| Summary: | Prolific growth of Ulva lactuca L. in some marine environments into which sewage is discharged causes many problems. An understanding of the reasons for this growth is required so that it can be controlled or prevented. The thesis is concerned with tackling this problem and also investigating whether Ulva lactuca can be used as an indicator organism for pollution. Prior to studies on growth of Ulva in polluted conditions, its pattern of life-history and mode of growth had to be elucidated. These findings led to the development of suitable laboratory culture facilities and development of a technique for studying growth by use of excised discs of thallus tissue. Laboratory culture was based upon the use of Erdschreiber medium and growth measured by percentage increase in area after two weeks in culture. Laboratory nutrient bioassays using growth rate measurements were used to determine the effects of likely growth rate stimulating substances found in sewage, including three nitrogen forms, (nitrate-N, nitrite-N and ammonium-N), orthophosphate, acetate, adenine and kinetin, and sewage-contaminated mud itself. Only ammonium-nitrogen within the range 0.4 to 7.8 mg dm -3, and sewage-contaminated mud, were found to significantly stimulate the growth rate of Ulva above that of Erdschreiber control medium. Further growth rate assessments were made with samples of field seawater collected at points throughout two sewage-polluted harbours, Poole in Dorset with an Ulva problem, and Langstone in Hampshire at present without a problem. The results obtained were correlated with the concentrations of nutrients in the water samples, and with results of the earlier artificially enriched seawater experiments. Ammoniumnitrogen was confirmed to be the only compound in field water which significantly stimulated growth of Ulva over that obtained with the control medium. The regions where prolific growth of Ulva occurred in Poole Harbour were the only areas which had field water with ammonium-nitrogen levels elevated to the range &aown in laboratory studies to cause significantly higher growth rates of Ulva discs. Water from Langstone Harbour although sewage-polluted did not stimulate growth of Ulva discs and water analysis revealed. ammonium- nitrogen levels below those determined as growth stimulatory. The ability of Ulva to act as a test organism for a range of toxic substances including industrial/domestic sewage sludge, detergent, copper, zinc, lead, cadmium and mercury was studied. Graded growth responses were obtained for these components, and toxic levels of them were found to be similar to those for other macro-algae. Ammonium-nitrogen was thus found to be the factor responsible for prolific growth of Ulva in the field. In order to prevent or minimise nuisance caused by enhanced Ulva growth it is recommended that the level of ammonium-nitrogen is kept below 0.3 mg dm -3 in seawater at all times. Because of specificity of response to a siaagle compound it is concluded that Ulva cannot be used as a general pollution indicator species, but its prolific growth in the field certainly indicates elevated ammonium-nitrogen levels.
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