We experimentally reduced benthic microalgae using opaque rubber canopy covers and observed dietary responses of Corophium spinicorne (amphipod) and Macoma balthica (bivalve) based on δ 13 C and δ 15 N measurements in 2 estuarine tidal flats. Each of the tidal flats (MA and TE) was predominated by marine-or terrestrial-origin organic matter (OM), respectively. We hypothesised that the reduction of benthic microalgae correspondingly decreases the proportion of macrobenthos diet attributable to

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... nthic microalgae, and enhances their assimilation of allochthonous OM. Our canopy treatment reduced chlorophyll a in the surface sediment by 35 to 40%. While δ 13 C and δ 15 N of M. balthica were not affected by the canopy treatment, the signatures of C. spinicorne were significantly altered at both sites. In MA, the canopy treatment reduced the nutritional dependence of C. spinicorne on benthic microalgae and maintained their substantial dependence on the marine-origin OM predominating in the sediment. On the contrary, in TE, the canopy treatment enhanced the nutritional dependence of C. spinicorne on benthic microalgae and lowered their dependence on the terrestrial-origin OM predominating in the sediment. The canopy treatment significantly changed the C:N and δ 15 N of the sediment OM in TE. This suggests that benthic microalgae affected biogeochemical processes and the chemical properties of OM in the sediment subsurface. Benthic microalgae may have indirectly controlled nutritional utilisation of terrestrial-origin OM in the sediment by macrobenthos. Overall, the dietary compositions of macrobenthos did not always vary in proportion to the relative abundance of OM sources; the chemical properties of allochthonous OM, its biogeochemical alterations and species-specific food selectivity are also important factors. KEY WORDS: Stable isotopes · Benthic microalgae · Canopy experiment · Corophium spinicorne (amphipod) · Macoma balthica (bivalve) · Trophic shift · Estuary · Forested and agricultural watersheds Resale or republication not permitted without written consent of the publisher Editorial responsibility: Hans Heinrich Janssen,