Marine Algae and Fouling: A Review, with Particular Reference to Ship-Fouling

The fouling of the submerged parts of ships, in particular by marine algae, is an age-old and costly economic problem, and conventional methods do not provide sufficient protection against the highly-resistant organisms involved. Research has been carried out over a number of years to provide new information on the biology of fouling algae, as a basis for improved methods of control, and this review summarises some of the more important findings which have been made, against a background of developing antifouling technology. Marine fouling is the result of settlement and growth of algae and invertebrate animals on the surfaces of sub· merged objects, and such fouling can create a number of important and costly economic problems. The fouling of the underwater areas of ships' hulls and harbour installations is well-known, but problems are also caused by colonization of other man-made structures. For example, fouling by marine organisms can markedly increase the loading forces caused by waves and currents on static offshore structures such as oil and gas platforms, and this could cause the design specification to be exceeded unless removed in time. In addition, marine growths severely hamper inspection and maintenance of such platforms, and may also accelerate localised corrosion (see paper by Terry and Edyvean in this volume). The patterns of colonization and abundance of species at different depths, the succession of communities which occurs and the growth rates, longevities and maximum sizes of the species involved therefore need to be determined and corordinated, from as many platforms and fields as possible. It would then be possible to determine whether loading forces are likely to exceed design specifications, and, in the longer term, such data may . provide the basis for some measure of biological control of the organisms concerned (Ralph and Goodman 1979; see also papers by Hardy; and Moss, Tovey and Court in this volume). Other structures where marine fouling accumulates include the effluents from power plants, and data by Thorhaug and Marcus (see paper in this volume) also suggests that extensive fouling by brown and red tropical macroalgae may occur in the new ocean thermal energy conversion machines (OTEC), which are planned for use in tropical marine waters as an alternative energy source. Fouling communities will occur on any substrates suspended in the sea, and where this affects economic efficiency, it is necessary to consider the development of a suitable long-term system for the protection of immersed surfaces against fouling. One of the most well-known effects of fouling is on the efficiency of movement of ships. Fouling can seriously increase the roughness of underwater parts of ships. This results in a considerable increase in the factional resistance to movement through the water, and thus an appreciable loss of speed, or, an increased use of fuel to maintain the normal operating speed. In the case of a large supertanker, where running costs are very high, an increase in costs of some 0.5% (£80,000 a year in 1972) would result from a loss of 0.1 knot on a service speed of 16 knots (Banfield 1972). Modern supertankers, because of their mode of operation (short stays in port, with rapid transition from temperate to tropical waters), are not on the whole subject to animal fouling, and seaweeds of different sorts are the principle fouling agents. The commonest of these is Enteroniorpha species (green, and often called "grass"), followed by Ectocarpus (brown), with other weeds e.g. Cladophora (green), Chaetomorpha (green), Ulothrix (green) and Polysiphonia or Ceramium (red) occurring in less significant quantities. The dominance ofEnteromorpha as a cosmopolitan fouling alga is the result of its ability to withstand wide fluctuations in environmental conditions such as temperature and salinity (Biebl 1962, Christie and Shaw 1968), coupled with a rapid and highly effective spore attachment mechanism (see later), in response to surface contact. In addition, the 0006-8055/81/0024-0167$02.00· · © by Walter de Gruyter & Co. · Berlin New York 168 Evans: Marine algae and fouling plant shows a considerable capacity for regeneration on tanker hulls, following mechanical underwater scrubbing techniques; basal parts of the thallus left behind after such cleaning, as well as detached pieces, were shown by Moss and Marsland (1976) to give rise to new thalli in the so-called "bottle-brush" formation, giving an increased amount of fouling following scrubbing. Various methods of reducing fouling on ships have been proposed and used down the ages, many based on the use of copper in one form or another. Originally used as metallic sheathing on wooden ships, the copper was later incorporated into paints known as antifouling compositions. Other toxic compounds are now used also, and re^ lease of these at a controlled rate results in the formation of a layer of high toxin concentration (the boundary layer) at the paint surface, and this poisons settling organisms. The thickness of the boundary layer depends on the leaching rate of the toxin, which in turn depends upon the temperature, pH and salinity of the seawater and on the water flow rate over the surface, being minimal under stationary conditions. As stated, for many years, copper compounds have been the main toxic components in antifouling compositions. The copper is used in the form of cuprous oxide (red), cuprous thiocyanate (white), copper acetoarsenite (green) or copper flake (copper bronze), and compounds of mercury, arsenic, tin, lead or zinc are sometimes included as boosters. Nowadays, these are largely excluded on the grounds of being health hazards, and organotin compounds are the main boosters used in copper based antifouling compositions. Marine organisms vary in their tolerance of copper: a leaching rate of 1-2 jug Cu cm" day" will prevent settlement of sensitive orgnisms such as mussels, whilst 10 Mg cm" day" is necessary to prevent settlement of barnacle larvae (Banfield 1980). Higher rates (approximately 20 jug cm" day") are reported to be required to prevent accumulation of diatoms and bacterial slimes, but such rates would mot be economic and the critical leaching rate for antifouling compositions based on copper used in practice is 10 jug cm" day" ^Banfield 1980). Until recent years, two types of antifouling composition were in widespread use, separated according to the mechanism by which toxins are released into the seawater. In the first, known as the soluble matrix type, (Fig. 1), the toxin particles are distributed in a binder, based on rosin, which is acidic and therefore slowly dissolves in the slightly alkaline seawater (pH 8.0—8.2), thus exposing the toxin particles which then dissolve. Merchant ships, drydocking every 12 months or so, commonly use this type of antifouling composition. The second type, the contact leaching type, has a higher copper content and the copper particles are closely packed (Fig. 2), so that as particles at the surface are dissolved so deeper ones become exposed. The matrix is largely insoluble and an intact skeleton is left, through which water penetrates. Contact leaching antifoulings are harder and thus better suited for faster vessels. Their active life is about