Browsing Irish Fisheries Investigations Series A - Freshwater by Title
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Water Quality Investigations in the River Blackwater and River Martin, Co. Cork--1966-1969(Department of Agriculture and Fisheries [Fisheries Division], 1971)Investigations in the period 1966 to 1969 showed that the bulk of the waste discharged to the Blackwater and Martin is of an organic nature and arises mainly from industries processing milk, sugar-beet and other foods and to a smaller extent from domestic sewage. Pollution, indicated by increases in the biochemical oxygen demand and suspended solids and depletion in dissolved oxygen, was detected below the towns of Rathmore and Mallow on the main Blackwater, Mitchelstown, on a tributary of the Blackwater, the R. Funcheon, and Rathduff on the R. Martin. Depletion of dissolved oxygen sufficiently large to constitute lethal conditions for fish and other aquatic life was recorded only at Mitchelstown and it appears that the high rates of reaeration operating in most cases prevented more widespread deoxygenation and also reduced the extent of diurnal variation. The concentrations of suspended solids recorded in polluted reaches were well below those which are directly injurious to fish but the accumulation of this material as sludge in slow flowing stretches below outfalls constitutes an extra demand on dissolved oxygen which in combination with the B.O.D. of the water may lead to lethal conditions. In the Funcheon, the presence of this material and of sewage fungus on the substratum invalidates the application of the standard theoretical method for predicting the variation of dissolved oxygen. The partial or complete elimination of the normal flora and invertebrate fauna from riffles below the main waste outfalls, and replacement of these by biocoenoses typical of slow flowing silted reaches were recorded in each area. The extent of such changes seemed to be related more to the intensity of sewage fungus growth on the substratum than directly to the chemical quality of the water. Complete elimination of fish was only recorded at Mitchelstown, the stretch affected being half a mile in length. Trout and coarse fish were present in all af the other polluted stretches investigated though in some of these young salmon were absent and trout very few in number. Trout appeared to make better growth in polluted than in unpolluted water, especially at Rathmore, and this is due in part to a greater food supply in the former reaches. Short surveys of the Blackwater estuary indicated that this reach may be slightly polluted. While the investigations indicated that severe pollution occurred in several reaches of the Blackwater and Martin, it was considered that the overall productivity of fish in the rivers was unlikely, at that stage, to have been adversely affected by such pollution.