Image source: http://www.stjohnsriverkeeper.org/issues
Human development and waste loads excessive amounts of nutrients into watersheds, and those nutrients eventually make their way to the ocean. This nutrient overload is called eutrophication, and it leads to blooms of phytoplankton that subsequently die out and cause conditions of low or no oxygen. Smith (2003) summarizes the process. The primary nutrients that lead to eutrophication are nitrogen and phosphorus, and they are limited in many aquatic ecosystems. Additions of N and P lead to temporary and occasionally toxic blooms of phytoplankton in both freshwater and saltwater. When the organisms die, they sink to the lower layers of the water column and decompose, removing oxygen from those layers. The effect is especially pronounced in coastal areas where there is a lot of stratification (Diaz and Rosenberg 2008). Low-oxygen conditions, known as hypoxia, often give way to no-oxygen conditions, known as anoxia, for some periods of the year, causing all organisms that depend on aerobic respiration to die or move somewhere else.
Crop fertilizer is a non-point source of nutrient loading (Image source: http://www.flickr.com/photos/48722974@N07/5114813333/)
Carpenter et al. (1998) describe the main sources of nitrogen and phosphorus. These sources fall into two categories: point sources and non-point sources. Point sources are limited to a specific location, and they include sewage treatment plants, mines, oilfields, and animal feedlots. These sources are fairly continuous and simple to regulate. Non-point sources are not limited to a specific location, so they are sporadic and difficult to regulate. Non-point sources include run-off from cropland and pastures as well as atmospheric deposition of nitrogen onto the water surface. Intensive agricultural practices and urban expansion, which are key components of an industrial society, are also underlying causes of eutrophication. In order to improve productivity, intensive agriculture relies on artificial fertilizers that supply vast quantities of nutrients, and the urban centers supported by intensive agriculture create vast quantities of waste. Essentially, a more productive society also tends to be a more wasteful society, and much of that waste eventually washes into the water supply, leading to eutrophication.
Solutions to eutrophication focus mainly on decreasing nutrient inputs for agriculture and urban waste. For more about these practices, see Sustainable Management: Eutrophication.