Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

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Processes of carbonate precipitation in modern microbial mats

The supply and accumulation of silica in the marine environment

The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.

The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water

A new scheme for the description and analysis of bioturbation and the resultant ichnofabrics is proposed. This system can be used in core and field-based studies and consists of two parts. (1) A bioturbation index in which a descriptive grade is assigned to the degree of bioturbation. This integrates the sedimentology and ichnology, where the higher grades of bioturbation result from increased burrow overlap and the subsequent loss of the primary sedimentary fabric. (2) An ichnofabric constituent diagram which records the detail of the ichnofabric by graphically plotting the dimensional data of the ichnotaxa and their order of emplacement against their coverage. This scheme differs from previous attempts to characterize bioturbation using semi-quantatitive methods and visually portrays the ichnofabric so that comparative studies can be carried out. This is of particular use in facies analysis, the establishment of the ichnocoenoses and tier diagrams, and within sequence stratigraphic studies so that hiatal surfaces can be recognized.

Description and analysis of bioturbation and ichnofabric

Biological mixing in deep-sea sediments is described in terms of a time-dependent eddy diffusion model where mixing takes place to a depth L at constant eddy diffusivity D. The differential equation that describes this model has been solved for an impulse source of tracer delivered to the plane surface that forms the top of the mixed layer. The solution then serves as a Green's function, which can be used to determine the distribution of tracer in depth and in time for a surface input of tracer specified as any arbitrary function of time. The characteristic properties of the solution are dependent on the dimensionless parameter D/Lυ, where υ is the sedimentation rate. If D/Lυ is greater than 10, the surface layer becomes homogeneous, and the model is identical to the homogeneous layer model proposed by Berger and Heath (1968). If D/Lυ is less than 0.1, little mixing can take place before the sediments are buried, and so the surface concentration propagates downward into the sediments with little dispersion. For all values of D/Lυ the weighted mean depth of the concentration distribution is the depth at which an impulse source would be found in the sediment if no mixing had taken place. The microtektite data of Glass (1969, 1972) and Glass et al. (1973) indicate that abyssal sediments are mixed from the surface to a maximum mixing depth that ranges between 17 and 40 cm below the surface. Mixing occurs at rates between 1 and 100 cm2 kyr−1. Higher mixing rates may occur nearer the surface, but microtektite distributions cannot be used to estimate these rates in the presence of the deeper, slower mixing. Estimates for D based on dimensional analysis of sediment reworking rates for nearshore organisms (103–106 cm2 kyr−1) are used to predict abyssal mixing rates between 1 and 103 cm2 kyr−1 by invoking the assumption that mixing is proportional to biomass. Plutonium distributions in deep-sea sediments (Noshkin and Bowen, 1973) indicate abyssal mixing rates ranging from 100 to 400 cm2 kyr−1.

Quantitative estimates of biological mixing rates in abyssal sediments

A model for interstitial silica concentrations is derived, incorporating biological mixing of sediments. This model predicts concentrations and gradients and can account for the observed geographical variations in interstitial silica on the basis of a dynamic balance between solution of silica particles and diffusion from the sediments. The flux of particulate biogenous silica into the sediments is confirmed as an important parameter controlling interstitial silica concentrations. Biological mixing of sea floor sediments also has an important influence on interstitial composition by modifyirig the depth at which dissolving particles react. Faster mixing raises the interstitial concentration. The rate at which siliceous particles dissolve also plays a role; the slower they dissolve, the greater the interstitial silica concentration. Measurements on near-bottom waters of the Atlantic show no consistent gradients in dissolved silica, but antarctic bottom water seems significantly more variable in the benthic boundary layer than in the water mass above or in the benthic zone of North Atlantic deep water.

Processes affecting the concentration of silica at the sediment-water interface of the Atlantic Ocean

Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. The study area is characterized by low water temperature (mean = 7 C), high pressure (about 5,400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH{sub 4}) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in {sup 13}C by as much as 3.8% PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37% SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO{sub 2}) is depleted in {sup 13}C by as much as 22.4% PDB. Hydrate-associated authigenic carbonate rock is also depleted in {sup 13}C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.

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David R. Schink

Papers

Quantitative estimates of biological mixing rates in abyssal sediments

Processes affecting the concentration of silica at the sediment-water interface of the Atlantic Ocean

Bacterial methane oxidation in sea-floor gas hydrate: Significance to life in extreme environments

129I in Gulf of Mexico waters

Radium, thorium, and actinium extraction from seawater using an improved manganese-oxide-coated fiber