The nitrogen isotopic composition of organic matter contained within diatom frustules of different sediment size fractions from the Guaymas Basin, Gulf of California, suggests that multiple nutrient cycling processes can be examined from a single sediment sample. The diatom-bound delta(15)N values from the greater and less than 63 mu m size fractions were analyzed from six intervals of core MD02-2517, each representing similar to 30-40 years. The diatom-bound delta(15)N values are compared to bulk sedimentary delta(15)N values, weight percent biogenic opal, and weight percent organic carbon. Bulk sedimentary delta(15)N values range from 10 parts per thousand to 12.5 parts per thousand and diatom-bound delta(15)N values of the <63 mu m size fraction from 7 parts per thousand to 13 parts per thousand. In contrast, diatom-bound delta(15)N values for the >63 mu m fraction range from 1 parts per thousand to 7 parts per thousand. The best explanation for the low diatom delta(15)N values in the >63 mu m fraction is that the large diatoms grew at the base of the euphotic zone in excess nitrate and under low light conditions. Large diatom delta(15)N values correlate significantly with bulk sedimentary delta(15)N values (r = 0.68, 0.82), indicating that large diatoms, such as Thalassiothrix longissima, are an important component of export production fueled by a nonupwelling source of nitrogen. The >63 mu m diatoms are estimated to contribute between 3% and 15% of the total nitrogen in the sediments based upon an assumed Si-to-N ratio of 1.2.