Spacial Variability in Plankton Size Structure and Community Composition Along Biogeochemical Gradients in the Pacific Ocean
UNIVERSITY OF HAWAII LIBRARY
SPACIAL VARIABILITY IN PLANKTON SIZE STRUCTURE AND COMMUNITY COMPOSITION ALONG BIOGEOCHEMICAL GRADIENTS IN THE PACIFIC OCEAN
A TIIESIS SUBMIITED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'! IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
IN
OCEANOGRAPHY
AUGUST 2007
By Tara M. Clemente
Thesis Committee:
David M. Karl, Chairperson Matthew J. Church Karin M. Bjorkman We certify that we have read this thesis and that, in our opinion, it is satisfactory in scope and quality as a thesis for the degree of Master of Science in Oceanography. ACKNOWLEDGEMENTS
I would like to thank my committee members for their time, encouragement, support and advice. My primary advisor, Dave Karl, who has challenged me to think critically and guided me throughout this study; Matt Church, for always leaving his door open and for giving me insight throughout this project; and Karin BjOrkman, who has been instrumental throughout this entire project especially in both the field and lab. I would like to thank the members of the HOT team for all their support and for their friendships.
I'd like to thank the officers and crew of the RfV Kilo Moana for providing a safe and comfortable work place. Thanks to all the BEACH-BASH participants for their hard work and assistance with sample collection and data analysis, in particular Chief Scientist
Karin BjOrkman, Eric Grabowski and Dan Sadler. For help in analysis I wish to acknowledge Bob Bidigare and Stephanie Christensen for HPLC, Karin BjOrkman, Susan
Curless and Claire Mahaffey for dissolved nutrients and Karen Selph for flow cytometry.
I'd like to thank my friends who stood by me throughout these last couple of years and gave me countless hours of advice, support and ice cream. I'd especially like to thank my family, my mom and dad for always supporting my goals no matter how strange they may have been at times; and for my sisters Jamie and Tracey, who were always available for those late night phone calls. I wish to give a special thanks to David Nichols whose support, friendship, and ability to make me laugh throughout these many years has been incredible. Lastly, I acknowledge the National Science Foundation, the Gordon and
Betty Moore Foundation, and the Hawaii Ocean Time-series for providing financial support.
iii ABSTRACT
The size structure of upper ocean plankton assemblages appears to play an important role in determining the efficiency of the biological carbon pump and the resulting magnitude of biologically-mediated carbon export to the deep sea. To evaluate spatial variability in the size structure and community composition of plankton assemblages in the upper ocean measurements of size-fractionated chlorophyll a (Chi a), phycoerythrin and adenosine 5' -triphosphate (A1P) concentrations along with picoplankton abundances and taxonomic pigment biomarkers were assessed during the BEACH-BASH transect cruise from American Samoa to Honolulu, Hawaii in March 2005. Sampling for this study spanned four different biogeochemical provinces including the oligotrophic South Pacific
Subtropical Gyre (SPSG), the relatively nutrient-enriched equatorial upwelling provinces, the Pacific Equatorial Divergence (PEQD) and the North Pacific Equatorial
Countercurrent (PNEC), and the oligotrophic North Pacific Tropical Gyre (NPTG).
Overall, nutrient concentrations were highest in the PEQD province, coinciding with the strong westerly South Equatorial Current (SEC) and the region of equatorial upwelling, and lowest in the SPSG and NPTG. Chi a and phycoerythrin concentrations throughout the transect were overwhelmingly dominated by picoplankton (0.2-2 f.UD.), accounting for
45 - 60 % of the total Chi a and 80 - 95 % of the total phycoerythrin throughout the transect. Flow cytometry measurements indicate that the picoplankton community composition was dominated by cyanobacteria of two genera; Prochlorococcus spp. and
Synechococcus spp. and these groups accounted for most of the Chi a biomass throughout the transect. Picoeukaryotes also contributed to picoplankton abundances; however
iv despite the likelihood of many species being present they remain largely unidentified.
Total microbial biomass estimates via particulate ATP (P-ATP) measurements showed high variability with the occasional occurrence of larger heterotrophic organisms
(>20 J.Lffi and 2-10 J.Lffi) which are not accounted for in Chi a biomass estimates.
However, their distribution is sure to play an important role in regulating size structure and food web dynamics. Measurements of taxonomic pigment markers by HPLC confirmed the overwhelming dominance of prochlorophytes and cyanobacteria throughout the transect, however a transition in community composition in the PEQD province was observed. This transition was attributed to the increase in taxonomic pigment markers of chromophyte microalgae (i.e. prymnesiophytes, pelagophytes and diatoms).
v TABLE OF CONTENTS
Section
Acknowledgements ...... ii
Abstract ...... iv
List of Tables ...... viii
List of Figures ...... ix
Chapter I. Introduction ...... 1
Chapter 2. Spatial variability in plankton size structure and community composition
along biogeochemical gradients in the Pacific Ocean ...... •...... 5
2.1 Introduction ...... 5
2.1.1 Plankton size structure and community composition ...... 5
2.1.2 Biogeochemical provinces ...... 8
2.1.3 Equatorial Pacific Ocean ...... 11
2.1.4 Size-fractionation ...... 16
2.1.5 Objectives of this study ...... 17
2.2 Methods ...... 18
2.2.1 BEACH-BASH data ...... 18
2.2.2 Sample collection and analysis ...... 18
2.2.2.1 Nutrient analysis ...... 18
2.2.2.2 Chlorophyll a ...... 21
2.2.2.3 Phycoerythrin ...... •...... 22
2.2.2.4 Adenosine 5'-triphosphate (ATP) ...... 23
2.2.2.5 Flow cytometry ...... 24
vi 2.2.2.6 HPLC pigments ...... 25
2.3 Results ...... 28
2.3.1 Habitat characteristics ...... 28
2.3.1.1 Biogeochemical provinces ...... 28
2.3.1.2 Latitudinal distribution of temperature, salinity
and Chl a ...... 31
2.3.2 Nutrient distributions ...... 33
2.3.2.1 Latitudinal distribution of dissolved inorganic nutrients.. 33
2.3.2.2 Dissolved inorganic nutrient ratios ...... •...... 35
2.3.3 Plankton size structure ••••••••••••••••••.•.•••••••••••••••••••••.•.•.••.• 37
2.3.3.1 Chlorophyll a ...... 37
2.3.3.2 Phycoerythrin ...•...... •..42
2.3.3.3 Adenosine 5'-triphosphate (ATP) ...... 44
2.3.4 Picoplankton community structure and taxonomic distribution ...... 47
2.3.4.1 Flow cytometry ...... •...... 47
2.3.4.2 HPLC pigments ...... 51
2.4 Discussion ...... 59
2.4.1 Nutrient dynamics ...... 59
2.4.2 Phytoplankton size structure ...... 62
2.4.3 Community structure and taxonomic composition ...... 67
2.4.4 Implications for the biological pump ...... •...... 75
Chapter 3. Conclusions ...... 77
References ...... 79
vii LIST OF TABLES
1. Detection limits, precision estimates and observed ranges .•...... 20
2. Abbreviations and taxonomic affinities of photosynthetic pigments ...... •...... 26
viii LIST OF FIGURES
Figures
1. Conceptual view of major controls on recycled vs. export production ...... • 6
2. Classification of planktonic organisms based on size ...... 7
3. BEACH-BASH transect cruise track ...... 9
4. Biogeochemical provinces sampled during BEACH-BASH ...... 10
5. ADCP data showing current velocities ...... 12
6. Sea surface temperature, wind and anomalies from TAD arrays ...... 15
7. Depth profiles of temperature, chlorophyll a and soluble reactive phosphorus in each of the biogeochemical provinces ...... 29
8. Latitudinal distribution of sea surface temperature, chlorophyll a and salinity ...... 32
9. Latitudinal distribution of surface nutrients nitrate + nitrite, soluble reactive phosphorus, and silicic acid ...... 34
10. Latitudinal distribution of surface nutrient ratios ...... 36
11. Latitudinal distribution of surface size-fractionated chlorophyll a ...... 38
12. Chlorophyll a regression analysis of GFIF vs. summed PC filters ...... 40
13. Correlations between chlorophyll a size fractions and nutrients -...... 41
14. Latitudinal distribution of surface size-fractionated phycoerythrin ...... 43
15. Latitudinal distribution of surface total particulate A TP (P-A TP) ...... 45
16. Latitudinal distribution of surface size-fractionated P-ATP ...... 46
17. Latitudinal distribution of picoplankton abundances ...... 48
18. Correlations between surface abundance of plankton groups and nutrients ...... 49
19. Latitudinal distribution ofHPLC TChl a, TChl band TChl c ...... 53
ix Figures
20. Chlorophyll a regression analysis ofTD-700 vs. HPLC ...... 54
21. Latitudinal distribution ofTChl a constituents MVChl a, DVChl a and MVChld a ...... 55
22. CyanoDP and ChromoDP as a proportion of total diagnostic pigments ...... 57
23. Temporal distribution of Prochlorococcus spp. abundance at Station ALOHA between 1991-2005 and the Southern Oscillation Index (SOl) ...... 70
x CHAPTER 1. INTRODUcnON
The production of organic matter in the sea is sustained by the continuous supply of the bioessential elements including carbon (C), nitrogen (N), phosphorus (P), and silica (Si) as well as minor nutrients such as iron (Fe) and zinc (Zn). Redfield et al. (1963) summarized the average bulk elemental stoichiometry of plankton particulate organic matter as a ratio ofl06C: 16N: IP; however, the chemical composition of living organisms varies as a function of growth rate, energy availability, ambient nutrient concentrations and nutrient concentration ratios (Sakshaug and Holm-Hansen, 1977;
Rhee, 1978; Laws and Bannister, 1980; Tett et af., 1985). In large areas of the world's oceans, low concentrations ofbioavailable nutrients control ecosystem production and plankton biomass. Liebig's Law of the Minimum states that ecosystem productivity is limited by the nutrient which is exhausted first relative to its demand (Liebig, 1840). The supply of nutrients to the productive surface layers of the open ocean and marine microorganism inhabitants can occur by large-scale processes such as eddy diffusion, upwelling, atmospheric deposition, land runoff or by loca1ized process such as the flux of remineralized nutrients within the microbial food web (Azam et al., 1983; Dugdale and
Goering, 1967).
Microorganisms in pelagic ecosystems are generally classified on the basis of size, nutritional and physiological characteristics or phylogeny (Karl, 1999). The size of a cell can have many consequences on the fitness of an organism in a habitat and on its ability to use resources like light and nutrients toward a competitive advantage (Raven, 1986).
