Microbial food web structural and functional responses to oyster and fish as top predators

Abstract

The impact of fish and oysters on components of the pelagic microbial food web (MFW) was studied in a 10 d mesocosm experiment using Mediterranean coastal waters. Two mesocosms contained natural water only ('Controls'), 2 contained natural water with Crassostrea gigas ('Oyster'), and 2 contained natural water with Atherina spp. ('Fish'). Abundances and biomasses of microorganisms (viruses, bacteria, phytoplankton, heterotrophic flagellates, and ciliates) were measured to estimate their contribution to the total microbial carbon biomass. Two MFW indices, the microbial autotroph:heterotroph C biomass ratio (A:H) structural index and the gross primary production:respiration ratio (GPP:R) functional index, were defined. In the Fish mesocosms, selective predation on zooplankton led to a trophic cascade with 51% higher phytoplankton C biomass and consequently higher A:H and GPP:R than in the Controls. By the end of the experiment, the Oyster mesocosms had a bacterial C biomass 87% higher and phytoplankton C biomass 93% lower than the Controls, giving significantly lower A:H and GPP:R (<1). Overall, the results showed that wild zooplanktivorous fish had a cascading trophic effect, making the MFW more autotrophic (both indices >1), whereas oyster activities made the MFW more heterotrophic (both indices <1). These MFW indices can therefore be used to assess the impact of multiple local and global forcing factors on the MFW. The results presented here also have implications for sustainable management of coastal environments, suggesting that intense cultivation of filter feeders can be coupled with management to encourage wild local zooplanktivorous fishes to maintain a more resilient system and preserve the equilibrium of the MFW.

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

“…In parallel to oyster mortality, temporal variability of nutrient concentrations and PMC were explored. Dissolved Inorganic Nitrogen (DIN), Phosphorus (DIP) and N:P ratio were low and consistent with those already observed in the Thau lagoon , Mostajir et al 2015. The Thau lagoon was in N limitation according to the criteria of Justic et al 1995 (DIN <1 μM and DIN/DIP <10), as already observed in spring .…”

“…The phytoplankton Chla biomasses, heterotrophic prokaryote abundances and biovolumes of HF and ciliates were converted to equivalent carbon (C) biomass as in Mostajir et al (2015). Chla data were converted to C biomass using a value of 57 (Latasa et al 2005), as carried out previously in a mesocosm experiment in the Thau lagoon (Vidussi et al 2011).…”

“…(i) filtration (Dupuy et al 2000, Trottet et al 2008), (ii) excretion (Mazouni 2004, Richard et al 2006, Jansen et al 2011, and (iii) biodeposition (Callier et al 2006, 2009, Robert et al 2013. At high stocking densities in confined environments, shellfish such as mussels and oysters can affect the seston biomass load via filtration (Smaal et al 2013, Filgueira et al 2014a, stimulate primary production via nitrogen excretion (Chapelle et al 2000, Souchu et al 2001, Mazouni 2004) and modify the microbial plankton community structure (Dupuy et al 1999, Froján et al 2014, Mostajir et al 2015. During filtration processes, oysters can remove particles from the water column using pallial organs (gills, labial palps and mantle).…”

“…Heterotroph C biomass ratio (A:H) structural index, oyster activities make the microbial food web more heterotrophic in a confined environment (Mostajir et al 2015).…”

Shellfish culture: a complex driver of planktonic communities

“…In the same way, in a mesocosm experiment in Mediterranean coastal waters, Mostajir et al . () demonstrated that oyster culture induces lowers autotrophy‐heterotrophy ratio, highlighting the occurrence of a profound alteration in trophic food‐web functioning. However, consequences for planktonic food‐web alteration are still poorly documented and further investigations are needed.…”