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Optical backscatter of marine flocs
Journal of Sea Research 46 (2001) 1±12
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Optical backscatter of marine ¯ocs Annamarie Hatcher*, Paul Hill, Jon Grant Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4J1 Received 5 September 2000; accepted 3 April 2001
Abstract Marine ¯ocs (de®ned here as aggregated particles .10 mm diameter) derived from phytoplankton culture (Phaeodactylum tricornutum; PT) and water-based drilling mud (WBDM) waste were generated in an enclosed upwelling system in order to examine how optical backscatter coef®cients at six wavelengths changed as ¯ocs matured. Equilibrium was reached after death of the phytoplankton cells and was identi®ed as a consistent particle projected-area concentration of ¯ocs (.10 mm diameter) despite compensating changes in the number and average size of ¯ocs. As the suspension of ¯ocs matured, the particle backscatter coef®cients at six optical wavelengths increased as a function of increasing particle projected-area concentration, despite a drop in the mass of suspended particulate matter (SPM). In this ¯occulating suspension, the mass of suspended particles and the particle projected-area concentration were not co-variates. These results indicate that the strong response of optical backscatter (OBS) to particle size in many previous studies may simply be due to the inappropriate use of weight concentration as the independent variable, rather than particle projected-area concentration. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Multispectral backscatter; Flocculation; Phaeodactylum tricornutum; Water-based drilling mud; Particle size; Aggregation
1. Introduction It has been recognised that ¯occulation of marine particles can have a profound effect on the optical properties of seawater by altering the size distribution and abundance of particles available to scatter light (Alldredge and Jackson, 1995). However, there are few empirical or theoretical analyses of the interaction between ¯oc formation and optical characteristics (Costello et al., 1995). As pointed out by Bunt et al. (1999), `light scattering by ¯ocs is extremely complex'. This complexity may be in¯uenced by re¯ection and refraction within the matrix of the ¯oc (Zerull and Weiss, 1974), or enhanced absorption by the organic materials which hold the ¯oc together, or * Corresponding author. E-mail address: [email protected] (A. Hatcher).
changes in the refractive index of the aggregate because of re-orientation of components (Meyer, 1979). To increase understanding of the controls on optical properties of ocean waters, it is critical that the complex relationship between the natural process of ¯occulation and the inherent and apparent optical properties of the water masses within which the ¯ocs are suspended be understood. This study represents one of the ®rst steps in that direction. In many marine environments, the majority of particles exist as components of ¯ocs (Hill, 1998). Flocculation, the repackaging of small, discrete particles into larger, aggregated particles, occurs through the physical processes of collision and sticking. A myriad of factors such as availability of particles and turbulent energy dissipation rate control ¯oc formation, and the types of ¯ocs and their physical characteristics vary under different conditions (Dyer
1385-1101/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(01)00066-1
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A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
¯ocs developed as the drill mud waste aggregated with the dying diatom cells. We then followed the ¯occulation to an equilibrium size distribution, which is a function of turbulent energy dissipation rate and the yield strength of the ¯ocs (Winterwerp, 1998). The results clearly emphasise the strong positive relationship between the projected surface area concentration, or `target area' (Bale et al., 1994) of the ¯ocs (10±100 mm in diameter) and the magnitude of the optical backscatter coef®cients. 2. Methods 2.1. Floc-a-tron
Fig. 1. Schematic diagram of the Floc-a-tron experimental tank.
and Manning, 1999). Micro¯ocs, which are less than 150 mm in diameter, are strong and relatively resistant to disaggregation, and under some conditions will aggregate to form larger macro¯ocs which are more loosely packed (Eisma, 1986; Dyer and Manning, 1999). The packing is one of the many factors, which could affect the optical characteristics of the ¯ocs. The aim of our study was to examine the relationship between maturing ¯ocs in a controlled laboratory setting and the magnitude of backscatter coef®cients at six optical wavelengths. This research is a component of a larger study, which is directed toward developing optical monitoring strategies for discharge plumes released as a result of offshore drilling activities. For this study, we engineered a simple system with two major components, an organic one (phytoplankton culture; Phaeodactylum tricornutum (PT)) and an inorganic/organic mixture (water-based drill mud waste (WBDM) collected at an offshore oil production rig). We designed an upwelling tank with a gentle impellor-driven water current in which
The experimental tank (Floc-a-tron; 0.23 m 3 volume) was constructed of marine grade plywood and ®nished with epoxy and a smooth ®breglass coating on all transitional surfaces (Fig. 1). A plastic cone was installed in the bottom and connected to a rigid hose and impellor pump (Little Giant 2E-38N, 779 dm 3 h 21 at 1.5 m) designed to produce an upwelling water current in the mid-tank of approximately 1 cm s 21. It was found during pilot experiments that this ¯ow rate allowed suspension of marine ¯ocs for a period that exceeded the proposed experimental sampling time periods. Plate glass windows installed on two sides at right angles, approximately 18 cm below the water surface, provided an opportunity to videotape suspended particles without any physical disruption. A pulley system above the tank allowed gentle introduction and placement of the backscatter instrument, with LED sources and sensors placed to allow sampling of the water at the level of the windows. The tank was placed in an air-conditioned laboratory at 208C. The laboratory was kept in the dark throughout the whole experiment. 2.2. Medium On 8th April 1999, the clean Floc-a-tron was ®lled with seawater pumped from the Dalhousie University Aquatron supply system after being drawn through a sand ®lter, and 1-mm and 0.2-mm cartridge in-line ®lters. The circulating pump system was installed, the top of the tank covered, and ®ltered water was allowed to circulate for 5 days. At this time, the blank measurements were made, after
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
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Table 1 Schedule of experimental measurements Time designate
No. of days since TP added
Date
Treatment
Measurements made
Period number
Blank T0 T1 T2 T3 T4 T5 T6 T7 T8 T9
20.5 0 1 2 6 7 8 13 21 28 37
April 13, 1999 April 13 April 14 April 15 April 19 April 20 April 21 April 26 May 3 May 10 May 19
Filtered, aged water Phytoplankton added Drill mud added Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates
Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments
Pre-¯oc Pre-¯oc Pre-¯oc Pre-¯oc 1 1 1 1 2 2 2
which 21.7 dm 3 of log-phase Phaeodactylum tricornutum diatom culture (approximately 10 6 cells cm 23) (PT; Provasoli lab details CCMP 632 isolate) were added, and the suite of experimental measurements initiated after an equilibration period of 1 h (T 0; Table 1). On the following day, 10 cm 3 of waterbased drill mud waste (WBDM) (approximately 220 g dry wt dm 23) were added. This mud was originally collected at the Cohasset/Panuke well on the Scotian Shelf and kept in cold storage (White, 1997). Water-based drill mud is a slurry largely composed of very ®ne bentonite particles, with some barite, in a carrier ¯uid. The PT/WBDM mixture was left for 1 h to mix and then the suite of experimental measurements made (T 1; Table 1). After this time, no further particles were added and experimental measurements were conducted regularly over a 37 day period as the PT/WBDM mixture aggregated into marine ¯ocs and matured in the upwelling water currents at 208C (Table 1). The suspension volumes added were determined based on the results of pilot experiments, so that backscatter coef®cients later measured would be low enough to avoid problems of multiple scattering. 2.3. Hydroscat-6 multispectral backscatter sensor A product of Hydro-Optics, Biology, and Instrumentation Laboratories, Inc. (HOBI Labs, Watsonville, CA), the Hydroscat-6 is a six-channel optical backscattering sensor measuring at wavelengths of 442, 470, 510, 589, 620, 671 nm with source beams
originating from colour LEDs. The geometry gives scattering measurements centred around an angle of 1408. A detailed description of the instrument, the calibration constants and s correction for water attenuation is published in Maf®one and Dana (1997). An assessment of calibration procedures using Hydroscat-6 is presented in Hatcher et al. (2000) and Hildebrand (1999). The backscatter coef®cient (bb; units are m 21) is the product of the geometrical cross section of the `target' particles, the dimensionless ef®ciency factor for backscattering, and the particle concentration (Morel, 1994). It is a measure of the total amount of light scattered in the backward direction (from u 908 to 1808, where u is the angle of scattering relative to the incident beam), de®ned as b b 2p
Z180 90
b u sin u du
1
In Eq. (1), b u is the volume scattering function of the target volume, or the angular distribution of single-event scattering around the direction of a parallel incident beam. Output from the Hydroscat-6 is the backscattering coef®cient, which is calculated at six wavelengths using a scaling factor and the measurement of the volume scattering function at 1408. The particle backscatter coef®cients (bbp) are calculated by subtracting the backscatter coef®cients due to water at each wavelength from the total as measured by the Hydroscat-6 (Smith and Baker, 1981).
