Short answer: we do not fully understand it yet

Long answer: There are differences in the biogeochemical processes occurring in subtidal and intertidal sites, from my research we have seen that intertidal areas are more efficient at nutrient removal than subtidal areas.

Biogeochemical processes are also dependent on the sediment type (muddy vs sandy) as muddy sediments store more OC than sandy ones, and are more biogeochemically active. The benthic community is also highly important because some animals like Corophium volutator and N. diversicolor are known to promote nitrification-denitrification processes. In addition to these factors, sediment resuspension, and the exposure of a given site to wave action also determine the concentration and fluxes of nutrients in the sediment and can therefore influence the biogeochemical processes occurring in a certain site. It is also important to remember that salinity and the interaction between nutrients and of nutrients with sediment particles can also influence their dynamics

To conclude, biogeochemical processes in coastal areas are highly complex and depend on many biological, chemical and physical aspects