Title: Estuary
Content:  sediment which has a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux. Phytoplankton are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependent upon the turbidity of the water. The main phytoplankton present is diatoms and dinoflagellates which are abundant in the sediment. It is important to remember that a primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation.
Title: Yaquina Bay
Content:  Major nitrogen sources for the Yaquina Bay estuary change with season. During drier seasons, the ocean is the major nitrogen source while the river is the major source in wetter seasons, where river discharge can be up to five times higher than drier seasons. Samples taken at the surface with varying salinities showed dissolved inorganic nitrogen is found mostly at the two salinity extremes; the mouth of the estuary and up the Yaquina River. The input at the mouth of Yaquina Bay indicates input from upwelled waters during the summer season. The nutrient flux into the estuary is enough to sustain primary productivity during the summer months.
Title: Estuary
Content:  An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the Holocene epoch with the flooding 
Title: Nutrient cycling in the Columbia River Basin
Content:  The primary source of iron into the estuary is from geologic rock weathering and subsequent runoff delivery to the system. In addition, some iron may come from the Pacific Ocean and is transported into the estuary during tidal oscillation, enhanced seasonally during periods of upwelling. Iron inputs derived from riverine and oceanic origins are typically on the magnitude of 14-30 nM and 6 nM in concentration respectively. These sources of iron are generally sufficient to meet biological demands, including those of primary production, in the estuary and upriver. Consequently, iron is commonly not the limiting nutrient in the system; rather, the river depending on season and flow may serve as a net source of iron to iron-deficient coastal regions of the Northeast Pacific, sometimes referred to as areas of High Nutrient, Low Chlorophyll.
Title: Marine biology
Content:   Estuaries are also near shore and influenced by the tides. An estuary is a partially enclosed coastal body of water with one or more rivers or streams flowing into it and with a free connection to the open sea. Estuaries form a transition zone between freshwater river environments and saltwater maritime environments. They are subject both to marine influences—such as tides, waves, and the influx of saline water—and to riverine influences—such as flows of fresh water and sediment. The shifting flows of both sea water and fresh water provide high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
Title: Dee Estuary
Content:  The estuary supports some important natural fisheries, including salmon and trout on their way to and from the freshwater river, as well as sea-fisheries and shell-fisheries, especially cockles.
Title: Ecology of the San Francisco Estuary
Content:  The main source of photosynthetically derived energy is phytoplankton. Generally speaking, diatoms and microflagellates produce most of the bioavailable carbon in the estuary. Other types, notably the dinoflagellates, may produce harmful algal blooms, or red tides, that are less readily available for assimilation into the foodweb. Primary production from phytoplankton is a function of two different factors: growth rates and accumulation (Fig. 1). Although the LSZ is a sink for high concentrations of nutrients from urban and agricultural sources, phytoplankton production rates are quite low. Nitrate is optimally used by phytoplankton for growth, but ammonium (largely derived from sewage outfalls) has a suppressive effect on growth 
Title: Estuary freshwater inflow
Content:  phosphorus. Freshwater inflows carry 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. Estuarine ecosystems are fundamentally important and necessary to protect through the management of freshwater inflow because of the many ecosystem services they produce. An ecosystem service is 
Title: Nutrient cycling in the Columbia River Basin
Content:  the estuary relative to the overall local nitrogen budget. Rather, the primary source of nitrate into the estuary is from ocean exchange caused by wind-driven seasonal upwelling off the Oregon and Washington coasts. This influx of nitrate, generally the primary limiting nutrient to biological communities in the estuary, is an important driver of primary productivity in estuarine plumes of river discharge. Moreover, large scale upwelling on the Oregon-Washington coast typically occurs in spring and summer, and as discussed previously, is coupled with the PDO and ENSO, this seasonal upwelling further contributes to diatom bloom in the estuary. Finally, without this source of nitrogen through upwelling, the estuary quickly becomes nitrate limited and hinders further facilitation of biological activity.
Title: Nutrient cycling in the Columbia River Basin
Content:  Due to coastal upwelling, ocean sources of nitrate (an oxidized form of nitrogen) exceed river sources by a magnitude of roughly 3 to 1, and is a primary source of nitrogen for the Columbia River Estuary. This is evident within the estuary by the positively correlated relationship of nitrate availability and salinity, the latter an identifying characteristic of highly saline marine waters. As a result of differences in delivery to the system, nitrate is the primary limiting nutrient in the estuary. Consequently, during seasonal periods of downwelling when less ocean water is mixed into the estuary, local nitrate concentrations may be very low. Thus, nutrient cycling and primary production in the estuary are heavily linked with the seasonal winds off the Oregon and Washington coasts that control local ocean upwelling and downwelling.

Question: what is the main source of nutrients in an estuary?
Answer: sea water.
fresh water.
detritus.