Favorite Lake Project
Limnology
December 2008
The Basic Limnology of Lake Malawi/Nyasa, Future Research, and Environmental Threats J. H. Fillingham1 1
Atmospheric Science Department, University of Wisconsin Milwaukee 2 December 2008
1. Executive Summary Lake Malawi, also known as Lake Nyasa and Lake Niassa, is located in the southeast corner of continental Africa and is bordered by the nations of Malawi, Tanzania, and Mozambique. The lake is one of the worlds few Great Lakes, with a surface area of 29, 500 square kilometers and an immense volume of 7,775 cubic kilometers. Lake Malawi was formed nearly 20 million years ago in the belly of a rift valley giving it a steep, rocky shoreline. The lake is tropical with a moderate seasonal variation in temperature. The lake never mixes to its full depth reaching nearly 700 meters, due to strong density contrasts between layers. Being eutrophic, Lake Malawi has an anoxic hypolimnion which is just slightly more acidic than its epilimnion. Primary producers are not very species rich, with only a few types of phytoplankton and zooplankton. These plankton species feed the numerous cichlid fishes who call Lake Malawi home. Although Lake Malawi is an extremely interesting source for biological research, as a Great Lake, it is also a great resource for studying physical aspects like those in larger ocean basins. Although there is a fair amount of physically oriented research published on Lake Malawi, surface wind wave topics are left out. Lake Malawi is threatened by several environmental and social pressures. These problems include over fishing, species introduction, and environmental development, from an ever expanding population who rely on it as a basic recourse, and international social dynamics. 2. Introduction Lake Malawi, also known as Lake Nyasa and Niassa, is one of the few Great Lakes of the world. The lake was formed in the belly of a rift valley through tectonic activity nearly 20 million years ago and sits with a surface elevation of 474 meters above mean sea level (Bootsma and Hecky, 2003). Due to the process of its formation, the lake is extremely deep averaging 264m and with a maximum depth of 700m (Figure 1, Appendix 1). The lake is located in the southeast corner of continental Africa, bordered primarily by the country of Malawi, with smaller shorelines in Tanzania, and Mozambique. 1
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Limnology
December 2008
Lake Malawi is set between 9° 30’ S and 14° 30’ S giving it a tropical environment with moderate seasons (Appendix 1) (Eccles, 1974). A hot, wet season begins in November and lasts till April, with monsoon winds from the north. From April to September the climate becomes cooler and dryer with strong south-southeasterly trade winds. The weather remains dry through November, but begins to warm up after September with strong southerly trade winds still dominating. The lake itself is long and narrow, oriented parallel to the dominant wind directions, north and south, with a length of 560 km and a mean breadth of 52.7 km (Appendix 1),(Eccles, 1974). This relatively long shape and predominantly north and south winds forces the development of fairly large seas, with a mathematically derived maximum wave height of 7.9 m. Lake Malawi, although tropical, does experience some moderate variation in its thermal status throughout the year. During the hot, wet, summer season the surface of the lake can get as warm as 29 degree Celsius, with a deep mixed epilimnion reaching to 50-100 m depth. The hypolimnion rarely mixes with surface layers, and holds a fairly consistent temperature below 23 degrees (Figure 2, Appendix 2), (Eccles, 1974; Irving, 2001). In the windier, cooler months, Lake Malawi has a cooler surface close to the temperature of the hypolimnion although it retains its density stratification not allowing much mixing. The temperature of the hypolimnion has risen slightly during the middle part of the twentieth century from 22 degrees to near 23 (Eccles, 1974). This effect may be a result of global climate change, or simply a result of seasonal wind patterns forcing