Photo 1. Microcystis bloom in Matilda Bay, Swan-Canning Estuary during February 2000. Photo by Dennis Sarson (University of Western Australia). Used with permission of the Waters and Rivers Commission of Western Australia.
The main cause of eutrophication in coastal waterways is nutrient overenrichment (nitrogen, phosphorus and silica). Other factors influence plant growth and the build-up of nutrient concentrations, and hence modify (or buffer) the response of a system to increased nutrient loads. These factors include hydrologic residence times, mixing characteristics, water temperature, light climate and grazing pressure.
Nutrients are derived from point sources and nonpoint (or diffuse) sources.
Nonpoint sources of nutrients are usually of greater concern than point sources because they tend to be larger and more difficult to control. Important nonpoint sources of nutrients include:
The Catchment Condition Index and Extent of Native Vegetation give a general indication of the overall catchment condition, and may also be useful for identifying coastal waterways that are at risk of eutrophication.
Photo 2. Microcystis aeruginosa bloom in Crawley Bay, Swan-Canning Estuary, WA. Photo courtesy of West Australian Newspapers and used with permission of the Waters and Rivers Commission of Western Australia.
Eutrophication is a major national  and international problem  because it can lead to:
An increase in macroalgae or phytoplankton is usually the first 'visible' sign that a system is becoming eutrophied. Key indicators of increasing trophic status in the ANZECC/ARMCANZ Australian and New Zealand Guidelines for Fresh and Marine Water Quality  include:
However, sediments (and benthic communities) are probably the most sensitive part of coastal ecosystems to nutrient enrichment . Therefore, some scientists feel that management criteria for defining sustainable carbon loading rates are best based on indicators found in sediment . Some sediment indicators of eutrophied conditions include:
TOC:TS ratios of < ~5  and Degree of Pyritisation values in the range from 0.55 - 0.93  also indicate the predominance of organic matter decomposition by sulfate reduction in sediment - a condition which is often also associated with elevated water column productivity. Increased sedimentation rates are also sometimes associated with eutrophication.
Other potential indicators of changing trophic status include:
More information on nutrients (changed from natural).