Particle flux and properties affecting the fate of bacterial productivity in the benthic boundary layer at a mud-bottom site in South-Central Gulf of Riga

At a deep-water station in South-Central Gulf of Riga (Stn. 119), the use of a multitrap for high time-resolution in the benthic boundary layer (BBL) enabled a refinement of the analytical endmember approach of Gasith [Gasith, A., 1975. Tripton sedimentation in eutrophic lakes — simple correction for the resuspended matter. Verh. Internat. Verein. Limnol., 19, pp. 116–122.] by which vertical flux emanating from surface waters may be distinguished from resuspension flux. As a consequence, the quantity of both slow- and fast-falling particles could be resolved also in bottom-water samples. Among events recorded during three intensive field studies were (1) a surface water export event in early May 1995, whereby net vertical flux increased from around 1 to 2–5 g m−2 d−1; (2) friction velocities at 1.4 cm s−1 causing a marked nepheloid layer, strained oxygenation and a characteristic high bacteria/chlorophyll ratio; (3) strong current-induced resuspension as the immediate response of a low-pressure passage around mid-summer 1994; and (4) a distinctly green and fresh pulse of phytoplankton arriving in less than 12 h in late August 1993. Results confirm previous observations of (1) strong benthic–pelagic coupling subject to event-controlled pulses of surface water export; furthermore, (2) the confined nature of the nepheloid layer with a bimodal falling-velocity distribution showing on the one hand rapidly falling (at 10 m d−1) aggregates and on the other hand slower-falling (at <1 m d–1) particles, the latter with a lower nitrogen content but both varying in concentration and presumably interacting through scavenging and fluff layer disaggregation; (3) how weather-controlled bottom currents regularly and easily entrain organic matter from the sediment–water interface. They also (4) suggest microbial activity in the BBL to be more associated with the slow-falling particles. Findings seem reconcilable with the notion of microbial benefits from staying suspended in the BBL rather than inside aggregates and the fluff layer.