mixing with the hypolimnion in the years analyzed. The heat budget of Lake Malawi, and other tropical lakes, is very dependent on wind and air temperature. Unlike in temperate lakes, where variation in annual, solar radiation play a key role in heat budgets, the evaporation and instability caused by high year round temperatures, relatively low humidity, absence of ice cover, and the intensity of seasonal trade winds are all much more critical factors in tropical lakes (Bootsma and Hecky, 2003). High evaporation rates can also be blamed for the relatively small amount of water that flows out from the basin. In fact, the major source and sink of water to the basin is precipitation and evaporation. Lake levels, as a result, are very sensitive to changes in climate. Throughout the twentieth century Lake Malawi has seen lake levels change by almost 4 m (Bootsma and Hecky, 2003). Lake Malawi can be considered eutrophic. Although the definition of a eutrophic lake is highly subjective, here it seems to fit. The lake is characterized by an oxic-anoxic boundary which separates surface layers from deep layers (Figure 3, Appendix 3). The zero boundary for oxygen occurs at about 200m, giving a vertical profile of oxygen a distinct clinograde shape (Appendix 3, Figure 3). The clinograde O2 curve would lead one to expect an orthograde CO2 curve, although no data was available to
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confirm. The pH of Lake Malawi varies with depth, reaching 8.6 at the surface and sinking to about 7.3 in the hypolimnion (Appendix 3, Figure 4). This fact agrees with the estimation of an orthograde carbon dioxide profile because of the pH buffering due to increased concentrations of DIC in hypolimnion. Lake Malawi is very productive. The lake supports a booming fish population. According to Bootsma and Hecky, 2003, Lake Malawi is the most species rich lake in the world with 500 to 1,000 species of fish. In rocky near-shore waters, more than 500 individuals and 22 species of cichlids can be found in a 50 m2 area. Cichlids are the dominant family of fish in the lake, making up 90% of the total population with a total of eleven different families. Phytoplankton are abundant throughout the lake, although chlorophyll a concentrations are relatively low with very few different species present (Irving, 2001). Zooplankton are even less species rich, providing a major food source for the diverse fish population. Appendix 4 outlines the major species of phytoplankton, zooplankton, and fishes. As a Great Lake, Lake Malawi offers many opportunities for interesting research. One field of particular interest is the lake’s physical limnology. In a lake the size of Lake Malawi, the physics disserve due attention as they quite often regulate the biogeochemical cycles so often researched. In a changing physical world, it is also important to discuss the changing climate of any aquatic system. Studies in the this area span many fields, however, around Lake Malawi, the ever expanding human population and growing need for food is putting pressure on the fragile ecosystem which hosts such a great diversity of aquatic species. 3. Assessment and Evaluation a) The importance of Wind Wave Research There is extensive research available on the physical limnology of Lake Malawi (Eccle, 1974; Wuest and Piepke 1996; Irving, 2001; Bootsma and Hecky, 2003). However, much of this research, with a few exceptions, is on internal dynamics related to biological systems. Without negating the importance of biological research on aquatic science, the magnitude of purely physical processes needs to be emphasized. There is little to no research done on Lake Malawi directly related to wind wave development, and its effects on the basin morphology, thermal structure, human activities, and geochemistry. Eccles, 1974, mentions the ability of Lake Malawi to develop a large sea state, and Dr. Harvey Bootsma, of the Great Lakes Water Institute, who has studied Lake Malawi extensively, has recognized the importance of such parameters (Bootsma and Hecky, 2003; Bootsma, 1993, and personal coorespondance).
