DIATOM SOURCES OF C-13-RICH CARBON IN MARINE FOOD WEBS

We used stable isotope measurements to investigate the possible nutritional importance of diatoms for consumers in planktonic food webs. Several lines of evidence indicated that rapidly growing diatoms had C-13-rich isotopic compositions in the Georges Bank (USA) ecosystem. Diatoms in spring blooms and in well-mixed summer waters were relatively rich in C-13, with delta-C-13 values in the -15 to -19 parts per thousand range, while other phytoplankton and most particulate organic matter collected over a 13 mo period had C-13-depleted values of -21 to -25 parts per thousand. Culture experiments with nutrient-enriched seawater performed on Georges Bank and in Woods Hole Harbor (MA, USA) also showed a C-13 distinction between fast-growing diatoms with C-13-rich contents and other algae that were depleted in C-13. Zooplankton from central Georges Bank where diatoms are abundant had relatively high delta-C-13 values, consitent with an important nutritional role for C-13 diatoms. We estimate that a minimum of 40% of the carbon present in zooplankton consumers of central Georges Bank is derived from diatoms. Bloom diatoms from the North Atlantic, northeastern Pacific, and the nearshore Gulf of Mexico also had C-13-rich compositions, indicating that diatoms can be a source of C-13-rich carbon in many marine food webs.