Freshwater Inflow

Environmental flows can be broken down into instream flow, freshwater inflow, and outflow, as shown in the depiction below. Instream flow is the freshwater water flowing in rivers or streams. Freshwater inflow is the freshwater that flows into an estuary. Outflow is the flow from an estuary to the ocean. This article's focus is upon freshwater inflow.

Freshwater inflow: Freshwater flows into an estuary and mixes with the marine water of the estuarine ecosystem. The mixing of freshwater inflow and the marine water of the estuarine ecosystem occurs spatially and temporally from climatic influences including tidal action, seasonal variability and storms. Mixing is affected by the amount of seawater in the estuarine system. The tides and the volume of the receiving estuary govern the volume of seawater. Tides are defined as the periodic rise and fall of the surface of the sea along the coast that are driven by the gravitational pull of the moon and of the sun. Although estuaries are influenced by the tides, they are often somewhat protected from storms and tidal action by buffers further offshore including barrier islands and peninsulas.

The low-salinity conditions of the marine water are created when freshwater mixes with the saltwater of the estuarine system. Average [salinity] for freshwater is around 0.5 parts per thousand (ppt) whereas average salinity of the ocean is 35 ppt. The salinity will not be consistent throughout a particular estuary and will be differ from one estuary to the next but its level will range from 0.5 ppt to 35 ppt. In estuaries, primary producers take up nutrients such as nitrogen and phosphorus. Freshwater inflows carry nutrients into estuaries, replenishing the stock for phytoplankton use.Sediments settling out and form banks, offshore peninsulas, and barrier islands that protect the estuary from strong tidal action and currents. The sediment also supports beaches and provision the inter-tidal wetlands. Organic material delivered to estuaries by freshwater inflows the primary energy source for organisms living in the estuary. Estuarine ecosystems need freshwater inflow to maintain their productivity, or to continue to produce biomass.

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