1 Sheldon el a1. (1972) were the first to supply an image of size distributions of plankton across a wide expanse of the world ocean. Sheldon put forth the Linear Biomass
Hypothesis, which states that the amount of biomass in equally-sized logarithmic size classes remains constant through the food web from bacteria to whales (Sheldon et al.,
1972). This hypothesis, promoted before the microbial loop was envisioned, provided a simple image of community structure, and provided a means for predicting fish stocks given the biomass of plankton (Sheldon et a!., 1977). Since the Sheldon et al. (1972) study, which used a coulter counter to investigate size structure, improved methods such as flow cytometry have been developed which has the ability to characterize small cells rapidly to generate size structure data (platt and Denman, 1977; Blanco et a1., 1994;
Vidondo et al., 1997). This enabled the discovery of two genera of picoplankton
Prochlorococcus spp. and Synechococcus spp. which are now known to contribute significantly to overall primary production and autotrophic biomass in oligotrophic marine environments (Chisholm, 1992). Prochlorococcus spp. was discovered about two decades ago in the Sargasso Sea by the use of on-board flow cytometry (Chisholm et al.,
1988; 1992). It was 1ater reported in the equatorial Pacific (Chavez et a1., 1991) and is now considered to be of global significance (partensky et al., 1999). Synechococcus spp. was first described in 1979 (Johnson and Sieburth, 1979; Waterbury et a1., 1979) and is now known to be an important component of the picoplankton community in temperate to tropical oceans.
Plankton cell sizes are typically characterized as picoplank:ton (0.2-2 1IJIl); nanoplankton
(2-20 1IJIl); microplankton (20-200 1IJIl); and mesoplankton (0.2-2 mm; Sieburth et al.,
2 1978}. Large cells, such as diatoms are observed to dominate on a biomass basis in high nutrient areas while small cells such as cyanobacteria tend to dominate oligotrophic areas of the world's oceans (Li et al., 1983; Hagstrllm et al., 1988). The size structure and biomass of upper ocean plankton assemblages can help provide an understanding of trophic structure, energy flow and export production within an ecosystem (Karl and
Dobbs, 1998). Legendre and Le Fevre (1989) suggest that export production is maximized iflarge phototrophs grow, aggregate and then sink and that export is minimized when phototrophic picoplankton fuel a complex microbial food web. Based on this conceptual framework, it appears that the size structure of plankton assemblages in the open ocean can play an important role in determining the efficiency of the biological carbon pump and the resulting magnitude of biologically-mediated carbon export to the deep sea.
There have been a large number of investigations on the spatial distribution and temporal variability of phytoplankton around the world's oceans. In the Atlantic Ocean the
Atlantic Meridional Transect (AMT) research program conducted several oceanographic cruises along a transect from 50' S to 50' N encompassing various biogeochemical provinces. Several of these studies focused on phytoplankton size structure and productivity (Marafl.6n et al., 2000; 2001; 2003; Fernandez et al., 2003; perez et al.,
2005; San Martin et al., 2006) as well as microbial abundance and taxonomic composition (Zubkov et al., 1998; 2000; Gibb et al., 2000; Barlow et al., 2002; 2004;
Poulton et al., 2006). In the North Atlantic, Li and Harrison (2001) partitioned bacteria and picophytoplankton into ecological provinces as specified by Longhurst (1998), and
3 the relationships of nutrient gradients to microbial community structure were investigated in Cavender-Bares et al. (2001).
In the Pacific Ocean no one study has encompassed as wide of a latitudinal range as that sampled in the Atlantic Ocean during the AMT program. However, several cruises have been conducted by the Joint Global Ocean Flux Study (JGOFS) in the equatorial Pacific
Ocean. The objectives of the JOOFS studies were to determine the fluxes of carbon and related elements and processes controlling those fluxes between the euphotic zone, the atmosphere, and the deep ocean. The equatorial JGOFS cruises include EqPac and
Surtropac (1992), FLUPAC and OLIPAC (1994), and EBENE (1996) and together encompass a latitudinal range between 20° S - 12° N. These JGOFS cruises along with the Iron Ex II (1995) cruise, helped capture microbial dynamics at various temporal and spatial scales of variability (Landry and Kirchman, 2002). In the North Pacific Ocean, the consistently low nutrient subtropical waters of the long term sampling Hawaii Ocean
Time-series (HOn site, Station ALOHA, have previously been used as a frame of reference for Variability in equatorial regions. To my knowledge the study presented in this thesis is the first in the equatorial Pacific to investigate the spatial dynamics of phytoplankton along biogeochemical gradients over a wide latitudinal range that includes the HOT site Station ALOHA.
4 CHAPTER 2. SPATIAL VARIABLITIY IN PLANKTON SIZE STRUCTURE
AND COMMUNITY COMPOSmON ALONG BIOGEOCHEMCIAL
GRADIENTS IN THE PACIFIC OCEAN
2.1 INTRODUCTION
2.1.1 Plankton size structure and community composition
The size structure and community composition of phytoplankton assemblages in the open ocean are important biological factors that regulate the functioning of pelagic food-webs and the export of organic carbon to the deep ocean (Azam et aZ., 1983; Tremblay and
Legendre, 1994). Large cells, such as diatoms are observed to dominate on a biomass basis in high nutrient areas, have rapid sinking rates and are believed to be large contributors to carbon export (Michaels and Silver, 1988), while small cells such as cyanobacteria tend to dominate oligotrophic areas of the world's oceans (Li et aZ., 1983;
Hagstr6m et aZ., 1988) and are believed to contribute relatively little to carbon export because of their small sizes, slow sinking rates and rapid utilization in the microbial loop
(Legendre and Le Fevre, 1989; Michaels and Silver, 1998; Figure 1). Phytoplankton may be classified based on cell size and include small picoplankton (0.2-2 f.Illl); medium-sized nanoplankton (2-20 f.Illl); large microplankton (20-200 f.Illl); and even larger mesoplankton (0.2-2 mm; Sieburth et al., 1978; Figure 2). This study took place in the
Pacific Ocean and focuses on three of the four previously stated size classes; picoplankton (e.g. prochlorophytes, Synechococcus spp., small eukaryotes), nanoplankton
(e.g. prymnesiophytes, pelagophytes, small diatoms and dinoflagellates) and microplankton (e.g. diatoms and dinoflagellates). It also further classifies nanoplankton 5 IDIA TOMS I IPROCHLOROCOCCUS I
MICROBIAL LOOP
IEXPORT FLUX I
Adapted fromiLesn1ruud Le.FiMe (1989)
Figure I. Conceptual view of major controls on recycled vs. export production in the sea. Image courtesy of David M. Karl.
6 AUTOTROPHS HETEROTROPHS Plankton Size categories (pm) 2000 0en w :E
200
-&J ~ ~ @~ f1 ~j1 ~ ~ 20 e wv- ~~ CI"V- ~ ,. ~~ ~ ~ ~ oJ'- --~-- 2
• • ~ • ~ • • .. ~ • • . . • • ~ • • • . . .. • .. • ... • • .. . . • • 0.2
Figure 2. Classification of planktonic organisms by size based on Sieburth el al. (1978) Limnology and Oceanography 23 : 1256-1 263 taken from Karl ( 1999). This study focused on three oftbe four size classes shown above; picoplankton (0 .2-2 flm) (e.g. prochlorophytes, Synechococcus spp., and small eukaryotes), nanoplankton (2-20 flm) (e.g. prymnesiophytes, pelagophytes, small diatoms and dinoflagellates) and microplankton (20-200 flm) (e .g. diatoms and dinoflagellates) and it further classifies nanoplankton into two size classes (2-10 flm and 10-20 flm).
7 into two size classes (2-10 pm and 10-20 pm). Nutrients and light aVailability along with predation and physical processes such as upwelling are factors that can influence the composition and dynamics of the phytop1ankton community. These factors all vary in time and space and can lead to variability in phytoplankton community composition and growth rates. Typically, the upper ocean is considered to be nutrient limited and well illuminated and the deep ocean is considered to be nutrient rich and light limited.
2.1.2 Biogeochemical provinces
To compare data over extensive spatial scales, stations sampled during the BEACH
BASH (Biogeochemical and Ecological Analysis of Complex Habitats-Between
American Samoa and Hawaii) transect cruise (Figure 3) were grouped according to the biogeochemical provinces proposed by Longhurst (1998), which are based on global scale productivity inferred from remotely sensed ocean color. According to this approach, four different biogeochemical provinces were sampled (Figure 4). These included the South Pacific Subtropical Gyre (SPSG) province, Pacific Equatorial
Divergence (PEQD) province, North Pacific Equatorial Countercurrent (PNEC) province, and the North Pacific Tropical Gyre (NPTG) province. The SPSG province as defined by
Longhurst (1998) comprises the central and southern part of the subtropical gyre of the
South Pacific Ocean extending from 35° S to 5° S. It is bounded by the chlorophyll enhancement of the Subtropical Convergence Zone (SSTC) in the south and the southern edge of chlorophyll enhancement caused by equatorial divergence in the north. The
PEQD province corresponds with the zone of nitrate-replete water of the tropical Pacific and extends from 5° S across the equator to 5° N (Longhurst, 1998). The surface
8 American Samoa to Hawaii 0.3
0.25
::::: ~ 0.2 >- QI ~ "C Q. ::l ...0 ~ 0 -ns ~ ...J 0.15 () QI U ~ en::l ns 0.1 enQI
0.5
o -180 -175 -170 -165 -160 -155 -150 Longitude
Figure 3. MODIS satellite derived sea surface chlorophyll concentrations along the BEACH-BASH transect cruise track (April 29 - May 9, 2005).
9 PEQD
SPSG
,r"'l Longhurst 1998
Provinces Acron ym Location Stations sampled
South Pacific Subtropical Gyre SPSG 35°S - 50S N=5 - Pacific Equatorial Divergence PEQD 50S - 5°N N = 12 5°N - 100N N =4 North Pacific Equatorial Countercurrent PNEC - North Pacific Tropical Gvre NPTG 100N - 32°N N=6
Figure 4. Biogeochemical provinces sampled during the BEACH- BASH transect (shown in black) as defined by Longhurst (1998). SPSG (B lue), PEQD (White), PNEC (Yellow) and NPTG (Grey) 10 circulation in the PEQD is dominated by the flow of the South Equatorial Current (SEC) which moves westwards across the ocean (Figure 5). Within this circulation occurs a strong linear divergence and upwelling along the equator (Longhurst, 1998). This divergence is forced by the change in sign of the Coriolis force at the equator polewards in both hemispheres for a westward flow. The PNEC province lies between the northern and southern Doldrum salinity fronts at 5· N and 10· N (Longhurst, 1998). In this province the curvature of wind stress not only forces the flow of the North Equatorial
Countercurrent (NECC). but is also thought to enhance algal growth and therefore
Chlorophyll a (Chl a) concentrations (Longhurst, 1993; Figure 5). The NPTG province is very large and lies between the Subtropical Convergence at 32· N and the northern
Doldrum Front (Roden, 1975) at 10· N (Longhurst, 1998).
