ECOSYSTEM EFFICIENCY
The efficiency of energy transfer within an ecosystem can be estimated as its ‘trophic
transfer efficiency’, the fraction of production passing from one trophic level to the next.
The energy not transferred is lost in respiration or to detritus. Knowing the trophic
transfer efficiency of an ecosystem can allow researchers to estimate the primary
production required to sustain a particular trophic level. For example, in aquatic
ecosystems, trophic transfer efficiency can vary anywhere between 2% and 24%, and
average 10%. Assuming a trophic efficiency of 10%, researchers can estimate how much
phytoplankton produc tion is required to support a particular fishery. Consider the open
ocean fishery for tuna, bonitos, and billfish.
These are all top predators, operating at the fourth trophic level. According to world catch
statistics recorded by the Food and Agriculture Organization, in 1990, 2 975 000 t of these
predators were caught, equivalent to 0.1 g of carbon per m 2 of open ocean per year. To
support this yield of tuna, bonitos, and billfish, researchers can calculate the production
rates of the trophic levels below, assuming a trophic efficiency of 10% and equilibrium
conditions. Essentially, to produce of 0.1 gC m 2 yr 1 of harvested predators (tuna, bonitos,
and billfish) requires 1 gC m 2 yr 1 of pelagic fish to have been consumed by the top
predators, 10 gC m 2 yr 1 of zooplankton to be consumed by the pelagic fishes, and 100
gC m 2 yr 1 of phytoplankton. Note that these values represent the pro duction that is
transferred up trophic levels. They do not represent the standing stock of biomass at each
trophic level.
Studying Ecosystem Dynamics Stable Isotopes
Important insights into ecosystem dynamics can be revealed through the use of naturally
occurring ‘stable isotopes’. These alternate forms of elements can reveal both the source
of material flowing through an ecosystem and its consumer’s trophic position. This is
because dif ferent sources of organic matter can have unique isotopic signatures which
are altered in a consistent manner as materials are transferred throughout an ecosystem,
from trophic level to trophic level. Consequently, stable
fractionation’. Although carbon fractionates very little (0.4‰, 1SD ¼ 1‰), the mean
trophic fractionation of 15 N is 3.4‰ (1 SD ¼ 1‰), meaning that 15 N increases on
average by 3.4‰ with every trophic transfer. Because the 15 N of a consumer is typically
enriched by 3.4‰ relative to its diet, nitrogen isotopes can be used to estimate trophic
position. Stable isotopes can provide a continuous measure of trophic position that
integrates the assimilation of energy or material flow through all the different trophic
pathways leading to an organism. In contrast, 13 C can be used to evaluate the ultimate
sources of carbon for an organism when the isotopic signatures of the sources are
different.
Stable isotopes can track the fate of different sources of carbon through an ecosystem,
because a consumer’s iso topic signature reflects those of the key primary producers it
consumes. For example, in both lake and coastal marine ecosystems, 13 C is useful for
differentiating between two major sources of available energy, benthic (nearshore)
production from attached macroalgae, and pelagic (open water) production from
phytoplankton.
This is because macroalgae and macroalgal detritus (specifically kelp of the order
Laminariales) is typically more enriched in 13 C (less negative 13 C) relative to
phytoplankton due to boundary layer effects. Researchers have exploited this difference
to answer many important ecosystem level questions. Below are two examples. During
the late 1970s and early 1980s, in the western Aleutian Islands of Alaska, where sea otters
had recovered from overexploitation and suppressed their herbivorous urchin prey,
productive kelp beds dominated.
There, transplanted filter feeders, barnacles and mussels, grew up to 5 times faster
compared to islands devoid of kelp where sea otters were scarce and urchin densities
high. Stable isotope analysis revealed that the fast growing filter feeders were enriched
in carbon suggesting that macroalgae was the carbon source responsible for this
magnification of secondary production. In four Wisconsin lakes, experimental
manipulations of fish communities and nutrient loading rates were con ducted to test the
interactive effects of food web structure and nutrient availability on lake productivity and
carbon exchange with the atmosphere.
The presence of top pre dators determined whether the experimentally enriched lakes
operated as net sinks or net sources of atmospheric carbon. Specifically, the removal of
piscivorous fishes caused an increase in planktivorous fishes, a decrease in large bodied
zooplankton grazers, and enhanced primary production, thereby increasing influx rates of
atmospheric carbon into the lake.
Atmospheric carbon was traced to upper trophic levels with 13 C. Here, naturally
occurring stable isotopes and experimental manipulations con ducted at the scale of
whole ecosystems illustrated that top predators fundamentally alter biogeochemical pro
cesses that control a lake’s ecosystem dynamics and interactions with the atmosphere.
Whole Ecosystem Experiments
Large scale, whole ecosystem experiments have contrib uted considerably to our
understanding of ecosystem dynamics. With its beginnings in wholesale watershed
experiments in the 1960s, ecosystems are now being stud ied experimentally and
analyzed as system of interacting species processing nutrients and energy within the con
text of changing abiotic conditions. This is particularly relevant these days given the
effects of anthropogenic climate forcing and pollution in both terrestrial and ocea nic
ecosystems.
A classic series of whole lake nutrient addition experi ments conducted in northwestern
Ontario by David Schindler and his research group illustrated the role of phosphorus in
temperate lake eutrophication. To separate the effects of phosphorus and nitrate, the
researchers split a lake with a curtain and fertilized one side with carbon and nitrogen and
the other with phosphorus, carbon, and nitro gen. Within 2 months, a highly visible algal
bloom had developed in the basin in which phosphorus had been added providing
experimental evidence that phosphorus is the limiting nutrient for phytoplankton
production in freshwater lakes. Certainly, algae may show signs of nitro gen or carbon
limitation when phosphorus is added to a lake; however, other processes often
compensate for these deficiencies.
For instance, CO 2 is rarely limiting because physical factors such as water turbulence and
gas exchange regulate its availiblity. Further, nitrogen can be fixed by blue green algae.
These species, which are favored when nitrogen is in short supply, increases the
availability of nitrogen to algae, and the lake eventually returns to a state of phosphorus
limitation. The practical significance of these results is that lake europhication can be
prevented with management policies that control phosphorus input into lake and rivers.
