Determining Longevity and Growth Rates

The species in maerl biotopes for which these measurements are most urgently required are the maerl species themselves. Growth rate determination requires some way of repeatedly retrieving and identifying individual thalli in the natural habitat. Various workers have used tagged thalli placed in marked areas; tagged thalli tied to fixed lines; and maerl thalli placed in baskets. Two main methods have been used to measure growth rates of maerl. Growth of bulk samples can be determined by buoyant weight measurements. Baskets of maerl are suspended from a balance in a medium of known density. Accuracy is low due to the weight of epiphytic animals and algae and attached sediment. Growth of individual thalli can be obtained by repeated photographic or caliper measurements of tagged thalli. Accuracy is high but problems arise due to fragmentation of thalli between measurements, for example. A further potential difficulty has been identified: growth rates may decrease due to effect of making the measurements, i.e. the desiccation involved is detrimental to the thalli.

Maerl thalli are thought to be very long-lived. Determination of longevity can be made by extrapolating from growth rates or by aging of individual thalli. It is not currently known whether thalli can be aged by 14C methods but these would appear to have considerable potential.

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