Longitudinal macroinvertebrate assemblages in contrasting discontinuities: the effects of damming in tropical streams

The study assessed the impact of damming on water quality and macroinvertebrate assemblages. It also assessed the response of macroinvertebrate‐based indices of water quality to damming. Macroinvertebrate community and physicochemical variables data were collected from 86 sites. Twenty‐nine sites downstream of dams were compared with 27 sites above impoundments and 30 sites on nearby unregulated streams. Of the downstream sites, 13 were situated <1 km from a dam while the other 16 were situated >1 km from a dam. A decrease in temperature, dissolved oxygen, conductivity and total dissolved solids was observed in sites immediately downstream of impoundments. Macroinvertebrate community structure and South African Scoring System (SASS) scores closely followed the damming‐induced changes in water quality. However, water quality variables, macroinvertebrate community structure and SASS scores reverted back to typical upstream conditions in distances around 1 km from dams. Stream recovery from dam‐induced changes was demonstrated with streams recovering at distances around 1 km from the point of regulation in corroboration with predictions of the serial discontinuity concept (SDC). These dam‐induced changes also reflected themselves in SASS scores suggesting potential usefulness of SASS in monitoring ecological integrity of tropical rivers following disturbances like damming.     

Spatial variations of microinvertebrates across different microhabitats of temporary floodplains of lower Okavango Delta,Botswana

Temporary floodplains of the Okavango Delta have been suggested as important nursery sites for juvenile fish. Fish production in the Delta is dependent on how well temporary floodplains are maintained. A study was conducted to understand how floodplains function so as to put measures in place that will sustain temporary floodplains in a high productive state. During high floods, distinct microhabitats (open water, sedges and inundated terrestrial grasses) develop in which microinvertebrate community structure was assessed across such habitats during the 2009–2010 flooding season. Depth was highest (ANOVA, P < 0.05) in the open‐water microhabitat and lowest within inundated grasses. Conductivity, total phosphorus and chlorophyll a were all highest (ANOVA, P < 0.05) within inundated grasses than in other microhabitats. A total of 46 microinvertebrate taxa were identified, and they were most diverse within the sedges. The abundance of microinvertebrates was highest (ANOVA, P < 0.05) within the inundated grasses and lowest within the open‐water zone. Given that microinvertebrates are an important food source for juvenile fish, their presence in high densities within inundated terrestrial grasses is a key determinant of fish production. This suggests the need for serious consideration of the importance of temporary floodplains when formulating conservation strategies for the Okavango Delta.     

Observations on limnological conditions associated with a fish kill of Oreochromis niloticus in Lake Chivero following collapse of an algal bloom

Possible causes of deaths of Oreochromis niloticus in Lake Chivero were examined in relation to changes in limnological conditions monitored over a 25‐month period. The fish deaths coincided with the collapse of an algal bloom that had developed and builtup in the lake for 8 months. Chlorophyll a and dissolved oxygen increased to average concentrations of 42.4 μg l^?1 and 10.9 mg l^?1 respectively prior to the collapse of the bloom. Dissolved oxygen decreased when the bloom started to die off and coincided with the fish deaths when the average surface dissolved oxygen concentration in the lake was 3.9 mg l^?1 and was at a depth of 5 m <2 mg l^?1. Mortality probably resulted from depressed oxygen levels caused by the high oxygen demand from the massive algal die‐off and released algal toxins. This is the first time that die‐off of algae has been linked to fish‐kills in Lake Chivero as occurs in other hypereutrophic systems.