“L1615_C011” — 2004/11/20 — 14:11 — page 237 — #1 11 Organic Rich Aggregates in the Ocean: Formation, Transport Behavior, and Biochemical Composition Laurenz Thomsen CONTENTS 11.1 Introduction 237 11.2 Formation of Organic Rich Aggregates 238 11.3 The Descent through the Water Column 239 11.4 Transport within the Benthic Boundary Layer: The Resuspension Loop 241 11.5 Degradation and Decomposition of the Aggregates 243 11.5.1 Bacteria 243 11.5.2 Fauna 244 11.6 Conclusions 245 References 246 11.1 INTRODUCTION The biogeochemical significance of organic rich aggregates (marine snow) in the vertical flux of organic matter into the oceans’ interior and sea floor is widely acknowledged. 1,2 The aggregates, which form during phytoplankton blooms and, to a lesser extent, by the resuspension of benthic biofilms, are a primary source of mar- ine snow. 3 A considerable part of the aquatic primary production is removed from the surface through processes of particle aggregation and sedimentation. 4–6 These aggregates are the most important components of the organic matter flux to the deep sea 7 and appear to be hotspots of heterotrophic activity in the water column, being an important carbon source for free-living bacteria throughout their descent. 8 After sedimentation and during an extended period of resuspension loops, almost all of the remaining carbon is then remineralized. Nevertheless, a part of this organic matter is too refractory to be recycled, thus becoming buried in ocean sediments, sequester- ing carbon and so influencing atmospheric carbon dioxide concentrations. 9,10 This chapter will concentrate on the fate of organic aggregates in the size range of tens to 1-56670-615-7/05/$0.00+$1.50 © 2005byCRCPress 237 Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 238 — #2 238 Flocculation in Natural and Engineered Environmental Systems thousands of micrometers, their production and descent through the water column as well as theirresidence and further modification within the benthic boundary layer. The term “benthic boundary layer” (BBL) is used for the water layers above the sediments although in sedimentological/physical-oceanography terminology “bottom boundary layer” would be the right phrase to use. Most examples will be given from continental margin studies. Continental margins can be defined as the region between the upper limit of the tidal range and the base of the continental slope. The burial of aggregate- associated organic matter in continental margin sediments is directly linked to the global cycles of carbon over geologic time. 11 Although continental margins account only for ≈15% of total ocean area and 25% of total ocean primary production, today more than 90% of all organic carbon burial occurs in sediments built up by particle deposition on continental shelves, slopes, and in deltas. 12 In this whole chapter, useful new references are predominantly cited which lead the interested reader to important previous work on the topic. 11.2 FORMATION OF ORGANIC RICH AGGREGATES In their review of the microbial ecology of organic aggregates, Simon et al. gave an overview of the present knowledge of macroscopic organic aggregates (>500 µm). 5 These macroaggregates are heavily colonized by bacteria and other heterotrophic microbes and greatly enriched in organic and inorganic nutrients as compared to the surrounding water. The authors point out that during the last 15 years, many studies have been carried out to examine the various aspects of the formation of aggregates, their microbial colonization and decomposition, nutrient recycling, and their signific- ance for the sinking flux. The significance of aggregate-associatedmicrobialprocesses as key processes and also for the overall decomposition and flux of organic matter var- ies greatly among limnetic and oceanic systems, and is affected by the total amount of suspended particulate matter. A conclusion from these studies is that the significance of bacteria for the formation and decomposition of aggregates appears to be much greater than previously estimated. For a better understanding of the functioning of aquatic ecosystems it is of great importance to include aggregate-associated processes in ecosystem modeling approaches. Knoll et al. studied the early formation and bac- terial colonization of diatom microaggregates (<150 µm) during the phytoplankton spring bloom and showed that these are colonized by bacterial populations that differ from those in the surrounding water. 13 They conclude that the bacterial community on aggregates develops largely from seeds on their precursor microaggregates. Theoretical analyses of particle coagulation processes predict that aggregate formation depends on the probability of particle collision and on the efficiency with which two particles that collide stick together afterwards (stickiness). 