Bioirrigation

Abstract: The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn4+, NO3 -, Fe3+, SO4 2- and CO2. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.

Abstract: Advective flows rapidly transport water, solutes, and particles into and out of permeable sand beds and significantly affects the biogeochemistry of coastal environments. In this paper, we reviewed the drivers of porewater and groundwater advection in permeable shelf sediments in an attempt to bridge gaps among different disciplines studying similar problems. We identified the following driving forces: (1) terrestrial hydraulic gradients, (2) seasonal changes in the aquifer level on land moving the location of the subterranean estuary, (3) wave setup and tidal pumping, (4) water level differences across permeable barriers, (5) flow- and topography-induced pressure gradients, (6) wave pumping; (7) ripple and other bed form migration, (8) fluid shear, (9) density-driven convection, (10) bioirrigation and bioturbation, (11) gas bubble upwelling, and (12) sediment compaction. While these drivers occur over spatial scales ranging from mm to km, and temporal scales ranging from seconds to years, their ultimate biogeochemical implications are very similar (i.e., they are often a source of new or recycled nutrients to seawater and transform organic carbon into inorganic carbon). Drivers 2–12 result in no net water input into the ocean. Taking all these mechanisms into account, we conservatively estimate that a volume equivalent to that of the entire ocean is filtered by permeable sediments at time scales of about 3000 years. Quantifying the relative contribution of these drivers is essential to understand the contribution of sediments to the global cycles of matter.

Abstract: In aquatic ecosystems, invertebrate bioturbation significantly influences microbial activities and biogeochemical processes in sediments by modifying water and sediment fluxes at the water-sediment interface. We apply the concept of ecosystem engineering to develop a qualitative general understanding of the role of bioturbation on microbial processes in different benthic environments. We hypothesized that the effects of the bioturbation mode (sediment reworking, biogenic structure building, bioirrigation) on microbial processes vary between diffusion- and advection-dominated benthic environments because bioturbation does not have the same influences on hydrological exchanges (and the flux of resources for micro-organisms living in sediments) at the water-sediment interface of the two systems. To test this hypothesis, we experimentally compared the influence of three bioturbation modes (fine-sediment reworking, U-shaped structure burrowing, and gallery-network burrowing) in a diffusion-dominated system (fine sediments/low interstitial flow rates) and an advectiondominated system (coarse sediments/advection of water in sediments). Our analysis demonstrated that bioturbation modes in the two systems had different impacts on microbial activities. For instance, U-shaped tube burrowing by animals increased O2 consumption in the diffusion- dominated system but produced the opposite effect in the advection-dominated system. The influence of bioturbation was also negatively related to interstitial flow rate, the bioturbation having a higher influence on O2 consumption in the diffusion-dominated system than in the advection-dominated system. According to our hypothesis, bioturbation modified microbial processes in sediments depend on the hydrological characteristics of the system. In the diffusion-dominated system, invertebrate bioturbation can produce water fluxes at the watersediment interface that may strongly influence microbial processes in sediments. In contrast, in the advectiondominated system, invertebrate bioturbation can only modify the water circulation patterns in sediments, moderately affecting microbial processes. Consequently, it is necessary to use a conceptual framework which takes into account the features of sediment habitats in order to allow a better prediction of bioturbation effects on sediment biogeochemistry. With this aim, the conceptual scheme of ecosystem engineers can be an organizing principle to integrate the complex relationships among physical habitat, bioturbation mode and microbial activity.