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2.4. Experimental measurements
2.6. Size/area of ¯ocs (video and image analysis)
On all days outlined in Table 1, a standard set of measurements was made according to the following protocol. First, the Hydroscat-6 backscattering sensor was gently lowered into the water into a sampling position which was 10 cm below the surface of the water. After a 30-min acclimation period, the output from the Hydroscat-6 was logged directly onto a laptop computer for at least two minutes at a sampling frequency of 2 Hz. Backscatter coef®cients are presented as the mean value for the last 50 s of the logging period n 100: After logging, the Hydroscat-6 was left in place and water samples were withdrawn by siphon from the sampling volume outlined by the beams, and replaced by aged, ®ltered seawater. The samples were ®ltered immediately for later analysis of suspended particulate material (SPM) and pigments on the particular sampling days as outlined in Table 1. The Hydroscat-6 was then removed, and the video camera was focussed on the volume targeted by Hydroscat-6, illuminated by quartz±iodide lights. The video footage was used to characterise ¯oc size and concentration, as described below.
After sampling using the Hydroscat-6 and extraction of siphoned samples for ®ltration, ten minutes of video footage were recorded on tape using a JVC VHS video recorder. Lighting was provided by two 500-W quartz/iodide lamps directly above the water surface, angled toward the camera window. It was determined during pilot experiments that this lighting array allowed adequate de®nition of ¯ocs. The video camera was a Pulnix TM-745 E high resolution CCD shutter camera with edge enhancer ®tted with a Zoom 6000 II Navitar lens, giving 29.2 times enlargement. All images were taken at the same focal distance, and careful calibration with independently measured objects (graph paper, mono®lament line) indicated a sampling area of 0.64 cm 2 over a 3.2 cm focus ®eld, for an effective sampling volume of 2.03 cm 3. From the video footage, ten frames in which visible ¯ocs were present were sampled randomly during each measurement period using a `Snappy' frame grabber. Flocs became visible on day 6. The sampling days on and after day 6 were divided into two periods based on the dry weight of suspended particulate matter, as described in Section 3. If a randomly sampled frame contained no observable particles because of heterogeneity in the particle ®eld, more frames were sampled until a total of ten for each sampling period was stored. The number of empty frames (,3% in all sampling periods) was low and did not change consistently with ¯oc number. Captured images were analysed in `Sigma Scan Pro 5.0 (SPSS software)' on a PC. Measurements made on images of the ®ltered water during the suite of blank measurements were used to re®ne later images that contained ¯ocs. Three replicate images taken before particles were added had grey-scale thresholds of 53, with all pixels having values less than 90 (Fig. 2). The grey-scale threshold is de®ned here as the mode on the grey-scale histogram of the converted 8-bit monochrome image (0±255). In images taken after particle addition, particles were de®ned as having grey-scale values .90 (particles were white on a black background). This effectively eliminated the `noise' (Costello et al., 1995). Using this procedure, the highlighting of particles corresponded to visual perceptions while watching the video footage. The image
2.5. Filtration and pigment analyses On all sampling days, water samples were taken in triplicate and immediately ®ltered through pre-combusted, pre-weighed Millipore ®lters (APFB04700). Samples of 500 cm 3 were ®ltered on day 0 and day 1, and thereafter samples comprised a total volume of 200 cm 3. Filters were dried to constant weight at 608C in a drying oven, weighed and ashed at 5008C for 4 h. The percent loss on ignition was calculated as 100 times the ratio of the dry minus ash weights to the dry weight of the material on the ®lter. Using a Coulter Multisizer and a 100-mm-aperture tube, particle size distributions were determined on untreated sub-samples of the water. On some sampling days, water samples were taken in triplicate for analysis of chlorophyll a and total phaeophytin (Table 1). Samples (50 cm 3, day 1 to day 28; 100 cm 3, day 37) were ®ltered onto GFC glass ®bre ®lters, extracted in buffered acetone and measured using a Turner ¯uorometer and the standard technique as described in Parsons et al. (1984).
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
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3. Results 3.1. Size distributions Ð Coulter Multisizer
Fig. 2. Grey-scale intensity diagrams of three replicate frames taken of `blank' conditions in the Floc-a-tron, before addition of particles.
analysis software correspondingly outlined all of the ¯ocs observable on the video. A standard suite of measurements was made on the particles including number per frame, length, width, and particle area. Only particles .3 pixels (corresponding to an equivalent spherical diameter of approximately 10 mm) were included in analyses, to minimise systematic errors in size estimation for small particles (Russ, 1995). Data from the image analysis were transferred to SYSTAT 9.0 (SPSS software) for statistical analysis.
Fig. 3. Plots of Multisizer output from culture of Phaeodactylum tricornutum (PT), from the Floc-a-tron after the PT culture 1 waterbased drilling mud (WBDM) was added (day 1) and from the Floca-tron on day 13.
The size distributions of particles measured using the Coulter Multisizer were different than those measured in situ using the video camera due to the break-up of ¯ocs as they were drawn through the aperture in the tube of the Multisizer. The Multisizer size distributions showed two distinct patterns (Fig. 3). Maximum size was less than 15 mm (equivalent spherical diameter) on all sampling days. Between days 0 and 8, a size distribution similar to that of the phytoplankton culture (Fig. 3) was observed (day 1 (Fig. 3) is similar to all plots between days 0 and 8). The peak at 4.5 mm, which persisted until day 8, is clearly caused by the algal cells. The peak was not visible on day 13, indicating disintegration of the algal cells sometime between days 8 and 13. A distribution similar to that on day 13 (Fig. 3) persisted until the experiment was terminated. 3.2. Characteristics of suspended particulate matter The dry weight of suspended particulate material in the Floc-a-tron was about 18 mg dm 23 after addition of the TP culture, and increased to about 48 mg dm 23 immediately after addition of the WBDM (Fig. 4A). One day after addition of the WBDM, the dry weight of suspended particulate matter increased dramatically. This may be due to an increase in the ef®ciency of capture by the ®lter paper as the primary particles aggregated in suspension or aggregation of dissolved organic matter and suspended colloids into the suspended ¯ocs. The weights of SPM between days 2 and 13 mean 129:58 mg dm23 were not signi®cantly different from each other, but were signi®cantly higher than the weights on days 21±37 mean 70:23 mg dm23 (Oneway ANOVA, F 197:111; p , 0:001). The appearance of visible ¯ocs on day 6 (Fig. 4B) was used to demarcate two periods, with period 1 with higher dry weight concentrations of SPM, covering the time from day 6 to 13, and period 2 with lower dry weight concentrations of SPM, covering the time from day 21 to 38 (Table 1). The organic content of the SPM was estimated using percent weight loss on ignition. The highest organic content was measured just after the PT culture
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A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
Fig. 5. Particle backscatter coef®cients at 671 nm during ten sampling times over the course of the experiment. Box plots summarise 100 measurements of the particle backscatter coef®cient during each sampling period.
was added (Fig. 4C), with a distinct decrease noted with the initial stages of ageing (day 3), which was probably due to a relative dilution of the organic material with bentonite particles. During the period when ¯ocs were visible, days 6±37, a slight increase in loss on ignition was measured, but it was not statistically signi®cant. Algal cells were apparent until day 8 in the Multisizer distributions and the loss of chlorophyll a was measured over a longer time period. Chlorophyll a in suspended particles becomes asymptotically low after day 8, corresponding to the disappearance of cells (Fig. 4D). Phaeopigments, breakdown products of chlorophyll, were measurable from days 21 to 37 (Fig. 4D). 3.3. Flocs (days 6±37) Flocs were ®rst observed in the images on day 6.
