Limnol. Oceanogr., 34(8), 1989, 1694-1705 0 1989, by the American Society of Limnology and Oceanography, Inc. A bridge between ocean optics and microbial ecology Charles S. Yentsch Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine 04575, and Center for Remote Sensing, Boston University, Boston, Massachusetts 02215 David A. Phinney Bigelow Laboratory for Ocean Sciences Abstract Phytoplankton cell size is believed to be closely regulated by the nutrient regime of water masses in the western North Atlantic Ocean. Since particle size affects attenuation of light in ocean water, we argue that a bridge between classical ocean optics and microbial ecology has formed whereby the physics of ocean color must include consideration of ecological factors important to the diversity of phytoplankton species. Observations of the specific absorption coefficient, a*, have been made during 10 oceanographic cruises in the western North Atlantic. The range in Chl a concentration for ocean samples from the various water masses in this region was 0.1-8.0 pg liter-l. Diffuse attenuation spectra of particles measured by a glass-fiber filter technique show strong correlations between extracted chlorophyll and blue (440 nm) or red (670 nm) absorption. Of the two, a*440 is the most variable. Average specific absorption coefficients are calculated to be 0.049 m2 mg-’ at 440 nm and 0.028 at 670 nm, but both relationships are seen to be nonlinear. The nonlinearity observed is attributed to differences in cell size “packaging” within natural phytoplankton populations. Variation in cell size and a* are primarily related to the availability of nitrate-N. In addition, other factors contribute to the variability in a*440, such as detritus and short wavelength UV-absorbing pigments. Particles that scatter and absorb light in the sea come from various sources; however, a major source in the open ocean is primary production. Changes in optical characteristics of water masses are now related to biochemical processes (Pak et al. 1988; Mitchell and Kiefer 1988a, b; Morel and Prieur 1977; Smith and Baker 1978; Yentsch 1962) and have given rise to a need for better understanding of the causes of ocean color (Bricaud et al. 1983). Thus, the need to interpret ocean color as measured by satellite calorimetry has moved classical optics into the arena of marine biology. It is clear that a bridge has formed between the two disciplines presenting challenges to relate ocean optics and microbial ecology. For satellite calorimetry to be an accurate measure of phytoplankton chlorophyll, one must expect a linear relationship between Acknowledgments Research supported by NASA, NSF, ONR, and the W. M. Keck Foundation. We acknowledge helpful discussions with R. W. Eppley and C. M. Yentsch. S. Sathyendranath and an unidentified reviewer provided many suggestions. Graphics were provided by James Rollins and manuscript preparation by Frances Scannell and Margaret Colby. light attenuation and chlorophyll concentration. The correlation between upwelled radiance and chlorophyll concentration has been shown to be reasonably strong (Gordon and Clark 1980), but other observations show considerable nonlinearity in the relationship between attenuation and phytoplankton chlorophyll concentration (Smith and Baker 1978). This nonlinear relationship is significant and associated with the diversity in optical properties (size, refractive index, shape) of biogenic particles (Morel and Bricaud 198 1; Sathyendranath et al. 1987; Spinrad and Brown 1986). In this paper, we explore the relationship between particulate light absorption and total amount of Chl a in the water masses of the western North Atlantic. We will attempt to demonstrate that the specific absorption coefficient for natural phytoplankton populations (a”), known to be a function of many factors, is related to cell size. We propose that the change in particle size spectrum is related to the nutrient regimes of a water mass. Methods Samples for analysis were collected with water bottles or by submersible pump from 1694 1695 Ocean optics and microbial ecology 39 Fig. 1. Station locations. depths within the euphotic zone. Generally speaking, our observational program concentrated on regions of the western North Atlantic where sharp baroclinicity occurred, i.e. the cold wall of the Gulf Stream, warm core rings, and tidal fronts in the Gulf of Maine. The data presented do not attempt to delineate spatial or temporal differences (Fig. 1). The treatment is to relate a specific optical parameter to phytoplankton population biomass expressed as chlorophyll concentration. The range of observations is set high and low by chlorophyll concentrations in coastal and oligotrophic waters. The