Estimating periphyton standing crop in streams: a comparison of chlorophyll a sampling and visual assessments

Figures & data

Table 1  Locations and characteristics of the three study sites on the Cust, Selwyn and Waipara Rivers, Canterbury, New Zealand.

	Cust River	Selwyn River	Waipara River
Easting	1569445	1510495	1582120
Northing	5198200	5187565	5224810
Stream order	5	5	6
Distance from sea (km)	8.2	85.6	4.1
Elevation (m)	8	294	21
Catchment area (km^2)	202.88	164.51	719.09
Mean flow (m^3 s^−1)	2.35	3.56	6.14
Mean DRP (mg L^−1)	0.0262	0.0037	0.0065
Mean DIN (mg L^−1)	4.6251	0.2578	0.1872
Mean river width (m)	8.70	12.53	11.88
Mean transect width (m)	6.79	8.26	10.65
% pasture	95.00	54.44	82.53
% indigenous forest	1.65	14.92	2.65
% exotic forest	1.04	9.79	8.78

Table 2  Categories of periphyton distinguished in the visual assessments.

Algae type	Code	Description
Visually absent	No algae	No algae visible (no colour perceived); stone surface not slimy or slippery
Biofilms	Film	Fine, slightly slimy black, brown or greenish colouration,<0.5 mm thick (thin layer containing various algae including diatoms and Chlorophyta, but also bacteria and organic matter)
Algal mats	Mats	Definite consolidated layer of algae from 0.5 mm to > 5 mm thick, variable colours; often dominated by diatoms but can be a mixture of diatoms, Cyanobacteria, Rhodophyta, Chlorophyta. Note that Phormidium-dominated mats are recorded as a separate category (Cyano)
	Sludge	Loose, unconsolidated algae, easily dislodged; e.g. mucilage-rich accumulations of diatoms such as Cymbella
	Cyano	Cyanobacterial mats, especially Phormidium; distinctive black, brown or whitish flecked with black/brown, smooth surface
	Didymo	The bloom-forming diatom Didymosphenia geminata; distinctive brown to whitish nodules or mats, usually a tough and woolly texture
Filaments	Green_fil	Green filamentous algae, usually slimy and more than about 1 cm long; mainly Chlorophyta with some Xanthophyceae (Ochrophyta)
	Other_fil	Brown, reddish or other filaments, slimy or coarse, more than about 1 cm long. Includes Rhodophyta, filamentous diatoms and Chlorophyta covered with epiphytic diatoms

Figure 1  Summary of measurements made by three observers at three rivers sites on three occasions. A, Estimates of mean periphyton cover in seven visual categories with each observer viewing 40 different points. B, Mean chlorophyll a determined from quantitative samples collected from the same 40 points viewed in A. C, Mean chlorophyll a derived from visual assessments of percentage cover weighted by a conversion factor (see text for details). D, Mean size of rocks sampled for chlorophyll a, as surface area estimated from x, y and z dimensions.

Table 3  Summary results (P-values) of 3-way ANOVAs to determine differences in estimates of % cover of seven visual categories of periphyton, across observers, survey occasions and sites, and showing interactions between observers and survey occasions and sites. (a) Data from a 120-point dataset (all observers viewed the same locations); (b) data from a 40-point dataset (observers all viewed different locations). Data were mean percentage cover estimates along transects of 10 views. Significant differences are in bold type.

Test	No algae	Film	Sludge	Cyano	Mats	Green_fil	Other_fil
(a) 120-point dataset (12 transects)
Observer	<0.001	0.78	0.87	0.80	<0.001	0.22	0.53
Survey	<0.001	0.06	0.69	<0.001	<0.001	<0.001	<0.001
Site	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Survey×observer	<0.001	<0.001	0.90	0.57	<0.001	0.12	0.06
Site×observer	<0.001	0.004	0.54	0.97	<0.001	0.42	0.58
(b) 40 point dataset (4 transects)
Observer	<0.001	0.94	0.94	0.35	0.026	0.15	0.67
Survey	<0.001	0.10	0.80	<0.001	<0.001	0.001	<0.001
Site	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Survey×observer	0.001	0.10	0.95	0.71	0.001	0.29	0.90
Site×observer	0.072	0.10	0.93	0.50	0.024	0.73	0.74

Figure 2  Cover by mats at the Selwyn River site in June 2011 (visit 2) assessed independently by three observers at 120 pre-determined viewing locations. Each box plot is derived from 100 random selections (with replacement) of 2, 4, 6, etc. observations (views).

Figure 3  Minimum numbers of views required to achieve 95% confidence of being within 10% of the true mean, calculated for every survey combination and visual assessment category using bootstrap resampling on the 120 views assessed by each observer in each survey.

Figure 4  Proportion of the means of 100 randomly re-sampled sets of observations of chlorophyll a (for each site, occasions and observer combination), which were within 20% of the mean of all 120 observations. Numbers of samples in each resampled set ranged from two to 40. Each row shows the results from one observer.

Figure 5  Proportion of the means of 100 randomly resampled sets of observations of chlorophyll a (with data from three observers combined), which were greater than 15, 50, 120 and 200 mg m⁻² (bottom row to top row). Numbers of samples in each resampled set ranged from two to 40.

Table 4  Summary results (P-values) of 3-way ANOVAs to determine differences in observed and derived chlorophyll a across observers, survey occasions and sites. Chlorophyll a was estimated from the visual assessments using conversion factors determined from cover on individual rocks (see Table 5). Significant differences are in bold type.

	Means at each transect		Random selection, n=12
Test	Observed	Derived	Observed	Derived
Observer	0.27	0.22	0.32	0.59
Survey	<0.001	<0.001	<0.001	<0.001
Site	<0.001	<0.001	<0.001	<0.001
Survey×observer	0.44	0.87	0.17	0.93
Site×observer	0.59	0.74	0.44	0.89

Figure 6  Chlorophyll a derived from visual estimates made by three observers at three sites on three occasions, using the conversion factors in Table 5, plotted against measured chlorophyll a. Each point is the mean of chlorophyll a samples and visual assessments at 20 points. The solid line indicates the best fit regression (R²=0.889, P<0.001) and the dashed line is the 1:1 relationship.

Table 5  Chlorophyll a equivalents for the seven periphyton categories, determined from measurements of chlorophyll a from samples with 100% cover by each category.

Category	Chlorophyll a equivalent (mg m^−2)
No algae	1
Film	9
Sludge	72
Cyano	599
Mat	118
Green_fil	404
Other_fil	517

Table 6  Summary results (P-values) of pairwise comparisons (following significant ANOVAs) of mean rock size in collections made by three observers (obs 1, 2 and 3) for chlorophyll a samples, and by a fourth person as a separate random collection (random). Significant differences are in bold type.

Test	Cust	Selwyn	Waipara
Random vs. obs 1	<0.001	<0.001	<0.001
Random vs. obs 2	<0.001	<0.001	<0.001
Random vs. obs 3	<0.001	<0.001	<0.001
Obs 1 vs. obs 2	0.12	0.12	0.95
Obs 1 vs. obs 3	0.003	0.29	0.051
Obs 2 vs. obs 3	0.59	0.97	0.18

Figure 7  Distributions of rock sizes (as estimated exposed surface area calculated from x, y and z dimensions) collected at the Cust, Selwyn and Waipara River survey sites by three observers for sample collection for chlorophyll a, plus a random collection made by a fourth person. Total n=120 for observers 1, 2 and 3, and n = 360 for the random collection, pooled from three survey occasions.