Particle–water interactions of organic nitrogen in turbid estuaries

The hypothesis that particle–water interactions of organic nitrogen (ON) in turbid estuaries are controlled by specific fractions of suspended particulate matter (SPM) has been tested. Suspended particles from the turbidity maximum zone of a temperate, macro-tidal estuary were separated by gravitational settling to yield permanently suspended (PSPM; 46–51% of the total SPM load) and resuspendable (RSPM) particle fractions. Abiotic and biotic laboratory incubations were carried out in triplicate over 48 h, on the separate fractions to quantify the partitioning of ON under representative estuarine conditions, using acidic, basic and neutral 14C-labelled amino acids as proxies for ON. Under abiotic conditions, about 10% of arginine (basic) sorbed to both SPM fractions within 48 h, while sorption did not occur for aspartic acid (acidic) and glycine (neutral). Partitioning of the amino acids with both SPM fractions under biotic conditions was much greater and most uptake had occurred by 24 h, when about 44% of added amino acid was in the particulate phase. By 48 h, 33–60% of activity on the particles had been lost as 14CO2, and the mean amount of dissolved amino acid remaining, in all biotic experiments, was 14 ± 3%. For abiotic experiments with arginine, Kds at 48 h were of the order 102 mL g− 1, although were much higher (103 mL g− 1) when RSPM concentrations were markedly increased. The biotic experiments consistently gave Kds that were of the order 103 mL g− 1. Thus, uptake of DON onto the SPM was bacterially-controlled and the general paradigm, that the removal of DON to particles is an abiotic process that restricts its bioavailability, does not hold for estuarine SPM.