Diatom sources of 13C-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 13c-rich isotopic conlpositions in the Georges Bank (USA) ecosystem. Diatoms in spring blooms and in well-mixed summer waters were relatively rich in 13c, with 8I3c values A n the -15 to -19%0 range, while other phytoplankton and most particulate organic matter collected over a 13 mo period had I3C-depleted values of -21 to -25 %. Culture experiments with nutrient-enriched seawater performed on Georges Bank and in Woods Hole Harbor (MA, USA) also showed a I3c distinction between fast-growing diatoms with l3C-rich contents and other algae that were depleted in I3C. Zooplankton from central Georges Bank where diatoms are abundant had relatively high b13C values, consistent with an important nutritional role for I3C-rich 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 13Crich compositions, indicating that diatoms can be a source of I3c-rich carbon in many marine food webs.