Sources, transport and sinks of organic matter in Lake Malawi and Lake Victoria, East Africa.
The rate of forest loss in Malawi has been estimated to be from 1.8 to 3.5 % per year with a decline in the total forest cover of 41% between 1972 and 1990. With loss of forest cover to provide land for agriculture, erosion rates have increased leading to increased deposition of nutrients and changes in water clarity of the lake. Three questions were posed to determine the importance of allochthonous inputs to Lake Malawi: (1) How much allochthonous material is added to Lake Malawi? (2) Are the changes in the watershed due to anthropogenic disturbances apparent in the riverine chemistry and the delta 13C and delta 15N? and (3) What happens to the allochthonous material in the lake?
It was found that atmospheric deposition of organic C to the lake constitutes 36% of the organic C from outside sources. The sediment load of particulate organic carbon (POC) from rivers contributes over 50% of the allochthonous organic C. Seasonal effects in Lake Malawi, and its surrounding catchment, are controlled by the monsoonal rainy season. The delta 13C of POC exported from the rivers was strongly related to the catchment yield of total suspended sediments (TSS). In the disturbed catchments with high export of TSS the delta 13C of POC was more enriched than the material exported from forested catchments. Because of the extensive planting of the C-4 crop, maize, it was possible to evaluate the degree of disturbance in different catchment types based on the delta 13C of the POC exported to the lake through the rivers. The C:N in the particulate material from catchments with extensive agriculture and high populations, were variable within the rainy season. Initially the C:N was >16, indicative of terrestrial vegetation, and the C:N decreased throughout the duration of the rainy season as soil was eroded and transported by the rivers. Thus, in the many rivers flowing into Lake Malawi, the degree of disturbance in the catchments of the rivers is reflected in the amount and quality of material transported by the rivers to the lake. In Lake Malawi, both POC and DOC organic carbon are lower than observed in other Great Lakes. Within the annual cycle of the lake, the total lake DOC concentrations can fluctuate by almost 50%. Studies using chemistry and stable isotopes in Lake Malawi indicate that inputs of terrestrial DOC and POC are not currently important to the overall concentration of carbon in the lake, although other nutrients associated with both may be important in autochthonous production. Data from sediment cores indicate that organic matter cycling in the lake has changed in the last 80 years and continues to change, as autochthonous production increases in southern Lake Malawi. Due to the importance of primary production in Lake Malawi, land use changes that increase nutrient export the lake will also alter the storage of organic carbon.
Stable isotopes were also used to study the possible sources of C and N in Lake Victoria.
The changing source of C to the POC was reflected in the delta 13C and concentrations of
dissolved CO2. The inshore waters may become limited in
CO2, which can cause a shift in
the algal population to those capable of using other forms of DIC. The delta 15N of the
particulate matter increased from inshore to offshore, as the algal population switched
from using atmospheric N (N-fixation) to having sufficient DIN to sustain the algal growth.
There was a strong negative correlation between the delta 15N of the particulate material
and the rate of N-fixation.