Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve,Kenya

Lake Bogoria, in the Rift Valley of Kenya is an extreme saline lake (conductivity 40–80 mS cm^–1, alkalinity 1500 m equ l^–1). It is hydrologically more stable than the other, endorheic lakes in Kenya, because it is deep – maximum depth at present just over 10 m in an area of 3000 ha – and so does not have periods when it is dry. It is ecologically simple, with only one species dominating the phytoplankton – the cyanobacterium `spirulina', Arthrospira fusiformis. Its biomass and productivity were very high – biomass between 38 and 365 g l<sup>–1</sup> chlorophyll `a' and 3.4–21 × 10³ coils ml^–1 and net production between 0.24 and 1 gm C m³ h, the latter in a narrow zone of less than a metre. There were no macro-zooplankton in the plankton and the only grazer of A. fusiformis was the lesser flamingo, Phoeniconaias minor,which occurred irregularly in very high concentrations (in excess of 1 × 10⁶). Detritivory in the benthos was effected by a single chironomid species, Paratendipes sp., at a maximum density of 4 × 10⁴ m^–2. The mean daily emergence of adult chironomids was estimated to be 1 × 10³ m^–2, the maximum 3. There was no littoral plant community within the lake but 44 dicotyledonous and 31 monocotyledonous plant species in the drawn-down zone and adjacent to it. A diverse draw-down terrestrial invertebrate fauna, only superficially described here, processed the flamingo feathers and carcasses, with other detritus such as chironomid pupal exuviae and decaying A. fusiformis scum. About 50 bird species depended upon the chironomids, either as they emerged through the water column as flying adults or later on the shoreline as floating pupal exuvia and dead adults. The lake has high conservation value because of three bird species in particular – lesser flamingo, Cape teal and black-necked grebe. The former provides real economic value in a region otherwise impoverished, because of the spectacle of tens of thousands of flamingos set against the landscape of hot springs and fumaroles at the lake edge, which draws 15000 visitors per annum. P. minor has experienced three periods during the past ten years when major mortalities have occurred, the last of which killed 700 birds day^–1. This could have involved as many as 200000 birds (about 1/5th of the maximum population at this lake) if mortality was at a constant rate for the nine months it was observed. Causes of mortality have been suggested as avian tuberculosis, poisoning from cyanobacterial toxins or from heavy metal contamination at Lake Nakuru, but it is still not yet clear what contribution each makes to the problem.     

Patterns of variation in waterbird numbers on four Rift Valley lakes in Kenya, 1991–1999

Waterbird populations were censused each January from 1991 to 1999 at Lakes Naivasha, Elmenteita and Nakuru and from 1992 at Lake Bogoria. These shallow lakes in the Kenyan Rift Valley fluctuate greatly in water level and alkalinity. All but Naivasha are usually saline; Nakuru and Elmenteita at times support fish, while Bogoria is fishless. A standardized logarithmic index of relative abundance (value 1.0 for the mean) was calculated for each major waterbird group at each lake, and for Naivasha, Elmenteita and Nakuru combined (combined lakes). Its variance was used to compare levels of variation within and across lakes. For the combined lakes, there was high variance in large piscivores (whether combined or separated into groups), grebes, rallids and flamingos. There was low variance in Palaearctic waders (combined or separated into groups), ibises and spoonbills and birds of prey. However, the lakes generally showed idiosyncratic patterns of variation across the different groups. Variance in the indices for birds of prey and kingfishers were consistently low (max. 0.036 and 0.042, respectively), but no group had consistently high variance across all sites. The variance for all birds (other than flamingos) combined was low (0.018 – 0.085) and similar across all lakes and for combined lakes (0.018). For the combined lakes, the variance for flamingos was five times higher than for all other birds (p<0.05), though the two variances were almost equal for Bogoria. Flamingos were the most variable at Naivasha (variance 0.281) followed by Elmenteita (0.177), Nakuru (0.101) and Bogoria (0.024, and significantly lower than all the rest, p<0.05). This was opposite in order to the mean numbers of flamingos recorded at each site. Large piscivores were relatively stable at Naivasha (variance 0.005) but much more variable at Elmenteita (0.199) and Nakuru (0.269). Patterns of variation within lakes were correlated for some groups, such as waders at Naivasha and large piscivores at Nakuru. These correlations could be related to local ecological conditions. However, there were few large correlations across sites, and these were mainly direct. There was, therefore, no evidence that a fixed population of waterbirds was distributing itself across sites according to conditions. Each lake thus seems to represent and independent entity, while the waterbirds they host evidently move much more widely afield than this portion of the Rift Valley.     

Co-ordinated waterbird counts: the Kenyan experience

Nasirwa, O. & Bennun, L. A. 2000. Co-ordinated waterbird counts: the Kenyan experience. Ostrich 71 (1 & 2): 99–101.

In Kenya annual waterbird counts have been carried out consistently since 1991. The counts are a collaborative effort between the National Museums of Kenya, Kenya Wildlife Service and the Kenya Wetlands Working Group. Their aim has been to train and equip Kenyan volunteers to monitor the ecological status of wetlands. At the same time, data on waterbird numbers are collected in a standard way. Focal wetland sites are the major Rift Valley lakes. Increased volunteer response and experience in counting led to the expansion of the monitoring from an initial eight sites to 26 sites in 1996. Over 500 volunteers have so far been involved and around 100 have regularly taken part. Many counters who cannot participate regularly in the main counts remain in touch and provide information on waterbird numbers in other wetland sites. The high turn-over of volunteers has created difficulties but has also generated greater awareness of the counts and the importance of wetlands. One result is increased competition for the opportunity to take part in the training. Enough counters are now trained to expand the coverage of sites further, but this is constrained by limited resources (including equipment, transport and time), difficult logistics and the organisation burden. Ensuring sustained, long-term funding for the counts also remains a problem. Experiences arising from the organisation and co-ordination of the counts over the past six years are reviewed and a future strategy is presented.     

Trends in waterbird numbers in the southern Rift Valley of Kenya

Bennun, L. & Nasirwa, O. 2000. Trends in waterbird numbers in the southern Rift Valley of Kenya. Ostrich 71 (1 & 2): 220–226.

Each January since 1991, volunteer teams have counted waterbirds at major wetlands in the southern Kenyan Rift Valley. There has been consistent coverage at Lakes Naivasha, Elmenteita, Nakuru and (since 1992) Bogoria. These lakes are shallow and, except for Bogoria, fluctuate greatly in extent; all but Naivasha are saline. Lake levels were moderately high in 1991–1993 but have been generally low since. Flamingo totals for the three saline lakes combined were more than one million from 1992–1994, but roughly halved each year since then. Greater Flamingos Phoenicopterus ruber made up between 0.7 and 4.1% of total flamingo numbers; other waterbirds made up between 2.7 and 10.2% of the overall total. Lakes Naivasha, Elmenteita and Nakuru together hold most of the non-flamingo waterbirds in the southern Rift; to compare trends for other species, we pooled totals for these sites. Significant, or near-significant, declines were evident for grebes, pelicans, cormorants, storks, gulls, rallids, kingfishers, terns and raptors. No group showed an overall increasing trend. At Dandora, a smaller site with stable water levels, these groups showed large annual fluctuations but no obvious declines. In most waterbird groups where numbers decreased, the probable cause was sustained low lake levels at Lake Nakuru; numbers at Lake Naivasha remained stable. Exceptions were rallids, kingfishers and raptors, where numbers steadily declined at Naivasha. There is a need to investigate local environmental causes of these changes for kingfishers and raptors, and to assess possible loss of breeding sites for rallids.