for the first time the interactions between oyster juveniles and PMC during a mortality episode. Innovative 425-L pelagic chambers were deployed weekly in situ around oyster lanterns along a stocking-density gradient in the Thau Mediterranean lagoon (France) before, during and after an oyster mortality episode, from April to May 2015. This study reveals (i) significant changes of planktonic microbial community structure during mortality episodes, with a proliferation of picoplankton ( Balanion sp. , Uronema sp.) within 2 weeks when mortality rates and numbers of moribund juvenile oysters were highest. These changes were probably induced by oyster tissue leaching, decomposition and mineralization, which probably began during the moribund period, as suggested by an increase of PO 4 concentration and N:P ratio decrease, (ii) oyster juveniles mainly retained 3–20 μm plankton. In contrast to adults, picophytoplankton and small heterotrophic flagellates (