Nutrient Loading. Item Type Report Section Authors Mayanga, V.; Osio, J.; Mwebembezi, L.; Myanza, O.; Opango, P.; Majaliwa, J.; Oleko, C.; Okwerede, L.; Gor, S.; Byamukama, D.; Mathayo, A.; Masongo, F.; Kanyesigye, C.; Kitamirike, J.; Semalulu, O.; Hecky, R. Publisher Lake Victoria Environment Management Project (LVEMP) Download date 25/09/2021 02:38:00 Link to Item http://hdl.handle.net/1834/6906 Regional 103 Lake Victoria Environment Report CHAPTER 7 Nutrient Loading V. P. Mnyanga1, J.O. Osio2, L. Mwebembezi3, O.I. Myanza1, P.O. Opango2, J.G.M. Majaliwa4, C.O. Oleko2, L. Okwerede5, S.A. Gor2, D. Byamukama6, A.A. Mathayo7, F.O. Masongo2, C. Kanyesigye5, J.M. Kitamirike3, O. Semalulu4 and R.E. Hecky8 1 LVEMP, Water Quality and Ecosystem Management Component, Box 211 Mwanza, Tanzania 2 LVEMP, Water Quality and Ecosystem Management Component, Box 1922 Kisumu, Kenya 3. LVEMP, Water Quality and Ecosystem Management Component, Box 19 Entebbe, Uganda 4. LVEMP, Land-Use and Management Component, Box 7065, Kampala-Uganda 5. LVEMP, National Water and Sewerage Corporation, Box 7053, Kampala-Uganda 6. LVEMP, Makerere University Institute of Environment and Natural Resources, Box. 7298, Kampala Uganda 7. LVEMP, Water Quality and Ecosystem Management Component, Box 1112, Musoma, Tanzania 8. Biology Department, University of Waterloo, 200 University Ave. W., Waterloo, Antorio N2L 3GI ABSTRACT. It is now recognized by most of the scientific community that Lake Victoria is enriched with nutrients. There are, however, conflicting reports on the magnitude of nutrients received from different sources and the dynamics of nutrients in the Lake. Studies were carried out to determine the lake nutrient balance and suggest strategies for sustainable utilization of the resources. Lake nutrient balance is essential for understanding primary productivity and ecosystem function and for planning nutrient management strategies. The current findings identify major point and non point sources of nutrients and estimate the rates of sedimentation into Lake Victoria. The determination of pollution loads from point sources was limited to the Biochemical Oxygen Demand (BOD5), Total-Nitrogen (TN), and Total-Phosphorus (TP). For the non-point pollution sources emphasis was given to TN, TP and TSS, the loads from rivers and atmospheric deposition have been calculated, both due to their relevance as quality indicators and their contribution to eutrophication of the Lake. For the purpose of determining the nutrient balance of the lake, the sedimentation rates in the lake have also been calculated both fluxes per unit area and total lake bottom accumulation. Municipal effluent load was higher than industrial one, but they both represent a threat to the community downstream the discharge point and the littoral zone of the lake. Atmospheric deposition was the overall dominant source contributing about 39,978 and 167,650 tons of TP and TN respectively to the lake annually. The riverine loads are estimated at 9,270 of TP and 38,828 tons/y of TN respectively, and represented in both cases 80% of the total non-point load. Point sources are estimated to contribute about 4.3 of TN and 1.9 tons/year of TP. The cores dated and analyzed show dry weight accumulation rates of 100 g.m-2y-1 to 300 g.m-2y-1. Linear regression indicates sedimentation rates of 0.5 to 1 mm per year. Furthermore, highest rates of permanent sediment accumulation occur in the deepest areas of the lake. However, the study indicates that rate of nutrients regeneration is 90% for C and N and 60% for P. It is therefore recommended that pollution loading into the Lake be controlled by reducing point sourcing and providing tertiary treatments for removal of Water Quality and Ecosystems Component Lake Victoria Environmental Management Project Regional 104 Lake Victoria Environment Report P, use of phosphorus free detergents, cleaner production technologies, and addressing non-point sources by improved land management. An initial goal of reducing the anthropogenic phosphorus loading to the lake by 30% would reverse the current upward trend and achieve water quality conditions that occurred in the 1980’s when fish production was at its maximum. Trans-boundary efforts may be required to control atmospheric deposition into water bodies. INTRODUCTION Well-documented changes have occurred to the water quality in Lake Victoria and its ecosystem over the past five decades (Ref). Among them, the most widespread and currently most serious is eutrophication (Chapter 6). It is now evident that Lake Victoria has received increased loadings of some plant nutrients from the catchments due to population growth and associated land-use changes as well as industrial and urban development. The increased additions of