measurement of diffuse attenuation by particles has been previously described (Yentsch 1962; Mitchell and Kiefer 1984; Phinney and Yentsch 1986; Yentsch and Phinney 1988). For sea observations, 0.52 liters of sample are filtered through a 2.5cm Whatman GF/F glass-fiber filter. The wet filter with the particulate matter is held upright in a Lucite holder in the spectrophotometer cuvette compartment. An identical wet blank filter is used in the reference light path (Fig. 2A). The diffuse attenuation spectrum as a function of wavelength is scanned between 750 and 350 nm with a Bausch and Lomb Spectronic 2000 dualbeam spectrophotometer. These absorbance values (AX) measured by the glassfiber filter technique are empirically related to diffuse attenuation spectra for suspensions of phytoplankton cultures obtained by a modified “opal glass” technique (Shibata 1958). A wetted, blank, glass-fiber filter was placed over the end of two quartz lo-cm C A hv Sa I +-----L------4 I 350 I 450 I 550 I 550 0 750 l(nm) Fig. 2. Cuvette calibration for the filter technique. A. Filter technique used at sea. B. Cells in a 1O-cm cuvette, 1.9-cm i.d., with blank filters (GFF) for diffusers. C. Cells on filter at exit of cuvette; blank filter is diffuser reference. D. Absorption spectra of panels A-C for Phaeodactylum tricornutum. Filtered media used in reference cuvette. 1696 Yentsch and Phinney 625 nm is discussed later. The cuvette provided the measure of optical light path (L = 10 cm) for a sample volume of 25 ml. To calculate the specific absorption coefficient, a*& from the absorbance measured by the glass-fiber filter technique, AX, we assume that all particles in an ocean sample (0.5-2.0 liters) are concentrated into the cuvette volume (25 ml) such that the absorbance values have units of 10 cm-l : 0.6 0.5 0.4 s 3 0.3 0.2 Chl ( micrograms / liter ) 0.6 1 a*X [m- ‘(pg Chl a)-‘] [2.3xAX (10 cm-‘) x 10 X cuvette volume (ml)] = [sample volume (ml) x sample Chl (pg liter-l)] (1) where chlorophyll concentration is measured on 85% acetone extracts of the filter with the method of Yentsch and Menzel (1963). Pure Chl a from spinach (Sigma Chem. Co.) is used as a standard. A factor of 2.3 is used to convert log base- 10 values to natural logarithms. Flow cytometry measurements at seawere made with a Becton-Dickinson FACS ana4 6 0 2 6 lyzer. This instrument measures chloroChl ( micrograms / liter ) phyll fluorescence (68 5 nm) and Coulter Fig. 3. Absorbance (A440 and A670) vs. chloro- impedance volume on single cells in a flow phyll concentration. stream. Hence the size distribution of the phytoplankton population is measured. The optical cuvettes. Volumes of mixed phyto- instrument was configured as follows: 76plankton cultures were added to the sample pm orifice, 436-nm excitation wavelength, cuvette while filtered growth medium oc- Corning CS2-64 emission filter for chlorocupied the reference cuvette (Fig. 2B). The phyll fluorescence. Impedance volume curdiffuse attenuation spectrum of the suspcn- rent equaled 0.7 1 mA such that a range of sion between 750 and 350 nm was recorded. particle diameters between 2.5 and 35 pm The sample cuvette contents were filtered were measured. Four milliliters of sample onto a glass-fiber filter, the spectrum was were prescreened through 53-grn Nitex netmeasured with filtered growth medium in ting and placed in a vial. After 10,000 t,otal the sample cuvette and the sample filter events (particles) had been analyzed by flow placed on the end of the cuvette (Fig. 2C). cytometry, the volume of sample remaining Finally, the cuvettes were removed and the in the vial was measured with a micropipet spectrum recorded in the manner of mea- to calculate the sample volume analyzed. surements made at sea (Fig. 2A). Because Size spectra as equivalent spherical diamthe internal diameter (d) of the cuvette and eter of chlorophyll-containing particles repthe area of the filter with particles is con- resent a subset of the total particles anastant and care is taken to ensure even dis- lyzed. tribution of particulates on the filter, the three spectra measured in this manner are Results a*440, a*670-The range in chlorophyll identical for samples with high absorption (Fig. 2D). The departure of the curves in concentration is representative of the water the low absorption region between 525 and masses of the western North Atlantic; high 1697 Ocean optics and microbial ecology Table 1. Calculated means and SE, 8 = chlorophyll, ? = x4670. Chl range N 8 SE2 5 0.06800 0.00735 0.05-0.09 0.10-0.40 12 1 0.25099 0.00799 0.41-0.50 24 0.43917 0.00496 0.51-1.0 104 0.76903 0.01807 66 1.50348 0.035 11 1.1-2.0 29 2.48068 0.04330 2.1-3.0 8 3.40625 0.06614 3.1-4.0 4.1-7.0 4 4.39500 0.11510 >7.0 1 