Eutrophication and development, pollution from septic systems and sewers, sewage sludge spreading, and other human-related activities increase the flow of both inorganic nutrients and organic substances into ecosystems. Elevated levels of atmospheric compounds of nitrogen can increase nitrogen availability. Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to "point source" pollution from sewage pipes. The concentration of algae and the trophic state of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication.
Natural population growth Natural population growth Natural population increase (in contrast to total population increase) happens as people are born (in contrast to immigration) into a country, and decrease as people die (in contrast to emigrate). Rates of population growth, usually expressed as a percentage, vary greatly. The immigrating population has increased by a huge margin in the past few years. Due to this rising trend of migration it is important that we look into how migration has affected the population, and the trends that follow with it. The countries that are to be looked into are Denmark, Finland, Iceland, Norway and Sweden.