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December 2008
Lake Malawi has a total fetch of 560 km and a maximum breadth of 75 km (Bootsma and Hecky, 2003). This makes the lake surface area just larger than that of Lake Michigan, one of the largest Laurentian Great Lakes. Wave height is proportional to fetch, the time wind has been blowing over the fetch distance, and the stability of the atmospheric boundary layer (Liu and Ross, 1980). During the winter months, cool trade winds rip through the basin nearly parallel to the length of the lake (Eccles, 1974). Cool winds blowing over a warm water surface, setting up instability in the atmospheric boundary layer, and over the full length of the lake, forms a situation ideal for the formation of a large, boisterous sea. In 1980, Dr. David Schwab, a research scientist with the National Atmospheric and Oceanic Administration, compared and applied an oceanic wave forecast model to the Laurentian Great Lakes. This wave model is being used today to forecast for recreation, commerce, and shipping. The need for such a numerical model on the Great Lakes was made clear in 1975 after the sinking of the Edmund Fitzgerald, a 222m iron ore tanker, in Lake Superior. Wave heights were estimated to be 7m, close to 25 feet, when the Edmund Fitzgerald went down (Hulquist et. al., 2006). The mathematically derived value for the maximum possible wave height in Lake Malawi is 7.9m, with recorded wave heights of over 4m (Eccles, 1974). With a booming fishing industry, it seems apparent that research into the application of a numerical wave model on Lake Malawi is imperative. Lake Malawi may also serve as a test ground for improving wave forecasting for many other applications as it is unique in its size, thermal structure, climate, and geographic location. The rocky, mountainous coastline of Lake Malawi may also provide ample opportunity to take advantage of the sea state. With large points of land jutting out into the lake along its length, wave surfing and wind surfing would be outstanding. Although firsthand accounts of such activity are nonexistent for this lake, these sports are common place throughout the Laurentian Great Lakes. Lake Malawi may even be better suited for such activity as the lack of ice and winter cold make it much more pleasant arena than temperate Great Lakes. Surface wind waves play a very important role in geochemical interactions of a water body with the atmosphere. The importance of carbon dioxide exchange between the atmosphere and ocean has been well documented in the last half century as this compound has been associated with global climate change. Wind waves are the dominant process controlling the transfer velocity of carbon between the airwater interfaces (Donalen et. al., 2002). Only recently has this topic been approached in relation to lakes even though they are subject to numerous physical processes such as upwelling of nutrient rich bottom water and internal mixing which expose high concentrations of carbon dioxide to the atmosphere. This
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December 2008
topic is currently being studied in relation to wave processes and invasive species dynamics in Lake Michigan. Often lakes are treated as no more than corn fields when global carbon models are developed. Future research in Lake Malawi should focus on the collection of carbon dioxide exchange data in order to eliminate this large gap. Data collection should include carbon dioxide concentrations in atmospheric and lake pools including storage and flux properties within the lake and sediments. As population grows around the lake, the anthropogenic contribution to the cycle will grow as well. The foundation for this type of research has already begun as in Ramlal, 2003. As we learn more about Great Lake carbon dynamics, the more they influence the accuracy and development of future global climate models. b) Environmental Threats It is well documented that Lake Malawi is close to the point of being over fished. Deforestation and environmental issues surrounding the lake are putting pressure on the lake’s ecosystem as well (Bootsma and Hecky, 2003; Ramlal et. al., 2003). Fishing accounts for a huge proportion of the economies of countries in the East African region (Bootsma and Hecky, 2003). It is estimated that 70% of the dietary animal protein in the country of Malawi is from fish. Annual catchment from the lake is nearly 30,000 metric tons, with most of this coming from the near shore zone. Unlike its sister lake, Lake Tanganyika, which has a well developed offshore fishing economy, Lake Malawi retains a more modest system. Research shows that it may be economically viable for