Alice L. Alldredge

Characteristics, dynamics and significance of marine snow

Abstract Macroscopic aggregates of detritus, living organisms and inorganic matter known as marine snow, have significance in the ocean both as unique, partially isolated microenvironments and as transport agents: much of surface-derived matter in the ocean fluxes to the ocean interior and the sea floor as marine snow. As microhabitats, marine snow aggregates contain enriched microbial communities and chemical gradients within which processes of photosynthesis, decomposition, and nutrient regeneration occur at highly elevated levels. Microbial communities associated with marine snow undergo complex successional changes on time scales of hours to days which significantly alter the chemical and biological properties of the particles. Marine snow can be produced either de novo by living plants and animals especially as mucus feeding webs of zooplankton, or by the biologically-enhanced physical aggregation of smaller particles. By the latter pathway, microaggregates, phytoplankton, fecal pellets, organic debris and clay-mineral particles collide by differential settlement or physical shear and adhere by the action of various, biologically-generated, organic compounds. Diatom flocculation is a poorly understood source of marine snow of potential global significance. Rates of snow production and breakdown are not known but are critical to predicting flux and to understanding biological community structure and transformations of matter and energy in the water column. The greatest challenge to the study of marine snow at present is the development of appropriate technology to measure abundances and characteristics of aggregates in situ .

The abundance and significance of a class of large, transparent organic particles in the ocean

Polysaccharide-specific staining techniques reveal the existence and high abundance of a class of large, discrete, transparent particles in seawater and diatom cultures formed from dissolved exopolymers exuded by phytoplankton and bacteria. Transparent exopolymer particles (TEP), ranged from 28 to 5000 particles ml−1 and 3 to 100s μm in longest dimension at five coastal stations off California. A high percentage of seemingly free-living bacteria (28–68%) were attached to these transparent sheets and films, suggesting that they may alter the distributions and microenvironments of marine microbes in nature. Preliminary coagulation experiments demonstrated that TEP are major agents in the aggregation of diatoms and in the formation of marine snow. The existence of microbial exudates acting as large, discrete particles, rather than as dissolved molecules or as coating on other particles, suggests that the transformation of dissolved organic matter into particulate form in the sea can occur via a rapid abiotic pathway as well as through conventional microbial uptake. The existence of these particles has far reaching implications for food web structure, microbial processes, carbon cycling and particulate flux in the ocean.

Can Microscale Chemical Patches Persist in the Sea? Microelectrode Study of Marine Snow, Fecal Pellets

Microelectrode studies demonstrate the existence of persistent oxygen and pH gradients around flocculent, macroscopic marine particles known as marine snow. Oxygen is partially, but continuously, depleted within and around marine snow in the dark and can be completely depleted within large fecal pellets. Boundary layers hundreds of micrometers thick are maintained despite advection of fluid past the particles. The existence of chemical microhabitats on the scale of millimeters around macroscopic particles in the pelagic zone may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.

The role of particulate carbohydrate exudates in the flocculation of diatom blooms

Diatom blooms are frequently terminated by mass aggregation of cells into large, rapidly sinking aggregates. It has been hypothesized that transparent exopolymer particles (TEP), abundant particles formed from the polysaccharides exuded by living cells, may be essential for this mass flocculation processes. We investigated the abundance of TEP and their role in the aggregation of diatoms in laboratory cultures and during a natural diatom bloom off California. TEP and dissolved carbohydrates accumulated appreciably over the growth cycle of Chaetoceros gracilis in the laboratory. The flocculation of C. gracilis in a laboratory flocculator was dominated by TEP, not cells, and large flocs, consisting predominantly of particulate polysaccharides, formed at a rate more than an order of magnitude higher than predicted by coagulation theory for cells alone. The frequency of interparticle attachment was three orders of magnitude higher for TEP than for cells. The pattern of flocculation of a natural diatom bloom was similar to that of laboratory cultures. Prior to bloom flocculation the abundance and total quantity of TEP and the concentration of particulate carbohydrates increased, while dissolved carbohydrate concentrations decreased. During the flocculation stage TEP aggregated into fewer, but much larger particles and concentrations of dissolved carbohydrates decreased further. The percentage of diatom cells which were attached to TEP increased during the flocculation period from 3 to 90% and TEP formed the matrix of all the natural diatom aggregates observed. During the late flocculation stage the quantity of TEP and TEP aggregates did not increase further and concentrations of diatoms decreased, presumably because large flocs sank out. Our findings indicate that TEP should be included in models of particle aggregation in the ocean. The abundance, large size and high sticking coefficient of TEP make them essential to the aggregation of diatom blooms. The extracellular release of polysaccharides by growing cells may be an adaptation for aggregation. The abiotic formation of particulate organic matter (TEP) from dissolved organic matter (DOC) may help to explain the extremely high turnover rates of DOC observed during blooms.

