Freshwater Ecosystems: River Systems and Lakes

     Freshwater is scarce. While oceans cover approximately 70% of Earth's surface, only about 0.65% of all freshwater in found on the continents. The majority of this continental freshwater (0.62%) is groundwater, with only about 0.0001% located in streams and 0.017% located in lakes (Sand-Jensen, 2013). As freshwater is such a rare and crucial commodity, it is critical that we have a thorough understanding of its ecological processes.

     Higher trophic levels tend to be uncommon within freshwater ecosystems and predominantly consist of herbivorous zooplankton and benthic invertebrates (invertebrates that live on/in the bottom substrate). The next trophic level generally consists of fauna in the pelagic zone, such as predatory zooplankton and small fishes, followed by medium-sized piscivorous fishes and finally, large predatory piscivorous fishes (Wetzel, 2001).

Figure 1: Trophic levels in a typical freshwater river (image obtained via http://pie-lter.ecosystems.mbl.edu/)

     An interesting example of an indicator species being used to determine ecosystem trophic condition is a study that was conducted by Ristau and Traunsperger in 2011. This study focused on the relationship of nematode communities to the trophic state of southern Swedish lakes. Trophic level was found to strongly influence nematode species richness, as oligotrophic and mesotrophic lakes supported the greatest species numbers (Ristau et al. 2011).

     The introduction of Nile perch in Lake Victoria is a classic example of an invasive species altering nutrient cycles by shifting the relative proportion of each trophic level. Nile perch have been linked to dramatic food web alterations and major limnological changes throughout Lake Victoria (Hall et al. 2000).

Figure 2: The influence of Nile perch on the food web in Lake Victoria (Ligtvoet and Witte, 1991).

     Lake Victoria is Earth's largest tropical lake and the birthplace of almost 400 native fish species, the majority of which are endemic haplochromine cichlid species (Coucherousset et al. 2011). Nile perch were first introduced to the lake in the early 1960s, in hopes of boosting the local fishery economy. As Nile perch are very efficient predators, they quickly increased in number as they began to consume the smaller native fish species. Conservative estimates hold that approximately 60% of the endemic fish species have become extinct (Witte et al. 1992).

Animal Planet's Jeremy Wade posing with one of his river monsters: a Nile Perch (http://www.animalplanet.com/tv-shows/river-monsters/photos/killer-fish-nile-perch-pictures.htm).

References Cited:

Coucherousset, J. and J. D. Olden. 2011. Ecological impacts of nonnative freshwater fishes. Fisheries 36:215-230.

 Hall, S. R. and E.L. Mills. 2000. Exotic species in large lakes of the world. Aquatic Ecosystem Health & Management 3:105-135.

Ristau, K. and W. Traunsperger. 2011. Relation between nematode communities and trophic state in southern Swedish lakes. Hydrobiologia 663:121-133.

Sand-Jensen, K.. 2013. Freshwater ecosystems, human impact on. Encyclopedia of Biodiversity 3:570-586.

Wetzel, R. G.. 2001. Freshwater ecosystems. Encyclopedia of Biodiversity 3:560-569.

Witte, F., T. Goldschmidt, P. C. Goudswaard, W. Ligtvoet, M. J. P. van Oijen, and J. H. Wanink. 1992. Species extinction and concomitant ecological changes in Lake Victoria. Netherlands Journal of Zoology 42:214-232.