Sinking as a factor affecting phytoplankton species succession: The use of selective loss semi-continuous cultures

J. Exp. Mar. Biol. Ecol., 1986, Vol. 99, pp. 19-30

19

Elsevier

JEM 696

SINKING AS A FACTOR AFFECTING PHYTOPLANKTON SPECIES SUCCESSION: THE USE OF SELECTIVE LOSS SEMI-CONTINUOUS CULTURES

P. J. HARRISON Departments of Oceanography and Botany, University of British Columbia, Vancouver, B.C., Canada V6T 2Bl

D. H. TURPIN Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6

P. K. BIENFANG Oceanic Institute, Waimanalo, Hawaii 96795, U.S.A.

C.O.

DAVIS

Jet Propulsion Laboratory, Pasadena, CA 91109, U.S.A.

(Received 6 November 1985; revision received 18 March 1986; accepted 19 March 1986) Abstract: The interactions between sinking rate, nutrient-limitation and species succession of natural phytoplankton assemblages were studied in outdoor semi-continuous cultures under four treatments. Two cultures were NH,-limited and two were Si-limited. One NH_,- and one $&limited culture were subjected to selective loss (due to sinking) and the other two were subjected to random loss due to daily aliquot removals following mixing of the cultures. Under NH,-limitation in the random loss cultures, centric and pennate diatoms dominated while in the selective loss cultures, flagellates dominated by the end of the 25-day experiment. Under Si-limitation a mixture of flagellates, pennate and centric diatoms occurred in the random loss cultures, while the selective loss cultures consisted offlagellates and a very small centric diatom, Chaetocerosgracilis Lauder. Skeletonema costatum (Grev.) Cleve was very sensitive to Si-limitation and sank more quickly than Chaetoceros spp. Pennate diatoms had lower sinking rates than centrics and formed a background group in flagellate dominated communities in the selective loss cultures. Phytoplankton species competition and succession are influenced by growth and loss processes. Sinking rate is an important selective loss process in temperate coastal areas which can be simulated by using this selective loss, semi-continuous culturing technique. The advantages of using this technique to study the effects of sinking rate and its interaction with nutrients on succession in natural phytoplankton assemblages are discussed. Key words: sinking rate; silicate; nitrate; nutrient-limitation;

diatoms; species succession

INTRODUCTION

Increases in phytoplankton populations processes according to the equation dhr/dt

are controlled by both growth and loss where N = biomass, t = time,

= iV (p-d)

0022-0981/86/$03.50 0 1986 Elsevier Science Publishers B.V. (Biomedical Division)

20

P. J. HARRISON

ETAL

p = growth rate (day- ‘), and d = 1oss rate (day- ‘). To date most attention has been focussed on the measurement of growth related processes such as nutrient uptake and photosynthesis. Less effort has been directed towards the measurement of loss processes such as sinking and grazing, even though losses may be equally important in determining species dominance in natural assemblages, particularly in temperate coastal environments. Species succession is controlled by physical, chemical and biological factors. Factors such as light (Wall & Briand, 1979), temperature (Goldman & Ryther, 1976; Goldman & Mann, 1980), specific nutrient flux (Harrison & Davis, 1979; Turpin & Harrison, 1979), nutrient pulsing (Turpin & Harrison, 1979; Turpin & Harrison, 1980) and nutrient ratios (Titman, 1976; Schindler, 1977; Tilman, 1977) have been evaluated for their influence on species succession in natural phytoplankton assemblages. Sinking rates have been measured for many phytoplankton species (Smayda, 1970; Walsby & Reynolds, 1980; Bienfang etal., 1982; Bienfang & Harrison, 1984a) and for natural assemblages (Bienfang, 1981a, 1984; Bienfang & Harrison, 1984b), but there are no studies on the effects of differential sinking rates of nutrient-limited phytoplankton and how these rates affect the outcome of species competition. It is well established that nutrient-limitation causes an increase in the sinking rate of many diatoms (Bienfang et al., 1982). Consequently, the interaction between nutrient-limitation and sinking rate may be of major significance in mediating community structure. When phytoplankton competition experiments are run in continuous or semicontinuous cultures, the loss of cells is generally achieved randomly or non-selectively, with all cells in the community experiencing identical mortality or loss rates. In the present study, however, cells with high sinking rates were actively selected against by culturing in unstirred separatory funnels. Daily removal of medium from the bottom of these cultures, in the process of culture dilution, ensured that species succession was influenced not only by nutrient limitation but by differential sinking rates of the various populations. In this study, we report the effects of sinking on species succession in a natural phytoplankton assemblage grown under four experimental regimes: (1) stirred, N-limited semi-continuous cultures (i.e., random loss, since cultures were continuously stirred and loss of cells was non-selective); (2) unstirred, N-limited (selective loss) semi-continuous cultures; (3) and (4) were identical to treatments (1) and (2) except the cultures were Si-limited. The results of these experiments are discussed in light of the ecological significance of sinking rates during competition for limiting resources. Silicate-limitation is known to increase the sinking rate of diatoms much more than nitrogen-limitation (Bienfang et al., 1982). For this reason we have chosen to study both nitrate- and silicate-limitation to determine how these nutrients interact with sinking rate and to determine the rate and direction of species succession.