14,15 The former is a function of particle concentration, size, and the mechanism by which particles are brought into contact, for example, Brownian motion, shear or the differential settlement of particles. The latter depends mainly on the physicochemical properties of the particle surface and may vary with the particle type. Particle collision does not necessarily result in aggregation, as the stickiness or sticking efficiency is often only 10% or less but can increase up to 60% depending on the particle type involved. 4 Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 239 — #3 Organic Rich Aggregates in the Ocean 239 Depending on its intensity, shear can either increase particle collision or increase particle destruction. This is particularly the case at the base of the surface mixed layer, where internal waves, wind driven shear and tidal shear are pronounced; and within the benthic boundary layer where turbulence is increased again. In surface waters, changes in particle coagulation efficiency have been attributed to the abundance of single species or as part of the life cycle strategy of cells. 16,17 The occurrence of aggregates does not, for example, always coincide with the peak of phytoplankton abundance. Rather, it is often postponed toward the decline of the bloom. 18 This has been hypothesized to be due to an increase in particle stickiness. 19 A decade ago, a special class of particles was found to be readily abundant during phytoplankton blooms in water and in aggregates as well. These gels, called trans- parent exopolymer particles (TEP), 20 are thought to play a central role in coagulation processes. Laboratory experimentshave demonstrated that diatoms produce more gels under nutrient limitation, although little is known about how limitation by different nutrients affects the quantity and composition of the gels and subsequent stickiness. Because of the great abundance in shelf seas and in the open ocean and because of the stickiness of TEP, the probability of particle collisions is enhanced. 21 Logan et al. proposed two hypotheses 22 to account for the precipitous formation of large, rapidly settling aggregates at the termination of phytoplankton blooms in nature: aggregation due primarily to cell–cell collisions, and aggregation resulting from the presence of TEP. By comparing TEP and phytoplankton half-lives in these systems, it is concluded that the formation of rapidly sinking aggregates following blooms of mucous-producing diatoms is primarily controlled by concentrations of TEP, not phytoplankton. 22 Engel conducted measurements of diatom species composition, TEP,bulk particle abundance, as well as chemical and biological variables in order to reveal the determ- inants of coagulation efficiency. 19 The investigation showed that an increase in TEP concentration relative to conventional particles at the decline of the bloom signi- ficantly enhanced apparent coagulation efficiencies. High proportions of TEP led to apparent values of stickiness of 1, which indicates that collision rates can be substantially underestimated when the stickiness parameter alpha is calculated on the basis of conventional particle counting only, for example, with the Coulter Counter. 11.3 THE DESCENT THROUGH THE WATER COLUMN The physical and biological properties of the aggregates determine their transport behavior in the water column. The excess density over that of the surrounding water controls the speed with which aggregates descend to the sea floor. For particles with Reynolds numbers <1, Stokes’ law can be applied to determine the settling velocity: W S = d 2 (ρ)g 18ν Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 240 — #4 240 Flocculation in Natural and Engineered Environmental Systems where d is the particle diameter, ρ is the excess density of the particle over seawater, g is the gravitational constant and ν is the kinematic viscosity of the fluid. The kinematic viscosity is strongly temperature dependent and has an enormous influence on the behavior of particles of low Reynolds numbers. ν nearly doubles from warm surface waters (0.01 cm 2 sec −1 at 20 ◦ C) to cold bottom waters (0.018 cm 2 sec −1 at 1 ◦ C). As Stokes law was originally applied to rigid, impermeable spherical particles of known density, it is difficult to apply to nonspherical aggregated particles which virtually represent most particulate material in the ocean. However, Stokes can then be used to back-calculate the particle density of the aggregates as discussed by. 14 During the last two decades empirical particle-size/settling-velocity relationships have been developed for different oceanic regimes (Figure 11.1). The data reveal that organic rich aggregates from surface waters at continental margins show much lower settling velocities than those of similar size but enriched in ballast. This lithogenic ballast is added to the organic aggregates during resuspension events. The surface mixed layer at the top of the ocean varies in thickness from tens to hundreds of meters and aggregate concentrations inside this layer are related to the processes of production, destruction, and sinking. Peak concentrations are often located at the base of the