What is considered an ecological service of freshwater systems? 

A)  recreation 
B)  hydroelectricity 
C)  nutrient cycling 
D)  irrigation

Nutrient cycling is considered an ecological service of freshwater systems as they regulate the cycling of essential nutrients, such as nitrogen and phosphorus, which are necessary for the growth of plants and other organisms in the ecosystem. The natural processes of freshwater systems can help to prevent nutrient pollution and excess nutrient runoff, which can otherwise cause harmful algal blooms and disrupt the balance of the ecosystem. Recreation, hydroelectricity, and irrigation are all important economic or social benefits of freshwater systems but do not necessarily contribute directly to ecosystem health and functionality.