21

EFFECTS OF SINKING ON SPECIES SUCCESSION

MATERIALS AND

METHODS

INOCULUMAND INCUBATION Outdoor semi-continuous cultures were set up as described previously (Harrison & Davis, 1979; Turpin & Harrison, 1979). For Expt. 1, the inoculum for the experiments was collected in early July 1978 from 4 to 8 m in the experimental enclosure (CEE-2) located in Saanich Inlet, B.C., Canada. It was fiitered through 1.53~pmmesh net to remove large zooplankton. The experiment was repeated in August (Expt. 2) with an inoculum from the CEEs and large zooplankton were removed as described above. A small inoculum of Skeletonema costatum (Grev.) Cleve, Thalassiosira nordenskioldii Cleve, Chaetoceros socialis Lauder, and C. constrictus Gran were added to Expt. 2 inoculum so these species which were absent from the natural sample but present in the inoculum for Expt. 1 could be observed in the succession sequence (Table I). Two

TABLE I

A comparison of the initial inocula for Expts. 1 (July) and 2 (August). Expt. 1 ( x IO6 cells/l)

Expt. 2 ( x 10’ cells/l)

Skeletonema costatum Chaetoceros spp. T?zalassiosiraspp. Sie~~~o~~x~ turris (Grev. & Am.) Ralfs. Ceraiaulina beeonii Nitzschia spp. Flagellates Others

0.63 3.50 0.92 0.96 1.60 1.40

1.4 3.4 5.9 2.4 0.7 1.4 8.6

Total

9.01

23.8

hundred litres of Saanich Inlet surface water were collected and filter-sterilized using a 147~mm, 0.45pm Millipore filter for subsequent use in culture dilutions, This water was stored in a dark, cold room (4 “C) for the duration of the experiment. Aliquots were removed and enriched to the desired inflow nutrient concentrations. Two, 2-1flat-bottom boiling flasks (for random loss cultures) and two, 2-110 cm high, separatory funnels were fXled with this inoculum and placed in an outside water bath at 13 f 1 “C. Natural sunlight was attenuated and spectrally corrected to simulate Jerlov Type 3 coastal water at 5 m by surrounding the incubator with 0.25~cm blue Plexiglas (Rohm and Hass, No. 2069) (Davis et al., 1973). The blue Plexiglas decreased to the incident radiation by ~50% from a noon value of 1200~E~m-2~s-’ 600~E*m-2.s-‘. The incident photon flux density in these experiments has been previously reported (Turpin & Harrison, 1979; see their Fig. 8).

P. J. HARRISON

22 EXPERIMENTAL

ETAL.

