Enhanced settling velocities and vertical transport of particulate matter by aggregation in the benthic boundary layer

Abstract

The importance of aggregation processes in the benthic boundary layer (BBL) with regards to settling velocities and flux of organic carbon was investigated at four stations on a coastal-depositional area transect in the southwest Baltic Sea. Water depths ranged between 15 and 46 m and stations were visited during eight cruises between October 1996 and December 1998. A specially designed bottom water sampler equipped with a particle camera system was used to investigate in situ aggregate characteristics: settling velocity, size, abundance. POC, PON, and chlorophyll-A contents. BBL characteristics including particulate matter concentrations, residence times, Kolmogoroff length scale, fluid flow, and shear stress were studied as well. Results showed no clear seasonal variation in either chlorophyll-A or particulate organic carbon (POC) content in the bottom water. The sediment surface was permanently covered with a layer of fluff material consisting of settled organic rich aggregates. Bottom shear stresses ranged between 0.001 and 0.10N m−2 whereas experimentally determined critical shear stress varied over a more narrow range (0.013–0.024 Nm−2). The calculated size of the turbulent eddies was always larger than the mean aggregate size so no physical fragmentation of aggregates occurred in the BBL. The mean settling velocity of aggregates (> 50 μm) was calculated to be between 0.06 0.3 cm s−1 and for suspended material (< 50 μm) 0.023 0.054 cm s−1. Accordingly, residence times in the BBL ranged between 1.4 and 5.6 hours for aggregates and between 7.7 and 18.1 hours for suspended material. A seasonal variation was detected for aggregates, which comprised up to 90% of total particulate matter in March 1997 but only 6% in October 1997. Material transported towards the sediment surface was enhanced by aggregation between 5948% (March 1997) and 18% (October 1997). The vertical flux of POC by aggregated material was calculated to be between 0.06–4.11 g Cm−2 d−1 and for the POC in suspension this figure was between 0.04–1.27 g Cm−2 d−1.