Nutrients are inorganic materials necessary for life, whose supply is potentially limiting to biological activity within lotic ecosystems. Although many macro- and micronutrients are required for enzymatic activity and protein synthesis, P and N are the primary nutrients that limit biological activity. In addition, the supply of silica can be important to diatoms because of the high silica content of their cell walls. Phosphorus and N occur in numerous forms including dissolved and particulate, and inorganic and organic. They are most bioavailable in their dissolved inorganic forms, as phosphate, nitrate, and ammonium. Their concentrations are very low in unpolluted waters, but are greatly elevated in many areas including most large temperate rivers due to human inputs of agricultural fertilizers, human and animal sewage, atmospheric deposition, and industrial pollution. Nitrogen and P concentrations in rivers often exceed the levels that cause eutrophication in standing water, and consequently the load of nutrients delivered to lakes and coastal waters by river export is a serious management concern.
There are, broadly speaking, two perspectives on nutrient cycling in lotic ecosystems: how nutrient supply affects biological productivity, and how processes within the ecosystem influence the quantity of nutrients that are transported downstream. Metabolic processes likely to affect and be affected by the supply of nutrients include primary production and the decomposition of organic matter by bacteria and fungi, and thus the rate at which basal resources for stream food webs are produced. Because rivers export substantial quantities of nutrients to receiving lakes and oceans, instream storage and removal processes have the potential to influence large-scale element budgets and reduce the quantity of nutrients delivered to receiving water bodies.
Nutrient cycling describes the passage of an atom or element from dissolved inorganic nutrient through its incorporation into living tissue to its eventual remineralization by excretion or egestion and decomposition. In many ecosystems nutrients largely cycle in place, but in lotic ecosystems, downstream flow stretches cycles into spirals. The distance traveled by an atom as inorganic solute before its immobilization in the streambed is called the uptake length, Sw. Because uptake length depends strongly on discharge and velocity it is desirable to standardize nutrient uptake by estimating an uptake velocity, vf, which quantifies the velocity at which a molecule moves from the water column to retention sites on and within the streambed.
A number of biotic and abiotic processes influence nutrient spiraling. The immobilization of nutrients by autotrophs involves assimilatory uptake for the incorporation of nutrients into new tissue. Heterotrophs in biofilms and on organic substrates likewise require nutrients for synthesis of new structural compounds, but nitrification and denitrification are dissimilatory reactions where bacteria obtain energy by using ammonia as a fuel or nitrate as an oxidizing agent. In addition, both N and P can be removed from streamwater by abiotic processes. Sorption-desorption reactions, in which both inorganic and organic molecules are bound to the surfaces of sediments, can help to regulate nutrient availability by serving as temporary storage sites when a nutrient is present in streamwater at high concentrations, releasing it back into solution when concentrations decline. Under aerobic conditions, dissolved inorganic and organic P both may complex with metal oxides and hydroxides to form insoluble precipitates, which are released under anaerobic conditions.
The capacity of lotic ecosystems to influence the dynamics of nutrients during their downstream passage depends on the factors that influence biotic and abiotic uptake. Biological demand varies seasonally with environmental conditions that favor high rates of primary production and with the supply of organic substrate for heterotrophs, particularly leaf fall. During periods of high discharge, streams are in a throughput mode, exporting most of their annual load of nutrients in weeks to a few months of the year. During low flows, processing and retention are more important. Because the streambed and its interstices are locations of biofilm development and organic matter accumulation, subsurface flowpaths can retard the downstream passage of nutrients and increase their exposure to sites of uptake, thereby contributing to nutrient retention and utilization. Transient storage capacity, which accounts for the slow passage of a conservative tracer relative to water column flow, is a useful descriptor of the extent to which channel complexity affects downstream passage. Once nutrients are assimilated by primary producers and heterotrophic microbes, they can pass through multiple steps of food webs before their eventual mineralization to a bioavailable state. In some highly productive systems, recycling by consumers makes a significant contribution to nutrient availability, and in some circumstances selective retention of nutrients in consumer biomass may contribute to nutrient imbalances.
Nutrient budgets provide an accounting of all inputs, exports, and internal stores for some delineated spatial unit, such as a stream reach, catchment, or large river basin or region. Outputs are typically much less than inputs in budget calculations because storage in soils and river sediments is not accounted for and, in the case of N, denitrification can be a significant loss term. Nutrient budgets have proven to be especially useful in revealing the magnitude of anthropogenic inputs, which can vary substantially among forested, agricultural, and urban catchments. Nitrogen inputs have increased so greatly that river export is now nearly 20 times above estimated pristine conditions in some regions. Phosphorus yields have increased by a smaller amount but anthropogenic influences are nonetheless important, especially during summer low flows when inputs of bioavailable phosphate from sewage water can be a significant fraction of total flows.
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