2.1.3 EqUllloriol Pacific Ocean
Approximately one-half of global primary production is supported by the photosynthetic activities of microscopic plankton in the world's oceans (Falkowski, 1994). Oceanic provinces far removed from land account for most of the primary production. The
Pacific Ocean is the largest water body in the world and can be divided into two parts according to the nutrient availability: the eutrophic region and the oligotrophic region. In the Pacific Ocean the eutrophic regions constitute the central equatorial upwelling zone and coastal areas due to the input of nutrients from land. However, the majority of the open ocean is made up of oligotrophic waters. It is therefore of interest to investigate the spatial dynamics of plankton size structure along biogeochemical gradients from the
11 Kilo Moana L 25 III to 35 m
CC
SEC
"'..
20 0 S~ ______~
1800 W 1600 W .r''''''''''ri ... '''''''''1, o 100
Figure 5. ADCP data showing the current velocities between the 25 m to 35 m depth layer during the BEACH-BASH transect cruise. South Equatorial Current (SEC), North Equatorial Countercurrent (NECC) and North Equatorial Current (NEC). Biogeochemical provinces are represented by colors; SPSG (Blue), PEQD (White), PNEC (Yellow) and NPTG (Grey). Image courtesy of Eric Firing and Jules Hurnmon.
12 South Pacific, across the equator to the North Pacific to determine the factors controlling their standing biomass.
The equatorial Pacific is extremely dynamic; it is characterized by episodic upwelling events, elevated chlorophyll concentrations and carbon fixation rates. The north and southeast tradewinds set up a shallow thermocline in the eastern and central Pacific and drive equatorial upwelling. The equatorial Pacific plays an important role in the global carbon cycle. Upwelling of cool water from below the shallow thermocline results in a large supply of macronutrients including carbon dioxide (C02) to the surface (Chavez and Barber, 1987; Feeley et al., 1987). It supplies annually about 1 gigaton (Ot) of carbon dioxide (C~) to the atmosphere and is the largest natural source of C02 to the atmosphere (Tans et al., 1990; Feeley et aI., 1987). At the same time, it also contributes a significant fraction of the ocean's new production (0.8 - 1.9 Ot C year -I; Chavez and
Barber, 1987). Even though concentrations of macronutrients are well above those that saturate phytoplankton uptake (Barber and Chavez, 1986), phytoplankton in the equatorial Pacific rarely if ever bloom and the depletion time of nitrate is long (Dugdale et al., 1992; Fieldler et al., 1991; McCarthy et al., 1996). The equatorial Pacific is also considered a region of high nutrient low chlorophyll (HNLC; Minas et al., 1986; Cullen,
1991). Several hypotheses have been examined to explain HNLC conditions in the equatorial Pacific. These included inhibition of phytoplankton growth by trace metals such as iron (Barber and Ryther, 1969; Martin, 1990), physical processes (Thomas,
1972), grazing control (Walsh, 1976; Pella et aI., 1990), regulation by silicate (Dugdale et al., 1995), and lack of bloom forming diatoms (Chavez, 1989).
13 The equatorial Pacific is also an area of dramatic variability in the physical environment.
Periodically the east-west slope in the equatorial Pacific thennocline is affected by El
Nifio or La Nifia (philander, 1990). During El Nifio the thennocline is depressed in the eastern Pacific and the supply of macronutrients to the surface waters is lower than normal, warm sea surface temperatures extend throughout the equatorial region due to relaxed tradewinds and reduced upwelling and the anomaly shows a large change in sea surface temperature that depicts warming in the eastern equatorial Pacific (Figure 6a).
During La Nifia the thennocline is even shallower than normal and the supply of macronutrients to surface waters is enhanced, cool sea surface temperatures extend throughout most of the basin due to increased winds and upwelling and the anomaly shows a large change in sea surface temperature that depicts cooling in the eastern equatorial Pacific (Figure 6b). Besides being influenced by the El Nifio-Southern
Oscillation (ENSO) cycle, the equatorial Pacific waters experience physical forcings that include Kelvin waves, Tropical instability waves (TIWs) and wind induced upwelling.
These physical phenomena can affect organism concentrations and their physiological characteristics on considerably different time and space scales making it important to employ appropriate sampling strategies (Dickey, 1991).
The equatorial Pacific is frequently divided into three different biogeochemical provinces
(Longhurst, 1998). Two of which I have mentioned previously, the PNEC and PEQD provinces which are located in the eastern and central equatorial Pacific and a third known as the Western Pacific Warm Pool (WARM) province which is located in the western Pacific. In the eastern and central equatorial Pacific upwelled waters are cold
14 TAO/TRITON Monthly SST ('C) and Winds (m s-l) (a) 1400E 160"E 180' 160'W 120'17 100' W lOON 31 2. S'N 27 0' 25 23 5'S 21 10'S 1I.
December 1997 An omalies
tolay JO 2007
TAO/TRITON MonthLy SST ('C) and Winds (m .-1)
(b) 1400E 160'E 120'17 100'W 10'N .,-~~-~ 31 S' N 2. 27 D· 25 23 5'S 21 10'S '-'='JIoo,--+---'--~-=F~ 119 ~ 10. m e-1
1~ , 1;~~ C!l~=-::==~~~~~~g;~~~~~~C;:~;;~1 1 ~ December 1998 Anomalies
TAO/TRITON Mon th Ly SST ('C) and Winds (m .-' )
(c) 140'E 100'W lOON 31 2. S' N 27 0' 25 23 5'S 2' 10 0 S ,. May 2005 Means
TN:! ProJ~t OIrlc./ PIotEL/NCWI tolay ZIt 2007
Figure 6. Mean monthly sea surface temperatures, winds and anomalies between 10' S and 10' N from the Tropical Atmosphere Ocean (TAO) arrays located in the equatorial Pacific. (a) EI ino, December 1997, (b) La ina, Dec 1998 and (c) Normal, May 2005.
15 and rich in nutrients leading to HNLC conditions while in the west the upwelled waters are nutrient-poor or oligotrophic. The transition from the warm pool to the HNLC waters is characterized by a marked increase in surface salinity (picaut et aI., 1996), an increase in the concentration of surface nutrients, a decrease in temperature and an increase in the surface partial pressure of C02 (Inoue et a!., 1996; Stoens et aI., 1999; Rodier et al.,
2000; Le Borgne et al., 2002). The location of the front between the warm pool and
HNLC water varies greatly depending on whether EI Nifio or La Nifia climatic conditions prevail (Inoue et al., 1996; Picaut et al., 1996; Delcroix and Picaut, 1998). This study transected the equator at approximately 167" W in April-May 2005, in the HNLC waters of the central equatorial Pacific under normal climatic conditions in which cooler sea surface temperatures occur in the eastern equatorial Pacific due to upwelling and the anomaly showed very little change from mean c1imatology (Figure 6c).
2.1.4 Slze-/ractionation
In general, methods for measuring total particulate material in aquatic ecosystems use filters while the methods for measuring size fractions of the particulate material use screens (Sheldon, 1972). Sheldon (1972) compared particle removal by cellulose
(MiIlipore) and glass-fiber (Wbatman) filters with particle removal by polycarbonate membrane (Nuclepore) filters. He found that polycarbonate membrane filters produce the sharpest separation of particles into size classes smaller than their pore size. Glass fiber and cellulose filters removed particles over a much wider size interval and were not recommended for separating particles by size. In the literature there has been extensive debate on Chi a retention properties of glass-fiber fine (GFIF; Wbatman) filters and
16 0.2 J.LIIl membrane filters (Dickson and Wheeler, 1993; Chavez et al., 1995). Dickson and
Wheeler (1993) reported significantly bigher concentrations ofCbl a for 0.2 J.LIIl membrane filters compared to GFIF filters. However, based on several independent open ocean comparisons of these two filter types, Chavez et al. (1995) found contradictory results to Dickson and Wheeler (1993), showing little or no difference between their retention properties. Throughout this study, total particulate material was collected using
GFIF (Whatman, nominal pore size 0.7 J.LIIl) filters and size-fractionated particulate material was filtered using polycarbonate membrane (Nuclepore) filters with pore diameters of20, 10, 2 and 0.2 J.LIIl as a way of separating phytoplankton assemblages into various size groups.
2.1.5 Objectives ofthis study
The size structure and taxonomic composition of the phytoplankton community in the open ocean are important factors in regulating organic carbon export to the deep ocean
(Azam et al., 1983; Tremblay and Legendre, 1994). The main objective of this study was to understand the spatial variability of the phytoplankton size structure and community composition across biogeochemical provinces in terms of nutrient availability. Here I present the spatial distributions of surface nutrients, phytoplankton size structure, abundance and taxonomic composition from one cruise within the Pacific Ocean. The cruise transected nutrient gradients within and across the south Pacific, central equatorial
Pacific and north Pacific waters and encompassed four biogeochemical provinces as defined by Longhurst (1998).
17 2.2 METHODS
2.2.1 BEACH-BASH Data
Sampling was conducted from the RN Kilo Moana during, BEACH-BASH, a latitudinal transect (12·S - 22·N) cruise across the Pacific Ocean from Pago Pago, American Samoa to Honolulu, Hawaii in May of2005. A total of28 stations were occupied during the transect (Figure 3). The stations were spaced at approximately equidistant intervals, 1_2° apart starting at 12°41.4'S latitude, 1700 26.5'W longitude. The last station occupied coincided with the Hawaii Ocean Time-series (HOT) Station ALOHA (22°45'N, 158°W)
(Karl and Lukas, 1996). Hydrographic data (temperature, salinity, pressure, density, fluorescence and oxygen) and in situ water samples were acquired from the near surface
(-5 m depth) ocean using either a SeaBird SBE 9/11 + CTD system plus rosette sampler fitted with 24 x 12 L Niskin-type water bottles or from the hull-mounted underway uncontaminated seawater intake system positioned at 7 m. The CTD fluorometer was calibrated against Chi a concentrations extracted from water samples and determined by fluorometric analysis. Current velocities were measured between the 25 m to 35 m depth layer using an RD Instruments 300 kHz WorkHorse Acoustic Doppler Current Profiler
(ADCP), mounted in the port hull of the ship at approximately 7 m.
