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We examined the hypothesis that the d13C of benthic consumers is depleted in offshore, deeper waters where kelp and benthic microalgae are absent by conducting three analyses: (1) we analyzed d13C (and d15N) of consumers along a nearshore–offshore gradient in the field, (2) we analyzed d13C (and d15N) of consumers placed in cages at various depths and distances from shore, and (3) we reexamined published stable isotope values for consumers taken at various depths from studies around the world. In all three situations, we did not observe the predicted change in d13C values of benthic consumers. In both the literature and in our field measurements, the d13C values for consumers were on average separated from the values for suspended particulate organic matter by 4%, irrespective of depth, indicating that factors other than the feeding on 13C-rich benthic primary producers may explain the enriched carbon ratios in benthic consumers (e.g., selective feeding on enriched particles). The hypothesis that the high d13C ratio in suspension and deposit feeders reflects feeding on benthic primary producers is thus tenuous and requires further investigation. In coastal temperate and polar oceans, there exist three potentially important sources of primary production: phytoplankton, kelp, and benthic microalgae (microphytobenthos). Although it is generally acknowledged that phytoplankton are the main food source available to benthic consumers, productivity measurements suggest that kelp and benthic microalgae may also be an important food source. Kelps are among the most productive plants on the planet, and up to 90% of their net production may end up as detritus (Mann 1988), which can be ingested by suspension and deposit feeders (Duggins and Eckman 1997). In a review of benthic microalgae productivity measurements in coastal ecosystems, Cahoon (1999) showed that they also can markedly contribute to overall production in temperate regions (around 60 g C m22 yr21), even at depths of .20 m. For example, Glud et al. (2002) found that the net photosynthetic rate of benthic microalgae at depths ,30 m in a Greenland fjord was twofold greater than that of phytoplankton in overlying waters. Although certain studies have shown an increased growth rate of suspension feeders in the presence of kelp both in the field (Duggins et al. 1989) and in controlled feeding experiments (Duggins and Eckman 1997), direct evidence for the use of benthic algae by suspension and deposit feeders comes from the analysis of stable isotopes. Observations from more straightforward approaches, such |
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