Lake Malawi to grow matching the development of Lake Tanganyika, but it is not known what effect this will have on pelagic fish stocks (Menz and Thompson, 1995; Bootsma and Hecky, 2003). Lake Malawi’s primary source of water is rain to the lake basin; however, there is significant inflow from streams and rivers which contain high nutrient loads (Ramlal et. al., 2003). With increasing deforestation and intensive agriculture in the Malawi water shed in the past 30 to 40 years, there has been a dramatic increase in allochthonous organic material entering the lake. Significant levels of dissolved organic carbon, particulate carbon, phosphorus and nitrogen are all inputs to the lake from river sources (Bootsma and Hecky, 1999; Ramlal et. al., 2003). It is apparent that there is a need for stringent management practices as the world develops around Lake Malawi. The lake is bordered by three countries, and a lot like the Laurentian Great Lakes, is subject to problems in organizing and implementing management practices (Bootsma and Hecky, 2003). Because the countries bordering Lake Malawi do not have a common colonial government, as is the case for the countries bordering Lake Victoria for example, collaboration at an international level is relatively
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December 2008
difficult. Research has begun along these lines, Bootsma and Hecky, published a water quality report in 1999 for the Lake Malawi/Nyasa Biodiversity Conservation Project. In nine chapters this report discusses issues in nutrient cycling, pollutant inputs, and physical processes critical to Lake Malawi water quality. 4. Summary As one of the worlds few Great Lakes, Lake Malawi plays a vital role in regional and global issues. The lake itself is very large with a surface area of 29,500 square kilometers and a total fetch of 560 kilometers. Forming in a rift valley from tectonic processes, the lake has a characteristic rocky shoreline. Seasonal weather patterns bring wet and dry seasons with predominantly north and south winds which whip up large waves. The lake is long and narrow oriented parallel to the common wind directions with a mean breadth of over 50 kilometers. The lake is extremely productive with a very diverse fish population made up of mainly cichlid fishes, who feed on the limited abundance of plankton. Lake Malawi holds a eutrophic status with small seasonal variation in temperature, with temperatures hovering around 25 degrees C. Lake Malawi is well researched in most limnologic fields. One area of lacking research is on the lake’s ability to produce very large wind waves. These large waves most certainly have a pronounced effect on local fishing and recreation, as well as influencing geochemical cycles related to global climate. Carbon exchange between the lake and atmosphere may play a vital role in future climate models as carbon dioxide gas influences the green house effect in the atmosphere. The development of a numerical, wave prediction model like ones implemented in the Laurentian Great Lakes is essential for full understanding of Lake Malawi limnology. Environmental threats and over fishing along with social dynamics are putting a strain on the fragile Lake Malawi ecosystem. The development of an offshore fishing fleet may dramatically affect fish stocks and species diversity within the lake, while loading of nutrients from runoff of expanding deforestation and agriculture threaten as well. The need to bring the bordering countries around Lake Malawi together and practice crucial management practices is essential to the sustainability of the Lake Malawi system.
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Appendix #1 Physical Parameters Lake Malawi (Nyasa / Niassa) Location of Lake: • •
South East Africa Bordering Countries: o Malawi o Tanzania o Mozambique
Latitude: 9° 30’ S to 14° 30’ S Longitude: ~ 34° 00’ E to ~ 35° 09’ E Elevation: 474 m amsl Formation and Age: • • •
Tectonic Basin; Rift Valley 10 – 20 million years old Core Dates: > 46,000 BP
Major Water Sources and Sinks: •
Sources: 1. Rainfall to catchment basin 2. Inflow from rivers Sinks: 1. Evaporation 2. Outflow through Shire River
•
Morphometry: Length (l): 560 km
Area (A): 29,500 km2
Breadth (b): 75 km
Mean Breadth (b’): 52.7 km
Shoreline (L): ~ 1500 km
Shoreline Development (DL): ~ 2.46
Maximum Depth (zm): 700 m
Mean Depth (z’): 264 m
Depth of Cryptodepression (zc): 226 m Volume (V): 7,775 km3
Volume Development (DV): 1.13
Area of Contributing Watershed: 100,500 km2
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Figure 1: Bathymetry chart and regional map of Lake Malawi. Images from Bootsma and Hecky, 2003 and Eccles, 1974, respectively.
