Upper Midwest Environmental Sciences Center

UMESC - LTRMP Reports - Vegetation - Methods
Vegetation - Stratified Random Sampling 2000

Methods

Sampling procedures were described in detail in Yin et al. (2000). The following is a brief description of the overall design.


Stratification

Sampling sites were distributed in shallow water areas where water depth was less than or equal to 3 m deep at flat-pool condition in 1998. After 1998, sampling sites were distributed in less than or equal to 2.5 m deep at flat-pool condition. Deeper water areas most likely do not support aquatic vegetation. Shallow water areas were divided into general habitat types (strata), including main channel borders, secondary channels, contiguous backwaters, isolated backwaters, and impounded areas. Sampling efforts were generally proportional to acreage and perceived habitat heterogeneity of each stratum, except for the isolated backwater areas whose sampling sizes were kept small to ensure a timely completion of the investigation. Some areas were excluded from the sampling areas because of safety concerns and accessibility difficulties.


Site Selection

Sites to be investigated were selected on a computer using a random number generator. A 50- X 50-m grid was generated and overlaid onto the stratified aquatic areas. Nodes of the grid were geo-spatially registered (Universal Transverse Mercator coordinates), and nodes that fell in the sampling strata were eligible for selection as sites. We navigated to the general area of a site using an enlarged hard-copy map and then switched to global positioning system (GPS) equipment with differentially corrected signals as the boat approached the targeted location. The boat was anchored at bow and stern when both the easting and northing coordinates displayed on the GPS unit were within 10 m (- or +) of their respective target readings. The actual GPS coordinates were read and recorded twice at each site, once immediately after the boat had been anchored and again before the boat was released for departure.


Equipment and Definitions

equipment - rakeSubmersed aquatic vegetation was collected using a long-handled, double-headed rake modified from Jessen and Lound (1962) and Deppe and Lathrop (1992). The rake is 36 cm (14 inches) wide, has 14, 5 cm (2 inches) long teeth on each side, and was made by welding two square-headed garden rakes together. The teeth are divided and marked into five equal parts (or 20% increments). The handle is about 3 m long, with a rope extension, and is scaled at 10-cm increments. Aquatic vegetation or aquatic species refer to the following plant types or life forms: submersed (S) and rooted floating–leaf (F).


The Site and Subsampling Areas

subsampling areas


Each site had six subsampling areas, each of which was a rectangular area approximately 1.5 m long and 0.35 m wide (the width of the rake head). One subsampling area was located off each corner of the boat and the other two were located, one each, off the left and right sides.


Sampling and Data Recording

Individual species and different life forms of aquatic vegetation were recorded as either present or absent at each subsampling area based on visual examination and a rake sample. When present, submersed species and the filamentous algae were given a density rating (see table at lower left) based on their thickness on the rake teeth. When present, rooted floating–leaf and emergent species were given a percent cover rating (see table at lower right). Species that had not been recorded in the six subsampling areas but were observed at the site were recorded and marked as "additional species." Fassett (1957), Voss (1972, 1985) and Gleason and Cronquist (1991) were the primary references used for plant identification. Scientific nomenclature and common names are based on those found in the U.S. Department of Agriculture's PLANTS Database on the Internet (http://plants.usda.gov/).

Submersed vegetation
density ratings
Percent of rake teeth filled
Density rating
81-100
5
61-80
4
41-60
3
21-40
2
1-20
1
no plants retrieved
0
Rooted floating–leaf vegetation
cover ratings
Percent of area covered
Cover rating
81-100
5
61-80
4
41-60
3
21-40
2
1-20
1
none
0


Computation of Summary Indexes

Frequency

Frequency values in this report are computed by dividing the number of sites where a species was recorded by the number of sites investigated in the stratum, and then multiplied by 100 to convert it into a percentage.
mathematical formula: Frequency values .
The frequency value in a pool is computed by averaging the frequency values of the shallow water strata, weighted by acreage:
mathematical formula: frequency value in a pool where Fj is the frequency in stratum j and Sj is the acreage of stratum j.

Abundance Index

An index is created to measure the quantity of a submersed species using both presence or absence and plant density rating data recorded in the six subsampling areas. We call it the abundance index to differentiate it from the frequency index. The abundance index is computed according to the following formula:
mathematical formula: abundance index
where Vi is the presence or absence (1,0) and Ri is the plant density ranking (0,1,2,3,4,5) data for the ith subsampling areas at the site (i=1,2,3,4,5,6). Data are treated before computation so that Vi=1 if Ri>=1 and, vice versa, Ri>=1 if Vi=1. The abundance index for a stratum is computed as the simple average of all its sites mathematical formula: abundance index for a stratum. The abundance index for a pool is computed as the average of all shallow water strata, weighted by acreage:
mathematical formula: abundance index for a pool where Aj is the abundance index of the species in stratum j and Sj is the acreage of stratum j.

Percent Cover (Rooted floating–leaf life form)

The percent cover of rooted floatingleaf life forms in a stratum is computed using the following formula: mathematical formula: percent cover of rooted floating-leaf life forms in a stratum where Lj is the cover rating at individual sites and A is the midpoint of the corresponding percent cover (Table 3), and M is the total number of sites in the stratum. Percent cover in a pool is computed as the average of all shallow water strata, weighted by acreage:
mathematical formula: percent cover of rooted floating-leaf life forms in a pool where Cj is percent cover in stratum j and Sj is the acreage of stratum j.


Content manager: Danelle Larson


Page Last Modified: April 17, 2018