The origin of transparent exopolymer particles (TEP) and their role in the sedimentation of particulate matter

Seasonal changes in the concentration of suspended transparent exopolymer particles (TEP) and flux rates of TEP and other particles at 500m were measured at 2-week intervals at a station in the Santa Barbara Channel between May 1995 and June 1997 in order to investigate the hypothesis that the presence of TEP is necessary for the aggregation and subsequent sedimentation of particles from the pelagic zone. During the 2-year period phytoplankton appeared to be the most significant source of TEP. However, in association with phytoplankton, the concentration of TEP was also positively aected by bacteria abundance. Possibly bacteria enhance the production of TEP by phytoplankton. The presence of TEP was necessary for the sedimentation of diatoms. However, only 67% of the peaks in particulate organic carbon flux corresponded to peaks in TEP flux. Lithogenic silica sedimented only when scavenged by marine snow; either by TEP-rich diatom aggregates or by zooplankton-derived snow (larvacean houses). TEP were not involved in the sedimentation of foraminifera. Although sedimentation was the dominant loss processes of TEP out of the euphotic zone, other loss process must have been important at greater depth, as only a small fraction of the standing stock of TEP arrived at 500m. # 2001 Elsevier Science Ltd. All rights reserved.

Rapid formation and sedimentation of large aggregates is predictable from coagulation rates (half-lives) of transparent exopolymer particles (TEP)

Two hypotheses have been proposed 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 abundant transparent exopolymer particles (TEP), a recently discovered class of particles formed from polysaccharides excreted by phytoplankton. The hypothesis of TEP-driven coagulation in three disparate systems, a freshwater lake, a coastal ocean, and a saltwater mesocosm was evaluated, by comparing TEP abundance to several related factors including phytoplankton concentrations, measured sediment fluxes, and abundances of large aggregates. The timing of large aggregate formation and sedimentation events was related to coagulation rates expressed in terms of particle half-lives, t12, calculated as the time for TEP or phytoplankton to decrease to half their concentration through shear coagulation. While TEP have been previously investigated only in marine systems, it is reported here that TEP also can be present in high concentrations (860 ml−1) in freshwater lakes (Lake Constance, Germany) and that high fluxes of particulate organic matter at depth coincide with the disappearance of abundant TEP from overlying waters. The half-lives of TEP in the three different systems indicate that large aggregate formation and massive sedimentation events following diatom blooms occur when the TEP half-life decreases to less than a few days. 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.

Effects of moonlight on the vertical migration patterns of demersal zooplankton

The diel vertical migration patterns of demersal zooplankton, those organisms which habit bottom substrates but periodically emerge to swim freely in the water column, water determined throughout the lunar cycle. Demersal zooplankton were quantitatively sampled on a subtidal sand flat in the Gulf of California every 2 h for 24-h periods at new, full, first, and last-quarter moons, both as they emerged into the water column and as they returned to the benthos. Demersal zooplankton rarely migrated during daylight. Three general patterns of migration were observed. (1) Polychaetes and cumaceans emerged from the benthos at dusk, regardless of the phase of the moon. Polychaetes returned to the benthos throughout the night while cumaceans returned near dawn. (2) Species of amphipods and isopods exhibited significant avoidance of moonlight, delaying emergence until moonset or returning to the benthos at moonrise. (3) Species of copepods, mysids, shrimp, Branchiostoma (cephalochordate), and tanaids emerged into the water column throughout the night. The timing of migration was highly variable and did not correlate with the presence or absence of moonlight. Large zooplankton migrated less frequently into the water column during moonlit periods than small forms, suggesting that nocturnal predation by visually oriented planktivorous fish may be an important selective pressure. Demersal zooplankton emerged into artificially darkened emergence traps in significantly higher numbers during daylight and during full and quarter moons than into undarkened control traps, demonstrating that absence of light is a major cue stimulating migration. Reentry traps resting on the bottom captured higher densities of demersal zooplankton than either emergence traps or reentry traps suspended off the bottom. Thus, many demersal zooplankton remain near the bottom, rarely swimming far into the water column. Some trap avoidance was observed and current methods for collecting demersal zooplankton are evaluated. Since most demersal zooplankton remained in the water column only a short time, dispersal, particularly over short distances, may be a major advantage of migratory behavior. Migration facilitates rapid recolonization of disturbed or defaunated sites, disrupts and mixes bottom sediments, and results in daily variation in the microdistribution, patchiness, and species composition of the benthic fauna.

