Ship-board flume experiments at the Oyster Grounds (central North Sea) showed that resuspending cohesive sediments releases far more dissolved nutrients — especially dissolved silicon (DSi, up to +125%) — than pore-water injection alone can explain (pore water contributed only about 6% of the observed DSi increase at the largest erosion step). The authors left open three linked questions: how bottom-water oxygen, organic carbon, and temperature interact seasonally to control the magnitude and chemical form of the release; how repeated or prolonged resuspension events affect ambient water-column nutrient concentrations and the rate at which pore-water and fluff-layer nutrient reservoirs are replenished between events; and which mechanisms (biogenic opal dissolution, bacterial ectohydrolase activity, adsorption/desorption) produce the large non-pore-water release fraction, particularly for DSi. Later work has advanced neighboring aspects — seasonal erodability of UK shelf sediments, redox and adsorption controls in Baltic and estuarine settings, disturbance-driven release at trawled and intertidal sites — but the mechanisms of resuspension-driven DSi release, the seasonal oxygen-carbon-temperature interaction at a cohesive shelf site, and the replenishment timescales of the pore-water and fluff reservoirs remain unresolved.

Background

The exchange of dissolved nutrients across the sediment–water interface (SWI) — the boundary between the seabed and the overlying water — is a major component of benthic–pelagic coupling, the two-way exchange of matter and energy between the seafloor and the water column that supplies nutrients fueling shelf-sea primary production. At the Oyster Grounds, a ~40 m deep cohesive-sediment (muddy) site in the central North Sea, the bottom water warms from about 7.6 °C in spring to 17.0 °C in late summer while dissolved oxygen drops from about 9.5 to 6.2 mg/L, and the summer water column is density-stratified so the bottom layer is isolated from the atmosphere; summer oxygen depletion there is a recurring feature driven by this stratification combined with benthic oxygen demand (Queste et al. 2016).

Nutrients cross the SWI by molecular diffusion out of sediment pore water (the fluid filling the spaces between sediment grains), by faunal activity (bioturbation, particle reworking, and bioirrigation, the pumping of water through burrows), and by resuspension: when currents or storms exceed a critical threshold of bed shear stress — the friction that flowing water exerts on the seabed, commonly expressed through the friction velocity U∗U^{*} acting on the bed — first the low-density "fluff" layer of recently settled phytodetritus is mobilized, and then the consolidated bed itself erodes. Tidal currents resuspend benthic fluff even in deep stratified waters of the northern North Sea (Jago et al. 2002), and since the proposal of Fanning et al. (1982), resuspension has been discussed as a potential nutrient-recycling mechanism on ocean margins. Because marine ecosystem models of the North Sea (for example ERSEM, the European Regional Seas Ecosystem Model) conventionally represent benthic nutrient supply only as diffusive flux, quantifying the resuspension pathway matters for predicting nutrient availability before and during phytoplankton blooms.

Couceiro et al. (2013) measured diffusive fluxes from intact cores in April and August 2008 (for nitrate, nitrite, phosphate, and dissolved silicon — DSi, the silicic acid H4SiO4\mathrm{H_4SiO_4} released when biogenic opal, the amorphous silica of diatom frustules, dissolves) and then eroded intact box cores in a ship-board annular flume — a ring-shaped channel in which rotating lids drive a controlled current over an intact sediment core — increasing bed shear in steps while continuously sampling the water column. Fluff resuspension caused small but significant nitrate and phosphate releases; bed erosion raised DSi by up to 125%, nitrate by up to 67%, and phosphate by up to 66%. A mass balance combining measured suspended particulate matter (SPM), porosity, computed-tomography (CT) derived sediment density, and pore-water nutrient profiles showed that direct injection of pore water accounts for only a small fraction of the observed increase (about 6.3% for DSi at the highest erosion step). Prior work had posed the same puzzle: in Gulf of Finland resuspension experiments (Almroth et al. 2009) and Boston Harbor flume experiments (Kalnejais et al. 2010), dissolved nutrient release exceeded the pore-water contribution, with ion exchange and desorption invoked as candidate explanations. Candidate mechanisms for the rapid (minutes-to-hours) release include accelerated dissolution of biogenic opal on newly exposed particle surfaces; bacterial ectohydrolases — extracellular hydrolytic enzymes by which bacteria degrade the organic coating of diatom detritus and thereby expose opal to dissolution (Bidle & Azam 2001); and adsorption/desorption equilibria on resuspended particles, which can either release or scavenge nutrients depending on redox conditions and surface chemistry. Couceiro et al. could not partition these contributions, measured only two seasons at single near-instantaneous conditions, and observed only single resuspension events, and they identified exactly these gaps as the unresolved issues of their study.

Problem Statement

Provide a quantitative, mechanistic, and seasonally resolved account of resuspension-driven nutrient release from cohesive shelf-sea sediments, of the kind left open by the Oyster Grounds (North Sea) flume study of Couceiro et al. (2013). The common objective is a predictive understanding of how much nutrient (nitrate, nitrite, phosphate, dissolved silicon) is released to the water column when a cohesive shelf bed is resuspended, which processes produce it, and how the reservoir recovers — so that resuspension can be represented alongside diffusive fluxes in shelf-sea ecosystem models. Within that objective, three linked questions must be answered:

  1. Mechanism partitioning: identify and quantify, on the minutes-to-hours timescale of a resuspension event, the relative contributions of pore-water injection, biogenic opal dissolution, bacterial ectohydrolase-mediated degradation of particulate organic and opal phases, and adsorption/desorption reactions to the observed dissolved-nutrient increase — in particular for the large dissolved-silicon (DSi) release that pore-water injection alone (≈6.3% of the observed increase at the highest erosion step) cannot explain, and under both fluff-layer and consolidated-bed erosion.
  2. Seasonal controls: determine how the interaction of bottom-water oxygen concentration, sediment organic-carbon availability, and temperature controls the magnitude and chemical speciation of nutrient release, resolving the seasonal dependence beyond the single spring and single late-summer snapshot available in the source study.
  3. Reservoir dynamics: quantify the long-term effects of repeated or prolonged resuspension on ambient water-column nutrient concentrations, and the rates at which pore-water and fluff-layer nutrient reservoirs are replenished (re-equilibrate) after nutrient loss during successive resuspension events.

The setting of interest is cohesive shelf-sea sediments comparable to the Oyster Grounds; the three components together support the single objective of a mechanistically based, seasonally varying parameterization of resuspension-driven benthic nutrient supply.

References

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