REGIMES

Two stirred, semi-continuous random loss cultures were diluted daily at 1300, yielding a dilution rate of 0.33 . day - ’ and were gently stirred with a magnetic stirring bar at 60 rpm. Two more semi-continuous cultures were maintained in separatory funnels and were not stirred (selective loss cultures). They were also diluted at the same rate, D = 0.33 * day- ‘. This was achieved by removing one-third of the culture medium from the bottom of the separatory funnel; this sample contained the cells that had sunk. New medium was added and the culture gently mixed by hand and allowed to stand until the next dilution 24 h later. This culture device was termed a selective loss semi-continuous culture because loss of cells or removal of species during the dilution process was selective and based on differences in the sinking rates of different species. One random loss and one selective loss culture were run as NH,-limited cultures with the ratio of nutrients in the inflow medium of 3.3 : 15 : 1, N : Si : P by atoms. The actual concentrations were 10 PM NH,’ ,45 PM SiO; 4, and 3 PM PO; 3. The other selective and non-selective loss cultures were run as Si-limited cultures with the ratio of nutrients in the inflow medium of 11 : 3.7 : 1, N : Si : P by atoms. The actual concentrations were 39.1 PM NH,‘, 13.4 PM SiO, 4, and 3.6 PM PO, 4. Vitamins and trace metals were added as f/25 (Guillard & Ryther, 1962). Following culture inoculation, the experiments were run for z 3 wk. This period was arbitrarily chosen. It was not long enough for steady state to occur but it does represent a period over which the initial major response to the treatments could be observed without getting into confounding effects of grazing. During this period, cultures were sampled regularly and the following measurements were obtained: (1) cell numbers and species identification; (2) in vivo fluorescence; (3) nitrate, ammonium, and occasionally soluble reactive phosphate and silicate; and (4) sinking rate. At the termination of the experiments, the uptake rate of the limiting nutrient was measured along with particulate nitrogen, carbon, and silica. MEASUREMENTS

Cell samples were preserved with Lugol’s solution, settled and counted with an inverted microscope. Nutrient analyses were undertaken using a Technicon Autoanalyzer as previously described (Davis et al., 1973). Particulate nitrogen and carbon samples were run on a Hewlett-Packard nitrogen analyzer and particulate silica was determined by fusion with Na,CO, (Conway et al., 1977; Paasche, 1980). Sinking rates were determined by measuring the accumulation of radioactively labelled cells at the bottom of a 5 cm column (Rothwell & Bienfang, 1978; Bienfang, 1979). Nutrient uptake rates were determined at the end of the experiment using a perturbation technique (Caperon & Meyer, 1972; Conway et al., 1976). The perturbation was conducted at 1230 which represents the interval just prior to the normal daily nutrient addition. Generally, a saturating amount (4 to 5 PM) of the limiting nutrient was added to the culture and its concentration was followed by sampling at 5-min intervals until the limiting nutrient was exhausted.

EFFECTS OF SINKING ON SPECIES SUCCESSION

23

RESULTS COMMUNITY

STRU~URE

Random ioss cultures

In Expt. 1 the NH,-limited culture was dominated by Skeletonema costatum (52% of total cell numbers) at the end of the 3-wk experiment (Fig. 1A; Table II). Pennate diatoms (e.g. Nitzschia spp. and Cylindrotheca fisifbmis Reimann dz Lewin) were of secondary importance. Community structure in a replicate experiment repeated the foilowing month (Expt. 2) was similar even though starting inocula were different (Table I). ~ke~etonema costatum, other cent&s (e.g. ~ratuu~~na ~ergonii H. PCragallo) and pennates (e.g. Nitzschia spp.) were of similar abundance while microflagellates were of minor importance. Si-limited cultures were in sharp contrast to the NH,-limited cultures. In Expt. 1, microflagellates (< 15 pm) dominated (64% of total cell numbers) while Chaetoceros spp. and pennate diatoms were less important (Fig. 1C; Table II). The replicate S&limited culture (Expt. 2) exhibited a predominance of pennate diatoms (Table II); some ciliates were noted in- this culture at the end of the experiment and this may be the reason why fewer flagellates were observed compared with the replicate (Expt. 1). Selective loss cultures

Community structure in the selective loss cultures was markedly different than in the non-selective loss cultures (Fig. 1; Table II). In Expt. 1, flagellates such as Chrysochromulina sp. dominated in the NH,-limited cultures (73% of total cell numbers). The replicate study (Expt. 2) gave similar results but with an increase in the number of centric diatoms. Silicate limitation caused even more pronounced dominance of IIagellates in Expt. 1 (99% of total cell numbers). In Expt. 2, the small, single-celled Chaetoceros gracilis Schuett co-dominated with flagellates such as Heterosigma akashiwo (Hada) Hada ( = Olithodiscusluteus auct. nonull.) and Ochromonas sp. Some ciliates were noted in the cultures near the end of the experiment, particularly at the end of Expt. 2 in the silicate-limited cultures. This may have’been the reason why fewer flagellates were observed in the Si-limited cultures compared with Expt. 1, as ciliates are known to prefer small fl~e~ates over diatoms (He~bokel8z Beers, 1978). SINKING RATES

sinking rates of the various components of the cultures are given in Table III. In the selective loss cultures, the “pelagic fraction” refers to the upper portion of the culture, while the “settled fraction” refers to the lower portion that was removed each day. Generally there was no difference in sinking rates between the pelagic and settled The

TABLE

II

~--.--Skelelonema coftatum Chaetoceros spp. Other centrics Pennates Micro~agellates

-x____--

~---

NW, 5.5 3.1 1.7 16.3 73.4

21.3 14.6 64.1

0.3 99.7

Si

Initial inoculum _.__-._ 5.9 14.3 31.8 5.9 36.1

__-

y0 of total cell numbers _..