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Appendix #2 Dynamics Lake Malawi (Nyasa / Niassa) Hydrodynamics: • • • • •
Maximum Fetch: 560 km Maximum Surface Wave Height: 7.9 m Period of transverse uninodal standing wave: 6.12 hours Period of longitudinal uninodal standing wave: 34.5 min Upwelling is observed
Thermal Characteristics: • • • •
Monomictic Tropical Lake Minimum Water Temperature Observed: ~ 16 °C Maximum Water Temperature Observed: ~ 29 °C No annual ice cover
Light Data: •
•
Daily total undepleted solar radiation by season (mEins/m/day): o Summer: 60,000 o Autumn: 50,000 o Winter: 40,000 o Spring: 50,000 Extinction Coefficient (k): ~ 0.14 – 0.18
Figure 2: Seasonal temperature vs. depth profiles from Eccles, 1974. Data collected from Nikhata Bay deep water station. From left to right, winter, early spring, summer, fall, late fall.
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December 2008
Appendix #3 Chemistry Lake Malawi (Nyasa / Niassa) Dissolved Gases: Oxygen: •
•
Calculated Equilibrium Concentration of Dissolved Oxygen for Lake Malawi: o Based on Ideal Gas Law: , with P = 101.3 kPa, R = 8.31 J/mol K Temp (°C)
mol / L
g/L
25
0.041
1.312
20 10
0.042 0.043
1.344 1.376
4
0.044
1.408
Oxic-Anoxic Boundary at ~ 200 m
Figure 3: Temperature vs. oxygen profiles for summer and winter. Estimates based on data from Irving, 2001. Profiles illustrate oxic-anoxic boundary at approximately 200 m depth.
Carbon Dioxide / pH: •
pH: o o
•
Epilimnion Hypolimnion
= 8.6 = 7.3
Relative proportion of carbon compounds: o CO2 = 2.5 % o HCO3- = 97.2 % o CO3- = 3.2*10-3 %
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Figure 4: Temperature, pH, and carbon dioxide profiles with depth. Data estimated from Irving, 2001. Profiles show decrease in pH to the hypolimnion, and eutrophic, orthograde carbon dioxide profile.
Dissolved Solids: Inorganics: • •
Specific Conductance: 230 µS/cm @ 25 °C o Conductivity generally increases with depth Concentration of major ions (µmol/L): Ca+: 450
•
Mg++: 300
Na+: 840
K+: 150
Concentration of major anions (µmol/L): SO4- : 30
Cl- : 100
Nutrients: (Bootsma and Hecky, 2008) (µM, Layer Volume = 2780, 2330, and 2719 respectively) Layer
NO3-
NH4+
TDN
SRP
TDP
SRSi
Epilimnion
1.95
0.36
6.5
0.19
0.60
19.6
Metalimnion
4.22
0.66
8.1
0.74
1.49
79.0
Hypolimnion
0
18.31
22.6
1.74
2.20
190.0
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December 2008
Figure 5: Profiles of nutrients in Lake Malawi, from Bootsma and Hecky, 1999. Profiles show a general trend of increasing nutrient concentrations with depth with a sharp increase at the anoxic boundary. There is a peak in oxidized nitrogen in the metalimnion.