2.2.2 Sample coUection and analysis
2.2.2.1 Nutrientanalysis
Water samples for nutrient analysis (nitrate + nitrite [N+N], soluble reactive phosphorus
2 [SRP] and silicic acid [Si(OH)4- ]) were collected in acid-washed 125 ml or 500 ml high density polyethylene (HOPE) bottles, immediately frozen (-20· C), and stored upright
18 until analyzed. Because nitrogen and phosphorus dynamics in the oligotrophic surface waters of the North Pacific Ocean are known to have ambient concentrations typically at or below the detection limits of standard colorimetric methods (Karl, 1999 and 2002), high-sensitivity analytical methods were used to determine surface concentrations of
N+Nand SRP.
Surface N+N determinations were perfonned using an Antek model 720 chemiluminescence nitrogen analyzer following the technique of Cox (1980) as modified for seawater by Garside (1982). This technique relies on the wet chemical reduction of nitrate (N03) and nitrite (NCh) to nitric oxide (NO) using ferrous ammonium sulfate, ammonium molybdate and a highly acidic solution of sulfuric acid. The limit of detection for N+N was approximately 0.001 pM with a triplicate precision and accuracy of ± 3 % (Table 1).
Surface SRP concentrations were analyzed using the magnesium induced co-precipitation
(MAGIC) procedure (Karl and Tien, 1992), which has been used extensively for analysis at the HOT site (Thomson-Bulldis and Karl, 1998; Karl and TieD, 1997). Frozen samples were thawed in a water bath and brought to room temperature. Subsamples for MAGIC
(3 replicates, 50 ml each) were decanted into disposable 50 ml polypropylene centrifuge tubes. A high purity NaOH solution was added at 1 % (NaOH/sample; vol/vol basis) to fonn the mineral brucite Mg(0H)2. Samples were than capped, inverted to mix the flocculate produced by NaOH and centrifuged at 1000 x g for 60 minutes. During this process the insoluble Mg(OHn containing the adsorbed phosphorus forms a pellet.
19 Table 1. Detection limit and precision estimates, volume sampled and observed ranges for all assays during the BEACH-BASH transect.
Detection limits & AssaI Precision Volume Values observed P-ATP GFIF 5ngll± 12% IL 15 - 87 ngll >20 !UD 5ngll± 12% IL
20 Following centrifugation, the supernatant was aspirated using a Pasteur pipette attached by Tygon@tubingto a vacuum line, being careful not to disturb the small pellet. The pellet was then dissolved in 5 ml of high purity O.IM HCI. The arsenate reduction mix was then added to each 5 ml sample in aliquots of 0.5 ml and allowed to react for 15 minutes. After 15 minutes 0.5 ml of the molybdenum blue reaction mix was added to each 5 ml sample, mixed and allowed to react for another 15 minutes to ensure full color development Samples were then read at 880 nm in a 10 em cell using a Beckman DU
640 Spectrophotometer. Typical precision estimates for triplicate determinations of SRP were ± 2 % with a detection limit of 0.003 J.LM (Table 1).
Triplicate surface Si(OH)4·2 concentrations were measured spectrophotometrically. This analysis was based upon the formation of yellow silicomolybdic acid from the reaction of ammonium molybdate and silica at acidic pH. As modified from Strickland and Parsons
(1972), the sensitivity of the analysis was increased by a further reduction of the yellow silicomolybdic acid using ascorbic acid. in order to produce "molybdenum blue." The silicomolybdenum blue complex was then measured at 810 nm in a 10 em cell using a
Beckman DU 640 Spectrophotometer. The limit of detection for Si(OH)4-2 was approximately 0.1 J.LM with a triplicate precision and accuracy of ± 5 % (Table 1).
2.2.2.2 Chlorophyll a
Surface Chi a concentrations were determined by both standard fluorometric techniques
(Holm-Hansen et al.• 1965; Strickland and Parsons, 1972) and by high-pressure liquid
chromatography (HPLC; Wright et al., 1991; Bidigare et al., 2005). The later method is
21 described in further detail in section 2.2.2.6. Samples for tluorometric analysis of total
Chi a (TChI a) were made by filtering 155 ml of seawater through a 25 mm diameter glass fiber filter (Whatman GFIF, nominal pore size 0.71=) via vacuum filtration.
Surface size-fractionated Chi a samples were measured by sequentially filtering 1 liter of seawater through a 25 mm diameter polycarbonate membrane filter (Nuclepore) with pore diameters of20, 10,2 and 0.21= via pressure filtration. Immediately following filtration, the filters were transferred to a glass screw cap tube containing 5 ml of cold
(-20'C) 100 % acetone. The tubes were then wrapped in aluminum foil and stored at
-20'C to prevent photodegradation of pigments. Chi a concentrations were measured by tluorometric analysis on a Turner Designs TD-700 with 436 nm (Ex) and 680 nm (Em) wavelength filters.
2.2.2.3 Phycoerythrin
Samples for the measurement of chlorophylls and photosynthetic accessory pigments are extracted in acetone (see section 2.2.2.6) and therefore select against phycoerythrin, a water soluble pigment which can be used as a biomarker for cyanobacteria such as
Synechococcus spp. and Trichodesmium spp. Surface seawater samples for phycoerythrin analysis were collected in a 5 L carboy and size fractioned sequentially through a 25 mm diameter polycarbonate membrane filter with pore diameters of20, 10, and 2 1= via pressure filtration following the in vivo glycerol-uncoupling method of
Wynam (1992) as modified by Dore et al. (2002). One liter of filtrate was then collected and passed through a 25 mm diameter 0.2 1= polycarbonate membrane filter. Filters were placed in 20 ml glass scintillation vials containing 5 ml of a saline 50 % glycerol
22 solution. The filters were placed up against the inner wall of the vial so that particles from the filters could become suspended in the solution during gentle shaking.
Phycoerythrin concentrations were measured by fluorometric analysis using a Turner
Designs TD-700 with 544 nm (Ex) and 577 nm (Em) wavelength filters. The TD-700 fluorometer was calibrated using buffered solution of a commercial R-phycoerythrin standard (Cyanotech Corp.).
2.2.2.4 Adenosine 5 '-triphosphate (ATP)
The amount of living microbial biomass in the surface water was determined by the measurement of particulate adenosine 5' -triphosphate (P-A TP) concentrations (Holm
Hansen and Booth, 1966). Seawater samples for total and size-fractionated P-A TP were passed through a 202 IJ.IIl mesh Nitex® screen to remove large plankton and collected in
lUter carboys. Surface total P-A TP samples were filtered onto 47 mm diameter GFIF filters. Surface size-fractionated P-ATP samples were collected and filtered through a
47 mm diameterpolycarbonate membrane filter with pore diameters of20, 10,2 and
0.2 IJ.IIl via vacuum filtration. The filtrate from each size fraction was then collected and filtered through 47 mm diameter GFIF filters. Upon completion of filtration P-ATP was extracted by placing the GFIF filters in 5 m1 of boiling TRlS buffer (20 mM, pH 7.4) for
S. minutes. Extracted ATP concentrations were then analyzed in a Turner Biosystems
20120· Luminometer using the firefly bioluminescence reaction (Karl and Holm-Hansen,
1978; Karl, 1993). To estimate the contribution of each size fraction to the total P-A TP pool, the P-ATP concentrations in individual fractions were calculated by difference.
Because of unidentifiable errors in size fractioning and calculating P-A TP concentrations
23 in each fraction by difference the absolute P-ATP concentrations in these operationally defined size classes should be viewed as estimates. The relative changes and spatial trends in P-ATP concentrations for each size class provide infonnation on the distributions of microbial assemblages across the BEACH-BASH transect.
2.2.2.5 Flow Cytometry
Surface flow cytometry samples were collected for enumeration ofpicoplankton
(Prochlorococcus spp., Synechococcus spp., and picoeukaryotes) and heterotrophic bacteria. Seawater samples were taken in 2 ml aliquots, placed into cryoviaIs and preserved using 40 !Jl of 0.5 )Ill1 filtered 10 % paraformaldehyde. Cryovials were then stored in a -80°C freezer until analyzed on the flow cytometer. Samples were thawed in batches, then stained with Hoechst 33342 (1 JLglmJ, final concentration) (Monger and
Landry, 1993; Campbell and Vaulot, 1993; Campbell et al., 1994). Anintemal standard to normalize scatter and fluorescence signals was added to each sample. The standard was a mixture of anto-fluorescent polystyrene beads, consisting of 0.5 JLm diameter yellow-green fluorescence beads, 0.5 JLm diameter UV beads and 1 JLm yellow-green fluorescence beads. The flow cytometer used was a Beckman-Coulter A1tra, mated to a syringe pump for quantitative analyses, and was equipped with two argon ion lasers, tuned to UV (225 mW) and 488 run (1 W) excitation. Scatter (side and forward) and fluorescence signals were collected using filters as appropriate, including those for
Hoechst-bound DNA, phycoerythrin and Chi a. Population designations, based on the scatter and fluorescence signals, were generated from the Iistmode files using FlowJo software (Tree Star, Inc., www.flo~o.com).
24 2.2.2.6 HPLC Pigments
Surface water samples for the measurement of chlorophylls and taxonomic biomarker accessory pigments were collected from 24 stations during the BEACH-BASH transect starting from 6.440 S. Samples were collected in 4 liter HOPE bottles and pressure filtered through 25 rom GFIF filters. The filters were placed in cryovials, flushed with N2 gas, and stored in liquid nitrogen until analysis. Pigments were extracted and analyzed by reverse-phase high performance liquid chromatography (HPLC; Wright et a/., 1991;
Bidigare et a/., 2005). Filters for pigment analysis were extracted in 3 ml of HPLC-grade acetone containing a known amount of an internal standard (cantbaxanthin) at 4"C for 24 hours. For each sample mixtures of 1 ml extract (containing cantbaxanthin) and 300 J1l
HPLC grade water were injected into the autosampler of the Varian 9012 HPLC system.
Photosynthetic pigments were separated on a reverse-phase Waters Spherisorb@ 5 !1111
ODS-2 (4.6 x 250 rom) CIS column with a corresponding guard cartridge (7.5 x 4.6 rom) and a Timberline column heater (26"C). Pigment identifications were based on absorbance spectra, co-chromatography with standards and relative retention time.
Abbreviations and significance of photosynthetic pigments discussed in this study are described in Table 2.
To facilitate a broad assessment of the relative importance of two functional classes of phytoplankton along the BEACH-BASH transect a diagnostic pigment (DP) index
(Bidigare pers corom.) was defined as the total of eight pigments selected for their taxonomic affinity: [DP ()LgIl) = Zea + DVChl a + TChl b + a-Car + Hex + But + Fuco +
TChl c], where zeaxanthin (Zea) is indicative of cyanobacteria, divinyl chlorophyll a
25 Table 2. Abbreviations and taxonomic affinities ofphotosyntbetic pigments separated in this study using high performance liquid chromatography (HPLC).
