We only spot sample nutrients and that doesn’t take into account multiple stressors/influences on aquatic bug communities. The best way to know if a community is degraded is to sample it directly – they are ones living in the polluted water after-all.
Published in the Gisborne Herald by Michael Nielson:
“Freshwater ecology doctoral researcher Adam Canning said measuring nutrient levels only would not tell the full story of how healthy a waterway was. Instead they should measure the presence and health of freshwater bugs.”
From my Evidence in Chief:
” Within rivers and streams there is Periphyton, Detritus, Terrestrial Plant and Animal matter, Aquatic Invertebrates, and Fish. Periphyton (the coating of slightly furry green or brown algae on rocks) and detritus (both in-stream and terrestrial derived plant matter, e.g., leaves) form the basis of the stream food web. Some periphyton is required as food for many aquatic invertebrates; however, too much algal growth can dramatically change the ecology and habitat conditions of a river. Aquatic invertebrates consume the periphyton and plant matter either directly (along with other organic sources) or by predating the smaller grazing invertebrates. Native and sport fish eat these invertebrates and some terrestrial inputs. All of the biological components of a river food web require the correct habitat, water quantity and water quality in order to maintain healthy populations and functioning ecosystems. A change in a single constituent can alter the entire community composition as a result of trophic cascades and resource competition.
Macroinvertebrates are important contributors to a river food web’s functioning and stability (important aspects that comprise ecosystem health). However, not all macroinvertebrates are equal contributors, contrast those presented in figure 10. Some invertebrates are more energetically rewarding with lower foraging costs for fish. Maintaining the diversity of these energetically rewarding invertebrates is important for the stability of fish diet. Large grazers are also important for down-cutting periphyton. Rivers with good water quality are dominated 14 by mayflies, stoneflies and caddisflies, whereas rivers with poor water quality are dominated by worms, snails and midges and do not support the same abundance, biomass or diversity of fish that the former communities do. Fish that feed on poor invertebrate communities become stressed, susceptible to disease and develop poor condition as a result of undesirable dietary changes (Dean & Richardson, 1999; Franklin, 2013).”
A more elaborate explanation found within my Evidence in Chief.