Whether through carbon dioxide emissions or nutrient pollution from fertilizers used in agriculture, human activities have a profound impact on ecosystems  often throwing them out of balance. By adding excess nutrients to crops, some are very likely to end up in rivers, lakes and streams. But not all of the carbon, nitrogen and phosphorus that enter waterways end up downstream. Freshwater ecosystems filter some of the excess pollutants out of the water before it reaches the ocean, according to a new study.

Published in L&O Letters, researchers discovered that:

• plants and algae process much of the carbon that enters freshwater ecosystems from land using photosynthesis;
• if there is too much carbon and nitrogen for plants and algae to use, freshwater systems are more likely to remove it by releasing it into the atmosphere;
• if phosphorus is in excess, freshwater ecosystems can remove it by burying it into the bottom of lakes and reservoirs.

By burying and "exhaling" excesses nitrogen and phosphorus, what is delivered to the oceans has a much more balanced composition than what is delivered to freshwater systems in the first place. A key factor determining how effective freshwater ecosystems are at filtering excess nutrients before they reach the ocean has to do with how long the water resides in rivers, lakes and reservoirs.