Among the biogeochemical cycles in the oceans carbon is one of the most significant. Oceans use carbon dioxide from the atmosphere as its source and can affect such important issues as global warming. Carbon is fixed through phytoplankton then distributed through grazers and passed further up the food chain by predators. To measure carbon content in the ocean water samples are taken and analyzed from various locations. However, this method is labor intensive and provides a limited geographic distribution. Current global methods of quantifying carbon involve estimation of phytoplankton populations through remote sensing satellites. Satellites measure the amount of light absorbed and reflected in different parts of the oceans. The measured absorption is correlated to the amount of chlorophyll present hence the amount of phytoplankton. Phytoplankton are the only organisms to contribute significantly to absorption of light. However, fluctuations in the carbon to chlorophyll ratio and the ratio of phytoplankton to other species can sometimes lead to an inaccurate picture of populations in the oceans. A new method proposed to estimate carbon utilizes scattering of light to estimate populations. One benefit of this method is that light is scattered by all organisms in the oceans, not just phytoplankton. Therefore, scattering gives a view of the entire populations. The total amount of scattering per group is dependent on several factors including the refractive index, size and shape of each species and the concentration of the group occurring in the oceans. Only phytoplankton and bacteria ranging in size from 0.1 to 100 µm are generally considered. Scattering rates for these groups of phytoplankton and bacteria are considered significant due to their reported high concentrations in the oceans. Zooplankton ranging in sizes from 100-1000 µm are not considered to cause significant scattering because they do not exist in high enough concentrations (Stramski and Keifer 1991). However, there are several reasons larger zooplankton could contribute significantly to scattering of light. They include:

• Hard exteriors made of chitin (refractive index =1.53) and calcium carbonate (1.66) are more refractive than phytoplankton (1.05-1.10).• Larger sizes of zooplankton would require smaller populations to contribute significantly to scattering.• Zooplankton populations fluctuate in response to environmental conditions leading to occurrences of high populations following an algal bloom (Nybakken 1996).

Attenuation of light by a natural assemblage of zooplankton.

By: Amy Overfelt and Dr. Jessica Nolan

Absorption

Attenuation

Methods

Preliminary Set up

Test sensitivity of the spectrophotometer and sample placement on phytoplankton with known values of absorption. Make modifications to spectrophotometer to allow measurements of attenuation.Purification of Sample

Performed plankton tow with 65 µm net to obtain sample of zooplankton. Siphoned zooplankton from sample into container to remove various debris. Transferred zooplankton by pipette and light through a series of beakers filled with 0.2 µm filtered sea water. Concentrated sample by siphoning out water.Fixed sample with formalin.

Obtaining Measurements

Utilized integrating sphere with spectrophotometer to measure absorption. Placed sample against beam origin in spectrophotometer to measure attenuation. Calculated scattering of sample by attenuation – absorption at 550 nm. Calculated scattering per individual and total scattering over a range of naturally occurring concentrations found in the oceans.

Discussion The methods used in this experiment allowed examination of attenuation of light due to scattering and absorption for a natural assemblage of zooplankton. Although zooplankton do not contribute significantly to absorption because they lack chlorophyll, their high refractive index can cause considerable scattering of light. Purification of the zooplankton sample ensured that any attenuation and scattering would be caused by zooplankton alone. Measurements of attenuation of light at 550 nm indicates that most of the attenuation is due to scattering of light. At a high concentration zooplankton ranging in size from 100-1000 contribute as much to total scattering as prochlorophytes and ultrananoplankton. This is significant because zooplankton generally reach high concentrations following the crash of an algal bloom. During this time use of absorption for carbon estimations would result in low values which would not represent carbon present from high concentrations of zooplankton. Therefore, use of scattering for carbon estimation would include zooplankton which do contribute significantly to total scattering in the oceans. Future research using scattering measurements and zooplankton could include examination of other types of zooplankton as the sample used was comprised of mainly copepods, larval clams and larval gastropods. Additionally, future research could include measurements taken in the field with specialized equipment.

Results

Literature CitedEsaias, W. and Thomas, D. 2005 September 26. SeaWiFS Biosphere Globes. Available from: http://oceancolor.gsfc.nasa.gov/cgi/biosphere_globes.pl Accessed 2005 October 30.Nybakken, J. 1997. Marine biology an ecological approach. 4th ed. Addison-Wesley Longman, Inc., Reading, MA.Stramski, D. and Kiefer, D. 1991. Light scattering by mircroorganisms in the open ocean. Prog. Oceanography 28: 343-383.

Acknowledgements: We would like to thank Dr.Tiffany Moisan and Dr. William Steele for use of equipment and NASA/NOAA for funding use of the research vessel.

Figure 1: Attenuation, scattering, and absorption by the zooplankton sample between the wavelengths of 450-700 nm. A vertical line indicates values used at 550 nm.

Figure 2: Total scattering by zooplankton sample and other organisms previously analyzed and considered significant contributors to scattering (Stramski and Keifer 1991).

Table 1. Concentrations of organisms and scattering (b) per individual resulting in total scattering at 550 nm (Stramski and Keifer 1991).

Introduction

The purpose of this study is threefold: 1. Determine the scattering per individual of a sample of zooplankton in the 100-1000 µm size range. 2. Calculate the total scattering for a range of concentrations of zooplankton. 3. Compare the scattering of zooplankton to scattering of phytoplankton and bacteria that significantly contribute to the total scattering of light. It is hypothesized that larger zooplankton ranging from 100-1000 µm will contribute significantly to the total scattering of the oceans.

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Organisms b per individual Concentration Total b

_________________________(m2/ind)___________(m-3)________(m-1)____

Viruses 2.07 10-18 7.0 1012 1.45 10-5

Heterotrophic

Bacteria 3.84 10-14 7.0 1011 2.69 10-2

Prochlorophytes 1.08 10-13 5.0 1010 5.40 10-3

Cyanobacteria 8.06 10-13 5.0 1010 4.03 10-2

Ultrananoplankton (2-8 µm) 1.60 10-11 5.0 108 8.0 10-3

Larger nanoplankton

(8-20 µm) 1.73 10-10 7.50 106 1.3 10-3

Microplankton 1.20 10-9 1.0 106 1.2 10-3

Natural Assemblage 1.0 105 7.76 10-3

(100-1000 µm) 7.76 10-8 1.0 104 7.76 10-4

1.0 103 7.76 10-5

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SEM of attenuation at 550 nm is 7.03 10-9.

Zooplankt

on

Viruse

s

Heter

trophic

Bac

teria

Proch

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phytes

Cyanobac

teria

Ultran

anopla

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Larger

nan

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Mic

ropla

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0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045Figure 2

To

tal

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-1)

450 500 550 600 650 7000

3e-008

5e-008

8e-008

1e-007

1e-007

2e-007

2e-007

AttenuationAbsorptionScattering

Figure 1

Wavelength

Sca

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(m

2in

d-1

)