Lake Production, Succession, and Eutrophication - Hydro-Environmental Analysis: Freshwater Environments - page 459

stormwater runoff, point sources such as wastewater discharges, livestock, pets, fertilizers used
on lawns, golf courses, croplands, etc. While sediments are important, the U.S. Environmental
Protection Agency (U.S. EPA) identiied excess nutrients as a major reason for impaired water qual-
ity in the nation's waters, indicating that “more lake acres are affected by nutrients than any other
pollutant or stressor…. States reported that excess nutrients pollute 3.8 million lake acres (which
equals 22% of the assessed lake acres and 50% of the impaired lake acres)” (USEPA 2002). As a
result, the U.S. EPA directed that all states and authorized tribes develop numeric criteria for nutri-
ents to protect the designated uses of the nation's waters. The purpose of numeric nutrient criteria is
to address cultural eutrophication (waters enriched with nutrients because of human activities) and
guidance for criteria development is provided in the U.S. EPA's (2000) “Nutrient criteria technical
guidance manual: Lakes and reservoirs.”
While reservoirs are not natural waterbodies, their ontogeny is similar to that of natural lakes,
but it is more rapid (Kimmell and Groeger 1986). The ontogeny of reservoirs can also be accelerated
by cultural eutrophication.
Reservoirs commonly become more productive soon after illing, partially as a result of the
leaching of nutrients from looded soils and the decomposition of inundated vegetation (Kennedy
and Walker 1990), although ontogeny does not always lead to reservoir eutrophication (Kimmel and
Groeger 1986; Hall et al. 1999). If the external load remains constant after the initial upsurge period,
the reservoir productivity will decline.
Reservoirs are also impacted by relatively rapid (in comparison to lakes) sediment accumulation.
As discussed in Chapter 10 (see Table 10.2), reservoirs typically have higher drainage to surface
area ratios (usually greater than 10:1), higher variations in water levels, greater changes in shoreline
(due to changes in water levels and shoreline erosion), and usually have larger nutrient loadings, all
of which contribute to sedimentation and aging. Seasonal water-level luctuations are important
components of reservoir ecosystems, affecting nutrient dynamics, underwater light climate, plank-
ton dynamics, and littoral development (Kimmel et al. 1990). The rate of sedimentation is related to
the trapping eficiency of speciic reservoirs, but a regional approach to estimating sediment dam-
ages in lakes and reservoirs indicated that 0.22% of the nation's water storage capacity is lost annu-
ally (Crowder 1987). Crowder also indicated that of this, an average of 24% is due to soil erosion on
cropland. Crowder also indicated that in the central United States, the greatest water storage capac-
ity losses were due to deposited sediment originating from cropland. However, Graf et al. (2010)
indicated that in western reservoirs, the rates of sedimentation have often been overestimated, and
sustainability has been underestimated.

16.4 METRICS FOR EUTROPHICATION

16.4.1 S InGLe M etrIc M etHodS
A variety of methods have been developed based on single metrics related to lake productivity.
Some of the more commonly used metrics include:

• Phosphorus loading

• Phosphorus concentrations

• Nitrogen loading

• Nitrogen concentrations

• Algal productivity

• Algal biomass

• Chlorophyll concentrations

• Secchi depth

• Hypolimnetic oxygen

• Typology