Pigment name Abbreviation Primary pigment in:
Total chlorophyll a* TChla All phytoplankton Monovinyl chlorophyll a MVChla All phytoplankton (except Prochlorococcus spp.) Divinyl chlorophyll a DVChla Prochlorococcus spp. Monovinyl chlorophyllide a MVChlda Senescent diatoms Monovinyl plus divinyl chlorophyll b TChlb Prochlorococcus spp. Chlorophyll c-like pigments TChlc Chromophytes 19' - Butanoyloxyfucoxanthin But Pelagophytes 19' - Hexanoyloxyfucoxanthin Hex Prymnesiophytes Fucoxanthin Fuco Diatoms Peridinin Per Dinoflagellates Prasinoxanthin Pras Prasinophytes Violaxanthin Viola Chrysophytes Diadinoxanthin DDX Chromophytes Lutein Lut Chlorophytes Alloxanthin Allox Cryptophytes Zeaxanthin Zea Cyanobacteria 13,e - Carotene a-Car Prochlorococcus spp., cryptophytes 13,13 - Carotene j3-Car All phytoplankton groups (except Prochlorococcus spp.)
>I< TChl a = MVChl a + DVChl a + MVChld a Pigment description form Jeffery et aI. (1997)
26 (DVChl a), total chlorophyll b (TChl b), and alpha carotene (a-Car) of Prochlorococcus spp. and prochlorophytes, 19' -hexanoyloxyfucoxanthin (Hex) of prymnesiophytes,
19'-butanoyloxyfucoxanthin (But) of pelagophytes, fucoxanthin (Fuco) of diatoms and total chlorophyll c (TChl c) of chromophytes. The proportion ofDP attributed to each of the functional classes of phytoplankton is defined as:
• [Zea + DVChl a + TChl b + a-Car] / DP x 100 or [CyanoDP] -used to indicate
the proportion of prokaryotic picoplankton i.e. cyanobacteria and
prochlorophytes.
• [Hex + But + Fuco + TChl c) / DP x 100 or [ChromoDP] - used to indicate the
proportion of chromophyte microalgae i.e. prymnesiophytes, pelagophytes, and
diatoms.
While these identified groups must be viewed with caution they have been chosen based on the community structure found in previous studies, in which cyanobacteria and prochlorophytes are known to contribute to phytoplankton biomass in oligotrophic waters
(Chisholm, 1992; Campbell and Vaulot, 1993) and chromophyte microalgae such as prymnesiophytes, pelagophytes and diatoms are known to contribute to phytoplankton biomass in the Pacific equatorial waters (Bidigare and Ondrusek, 1996). The grouping of different diagnostic pigments has been used in other studies to look at the size and taxonomic composition of the phytoplankton community (Vidussi et al., 2001; Barlowet al., 2004; Poulton et aZ., 2006).
27 2.3 RESULTS
2.3.1 Habilllt Characteristics
2.3.1.1 Biogeochemical provinces
ADCP current velocities between the 25 m to 35 m depth layer are shown in Figure 5, and representative vertical distributions of temperature, fluorometric TChi a and SRP from each province are shown in Figure 7. During the BEACH-BASH cruise the SPSG province was sampled starting from 12°41.4'S and encompassed a total of 5 stations
(Figures 3 and 4). The vertical distribution of temperature, TChi a and SRP in the SPSG province are shown in Figure 7a The SPSG exhibited a mixed layer depth of approximately 75 m based on a 0.125 unit change in potential density. Surface temperatures were approximately 30' C and decreased with increasing depth to a temperature of22' C at 200 m. TChi a concentrations were 0.10 IJ.gI1 at the surface and exhibited a subsurface maximum of 0.30 IJ.gI1 at the base of the mixed layer depth (75 m).
Surface water SRP concentrations were fairly low (0.14 )JM) but increased below the mixed layer to a maximum concentration of 0.66 !1M at a depth of 150 m.
The surface circulation in the PEQD province was dominated by the flow of the SEC which moved westward across the ocean and the region of divergence located at the equator (Figure 5). During the BEACH-BASH cruise the PEQD province encompassed a total of 12 stations (Figures 3 and 4). The vertical distribution of temperature, TChi a and SRP in the PEQD province are shown in Figure 7b. The PEQD exhibited a mixed layer depth of approximately 55 m. Surface temperatures were 28" C and decreased with increasing depth reaching a minimum temperature of 14° C at 200 m. TChi a 28 SPSG PEQO 0 0 (a) (b)
50 Ul 50 ~ nl .0 ~ ~ 100 100
!/)=> !/) Q) ~ a.. 150 150
200 200
PNEC NPTG 0 0 (c) (d --- 50 50 Ul ~ nl .0 ~ ~ 100 100 => !/) !/) Q) ~ a.. 150 150
12 14 16 18 20 22 24 26 28 30 12 14 16 18 20 22 24 26 28 30 Temperature (0G) Temperature (OC)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Chlorophyll a (I'!I/I) Chlorophyll a h'9/1) ,- - ,- .---r---,---~--~--~' ~ 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
SRP (~M) SRP (~M)
Figure 7. Depth profiles of temperature (blue), fluorometric Chi a (green), and soluble reactive phosphorus (SRP) (red) at representative stations in the biogeochemical provinces sampled during the BEACH-BASH transect cruise. The dashed line indicates the mixed layer depth based on a 0.125 unit change in potential density. (a) SPSG (10.51 ° S), (b) PEQD (0.63° N), (c) PNEC (6.00° N) and, (d) NPTG (J 5.85° N).
29 concentrations at the surface were 0.15 p.g/l and exhibited a subsurface TChl a maximum of 0.30 p.g/l at 60 m. Surface SRP concentrations were 0.45 JlM. These concentrations were higher than those found in the SPSG province and steadily increased with depth to a maximum concentration of 1.0 JlM at 175 m.
In this PNEC province the curvature of wind stress not only forces the flow of the eastward NECC, but is also thought to enhance algal growth and therefore Chl a concentrations (Longhurst, 1993; Figmes 3 and 5). During the BEACH-BASH cruise the
PNEC province encompassed a total of 4 stations (Figmes 3 and 4). The vertical distribution of temperature, TChl a and SRP in the PNEC province are shown in
Figure 7c. The PNEC exhibits a mixed layer depth of approximately 100 m. Surface temperatures of 29" C extended throughout the mixed layer (100 m) with temperatures reaching a minimum of 12° C at a depth of200 In. TChl a surface concentrations were
0.28 J,tgll. These concentrations were higher than those found in the PEQD province and extended down to a depth of 100 m. No subsurface TChl a maximum occurred. Surface
SRP concentrations were 0.2 JlM. These concentrations were lower than those found in the PEQD province. SRP concentrations increased below the mixed layer depth to a maximum concentration of 1.2 JlM which occurred at a depth of 175 m.
The circulation in the NPTG province between 10' N and 15' N was dominated by the westward flowing North Equatorial Current (NEC; Figure 5). During the BEACH-BASH cruise the NPTG province encompassed a total of 6 stations with the last station coinciding with Station ALOHA at 22°45' N (Figmes 3 and 4). The vertical distribution
30 of temperature, TChl a and SRP in the NPTG province are shown in Figure 7d. The
NPTG exhibits a shallow mixed layer depth at approximately 35 m. Surface temperatures were approximately 26° C and decreased steadily with increasing depth to a temperature of ISO C at 200 m. TChl a concentrations were 0.07 !LW'l at the surface and exhibited a deep subsurface maximum of 0.27 !LW'l at a depth of 150 m. SRP concentrations in surface waters were fairly low (0.15 !J.M) and remained low throughout the upper 175 m.
2.3.1.2 Latitudinal distribution o/Temperature, Salinity and Chlorophyll a
The latitudinal distribution of surface temperature, salinity and fluorometric TChl a are shown in Figures Sa and b. In general, surface temperatures decreased from 30· C at the southern extent of the transect to 25.5" C at the northern end of the transect. At the equator there was an abrupt drop in temperature to 2S· C followed by a rapid increase in temperature just northward (Figures Sa and b). Surface salinity ranged from 34.25 to
35.5 Practical Salinity Scale (PSS; Figure Sb). Surface salinity was highest at the southern and northern most ends of the transect as well and just south of the equator. The sea surface temperature was relatively low and the salinity relatively high in the equatorial area which indicated upwelling. Fluorometric TChl a values ranged from 0.06 to 0.28 !LW'l with the highest concentrations occurring in the equatorial provinces (PEQD and PNEC; Figure Sb). TChl a concentrations peaked north of the equator in the PNEC province and were followed by an abrupt drop in concentration from 0.2S to 0.08 !LW'l.
TChl a concentrations in the SPSG province (0.07 to 0.10 !LW'l) were slightly higher than
TChl a concentrations measured in the NPTG province (0.06 to 0.09 !LW'l).
31 Surface Temperature, Salinity and Chlorophll a SPSG PEQD PNEC NPTG 31 -.------="----='-=--..,-----'---= -=-----.,.-'--'c..:=.=-r---'-.:..:..--'-=------, 0.30 (a) -- Temperature 30 - Chlorophyll a 0.25
~ ~ ()o 29 " ~ ~ ~ 0.20 -; :::l > "§ 28 .s:::. Q) a. a. 0.15 E eo ~ 27 .s:::. () 0.10 26
25 +-----.--I-----,---+--t---,-----.--+ 0.05 31 36.0 (b) -- Temperature 30 -- Salinity 35.5
oG' 29 ~ if) (/) ~ a.. :::l ~ 'iii 28 35.0 ~ ~ Q) a. c: ro E (/) ~ 27 34.5 26
25 +-- ---.--I-----,---+--t---,-----.--+ 34.0 -15 -10 -5 o 5 10 15 20 25 Latitude (Oeg)
Figure 8. Latitudinal distribution of surface (a) temperature (0C) and chlorophyll a (!!g/l) and (b) temperature (OC) and salinity (PSS).
32 2.3.2 Nutrient distributions
2.3.2.1 LatitudiTUll distribution ofdissolved iTUlrganic nutrients
The latitudinal distribution of surface inorganic nutrients N+N, SRP and Si(OH)4"2 are shown with their standard deviations in Figure 9. In most cases standard deviations were less than or equal to the size of the data point and are not visible. All nutrient concentrations exhibit a pronounced increase at the start of the PEQD province (5° S) which coincided with the start of the strong westerly SEC and region of equatorial divergence (0") where nutrients are upwelled to the surface layer from below the thermocline (about 75 m, refer to Figure 7b). N+N concentrations ranged from
0.0025 - 3.73 JIM throughout the transect. Consistently low N+N values were found in
SPSG, PNEC and NPSG provinces. N+N concentrations increase sharply at the start of the PEQD province (5° S) and remain relatively high until reaching the equator where a pronounced drop in N+N concentration occurred. N+N concentrations exceeded SRP concentrations between 5° S and 0°. Surface SRP concentrations ranged from
0.06 - 0.43 JIM throughout the transect. SRP concentrations steadily increased at the start of the PEQD, peaked at the equator and then steadily decreased through the PNEC.