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The number of visible ¯ocs increased rapidly from day 6 to 8. A further increase was measured from day 8 to the end of the experiment (Fig. 4B). A comparison of means per sampling date in the two periods as outlined above indicates a higher number in period 2 (26 per cm 3) than period 1 (9 per cm 3) (One-way ANOVA, F 6:215; p 0:05). The average size of ¯ocs (de®ned by the length of the longest axis) increased from day 6 to 13, did not change from day 13 to 28 and decreased from day 28 to 37 (Fig. 4E). However, variability was always high, and there was no signi®cant difference between the two periods as outlined above. The particle projected-area concentration of ¯ocs increased rapidly from days 6 to 8, and then remained at a steady value with high variability (Fig. 4F). The particle projected-area concentration was signi®cantly higher in period 2 (0.046 mm 2 cm 23) than in period 1 (0.015 mm 2 cm 23) (Oneway ANOVA, F 33:205; p 0:002). 3.4. Particle backscatter The magnitude of the particle backscatter coef®cients increased dramatically when WBDM was added to the PT in the Floc-a-tron (as can be seen for day 0 to 1 in Fig. 5 for 671 nm). An increase of approximately fourfold in the magnitude of the backscatter coef®cients (Fig. 5) at all six wavelengths was the result of a 2.7 fold increase in SPM dry weight from day 0 to 1 (Fig. 4A). Particle backscatter at all wavelengths was signi®cantly higher in period 2 than period 1. As an example, for 671 nm (Fig. 5), the mean particle backscatter coef®cient was 0.01561 m 21 for period 1 and 0.02157 m 21 for period 2 (Oneway ANOVA F 719:107; p , 0:0001). The strong relationship between the magnitude of the particle backscatter coef®cient and particle projected-area concentration in the present experiment is demonstrated by the plot in Fig. 6 showing the backscatter coef®cient at 671 nm plotted against
Fig. 4. Characteristics of suspended particulate matter (SPM) in the Floc-a-tron over time; means per sampling day (vertical bars represent 1 standard deviation); (A) dry weight concentration n 3; (B) number of visible ¯ocs observed in video frames n 10; (C) loss on ignition n 3; (D) pigment concentration n 3; (E) mean length of visible ¯ocs n 10; (F) particle projected-area concentration of visible ¯ocs n 10: Days represents the number of days since addition of PT culture. The data has been divided into three groups, designated as pre-¯oc (before day 3), period 1 and 2, based on a posteriori comparisons of dry weight concentrations which are more similar within periods than between.
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Fig. 6. Scatterplot showing the mean particle backscatter coef®cient at 671 nm n 100 and the mean particle projected-area of ¯ocs n 10 over the seven measurement periods when measureable ¯ocs were present in the Floc-a-tron.
projected surface area. The backscatter coef®cients at all wavelengths showed similar relationships. Thus, a 28% increase in the particle backscatter coef®cient with a 1 mm 2 cm 23 change in projected surface area is expected at all visible wavelengths, based on the results at 671 nm. In distinct contrast, there is a negative relationship between the particle backscatter coef®cients at six wavelengths and the weight of SPM in the two periods after ¯ocs were visible (Fig. 7). These results indicate that the expected
Fig. 8. Three dimensional plot of spectral signatures, plotted on the natural log scale of the normalised particle backscatter coef®cient (normalised to the backscatter coef®cient at 442 nm) (Y) and the natural log scale for the normalised wavelength (normalised to 442) (X) over time since addition of TP (Z).
increase in backscatter with turbidity is offset by the reduced backscatter of ¯ocs compared to their disaggregated component grains. A change in the spectral signature of backscatter was apparent after the addition of the WBDM to the TP (Fig. 8). The difference can be ascribed to a relative decrease in the 620/442 ( 1.4) and 670/442 ( 1.5) compared to the 589/442 ( 1.3) ratios (Fig. 8). With a relative decrease in pigmented particles, the spectral signature thus becomes ¯atter with the addition of the inorganic matter (WBDM). A further ¯attening is obvious after SPM is packaged into ¯ocs and a sloping toward the red end of the spectrum occurs near the end of the experiment. 4. Discussion
Fig. 7. Scatterplot showing the mean particle backscatter coef®cient at 671 nm n 100 and the mean dry weight of SPM n 3 over the seven measurement periods when ¯ocs were visible in the Floca-tron.
In the Floc-a-tron, ¯ocs grew rapidly and became visible to the video camera (.10 mm equivalent circular diameter) on day 6. Over time, maturation of the ¯ocs was characterized by an initial rapid increase in mean size, a trend to looser packing of the ¯ocs (higher particle projected-area with lower dry weight of SPM in period 2), and a distinct increase in particle projected-area concentration on day 21, which then remained constant (Fig. 4). The
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
development of this `equilibrium' ¯oc population followed the loss of the phytoplankton cells and the chlorophyll a. Despite changes in size distributions and refractive index of the ¯ocs, the evidence points strongly to the dominant in¯uence of particle projected-area concentration of the ¯ocs on the magnitude of backscatter, with minimal spectral differences within the optical range. We know little about the spectral responses of the backscatter coef®cient to natural particles in the ocean. The cause and signi®cance of the small change in slope of the spectral signature at the end of the experiment are unknown. 4.1. Floc formation in the Floc-a-tron Flocs form in the ocean largely due to the physical processes of collision and sticking of small particles. These small particles are bound by organic material such as transparent exopolymer particles (TEP), produced by bacteria (Alldredge and Jackson, 1995). Flocs are a matrix of inorganic and organic fractions with sizes largely controlled by stresses related to turbulence or sinking (Hill, 1998). The particular set of conditions present in the Floc-a-tron facilitated the formation of a persistent population of ¯ocs. These ¯ocs were present for at least 63 days in the Floc-atron and at that time were used to examine the in situ size distribution measured using the video imagery and a new type of particle sampler (Archambault et al., 2001). The trend toward increasing loss on ignition of the SPM with time may have been due to a differential settling out of the inorganic particles, a relative increase in bacterial biomass, or both. The drop in dry weight of SPM lagged the loss of phytoplankton cells by about a week, perhaps being associated with the loss of the more labile exudates and associated microbes of the live and recently dead phytoplankton cells. Between day 28 and 37, mean ¯oc size decreased and mean number of ¯ocs increased. This was probably due to disaggregation, which may have been caused by a change in the cementing qualities of the ageing ¯oc components. On day 37, the average size of the ¯ocs resembled that of period 1 more closely than the previous 2 sampling days of period 2. However, this change did not affect the total area of
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¯ocs in suspension, and the particle projected-area concentration was not signi®cantly different during the three measurement days in period 2 despite an increase in the number and a decrease in mean size on day 37. We maintain that an equilibrium exists, with a consistent particle projected-area concentration of ¯ocs under given conditions, and compensating changes in ¯oc abundances and sizes. Because of the larger and more numerous ¯ocs and the lower dry weight of SPM in period 2, the projected surface area normalised to dry weight was much higher in period 2 than period 1. These distinct differences between periods can be used as a conceptual framework within which we can assess patterns of spectral backscatter. 4.2. Optical backscatter, particle size and particle projected-area concentration The study of backscatter from particles suspended in the ocean has been the emphasis of several disciplines, including sedimentology and applied optics. Since 1985, the optical backscatter sensor (OBS) has been used by sedimentologists to estimate the concentration of suspended particles in seawater. Instrument calibration has been problematic because of the greater sensitivity of the instrument with decreasing particle size (Ludwig and Hanes, 1990; Bunt et al., 1999). This observation is largely due to the measurement scale of the independent variable. Increasing particle concentration is almost always based on weight, with no regard to changes in particle numbers or particle projected-area concentration. Calibration slopes which differ when weight concentration is used as the independent variable often do not differ when particle `target area' is used (Bale et al., 1994). A higher OBS response of smaller particles can often be viewed as an artefact of the measurement scale of the independent variable. When viewed in a theoretical framework, the size dependence of the backscatter coef®cient is a function of the product of the particle projected-area concentration and the backscattering ef®ciency, Qbb (Stramski and Kiefer, 1991). This relationship can be seen as: bbp j
ZDmax Dmin
Qbb l; D; mj pD2 =4Fj D dD
2
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A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12 2
