J . Chang J. Exp. Mar. Biol. Ecol. 245 2000 215 –224
217
2. Materials and methods
During a cruise on board R V Ocean Researcher II to the East China Sea on 31 August 1994, water samples containing Trichodesmium trichomes were collected by
both Go-Flo bottles and a plankton net from a station at 25813.39N, 122851.49E. The size of the Go-Flo bottles was 2.5 l General Oceanics, Miami, FL, USA, and a total of five
samples were taken from a depth of 20 m. From each Go-Flo bottle, two subsamples, with a volume of 1 l each, were retrieved sequentially and fixed with acidic Lugol’s
solution Throndsen, 1978. Trichomes in the subsamples were concentrated by settling Sukhanova, 1978. First the 1-l bottle was allowed to settle on a bench for at least 2
days. The supernatant sea water was then carefully removed using an aspirator until about 50 ml of sample remained. The concentrated sample was transferred to a conical
centrifugation tube and went through a second round of settling. Finally, the subsample volume was reduced to 1 ml, and trichomes were counted in a Sedgwick–Rafter
counting cell at 100 3 . The entire counting cell was scanned and total number of trichomes was recorded.
Net-collected samples were obtained using a plankton net with 20-mm mesh size and a mouth diameter of 0.5 m. The length of the net was 2.5 m. A flowmeter Model
2030R, General Oceanics was attached to the center of the mouth ring as a means to estimate the volume of water filtered. The net was towed at 20-m depth with a speed of
1 knot for 10 min, and three net tows were conducted at each station. The materials in a receiving bucket were subsampled into three scintillation vials volume 5 17 ml each
and preserved with Lugol’s solution. For counting the trichomes, two aliquots of 1 ml in volume were retrieved from a scintillation vial after thorough mixing. Each aliquot was
placed in a Sedgwick–Rafter cell, and the entire counting cell was examined. The number of trichomes observed in each counting cell was converted to natural abundance
in the water column, and the abundance data was used for further analysis.
Identical sampling procedures were repeated on 2 July 1995 at a nearby station 25825.79N, 122817.89N. During this cruise, in addition to the five Go-Flo samples for
settling, another five Go-Flo samples were obtained, and each of the two subsamples from a Go-Flo bottle was immediately filtered through a 2-mm pore size Nuclepore filter
on board Capone et al., 1997. The Nuclepore filter was then mounted on a microscope slide with a small amount of immersion oil, and was stored at 2 158C. Trichomes on the
Nuclepore filter were examined with an Nikon epi-fluorescence microscope using 450 to 490 nm as the excitation wavelengths for phycoerythrin Brock, 1978; Carpenter et al.,
1990. The entire filter membrane was scanned to obtain total number of trichomes in a 1-l subsample.
The statistical method of analysis of variance ANOVA was employed to estimate the variances from different sources Venrick, 1978. For bottle-collected samples, two
sources of variation were identified in the final estimate of trichome concentration Table
2
1. One came from taking two subsamples in a Go-Flo bottle s , which was estimated
2
by the mean square among subsamples MS . Since a significant portion of the Go-Flo
2 2
sample was used as a subsample, MS actually estimated the product of s and FPC2,
2 2
the finite population correction. The equation for FPC2 is Venrick, 1978: V 2 v
]] FPC2 5
1 v
218 J
. Chang J. Exp. Mar. Biol. Ecol. 245 2000 215 –224 Table 1
a
Distribution of variance components at each level of the Go-Flo bottle subsampling procedure Level
Source of variation n
df MS
MS estimates
i i
i 2
2
1 Between five Go-Flo
5 4
MS s FPC11n s
1 2
2 1
bottles
2
2 Between two subsamples
2 5
MS s FPC2
2 2
within each Go-Flo
a
n : Number of samples taken from a population at level i 21, df: degrees of freedom, MS : mean square
i i
2
of level i, s : variance of level i.
i
where v is the volume of a subsample 1 l, and V is the volume of a Go-Flo bottle 2.5
2
l. The other source of variation came from different Go-Flo bottles s which
1
represented the natural variation of trichome distribution at the sampling site. However,
2 2
the mean square among Go-Flo bottles MS estimates a combination of s and s
1 1
2 2
Table 1, so s was computed as:
1
1 FPC1
2
] ]]
S D
s 5 MS 2
MS 2
1 1
2
n FPC2
2
where n is the number of subsamples taken from a Go-Flo bottle. Again, FPC1 was
2
used to adjust the effect of large subsample volume, and the equation is Venrick, 1978: V 2 n v
2
]]] FPC1 5
3 V
The mean squares at each level was obtained by constructing a standard ANOVA table Sokal and Rohlf, 1981.
The variance components at each level were used to calculate the coefficient of variation C.V. of an abundance estimate x based on a single 1-l subsample from a
single Go-Flo bottle Woelkerling et al., 1976: ]]]
2 2
s 1 s
œ
1 2
]]] C.V. 5
3 100 4
x The procedure to separate the variance components in net-collected samples was very
similar to that for bottle-collected samples, and detailed descriptions can be found in Venrick 1978. With this sampling design, three sources of variation came from the
2
variation between counting cells within scintillation vials s , variation among
2 2
scintillation vials within receiving buckets s , and natural variation reflected by
1 2
individual tows s , respectively. No finite population correction was needed because
all subsamples occupied a very small fraction of the upper-level samples. The performance of the two methods was further examined during three other cruises
conducted in 1994 and 1995 to the southern East China Sea. At each of the 15 stations, bottle-collected and net-collected samples were taken with no replicates Table 2. The
product–moment correlation coefficient was computed to determine if the two sampling programs showed a common trend Sokal and Rohlf, 1981.
J . Chang J. Exp. Mar. Biol. Ecol. 245 2000 215 –224
219 Table 2
Trichodesmium abundance estimated with bottle- and net-collected samples at various locations in the southern East China Sea
21
Cruise Station
Sampling Trichomes l
Volume filtered depth m
by net l Bottle
Net March 1994
25823.09N, 122815.99E 10
3 44.3
11 681 25838.59N, 123807.79E
10 0.9
39 628 25817.89N, 122830.19E
10 3.5
18 280 25814.09N, 122839.99E
10 11.9
12 646 25811.19N, 122850.79E
10 3
2.8 23 172
25804.59N, 122800.09E 10
3 9.5
15 052 25826.39N, 121834.69E
10 5
0.1 13 918
25852.19N, 121820.09E 1
9 0.1
48 431 May 1994
25855.99N, 121819.99E 10
99 142.3
9370 25840.29N, 121849.99E
10 48
5.3 28 572
25830.09N, 122810.29E 10
25 57.5
8602 25825.09N, 122820.09E
10 1
11.3 6911
25819.89N, 122830.29E 10
7 7.9
5793 April 1995
26800.69N, 121810.39E 15
2 14.6
7413 25810.19N, 122850.09E
2–5 138
41.6 11 726
3. Results
