The biogeochemical cycling of ammonium and methylamines in intertidal sediments

The methylamines (MAs) are chemical analogues of ammonia and contain one, two or three methyl groups. This study looked at their occurrence in inter-tidal sediments and at changes in their abundance during tidal cycling, including forced and naturally occurring sediment resuspension. Two sites in the UK, Burnham Overy Staithe (BOS) and the Thames Estuary (TE), and the Ria Formosa (RF), Portugal,
were chosen for the study.
The MAs were abundant in all samples collected. MA concentrations were compared to NH ₄⁺ at BOS and TE. A consistent trend emerged, with NH ₄⁺ more abundant in the pore-waters and the MAs dominating the solid phase. Most NH ₄⁺ and MAs were found on the solid-phase, and the general magnitude of adsorption was:
TMA > DMA > MMA > NH ₄⁺. This was inconsistent with their pKb values but could be explained by the ability of each cation to form hydrogen bonds with water.
Pore-water MA concentrations at BOS were compared with salinity but no correlation was observed. However, the clam Ruditapes decussatus (L.) released TMA during tidal inundation. The mechanism of release is unclear as these organisms do not osmoregulate, but the calculated TMA loss from these sediments (169 mmol m⁻² per tide) could be increasing dissolved organic nitrogen concentrations in the Ria
Formosa.
TE sediments were used in desorption experiments. Desorption of NH ₄⁺ was more rapid than the MAs and their mean chemical response times were 15 and 25 minutes, respectively. Increases in concentrations of dissolved
NH ₄⁺ and MAs over a tidal cycle were coincident with remobilisation of seabed sediments. Desorption of NH ₄⁺ and MAs from the remobilised sediments accounted for approximately 50% and > 90% of the increase, respectively. The results are proposed as a predictor for the sorption behaviour of other ON compounds and emphasise the importance of sediment resuspension as a mechanism of ON release to the water column.