The migratory movements of reproductive potamodromus fish established in man-made tropical lakes in the dry zone of Sri Lanka were determined. It was shown that the nine indigenous riverine species do not spawn in these lakes but move into the upstream channels when sexually mature. The pattern of upstream movement was found to be species-specific although most species showed a diurnal periodicity. It is suggested that the development of totally lacustrine forms of riverine species is very unlikely to occur in the man-made lakes of Sri Lanka because of their relatively short duration time.

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The die1 pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m-2 d-' but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m-2 d-I is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m-2 d-'. Mean maximum photosynthetic rate was 0.30 mg C m-3 h-I but was lower in the reservoirs than in the temporary ponds and lagoons.

The relationship between phytoplankton primary production and seasonality of physico-chemical parameters were examined for five man-made lakes in the dry-zone of Sri Lanka. Sri Lanka experiences two monsoons dividing the year into four meteorological seasons: -the North East (October-December) and South West (April-June) monsoons and the two inter-monsoons. A significant log linear relationship was found between Secchi disc depth and the depth of the euphotic zone which was lowest during the NE monsoon. Maximum mean photosynthetic rate ranged from 0.935 t-0.067 SE to 0.479 mg 021-I h-I k 0.115. Gross primary productivity which ranged from 0.378 g 0, m-2 h-I in the NE monsoon to 0.980 g O2 rn-, h-' in the SW monsoon showed significant season variation. This is shown to be determined either directly or indirectly by the light regime.