Influence of cockle bioturbation on microphytobenthic primary producers: habitat and density-dependent effect

The purpose of this study was to better understand the non-trophic interactions of benthic macrofauna, especially through their bioturbation activity, on microphytobenthos (MPB), which remain poorly studied and understood. For this purpose, a mesocosm experiment was performed, using the common cockles Cerastoderma edule. This species plays a key role in coastal ecosystem, especially impacting sediment characteristics and biogeochemistry through their bioturbation, including sediment reworking and bioirrigation. For the first time, bioturbation rates, biogeochemical fluxes at the sediment-water interface and MPB biomass and photosynthetic variables were measured at the same time. The effect of cockles density and sediment type were also investigated.
This mesocosm experiment took place at the marine station of Arcachon. Experimental units consisted of PVC tubes filled with 2 types of sediment (medium sand or fine sand; samples in Arachon Bay and Baie des Veys in France). Then, 4 density of cockles were added in triplicate (0, 288, 720 and 1,297 ind. m-2), for each sediment type. The whole design was repeated twice, because in one of them luminophores were added at the top to measure sediment reworking, and could interfere with fluorescence measurement of MPB variables. All experimental units were incubated 6 days into a big tank, with artificial tide and light. Dissolved tracers were added into the natural seawater (close system) to measure bioirrigation rates of cockles.
After 6 days, regarding experimental units with luminophores, they were slices and porewater was extracted to quantify bioturbation rates. Regarding units without luminophores, surface biomass and photosynthetic parameters of MPB were first measured in all unit with an IMAGING PAM. Then, oxygen and nutrient fluxes at the sediment water interface were measured using incubations. And finally, the first centimeter was sliced to measure total MPB biomass.
This study demonstrated that bioturbation intensity of cockles was dependent of their density and the sediment type. There was a strong cascading effect of biogeochemical fluxes, and MPB, with a difference between the 2 sediment. The sediment reworking generated by cockles, stronger in fine sand, had a more negative impact on MPB, compare to medium sand where cockles generated higher bioirrigation rates and nutrient fluxes. The non-trophic interaction was overall a balance between positive and negative impact, depending on the cockle density.