where (pD /4) is the projected-area of the particle, Fj D is the size distribution in the jth component of particles with the refractive index mj, and Qbb l; D; mj ) is a dimensionless ef®ciency factor for backscattering (Stramski and Kiefer, 1991). In optically small particles (around 1 mm or less), such as the primary bentonite particles that were added in the drill mud during the present experiment, backscattering is more ef®cient than in larger particles. This higher ef®ciency caused the fourfold increase in the magnitude of the backscatter coef®cients with the 2.7 fold increase in dry weight of SPM at the beginning of the present experiment. In particles with a low refractive index, such as marine ¯ocs in the narrow size range of approximately 10±100 mm, backscattering ef®ciency is not expected to change signi®cantly, and the Stramski and Kiefer model predicts a linear relationship between the magnitude of the backscatter coef®cient and particle projected-area concentration. Thus, there is no basis to expect a strong relationship between the backscatter coef®cient and weight of SPM with either optically small or large marine particles. 4.3. The in¯uence of ¯occulation on optical backscatter The positive relationship between the particle backscatter coef®cients and particle projected-area concentration and not the mass of SPM is likely a function of the fractal geometry of ¯ocs. In volume conserving, ¯occulating suspensions, coalescence of two particles of volume v produces a particle of volume 2v. The density of the new particle is the same as the density of the two smaller particles, but the particle projected-area is less than the sum of the two projected areas measured independently. The ratio of the projected area of the new particle to the sum of the projected areas of the smaller particles is 2 21/3. Therefore, in a volume-conserving suspension, ¯occulation causes a systematic, size-invariant drop in projected area, which produces a decrease in the intensity of backscatter independent of the mass of the SPM. In suspensions of natural particles, volume is not conserved during ¯occulation, and the systematic relationship described above breaks down. Each coalescence produces a particle with volume, including void space, that is larger than that of the parent
particles. The fundamental relationship of fractal geometry is the scaling between particle mass and diameter (Orbach, 1986), which states D3 d 3 m d m0 d0 In Eq. (3), m d is the mass of a ¯oc of diameter d, and m0 and d0 are the mass and diameter of the component particles. The exponent D3 is the fractal dimension. In a volume conserving, constant-density system, D3 is 3, but in typical marine suspensions D3 ranges from 1.5 to 2.5 (Li and Logan, 1995). The effect of ¯occulation on particle projected-area is demonstrated most simply by considering two mono-dispersions. In one, all mass is packaged within ¯ocs of diameter d1 and in the other mass is packaged in ¯ocs of diameter d2. Assuming that the total suspended mass and fractal dimension in the two suspensions are the same D3 D3 d d N1 m0 1 N2 m0 2 4 d0 d0 In Eq. (4), N1 and N2 are the ¯oc number concentrations in the suspensions, whose ratio is described as D3 N1 d2 5 N2 d1 The ratio of the particle projected-areas in the two suspensions is described by p N1 d12 A1 4 6 p A2 N2 d22 4 and substituting Eq. (5) into Eq. (6) gives D322 A1 d2 A2 d1
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If d2 is greater than d1, Eq. (7) shows that decreasing the fractal dimension decreases the ratio of projected areas. If D3 2, then the projected areas in the two suspensions equal each other. If D3 , 2, then the projected area is actually greater in the suspension with larger particles. Such low fractal dimensions are commonly observed for suspensions of diatoms following blooms (Li and Logan, 1995), and can explain why the fractal geometry of ¯ocs underlies the observed dependence of optical
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
backscatter on particle projected-area concentration, and not mass of SPM. It has long been recognised that standard OBS output varies as a function of the particle size distribution (Bunt et al., 1999). With a given particle size distribution, the particle projected-area concentration co-varies with mass of the SPM. The effective use of an OBS assumes that the ratio of the particle projected-area concentration to mass of SPM remains constant between the calibration suspensions and the target concentrations calculated from the OBS output. This assumption may not be supported if differential settling or ¯occulation changes this ratio. Because both of these processes are common in seawater, the accurate use of OBS output to calculate mass of SPM should only be used in certain specialised applications, such as a suspension of small sands with a low organic content in a turbulent medium. This caution applies equally to the use of other optical instruments, such as the transmissometer, to calculate mass of SPM from instrument output. 4.4. Flocculation and the spectral response of optical backscatter The spectral characteristics of backscatter coef®cients are still not well known because the sensors to measure spectral backscatter have only been available for the last few years. Absorption by pigmented cells has an effect on spectral backscatter, generally by decreasing the amount of light available for backscatter in the strongly absorbing wavelength bands, around 440 and 660. A comparison of spectral signatures of the backscatter coef®cient before and after the death of the phytoplankton cells shows no easily explainable pattern, indicating the relative unimportance of the chlorophyll. The only trend that is obvious is a general ¯attening of the spectral signature after ¯ocs were present compared to those of the primary particles. With the addition of the small, non-absorbing particles of the WBDM, a peak of the normalized particle backscatter coef®cient at the longer wavelengths in the PT only addition was ¯attened. The distinct grouping of period 1 and period 2 is not evident in the examination of the spectral signatures. The relationship between particle projectedarea concentration and the magnitude of backscatter occurs among all six wavelengths. It appears that
11
spectral signatures of the backscatter coef®cient do not relate uniquely to particle packaging, which means that spectral responses of backscatter offer a poor diagnostic for particle characteristics in natural marine suspensions. 5. Conclusions In conclusion, this study has provided signi®cant insight into the particle projected-area dependence of the OBS response. We have generated marine ¯ocs in the lab using an organic component (dying TP culture) and an inorganic/organic mixture (WBDM). Flocs grew rapidly, with an `equilibrium' population of ¯ocs developing shortly after the death of the PT cells and the degradation of the chlorophyll. The equilibrium ¯oc population was identi®ed as a consistent particle projected-area concentration, irrespective of changes in number and average size. The intensity of backscatter at 6 optical wavelengths was a function of the particle projected-area concentration, irrespective of changes in ¯oc size distributions and refractive indices of the SPM. This ®nding has signi®cant rami®cations in the reassessment of spurious conclusions of size-related OBS responses found in many previous studies. We propose that conclusions from many of the previous particle size/OBS response studies may have been simply due to the incorrect use of weight concentration as the independent variable. In natural marine suspensions, the mass of suspended particles and the particle projected-area concentration are not expected to co-vary because of the process of ¯occulation. The functional relationship between the magnitude of backscatter and suspended particulate matter in the ocean is largely dependent on the particle projected-area concentration, not the weight. Acknowledgements We are grateful to Paul MacPherson, who maintained the Floc-a-tron and conducted water sampling and analysis. We would also like to thank Gary Maillet, who grew the phytoplankton culture, and Mark Merriman, who coated and plumbed the Floc-a-tron. David Dana and Robert Maf®one of HOBI labs provided generous advice and guidance. The laboratory facilities are those of the Aquatron,
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Dalhousie University. Support for this research was from an NSERC Strategic Grant to P. Hill and J. Grant. References Alldredge, A.L., Jackson, G., 1995. Aggregation in marine systems. Deep-Sea Res. II 42, 1±7. Archambault, M.C., Grant, J., Hatcher, A., 2001. The small volume particle microsampler (SVPM): a new approach to particle size distribution and composition analysis. Deep-Sea Res. I 48, 2331±2346. Bale, A.J., Tocher, M.D., Weaver, R., Hudson, S.J., Aiken, J., 1994. Laboratory measurements of the spectral properties of estuarine suspended particles. Neth. J. Aquat. Ecol. 28, 237±244. Bunt, J.A.C., Larcombe, P., Jago, C.F., 1999. Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter. Cont. Shelf Res. 19, 1199±1220. Costello, D.K., Carder, K.L., Hou, W., 1995. Aggregation of diatom bloom in a mesocosm: bulk and individual particle optical measurements. Deep-Sea Res. II 42, 29±45. Dyer, K.R., Manning, A.J., 1999. Observation of the size, settling velocity and effective density of ¯ocs, and their fractal dimensions. J. Sea Res. 41, 87±95. Eisma, D., 1986. Flocculation and de-¯occulation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183±199. Hatcher, A., Hill, P., Grant, J., Macpherson, P., 2000. Spectral optical backscatter of sand in suspension: effects of particle size, composition and colour. Mar. Geol. 168, 115±128. Hildebrand, E.N., 1999. The effect of particle size distribution on spectral backscattering coef®cient. MSc Thesis. Dalhousie University, Halifax, Canada, 114 pp. Hill, P.S., 1998. Controls on ¯oc size in the sea. Oceanography 11, 13±18. Li, X., Logan, B.E., 1995. Size distributions and fractal properties of
particles during a simulated phytoplankton bloom in a mesocosm. Deep-Sea Res. II 42, 125±138. Ludwig, K.A., Hanes, D.M., 1990. A laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand-mud mixtures. Mar. Geol. 94, 173±179. Maf®one, R.A., Dana, D.R., 1997. Instruments and methods for measuring the backward-scattering coef®cient of ocean waters. Appl. Optics 36, 6057±6067. Meyer, R.A., 1979. Light scattering from biological cells: dependence of backscatter radiation on membrane thickness and refractive index. Appl. Optics 18, 585±588. Morel, A., 1994. Optics from the single cell to the mesoscale. In: Spinrad, R.W., Carder, K.L., Perry, M.J. (Eds.). Ocean Optics. Oxford University Press, New York, pp. 93±102. Orbach, R., 1986. Dynamics of fractal networks. Science 231, 814±820. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A Manual of Chemical and Biological Methods of Seawater Analysis. Pergamon Press, Oxford, 173 pp. Russ, J.C., 1995. The Image Processing Handbook. 2nd ed. CRC Press Inc., Boca Raton. Smith, R., Baker, K., 1981. Optical properties of the clearest natural waters (200±800 nm). Appl. Optics 20, 177±184. Stramski, D., Kiefer, D.A., 1991. Light scattering by microorganisms in the open ocean. Prog. Oceanogr. 28, 343±383. White, M., 1997. The effect of ¯occulation on the size-selective feeding capabilities of the sea scallop Placopecten magellanicus. MSc Thesis. Dalhousie University, Halifax, Canada, 77 pp. Winterwerp, J.C., 1998. A simple model for turbulence induced ¯occulation of cohesive sediment. J. Hydraul. Res. 36, 309±326. Zerull, R.H., Weiss, K., 1974. Scattering properties of irregular dielectric and absorbing particles. In: Atmospheric Aerosols: their Optical Properties and Effects. Optical Society of America. NASA, Washington, DC.