SRP concentrations were lowest in the SPSG and NPTG provinces but exceeded N+N concentrations in the SPSG, PNEC and NPTG province. Surface Si(OH)4"2 concentrations ranged between 0.55 - 2.65 JIM throughout the transect and followed the same pattern as N+N. Surface Si(OH)4"2 concentrations were lowest in the SPSG and increased sharply at the start of the PEQD province, remaining high until reaching the equator where a pronounce drop occurred. Si(OH)4"2 concentrations in the PNEC and
NPTG provinces were greater than those found in the SPSG province.
33 Nutrients SPSG PEQO PNEC NPTG 4 2.0 - N+N - SRP ~ 3 1.5 :2 - Silicic Acid :::1. ~ -0 Tj ~ rn 2 1.0 :2 u :::1. ~ : ~ a.. en ...... ~ 0::: o(l 1 0.5 en z + ~- z 0 0.0
-15 -1 0 -5 o 5 10 15 20 25 Latitude (Oeg)
Figure 9. Latitudinal distribution of surface nutrients nitrate + nitrite [N+N], soluble 2 reactive phosphorus [SRP] and silicic acid [Si(OHk ] along the BEACH-BASH transect.
34 2.3.2.2 Dissolved inorganic nutrient ratios
The dissolved inorganic N+N : SRP ratios in the surface waters throughout the transect never exceeded the Redfield ratio 16:1 (Redfield et aI., 1963; Figure lOa). The
N+N : SRP ratios were highest in the PEQD province between 5° S - 0° (mean = 7.6
± 1.18, n = 6), where the highest N+N concentrations were observed. The N+N : SRP ratios were significantly lower in the SPSG, PNEC and NPTG provinces (mean = 0.08, n = 16).
The dissolved inorganic Si(OH),,2 : N+N ratio in the surface waters throughout the transect almost always exceeded the 1: 1 ratio of growing diatoms (Brzezinski, 1985), except in the PEQD province between 5° S - 0°, where equatorial upwelled waters were observed (Figure lOb). In this region the ratio of Si(OH)4·2 : N+N approached 1: 1
(mean = 0.68 ± 0.09, n = 6). The Si(OH)4·2 : N+N ratios dramatically increased between the equator and 5° N exhibiting a maximum of 500 at approximately 4° N. The mean
Si(OH)4·2 : N+N ratios in the SPSG, PNEC and NPTG provinces were 90, 135 and 185 respectively.
The dissolved inorganic Si(OH)4·2 : SRP ratio in the surface waters throughout the transect almost never exceeded the ratio 16: 1 of diatom growth, except in the northern most Station ALOHA (Si(OH)4·2 : SRP = 18; Figure lOe). The Si(OH)4·2 : SRP ratio gradually increased along the transect from south to north. The mean Si(O~'2 : SRP ratio in the SPSG, PEQD, PNEC and NPTG provinces were 4, 4.5, 7 and 12.5 respectively.
35 SPSG PEQD PNEC NPTG 100 (a) ------f------10 ...... a.. a:: en 1 • • Z • • + 0.1 1 • • •• ~ • • Z • •• • • • • • 0.01 •• 0.001 -15 -10 -5 o 5 10 15 20 25
1000 (b) •••• • Z •• ~ • • + 100 , • . • Z • • • • • "0 '(3 10 ttl • • U : ~ • • en 1 c------.-:••••------
0.1 -15 -10 -5 o 5 10 15 20 25
100 (c) a.. a:: en f------"0 '(3 10 , • • • • • ttl • • U • •• :~ • ., ... ' .... Ui • •• • •
1 -15 -10 -5 o 5 10 15 20 25 Latitude (Oeg)
Figure 10. Latitudinal distribution of surface nutrient ratios; (a) ratio ofN+ to SRP, with dashed line at a ratio of 16: I, (b) ratio of silicic acid to N+N, with dashed line at a ratio of I: I, (c) ratio of si licic acid to SRP, with dashed line at a ratio of 16: I across BEACH-BASH transect. 36 2.3.3 Plankton Size Structure
2.3.3.1 Chlorophyll a
The latitudinal distributions of fluorometric surface size-fractionated ChI a concentrations are shown in Figures lla and b. Highest levels of phytoplankton ChI a were observed in waters located in the PEQD and PNEC provinces between 5' S and
10' N with a peak in all size fractions occurring in the PNEC province at approximately
6' N. The latitudinal distribution of ChI a was dominated by picoplankton (0.2-2 lUll) in all provinces (Figure II a). Picoplankton Chi a closely reflected the patterns of abundance of total phytoplankton, given that this fraction accounted for 45 - 60 % of the
TChi a during most of the transect (Figure 11 c). Lowest levels of phytoplankton ChI a were observed in waters located in the oligotrophic SPSG and NPTG provinces with picoplankton values below 0.1 )1gI1. Flow cytometry analysis indicated that the cyanobacteria Prochlorococcus spp. and Synechococcus spp. constituted the bulk of the picoplankton abundance during the transect (Figure 17).
Nanoplankton (2-10 lUll and 10-20 lUll) Chi a concentrations were markedly lower than those of picoplankton. Typical surface values ranged between 0.027 - 0.036 )1gI1, with nanoplankton of 2-1 0 lUll in size having slightly higher concentrations than nanoplankton of 10-20 lUll in size throughout the transect. This can be seen most noticeably in the
PEQD and PNEC provinces (Figure lib). The highest concentration of nanoplankton
ChI a (2-10 lUll) was 0.038 )1gI1 and occurred in the PNEC province at 6' N. The highest concentration of nanoplankton ChI a (10-20 lUll) was 0.031 )1gI1 and occurred in both the
PEQD and PNEC provinces. Each of the nanoplankton size fractions typically
37 Size - Fractionated Chlorophyll a 0.25 -,----=-==----SPSG -,----'PEQD'-==--r'- PNEC:....==-, ----'--'''-'-=-NPTG ----, (a) -- > 20 I'm ::::::~ 0.20 -- 10-20 I'm ~ -- 2-10 I'm ~ (\) 0.15 -- 0.2 -21'm >- .r:: a. 0 0.10 ~ 0 .r:: • .... ---- - () 0.05 . •
0.00 0.040 (b) -- >20 I'm ~ -- 10- 20 l'm ""~ 0.036 -- 2 -10 I'm ~ (\) 1= 0.032 a. 0 ~ 0 .r:: 0.028 ()
0.024 -15 -10 -5 o 5 10 15 20 25 Latitude (Deg) 70 (C) _ >20 I'm 60 _ 10- 20 l'm (\) _ 2- 10 l'm >. .r:: 50 _ 0.2 -2 I'm a. 0 ~ 0 40 .r:: () 30 til 0 I-- 20
0~ 10
0 SPSG PEQD PNEC NPTG Province Figure II . Latitudinal di stribution of surface size-fracti onated chlorophyll a (a) >20 I!m, I 0-20 ~ , 2- \0 I!m and 0.2-2 I!m (b) >20 I!m, 10-20 I!m, and 2-1 0 I!m, and (c) Regionalized percent (%) total surface chloropbyll a.
38 represented 15 - 20 % of the TCbl a throughout the transect (Figure llc), therefore total nanoplankton (2-20 11IIl) represented 30 - 40 % of the TCbl a.
Microplankton (>20 11IIl) Cbl a concentrations were also markedly lower than those of picoplankton and in most cases lower than both nanoplankton (2-10 and 10-20 11IIl) size fractions (Figure 11 b). Microplankton Cbl a was present in surface waters with typical concentrations ranging between 0.026 - 0.030 j.tgIl. Concentrations of microplankton
Cbl a peaked in the PEQD province at approximately 4° N with a value of 0.038 p.g/l.
Microplankton Cbl a typically represented 12 - 18 % of the TChI a throughout the transect (Figure Ilc). TCbl a measurements made using glass fiber filters (GFIF) were well correlated with TCbl a measurements made using the sum of the polycarbonate filters (PC); (pC = 0.82 (GFIF) + 0.083, ~ = 0.987, p < 0.001, n = 27; Figure 12).
Correlations between ChI a size fractions and nutrients were observed (Figures 13a, b and c). N+N showed the weakest correlation to all Cbl a size fractions with Pearson correlation coefficients ranging berween r = 0.077 - 0.334 (Figure 13a). Microplankton
Cbl a (>20 11IIl) appeared only weakly dependent on N+N (r = 0.077) while nanoplankton
Cbl a (10-20 11IIl) demonstrated greater dependence (r = 0.334). Similar to the ChI a relationships to N+N, nanoplankton Cbl a (10-20 11IIl) concentrations appeared most strongly dependent on Si(OH)4·2 and SRP concentrations (r = 0.594 and r = 0.732, respectively; Figures 13b and c).
39 0.35 -,------, • 0.30
E" ~ 0.25 • • :c ~u 0.20 Q. •
0.15 y =0.82 x + 0.083, R2 =0.987 0.10 +-----,---,----.------r----j 0.05 0.10 0.15 0.20 0.25 0.30 GFIF TChl a (Ilg/l)
Figure 12. Chlorophyll a regression analysis. TD-700 fluorometric TChl a measured using GFIF filters versus TChl a measured using summed PC filters.
40 •
(a)
~ Ol '"::t ~ Total Chi a (r = 0.294) (1) 0.1 • > 20 ~m Chi a (r = 0.077) >- .c • '" 10-20 ~ m Chi a (r = 0.334) a. 0 .II. 2-10 ~ mChla(r=0 . 198) ~ 0 :c • 0.2-2 ~m Chi a (r = 0.179) u
0.01 0.001 0.01 0.1 1 10 N+N (b)
~ '"Ol 3 o Total Chi a (r = 0.682)" (1) 0.1 - • > 20 ~ m Chi a (r = 0.48Qf >- .c '" 10- 20 ~m Chi a (r = 0.732)" a. 0 .II. 2-1 0 ~m Chi a (r = 0.603)' ~ ..Q • 0.2-2 Chi a (r = 0.656)' .c ~m U
0.01 0.01 0.1 1 SRP (c)
~ Ol Total Chi a (r = 0.390) '"3 • > 20 Chi a (r = 0.281 ) (1) 0.1 ~m '" 10-20 ~mChl a (r = 0.594)' >- .c a. .II. 2-10 ~ m Chi a (r = 0.517)' 0 ~ • 0.2-2 ~l m Chi a (r = 0.341) :c0 u
0.01 0.1 1 10 Silicic Acid (~M)
Figure 13. Correlations between (a) Chl a and N+N, (b) Chi a and SRP, and (c) Chi a and si licic acid. (*), significantly different from zero at the 99 % significance level. (+), significantly different from zero at the 95 % signjficance level.
