Biologically significant properties of submerged marine sands

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Abstract
The distribution, of Branchiostoma lanceolatum does not appear to be related to the texture of the sand in which it lives. At Helgoland there is a great variety of different sands some of which contain large numbers of lancelets and others few or none. In contrast at Le Racou on the south coast of France the same species is present in varying numbers in apparently uniform sands. It does not seem that coarseness of the deposit is in itself a limiting factor but rather that the animals react to some more subtle properties. The nature of these properties in Helgoland and Le Racou sands has been investigated. A system of sieves has been used to separate the sand grains into grades in which the ratio between the diameters of the smallest and largest grains in each grade approximates to 1:1.5. An estimate of the error involved in grading sands is given. A formula is suggested for determining grain surface area (specific surface) that takes account of differences in grain shape. A new falling head permeameter is described in which consolidation of the sand is in­duced by placing the sand column in an ultrasonic wave field and controlled by the length of exposure to the radiation. Measurements of porosity and the rate of flow of water (drainage factor) through a sand-column are obtained at different degrees of compaction from loose to close packing of the grains and are closely replicable. A linear relationship between porosity and drainage factor has been found to exist for the whole or part of the process of consolidation and appears to be due to the maintenance of grain orientation in the sand grain lattice during compression until (if it happens) a sudden reorientation of the grains takes place and a new lattice with a different geometry is formed. Sands are thus recognized to have single-phase, two-phase or three-phase consolidation according to the number of such changes in the geometry of the lattice. As the rate of change in drainage factor with porosity is constant within a phase of consolidation, this ratio, here called the specific permeability, can be used to compare the different phases in different sands without complications arising from differences in degree of compaction. It has been demonstrated that B. lanceolatum and also the interstitial harpacticoid copepod, Evansula incerta , both react not only to differences in specific permeability but also to differences in that part of the void that changes with the drainage factor and has been called the capillary space. Capillary space is envisaged as pathways through the sand of preferential flow, the re­mainder of the void being relatively stagnant and termed cavity space. Specific permeability is thought to represent the relative rate at which capillary water can be exchanged with water above the sand. Capillary space changes continuously with consolidation, whereas cavity space is constant for a given phase in consolidation, but changes with the phase. Porosity, specific permeability and cavity and capillary space have been measured first in graded sands at temperatures from 5 to 30°C and then in various simple mixtures of two grades at 20°C. Except for porosity which is temperature independent, changes in temperature and, for the mixtures, small changes in composition caused fluctuating and often relatively large changes in these characteristics of a nature that would have been difficult to predict. The fluctuations appear to have been due first to differential effects of changes in fluidity on the rates of flow of water through pores of different size and, secondly, to alterations in the form of the sand grain lattice brought about by changes in the com­position of the mixtures so small as to be barely recognizable if at all by sieving. Experiments with sands on which epipsammic films of bacteria have been grown by irrigating the sand with nutrient solution have shown that their properties are highly modified by the extent and thickness of the film which evidently changes the adhesiveness of the grains and hence the geometry of the lattice they form. Any factor that determines organic content should modify the properties of sands from this cause. In addition, stratifica­tion in a sand arising from differences in deposition or from the activity of burrowing animals also affects the spatial and flow characteristics. Thus sand deposits of similar particle size composition but with different non-random in situ arrangements of grains and dif­ferences in their organic component, neither being characters that appear in grain-size analysis, may differ in their properties from place to place and seasonally and may support different faunas both in numbers and species. Moreover, the capillary characteristics of sands, being measured with a permeameter using a standard Water flow, only have meaning in an irrigated sand. Such characteristics cannot exist in sands in which there is no movement of water but appear as soon as forces operate that cause interstitial flow such as pressure differences due to animal activity, wave movement and convection. The biologically im­portant conditions in a natural Sand deposit depend in part on the magnitude of the forces that cause irrigation and therefore cannot properly be estimated without information on the water movements in its neighbourhood both in the short and in the long term.