an ecosystem is composed of biotic communities that are structured by biological interactions and abiotic environmental factors. some of the important abiotic environmental factors of aquatic ecosystems include substrate type, water depth, nutrient levels, temperature, salinity, and flow. it is often difficult to determine the relative importance of these factors without rather large experiments. there may be complicated feedback loops. for example, sediment may determine the presence of aquatic plants, but aquatic plants may also trap sediment, and add to the sediment through peat.