Yes it will occur but much at a much slower rate. Please remember that the sub aqueous content is almost always lower than that of the water column. You may have 8.4 in the water column but I am sure that is not what it is in the sediment if you have excessive organic matter.
Water pH fluctuates daily due to photosynthetic activity, being low at dawn and high mid afternoon. Sediment pH is less variable on a daily basis. Anoxic conditions lead to lower pH sediments due to increased CO2 levels. When these anaerobic conditions occur other oxidants become more important, sulfate reduction (SO42-) forming H2S then FeS
Denitrification (NO3-) releasing NH3 which accumulates. Fe reduction (FeOx) forming Fe2+ releasing P. Mn reduction (MnO2) forming Mn2+ releasing P
