Recycle of buried macroalgal detritus in sediments: use of dual-labelling experiments in the field

Abstract

In intertidal sediments, burial and decomposition of macroalgal detritus can fuel the sediment of carbon (C) and nitrogen (N), which can be either promptly mineralised or assimilated to enter the food web. This study investigates the transfer of algal-derived C and N to the sediment and to the infauna feeding primarily on benthic diatoms. Thalli of Ulva spp. were 13C- and 15N-labelled in the mesocosm and frozen to create detritus. Thawed macroalgae were, then, buried in the sediment of an intertidal sand-flat forming a mosaic of small patches (50 × 50 cm2) enriched with the macroalgae interspersed with bare sediment. The area was dominated by Corophium volutator and Hydrobia ulvae. The uptake of 13C and 15N was measured in the residual macroalgae, in the sediment and in those animals. Decomposition of detritus was rapid and after 4 weeks the residual biomass was 3% the amount added. Algal-derived 13C and 15N were moved to the sediment. The total amount of 13C and 15N retained in the sediment after completing the decomposition was 3.4 ± 0.5% 13C and 2.7 ± 0.6% 15N the amount decomposed. During the first 2 weeks more N than C was assimilated (1.7% 13C and 13.5% 15N). During the remaining two weeks, N was released from the sediment, while there was little accumulation of C (+6.4 ± 2.0 % 13C and −7.7 ± 3.8% 15N). At the end of the decomposition, animals were 15N- and 13C-labelled. Considering the total accumulation of label in the sediment, they accounted for 3.5 ± 1.8% 13C and 25.8 ± 12.9 % 15N. Similarly, considering the mass of the heavy isotopes gained (13C) or lost (15N) during the remaining 2 weeks, the animals accounted for 4.7 ± 2.1% of the 13C in excess and for 18.6 ± 9.1% of the 15N loss. The transfer of C and N to the sediment and to the surface deposit-feeders can be a relevant mechanism to remove the excess of detritus from the sediment.