Nutrient loading

Nutrient inputs to marine and coastal ecosystems have increased dramatically over the past three decades due to large increases in population density, fossil fuel use, sewage inputs, livestock production and fertilizer use (EC 2000). These activities release nitrogen and phosphorus, which can enhance plant growth in aquatic systems and lead to oxygen depletion and multiple effects on the ecosystem including destroyed fish habitat, coastal pollution and harmful algal blooms (EC 1999, 2000).

In many parts of North America, nutrients from non-point sources come mainly from fertilizer and manure run-off. Over the past 30 years, fertilizer use has increased by almost 30 per cent while a trend towards rearing livestock in intensive feedlots has led to the release of large amounts of manure to surface and coastal waters (Mathews and Hammond 1999). Atmospheric inputs of nitrogen derived from manure, as well as from vehicles and electric utility power plants, are also significant (NOAA 1998a).

Since the early 1970s, anti-pollution legislation has greatly reduced point sources of nitrogen and phosphorus, principally from the discharge of municipal sewage and industrial wastes and the control of phosphates in laundry detergents (NOAA 1998a, EC 2000). However, most municipal wastewater discharged into Canada's coastal waters is still either untreated or only partially treated (EC 2000). Canadian estuaries in the North Atlantic are less severely affected by nutrient loading than more southerly ones due in part to a cooler climate and significant flushing of coastal waters (NOAA 1998b). Along the north Atlantic coast, non-point sources of nitrogen are some ninefold greater than inputs from wastewater treatment plants (EC 2000).

In 1998, more than 60 per cent of US coastal rivers and bays were moderately to severely degraded by nutrient contamination, and nitrogen was found to be the single greatest environmental threat in some 'trouble' spots on the Atlantic coast (NOAA 1998b, Howarth and others 2000). The US Clean Water Act and the 1972 Coastal Zone Management Act directed states to develop management plans for non-point contamination sources and provided funding and incentives to implement them (NRC 2000). The 1987 US National Estuary Program aims to minimize regional nutrient contamination (see box).

Nutrient enrichment is probably a contributing factor in the recent dramatic increase in the intensity, frequency and spatial extent of algal blooms or red tides, causing increased economic losses and health impacts. The number of coastal and estuarine locations in the United States with major recurring incidents of Harmful Algal Blooms (HABs) doubled between 1972 and 1995 (US Senate 1997).

The impacts of HABs can include human illness and death from eating contaminated fish or shellfish, mass mortalities of wild and farmed fish, and changes in marine food webs. In response to incidents of human illness from contaminated shellfish, both Canada and the United States have developed testing and water quality programmes to identify phytoplankton toxins and to provide information about them to the public.

Ocean acts in both countries (1997 in Canada and 2000 in the United States) establish frameworks for improving the stewardship of North America's coastal and ocean waters (EC 1999). Since 1996, the North American Commission for Environmental Cooperation has been facilitating regional implementation of the Global Programme of Action for the Protection of the Marine Environment from Land-based Activities in North America (CEC 2000b).

As yet, there is no regional strategy to address nutrient loading in North America's coastal waters, and coordination among the various agencies responsible for their management is inadequate (NRC 2000). Evidence suggests that the situation can be reversed, but the need remains for increased political action and changes in the activities in the watersheds and airsheds that feed coastal streams and rivers.