@article {PIE3, title = {Measuring 15N-NH4 in marine, estuarine, and freshwaters: an adaptation of the ammonium diffusion method for samples with low ammonium concentrations.}, journal = {Marine Chemistry}, volume = {60}, year = {1998}, note = {PI Coastal}, pages = {235-243}, keywords = {ammonium, analytical methods, diffusion, estuarine, fresh water, inorganic nutrients, N-15, PIE LMER, sea water}, doi = {10.1016/S0304-4203(97)00099-6}, author = {Holmes, R.M. and McClelland, J. and Sigman, D.M. and Fry, B. and Peterson, B.J.} }