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The mean annual transfer (loss and retention) of nitrogen in a river system was estimated using a conceptual approach based on water surface area and runoff. Two different approaches for the calculation of water surface area were applied to determine riverine nitrogen retention in four European catchments, ranging between 860-14,000 km2 in area, and differing considerably in the proportion and distribution of surface waters, specific runoff and specific nutrient emissions. The transfer rate was estimated sequentially as either the mean value for the total catchment, on a sub-catchment scale, or considering the distribution of water surface area within a sub-catchment. For the latter measure, nitrogen retention in larger lakes was calculated separately. Nitrogen emissions modelled with MONERIS and HBV-N were used to calculate nitrogen river loads and compare those with observed loads. Inclusion of the proportion of water area within a sub-catchment improved modelled results in catchment with large lakes in sub-catchments, but not where there was a homogenous distribution of surface waters among sub-catchments.

S U M M A R Y 1. Studies of diel feeding periodicity, rates of food consumption and diet composition of pelagic fish were undertaken to resolve the dispute regarding the existence of a vacant niche for a pelagic zooplanktivore in Lake Malawi. 2. Six species of zooplanktivore were abundant in the offshore pelagic zone, these were: Diplotaxodon limnothrissa, D . 'bigeye', Rhumphochromis longiceps and Copadichromis quadrirnaculatus (all haplochromine cichlids), Synodontis njussae (Mochokidae) and Engraulicypris sardella (Cyprinidae). The main piscivores were cichlids of the genus Rhamphochromis. 3. All cichlids were daytime feeders; some showed peaks in feeding activity at dawn and dusk that were related to vertical migration patterns of their prey. Synodontis njassae was able to feed throughout the day but fed most actively at night. 4. Food consumption: biomass ratios ( Q I B y r l ) calculated from diel stomach contents data were 5.87-7.13 for D. limnothrissa, 12.79 for 0. 'bigeye', 4.20-24.7 for R. longiceps and 6.45 for S. njassae. Annual Q / B ratios for other species, which ranged from 4.74 to 9.28, were obtained from an empirical model relating food consumption to fish morphology. Published estimates were used for E. sardella and Opsaridium microcephulum (Cyprinidae). An estimate of total prey consumption by the pelagic fish population was obtained from Q / B ratios and fish biomass estimates from acoustic surveys covering the entire offshore pelagic zone of the lake (24 000 km2). 5. Diplotaxodon limnothrissa, R. longiceps and C. quudrimaculatus had a broad range of diets,with the ability to switch from crustacean zooplankton to larvae and pupae of Chaoborus edulis (Diptera) or larval and juvenile E. sardella. Diplotmodon 'bigeye' and S . njassae fed almost exclusively on fourth-instar larvae and pupae of C . edulis. Engraulicypris sardella fed mainly on crustacean zooplankton. The main prey of the large piscivorous Rhamphochromis species were Diplotaxodon spp. and E . surdella. 6. The fish community consumed only 3% of estimated crustacean zooplankton production directly. Predation pressure was intense (> 80% of estimated production consumed by predators) on late instars of the zooplanktivorous C . edulis larvae and on E . sardella larvae. Rhamphochomis spp. consumed 2243% of estimated production of the zooplankton-eating fishes. 7. Although many members of the fish community do not feed directly on crustacean zooplankton, so that the food-chain supporting fish production involves an extra trophic level, it is considered that the endemic fish community is well adapted to feeding on the low-density prey in the pelagic zone, and that there is no vacant feeding niche. 0 1996 Blackwell Science Ltd 489 490 E. H. Allison et al.

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