Organics: • • •
Total Dissolved Carbon: 41.7 – 76.5 (1010 mol) Total Particulate Carbon: 5.6 – 6.4 (1010 mol) Chlorophyll a: 0.51 (µ/L)
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Appendix #4 Biology Lake Malawi (Nyasa / Niassa) Bacteria: (Bootsma and Hecky, 1999) •
•
Nitrogen Fixation: o ~ 78 mmol m-2 yr-1 o Epilithic Littoral Zone o Heterocystuous cyanobacteria: Calothirx and Rivularia (most common genera) Denitrification: o Occurs in the water column o Oxic-anoxic interface o Difficult to measure
Phytoplankton: (Irving, et. al., 2001) • • •
Summer: Cyanophytes; Anabaena flos-aquae, Cyclindrospermopsis Winter: Bacillariophyta; Stephanodiscus, Cyclostephanos Areal Biomass (mg Chlorophyll a m-2) o Summer: ~ 110 – 160 o Winter: ~ 50
Zooplankton: (Irving, et. al., 2001) • •
•
Species poor open waters Crustacean Species: o Calanoid copepod: Troppdiaptomus cunningtoni o Cyclopoid copepod: Mesocyclops aequatorialis & Thermocyclops neglectus o Cladoceran: Diaphanosoma excisum & Bosima longirostis Larvae of the depteran Chaoborus edulis
Fish: (Irving, et. al., 2001) • •
• •
Standing biomass of fish: ~ 1.1 – 1.2 g dry wt. m-2 Planktivorous fish o Cichlidae Diplotaxondon limnothirissa & cheironym Copadichromis quadrimaculatus Rhamphochromis longiceps & ferox o Cyprindae o Engraulicypris sardella Clariidae Mochokidae o Synodontis njassae
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References Bootsma H. A., 1993. Algal Dynamics in an African Great Lake, and their Relationship to Hydrographic and Meteorological Conditions. PhD Dissertation, Department of Botany, University of Manitoba. Bootsma, H.A., and Hecky, R.E. 2003. A Comparative Introduction to the Biology and Limnology of the African Great Lakes. J. Great Lakes Res. 29 (Suppliment 2): 3-18. Bootsma, H.A., and Hecky, R.E., 1999. Nutrient Cycling in Lake Malawi/Nyasa. Water Quality Report, SADC/GEF Lake Malawi/Nyasa Biodiversity Conservation Project [online]. Available at http://www.uwm.edu/~hbootsma/Lake%20Malawi/WQR/WQR.htm [accessed 25 November 2008]. Donelan, M. A., Drennan, W. M., Saltzman, E. S., Wanninkhof, R., 2002. Gas Transfer at Water Surfaces. Geophysical Monographs 127. American Geophysical Union, Washington DC. Eccles, D.H. 1974. An Outline of the physical limnology of Lake Malawi (Lake Nyasa). Limnology and Oceanography, V. 19(5). Guilford, S.J., Bootsma, H.A., Fee, E.J., Hecky, R.E., Patterson, G. 2000. Phytoplankton nutrient status and mean water column irradiance in Lake Malawi and Superior. Aquatic Ecosystem Health and Management 3, 35-45. Hultquist, T. R., Dutter, M. R., Schwab, D. J., 2006. Reexamination of the 9-10 November 1975 “Edmund Fitzgerald” Storm Using Today’s Technology. American Meteorological Society. Access online: www.glerl.noaa.gov, 29 November 2008. 620-622. Irvine, K., Patterson, G., Allison, E. H., Thompson, A. B., Menz, A., 2001. The pelagic ecosystem of Lake Malawi, Africa: Trophic structure and current threats. The Great Lakes of the World (GLOW) Food Web, Health & Integrity. Backhuys Publishers, Leiden, The Netherlands. Liu, P. C., Schwab, D. J., Bennett, J. R., 1984. Comparison of a Two-Dimensional Wave Prediction Model with Synoptic Measurements in Lake Michigan. J. Physical Oceanography. V. 14: 1514-1518. Liu, P. C., Ross, D. B., 1980. Airborne Measurements of Wave Growth for Stable and Unstable Atmospheres in Lake Michigan. J. Physical Oceanography. V. 10: 1842-1853. Menz, A., Thompson, A. B., 1995. Management report of the UK/SADC pelagic fish resource assessment project, Lake Malawi/Nyasa. Chatham, UK: Natural Resources Institute. Via Bootsma and Hecky, 2003. Ramlal, P.S., Hecky, R.E., Bootsma, H.A., Schiff, S.L., and Kingdon, M.J. 2003. Sources and Fluxes of Organic Carbon in Lake Malawi/Nyasa. J. Great Lakes Res. 29 (Suppliment 2): 107-120. Wuest, A., Piepke, G., 1996. Combined Effect of Dissolved Solids and Temperature on the Density Stratification of Lake Malawi. The Limnology, Climatology, and Paleoclimatology of the East African Lakes. Gordon and Breach Science Publishers, Netherlands.
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