Updated: Mar 15, 2020
In many third world countries yields from coastal fisheries have decreased due to poor fishing practices such as overfishing or destructive fishing measures, e.g. the use of explosives or poisons, like cyanide. (FAO 2005-13). In the poorer areas with inadequate road access and little cash, the sea nearby is the major source of protein, but the available fishing equipment remains relatively simple. Artificial reefs have been used in many locations (Pickering and Whitmarsh, 1997) to increase fish catches, but these may just increase fish yields in the vicinity of the reef without an overall increase in numbers of fish, in which case they would easily lead to more over-fishing. The proposal being examined is for an artificial reef, which could enclose an amount of added nutrient (e.g. urea or a locally available fertiliser), held for sufficient time to grow phytoplankton within it to form the base of a local food chain, so as to increase overall productivity. This would provide a source of increased protein for a local community. The proposal is for this to be made, as far as possible, from locally available resources, using local skills and labour.
The aim is to construct an artificial reef in the photic zone of the coastal ocean that acts as a bioreactor able to produce organic carbon for consumption by organisms higher in the food train. A potential site being investigated for such a reef is in the Beqa Lagoon, near Yanuca Island, Fiji (Calamia et al., 2010 and website of Pacific Blue Bequa Lagoon Marine Reserve Project.) The rate of growth of phytoplankton concentration depends, amongst other things, on the assemblage of phytoplankton species, the photosynthetically available radiation (PAR) the available nutrients and the dilution rate. Growth is achieved by cells dividing and so the greater the concentration of cells, other things being the same, the more organic carbon is produced from a given volume of sea water. If this volume of sea water in the bioreactor is continuously being exchanged with surrounding water of low phytoplankton concentration, the amount of phytoplankton produced in the bioreactor is reduced. The aim of a nutrient retaining artificial reef is to have a dilution rate that carries away the phytoplankton produced at a rate that maintains a high concentration of phytoplankton in the reef and consequently high production rates of organic carbon.
As well as limiting the dilution rate, the artificial reef needs to capture enough PAR to provide the energy needed to support photosynthesis. If we imagine cavities of simple shape admitting PAR from above, the relationship between upper open area and the volume of the cavity becomes an important consideration. It may be that to sufficiently restrict dilution in the cavity while maintaining high PAR within it, it will 2 be necessary to use an upper surface for the cavity that is light transparent. This second line of investigation is left for a further study if a simple cavity does not appear to achieve sufficient PAR while trapping a volume of water.
Laboratory experiments have been carried out to examine the design features necessary for an artificial reef to retain an amount of nutrient, in our case urea, for sufficient time for phytoplankton to grow. The report first looks at the physical parameters which need to be considered in order to correctly scale up from laboratory models to full scale structures. This involves the use of non-dimensional groups of relevant parameters because retention times found at laboratory scale need to be adjusted to scale up to full scale. Retention times found in the laboratory have to be scaled up correctly to give valid results for full-scale structures. The report presents the laboratory equipment and measurement methods used. The experimental results are then presented. These results are then discussed.