General Background

Movement of Nutrients in Ecosystems

An important characteristic of ecosystems is the movement of nutrients (elements contained in molecules, e.g., C, N, P, and others) among components of an ecosystem and between different types of ecosystems. Much of the physical movement of nutrients is driven by the movement of water, which carries nutrients both directly dissolved in the water and as particles carried along in the flow. This water-mediated movement is powered by gravity and so is largely one way, downhill or downstream, in most cases ultimately ending in the ocean.

There are, however, some important examples of reversals of this flow of nutrients. For example, river floods can redistribute nutrient-rich sediments back “up-hill” onto what is in other conditions dry land. In the Pacific Northwest of North America, spawning salmon move from the ocean back upstream into freshwater. Scientists suspect that this salmon migration functions as a nutrient “conveyor belt,” carrying marine nutrients back into freshwater and even to surrounding terrestrial ecosystems.

In this case study, you will examine evidence from a growing body of research investigating the role of Pacific salmon in ecosystems in and near where they spawn. The figures examine the movement and impact of salmon-delivered nutrients in the stream and riparian (= streamside) ecosystems. Before you begin examining the data yourself, you should familiarize yourself with the basic life cycle of Pacific salmon and the techniques scientists use to track the nutrients salmon deliver by reading the sections below.

Pacific Salmon Life Cycle

There are five species of Pacific salmon (genus Oncorhyncus) that spawn in northwestern North America: chinook (O. tshawytscha), sockeye (O. nerka), pink (O. gorbuscha), chum (O. keta), and coho (O. kisutch). Although they vary in the details of their life history and ecology, all five species share a general life cycle.

A Pacific salmon starts life as a pea-sized egg laid in a nest or redd, a shallow depression excavated from the gravel bottom of a stream (or sometimes a lake) by the mother. After a period of development, the egg hatches as an aelvin, which remains attached to the yolk and buried in the gravel until the yolk is completely absorbed. The salmon is then a parr, which spends weeks or months in the stream feeding and growing. When ready, the young salmon, now a smolt, heads downstream, undergoing physiological changes necessary for the switch to marine conditions. A salmon spends one or more years in the ocean, where it gains > 95% of its final body mass by eating crustaceans, fish, and other marine animals.

At sexual maturity, salmon migrate in some cases long distances through the ocean and then travel upstream, without feeding, to return to the same area where they were born. Along the way and at spawning sites, many salmon are captured by predators such as eagles, seals, bears, and humans; terrestrial predators such as bears and birds often move a captured salmon to land before consuming all or part of it. At the spawning site, a female excavates a redd, then releases her eggs over it; at the same time, the dominant male in the area releases sperm over the eggs. The fertilized eggs settle into the gravel at the bottom of the redd, and the life cycle starts again.

After spawning, the adult salmon soon die, depositing 2–20 kg (sometimes up to 50 kg!) each of organic material (biomass) in the streams at or near the spawning site. Carcasses are eaten by aquatic scavengers, terrestrial scavengers (such as bears, other mammals, and birds, which sometimes move carcasses to land), or microbial decomposers. Since most of this biomass is from growth that occurred in the ocean, and spawning runs in a single river system can number in the tens of millions, this represents a huge amount of ocean-derived organic material being transported upstream into freshwater and the surrounding riparian ecosystems.

In summary, Pacific salmon share three important life history characteristics: anadromy, starting life in freshwater, moving to the ocean, then returning to freshwater to reproduce; homing, returning to the natal stream to reproduce; and semelparity, reproducing once and dying (as opposed to iteroparity, having more than one reproductive bout during the life cycle).

Stable Isotope Analysis

Simple observation of salmon spawning runs reveals that large amounts of marine-derived organic material is being deposited in and around spawning streams. But do these organic molecules of marine origin (marine-derived nutrients, or MDNs) actually get into these upstream ecosystems, and, if so, what parts? Scientists studying these questions have been able to trace the path of salmon-delivered MDNs using stable isotope analysis.

Many elements have different isotopes, which are versions of the element with different atomic weights because they have different numbers of neutrons in the nucleus. Unstable isotopes are radioactive (emit atomic particles) and change from one isotope to another; these are not what we are interested in here. Many elements important in organic molecules have one or more stable isotopes (i.e., they don’t change into other isotopes). The two elements most important to the study of MDNs are nitrogen and carbon, both of which have two stable isotopes—¹⁴N and ¹⁵N, ¹²C and ¹³C (the superscripted number refers to the atomic weight of the isotope). For both elements, the heavy isotope is much less abundant than the light version (less than 2% of molecules). However, the ratio of the two isotopes is not uniform throughout the Earth. This is because physical and biological processes can select among (or fractionate) isotopes of different weights. For example, evaporation discriminates against heavy isotopes. On the other hand, heavy isotopes of nitrogen tend to accumulate in consumers (relative to what they eat). The result is that different ecosystems and different components of ecosystems will have different ratios of heavy: light isotopes of some elements. The isotopic ratio of a sample of say a fish or a plant can be measured using a technique called mass spectroscopy.

The key to tracing MDNs is that the oceans tend to be enriched in heavy isotopes of nitrogen and carbon relative to freshwater or terrestrial ecosystems. The bodies of salmon returning to streams to spawn likewise have higher heavy: light isotope ratios than their surroundings. This means that scientists can trace the path of MDNs in stream and riparian ecosystems by looking for enrichment of the heavy isotopes ¹⁵N and ¹³C.

In ecological studies, isotopic ratios are usually presented as δ (delta) values, which are differences in ratios between a sample (e.g., a salmon) and a reference standard, usually given in parts per thousand (‰). For example, for nitrogen, δ would be calculated as:

δ¹⁵N = (R sample − R standard) / R standard × 1000

where R = the ratio of ¹⁵N:¹⁴N. Because the standard typically used for calculating δ¹⁴C has a relatively high ¹⁴C:¹³C ratio, δ¹⁴C values usually are negative. For both N and C, higher delta values mean higher amounts of the heavy isotope; so higher δ¹⁵N or δ¹⁴C is an indicator of enrichment from marine sources.

Date Posted: March 27, 2009.

Credit: Licensed image of Native American salmon ©Rich Harris | iStockphoto.

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