Fate of phytodetritus in marine sediments: functional importance of macrofaunal community

A 54 d incubation of intact sediment box-cores from 2 different macrofaunal sediment com- munities from the Swedish west coast was performed under controlled laboratory conditions in April to June 1999. One community was dominated by passive suspension feeders the other by subsurface de- posit feeders. The box-cores were seeded with 14 C-labelled detritus of the diatom Skeletonema costa- tum in order to mimic a post-spring-bloom situation. In addition to the labelled diatom phytodetritus, the box-cosms initially contained chlorophyll a with concentrations between 1 and 1.5 µg ml -1 in the top cm of the sediment. The experiment thus provided an opportunity to estimate degradation rates of the nat- urally occurring chlorophyll with a natural benthic fauna present over a ca. 2 mo period. Using a dia- genetic model to describe the vertical distribution of chlorophyll a (chl a), and using the same, previously published, degradation rate constant in both communities (0.03 d -1 ), gave mixing coefficients (DB, cm 2 d -1 ) that were on average >2 times higher in the community (L18) dominated by deposit feeders than in the suspension-feeding community (S3): 2.5 ± SD 1.7 and 0.86 ± SD 0.50, respectively. This indicates a higher mixing rate at L18. This difference in mixing between the 2 communities was supported by changes in vertical distribution of 14 C-labelled phytodetritus over the 54 d period. The mixing coefficients ( DBs) were positively correlated with biomass of subsurface deposit feeders but not with total biomass. Background chl a was higher at L18 than at S3. A quantitative comparison of the total chl a inventories at the start and end of the experiment suggested a low overall degradation rate (no significant overall change) in the chl a, far from the reaction rate constant of 0.03 d -1 often used in the literature. Similarly, the total 14 C activity in the cosms did not change significantly over the study period, suggesting a small loss of 14 CO2 from the cosms relative to the 14 C-pool size. The labelled algal matter distributions showed clear mixing over the 2 months in both communities with a higher mixing rate in the deposit-feeding community than the suspension-feeding community. Mixing also occurred deeper in the deposit-feed- ing community. Uptake of labelled matter by macrofauna was similar in the 2 communities, but differed markedly between species and trophic groups. At the end of the incubation, surface deposit feeders had an order of magnitude higher weight-specific 14 C activity than suspension feeders and subsurface de- posit feeders. The proportion of macrofaunal uptake of total 14 C activity in the cosms was small, on the order of 5%. The results support the idea that community species composition is important for the fate of sedimented phytodetritus and that macrofaunal influence on degradation of sedimentary chlorophyll is small at this time of the year. The initial fate of the bloom material was burial in the sediment rather than consumption by heterotrophs. The findings are thus in accordance with the hypothesis that a part of the spring phytoplankton bloom may be buried for a while in the sediment before being remineralised.