Resuspension of phytodetritus from the sea floor: A laboratory flume study

Mass settling of phytoplankton can result in the accumulation of a layer of phytodetritus on the sea floor. In order to determine the fate of this organic matter, we must know whether the phytodetritus may be returned to the water column by resuspension. This study was designed to measure the critical bed shear stress (t0crit) for resuspension of phytodetritus and to test whether t0crit differs (1) among types of phytodetritus with different phytoplankton species composition and (2) within type among ages. Phytodetritus was created from cultured Skeletonema costatum and from natural assemblages dominated by Chaetoceros spp., collected near the end of diatom blooms in Cape Cod Bay and Woods Hole, Massachusetts. Resuspension of phytodetritus decomposed 1to 11-d was measured in a laboratory flume. Overall, t0crit for the diatom-derived detritus ranged by a factor of four (0.018– 0.068 N m22), with significant differences in t0crit among types and ages of phytodetritus. Reasons for these differences are suggested, such as species-specific differences in size, shape, porosity, and stickiness of the detrital matrix, as well as changes in these parameters that occurred during decomposition. Values for critical shear velocity, which ranged from 0.4 to 0.6 cm s21 for Chaetoceros-derived detritus and from 0.5 to 0.8 cm s21 for Skeletonema-derived detritus, validated indirect estimates from field studies. An interesting finding with special relevance in lake and coastal systems was the importance of wave-forced resuspension, suggesting that coupling of settled phytodetritus and other sticky organic flocs to the water column is enhanced by waves. Phytoplankton, especially chain-forming planktonic diatoms, can settle en masse, yielding a pulse of organic matter to the sea floor. Ephemeral, fluffy, green carpets of phytodetritus may cover the sea bed in areas with low flow energy, including sandy beds at slack tide. These ‘‘fluff’’ layers have been observed in lakes (e.g., Carrick et al. 1993), shallow marine systems (e.g., Davies 1975; Williams et al. 1998), and the deep sea (e.g., Lampitt 1985; reviewed by Beaulieu 2002). Sedimentation of relatively short-lived phytoplankton blooms may result in a significant percentage of the annual carbon input to the benthos in some systems (e.g., Goedkoop and Johnson 1996; Keller et al. 1999). To evaluate the carbon budget in such systems, we must know the fate of phytodetritus on the sea floor. Local processes may occur such as microbial decomposition, consumption by benthic fauna, dissolution, or burial into the sediments. However, phytodetritus also may be resuspended and advected laterally, 1 Corresponding author ([email protected]).