Stefan Forster

Advective Transport Affecting Metal and Nutrient Distributions and Interfacial Fluxes in Permeable Sediments

Abstract Our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments, leading to specific and reproducible complex patterns of Fe, Mn, and nutrients. Oxygenated water, forced into the sediment when boundary flows were deflected by protruding sediment structures, generated distinct zones of nitrification and ferric iron precipitation. This inflow was balanced by ammonium-rich porewater ascending from deeper sediment layers, thereby creating an anoxic channel where dissolved Fe2+ and Mn2+ could reach the surface. Between the zones of ferric iron precipitation and Fe2+ upwelling, a layer with increased manganese oxide and solid phase Fe(II) concentrations formed, indicating redox reaction between these components. The establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes. While the smooth control core was found to be a sink for metals and nutrients, the sediment with mounds acted as a source for these substances. Our experiments show that in sandy sediment with an oxidised surface layer, reduced metal species can be released to the water column by flow-topography interactions. We conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds.

Impact of irrigation on oxygen flux into the sediment: intermittent pumping by Callianassa subterranea and “piston-pumping” by Lanice conchilega

O2-flux into sediments attributed to the pumping behaviour of two macrofauna species, Callianassa subterranea (Decapoda) and Lanice conchilega (Polychaeta) was investigated. Samples were obtained from the North Sea near Helgoland in 1989 and 1990. The two species were found to transport roughly similar amounts (3 mmolm-2d-1) of oxygen into the sediment although they displayed markedly different pumping behaviours. Irrigation by C. subterranea was intermittent and characterized by regularly recurring breathing currents which lasted 2.6 min and were separated by 40-min pauses. In addition to this regular intermittent irrigation, an irregular mode was observed. C. subterranea constructed a complex burrow system. At least half of the burrow wall was not in contact with oxygenated water, however, and thus not effective as additional interface for O2-exchange. Sediment expelled from the burrow increased the total oxygen uptake (TOU) relative to the surrounding sediment surface. L. conchilega moved water much more frequently (every 4 min) than C. subterranea. We suggest that L. conchilega acted as a piston when moving in its tube, exchanging burrow water with the overlying water. This mechanism, termed ‘piston-pumping’, is also potentially important in other smaller tube dwelling organisms. At a shallow water station in the southern North Sea 21 ind of C. subterranea constructed 1.6 m2 burrow surface per m2. L. conchilega (300 ind m-2) created only 0.37 m2m-2 tube surface. On the basis of the abundance and oxygen transport associated with pumping activity, it is calculated that the two species increase TOU by 85% compared to O2-flux across the sediment-water interface.

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

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.