surface mixed layer, which can extend up to a few hundred meters during winter. This layer is subject to rapid changes in heat, turbulence, nutri- ents, and depth of mixing. The peak concentrations at the base of the surface mixed 0.01 0.1 1 B 0.001 0.01 0.1 1 Diameter (cm) A BBL aggregates D Surface water aggregates C Settling velocity (cm s –1 ) E Aggregate size and settling velocity FIGURE 11.1 Particle settling velocities/particle diameter relationships of aggregates from ocean surface waters (A), intermediate and bottom nepheloid layers (B), and from the benthic boundary layer (C, D, E) determined by Alldredge and Gottschalk 4 for A, McCave 14 for B, Thomsen et al. 23 for C (Celtic margin), Thomsen et al. 24 for D (Iberian margin), and Sternberg et al. 25 for E (North East Pacific margin). Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 241 — #5 Organic Rich Aggregates in the Ocean 241 layer mainly coincide with the occurrence of pycnoclines, where rapid changes in seawater-density (and thus excess-density of aggregates) can reduce or even stop the vertical flux of the aggregates. The physical forcing in the mixed layer creates changes in the biological processes, which depend on them. Here, organic rich aggregates are formed which are derived from gelatinous housing of zooplankton species, mucous feeding webs used by others, faecal material, and from phytoplankton cells and their component particles. 26 The proportion of free waterwithin the aggregates, its porosity, determines how fast the internal environment changes in response to varying external conditions; the porosity also influences the rate at which small particles accumulate on the aggregates. After their slow descend through the pycnocline, differential set- tling is mainly responsible for additional aggregate formation as well as the migrating zooplankton, which consume the aggregates to depth of up to 1000 m. 11.4 TRANSPORT WITHIN THE BENTHIC BOUNDARY LAYER: THE RESUSPENSION LOOP Once on the sea floor, the aggregates are more easily remobilized into the benthic boundary layer than the bulk sediments beneath, 23 and are resuspended back into the water column, being again subjected to aggregation and disaggregation processes. 27 Long-term studies at different continental margins revealed that the bottom sedi- ments consist of a thin surface layer of organic rich aggregates (mean diameter 100 to 2500 µm). These resuspend under critical shear velocities [u ∗ c ] of 0.4 to 1.2 cm sec −1 (mean u ∗ c of 0.8 ±0.1 cm sec −1 ) and have median diameters of 140 to 450 µm and settling velocities of 0.05 to 0.35 cm sec −1 (Figure 11.1). The aggregates consist of up to 75% of organic matter, which is mostly refractory with a carbon/nitrogen ratio exceeding 10, and the lithogenic material is embedded in the amorphous matrix of the organic matter. The BBL aggregates contain remnants of faecal pellets, meiofauna organisms, and shell debris of foraminifera. Approximately 35 to 65% of the bacteria of the BBL are particle attached and live within the organic matrix of the aggregates and approximately 1% of the organic fraction is labile bacterial organic carbon. 23,24 The BBL aggregates in >100 µm size range are resuspended under similar flow conditions as particles of similar size but higher density (sand). However, they are transported over much greater distances due to their lower density and porous struc- ture, which reduces their settling velocity (Figure 11.2, compare Figure 11.1). These aggregates can subsequently be transported in tide-related resuspension–deposition loops over long distances. Table 11.1 summarizes typical particle characteristics from continental margin BBLs. A cohesion effect for the aggregates is visible at about 30 µm (Figure 11.2). Thus, organomineral aggregates with average sizes <30 µm behave in the same way as clay (<2 µm), and particles coarser than 30 µm should display size sorting behaviour. The last result is different from the calculations of McCave et al., who propose 10 µm as threshold between noncohesive and cohesive sediment behavior. 31 This difference seems due to particle stabilization from micro- bial exudates. 32 The erosion threshold data were mainly obtained in summer, when biological activity in surface sediments at the study site is high. 33 Evidence for the importance of biological adhesion on critical stress for incipient transport has been Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 242 — #6 242 Flocculation in Natural and Engineered Environmental Systems 1 10 100 1000 fine coarse Silt Particle size d 50 (m) Miller et al.,(1977) MKK data limits 30 0.01 0.1 1 BBL aggregates Beds hear stress  0 (N/m 2 ) Celtic sea, Rockall Iberian margin FIGURE 11.2 Critical bed shear stress for erosion of continental margin sediments showing the onset of a cohesion effect at about 30 µm. The black curve represents the modified Shields curve after Unsöld. 