Selective loss

Si

-

_---.-~~.--Expt. 1 ~--Random loss

.---~. Initial inoculum NH, __. ._“__ 52.3 7.0 38.8 4.6 4.9 25.1 36.3 10.6 17.1 1‘9

-------

__-..__

---____30.8 0.6 16.0 40.6 1.6 26.6 60.9 1.9 20.9

NH,

Si

-_-I-

Random loss

Expt. 2

24.8 25.9 1.6 47.8

NH,

l_l_

-

76.8 0.9 22.2

Si _-

Seiective loss

-

composition of the phytopiankton assemblages initially and at the end of two 3-wk experiments that commenced in July or August: cultures were either random loss (stirred semi-continuous) or selective loss (unstirred semi-continuous) cultures and they were either ammonium- or silicate-limited; a dash means no data are available.

The

h

g

E

yo L3 2:

EFFECTS OF SINKING ON SPECIES SUCCESSION

25

fractions for both ammonium- and silicate-limitation. Generaliy, the sinking rates of both the settled and pelagic fractions under s~icate-l~itation were higher than under ~onium-notation. The sinking rates for the NH~-l~it~ random loss culture on Day 17 in Expt. 2 was 0.55 m +day- ’ and, therefore, greater than for the pelagic fraction of the selective loss cultures (0.39 m * day- ‘). Unfortunately, the sinking rate measurement for the S-limited random loss culture in Expt. 2 was lost and therefore the same comparison cannot be made as for ammonium.

TIME

( days 1

Fig. 1. Changes in groups and species of phytoplankton during Expt. 1, expressed as cumulative cell numbers for NH,-limited, random loss (A), NH,-limited, selective loss (B), Si-limited, random loss (C), and S&limited, selective loss (D) cultures: categories of phytoplaukton are, I = flagellates, II = Skeletonemu costutum, III = Chnetoceros spp., IV = pennates, V = other centric diatoms.

26

P. J. HARRISON

ETAL.

TABLE III Sinking rates (m day ‘) for either pelagic or settled fractions of the selective loss cultures grown under silicate- or ammonium-limitation: Expt. 1 was conducted in July and Expt. 2 as a replicate experiment that commenced in August; * indicates the additions of 10 PM NH,’ or SiOL4 were made just before measurements of sinking rates; a dash means no data are available. Population

fraction

Si-limited Experiment

Pelagic

Settled

Pelagic

Settled

Expt. 1 Day 8 Day 9*

0.47 0.37

0.51 0.54

0.31 0.40

0.37 0.40

0.74 0.76

0.55

0.48 0.53 0.39

0.44

Expt. 2 Day 4 Day 4* Day 17

NUTRIENT

NH,-limited

0.25

UPTAKE

0.64

RATES

The ammonium uptake rates for the NH,-limited random loss cultures (Fig. 2A) were similar, except for the higher initial rapid uptake exhibited by the assemblage in Expt. 1. The specific uptake rate for the first 12 min was 0.8 . h - ’ compared with 0.17. h ’ for 5

3-

6 4; I 2

e 0 e f. 04
B 6-

D

_

32I -

0

0

30

60 TIME

90 lmin)

120

150

0

60

120 TIME

180 240

300

(min)

Fig. 2. Disappearance of ammonium (A and B) or silicate (C and D) during a perturbation experiment on the final phytoplankton assemblages of four cultures: A, NH,-limited, random loss; B, NH,-limited, selective loss; C, Si-limited, random loss; D, Si-limited, selective loss cultures; 0 = Expt. 1; 0 = Expt. 2; the particulate nitrogen and silica values are given in Table IV.