7.73000 0.0 P SE P 0.00920 0.02102 0.03583 0.05414 0.09489 0.13548 0.17125 0.21200 0.29900 0.00153 0.00084 0.00231 0.00157 0.00267 0.00427 0.01027 0.00745 0.0 concentrations are observed in coastal and slope waters and low values are typical of the central gyre. The variability in blue light attenuation by particles measured as A440 (Fig. 3) ranges between 0.02 and >0.50. In this plot a deviation from linearity occurs when chlorophyll values are < 0.5 pg liter-‘, which is consistent with the observations of Smith and Baker (1978). The mean a*440 for the entire data set is 0.049 m2 mg-l. Between 0.2 and 0.5 ,ug liter-‘, a*440 appears to be more variable than at concentrations above or below these values. Figure 3 shows a strong linear correlation (r2 = 0.88 1) between chlorophyll absorption at 670 nm and extracted chlorophyll. (The attenuation maxima for the red band of chlorophyll ranges between 670 and 675 nm. For convenience we have labeled this band 670 nm.) The data fitted with a straight line gives an average value for a*670 of 0.028 m2 mg-l. As in the case of the blue attenuation, an inflection exists (deviation from linearity) around 0.5 pg liter-l of chlorophyll. It is apparent that the variability in the data increases with increasing concentration of chlorophyll, at least in the case of the region of l-4 hg liter-l chlorophyll. Even though the data shown in Fig. 3 are linearly correlated, there are distinctive features in the plot that suggest that it is not linear (nonrandom distribution of errors) as previously mentioned. The slope appears to change in the region indicated by the arrow on the plot. In an attempt to uncover the degree of nonlinearity, we have divided chlorophyll and absorbance into nine ranges and computed the means and standard errors (Table 1). When these mean values are plotted (Fig. 4), the degree of nonlinearity is accentuated. For chlorophyll con- .25 - -0 1 2 3 4 5 6 7 6 Chl ( micrograms / liter ) Fig. 4. Power curve_(Y = aP) fitted to the mean values (Chl = X, A = Y) shown in Table 1. centrations < 1.Opg liter-‘, the slope (A670 : Chl) is steeper than at chlorophyll concentrations > 1.O pg liter-‘. A power curve provides a best-fit of the means and yields the relationship A670 = 0.0667 Chl (pg liter-1)0.758. A relationship between chlorophyll, a*670, and cell size would be predicted for natural phytoplankton populations from previous theoretical and experimental work (Duysens 1956; Kirk 1975; Morel and Bricaud 198 1; Bricaud et al. 1983). We believe that the scatter and deviation from linearity shown in our data (Fig. 3) are largely due to differences in the mean cell size of phytoplankton species. Therefore, we view the range of observed chlorophyll (O-8 pg liter-l) not only as an index of a range in phytoplankton biomass but also as an index of mean cell size (i.e. low chlorophyll concentration corresponds to small cells and high chlorophyll concentration to the presence of large cells in addition to small ones). The data set, as a whole, shows a linear relationship with a high correlation coefficient suggesting that, in a statistical sense, light attenuation of natural populations tends to obey Beer’s law. However, by dividing the chlorophyll data into concentration ranges and obtaining mean values, we observe a nonlinear relationship that can be fitted by a power curve. The exponent (b) we obtain by y = aP suggeststhat spherical Yentsch and Phinney 1698 Sargasso Sea Gulf of Maine 2m Chl = 0.1 2.5 5 10 DIAMETER 15 20 2.5 ( pm ) 5 DIAMETER 10 15 20 ( pm ) 18m Chl = 0.90 2.5 10 15 20 DIAMETER ( pm ) 2.5 10 DIAMETER 24 m Chl = 2.55 Chl =0.50 2.5 5 10 DIAMETER 15 20 ( pm ) 15 20 ( pm ) 2.5 5 DIAMETER 10 15 20 ( pm ) Fig. 5. Size spectra of phytoplankton (cellular fluorescence) for the Sargasso Sea (4 July 1985) and Gulf of Maine (20 July 1985) obtained by flow cytometry. Ocean optics and microbial ecology 1699 0.5 volume is the important factor in regulating 7 cellular absorption of light. Phytoplankton cell size spectra -We have much more synoptic data on a* than phytoplankton sizing. Even with the added utility of flow cytometers, measurement of size spectra of phytoplankton at natural concentrations present in oligotrophic waters is time consuming. In Fig. 5 we show what we believe are two extremes in size spectra that lend support to the interaction of optics and V.” I 1’ 2’ 0 i ; Iii il cell size. At one extreme are the phytoplankChlorophyll ton populations of the oligotrophic waters 0.4 of the north central Sargasso Sea. At the 1 other extreme are the populations from the 1 chlorophyll-rich waters of the Gulf of Maine. In both cases, there is a subsurface chlorophyll maximum: at 58 m in the Sargasso Sea and at 24 m in the Gulf of Maine. The maximum cell size (mean spherical diameter) for the SargassoSeapopulation is < 5.O . w .