The mechanical grading method selected for the bottom-deposits is that used by E. J. Allen in his fine paper “On the Fauna and Bottom-Deposits near the Thirty-fathom Line from the Eddystone Grounds to Start Point” (Journal of the Marine Biological Association, Plymouth, vol. 5, pp. 365–542, 1899). As already described, the material was secured by means of either a naturalist's dredge or a conical dredge designed so that there was no appreciable loss of the finer silts on the way to the surface. A half-gallon jar from each dredging was set aside and was subjected to the sieving process while still in a wet state. The sieves, six in number, were especially made, and each had a diameter of 15 cm. The sizes of the circular perforations were 15 mm., 5 mm., 2.5 mm., 1.5 mm., 1 mm., and 0.5 mm. The material that passed through the 0.5 mm. sieve was further subdivided into two parts, one of which was considered as fine sand and the other as silt. The separation of the fine sand from the silt was made by taking the material that passed the 0.5 mm. sieve and stirring it up with a quantity of fresh water;* then at the end of one minute the material that had settled was considered to be fine sand and that still in suspension was regarded as silt. In this way eight grades of material were obtained from each dredging, and following for the most part Allen's original classification, they may be given the following nomenclature, which becomes a very satisfactory method of supplying a precise meaning to such common terms as gravel, sand, and silt.
The eight grades with their descriptive equivalents are as follows:
Material left on 15 mm. sieve = stones (if inorganic) or large shell-fragments.
Material left on 5 mm. sieve = coarse gravel or coarse shell-gravel.
Material left on 2.5 mm. sieve = medium gravel, etc.
Material left on 1.5 mm. sieve = fine gravel, etc.
Material left on 1 mm. sieve = coarse sand or coarse shell-sand.
Material left on 0.5 mm. sieve = medium sand, etc.
Material that passes 0.5 mm. sieve and settles in one minute = fine sand, etc.
Material that passes 0.5 mm. sieve and does not settle in one minute = silt.
The samples from the various grades were first thoroughly dried at a low temperature and then weighed, the results being expressed as percentages of the total dry weight. Allen (loc. cit.) has devised an ingenious method of reducing the entire eight grades of each dredging to a grade-number which indicates the general nature of the material, and can be used not only in comparing samples, but also indicating the range in substratum favoured by a particular organism. I quote Allen's method, which is as follows:—“The method adopted is founded on the determination of the average grade of the whole sample. The figures I, II, III, etc., are purely conventional, but may be conveniently adopted for the purpose of shortly stating the average grade of any sample. The actual method followed
[Footnote] * Salt water was used by Allen, but as most of the present work was done at a distance from the sea a day or so after the material was collected, the use of fresh water was far more convenient.
was to multiply each percentage in the table by the conventional number attached to its grade, add together the figures so obtained, and divide by 100; the result has been described as the ‘average grade’ of each sample, and affords a ready means of comparing the texture of any two deposits.”
An instance of this method of determination of the “average grade” is here given for dredging D.3:—
|1.||Material left on 15 mm. sieve||36.78×1 = 36.78|
|2.||" " 5mm. "||31.03 × 2 = 62.06|
|3.||" " 2.5 mm. "||13.70 × 3 = 41.10|
|4.||" " 1.5 mm. "||8.57 × 4 = 34.28|
|5.||" " 1 mm. "||5.73 × 5 = 28.65|
|6.||" " 0.5 mm. "||1.70 × 6 = 10.20|
|7.||" settled in 1 minute||2.29 × 7 = 16.03|
|8.||" not settled in 1 minute||0.20 × 8 = 1.60|
|230.70 / 100 = 2.307|
|2.307 the “average grade” of D.3.|
Hard-bottom grades vary between 1.5 and 3.5 and have the fine sand and silt grades constantly very low. Soft-bottom grades are between 5.4 and 7.9 and always the fine sand and silt grades make up the bulk of the sample.
