Solutes in rivers and streams are influenced by interactions with the
streambed, streambanks, and the land as they travel downstream. This
ongoing process interests stream ecologists because some of those solutes
essential to the lives of organisms, most notably phosphorus and nitrogen.Their
uptake, transformation, and release play an important role in determining
the amount of life an ecosystem can support. As nutrient
atoms are used in a stream they are displaced further and further downstream,
leading to the term "nutrient spiraling" being used to describe
the interdependent processes of nutrient cycling and downstream transport.
These interdependencies can be looked at in two ways: (1) how nutrient
supply affects biological productivity; and (2) how the stream
ecosystem influences the supply of nutrients being transported downstream.
Basic Nutrient Cycles
It is a basic rule of ecology that chemical
constituents of organisms are continually recycled between the biota
and the environment. Carbon, nitrogen,
phosphorus, and silicon are the elements most utilized by organisms as
nutrients. Carbon, being readily available as dissolved carbon dioxide,
is usually left out of consideration as one of nutrients considered to
be most ecologically important. As a group nitrogen, phosphorus,and silicon
are referred to as macronutrients.
Phosphorus is often the nutrient in most limited supply and thus the
nutrient most likely to limit the productivity of plants and other autotrophs
in stream environments. Plants
assimilate dissolved inorganic phosphorus (DIP) into cellular structures.
This process transforms phosphorus into particulate organic phosphorus
(POP). Dying cells may excrete or release particulate organic phosphorus
as dissolved organic phosphorus (DOP).
phosphorus is then broken down to DIP by bacterial activity, making it
available to autotrophs once again.
Major Forms of Phosphorus
- Dissolved Inorganic Phosphorus (DIP) (also known as orthophosphate
- Dissolved Organic Phosphorus (DOP)
- Particulate Organic Phosphorus (POP)
- Particulate Inorganic Phosphorus (PIP)
General availability of phosphorus is not only shaped by its abundance
in local rocks and soils, but also by physical-chemical transformations.
Sorption of phosphate ions onto charged
clay or organic particles occurs when dissolved inorganic phosphorus
(DIP) concentrations are relatively high, while release or desorption
is favored at
low DIP concentrations. Additionally, DIP and DOP may complex with metal
oxides and hydroxides to form insoluble precipitates which can be released
under anaerobic conditions.
Diatoms are the only lotic organism to which silicon is limiting.
The silicon cycle begins with silicic acid being dissolved from the weathering
of rocks and anthropogenic inputs (especially sewage). It is assimilated
by diatoms and eventually released by chemical dissolution over time.
Major Forms of Nitrogen
The cycling of nitrogen within ecosystems is complex due
to the many transformations in form it goes through as a result of microbial
processing. The principal forms of dissolved organic nitrogen (DON) include
urea, uric acid, and amino acids. Dissolved inorganic nitrogen (DIN)
is present as ammonium (NH4+), nitrate (NO3-), and nitrite (NO2-).
Nitrogen is also present in gases as nitrogen gas (N2) and nitrous oxide
The atmosphere, natural runoff, sewage, and agriculture all deliver various
forms of nitrogen to receiving bodies of water such as streams and rivers.
Dissolve Organic Nitrogen (DON)
Particulate Organic Nitrogen (PON)
- Dissolved Inorganic Nitrogen (DIN)
In order for nitrogen to enter and move through an ecosystem it must
be transformed by microbes a number of times. Initially, dissolved inorganic
nitrogen (DIN - N2,NH3, NO3-,NH4+)
must be fixed or assimilated by organisms to make it available for inclusion
in the synthesis of organic molecules.
Autotrophs, bacteria, and fungi utilize NH4+ preferentially
over nitrate and nitrite, while bacteria and cyanobacteria fix nitrogen
gas (N2) in
to NH4+ for its inclusion in structural organic
molecules. Following excretion and decomposition Ammonium (NH4+)
is converted to nitrite and
then nitrate by nitrifying bacteria in aerobic conditions. If conditions
are anaerobic, nitrate is converted to nitrite, and is eventually returned
to the atmosphere as nitrogen gas by denitrifying bacteria. Denitrifying
bacteria use ammonia as an energy source and nitrate as an oxidizing
agent in the breakdown of organic matter under anaerobic conditions.
In well-oxygenated streambeds, denitrification
is of little consequence, but in deep sediments or in oxygen depleted
denitrification can be more important, occaisonally leading to an accumulation
Nutrient Concentrations in Rivers and Streams
River chemistry is highly
variable over time and space, making nutrient concentrations difficult
to use as a predictor of biological activity and overall water quality.
human influence is thought to be minimal, such as in small temperate
streams or large tropical rivers, natural levels of nutrients can be
estimated. Nutrient concentrations from these systems are very similar
to levels found in rainfall, supporting the thought that they are unpolluted.
