Bio-optical observations of the North Atlantic Spring BloomBio-optical observations of the North Atlantic Spring Bloom Toby K. Westberry1, Giorgio Dall’Olmo1, Mike Behrenfeld1, Emmanuel Boss2

1Department of Botany & Plant Pathology, Oregon State University, USA, 2School of Marine Sciences, University of Maine, USA

1. Introduction 4. Data

3. Methods

2. Context

5. Validation of Measurements 6. Conclusions and Significance

What: Presented here are data collected aboard the R/V/ Knorr as part of the North Atlantic Bloom (NAB) Experiment

Where: A 10000 km2 area of the North Atlantic nominally centered on 61ºN, 26ºW

When: May 2-22 2008 (autonomous measurement platforms were deployed prior to and recovered after this time)

How: Continuous, underway measurements of spectral particulate beam attenuation (cp), absorption (ap), and backscattering (bbp) were measured simultaneously. Discrete biogeochemical measurements of phytoplankton pigment concentration (HPLC), particulate organic matter concentration (POC/PON), and bio-volume estimated phytoplankton carbon (Cphyto) were also made (but not presented here).

Why: Examine bio-optical relationships with one another and with relevant biogeo- chemical quantities during one of the largest mass “greenings” in the ocean. Provide validation for satellite observation at basin scale.

Specific questions being addressed:• Are the observed bio-optical relationships consistent with global models and data collected in other environments?

• Are the two scattering indices (cp and bbp) related to one-another?

• How are cp and bbp related to POC and Cphyto?

Chl-HPLC (mg m-3)

Ch

l-AC

s (mg

m-3)C

hl-

AC

s (m

g m

-3)

Chl-Fluor (mg m-3)

y=0.77 (±0.03) – 0.09 (±0.05)y=0.94 (±0.07) – 0.01 (±0.08)

% b

ias

Wavelength (nm)

- To provide some spatial and temporal context for our sampling:

1. SeaWiFS L3 Monthly composite Chl a for May 20082. Annual climatologies of Chl a, PAR, MLD, and bbp for SeaWiFS satellite era (1997-2008)

* Climatologies are calculated for a 2ºx2º box surrounding the NAB site Chl a, PAR, and bbp are derived from SeaWiFS data, MLD from the FNMOC model

* % bias = CTD – flowThru) / CTD * 100

* Regression fits (± 95% confidence) are the result of >1000 non-parametric bootstrap samples (Note difference in slopes)

A. Median bbp are similar to previously published studies (0.01±0.002)

B. Estimated n (real part of index of refraction) are in correct range (1.09±0.01)

C. Direct comparison of ap, cp, bbp , and particle size distribution (PSD) in flowthru system and surface CTD water

bb

p 5

26 (

m-1)

Chl a (mg m-3)

bb

p 5

26 (

m-1)

cp 526 (m-1)

Chl a (mg m-3)

c p 5

26 (

m-1)

Np

ts

2D histograms (N>30000)

cp_526 (m-1)bbp_526 (m-1)

PO

C (M

)

PO

C (M

)

* Loisel and Stramski relationships were employed by assuming -1 spectral cp slope** Regression fits (± 95% confidence) are the result of >1000 non-parametric bootstrap samples

NAB data “best fit” Loisel et al. (2001) Stramski et al. (1999) Stramski et al. (2008) Gardner et al. (2006)

“best fit” Loisel & Morel (1998) L & M (1998) w/ N.Atl Huot et al. (2008)

Data reported here are measurements conducted during the NAB 2008 cruise on the R/V Knorr using the ship’s clean flow-through seawater supply.

A. OPTICAL MEASUREMENTS: C-star beam attenuation (cp) at 526 and 660 nm

ECO-BB3 (bbp) at 470 and 526 nm The instrument was installed in a custom-made chamber that serves as a light trap while measuring flow-through backscattering. The chamber walls themselves do not contribute a measurable signal (Slade et al. 2008; Dall’Olmo et al. 2008).

