Degradation And Decomposition Of The Aggregates

11.5.1 Bacteria

The breakdown of the generally strongly degraded organic matter deposited on deep-sea sediments is mainly accomplished by bacteria. The rates of degradation depend largely on the proportion of biologically labile material which decreases with advancing decay.7 Despite the possible protection mechanisms, like bacterial community pressure inhibition and sorption to mineral surfaces,11 if the net vertical and downslope transport is too slow, it is likely that mainly refractory organic matter will reach the deep ocean floor. Once the aggregates enter the benthic boundary layer, their fate is to a large extent controlled by the benthic flora and fauna which play a major role in determining their geochemical behavior. The reworking of the aggregates may further inhibit the degradation of organic matter, since the sorption of organic matter to the larger amount of lithogenic material in these aggregates may provide some degree of protection against microbial activity.11 However, the resuspension of the particles can also enhance their remineralization. Ritzrau showed that microbial activities and concentrations of various parameters (particulate organic carbon, Chlorophyll a, utilization of 14C-amino acids) displayed distinct distribution patterns in the BBL and were up to a factor of 7.5 higher than in the adjacent water column and concluded that turbulence increases the microbial activity in the benthic boundary layer.35 For BBL aggregates, Lind et al. presented a comparison between phytoplankton and bac-terioplankton production36 with each modified by high concentrations of suspended clays. High clay turbidity caused light-limitation of water column phytoplankton production. However, the clay combined with DOC to form aggregates which supported bacterioplankton production. Leipe et al. collected particles from the water column, the bottom nepheloid layer, and the "fluffy layer" in the Baltic Sea and revealed that suspended particulate matter (SPM) in the bottom nepheloid layer and the "fluffy layer" overlying sediments was enriched in organic carbon and clay minerals, whereas the nonaggregated SPM was dominated by quartz and biogenic opal.37 It appeared that separation effects operate during aggregation of mineral particles and organic matter in repeated cycles of resuspension and settling. No clear seasonal variations in the composition of the SPM were found, in spite of high spatial and temporal variability of biological and physical variables. Their results suggest that preferential incorporation, possibly aided by microbiological colonization, of silicates into the organic flocs is a process that occurs under a wide range of conditions.

11.5.2 Fauna

In their classic study on the effects of benthos on sediment transport, Jumars and Nowell summarize that no consistent functional grouping of organisms as stabilizers vs. destabilizers, respectively decreasing or enhancing erodibility, is possible.38 Benthic organisms can affect erodibility in particular — and sediment transport in general — via alternation of (1) fluid momentum impinging on the bed, (2) particle exposure to the flow, (3) adhesion between particles, and (4) particle momentum. The net effects of a species or individual on erosion and deposition thresholds or on transport rates are not generally predictable from extant data. Furthermore, they depend upon the context of flow conditions, bed configuration, and community composition into which the organism is set. Suspension-feeding fauna actively remove the aggregates from the water column and deposit it as faeces either within or on top of the sediment, a process called biodeposition.39 Feeding pits, faecal pellet mounds, and tube-structures of the benthos locally can change the current regime and cause resuspension and passive biodeposition of particles.38,40 Bioturbation due to moving animals or due to bulk feeding by deposit feeders substantially modifies the physical and geochemical properties of the aggregates.41,42 Muschenheim and Milligan studied BBL characteristics in the Bay of Fundy and summarized that seston concentration and composition were found to vary greatly throughout the course of a tidal cycle, with periodic dilution of the organic content due to resuspended sand.43 Examination of the particle size distributions suggests that flocculation plays a major role in packaging the material ingested by these benthic communities.

Heip et al. summarize that at continental margins the overall metabolism in shelf and upper slope sediments is dominated by the macrofauna, which are responsible for 50% of the organic aggregate mineralization.44 At the lower slope and abyssal depth microbiota dominate in terms of total biomass (>90%) and organic matter respiration (about 80%). Because large animals have a lower share in total metabolism, mixing of the aggregates within the sediments is reduced by a factor of 5, whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf. The lability of the organic aggregates in the sediments at the upper slope and shelf is significantly higher than in sediments in the deeper parts. The residence time of mineralizable carbon which is mainly transported in form of organic rich aggregates is about 120 d on the shelf and more than 3000 d at the lower slope. These conclusions for the lower slope and deep sea are supported by studies of Smith et al.45 They carried out an important experiment on the biological reaction of incoming seasonal pulses of particulate matter in the open Pacific (4100 m depth, 220 km west of the central California coast) and hypothesized that the incoming aggregates would create localized regions of intense biological activity on the sea floor. However oxygen consumption of organic aggregates was similar to that of the background sediment and had no measurable influence on the chemical composition of the underlying sediment on time scales from 23 to 223 d or on sediment oxygen consumption after 222 d. The aggregates produced a minimal impact on sediment mineralization rates. The results are supporting the ideas of a fast benthic pelagic coupling, where the labile organic aggregates are rapidly consumed and elevated values of benthic activities are reduced to background values after a period of 2-3 weeks. There are, however, some areas in the oceans where the transport of aggregates can be enhanced: the submarine canyons.

Submarine canyons are areas where potentially the residence time can be short and net transport fast enough to supply the lower slope with labile material.24 Recent studies point to the existence of a fast and continuous downward sediment transport along the axis of the canyon, independently of the current regime operating on the shelf. Aggregates being transported down a canyon would be subjected to aggregation and disaggregation cycles in the benthic boundary layer, but also to a continuous variation of the pressure to which they are subjected. So, it is possible that the organic matter, present in aggregates transported down a canyon, might be partially preserved due to both mineral particle sorption and increasing hydrostatic pressure.

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