2000 - 2001

Committente: Università degli Studi dell’Insubria / Istituto Oikos / TANAPA (Tanzanian National Parks)

Aquatic ecosystems of Arusha National Park, Tanzania: limnological survey

Cesare M. Puzzi, Stefania Trasforini, Massimo Sartorelli, Beniamino Barenghi, Mauro Bardazzi, Silvia Porrini

The “Limnological survey of the aquatic ecosystems of Arusha National Park, Tanzania”, includes all the activities carried out in two seasons, the rainy and the dry season, at the Arusha National Park in October/November 2000 and April/May 2001. It is a chapter of the Project “Biological Conservation and Sustainable Management of Mount Meru natural system (Arusha National Park and surrounding areas, Tanzania)” – Mount Meru Conservation Project – MMCP –.

The aim of this limnological survey is contribute to the whole scientific assessment of natural resources of the Mount Meru natural system, in order to integrate resource conservation and economic development of the areas surrounding ANP. Specific objective of the survey is the limnological characterisation of ANP aquatic ecosystems, by the quality water monitoring (both lakes and rivers) and the qualitative assessment of biological communities.

The work is the result of a multidisciplinary researchers team, including biologists, naturalists, ichthyologists and environmental engineers.

On the base of chemical-physical analysis and of literature data, the peculiarity of the waters of ANP and of Mount Meru natural system can be described. In particular, hydrochemical features of the lakes are the following:

	high salinity and alkalinity. All investigated lakes, except Longil, show very high pH and dissolved salt concentration values, compared to common freshwater, especially for sodium carbonate and bicarbonate. These features found their explanation in geology and pedology of the territory and also in the concentration process of solutes after evaporation, that is determined determined by the low depth of these closed basins. This enrichment has been verified comparing literature data. Previous chemical analysis of these waters (Kilham, 1971), show concentration values lower than the ones recorded in 2000, indicating a more or less marked increase of different ions, even if proportions between ions are the same. Also Lake Longil shows predominance of sodium and carbonate ions, but it is characterised by lower salinity and pH values, probably because of the different nature of its basin and for its different genesis;
	high trophic level. With OECD trophic classification methods, all lakes are hypertrophic, although Longil shows a considerably lower total phosphorous content than other lakes. Considering the high trophic level that normally characterised saline and alkaline African lakes (Talling & Talling, 1965; Symoens, 1981), further study about trophic level of these waters can be useful, with the aim to suggest possible specific criterion of trophic classification for this hydrochemical typology;
	considerably high fluoride contents. With the exception of Lake Longil, all examined lakes show concentration values of this ion considerably up to the established limit for  drinkable use of water (1,5 mg/l), confirming available literature data. This matter is particularly important for its repercussions on human and wild animal health.

Also rivers of ANP show the same chemical-physical features of lakes, with high conductivity values, indicating high dissolved salts content, and with high pH, confirming the alkaline typology of these waters, typical of a volcanic area like this one.

Results of statistical analysis, about a sample of water bodies of Central-Eastern Africa (including investigated ones), confirm the chemical-physical characterisation results. About lakes, those of ANP show ionic concentration values among the highest in the sample, with the exception of calcium and magnesium values that are relatively low. This analysis confirms also the high fluoride content that characterised waters of the Park.

According to the hydrochemical classification through triangular diagram, all waters of ANP (also river waters) belong to class II (sodium-chloride waters), when the majority of the database rivers of Central-Eastern Africa are classified as calcium-bicarbonate waters (class III). In this hydrochemical classification the majority of the springs in the database is classified as sodium-chloride waters (class II), because of their high sodium chloride content.

A reduced abundance of taxa results from the analysis of biological communities, with the dominance of few tolerant species to considerably restrictive environmental conditions that define investigated waters. This features are also reported in literature for all African saline and alkaline waters, characterised by the presence of monotonous community with the dominance of few species. Results obtained from analysis of collected samples are synthesised as follow:

	in collected samples of phytoplankton population, the majority of algal biomass, in all lakes, is represented by a few genera of Cyanophyceae (mainly by Spirulina and Microcystis); in terms of relative abundance, Chlorophyceae follow previous class (mainly the genera Oocystis). Diatomeae are the class characterised by the highest number of collected genera, but with a relative abundance mainly occasional or rare. The lake with the most diversified phytoplankton community is Longil, followed by other lakes with more and more extreme environmental conditions;
	in collected samples, zooplankton population is mainly represented by Rotifera, which are the most diversified fauna group and, in the majority of lakes, also the most abundant one. Cladocera and Copepoda are not very represented in the zooplankton population, in terms of both number of taxa and (with the exception of lakes Longil and Small Momela) relative abundance; possible explanations of this reduced presence of Cladocera and Copepoda are their higher sensibility to high salinity and alkalinity, to high fluoride concentrations and to eutrophication; in addition to these factors there is also the possible harmful effect that some Cyanophyceae can have by releasing toxins;
	also macroinvertebrate community, sampled in Maksoro and Ngare Nanyuki rivers, is represented on the whole by few systematic units. An higher diversification has been recorded in Maksoro River, in terms of total number of taxa, probably because of its less restrictive environmental conditions; this river course is characterised by conductivity and pH values lower than the ones found in Ngare Nanyuki River;
	according to samplings and evidences of Park staff, among investigated lakes, only in Lake Longil has been fished the species Oreochromis spirulus in the subspecies form niger, that belongs to the family of Cichlids. This species is reported for the first time in Tanzania; probably it was introduced by man in the past. In Maksoro River two species of genus Barbus have also been collected, but they need more studies by TAFIRI for final specific classification. In this River the presence of Clarias gariepinus has also been reported, but not confirmed by samplings.

Available data concerning phytoplankton and zooplankton samples have been utilised to analyse similarities in the composition of taxa between examined lakes by cluster analysis. Results of this analysis indicate strong similarity between lakes Big Momela and Tulusia and between lakes Small Momela and Lekandiro, very similar also in terms of chemical-physical features. Lake Longil confirms its diversity from other lakes, as chemical-physical characterisation indicates.

This limnological investigation represents a preliminary study to characterise the quality of aquatic ecosystem of ANP, in terms of hydrochemical conditions and biological communities. Further complementary investigations can be useful in the future to get better details of limnological aspects that were not studied during the survey 2000-2001. Examples are: a more complete study of the watershed and a nutrient loading assessment in the lake basins of ANP, coming from surrounding villages, that can give TANAPA useful suggestions for a correct management of the Park, considering also needs of local communities; also the presence of thermal stratification in the lakes of the Park can be interesting to be studied, through monthly measurements, contributing to increase the available information about tropical lake basins so characteristic like the investigated ones.