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1. Major reductions in catchment run-off, a result of frequent and prolonged drought conditions, together with extensive impoundment of rivers and streams, has led to concern about the possible negative effects on downstream estuaries. Preliminary studies have shown that changes in river flow and associated nutrient inputs have had a predominantly negative impact on the aquatic biota of Eastern Cape estuaries. Natural successions now have human imposed trajectories which are reducing variability and forcing both freshwater 'deprived' and freshwater 'enriched' systems into artificial cycles.2. The Kariega and Great Fish estuaries in the Eastern Cape Province are used in a comparative manner to illustrate how differences in riverine inflow can influence the structure and functioning of selected biotic components in permanently open systems. Maximum chlorophyll a values in the freshwater deprived Kariega Estuary were 1 mg L − 1 , whereas maximum values in the freshwater enriched Great Fish Estuary were 22 mg L − 1 . Mean zooplankton biomass in the lower, middle and upper reaches of the Kariega Estuary was always below 50 mg m − 3 , whereas in the same reaches of the Great Fish Estuary, these values ranged from 256 to 4253 mg m − 3 . Similarly, mean ichthyonekton densities in the mouth region of the Kariega Estuary were 49 individuals per 100 m 2 compared with 279 per 100 m 2 in the Great Fish Estuary.3. Temporal changes of riverine flow reinforce the importance of allochthonous inputs to the functioning of Eastern Cape estuaries. A comparison between a dry and wet period in the Kariega Estuary revealed mean nitrate and phosphate concentrations increasing from 5 to 101 mmol L − 1 and from 1 to 5 mmol L − 1 , respectively. Phytoplankton stocks responded positively to freshettes in both the Kariega and Great Fish estuaries. Similarly, peaks in zooplankton biomass in the Great Fish Estuary increased rapidly in response to high food resource availability resulting from elevated river discharge. The higher ichthyoplankton and ichthyonekton densities in the Great Fish Estuary, when compared with the Kariega Estuary, were attributed to a combination of stronger olfactory cues for larval immigrants from the sea and elevated food stocks in the former system.

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SynopsisJuvenile Tilapiu mossumbica Peters (2.5-10.0 cm standard length) in Lake Sibaya, South Africa, move daily from deep offshore waters to shallow (<0.5 m) littoral areas where they feed for several hours and then return to deep water. The timing of these movements varies in response to changing physical and biological features of the littoral environment. During this study (December 1973 -February 1976, lake level rose more than 1 m. At the start of the study, littoral areas visited by juvenile 7: mossambica were free of vegetation and debris, but at higher lake levels trees and brush from the eroding shoreline were common in nearshore littoral waters. When the littoral zone was free of debris, juvenile T. mossambica visited nearshore waters only during daylight hours. After a 0.65 m rise in lake level and considerable accumulation of debris, these fish were abundant in the littoral zone only at night. This reversal can be attributed to efforts by juvenile T. mossambica to avoid their principal predator, the catfish Clurias gariepinus. At low lake level C. guriepinus ventured into the littoral only at night but at higher lake levels this predator was found within flooded vegetation during the day. When further increases in lake level flooded marginal grasslands, juvenile T. mossambica abandoned the littoral zone in favor of this newly created eulittoral habitat where potential fish predators did not occur. With the change in habitat, the diet changed from benthic detrital aggkgate to periphyton. These observations suggest the importance of behavioral and throphic plasticity in l.he ability of tilapia to utilize unstable habitats.

Summary The vertical distribution and diel migratory behaviour of Pseudodiaptomus hessei in a freshwater lake is described. All stages showed a pronounced diel migration. During daylight naupliar and copepodite instars were almost exclusively benthic in shallow areas (<10m). Copepodite V and adult stages were predominantly benthic even at 40 m. The diurnal vertical distribution pattern implies age‐related differences in photosensitivity and a possible depth‐regulatory mechanism, based on the existence of differential photosensitivity, is offered to account for day‐depth control. Nocturnal vertical distribution, studied approximately fortnightly during 1970–71, showed no clear seasonal variation. Variations in pattern, most obvious in adult instars, correlated with prevalent lunar conditions. Nocturnal or midnight sinking was evident, particularly in post‐naupliar stages. Dusk rise and dawn descent were performed at very low light intensities and paralleled changes in light penetration. Dawn descent was less rapid than the dusk rise. An ephemeral dawn rise was sometimes observed but was not a consistent feature of all stages. An endogenous rhythm of locomotor activity was recorded under laboratory conditions. Activity was low during daylight, increased sharply at dusk and then decreased through the night to reach daytime levels at dawn. The stimulus provided by changes in light intensity and/or endogenous activity changes can account for the dusk and dawn movements of P. hessei but a definitive identification of the direct migratory stimulus is not possible. Attention is called to a closer examination of the role of endogenous activity rhythms in vertical migratory behaviour. The possible adaptive value of vertical migration to P. hessei is examined briefly.