www.elsevier.com/locate/seares
Optical backscatter of marine ¯ocs Annamarie Hatcher*, Paul Hill, Jon Grant Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4J1 Received 5 September 2000; accepted 3 April 2001
Abstract Marine ¯ocs (de®ned here as aggregated particles .10 mm diameter) derived from phytoplankton culture (Phaeodactylum tricornutum; PT) and water-based drilling mud (WBDM) waste were generated in an enclosed upwelling system in order to examine how optical backscatter coef®cients at six wavelengths changed as ¯ocs matured. Equilibrium was reached after death of the phytoplankton cells and was identi®ed as a consistent particle projected-area concentration of ¯ocs (.10 mm diameter) despite compensating changes in the number and average size of ¯ocs. As the suspension of ¯ocs matured, the particle backscatter coef®cients at six optical wavelengths increased as a function of increasing particle projected-area concentration, despite a drop in the mass of suspended particulate matter (SPM). In this ¯occulating suspension, the mass of suspended particles and the particle projected-area concentration were not co-variates. These results indicate that the strong response of optical backscatter (OBS) to particle size in many previous studies may simply be due to the inappropriate use of weight concentration as the independent variable, rather than particle projected-area concentration. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Multispectral backscatter; Flocculation; Phaeodactylum tricornutum; Water-based drilling mud; Particle size; Aggregation
1. Introduction It has been recognised that ¯occulation of marine particles can have a profound effect on the optical properties of seawater by altering the size distribution and abundance of particles available to scatter light (Alldredge and Jackson, 1995). However, there are few empirical or theoretical analyses of the interaction between ¯oc formation and optical characteristics (Costello et al., 1995). As pointed out by Bunt et al. (1999), `light scattering by ¯ocs is extremely complex'. This complexity may be in¯uenced by re¯ection and refraction within the matrix of the ¯oc (Zerull and Weiss, 1974), or enhanced absorption by the organic materials which hold the ¯oc together, or * Corresponding author. E-mail address: [email protected] (A. Hatcher).
changes in the refractive index of the aggregate because of re-orientation of components (Meyer, 1979). To increase understanding of the controls on optical properties of ocean waters, it is critical that the complex relationship between the natural process of ¯occulation and the inherent and apparent optical properties of the water masses within which the ¯ocs are suspended be understood. This study represents one of the ®rst steps in that direction. In many marine environments, the majority of particles exist as components of ¯ocs (Hill, 1998). Flocculation, the repackaging of small, discrete particles into larger, aggregated particles, occurs through the physical processes of collision and sticking. A myriad of factors such as availability of particles and turbulent energy dissipation rate control ¯oc formation, and the types of ¯ocs and their physical characteristics vary under different conditions (Dyer
1385-1101/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(01)00066-1
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¯ocs developed as the drill mud waste aggregated with the dying diatom cells. We then followed the ¯occulation to an equilibrium size distribution, which is a function of turbulent energy dissipation rate and the yield strength of the ¯ocs (Winterwerp, 1998). The results clearly emphasise the strong positive relationship between the projected surface area concentration, or `target area' (Bale et al., 1994) of the ¯ocs (10±100 mm in diameter) and the magnitude of the optical backscatter coef®cients. 2. Methods 2.1. Floc-a-tron
Fig. 1. Schematic diagram of the Floc-a-tron experimental tank.
and Manning, 1999). Micro¯ocs, which are less than 150 mm in diameter, are strong and relatively resistant to disaggregation, and under some conditions will aggregate to form larger macro¯ocs which are more loosely packed (Eisma, 1986; Dyer and Manning, 1999). The packing is one of the many factors, which could affect the optical characteristics of the ¯ocs. The aim of our study was to examine the relationship between maturing ¯ocs in a controlled laboratory setting and the magnitude of backscatter coef®cients at six optical wavelengths. This research is a component of a larger study, which is directed toward developing optical monitoring strategies for discharge plumes released as a result of offshore drilling activities. For this study, we engineered a simple system with two major components, an organic one (phytoplankton culture; Phaeodactylum tricornutum (PT)) and an inorganic/organic mixture (water-based drill mud waste (WBDM) collected at an offshore oil production rig). We designed an upwelling tank with a gentle impellor-driven water current in which
The experimental tank (Floc-a-tron; 0.23 m 3 volume) was constructed of marine grade plywood and ®nished with epoxy and a smooth ®breglass coating on all transitional surfaces (Fig. 1). A plastic cone was installed in the bottom and connected to a rigid hose and impellor pump (Little Giant 2E-38N, 779 dm 3 h 21 at 1.5 m) designed to produce an upwelling water current in the mid-tank of approximately 1 cm s 21. It was found during pilot experiments that this ¯ow rate allowed suspension of marine ¯ocs for a period that exceeded the proposed experimental sampling time periods. Plate glass windows installed on two sides at right angles, approximately 18 cm below the water surface, provided an opportunity to videotape suspended particles without any physical disruption. A pulley system above the tank allowed gentle introduction and placement of the backscatter instrument, with LED sources and sensors placed to allow sampling of the water at the level of the windows. The tank was placed in an air-conditioned laboratory at 208C. The laboratory was kept in the dark throughout the whole experiment. 2.2. Medium On 8th April 1999, the clean Floc-a-tron was ®lled with seawater pumped from the Dalhousie University Aquatron supply system after being drawn through a sand ®lter, and 1-mm and 0.2-mm cartridge in-line ®lters. The circulating pump system was installed, the top of the tank covered, and ®ltered water was allowed to circulate for 5 days. At this time, the blank measurements were made, after
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
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Table 1 Schedule of experimental measurements Time designate
No. of days since TP added
Date
Treatment
Measurements made
Period number
Blank T0 T1 T2 T3 T4 T5 T6 T7 T8 T9
20.5 0 1 2 6 7 8 13 21 28 37
April 13, 1999 April 13 April 14 April 15 April 19 April 20 April 21 April 26 May 3 May 10 May 19
Filtered, aged water Phytoplankton added Drill mud added Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates Ageing of aggregates
Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM Backscatter, video, SPM Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments Backscatter, video, SPM, pigments
Pre-¯oc Pre-¯oc Pre-¯oc Pre-¯oc 1 1 1 1 2 2 2
which 21.7 dm 3 of log-phase Phaeodactylum tricornutum diatom culture (approximately 10 6 cells cm 23) (PT; Provasoli lab details CCMP 632 isolate) were added, and the suite of experimental measurements initiated after an equilibration period of 1 h (T 0; Table 1). On the following day, 10 cm 3 of waterbased drill mud waste (WBDM) (approximately 220 g dry wt dm 23) were added. This mud was originally collected at the Cohasset/Panuke well on the Scotian Shelf and kept in cold storage (White, 1997). Water-based drill mud is a slurry largely composed of very ®ne bentonite particles, with some barite, in a carrier ¯uid. The PT/WBDM mixture was left for 1 h to mix and then the suite of experimental measurements made (T 1; Table 1). After this time, no further particles were added and experimental measurements were conducted regularly over a 37 day period as the PT/WBDM mixture aggregated into marine ¯ocs and matured in the upwelling water currents at 208C (Table 1). The suspension volumes added were determined based on the results of pilot experiments, so that backscatter coef®cients later measured would be low enough to avoid problems of multiple scattering. 2.3. Hydroscat-6 multispectral backscatter sensor A product of Hydro-Optics, Biology, and Instrumentation Laboratories, Inc. (HOBI Labs, Watsonville, CA), the Hydroscat-6 is a six-channel optical backscattering sensor measuring at wavelengths of 442, 470, 510, 589, 620, 671 nm with source beams
originating from colour LEDs. The geometry gives scattering measurements centred around an angle of 1408. A detailed description of the instrument, the calibration constants and s correction for water attenuation is published in Maf®one and Dana (1997). An assessment of calibration procedures using Hydroscat-6 is presented in Hatcher et al. (2000) and Hildebrand (1999). The backscatter coef®cient (bb; units are m 21) is the product of the geometrical cross section of the `target' particles, the dimensionless ef®ciency factor for backscattering, and the particle concentration (Morel, 1994). It is a measure of the total amount of light scattered in the backward direction (from u 908 to 1808, where u is the angle of scattering relative to the incident beam), de®ned as b b 2p
Z180 90
b u sin u du
1
In Eq. (1), b u is the volume scattering function of the target volume, or the angular distribution of single-event scattering around the direction of a parallel incident beam. Output from the Hydroscat-6 is the backscattering coef®cient, which is calculated at six wavelengths using a scaling factor and the measurement of the volume scattering function at 1408. The particle backscatter coef®cients (bbp) are calculated by subtracting the backscatter coef®cients due to water at each wavelength from the total as measured by the Hydroscat-6 (Smith and Baker, 1981).