28 The dashed lines refer to the limits of available high quality data evaluated by Miller et al. 29 and Self et al. 30 TABLE 11.1 Typical Flow and Particle Characteristics from Continental Margin BBLs u (cm sec −1 ) 2–50 u ∗ c (cm sec −1 ) 0.1–2 τ c (Nm −2 ) 0.01–0.4 Total particulate matter (gm −3 ) 0.1–8 Particulate organic carbon (mg m −3 ) 10–150 Chlorophyll equivalents (mg m −3 ) 0.01–0.3 BBL aggregate number (nm −3 )(10–1500) × 10 3 BBL aggregate diameter d 50 140–2400 Note: u =mean velocity, u ∗ c =critical shear velocity, τ c =critical bed shear stress, d 50 =median aggregate diameter (µm). demonstrated by various authors (e.g., ref. [32]) who showed that microbial exudates could increase the critical bed stress by a factor up to 5. During times of enhanced flow conditions, aggregates are formed and compacted by shear, which accounts for the fact that they do not disaggregate when they enter the viscous sublayer at mid- and lower slope sediments. The formation of these BBL aggregates and the minerals incorporated into the particles might be responsible for the organic matter preservation on continental slopes. Statistical analyses of Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 243 — #7 Organic Rich Aggregates in the Ocean 243 TABLE 11.2 Model Particle Parameters d (µm)ρ(gcm −3 ) W s (cm sec −1 ) u ∗ c (cm s −1 )τ 0 (Nm −2 ) ≈u 100 u ∗ li τ 0 (Nm −2 ) ≈u 100 r 2 = 0.8; n = 191 r 2 = 0.7; r 2 = 0.9; r 2 = 0.8; n = 31 n = 15 ∗ n = 6 50 0.139 0.013 0.98 0.10 16 0.7 0.050 11 100 0.137 0.050 0.90 0.084 14 0.9 0.083 15 185 0.091 0.114 0.89 0.082 14 0.8 0.066 14 200 0.086 0.125 0.89 0.081 14 0.8 0.066 14 400 0.047 0.275 0.86 0.076 13 0.6 0.038 10 500 0.038 0.350 0.84 0.073 12 0.5 0.026 8 1000 5.90 ×10 −3 0.215 0.76 0.060 11 2000 2.07 ×10 −3 0.303 0.61 0.038 10 0.5 0.026 8 4000 8.17 ×10 −4 0.477 0.50 0.026 8 Note: d =aggregate diameter, ∗ ρ =excess density with fluid density taken as 1.028g cm −3 5 ◦ C, salinity =36, w s =settling velocity, u ∗ c =critical shear velocity, τ 0 =bed shear stress, u 100 =flow velocity at 100 cm a.b. calculated after Middleton and Southard (1984) with z 0 taken as 0.1 cm, u ∗li =critical deposition velocity, n =15 ∗ =d 50 data from 15 stations with 50 to 200 single datapoints. the available BBL flow and particle and biogeochemical data of the Iberian margin (15 stations with u ∗ c data, 3 stations with u ∗ li data, 191 experimental w s measurements, 31 in situ w s measurements, ≈1200 aggregates analyzed), were used to determ- ine the basic parameters for a simple particle transport model (Table 11.2). For a particle size spectrum from 50 to 4000 µm in diameter, estimated excess densities, critical shear velocities, and critical deposition velocities decreased with increas- ing particle size, while the settling velocity increased over the same particle size spectrum. 24 11.5 DEGRADATION AND DECOMPOSITION OF THE AGGREGATES 11.5.1 B ACTERIA 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 advan- cing decay. 7 Despite the possible protection mechanisms, like bacterial community pressure inhibition and sorption to mineral surfaces, 11 if the net verticaland 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 Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 244 — #8 244 Flocculation in Natural and Engineered Environmental Systems 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, util- ization of 14 C-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 production 36 with each modified by high concentrations of suspended clays. High clay turbidity caused light-limitation of water column phytoplankton pro- duction. 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 stabil- izers 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 com- position 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 phys- ical and geochemical properties of the aggregates. 41,42 Muschenheim and Milligan studied BBL characteristics in the Bay of Fundy and summarized that seston con- centration 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. Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 245 — #9 Organic Rich Aggregates in the Ocean 245 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 trans- port 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. 11.6 CONCLUSIONS In conclusion there is now an increasing amount of information on the formation and transport behavior of organic rich aggregates but lack of knowledge on the com- positional changes over long time periods. The organic rich aggregates within the resuspension loop can still show a greenish color after several weeks or months but seem highly refractory and are thus exposed to low bacterial decomposition. Further studies are needed to investigate this effect of possible carbon protection within the BBL and the implication for long-term carbon storage in the ocean. Copyright 2005 by CRC Press “L1615_C011” — 2004/11/20 — 14:11 — page 246 — #10 246 Flocculation in Natural and Engineered Environmental Systems REFERENCES 1. 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Graf, G. and Rosenberg, R., Bioresuspension and Biodeposition—AReview.Journal of Marine