41 2.3.3.2 Phycoerythrin
The latitudinal distribution of surface size-fractionated phycoerythrin concentrations are shown in Figures 14a and b. Highest concentrations of phycoerythrin were observed in waters located in the SPSG, PEQD and PNEC provinces and exhibited peaks both north and south of the equator at approximately 6° S and 6° N. The latitudinal distribution of surface phycoerythrin was dominated by picoplankton (0.2-2 JIlll) in all provinces with picoplankton phycoerythrin accounting for 75 - 95 % of the total phycoerythrin throughout the transect (Figure 14c). Surface picoplankton phycoerythrin concentrations ranged between 0.7 - 31 ng/l with the lowest levels of picoplankton phycoerythrin being observed in the NPTG (Figure 14a).
Nanoplankton (2-10 JIlll and 10-20 JIlll) phycoerythrin concentrations were markedly lower than those of picoplankton (Figures 14a and b). The highest phycoerythrin concentrations of nanoplankton (2-10 JIlll) also exhibited peaks both north and south of the equator at approximately 3° S and 4° N with concentrations reaching 1.73 ng/l.
Nanoplankton (2-10 JIlll) phycoerythrin accounted for < 10 % of the total phycoerythrin in each of the provinces (Figure 14c). The highest concentrations ofnanoplankton
(10-20 JIlll) phycoerythrin occurred in the SPSG and NPSG with concentrations reaching
1.16 ng/l. Nanoplankton (10-20 JIlll) phycoerythrin accounted for 15 % of the total phycoerythrin in the NPTG and < 5 % of the total phycoerythrin in each of the remaining provinces (Figure 14c).
42 Size - Fractionated Phycoerythrin 35.-__~SP~S~G ~-, ___P~E~Q~D ~-,~P~N~E~C~ ____~N~P~T~G~ __-. ~ >20 11m :::: 30 ~ 10 - 20 11 m g> 25 ~ 2-10Ilm ~ c: 20 ~ 0.2 -2 11m .'".r= ~ 15 8 10 > .r= 5 (L o
2 . 0~~------r------.-----'------, 1.8 (b) ~ >20 11m ::::- 1.6 ~ 10-20Ilm Cl E. 1.4 ~ 2- 10 11m c: 1.2 ..r='" 1.0 ~ 0.8 Ql o 0.6 u >- 0.4 .r= (L 0.2 0.0
-15 -10 -5 o 5 10 15 20 25 Latitude (Deg) 100.------~~------_, (c)
c: 80 .'".r= ~ 2l 60 u > .r= (L CI3 40 o I- ?J? 20
01----.... SPSG PEQO PNEC NPTG Province
Figure 14. Latitudinal distribution of surface size-fractionated phycoerythrin (a) >20 ~m , 10-20 ~m , 2-10 ~ and 0.2-2 ~m (b) >20 ~m , 10-20 ~m , and 2-10 ~m, and (c) Regionalized percent (%) total surface phycoerythrin. 43 Microplankton (>20 !J.IIl) phycoerythrin concentrations were also markedly lower than those of the picoplankton (Figures 14a and b). Microplankton phycoerythrin was present in surface waters with concentrations reaching 0.81 ngIl. Microplankton phycoerythrin concentrations accounted for 10 % of the total phycoerythrin in the NPTG province and
< 1 % of the total phycoerythrin in each of the remaining provinces (Figure 14c).
2.3.3.3 Adenosine 5 '- triphosphate (ATP)
The latitudinal distribution of surface total P-ATP concentrations are shown in Figure 15.
Total P-ATP concentrations throughout the transect ranged between 15 - 87 ngIl with increased variability in the PEQD and PNEC provinces. Highest levels of total P-A TP were observed in the equatorial PEQD and PNEC provinces between 5· S and 10· N.
Lowest levels of total P-ATP were observed in waters located in the oligotrophic SPSG and NPTG provinces.
The latitudinal distributions of surface size-fractionated P-A TP concentrations are shown in Figures 16a and b. Picoplankton P-ATP concentrations throughout the transect ranged between 3 - 20 ng/l, with highest concentrations occurring in the equatorial PEQD and
PNEC provinces (Figure 16b). Picoplankton P-ATP concentrations dominated the SPSG accounting for - 45 % of the total P-ATP in that province (Figure 16c). Picoplankton
P-ATP in the PEQD, PNEC and NPTG provinces accounted for 20 - 30 % of the total
P-ATP.
44 Adenosine 5'-triphosphate PEaD PNEC NPTG 100 SPSG
-+- P-ATP
80
:::::~ 60 OJ c ~ h 40 \ --- 20 ~ 1IV o -15 -1 0 -5 o 5 10 15 20 25 Latitude (Deg)
Figure 15 . Latitudinal distribution of surface total particulate ATP (P-ATP) and along the BEACH-BASH transect cruise.
45 Size - Fractionated ATP 60.-__~S~P~S~G ~-r __~P~E~Q~O~-,~P~N~E~C~ ____~N~P~TG~ ____~ (a) -- > 20 llm 50 -- 10 - 20 llm
::::- 40 Cl c::: ~ 30 (L I-- « 20
10 o~~~~~~~~tl~~~~~~~
30.------.------,-----.------~ (b) 25
:::- 20 Cl c::: ~ 15 (L ~ 10 •
5
o+-----.-----+-----,-----~----~----_r----_r----~ -15 -10 -5 o 5 10 15 20 25 Latitude (Oeg) 50.------~~------~------, (c) _ > 20 llm _ 10 - 20 llm 40 _ 2-101lm
(L ~ 30 (5'" I-- 20 'J<
10
o SPSG PEQO PNEC NPSG Province
Figure 16. Latitudinal distribution of surface size-fractionated particulate A TP (P-A TP) (a) >20 Iilll and 10-20 11m (b) 2-\ 0 11m and < 2 Iilll, and (c) Regionalized percent (%) total surface P-A TP. 46 Nanoplankton (2-10 ~ and 10-20 ~) P-ATP concentrations ranged between 3 - 27 ng/l and below detection limit « DL) -11 ng/l respectively (Figures 16aand b). The 2-10 ~ nanoplankton P-A TP concentrations showed greater variability than the 10-20 ~ nanoplankton P-ATP concentrations. P-ATP concentrations in the PEQD, PNEC and
NPTG provinces were all dominated by 2-10 ~ nanoplankton accounting for 35 - 45 % of the total P-ATP in those provinces (Figure 16c). However, the least dominant were the 10-20 ~ nanoplankton accounting for 5 - 15 % of the total P-ATP.
Microplankton (>20 ~) P-A TP concentrations throughout the transect ranged between
< DL - 53 ng/l, exhibiting the widest range out of all the size fractions (Figure 16a).
Microplankton P-ATP concentrations were lowest in the SPSG accounting for 10 % of the total P-A TP (Figure 16c). However, throughout the remainder of the transect microplankton P-ATP concentrations accounted for 25 - 30 % of the total P-ATP.
2.3.4 Plcoplankton community structure and toxonomic distribution
2.3.4.1 Flow Cytometry
The flow cytometric analysis of surface samples along the transect allowed for the enumeration of picoplankton (Prochlorococcus spp., Synechococcus spp., and picoeukaryotes) and heterotrophic bacteria. Patterns of abundance ofpicoplankton and heterotrophic bacteria in surface samples along the transect (Figure 17) aligned with some or all of the prominent features in N+N, SRP and Si(0In4-2 (Figure 18). Among all stations the estimates of surface heterotrophic bacteria abundances were highest ranging
4 4 l between 52 x 10 - 88 X 10 cells m1- (Figure 17a). High densities of heterotrophic
47 Plankton Abundances SPSG PEQO PN EC NPTG _ 100.---~~---,--~~~--~~~----~~~-----. "0 (a) ';; 90 E ;; 80 -a.'" 70 e ~ e 60 /
I* 50 +-----._----+_----._----+_----+_~--~ ~~~----_r~ ~~~ -15 -10 -5 o 5 10 15 20 25 ,,~ 20 .-~------._------._----~------_. (b) >< E 15
I':+- ____ ,-____ +_--~._----+_----+_----,_----,_--~ -15 -10 -5 o 5 10 15 20 25
,,~ 2.0 -y------:;:r.:------r::----,_------, ~ (c) i 1.5 -~ 1.0 I:: __ ------e ill -15 -10 -5 o 5 10 15 20 25
~ 1.0 ._------._------~----.------_. ~ (d) i 0.8 -~ 0.6 J!!'" g. 0.4 ~ § 0.2
a:: 0.0 +-----._----+_----,_----+_----+_----.------.----~ -15 -10 -5 o 5 10 15 20 25 Latitude (Oeg )
4 Figure 17. The latitudinal di stribution of surface plankton abundances ( # / ml x 10 ) (a) heterotrophic bacteri a (b) Prochlorococcus, (c) Synechococcus and (d) picoeukaryotes during the BEACH-BASH transect. 48 r = 0.58' r = 0.76' r = 0.39 ' 105 +---.---.-~~~
1o6 ,------,
~ u - 5 •• •• g L 10 ., .i. .. ~ ~ l·l·r·I'·'·:..J'·~· ••--': ":"'-'-,~~ , ' •• J ~ ~ 10' ;"- ., . .. . , o- •• ct
103 +-__.- __.-~ r _= T- ~0~.2~4
10 5~------, to .,. . ••• "-§~ 10' • • .. t> E .. • • o Vl . • • ""5=Q) Q) , 3 <::~ 10 6J •... "..• r = 0.52 ' r = 0.79' r = 0.32 10 2 +---.-~~~~~
105 ,------.
