Annular flume experiments and high-performance liquid chromatography (HPLC)
were used to quantitatively and qualitatively measure benthic-pelagic exch
ange of microphytobenthos from natural sediments. Clear spatial and tempora
l differences in. microphytobenthos resuspension in response to stepwise in
creases in current velocity were observed. Resuspension of chlorophyll a (c
hl a) from sandy sediments containing low levels of microphytobenthos bioma
ss (<11.5 mg chl a m(-2)) occurred gradually and continuously over a range
of current velocities from 10 to 40 cm s(-1). In June, well-developed diato
m mats at siltier sites (>56 mg chl a m(-2)) displayed strong resistance to
erosion at currents <20 to 25 cm s(-1), above which there was a very rapid
increase in the amount of chi a in suspension following the stripping of t
he algal mat from the sediment surface. In September, when the diatom bloom
was over, these sediments were less resistant to erosion and resuspension
of microalgae occurred at current velocities above 15 to 20 cm(-1). Site 1,
situated at the edge of the flat, had a dense algal mat but low sediment s
tability. Microalgae were readily resuspended because the extracellular pol
ymeric substances (EPS) produced by the migratory diatoms were unable to co
nsolidate during the short emersion period. As a rule more chi a was resusp
ended from sandy sediments at current velocities 15 and 20 cm(-1), but abov
e this current velocity chi a resuspension was greater from silty sediments
. Although sandy sites have low biomass in the surface layer, the greater d
epth of sediment erosion during bedload transport exposes more chl a to the
surface. The percentage of sedimentary chl a lost at selected current velo
cities was estimated, and the implications for carbon supply to the pelagic
and benthic systems discussed. Much of the biomass resuspended may be depo
sited locally, particularly in sandier regions. During the flume experiment
s it was observed that settling of fine sediment and microalgae was extreme
ly rapid, because it was being biodeposited by suspension-feeding activity,
and 'stripped' out of the water column by rapidly sinking suspended partic
ulate matter (SPM). Qualitative changes in suspended material were measured
as % chl a (chl a/phaeopigments x 100%) and accessory pigment content. As
current velocity increased the relative proportion of phaeopigments increas
ed, which has important implications for benthic suspension feeders feeding
in the benthic boundary layer. The findings from the flume experiments hav
e been compared with in situ measurements of current velocity, SPM and chl
a.