Each of the main animal communities of the sea-bottom in the Auckland Harbour and vicinity is found in association with a distinctive bottom grading. Of course other physical conditions such as salinity, depth, and p.H. have a definite bearing on the distribution of organisms; but since the range in these factors is not great over most of the area, it follows that the grade of the substratum and the animal community suited to that grade coincide very well. It is only when considering the truly estuarine part of the harbour, where there is a fairly marked decline in the salinity, that the normal animal community associated with a similar mud from an area of normal salinity is found to be wanting. This means that with the physical conditions of the sea-water more or less uniform and no very great variation in depth there is a definite correlation between animal community and substratum.
The graph shown below gives the average curves for four distinct animal formations, three of them occurring in the Auckland Harbour and the fourth in the Manukau. So far as we know, this latter one is not represented in the Auckland area under investigation. Notice the characteristics of each curve. In (1) the Echinocardium formation, the heart-urchin and its associates can live only in a substratum that averages 293.81% mixture of fine sand and silt; and in (2), the Arachnoides formation, 94.79% of the sample is fine sand, there being practically no silt. No. (3), the Tawera-Glycymeris formation, has high percentages of the coarse grades, but diminishes rapidly over the remaining grades until there is a very small amount of fine sand and silt, while (4), the Maoricolpus formation, although similar to Tawera-Glycymeris in the coarse grades, has a comparatively large amount of fine sand and silt, the intermediate grades being least.
Diagram I. Showing Distinctive Texture-curve for Each of the Four Main Animal Communities of the Auckland and Manukau Harbours.
(a) Horizontal figures 1–8 = Texture-grades of Substratum.
1 = Material left on sieve with round holes of 15 mm. diameter.
2 = " " 5 mm. " "
3 = " " 2.5mm. " "
4 = " " 1.5mm. " "
5 = " " 1 mm. " "
6 = " " 0.5mm. " "
7 = Material that passes sieve 6 and settles in one minute = (fine sand).
8 = " " does not settle in one minute = (silt).
(b) Vertical figures 0–100 indicate percentages (i.e., percentage of each grade to the whole sample).
(c) Numbers enclosed in circles indicate the Animal Community (see below) of the particular Substratum-curve it appears against.
1 = Echinocardium formation.
2 = Maoricolpus formation.
3 = Tawera+Glycymeris formation.
4 = Arachnoides formation.
In Diagram III particulars of respective animal communities are arranged vertically above the community indicator numbers (enclosed in circles) on the base-line. The thin vertical line represents the maximum number of species at any one station in the community, and the thick line the average number of species for all stations of that community. Lateral extensions on the base line indicate rough quantitative estimates of the animals present in the community. The extension to the left represents the macrofauna, and the number beside the arrow is the total of the larger animals taken from the whole dredging. The extension to the left represents the microfauna, and the numbers quoted are not for the entire sample, but for a
Diagram III. Showing the relationship between the substratum-textures and the frequency of animals (both in species and in relative abundance) for the four main animal communities of the Auckland and Manukau Harbours.
(1) Echinocardium formation.
(2) Maoricolpus formation.
(3) Tawera+Glyoymeris formation.
(4) Arachnoides formation.
small quantity weighing about six ounces. The transverse lines are texture-curves for each of the eight grades. Dotted lines 1–6 are the coarser grades (representing finer grades in descending order), and continuous lines 7 and 8 are the fine-sand and silt grades respectively. The vertically arranged numbers at the sides may be applied numerically or as percentages. The graph illustrates how the maximum fauna both in species and quantitatively is associated with coarse textured bottom, and conversely that the minimum fauna is found in the fine textured bottom. The critical grades are thus shown to be fine sand (7), and silt (8). The substratum most unfavourable to animal life appears to be that which is almost entirely fine sand. The reason for the poor fauna of this type of bottom is probably the extreme mobility of the surface, for it does not bind, and in consequence no great accumulation of food material in the form of organic detritus is possible.