*a . . . . '. 0.1 .*..': * . . pm, centered at about 3.0 pm. In contrast, . . . -I the Gulf of Maine population exhibits three prominent maxima in cell size: 3.0, 5.0, and, in the case of the surface, 10.0 pm. The 0 1 2 3 4 5 6 comparison of the two regions demonNOB” strates that with increasing richness (oligo1.0 trophy to eutrophy) the size spectrum be. *. . . . ’ c comes more skewed due to the appearance of large cell sizes. It is important to recognize that this is not because of removal of small cells but rather to the addition of large 0 0.6 .cells (Yentsch and Spinrad 1987). Relationship between cell size a* and the 5 I 0.4 availability of new nitrogen -The primary x, departure from linearity in the plot ofA vs. Chl (arrow in Fig. 3) occurs with chlorophyll concentrations of ~0.5 pg liter-l. In Fig. 6 we directly compare a* with chlorophyll and nitrate concentrations measured at the same locations. Also included is the relationship between thef-ratio (NH3 : Fig. 6. Relationship between chlorophyll (pg liN03-) and nitrate concentration as obter-l), nitrate &g-atom liter *), and a*440. Also shown served by Platt and Harrison (1985). This is the relationship betweenf-ratio and nitrate concencurve is characteristic of observations made tration (Platt and Harrison 1985). in the mid-Atlantic Bight (Harrison et al. 1987). We believe that a* is closely coupled to the availability of nitrate-N. When this a* more than double. At low levels of nitrate nutrient is < 1.0 pg-atom liter-l (f-ratios the higher efficiency for nutrient absorption < 0.8), the principal source of nitrogen would by small cells, which have a large ratio of be in the form of ammonia, presumably surface area to volume favoring survival, is supplied by grazing herbivores. As nitrate consistent with the idea that a* is a function levels and f-ratios decrease, the values for of cell size. l 0.01 l 1700 Yentsch and Phinney a suspension of absorbing particles which Duysens devised as 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Transmission Fig. 7. Graphical representation of change in absorbance (A) with change in particle transmission (T,). With changing concentration of absorbing particles in suspension, absorbance equals (1 -- T,) instead of Beer’s law, ln( 1IT,). Discussion Cell size and packaging-The problem concerning the measurement of light absorption in suspensions of cells surfaced in the-early plant physiology literature. It was recognized that Beer’s law was not always obeyed by light-absorbing particles in suspensions. The obvious reasons are that some of the light passesunobstructed, that wavelengths that are weakly absorbed have longer light paths (detours) due to scattering, and that there is self-shading if particles are strongly absorbing. The geometric factors that influence particle absorption are not obvious. The example used by Duysens (1956) to demonstrate the geometry where particle absorption obeys Beer’s law consisted of a solid layer of particles illuminated by an optical cross-section. This is analogous to a molecular solution of pigment such as chlorophyll. In this case,Beer’s law can be useda change in concentration changes absorptionA = n X ln(l/T,) where A is absorbance, n the number of particles, and TP the transmittance of particles. This equation contrasts with that for A = n x (1 - TP). In the latter case, where the transmission of the particles is zero, absorbance will be 1.O. Changes in phytoplankton concentrations obey Beer’s law when particles (cells) are nearly nonabsorbing or when the suspension is extremely dense, but a marked deviation from Beer’s law [ln( 1/TP)] occurs between these extremes (Fig. 7). For example, if the transmission of the particles changes from 0.6 to 0.2, and Beer’s law is obeyed, the absorbance ln( 1/TP) change will be 0.48. The measured absorbance (1 - TP) of the suspension, however, is lower, changing to -0.4. When making spectral measurements of suspensions (i.e. changes from highly absorbed wavelengths to weakly absorbed wavelengths and vice versa), the problem is the same. Duysens called this spectral “flattening.” None of the techniques (diffusers, integrating spheres) designed to remove optical artifacts is perfect (Maske and Haardt 1987). The filter technique does come close to Duysens’ standard particle package: where all particles are distributed into a thin layer, absorption is hence equivalent to ln( 1/TP). There are some differences between spectra measured by diffusers and filters (see Fig. 2). These differences are at wavelengths where pigment absorption is weak; we suspect that these diflerences represent the longer light