Aggregation of a diatom bloom in a mesocosm: The role of transparent exopolymer particles (TEP)

The role of TEP (Transparent Exopolymer Particles) in the flocculation of a diatom bloom was studied under controlled conditions in a mesocosm. The concentration of TEP increased exponentially during growth, flocculation and senescence of the bloom. Aggregation began dominating the particle dynamics of TEP during the early growth phase of the bloom, several days prior to the appearance of large flocs and nutrient depletion. TEP aggregated with themselves and with phytoplankton due to the high stickiness of TEP, but phytoplankton was not observed to aggregrate with itself. The production of TEP, estimated from changes in concentration, did not increase after nutrients were depleted. The concentration of TEP was a linear function of chl a and particulate organic carbon (POC), indicating that production of TEP was linked to growth rather than standing stocks of phytoplankton. The ratio between TEP and phytoplankton appeared to be one of the factors determining the onset of the flocculation of the bloom. The concentration of TEP may have been decreased by bacterial degradation. Bacterial degradation of TEP may explain the low TEP to chl a values, the decrease in stickiness of particles as the bloom progressed, and the retarded onset of flocculation.

Distribution, abundance, and substrate preferences of demersal reef zooplankton at Lizard Island Lagoon, Great Barrier Reef

Demersal zooplankton, those plankton which hide within reef sediments during the day but emerge to swim freely over the reef at night, were sampled quantitatively using emergence traps planced over the substrate at Lizard Island Lagoon, Great Barrier Reef. Densities of zooplankton emerging at night from 6 substrate types (fine, medium, and coarse sand, rubble, living coral and reef rock) and from 5 reef zones (seaward face, reef flat, lagoon, back reef, and sand flat) were determined. A large population of nocturnal plankton including cumaceans, mysids, ostracods, shrimp, isopods, amphipods, crustacean larvae, polychaetes, foraminiferans and copepods are resident members of the reef community at Lizard Island. The mean density of plankton emerging throughout the reef was 2510±388 (standard error) zooplankton/m2 of substrate. Biomass averaged 66.2±5.4 mg ash-free dry weight/m2 of substrate. Demersal zooplankton exhibited significant preferences for substrate types and reef zones. The highest mean density of zooplankton emerged from coral (11,264±1952 zooplankton/m2) while the lowest emerged from reef rock (840±106 zooplankton/m2). The density of demersal plankton was six times greater on the face than in any other zone, averaging 7900±1501 zooplankton/m2. Copepods dominated samples collected over living coral and rubble while foraminiferans, ostracods and decapod larvae were most abundant from sand. Plankton collected with nets at night correlated only qualitatively with plankton collected in emergence traps from the same location. Although abundant, demersal plankton were not numerous enough to meet the metabolic needs of all corals at Lizard Island Lagoon. Demersal plankton appear especially adapted to avoid fish predation. The predator-avoidance strategies of demersal plankton and maintenance of position on the reef are discussed. Our results indicate that much of the zooplankton over coral reefs actually lives on the reef itself and that previous studies using standard net sampling techniques have greatly underestimated plankton abundance over coral reefs.

Particle size spectra between 1 μm and 1 cm at Monterey bay determined using multiple instruments

Abstract Particles are responsible for the vertical transport of material in the ocean. Size is an important characteristic of a particle, determining its fall velocity, mass content, scattering crosssection, and food value, as well as other properties. The particle size spectrum describes the distribution of particles in a volume of water as a function of their sizes. We measured particle size spectra in Monterey Bay, CA, using six different instruments that examined particles ranging from approximately 1 μm to 10 mm. Before the results could be combined, they had to be adjusted for the different particle properties actually measured. Results from different optical instruments were similar, although the spectral values were sensitive to minor variations in the diameter assigned to particles. Sample volume was crucial in determining the effective upper size limit for the different techniques. We used fractal scaling to piece the results together, deriving fractal dimensions of 2.26–2.36. Diver observations of visible particles showed that they were composed mostly of aggregated diatoms. The particle size spectra n I were remarkably well fitted with a power law function n I = ad I − b I , where d I is the image diameter and b I = 2.96–3.00 . The equivalent slopes for particles measured with an aperture impedance instrument were 3.50–3.61. The particle volume distribution showed that most of the particle mass was in the 0.1–3 mm range. This volume distribution is consistent with theories that assume particle sizes are controlled by simultaneous coagulation and disaggregation.