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2.4. Experimental measurements
2.6. Size/area of ¯ocs (video and image analysis)
On all days outlined in Table 1, a standard set of measurements was made according to the following protocol. First, the Hydroscat-6 backscattering sensor was gently lowered into the water into a sampling position which was 10 cm below the surface of the water. After a 30-min acclimation period, the output from the Hydroscat-6 was logged directly onto a laptop computer for at least two minutes at a sampling frequency of 2 Hz. Backscatter coef®cients are presented as the mean value for the last 50 s of the logging period n 100: After logging, the Hydroscat-6 was left in place and water samples were withdrawn by siphon from the sampling volume outlined by the beams, and replaced by aged, ®ltered seawater. The samples were ®ltered immediately for later analysis of suspended particulate material (SPM) and pigments on the particular sampling days as outlined in Table 1. The Hydroscat-6 was then removed, and the video camera was focussed on the volume targeted by Hydroscat-6, illuminated by quartz±iodide lights. The video footage was used to characterise ¯oc size and concentration, as described below.
After sampling using the Hydroscat-6 and extraction of siphoned samples for ®ltration, ten minutes of video footage were recorded on tape using a JVC VHS video recorder. Lighting was provided by two 500-W quartz/iodide lamps directly above the water surface, angled toward the camera window. It was determined during pilot experiments that this lighting array allowed adequate de®nition of ¯ocs. The video camera was a Pulnix TM-745 E high resolution CCD shutter camera with edge enhancer ®tted with a Zoom 6000 II Navitar lens, giving 29.2 times enlargement. All images were taken at the same focal distance, and careful calibration with independently measured objects (graph paper, mono®lament line) indicated a sampling area of 0.64 cm 2 over a 3.2 cm focus ®eld, for an effective sampling volume of 2.03 cm 3. From the video footage, ten frames in which visible ¯ocs were present were sampled randomly during each measurement period using a `Snappy' frame grabber. Flocs became visible on day 6. The sampling days on and after day 6 were divided into two periods based on the dry weight of suspended particulate matter, as described in Section 3. If a randomly sampled frame contained no observable particles because of heterogeneity in the particle ®eld, more frames were sampled until a total of ten for each sampling period was stored. The number of empty frames (,3% in all sampling periods) was low and did not change consistently with ¯oc number. Captured images were analysed in `Sigma Scan Pro 5.0 (SPSS software)' on a PC. Measurements made on images of the ®ltered water during the suite of blank measurements were used to re®ne later images that contained ¯ocs. Three replicate images taken before particles were added had grey-scale thresholds of 53, with all pixels having values less than 90 (Fig. 2). The grey-scale threshold is de®ned here as the mode on the grey-scale histogram of the converted 8-bit monochrome image (0±255). In images taken after particle addition, particles were de®ned as having grey-scale values .90 (particles were white on a black background). This effectively eliminated the `noise' (Costello et al., 1995). Using this procedure, the highlighting of particles corresponded to visual perceptions while watching the video footage. The image
2.5. Filtration and pigment analyses On all sampling days, water samples were taken in triplicate and immediately ®ltered through pre-combusted, pre-weighed Millipore ®lters (APFB04700). Samples of 500 cm 3 were ®ltered on day 0 and day 1, and thereafter samples comprised a total volume of 200 cm 3. Filters were dried to constant weight at 608C in a drying oven, weighed and ashed at 5008C for 4 h. The percent loss on ignition was calculated as 100 times the ratio of the dry minus ash weights to the dry weight of the material on the ®lter. Using a Coulter Multisizer and a 100-mm-aperture tube, particle size distributions were determined on untreated sub-samples of the water. On some sampling days, water samples were taken in triplicate for analysis of chlorophyll a and total phaeophytin (Table 1). Samples (50 cm 3, day 1 to day 28; 100 cm 3, day 37) were ®ltered onto GFC glass ®bre ®lters, extracted in buffered acetone and measured using a Turner ¯uorometer and the standard technique as described in Parsons et al. (1984).
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3. Results 3.1. Size distributions Ð Coulter Multisizer
Fig. 2. Grey-scale intensity diagrams of three replicate frames taken of `blank' conditions in the Floc-a-tron, before addition of particles.
analysis software correspondingly outlined all of the ¯ocs observable on the video. A standard suite of measurements was made on the particles including number per frame, length, width, and particle area. Only particles .3 pixels (corresponding to an equivalent spherical diameter of approximately 10 mm) were included in analyses, to minimise systematic errors in size estimation for small particles (Russ, 1995). Data from the image analysis were transferred to SYSTAT 9.0 (SPSS software) for statistical analysis.
Fig. 3. Plots of Multisizer output from culture of Phaeodactylum tricornutum (PT), from the Floc-a-tron after the PT culture 1 waterbased drilling mud (WBDM) was added (day 1) and from the Floca-tron on day 13.
The size distributions of particles measured using the Coulter Multisizer were different than those measured in situ using the video camera due to the break-up of ¯ocs as they were drawn through the aperture in the tube of the Multisizer. The Multisizer size distributions showed two distinct patterns (Fig. 3). Maximum size was less than 15 mm (equivalent spherical diameter) on all sampling days. Between days 0 and 8, a size distribution similar to that of the phytoplankton culture (Fig. 3) was observed (day 1 (Fig. 3) is similar to all plots between days 0 and 8). The peak at 4.5 mm, which persisted until day 8, is clearly caused by the algal cells. The peak was not visible on day 13, indicating disintegration of the algal cells sometime between days 8 and 13. A distribution similar to that on day 13 (Fig. 3) persisted until the experiment was terminated. 3.2. Characteristics of suspended particulate matter The dry weight of suspended particulate material in the Floc-a-tron was about 18 mg dm 23 after addition of the TP culture, and increased to about 48 mg dm 23 immediately after addition of the WBDM (Fig. 4A). One day after addition of the WBDM, the dry weight of suspended particulate matter increased dramatically. This may be due to an increase in the ef®ciency of capture by the ®lter paper as the primary particles aggregated in suspension or aggregation of dissolved organic matter and suspended colloids into the suspended ¯ocs. The weights of SPM between days 2 and 13 mean 129:58 mg dm23 were not signi®cantly different from each other, but were signi®cantly higher than the weights on days 21±37 mean 70:23 mg dm23 (Oneway ANOVA, F 197:111; p , 0:001). The appearance of visible ¯ocs on day 6 (Fig. 4B) was used to demarcate two periods, with period 1 with higher dry weight concentrations of SPM, covering the time from day 6 to 13, and period 2 with lower dry weight concentrations of SPM, covering the time from day 21 to 38 (Table 1). The organic content of the SPM was estimated using percent weight loss on ignition. The highest organic content was measured just after the PT culture
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A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
Fig. 5. Particle backscatter coef®cients at 671 nm during ten sampling times over the course of the experiment. Box plots summarise 100 measurements of the particle backscatter coef®cient during each sampling period.
