Our current best conceptual understanding of the stressor ‘nutrients’ is shown in Figure 1.
Figure 1. Potential causes of a change to ‘nutrients’ and the condition responses observed as a result of this change.
Erosion and run-off of fertilised urban and agricultural land, and discharges from Sewage Treatment Plants (STPs) can result in increase in nutrient loads to waterbodies. Nitrogen and phosphorus are the nutrients of greatest concern. Some species of algae and algae-like bacteria are particularly good at utilising nutrients and form ‘bloom’ when nutrient levels and other conditions are favourable. Blooms can have human health, amenity and odour risks. Blooms can also result in fish kills and threaten aquatic plants by reducing light penetration. Residence times/flushing rates/dilution efficiency, geomorphic setting (e.g. estuary type), estuary length and tidal range, bioavailability and speciation of nutrients, and light availability are natural mediating factors which influence the impact of nutrients on the ecosystem.