FRR, variable fluorescence (not shown here)

Coulter Counter particle size distribution (PSD)

ACs /AC9 (ap, bp, cp) every ~4nm from 400-750nm

To account for combined effects of dissolved signals, instrumental drifts, and biofouling, an automatic valve directed bulk seawater through a 0.2-m nylon cartridge filter for ten minutes every hour. cp and ap were computed by subtracting the interpolated 0.2-m filtered values from the bulk signal . This procedure allows determination of calibration-independent particulate beam-attenuation and absorption coefficients with uncertainties determined mostly by instrument precision (Boss et al., 2007; Slade et al., 2008; Dall’Olmo et al., 2008).

Periodic comparisons of flow through measurements with bulk seawater taken from the surface CTD were also conducted (see results in Section 5).

B. DISCRETE BIOGEOCHEMICAL MEASUREMENTS HPLC pigments

Particulate organic carbon (POC) Measurements made primarily following JGOFS protocols, but using multiple volumes

(0.5L,1L, 2L) to determine blank value from regression (c.f. Menzel, 1966; Moran et al., 1999; Turnewitsch et al., 2007)

D. Matchups of discrete and optics-based estimates of Chl a

~

A. Time Series B. Bio-optical Relationships

C. Matchups with POC

Chl-ACs is calculated by absorption baseline method (Davis et al., 1997; Boss et al., 2007) = [ap(676) (39/65ap(650) + 26/65ap(715))]/0.014

• Time series of underway optics show high frequency variability that spans the entire dynamic range observed during cruise

• High scattering (cp) per Chl a is observed, but is consistent with previous N. Atlantic data published

What causes this other than coccolithophorids?

• bbp per Chl a is higher than predicted based on relationships extrapolated from more oligotrophic environments

These are the first measurements made of this type in significant numbers.

• bbp and cp are highly correlated throughout entire record AND degree of relationship is consistent with previously collected data

This suggests that the two indices are sensitive to the same pool of particles OR that different pools of particles strongly covary (and the resulting PSD is conserved)

• Optics data collected with underway system are NOT significantly different than that measured outside of ship (+- 10%)

This is surprising because of differences observed in particle size spectra.

• Both HPLC and ap indices show changes in dominant phytoplankton groups

E. Modeling using remote sensing reflectance and optical closure (not done yet)

ESD (m)

Par

ticle

s m

l-1

m-1

% b

ias

May 2008 Chl (mg m-3)Annual Climatology (1997-2008)

Chl

(m

g m

-3)

0

45

PA

R (E

in m-2 d

-1)

450

0

MLD

(m)

0.007

0

bbp (m

-1)

~NAB site

Reykjavik

a p(4

90)/

a p(6

76)

Ch

l-A

Cs

(mg

m-3)

Data collected in Equatorial Pacific for comparison(**NOT** NAB data)

Month Day

Fucoxanthin/Chl a

19’ Hex-Fuc /Chl a p

igm

en

t ra

tio

(m

g m

-3)

Dia

tom

s ?

Prymnesiophytes ?

b bp(5

26)

(m-1) c

p (526

) (m-1)

0.70

0.56

0.42

0.28

0.14

- Time periods highlighted in yellow demonstrate the tight coupling between all optical indices (absorption and scattering)

- Peaks in bbp, cp, and Chl a correspond directly to maxima in [Fucoxanthin]/[Chl a], the result of high diatom biomass

- Variability in the ratio of ap(490)/ap(676) can be attributed to diatoms during the first half of the cruise, while the steady rise during the second half of the cruise can be attributed to other phytoplankton groups (Prymnesiophytes?)

Acknowledgments: The authors wish to thank to the crew of the R/V Knorr and all science personnel who participated in the 2008 NAB Experiment. Funding was provided by NASA grant NNX08AK70G. For further information please contact Toby Westberry (toby.westberry@science.oregonstate.edu). This poster is dedicated to the loving memory of Fredrick Charles Crebassa.