Bacteria, diatoms and detritus in an intertidal sandflat subject to advective transport across the water-sediment interface

This study focused on organic particles with respect to their transport and sedimentary mineralisation in a North Sea intertidal sandflat previously c haracterised as strongly influenced by advective transport across and below the water-sediment interface. Measured permeabilities of the sandy sedimen t ranged from 5.5 to 41.10(-12) m(2), and permeabilities calculated from gr anulometric data exceeded the measured values by a factor of 4.4 +/- 2.8. B acteria (2-9% of the POC) were highly variable in space and time. They were less mobile than interstitial fine (< 70 mum) organic and inorganic partic les, as part of the population lived attached to large, heavy sand grains. The vertical distribution of bacteria was closely related to the organic ca rbon content of the fine-grained interstitial material. In winter, bacteria l numbers in the uppermost 5 cm amounted to 39-69% of the summer ones. Carb on mineralisation rates ranged between 20 mg C m(-2) d(-1) in winter and 58 0 mg C m(-2) d(-1) in summer, keeping step with finer-grained sediments tha t contained an order of magnitude more organic carbon. Sedimentary carbohyd rates were mainly intracellular or tightly bound to particles, and their co ncentrations were depth-invariant in winter, but exponentially decreasing w ith depth in summer. Below 5 cm depth, the mean concentration was (1590 +/- 830) mug cm(-3), without major downcore or seasonal changes. Phytobenthos and phytodetritus were dominated by diatoms and comprised merely minor amou nts of other primary producers. Planktonic diatom depth profiles were relat ed to weather and phytoplankton conditions, and benthic diatoms showed simi lar depth distributions due to passive and active motion. The penetration o f relatively fresh phytodetritus down to at least 5 cm, shown by chloropigm ent composition, emphasised the close coupling between water column and san dy sediment, facilitated by advective interfacial and subsurface flows.