Cultivating Palmaria palmata - Bord Iascaigh Mhara

ecommended are relatively high, i.e. 3-4 kg FW m -2 (Demetropoulos & Langdon, 2004; Pang & Lüning,
2004; Morgan et al., 1980) because it is only with such dense cultures that the growth of opportunistic
algae can be suppressed. For a specific site and tank set-up, however, stocking densities may be
adjusted depending on the tank size and shape, light exposure and season, and nutrient application and
seawater flow-through rate. High stocking densities create more shading in the tank, and this is
advantageous for two main reasons: opportunistic algae are shaded out, and individual blades of
Palmaria are only briefly exposed to sunlight when tumbled in the water flow, so that there is less
danger of bleaching. On the other hand, high biomass in the tank reduces the flow along the fronds of
individual Palmaria plants, thereby reducing nutrient uptake and in turn reducing growth. Selecting a
stocking density therefore requires a balance between good growth rates stimulated by high nutrients at
low densities versus the potential damage to fronds caused by high light, and the competition from
opportunistic algae. The biomass per tank should certainly be adjusted to the available light, with higher
stocking densities in summer and lower densities in winter.
8.3 Nutrient supply
The enrichment of seawater with nutrients increases the growth of cultured algae in the same way that fertilisers
affect land plants. Several aspects have to be considered about nutrient addition, e.g. which nutrients to add, at
what concentration, how they can be applied in a flow-through system, the potential side effects of seawater
fertilisation.
Nutrients: A range of nutrients is essential for the growth of seaweeds, and these can be divided into
macro-nutrients such as nitrogen (N), phosphorus (P), potassium, calcium and magnesium, which are
required in higher concentration, and micro-nutrients such as cobalt, iron, manganese, zinc and
vitamins, which are needed in only very small amounts. Seawater provides all these nutrients, but the
concentrations of N and P and many of the micro-nutrients will be much lower during the summer
months than in the winter. The addition of nutrients ensures that all nutrients are in excess, so that
growth is not limited by a shortage of any nutrient. The application of N especially, in the form of either
nitrate or ammonium or both, has a visible effect on the appearance of Palmaria: it increases the
pigmentation of the fronds and thus intensifies the colour of the tissue to dark purple-red.
A mixture of all the essential nutrients can be applied in the form of self-prepared or ready-mixed
fertiliser solution or as a slow-release solid fertiliser. Some agricultural fertilisers may be useful.
However, the addition of nitrate and phosphate at a 10:1 ratio can be as effective as a complete nutrient
mix, because seawater contains an adequate supply of most of the metals required.
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