MAP Technical Reports Series No. 106 UNEP

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biogenic sestonic organic material. Biogenic organic seston enters the food-chain (zooplankton,
detritus feeders), is settleable (by sedimentation), mineralizeable (through autolysis and bacterial
activity), and its oxygen consumption potential can be estimated relatively reliably.
Adenosine-Tri-Phosphate (ATP) ATP is practically found only in living cells, and
represents a measure of stored biologically available energy, and hence, of active biomass. The
ratio between organic cell carbon and ATP is fairly constant in unicellular and multicellular
organisms (bacteria, phytoplankton, zooplankton), varying around 265. Therefore, ATP is not a
specific measure for phytoplankton, but if the bulk of zooplankton can be filtered off, and the
bacteria content vis-a-vis that of phytoplankton is small, it gives a fairly reliable measure of the
active particular organic carbon (aPOC) in phytoplankton even in the presence of high amounts
of detrital POC.
Main biochemical components. Information about the biochemical composition of
sestonic material is important for evaluating its nutritional value. There is a large number of
biochemical components, such as proteins, carbohydrates, lipids, amino acids, RNA, DNA, o.a.,
that can be measured directly by standard biochemical analytical techniques. Total proteins are
often estimated indirectly from PON, using a factor of 6.25. Carbon in proteins is about 3.3*PON.
Carbohydrates (present as assimilates and storage products, as components in cell walls
(crude fibres), and as gelatinous involucra of cells or colonies) plus lipids are estimated by
difference. However, the relative composition in these main compartments depends on a
number of factors: species specificity, age, activity phase, nutritional conditions, etc., and
therefore can vary significantly. Therefore, interpretation of indirect estimates has to be made
with caution.
Beside the components mentioned above, knowledge about, and measurement of other
biochemic compounds, such as toxins produced by algae, have become over recent years of
increasing practical interest. This field is in full progress of development (cf. Chapter 3.6).
b) Macrophytes and macroalgae
Much of what has been said above applies also to these categories albeit with variation
in methodology and relative importance. Still, the quantitative assessment of macroalgae and
macrophytes is by far more difficult than the quantification of phytoplankton, and hence, the
respective figures are subject to more uncertainties.
c) Zooplankton, nekton and bottom fauna
Biomass assessment of the typical zooplankton (like e.g. Calanus) follows similar
principles as exposed for phytoplankton, except, of course, that there is no pigment, like
chlorophyll, common to all component species, which could be used as a crude mean to
quantify the amounts of zooplankton present.
Macro-nekton, like nektic Cnidaria (Medusae), etc., and the vast array of bottom fauna
require special techniques that are not of direct interest in the context of the present report. For
the assessment of biomass of pelagic fish, instead, sonar techniques have given reliable figures.