Sediment Transport in Estuarine Areas
The Estuarine Sedimentation Research Group of King's College and Sir John Cass College has been studying the estuaries of the Thames, Medway and Taw-Torridge. Its members, who report some of their results here, have found that the sedimentary processes in the estuaries are varied and complex.
Solid phase partitioning of metals in managed retreat soils: field changes over the first year of tidal inundation.
Sequential extraction methods were used to determine the effect of saline inundation on partitioning behaviour of metals in soils at Orplands Managed Retreat site, Essex, UK. A suite of metals (Al, Ca, Cr, Cu, Fe, K, Li, Mn, Ni, Pb and Zn) were analysed in sequential extracts from samples of soil cores extracted immediately before breaching of sea walls in 1995 and 1 year after. Generally, partitioning of most metals was dominated by associations with the residual fraction and an absence of associations with the exchangeable fraction indicating a relatively uncontaminated environment. Changes in partitioning were predominantly limited to the top 8 cm of the soil profile where a decaying mat of vegetation from the pre-inundation surface resulted in high moisture contents and low redox potentials. In this zone, the predominance of metal associations with the residual fraction was replaced by associations with more labile fractions. In the surface layer deposited after breaching, Ca was dominantly associated with the carbonate fraction, which indicates biogenic import from adjacent estuarine sediments.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.