path in cuvettes due to scattering. Thus, agreement with Beer’s law occurs at high and low values of particle absorption, whereas deviations from Beer’s law occur between the extremes. Other causes for change in a*440-The attenuation of blue light by oceanic particles is much higher than can be accounted for simply by an “average” attenuation by chlorophyll and accessory pigments of phytoplankton. Some workers, by the use of difference spectra (phytoplankton-ocean particle spectra), have generated so-called detrital spectra. These are characterized by a monotonous increase in attenuation from red to blue regions of the spectrum (see Yentsch 1962). These spectra are recognized as being similar to that for dissolved 1701 Ocean optics and microbial ecology yellow substances, which in turn has reinforced speculation as to the nature of this material. The problem with the so-called detritus decomposition interpretation is that many substances, detrital and nondetrital, attenuate light in this region and there is no obvious optical way of identifying detritus alone. Second, there are large variations in the attenuation spectra for phytoplankton (Sathyendranath et al. 1987; Bricaud et al. 1988). Most of the variation concerns the blue region of the spectrum. The magnitude of these differences is seen in the spectra of Aureococcus and Skeletonema (Fig. 8) normalized at 670 nm. Aureococcus attenuates blue light 1.5 times greater than Skeletonema at 450 nm. One is tempted to assign such differences to optical geometry: Aureococcus is 2.0 pm in diameter, Skeletonema is 25 pm, yet there is a major difference in ’ accessory pigmentation. Both contain fucoxanthin, but Aureococcus has in addition 19’butanoyloxyfucoxanthin (Hooks et al. 1988), which suggestsreal differences in absorption between these two organisms. Finally, a group of compounds known to be in sizable abundance in some algae are the mycosporinelike UV-absorbing pigments (Chalker and Dunlap 1982). These pigments are believed to be UV blockers (i.e. DNA protectors) in phytoplankton. They are easily detected in the in vivo absorption of some species (Yentsch and Yentsch 1982) and can be characterized and estimated by extractions with aqueous methanol. In natural populations from specific areas, we have observed large increases in the short wavelength attenuation due to the presence of these pigments (Fig. 9). The environmental conditions in the surface waters of the two areas that are compared are quite different: in the Gulf of California, the water mass was vertically stable and bathed in sunlight from a cloudless sky; the North Atlantic population was in a water column where the depth of the mixed layer was 50 m. Solar radiation was moderate due to low sun angle and clouds. At the time of this writing it is uncertain how much of these blocking pigments occur in natural populations of phytoplankton, or whether the capacity to produce this pigment is confined - B .2 .l 400 500 600 700 h(nm) Fig. 8. Absorbance spectra for Aureococcu.ssp. and Skeletonema costatum. to certain groups or to all algae. One thing is clear-when these pigments are present, they can be a major source of the high attenuation of near-UV and blue light in the in vivo spectra of oceanic phytoplankton. Cell size and nutrient availability-Earlier we noted that a bridge had formed between classical ocean optics and marine microbial ecology. The substance of this bridge concerns nutrient enrichment of the upper layers of the oceans. We are accustomed to the idea that the overall biomass of phytoplankton populations is regulated by the level of nutrients in a water mass; however, the role nutrients play in determining species diversity is less clear (Banse 1976; Malone 1980). We have argued that changes in cell size and optical features such as a* are regulated by nutrient availability. It can now be asked, are these concepts borne out by measurements of the size spectra of marine phytoplankton? It is not apparent how the “availability” of nutrient influences size spectra; however, some clues come from study of the shape of these spectra. The cell size distributions generated by the flow cytometer suggest the existence of a log-normal spectrum (Co) (Fig. lo), which is common (basic) to all size 1702 Yentsch and Phinney Station 5 19 Mar 88 Gulf of California 2m Chl = 1.70 Station 53 14 Ott 88 Gulf Stream System 2m Chl = 0.55 in vivo 95 % MeOH 95% MeOH h (nm) h (nm) Fig. 9. Absorbance spectra and spectra of 95% methanol extracts for natural populations in the surface waters of the Gulf of California and the North Atlantic. spectra. We argue that the major differences in the spectra arise from “nutrient availability,” which is manifested by an increase of cells