was added (Fig. 4C), with a distinct decrease noted with the initial stages of ageing (day 3), which was probably due to a relative dilution of the organic material with bentonite particles. During the period when ¯ocs were visible, days 6±37, a slight increase in loss on ignition was measured, but it was not statistically signi®cant. Algal cells were apparent until day 8 in the Multisizer distributions and the loss of chlorophyll a was measured over a longer time period. Chlorophyll a in suspended particles becomes asymptotically low after day 8, corresponding to the disappearance of cells (Fig. 4D). Phaeopigments, breakdown products of chlorophyll, were measurable from days 21 to 37 (Fig. 4D). 3.3. Flocs (days 6±37) Flocs were ®rst observed in the images on day 6.
7
The number of visible ¯ocs increased rapidly from day 6 to 8. A further increase was measured from day 8 to the end of the experiment (Fig. 4B). A comparison of means per sampling date in the two periods as outlined above indicates a higher number in period 2 (26 per cm 3) than period 1 (9 per cm 3) (One-way ANOVA, F 6:215; p 0:05). The average size of ¯ocs (de®ned by the length of the longest axis) increased from day 6 to 13, did not change from day 13 to 28 and decreased from day 28 to 37 (Fig. 4E). However, variability was always high, and there was no signi®cant difference between the two periods as outlined above. The particle projected-area concentration of ¯ocs increased rapidly from days 6 to 8, and then remained at a steady value with high variability (Fig. 4F). The particle projected-area concentration was signi®cantly higher in period 2 (0.046 mm 2 cm 23) than in period 1 (0.015 mm 2 cm 23) (Oneway ANOVA, F 33:205; p 0:002). 3.4. Particle backscatter The magnitude of the particle backscatter coef®cients increased dramatically when WBDM was added to the PT in the Floc-a-tron (as can be seen for day 0 to 1 in Fig. 5 for 671 nm). An increase of approximately fourfold in the magnitude of the backscatter coef®cients (Fig. 5) at all six wavelengths was the result of a 2.7 fold increase in SPM dry weight from day 0 to 1 (Fig. 4A). Particle backscatter at all wavelengths was signi®cantly higher in period 2 than period 1. As an example, for 671 nm (Fig. 5), the mean particle backscatter coef®cient was 0.01561 m 21 for period 1 and 0.02157 m 21 for period 2 (Oneway ANOVA F 719:107; p , 0:0001). The strong relationship between the magnitude of the particle backscatter coef®cient and particle projected-area concentration in the present experiment is demonstrated by the plot in Fig. 6 showing the backscatter coef®cient at 671 nm plotted against
Fig. 4. Characteristics of suspended particulate matter (SPM) in the Floc-a-tron over time; means per sampling day (vertical bars represent 1 standard deviation); (A) dry weight concentration n 3; (B) number of visible ¯ocs observed in video frames n 10; (C) loss on ignition n 3; (D) pigment concentration n 3; (E) mean length of visible ¯ocs n 10; (F) particle projected-area concentration of visible ¯ocs n 10: Days represents the number of days since addition of PT culture. The data has been divided into three groups, designated as pre-¯oc (before day 3), period 1 and 2, based on a posteriori comparisons of dry weight concentrations which are more similar within periods than between.
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Fig. 6. Scatterplot showing the mean particle backscatter coef®cient at 671 nm n 100 and the mean particle projected-area of ¯ocs n 10 over the seven measurement periods when measureable ¯ocs were present in the Floc-a-tron.
projected surface area. The backscatter coef®cients at all wavelengths showed similar relationships. Thus, a 28% increase in the particle backscatter coef®cient with a 1 mm 2 cm 23 change in projected surface area is expected at all visible wavelengths, based on the results at 671 nm. In distinct contrast, there is a negative relationship between the particle backscatter coef®cients at six wavelengths and the weight of SPM in the two periods after ¯ocs were visible (Fig. 7). These results indicate that the expected
Fig. 8. Three dimensional plot of spectral signatures, plotted on the natural log scale of the normalised particle backscatter coef®cient (normalised to the backscatter coef®cient at 442 nm) (Y) and the natural log scale for the normalised wavelength (normalised to 442) (X) over time since addition of TP (Z).
increase in backscatter with turbidity is offset by the reduced backscatter of ¯ocs compared to their disaggregated component grains. A change in the spectral signature of backscatter was apparent after the addition of the WBDM to the TP (Fig. 8). The difference can be ascribed to a relative decrease in the 620/442 ( 1.4) and 670/442 ( 1.5) compared to the 589/442 ( 1.3) ratios (Fig. 8). With a relative decrease in pigmented particles, the spectral signature thus becomes ¯atter with the addition of the inorganic matter (WBDM). A further ¯attening is obvious after SPM is packaged into ¯ocs and a sloping toward the red end of the spectrum occurs near the end of the experiment. 4. Discussion
Fig. 7. Scatterplot showing the mean particle backscatter coef®cient at 671 nm n 100 and the mean dry weight of SPM n 3 over the seven measurement periods when ¯ocs were visible in the Floca-tron.
In the Floc-a-tron, ¯ocs grew rapidly and became visible to the video camera (.10 mm equivalent circular diameter) on day 6. Over time, maturation of the ¯ocs was characterized by an initial rapid increase in mean size, a trend to looser packing of the ¯ocs (higher particle projected-area with lower dry weight of SPM in period 2), and a distinct increase in particle projected-area concentration on day 21, which then remained constant (Fig. 4). The
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
development of this `equilibrium' ¯oc population followed the loss of the phytoplankton cells and the chlorophyll a. Despite changes in size distributions and refractive index of the ¯ocs, the evidence points strongly to the dominant in¯uence of particle projected-area concentration of the ¯ocs on the magnitude of backscatter, with minimal spectral differences within the optical range. We know little about the spectral responses of the backscatter coef®cient to natural particles in the ocean. The cause and signi®cance of the small change in slope of the spectral signature at the end of the experiment are unknown. 4.1. Floc formation in the Floc-a-tron Flocs form in the ocean largely due to the physical processes of collision and sticking of small particles. These small particles are bound by organic material such as transparent exopolymer particles (TEP), produced by bacteria (Alldredge and Jackson, 1995). Flocs are a matrix of inorganic and organic fractions with sizes largely controlled by stresses related to turbulence or sinking (Hill, 1998). The particular set of conditions present in the Floc-a-tron facilitated the formation of a persistent population of ¯ocs. These ¯ocs were present for at least 63 days in the Floc-atron and at that time were used to examine the in situ size distribution measured using the video imagery and a new type of particle sampler (Archambault et al., 2001). The trend toward increasing loss on ignition of the SPM with time may have been due to a differential settling out of the inorganic particles, a relative increase in bacterial biomass, or both. The drop in dry weight of SPM lagged the loss of phytoplankton cells by about a week, perhaps being associated with the loss of the more labile exudates and associated microbes of the live and recently dead phytoplankton cells. Between day 28 and 37, mean ¯oc size decreased and mean number of ¯ocs increased. This was probably due to disaggregation, which may have been caused by a change in the cementing qualities of the ageing ¯oc components. On day 37, the average size of the ¯ocs resembled that of period 1 more closely than the previous 2 sampling days of period 2. However, this change did not affect the total area of
9
¯ocs in suspension, and the particle projected-area concentration was not signi®cantly different during the three measurement days in period 2 despite an increase in the number and a decrease in mean size on day 37. We maintain that an equilibrium exists, with a consistent particle projected-area concentration of ¯ocs under given conditions, and compensating changes in ¯oc abundances and sizes. Because of the larger and more numerous ¯ocs and the lower dry weight of SPM in period 2, the projected surface area normalised to dry weight was much higher in period 2 than period 1. These distinct differences between periods can be used as a conceptual framework within which we can assess patterns of spectral backscatter. 4.2. Optical backscatter, particle size and particle projected-area concentration The study of backscatter from particles suspended in the ocean has been the emphasis of several disciplines, including sedimentology and applied optics. Since 1985, the optical backscatter sensor (OBS) has been used by sedimentologists to estimate the concentration of suspended particles in seawater. Instrument calibration has been problematic because of the greater sensitivity of the instrument with decreasing particle size (Ludwig and Hanes, 1990; Bunt et al., 1999). This observation is largely due to the measurement scale of the independent variable. Increasing particle concentration is almost always based on weight, with no regard to changes in particle numbers or particle projected-area concentration. Calibration slopes which differ when weight concentration is used as the independent variable often do not differ when particle `target area' is used (Bale et al., 1994). A higher OBS response of smaller particles can often be viewed as an artefact of the measurement scale of the independent variable. When viewed in a theoretical framework, the size dependence of the backscatter coef®cient is a function of the product of the particle projected-area concentration and the backscattering ef®ciency, Qbb (Stramski and Kiefer, 1991). This relationship can be seen as: bbp j
