Many lakes
can exist in two regimes, one characterized by clear water
and the other one turbid. The turbid, or eutrophic, condition
is caused by excessive fertilization with nutrients. By
fitting nonlinear time series models to data from Lake
Mendota, NTL researchers have estimated that the probability
of the turbid regime is about 75%. Furthermore, the probability
of irreversible turbidity (sustained by recycling from
sediments) is about 25%. We have also estimated the effects
of changes in phosphorus loading on the regime, and used
spatial models to study the effects of farming practices
on the phosphorus loading.
NTL interdisciplinary studies have
calculated the net economic value of water quality,
based on the economics of farming, value of housing
near the lake, and the recreational economy derived
from boating, fishing and so forth. These analyses suggest
that the economically optimal loading (which maximizes
net costs and benefits to society as a whole) is around
one-third of the current loading rate to the lake.
The long-term data also suggest that
even the economically-optimal phosphorus loads may incur
a high risk of shifting the lake into an irreversible
eutrophic state. This risk is related to the high variability
in loading caused by variable climate. It also depends
on the proportion of the watershed used for phosphorus-intensive
agriculture such as dairy or meat production.
If climate variability is ignored,
economic value appears to peak when phosphorus -intensive
farming covers about 75% of the watershed (red line
in Fig. 1A). If climate variability is accounted for,
economic value is maximum when phosphorus-intensive
farms cover about half the watershed (blue line in Fig
1A). When phosphorus-intensive farms cover a bit more
than 80% of the watershed, the irreversibly turbid regime
occurs. Mathematically, this corresponds to disappearance
of the stable attractor for the clear water state (
blue line in Fig. 1B). However, large inputs caused
by unusually wet years can also tip the lake into the
turbid regime. Variability of the loads increases with
the proportion of land used for phosphorus-intensive
farming (yellow line in Fig. 1B). Near the economic
optimum where about half the land is used for phosphorus-intensive
farming, the standard deviation of loading is large
enough to shift the lake into the turbid regime about
one year in ten.
NTL long-term data have shown, therefore,
that economically optimal phosphorus loads are considerably
lower than would be expected from models that ignore
climate variation. Even when climate variation is accounted
for, the probability of shifting to the turbid state
is surprisingly high.
References:
Carpenter, S.R. 2001. Alternate states
of ecosystems: Evidence and its implications. Pages
357-383 in M.C. Press, N. Huntly and S. Levin (eds),
Ecology: Achievement and Challenge. Blackwell, London.
Carpenter, S.R., W.A. Brock and P.C.
Hanson. 1999. Ecological and social dynamics in simple
models of ecosystem management. Conservation Ecology
3(2): 4. Available on the internet: URL http://www.consecol.org/vol3/iss2/art4.
Carpenter, S.R., D. Ludwig and W.A.
Brock. 1999. Management of eutrophication for lakes
subject to potentially irreversible change. Ecological
Applications 9: 751-771.
Dent, C.L., S.R. Carpenter and G. Cumming.
2001. Multiple states in river and lake ecosystems.
Philosophical Transactions of the Royal Society of London:
in press.
Scheffer, M., S. Carpenter, J. Foley,
C. Folke and B. Walker. 2001. Stochastic events can
trigger large state shifts in ecosystems with reduced
resilience. Nature: in press.
Wilson, M.A. and S.R. Carpenter. 1999.
Economic valuation of freshwater ecosystem services
in the United States, 1977-1997. Ecological Applications
9: 772-783.