Page 296 - Algae

Algae and Men 279

Maerl is the common name of a fertilizer derived from calcareous red algae; maerl beds are
characterized by accumulations of living and dead unattached non-geniculate calcareous rhodo-
phytes (mostly Corallinaceae but also Peyssonneliaceae, such as Phymatolithon calcareum and
Lithothamnion corallioides). Also known as rhodolith beds, these habitats occur in tropical, tem-
perate, and polar environments. In Europe they are known from throughout the Mediterranean
and along most of the Western Atlantic coast from Portugal to Norway, although they are rare
in the English Channel, Irish Sea, North Sea, and Baltic Sea. Maerl beds are often found in
subdued light conditions and their depth limit depends primarily on the degree of light penetration.
In the northeast Atlantic, maerl beds occur from low in the intertidal to ca. 30 m depth; in the West
Mediterranean they are found down to 90–100 m, while in the East they occur down to depths of
ca. 180 m. Maerl beds in subtidal waters have been utilized over a long period in Britain, with early
references dating back to 1690. In France also, maerl has been used as a soil fertilizer for several
centuries. Extraction of maerl, either from beds where live thalli are present or where the maerl
is dead or semifossilized, has been carried out in Europe for hundreds of years. Initially, the quan-
tities extracted were small, being dug by hand from intertidal banks, but in the 1970s about
600,000 tons of maerl was extracted per annum in France alone. Amounts have declined to
about 500,000 tons per annum since then, though maerl extraction still forms a major part of the
French seaweed industry, both in terms of tonnage and value of harvest. Live maerl extraction is
obviously very problematic with regard to growth rates for replacement. Dead maerl extraction
is liable to lead to muddy plumes and excessive sediment load in water that later settles out and
smothers surrounding communities.
The maerl is marketed mainly for use as an agricultural fertilizer, for soil improvement in hor-
ticulture, mainly to replace lime as an agricultural soil conditioner. There are conflicting reports on
the benefits of maerl use as opposed to the use of dolomite or calcium carbonate limestone. Other
uses include: as an animal food additive, for biological denitrification, and in neutralization of
acidic water in the production of drinking water, aquarium gravel as well as in the pharmaceutical,
cosmetics, nuclear, and medical industries. These uses are all related to the chemical composition
of maerl, which is primarily composed of calcium and magnesium carbonates. It is occasionally
used for miscellaneous purposes such as hardcore for filling roads, and surfacing garden paths.
Maerl beds are analogous to the sea-grass beds or kelp forests in that they are structurally and
functionally complex perennial habitats formed by marine algae that support a very rich biodiver-
sity. The high biodiversity associated with maerl grounds is generally attributed to their complex
architecture. Long-lived maerl thalli and their dead remains build up on underlying sediments to
produce deposits with a three-dimensional structure that is intermediate in character between
hard and soft grounds. Maerl thalli grow very slowly such that maerl deposits may take hundreds
of years to develop, especially in high latitudes. One of the most obvious threats is commercial
extraction, as this has led to the wholesale removal of maerl habitats (e.g., from five sites
around the coasts of Brittany) while areas adjacent to extraction sites show significant reductions
in diversity and abundance. Even if the proportion of living maerl in commercially collected
material is low, extraction has major effects on the wide range of species present in both live
and dead maerl deposits. Brittany is the main area for maerl extraction with about 500,000 tons
extracted annually; smaller amounts are extracted in southwest England and southwest Ireland.
Maerl beds represent a non-renewable resource as extraction and disruption far out-strips their
slow rate of accumulation. In France, maerl extraction is now considered to be “mining,” which
implies more constraints for the extractors and more controls on the impact of extraction. Scientists,
managers, and policy makers have been slow to react to an escalating degradation of these habitats
such that there is now an urgent need to protect these systems from severe human impacts.
Macroalgae extracts and suspensions have achieved a broader use and market than macroalgae
and macroalgae meal. They are sold in concentrated form, are easy to transport, dilute, and apply,
and act more rapidly. They are all made from brown macroalgae, although the species varies
between countries. Some are made by alkaline extraction of the macroalgae and anything that