ZDmax Dmin
Qbb l; D; mj pD2 =4Fj D dD
2
10
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12 2
where (pD /4) is the projected-area of the particle, Fj D is the size distribution in the jth component of particles with the refractive index mj, and Qbb l; D; mj ) is a dimensionless ef®ciency factor for backscattering (Stramski and Kiefer, 1991). In optically small particles (around 1 mm or less), such as the primary bentonite particles that were added in the drill mud during the present experiment, backscattering is more ef®cient than in larger particles. This higher ef®ciency caused the fourfold increase in the magnitude of the backscatter coef®cients with the 2.7 fold increase in dry weight of SPM at the beginning of the present experiment. In particles with a low refractive index, such as marine ¯ocs in the narrow size range of approximately 10±100 mm, backscattering ef®ciency is not expected to change signi®cantly, and the Stramski and Kiefer model predicts a linear relationship between the magnitude of the backscatter coef®cient and particle projected-area concentration. Thus, there is no basis to expect a strong relationship between the backscatter coef®cient and weight of SPM with either optically small or large marine particles. 4.3. The in¯uence of ¯occulation on optical backscatter The positive relationship between the particle backscatter coef®cients and particle projected-area concentration and not the mass of SPM is likely a function of the fractal geometry of ¯ocs. In volume conserving, ¯occulating suspensions, coalescence of two particles of volume v produces a particle of volume 2v. The density of the new particle is the same as the density of the two smaller particles, but the particle projected-area is less than the sum of the two projected areas measured independently. The ratio of the projected area of the new particle to the sum of the projected areas of the smaller particles is 2 21/3. Therefore, in a volume-conserving suspension, ¯occulation causes a systematic, size-invariant drop in projected area, which produces a decrease in the intensity of backscatter independent of the mass of the SPM. In suspensions of natural particles, volume is not conserved during ¯occulation, and the systematic relationship described above breaks down. Each coalescence produces a particle with volume, including void space, that is larger than that of the parent
particles. The fundamental relationship of fractal geometry is the scaling between particle mass and diameter (Orbach, 1986), which states D3 d 3 m d m0 d0 In Eq. (3), m d is the mass of a ¯oc of diameter d, and m0 and d0 are the mass and diameter of the component particles. The exponent D3 is the fractal dimension. In a volume conserving, constant-density system, D3 is 3, but in typical marine suspensions D3 ranges from 1.5 to 2.5 (Li and Logan, 1995). The effect of ¯occulation on particle projected-area is demonstrated most simply by considering two mono-dispersions. In one, all mass is packaged within ¯ocs of diameter d1 and in the other mass is packaged in ¯ocs of diameter d2. Assuming that the total suspended mass and fractal dimension in the two suspensions are the same D3 D3 d d N1 m0 1 N2 m0 2 4 d0 d0 In Eq. (4), N1 and N2 are the ¯oc number concentrations in the suspensions, whose ratio is described as D3 N1 d2 5 N2 d1 The ratio of the particle projected-areas in the two suspensions is described by p N1 d12 A1 4 6 p A2 N2 d22 4 and substituting Eq. (5) into Eq. (6) gives D322 A1 d2 A2 d1
7
If d2 is greater than d1, Eq. (7) shows that decreasing the fractal dimension decreases the ratio of projected areas. If D3 2, then the projected areas in the two suspensions equal each other. If D3 , 2, then the projected area is actually greater in the suspension with larger particles. Such low fractal dimensions are commonly observed for suspensions of diatoms following blooms (Li and Logan, 1995), and can explain why the fractal geometry of ¯ocs underlies the observed dependence of optical
A. Hatcher et al. / Journal of Sea Research 46 (2001) 1±12
backscatter on particle projected-area concentration, and not mass of SPM. It has long been recognised that standard OBS output varies as a function of the particle size distribution (Bunt et al., 1999). With a given particle size distribution, the particle projected-area concentration co-varies with mass of the SPM. The effective use of an OBS assumes that the ratio of the particle projected-area concentration to mass of SPM remains constant between the calibration suspensions and the target concentrations calculated from the OBS output. This assumption may not be supported if differential settling or ¯occulation changes this ratio. Because both of these processes are common in seawater, the accurate use of OBS output to calculate mass of SPM should only be used in certain specialised applications, such as a suspension of small sands with a low organic content in a turbulent medium. This caution applies equally to the use of other optical instruments, such as the transmissometer, to calculate mass of SPM from instrument output. 4.4. Flocculation and the spectral response of optical backscatter The spectral characteristics of backscatter coef®cients are still not well known because the sensors to measure spectral backscatter have only been available for the last few years. Absorption by pigmented cells has an effect on spectral backscatter, generally by decreasing the amount of light available for backscatter in the strongly absorbing wavelength bands, around 440 and 660. A comparison of spectral signatures of the backscatter coef®cient before and after the death of the phytoplankton cells shows no easily explainable pattern, indicating the relative unimportance of the chlorophyll. The only trend that is obvious is a general ¯attening of the spectral signature after ¯ocs were present compared to those of the primary particles. With the addition of the small, non-absorbing particles of the WBDM, a peak of the normalized particle backscatter coef®cient at the longer wavelengths in the PT only addition was ¯attened. The distinct grouping of period 1 and period 2 is not evident in the examination of the spectral signatures. The relationship between particle projectedarea concentration and the magnitude of backscatter occurs among all six wavelengths. It appears that
11
spectral signatures of the backscatter coef®cient do not relate uniquely to particle packaging, which means that spectral responses of backscatter offer a poor diagnostic for particle characteristics in natural marine suspensions. 5. Conclusions In conclusion, this study has provided signi®cant insight into the particle projected-area dependence of the OBS response. We have generated marine ¯ocs in the lab using an organic component (dying TP culture) and an inorganic/organic mixture (WBDM). Flocs grew rapidly, with an `equilibrium' population of ¯ocs developing shortly after the death of the PT cells and the degradation of the chlorophyll. The equilibrium ¯oc population was identi®ed as a consistent particle projected-area concentration, irrespective of changes in number and average size. The intensity of backscatter at 6 optical wavelengths was a function of the particle projected-area concentration, irrespective of changes in ¯oc size distributions and refractive indices of the SPM. This ®nding has signi®cant rami®cations in the reassessment of spurious conclusions of size-related OBS responses found in many previous studies. We propose that conclusions from many of the previous particle size/OBS response studies may have been simply due to the incorrect use of weight concentration as the independent variable. In natural marine suspensions, the mass of suspended particles and the particle projected-area concentration are not expected to co-vary because of the process of ¯occulation. The functional relationship between the magnitude of backscatter and suspended particulate matter in the ocean is largely dependent on the particle projected-area concentration, not the weight. Acknowledgements We are grateful to Paul MacPherson, who maintained the Floc-a-tron and conducted water sampling and analysis. We would also like to thank Gary Maillet, who grew the phytoplankton culture, and Mark Merriman, who coated and plumbed the Floc-a-tron. David Dana and Robert Maf®one of HOBI labs provided generous advice and guidance. The laboratory facilities are those of the Aquatron,
12
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