Effects of early tidal inundation on the cycling of methylamines in inter-tidal sediments

Abstract

Methylamine (MA) concentrations were measured at 2 sites in the Ria Formosa, Portugal, including a rearing plot for the clam Ruditapes decussatus (L), Sampling encompassed both sediment exposure and tidal inundation. Porewater MA concentrations at Site 1 (cohesive sediment with low numbers of clams) did not change significantly during sediment exposure, but increased at the onset of inundation, with desorption of solid-phase monomethyldmine (MMA) mainly responsible. Porewater dimethylamine (DMA) did not vary significantly, while porewater trimethylamine (TMA) concentrations decreased after inundation. Solid-phase MA concentrations at Site 1 were more variable, where TMA was consumed at a rate of 60 mu mol kg(-1) bulk sediment h(-1) during the first hour of sampling, suggesting that it was loosely bound to the sediment. The consumption of solid-phase TMA at Site 1 may contradict the notion of adsorption as a preservation mechanism for certain basic analytes. MA concentrations were considerably higher in the sediment and clam samples from Site 2, containing high densities of R. decussatus, where TMA was the most abundant MA, Its concentrations in clam tissue before inundation were 191 decreasing to 36 mmol kg(-1) after inundation. A concurrent increase in sediment solid-phase TMA from 0.1 to 0.9 mmol kg(-1) pointed to release of TMA by the clams. Excretion of TMA from the clam tissue should have increased the sediment concentration by 2.19 mmol kg(-1), but the measured value was 0.84 mmol kg(-1). The low adsorption capacity of sandy sediment and the fact that clams excrete their waste close to the sediment surface should increase the possibility of TMA flushing to the water column. The high density of clams, and the current preference for clam rearing in sandy sediments, could contribute to increased concentrations of dissolved nitrogen in the Ria Formosa lagoon.