Primary production

Measurements of primary production in the western English Channel were made around two decades ago using the carbon-14 method (see Section 5.2.4) and indicated an average value of about 120 gC-m-2-yr-1 for carbon fixation in particulate organic matter. We will take this to be a measure of net primary production (NPP), i.e. the formation of new plant tissue, and will ignore losses of dissolved organic matter. As conversion factors we will take 1 gC as equivalent to 10 kcal or to 2.3 g dry wt of organic matter. NPP can therefore be expressed as 1200 kcal m-2-yr-1 or 2.3 X 120 = 276 g dry wt-m-2-yr-1.

Estimates of energy loss in plant respiration fall between 10 per cent and 50 per cent of gross primary production (GPP). Taking the respiratory loss for phytoplankton as 20 per cent of GPP, then NPP is 80 per cent of GPP, and GPP = 1500 kcal -m-2-yr-1. The energy loss from the system by plant respiration is 300 kcal m-2- yr-1.

A mean figure for solar energy entering the earth's atmosphere has been given as 15.3 X 105 kcal-m-2-yr-1.

Much of this is reflected or absorbed on passage through the atmosphere, and the amount reaching the surface varies greatly with locality. We will take a value of 3 X 105 kcal-m-2-yr-1 as the energy at the surface of the English Channel. Here there are losses by reflection, and on penetrating the surface the light is rapidly absorbed by the water and causes heating. Estimates of the fraction of the incident radiation which is fixed by photosynthesis in aquatic environments usually fall within the range 0.1-0.5 per cent of the energy at the surface. If the GPP for the western English Channel is 1500 kcal-m-2-yr-1 and the energy of sunlight reaching the sea surface is 3 X 105 kcal-m-2-yr-1, then the proportion of this radiation fixed in particulate organic matter is 15 00/3 X 105 = 0.005 or 0.5 per cent. Evidently the English Channel is a relatively highly productive part of the sea.

Table 7.1 presents some estimates by Harvey of the mean annual biomass of several trophic levels in the English Channel, and we will use these figures in our analysis. The value for mean annual biomass of standing stock of phytoplankton for the English Channel is 4.0 g dry wt-m-2. Comparing this figure with our value for annual production, it can be seen that there is a high rate of turnover. The weight of new plant tissue produced in the year is nearly 70 times (276/4) the mean weight of standing stock.

The production of new phytoplankton does not increase the standing stock

Table 7.1 Estimates of the mean annual biomass of several trophic levels in the English channel. (From Harvey (1950) published by Cambridge University Press)

Trophic level

Dry wt of organic matter (g-m 2)

Phytoplankton

Ca. 4.0

Zooplankton

1.5

Pelagic fish

1.8

Demersal fish

1-1.25

Figure 7.1 Diagrammatic representation of hypothetical energy relationships in a marine ecosystem in coastal waters of the British Isles. Figures on arrows have units of kcal-m~2- yr~x.

from one year to the next because it is balanced by a corresponding loss of plants from the water. Death of plant cells occurs in two main ways, by sinking and by consumption by herbivorous zooplankton. Sinking cells die through lack of light and constitute a large part of the organic detritus reaching the sea bottom. This provides a major energy source to support the benthos. However, in contrast to most terrestrial ecosystems, it appears certain that in the sea a greater proportion of the vegetation is consumed by animals than is lost by death and decomposition, the main energy transfer from NPP going to grazing zooplanktonts. We will assume that 20 per cent of NPP contributes directly to sinking detritus, with 80 per cent consumed by pelagic herbivores. The energy content of detritus from this source is therefore 0.2 X 1200 = 240 kcal-m^-yr"1. This will be given further consideration below (see Section 7.3). The pelagic grazing population receives an energy inflow of 0.8 X 1200 = 960 kcal- m_2-yr_1.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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