The effects of water chemistry and lock-mediated connectivity on macroinvertebrate diversity and community structure in a canal in northern England

Freshwater ecosystems are under threat from habitat loss, partly due to urban expansion. However, some elements of urban freshwaters are already integral parts of the urban landscape and so are more resilient to loss, representing opportunities for the enhancement of freshwater resources within cities. This study investigated the biodiversity value of the Leeds-Liverpool Canal in Leeds, UK, in relation to its landscape context. Specifically, we tested the hypotheses that (i) biodiversity value is lowest nearest to the urban core, and (ii) the pattern of canal locks structured ecological communities. Nutrients, metals and dissolved carbon all existed at relatively low concentrations, contrary to what is often seen in urban water bodies, although concentrations were higher in the urban core. This gradient of chemical stress was associated with a decline in macroinvertebrate diversity towards the city centre, which manifested as pollution-sensitive taxa being excluded from this area. Community structures were found to vary between groups of sampling sites separated by locks, suggesting that locks may act as barriers for aquatic invertebrates by restricting dispersal. The results in this study indicate that canals in urban areas can be high-quality habitats, despite the associated anthropogenic stressors, and locks may represent a unique model for researching relationships between connectivity and community structure

Trophic interrelationships between the exotic Nile tilapia, Oreochromis niloticus and indigenous tilapiine cichlids in a subtropical African river system (Limpopo River, South Africa)

The stable isotope ratio and seasonal changes in diet of two indigenous (Oreochromis mossambicus, Tilapia rendalli) and one exotic (Oreochromis niloticus) tilapiine cichlids in the subtropical Limpopo River, South Africa were investigated to determine patterns of resource partitioning. Stomach contents of O. niloticus and O. mossambicus indicated high dietary overlap across size class, habitat and season, with both species primarily feeding on vegetative detritus. However, stable isotope analysis revealed that the two Oreochromis species had different stable isotope ratios derived from different food sources. The relatively δ13C-depleted O. niloticus indicates a phytoplankton-based diet, while the δ13C-enriched O. mossambicus indicates a macrophagous diet dominated by vegetative detritus and periphyton. The high similarity in stomach contents and the interspecific differences in isotopic composition reveal fine-scale patterns of food resource partitioning that could be achieved through selective feeding. Tilapia rendalli was largely macrophagous and fed mainly on aquatic macrophytes and had a low dietary overlap with both O. niloticus and O. mossambicus. In the Limpopo River, detritus and algae are probably the most abundant food resources and the causal factors responsible for the observed patterns of resource partitioning among the tilapiines are usually difficult to ascertain. Fish may be able to perceive food resources in terms of the dynamics that determine their availability. Detailed studies of variation in food resource availability and fish habitat use within the system are needed to evaluate this hypothesis.DST-NRF Centre of Excellence for Invasion Biology (CIB), University of Pretoria, South Africa.http://www.springerlink.com/content/102877/ab201