EFFECTS OF SINKING ON SPECIES SUCCESSION

27

the next 66 min. Uptake rates of the final assemblages in the selective loss cultures of Expts. 1 and 2 were not similar (Fig. ZB). Expt. 1 exhibited a much slower uptake rate which is probably a reflection of the dominance by microfl~e~ates in this culture compared with the dominance by centric diatoms in the culture from Expt. 2. The specific uptake rate of ammonium for the first 12 min was 0.3 +h - ’ and 0.1. h - ’ for the remaining 120 min for the final assemblage of Expt. 1. The uptake of silicate was very slow in all cultures. In the final assemblage from the random loss cultures of Expt. 1, there was a lag in uptake for almost 1 h, then it resumed for 2 h and then slowed considerably, suggesting the possibility of a cell synchrony effect. The silicate uptake rate of the assemblage from the selective loss culture was very slow because of the overwhelming dominance by microflagellates.

DISCUSSION

The results from the random loss cultures are consistent with previous studies. The dominance of Skeletonemu costatum in the NH,-limited non-selective loss cultures is similar to observations made by Turpin & Harrison (1979) who conducted their once per day ~onium pulsing experiments using the same inoculum. In experiments conducted at the same site 1 yr previous to the experiments reported here, Harrison & Davis (1979) also observed dominance of S. cost&urn in chemostats (i.e. stirred, continuous cultures). The dominance by the small single-celled Chaetocerosgracile along with pennates and microflagellates in the random loss, Si-limited cultures is in agreement with previous observations by Harrison & Davis (1979). Ske~to~ma costatum was particularly sensitive to silicate-limitation, being absent or nearly so by the end of the experiment (Fig. 1C) and this observation was also made in earlier experiments (Harrison & Davis, 1979; Bienfang & Harrison, 1984b). Imposing a sinking rate dependent loss on nutrient-limited temperate phytoplankton assemblages dr~atic~y a&&s the outcome in species competition experiments, but this is not true for subtropical assemblages (Bienfang & Harrison, 1984b). Under ammonium-limitation the per cent flagellates increased while the per cent centric and pennate diatoms decreased. Expt. 2 also showed a predominance of flagellates but some centric diatoms such as Chaetoceros gracilis and Cerataulina bergonii were still able to maintain themselves in the NH,-limited selective loss cultures. Under si~cate-citation, flagellates dominated, but in Expt. 2, C~uetoceros graciiis was a co-dominant, demonstrating that this very small, single-celled centric diatom was an excellent competitor and that its sinking rate was not adversely affected by silicate-limitation (Bienfang & Harrison, 1984b). This is in contrast to most other centric diatoms and some pennates. Under long term silicate-starvation (48 h) the sinking rate of C. gracG.shas been shown to increase (Bienfanget al., 1982). Other centric diatoms such as ~~t~yZ~~~ spp. and small Chaetoceros spp. have been noted to be relatively insensitive to silicate-depletion (Bienfang & Harrison, 1984b). The reason why some diatoms are much more sensitive

28

P. J. HARRISON ETAL.

to silicate-limitation than others is of great interest. For example Skeletonema costatum is one of the most sensitive and Chaetoceros gracilis is one of the least sensitive to s~icate-citation. This marked difference does not appear to be due to cell size (they are both x 200 pm3) nor the amount of silica per cell. Skeletonema costatum contains ~0.4 to 0.7 x lo-‘pm01 Si per cell (Harrison et al., 1977), while estimates for Chaetoceros gracilis from cell counts and particulate silica values from Table IV yield a TABLE

lV

Chemical composition of phytoplankton at the end of the species succession experiments: N-selective loss = N&limited unstirred semi-continuous culture; N-random loss = NH,-limited stirred semi-continuous culture; Si = silicate-limited cultures; PSi means particulate silica concentration; a dash means that no data are available.

Expt. 1

2

Conditions

Carbon (FM)

Nitrogen (PM)

PSI (FM)

C:N (by atoms)

N : Si (by atoms)