Dissolved phosphorus levels are low, approximately 0.01 mg/L for
orthophosphate and 0.025 mg/L for total dissolved phosphate. Levels of
DIN are about 0.12 mg/L, nitrate (84%) the major contributor
and ammonia (15%) and nitrite (1%) making
lesser contributions. It is important to note that human activity can
dramatically influence nutrient concentrations in rivers through industrial
emissions, sewage and agricultural inputs, and other alterations to watersheds
that facilitate the delivery of nutrients from the landscape. Non point
nutrient pollution is currently a topic of major interest in addressing
river water quality. For an introduction to this topic read "Nutrients
in the Mississippi River" at http://water.usgs.gov
and "Sediments and Nutrients
in the Mississippi" at www.americanrivers.org.
It is normal to find higher nutrient concentrations at downstream sites
along large rivers primarily due to increasing human influence. Nutrient
concentrations also vary seasonally due to seasonal hydrologic regimes,
the growing season, and seasonal variation in human inputs. If the input
of a nutrient is relatively constant it is normal then to find that low
flows will concentrate the nutrient and high flows will dilute it. Uptake
by organisms can also influence nutrient concentrations and can, for
example, explain why summer nutrient concentrations can be extremely
Land use has been shown to clearly influence nutrient levels in streams.
For example, as land loses forest cover, nitrogen and phosphorus levels
increase, with nitrogen levels increasing to a greater degree.
The ration of nitrogen to phosphorus in
water indicates which nutrient is likely to limit algal growth. In algal
tissues carbon, nitrogen, and
phosphorus are found in a very consistent ratio of atomic weights, approximating
106:16:1. This indicates that if nitrogen to phosphorus ratios fall below
16:1 algae will become limited by low nitrogen availability. Conversely,
if N:P ratios exceed 16:1 algae will be limited by phosphorus availability.
When ratios are between 10:1 and 20:1, evidence indicates that limitation
by both nutrients is occurring.
It is more common to find N:P ratios exceeding 16:1, indicating that
nitrogen is less commonly a limiting nutrient than phosphorus. This varies
regionally, however, with eastern streams more commonly showing high
nitrogen levels and phosphorus limitation (high N:P values), and western
streams showing nitrogen limitation and relatively higher phosphorus
levels (low N:P values).
As a nutrient molecule travels
downstream it changes form,
from being available as a dissolved nutrient, assimilated into living
tissue, possibly passing through several links in the stream´s
food web, being released via excretion or decomposition, and re-entering
of available, dissolved nutrients. As this process occurs in running
it is important to keep in mind that transport downstream is simultaneously
occurring - thus conceptually making it spiral-like in nature (Webster
and Patten, 1979).
Understanding nutrient spiraling requires quantifying the distance
that nutrient molecules travel within a river or stream. This is usually
estimated by measuring the uptake rate of nutrient molecules and the
distance traveled. The spiraling length (S) is the average distance a
molecule travels downstream during one cycle. The cycle begins with the
availability of the nutrient in the water column in inorganic
form, and includes
the distance traveled in the water (Sw) until its uptake (U)
and assimilation by an organism where the nutrient becomes part of an
Additional distance traveled as part of the biota (Sb) completes
the distance as the nutrient atom is re mineralized and released (Newbold,
Nutrient isotopes such as 32P are commonly used to track
dissolved nutrients along their spiraling path.
Image Courtesty of: Hebert, P.D.N, ed. Canada's Aquatic Environments
CyberNatural Software, University of Guelph.
The distance traveled in the biota (Sb) can be subdivided in
various ways. A nutrient atom is likely to be incorporated initially into
autotroph associated with the streambed and then be consumed by a microbe
or benthic invertebrate before eventually being released. As many as
12 compartments have been used to track the fate of nutrient molecules
in the water column and the biota, including: the water column, coarse
particulate organic matter, benthic and suspended fine particulate organic
matter, Aufwuchs, and an array of consumer compartments, including benthic
macro invertebrates (Newbold, et al., 1983a).
Newbold, et al. (1981) found that 32P traveled a total distance
of 193 m in a small Tennessee woodland stream, 167 m of the total distance
water column and 26 m within the biota. Spiraling length can be influenced
by many things, some of which are abiotic, including; physical-chemical
transformations, hydrologic regimes, and sediment characteristics. Other
influences are biotic in nature, including: abundance of periphyton,
abundance of heterotrophic microbes, uptake rates, and the composition
of the animal community. Hydrologic influences may be seasonal or annual.
Low flow conditions, especially when combined with a high ratio of streambed
area to channel volume, favor retention of nutrients within a stream
segment. High flow conditions favor export from a stream segment, increasing
spiraling distance. Retention of nutrients can be enhanced (downwelling)
or reduced (upwelling) by factors that favor interaction between the
water column and streambed.
Animal communities play a variety of roles in nutrient cycling. Direct
consumption of periphyton, microbes, and other animals reduces standing
stock of such organisms and may serve to stimulate or reduce their productivity
depending upon the severity of the reduction in standing stock. Movements
and migrations of animals can influence nutrient spiraling as well. Insect
emergence can reallocate nutrients within the stream or, perhaps, remove
nutrients from a stream if the emerging insects move a great enough distance.
migrations by fish can move significant amounts of nutrients, enough
to demonstrate assimilation of phosphorus and carbon from the fish into
periphyton, macroinvertebrates, and fish following the death and decomposition
of the migratory, spawning fish (Kline, et al, 1990).