of larger sizes C, and C, (Fig. 10). Furthermore, we believe that it is these additions to the basic spectrum (C,) that cause the variability in a* observed with increasing concentrations of chlorophyll (Table 1). Thus, optical diversity in particles results from species diversity, and changes in both time and space are lodged in the ecological theory of phytoplankton diversity (Hutchinson 1967; Kilham and Kilham 1980). The neo-Darwinian explanation of removal by natural selection does not seem appropriate. What does seem appropriate is the concept of opportunism, i.e. opportunistic growth. The important point is the surface-area-to-volume relationship and its role in nutrient uptake (see Smayda 1980; Kilham and Kilham 1980; Malone 1980). We propose a simple resource limitation model (Yentsch 1974; Tilman 1977) in the manner shown in Fig. 10. Assuming that the seasonal regime of the upper layers of the oceans offers a range of nitrate (O-5 j&M), one can visualize how size spectra could change in time and space. Inherent to this concept is that being large has advantages, yet most of the experimental data show small cell sizes out-competing large cells under any oceanic conditions (Geider et al. 1986). Such data are in contrast to field observations, where nutrient-rich and highly productive regions are characterized by large species (Davis 1982). This apparent contradiction has led others to postulate complex interactions of growth and grazing by herbivores (Sommer 1988; Walsh 1976). We believe that in the course of time algal species evolved into large, more complex cells, which provided cellular systems more flexibility or adaptability, such as storage, buoyancy regulation, resting stages, etc. All these, as well as those not stated (e.g. see Smayda 1980), become necessary for the 1703 Ocean optics and microbial ecology survival of the species, especially in coastal and shelf ecosystems where rapid environmental change is often the case. The relationship between turbulence and cell size (Margalef 1978; Legendre and LeFevre in press) appears to be one where most agree: turbulence convects nutrient which overcomes the uptake problems associated with large surface-area-to-volume cells. To say that information on evolutionary theory is required to explain the workings of particle optics may sound a bit much. However, those familiar with the difficulty of determining the causes of biogeographic boundaries recognize these formidable problems. Problems in identifying the cause of temporal and spatial change in particle optics (i.e. ocean optical geography) are just as severe. In the case of the biogeographer, the major tools are taxonomic, whereas the optical oceanographer uses ataxonomic tools. It remains to be seen whether these two disciplines progress in parallel, converge, or totally diverge. Conclusions In the western North Atlantic, the attenuation of light by oceanic particles is dominated by the absorption characteristics of natural populations of phytoplankton. Cross-sectional area values for a*440 and a*670 are strongly correlated with measured chlorophyll in vitro. Both a*440 and a*670 show that the relationship between measured absorbance and chlorophyll is nonlinear: for chlorophyll concentrations < 1.Opg liter- l the absorbance per unit chlorophyll is higher than for chlorophyll concentrations > 1.O pg liter-‘. Absorbance at 670 nm and chlorophyll are fitted with a power curve y = axb. The value for the exponent (0.75) suggests that the nonlinearity observed is the result of packaging due to differences in cell size. We did not try to analyze variability in a*440 because of the contribution by other colored substances in unknown concentrations which, in addition to the geometrical problems of packaging, increase variability. Comparison of cell size spectra, measured by flow cytometry, shows that spectra from extremes (oligotrophy or eutrophy) are dominated by a log-normal curve which has a maximum at -3.0 pm. However, for re- 0 I I I I I I I I I I 2 4 6 8 10 12 14 16 18 20 Diameter ( microns ) - GOM Fig. 10. Concept of“bio-particle” relationships. > size and nutrient gions where nutrients are plentiful, this spectrum is modified by including larger cell sizes-without any apparent loss of the small sizes. We believe that variability due to cell size is a consequence of differences in nutrient availability among water masses. For example, in oligotrophic water masseswhere nitrogen is recycled as ammonia, the populations are primarily small cells. In water masses where nitrate-N is plentiful, large cells prevail. We conclude that the factors regulating variability in species diversity apply to particle optics. 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