nitrogen (N) and especially phosphorus (P) have saturated the lake’s biological capacity to absorb these, and have caused the lake’s N-fixation rates and P concentrations to rise to the extent that algal growth is now light limited. The main sources of nutrients to the lake can be defined as point and non-point sources. The point sources have traceable and quantifiable origin, mainly from industrial establishment, municipal effluent, shoreline settlements and urban runoff. On the other hand non-point pollution sources generally originate from land run off, wet and dry atmospheric precipitation, and ground water which transfer nutrients, mobilized by broadly distributed human activities, into rivers and directly to the lake. Ground water inputs are considered minor based on water budget studies (McCann, 1972, Rwegoshora et al, (2004) and are not further considered in this chapter. Of the materials delivered to the lake via rivers and the atmosphere, only relatively small amounts of these nutrients are flushed out of the Nile due to minor portion of the total water budget leaving the lake at Jinja. Most of the dissolved and particulate nutrient material entering the lake is processed by biological organisms within the lake and eventually sedimented as detritus, processed into gaseous forms e.g. bacterial denitrification which produces nitrogen gas, or simply settles to the bottom of the lake e.g. mineral sediments. Hence knowledge of sedimentation rates and patterns is necessary to fully account for the materials brought to the lake. Sedimentation studies are critical in calculation of the nutrient mass budget, determining the fate of added nutrients (volatilization or sedimentation) and estimation of internal and external loading of nutrients. Although it is now recognized by most of the scientific community that the lake is enriched with nutrients, there are conflicting reports on the magnitude of nutrients received from different sources and the dynamics of nutrients in the Lake (LVEMP 2002) Therefore the objectives of this chapter are to identify and quantify sources of the nutrient loading which are essential for understanding primary productivity, determining algal community composition and resulting ecosystem function. If there is agreement that nutrient loading is excessive and should be reduced then the information on loadings will be critical to determining nutrient management strategies. Point Sources Point sources are, in principle, the easiest sources to estimate and the easiest to control. The nutrient loads from point sources enter receiving waters at defined and relatively confined outfalls that can be sampled and, if necessary, treated. Because these outfalls are collected from areas having large Water Quality and Ecosystems Component Lake Victoria Environmental Management Project Regional 105 Lake Victoria Environment Report populations and intensive water use, these point sources can be important components of the total loading to some lakes. In North American Great Lakes, the reduction of P at point sources accomplished major reductions in total loading and recovered the lakes’ from eutrophication (Barbiero et al. 2002). Because point source discharges are often dominated by products of human waste and agro-processing industries, they have high oxygen demand that can affect the oxygen status of receiving water and their suitability for sustaining higher life forms. Oxygen conditions can also affect the availability of N and P with hypoxia favoring release of P from mineral matter and loss of N by denitrification. The technological aspect of point sources also identifies them as a clear increment over natural nutrient loading; and therefore, directly attributed to anthropogenic (caused by humans) nutrient loading. Methods of Estimating Loads The determination of pollution loads from point sources was limited to the following parameters: Biochemical Oxygen Demand (BOD5), Total-Nitrogen (TN), and Total-Phosphorus (TP) because of their relevance as quality indicators and their contribution to eutrophication of the Lake. Along with these key parameters, pH, conductivity, Chemical Oxygen Demand (COD), Alkalinity, Chloride, Dissolved Oxygen (DO), Faecal Coliforms (FC), TSS (total suspended solids), and heavy metals were often analyzed and available in the national water quality reports. Two criteria were used for selection of point sources for the assessment. Only towns with more 10,000 inhabitants have been included, defining smaller towns as rural settlements. For the selection of industries, only those referred to as "wet" industries (using water in the production) were considered. Various methods have been used to quantify the industrial and municipal effluents depending on data availability. These