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Abstract

This study focused on organic particles withrespect to their transport and sedimentarymineralisation in a North Sea intertidalsandflat previously characterised as stronglyinfluenced by advective transport across andbelow the water-sediment interface. Measuredpermeabilities of the sandy sediment rangedfrom 5.5 to 41⋅10−12 m2, andpermeabilities calculated from granulometricdata exceeded the measured values by a factorof 4.4 ∓ 2.8. Bacteria (2–9% of the POC)were highly variable in space and time. Theywere less mobile than interstitial fine (<70 µm) organic and inorganic particles, aspart of the population lived attached to large,heavy sand grains. The vertical distribution ofbacteria was closely related to the organiccarbon content of the fine-grained interstitialmaterial. In winter, bacterial numbers in theuppermost 5 cm amounted to 39–69% of thesummer ones. Carbon mineralisation rates rangedbetween 20 mg C m−2 d−1 in winter and580 mg C m−2 d−1 in summer, keepingstep with finer-grained sediments thatcontained an order of magnitude more organiccarbon. Sedimentary carbohydrates were mainlyintracellular or tightly bound to particles,and their concentrations were depth-invariantin winter, but exponentially decreasing withdepth in summer. Below 5 cm depth, the meanconcentration was (1590 ∓830) µg cm−3, without major downcoreor seasonal changes. Phytobenthos andphytodetritus were dominated by diatoms andcomprised merely minor amounts of other primaryproducers. Planktonic diatom depth profileswere related to weather and phytoplanktonconditions, and benthic diatoms showed similardepth distributions due to passive and activemotion. The penetration of relatively freshphytodetritus down to at least 5 cm, shown bychloropigment composition, emphasised the closecoupling between water column and sandysediment, facilitated by advective interfacialand subsurface flows.