N-random N-selective Si-random Si-selective

loss loss loss loss

129 147 207 134

13.1 16.4 25.1 21.6

35.45 10.48 8.70

9.8 9.0 8.2 6.2

0.5 2.4 2.5

N-random N-selective Si-random Si-selective

loss loss loss loss

142 203 204 188

13.4 14.8 35.3 30.8

60.55 _ 14.55 10.00

IO.6

0.2

13.7 5.8 6.1

2.4 3.1

of cz2.8 x 10 --7 pmol Si per cell. A more detailed physiological study of these two species could help to reveal the physiologicai basis for regulation of sinking rates in diatoms, Observations on the effect of sinking on species succession have been made in large enclosures (mesocosms) by Eppley et al. (1978) and Oviatt (1981). The former investigators observed that stirred enclosures maintained a large Coscinodiscus sp. for 6 wk, in contrast to the disappearance of many of the centric diatoms in 2-4 wk in unstirred enclosures. In freshwater enclosure experiments, blue-greens dominated under the quiescent, stratified conditions of unmixed enclosures, whereas mixing generally favoured an increase in large diatoms such as FrugiZaria(Reynolds et al., 1983). The higher sinking rate of Si-limited than NH,-limited cultures in this study is similar to previous laboratory (Bienfang et al., 1982) and field measurements (Bienfang & Harrison, 1984b) for temperate ph~opl~kton. The lack of difference between pelagic and settled fractions of the selective loss cultures is probably due to the short length of the columns used. The largest differences between the pelagic and settled fractions would be expected over the first day or two of the experiment but unfortunately no measurements were made during this period. There was a general tendency for the sinking rates to decrease with time during the experiment. The decline of sinking rates with time in “captivity” for contained populations has also been observed in CEPEX enclosure experiments by Bienfang (198 la). value

EFFECTS OF SINKING ON SPECIES SUCCESSION

29

The ammonium uptake rates for the NH,-limited random loss cultures which were dominated mainly by centric diatoms were very similar to rates reported by Turpin & Harrison (1979). The uptake of silicate in the Si-limited selective loss culture in Expt. 1 (see PSI value in Table IV and Fig. 2B) was probably due to chrysophyte flagellates such as Chysochromulina sp. since there were few diatoms present. Many chrysophytes possess silica scales and are known to require silicon for growth (Simpson & Volcani, 198 1). In previous experiments in which factors affecting species succession of natural assemblages have been examined, removal of cells from the cultures when they were diluted with new medium has been random (Dunstan & Menzel, 1971; Harrison & Davis, 1979; Turpin & Harrison, 1979) and hence not representative of processes in nature (Jannasch, 1974). Grazing and sinking rates are both size-selective loss processes that influence species succession in the field (Bienfang, 1981a, 1984). Therefore selective loss cultures provide a novel way in which to examine species succession in natural phytoplankton assemblages in the laboratory. The separatory funnels used in this study can be improved by using long plastic tubes, similar in construction to the tubes at present used to measure sinking rates (Bienfang, 1981b). This would increase the column length and hence the depth interval over which sinking takes place and eliminate the settling of some material on the sloping sides of the funnel. This simple culturing technique is recommended for studying changes in natural phytoplankton assemblages with time when both growth and loss processes will influence the final species composition. ACKNOWLEDGEMENTS

This research was supported by the National Science Foundation, International Decade of Ocean Exploration, Grant No. OCE 77-27226 and the Natural Sciences and Engineering Research Council of Canada. We acknowledge the technical expertise of Mr. D. S. Scales for the nutrient analyses, Ms. R. Waters for cell counts and identification, Mr. B. Cochlan for drafting the figures and the CEPEX staff for logistic support. Discussions with C. Suttle were valuable. REFERENCES BIENFANG,P.K., 1979.A new phytoplankton sinkingratemethodfor fielduse.Deep-Sea Res., Vol. 26,

pp. 719-729.

BIENFANG,P. K., 1981a. Sinking rates of heterogeneous, temperate phytoplankton populations. J. Plankton Res., Vol. 3, pp. 235-253. BIENFANG,P.K., 1981b. SETCOL - a technologically simple and reliable method for measuring phytoplankton sinking rates. Can. J. FLh Aquat. Sci., Vol. 38, pp. 1289-1294. BIENFANG, P.K., 1984. Size structure and sedimentation of biogenic microparticulates in a subarctic ecosystem. J. Plankton Res., Vol.6, pp. 985-995. BIENFANG,P.K. & P.J. HARRISON, 1984a. Co-variation of sinking rate and cell quota among nutrient replete marine phytoplankton. Mar. Ecol. Prog. Ser., Vol. 14, pp. 297-300. BIENFANG,P. K. & P. J. HARRISON,1984b. Sinking rate response of natural assemblages of temperate and subtropical phytoplankton to nutrient depletion. Mar. Biol., Vol. 83, pp. 293-300.

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P. J. HARRISON

ETAL.

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