Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 435

in the substrate so they can take nutrients from the water column and sediments. Some, such as the
bladderworts, are not rooted. The submerged macrophytes are at some disadvantage to other forms,
since light, oxygen, and carbon dioxide are not as available as they are at the water surface, but they
have to invest less energy in structural support since that is provided by the water of which they are
primarily (95%) composed (LakeWatch 2007).
Macrophytes are both inluenced by, and they themselves inluence, lake characteristics. Dense
canopies may reduce the light availability to lower water levels by shading, so that productivity (and
oxygen) may be limited to only the surface. Photosynthesis and respiration in dense stands can result
in large diel variations in oxygen concentrations, as large as 12 mg L -1 , to occur in the surround-
ing waters (LakeWatch 2007). Conversely, they may also cause large variations in carbon dioxide
concentrations, and can change the pH by 2-3 pH units during a 24-hour period (LakeWatch 2007).
They may provide a substrate on which other organisms can grow (e.g., the microbiota), provide
shelter and escape habitats for organisms, impact lows, impact nutrient cycling and sediment chem-
istry, and impact other limnological characteristics of lakes. Horne and Goldman (1994) indicated
that emergent reeds and submerged macrophytes are the dominant primary producers and contrib-
ute the most biomass in small lakes.
The impact of macrophytes is proportional to their abundance and productivity, which are, in
turn, impacted by lake characteristics (e.g., lake levels and the extent of the littoral zone). Excess
biomass may eventually cause those littoral zones to be illed in, resulting in the succession of lakes
to wetlands. Macrophytes are commonly used as indicators of the health of lakes and reservoirs.
There may be too few macrophytes, or too few of the native or desirable species, as a result of toxic-
ity, pollution, or other factors. Another common problem is “too much of a good thing,” where mac-
rophytes may be in excess, essentially choking some shallow reservoirs. As a result, macrophyte
management is a large and expensive component of many lake management programs.
As indicated by the U.S. EPA's “Biological Indicators of Watershed Health,” macrophytes are
excellent indicators because they:

• Respond to nutrients, light, toxic contaminants, metals, herbicides, turbidity, water level

change, and salt

• Are easily sampled through the use of transects or aerial photography

• Do not require laboratory analysis

• Are easily used for calculating simple abundance metrics

• Are integrators of environmental conditions

Macrophytes may be used as a component of a tiered assessment approach, as described by the
U.S. EPA's (USEPA 1998) technical guidance on lake and reservoir bioassessment and biocriteria.
Also, a number of ecological indices based on macrophytes are in use, particularly in Europe. An
example is the free macrophyte index developed in Scotland (Free et al. 2007), which is based on
the

• Maximum depth of colonization

• Mean depth of presence

• Percent relative frequency of Chara (for lakes with an alkalinity ≥100 mg L −1 CaCO 3 )

• Percent relative frequency of Elodeids (which have grown profusely in some softwater

lakes in Northwest Europe; Roelofs 1983)

• Plant trophic index

• Percent relative frequency of tolerant taxa

Another example is the U.K. macrophyte assessment system (LEAFPACS; UKTAG 2009).
Penning et al. (2008a,b) discuss using macrophytes as indicators in European lakes. A disadvantage
of macrophytes as an indicator is that macrophytes are often controlled (cutting, poisoning, etc.)