Nutrient retention in wetlands hinges on the water moving slowly enough to stay in contact with the soil, sediments, and plant material long enough for processing to occur. This long residence time lets nutrients drop out of the water column and become bound in sediments or taken up by microbes and plants, rather than passing straight through the system. The saturated soils in wetlands create low-oxygen (anaerobic) conditions, which change how microbes process nutrients. Under these conditions, certain pathways immobilize nitrogen and phosphorus—such as microbial uptake and processes that convert dissolved forms into forms that stay put in the soil or are emitted as inert gases. The combination of slow flow and anoxic, waterlogged soil also promotes sediment deposition, so nutrients hitch a ride on settling particles rather than being exported downstream. In short, slow water movement, paired with saturated, low-oxygen soils that support active microbial processing, makes wetlands effective at retaining nutrients. When flow is fast or soils aren’t saturated, nutrients spend less time interacting with the wetland’s surfaces and organisms, so they are more likely to leave the system.

Light available for photosynthesis in the water column dictates how productive phytoplankton can be. The Secchi disk is a simple way to gauge how clear the water is by noting the depth at which the disk disappears from view. When water is turbid from sediments, algae, or dissolved substances, light is blocked and the disk vanishes at a shallow depth. That shallow light limit often limits primary production because phytoplankton deeper in the water can’t photosynthesize as effectively. In clearer water the disk remains visible to greater depths, meaning more light penetrates deeper into the water column. If nutrients are available, this extra light can support higher phytoplankton productivity because more of the water column becomes photosynthetically active. If nutrients are lacking, productivity may still be limited despite the increased light. The other options describe measurements that Secchi depth does not provide: dissolved oxygen, direct chlorophyll concentration, or water temperature. Secchi depth specifically reflects light penetration and water transparency, linking physical clarity to the potential for photosynthesis and phytoplankton growth.

Bottom-dwelling organisms living in sediments are called benthos. This term names the community that lives on, in, or right at the bottom of aquatic environments—the sediment layer and the interface just above it. Benthos includes a range of organisms, from burrowers that move through or within the sediment to those that live on the surface of the bottom. Periphyton, by contrast, are groups of algae, microbes, and detritus attached to submerged surfaces like rocks or aquatic plants, not specifically those living embedded in sediment. Carrion refers to dead animal matter, which is not a group of bottom-dwelling organisms. Decomposers describe organisms that break down dead organic material; many are bacteria and fungi, and while some operate in sediments, the term describes a function rather than the habitat or group name for bottom-dwellers.

Flowing-water systems are called lotic. The key feature of lotic ecosystems is the movement of water, as seen in streams and rivers, which creates currents, turbulence, and a gradient of habitats—from fast riffles to deeper pools. This flowing condition shapes which organisms can live there and influences oxygenation, nutrient transport, and sediment dynamics, all of which are defining aspects of these systems. In contrast, lentic ecosystems describe standing-water bodies like lakes and ponds where water movement is minimal. The riparian zone refers to the vegetated area along the watercourse that interacts with the stream but is not itself the water system. Ecosystem is a broad term for any living community and its physical environment, not specifically describing flowing water.