Production, decomposition, and heterotrophic utilization of the seagrass Halophila decipiens in a submarine canyon

Abstract

We examined the net production, decomposition, and microbial utilization of the seagrass Halophila decipiens during a 6.5 d period in May 1985 in the Salt River Canyon, St Croix, US Virgin Islands. H. decipienscovered 37 % of the Canyon floor between depths of 14 and 32 m with a biomass of 9.15 g dry \vt m-'; its net productivity was ca 0.145 g C m-' d -' Turnover time, estimated by 2 independent methods, was 10.7 d. After 6.5 d H decjpiens incubated in lltterbags buried in the sediment lost 5 6 % of their original ash free dry weight (AFDW) while litterbags incubated on the sediment surface lost only 28 % of their original AFDW. Bacteria grew rapidly on the detritus, doubling in 3.1 d in the surface bags and 3.7 d in the buried bags. Per-cell thymidine incorporation rates peaked within the first 13 h in both treatments but declined thereafter. Final incorporation rates were highest in surface bags. Mean bacterial cell size and bacterial abundance associated with degrading H. decipiens were larger in the buried litterbags. Bacterial biomass, however, was only 29.3 mg cell C g -' AFDW in buried bags and 17.5 mg C g-' AFDVV in surface bags Using bacterial production averaged for the 6.5 d, we estimate that only about 0.26 "10 of the daily detntal input from H. declplens is converted daily into bacterial biomass attached to the degrading plant material. We conclude that, unless the bacterial community on H. decipiens detritus were to use the organic matter more efficiently and were heavily grazed upon, attached bacteria would not make a significant contribution to a deposit-feeding detritivore's energy demands.