Vl 104 ~..-:>. .!.... •• ~ . E -. ... 3 ~ ~~.~---..~l ~ a; 103 a:o~ 10 2 +---.---.-~r _=~0~.3~3_1 r=O.73' r = 0.51 ' 10-3 10-2 10-1 1()O 10 1 10-2 10-1 10· 10 1 10-1 10· 101
Silicic Acid (I-lM)
Figure 18_ Correlations between the surface abundance of plankton groups (a) heterotrophic bacteria, (b) Prochlorococcus, (c) Synechococcus and (d) picoeukaryotes and nutrients along the BEACH-BASH transect. Left panels for +N, middle panels for SRP and right panels for Silicic acid. In some cases correlation coefficients (r) are indicated_
* Significantly different from zero at the 99 % significance level. + Significantly different from zero at the 95 % significance level. 49 bacteria were noted around the equator in the high-nutrient PEQD province. Lowest abundances of heterotrophic bacteria were observed in the low-nutrient SPSG and NPTG provinces. Plotting their abundance against all nutrient concentrations (Figure 18a) reveals a somewhat tighter relationship between SRP and abundance
2 (r = 0.76) than N+N and Si(OH)4- •
4 Most estimates of surface Prochlorococcus spp. abundance clustered within 1.5 x 10 -
4 l 10 X 10 cells mr\ with the highest density of15 x 104 cells mI· occurring in the PNEC province at approximately 6° N (Figure 17b). The lowest abundances of
Prochlorococcus spp. occurred just south of the equator in the high-nutrient PEQD province and in both the low-nutrient SPSG and NPTG provinces. Plotting their abundance against all nutrient concentrations (Figure 18b) reveals weak correlations with all nutrients. N+N and Si(OH)4-2 reveal a negative correlation and SRP reveals a slightly positive correlation.
Surface Synechococcus spp. abundances ranged between 0.04 x 104 - 1.88 x 104 cells mI-1 and exhibited peaks both north and south of the equator (Figure 17c). This distribution had a local minimum in Synechococcus spp. abundance centered about the equator with higher abundances of Synechococcus spp. to the north (6° N) and south
(5° S). The lowest Synechococcus spp. abundances were observed in the low-nutrient
NPTG province. The abundance of Synechococcus spp. along the entire transect correlated well with SRP and N+N (r = 0.79 and 0.52 respectively; Figure 18c).
so Surface picoeukaryote estimates among all stations were the lowest among the groups of
4 4 1 picoplankton studied ranging between 0.08 x 10 - 0.78 X 10 cells ml- (Figure 17d).
Picoeukaryote abundances exhibited a similar distribution to Prochlorococcus spp. with highest densities occurring in the PNEC province at approximately 6° N and lowest abundances occurring in both the low-nutrient SPSG and NPTG provinces. The abundance of picoeukaryotes correlated better with all nutrients compared to
Prochlorococcus spp. which showed weak correlations with nutrients. Picoeukaryotes correlated best with SRP and Si(OH)4-2 (r = 0.73 and 0.51 respectively; Figure 18d).
2.3.4.2 HPLC Pigments
The dominant pigments detected during the BEACH-BASH transect were monovinyl chlorophyll a (MVChl a), divinyl chlorophyll a (DVChl a), monovinyl plus divinyl chlorophyll b (TChl b), chlorophyll c-Iike pigments (TChl c), zeaxanthin (Zea), peridinin
(Per), diadinoxanthin (DDX), fucoxanthin (Fuco), 19'-butanoyloxyfucoxanthin (But),
19' -hexanoyloxyfucoxanthin (Hex) and a-carotene (a - Car). Concentrations of monovinyl chlorophyllide a (MVChld a), prasinoxanthin (Pras), alloxanthin (Allox), violaxanthin (Viola), lutein (Lut) and p-carotene (P - Car) were low and variable. or below the limit ofRP-HPLC quantification « DL). These results agree well with those found by Bidigare and Ondrusek (1996). Abbreviations and taxonomic affinities of photosynthetic pigments are shown in Table 2.
Distributions of total chlorophyll a (TChl a = MVChl a + DVChl a + MVChld a), total chlorophyll b (TChl b = MVChl b + DVChl b), and total chlorophyll c (TChl c) are
51 shown in Figure 19. The relationship between TChl a concentrations measured using the
TD-700 vs. TChl a concentrations measured by HPLC were also well correlated
(TD-700 = 0.89 (HPLC) + 0.013, ~ = 0.94,p < 0.001, n = 27; Figure 20). TChl a values ranged from 0.06 - 0.28 p.g/l with the highest concentrations occurring in the equatorial provinces (PEQD and PNEC). TChl a concentrations peaked north of the equator in the
PNEC province and were followed by an abrupt drop in concentration from 0.28 - 0.08 p.g/l. TChl b values ranged between 0.002 - 0.027 p.g/l with the highest concentrations occurring in the PEQD province and the lowest concentrations occurring in the NPTG province. TChl c values that ranged between 0.007 - 0.055 p.g/l. TChl c values followed a similar pattern to TChl a with highest concentrations occurring in the PEQD and PNEC equatorial provinces. TChl c concentrations also peaked north of the equator in the
PNEC province and were followed by an abrupt drop in concentration from 0.52 - 0.009 p.g/l. The lowest concentrations ofTChl c occurred in the NPTG.
The latitudinal distribution of surface TChl a pigment constituents MVChl a, DVChl a and MVChld a are shown in Figure 21a The chemotaXonomic marker MVChl a is representative of all phytoplankton (except Prochlorococcus spp.), DVChl a is representative of the prokaryotic pigment marker found in Prochlorococcus spp. and
MVChld a is representative of the degradation pigment found in senescent phytoplankton and fecal pellets. The latitudinal distribution ofMVChl a concentrations ranged between
0.029 - 0.146 p.g/l. The highest concentrations ofMVChl a occurred in the PEQD and
PNEC equatorial provinces and the lowest concentrations occurred in the oligotrophic
NPTG province. The latitudinal distribution ofDVChl a concentrations ranged between
52 HPLC Total Chlorophyll a, band c PEQD PNEC NPTG 0.30 SPSG 0.10 - TChia - TChlb 0.25 ~ TChic 0.08 ~ -=: ~ 0.20 ~ -=:~ <..l Ol 0.06 .c ::1. ~ () en 0.15 I- "0 .c c () 0.04 t'II I- .Q 0.10 .c () 0.02 I- 0.05 • 0.00 0.00 -10 -5 0 5 10 15 20 25 Latitude (Deg)
Figure 19. The latitudinal distribution of HPLC TChl a [MVChl a + DVChl a + MVChld a] , TChl b [MVChl b + DVChl b] and TChl c pigments.
53 0.30 • 0.25 • ~ ~ 0.20 ~ lIS :c (.) 0.15 • ~ • 0 • ....0 0 0.10 ~
0.05 y =0.89 x + 0.013, R2 =0.94 0.00 +------,---.------r----,.---,----I 0.00 0.05 0.10 0.15 0.20 0.25 0.30 HPLC TChl a ().tgll)
Figure 20. Chlorophyll a regression analysis. TChl a measured using the TD-700 versus TChl a measured by HPLC.
54 MVChl a, DVChl a and DVChld a
0.16 ,---..::..:...SPSG-=-:::.., __ --'--' PEQO==- _ -,---'--'PNEC-'==, ___ ----' NPTGc.::..-c....:::: ___----, (a) _ MVChla 0.14 -0 OVChl a 0.12 - MVChida -=::~ ~ 0.10
~ co 0.08 >. .c e0.. 0.06 .2 .c 0.04 () 0.02
0.00
-10 -5 o 5 10 15 20 25
90 ,----,------,----,------, (b) - MVChla 80 ~ OVChl a
<3 70 >o C/:I 60
:;:: 50 ~ :2 40 (ij ~ 30 :oR.o 20
10 +----+---r---~---4---._--_.--~ -10 -5 o 5 10 15 20 25 Latitude (Oeg)
Figure 21. The latitudinal distribution of (a) TChI a pigment constituents MVChl a, DVChl a and MVChld a, and (b) MVChl a and DVChl a as a proportion (%) ofTChl a [TChI a = MVChl a + DVChl a + MVChld aJ for the BEACH-BASH transect.
55 0.021- 0.1191l1Yl. The distribution of DVChl a closely follows the pattern oftlow cytometry Prochlorococcus spp. abundance (Figure 17b). DVChl a concentrations were highest in the PEQD and PNEC provinces and peaked in the PNEC province. Lowest
DVChl a concentrations occurred in the NPTG province. MVChld a concentrations were low and remained fairly constant throughout the transect with values of approximately
0.0021l1Yl. MVChld a concentrations show a single peak ofO.0351l1Yl at - 4.50 N latitude in the PEQD province.
The latitudinal distribution of the percent TChl a pigment markers MVChl a and
DVChl a are shown in Figure 21 b. MVChl a and DVChl a together accounted for
> 95 % of the TChl a throughout the transect. Percent total MVChl a and DVChl a both showed sinusoidal distributions, however they were inversely related to one another.
MVChl a accounted for approximately 50 - 60 % of the TChl a in the PEQD and NPTG provinces. DVChl a accounted for approximately 50 - 60 % of the TChl a in the SPSG and PNEC province suggesting that Prochlorococcus spp. was the dominant phytoplankton species in those provinces.
The latitudinal distribution of diagnostic pigments CyanoDP and ChromoDP as a proportion (%) of total diagnostic pigments [DP = Zea + DVChl a + TChl b + a-Car +
Hex + But + Fuco + TChl c] are shown in Figure 22. CyanoDP accounted for> 50 % total phytoplankton DP throughout the transect. CyanoDP accounted for approximately
70 - 80 % of total DP in the SPSG and NPTG provinces and 50 -70 % of total DP in the
PEQD and PNEC provinces. ChromoDP on the other hand accounted for < 50 % total
56 Cyanobacteria and Chromophyte Microalgae SPSG PEQD PNEC NPTG 90 - CyanoDP 80 - ChromoDP
70
c.. 60 0 III 50 -0 ~ ::§? 0 40
30
20
10 -10 -5 o 5 10 15 20 25 Latitude (Deg)
Figure 22. Diagnostic pigments CyanoDP and ChromoDP as a proportion (%) of the total diagnostic pigments [DP = Zea + DVChl a + TChl b + a-Car + Hex + But + Fuco + TChl c) for the BEACH-BASH transect cruise.
57 phytoplankton DP throughout the transect with 30 - 50 % occurring in the PEQD and
PNEC provinces and 20 - 30 % occurring in the SPSG and NPTG provinces. These results once again reinforce the dominance of picoplankton throughout the transect.
58 2.4 DISCUSSION
2.4.1 Nutrient dynll1llics
Nutrients in the oceans are circulated by biological and physical processes. In the euphotic zone of the surface ocean, nutrients are assimilated mainly by microorganisms, including phytoplankton and bacteria. Biogenic particles are then carried below the mixed layer by the sinking of detritus and the feeding activities of animals, this particulate matter is then subject to remineralization which causes dissolved nutrients to be released back into seawater at depth. As a result, nutrient concentrations are generally low near the surface but high in deeper waters, except in Polar regions and/or regions of intensive upwelling like the equatorial Atlantic and Pacific which brings cold, nutrient rich seawater back to the surface. This study's principal transect, which exhibits a gradient in nutrients, was carried out during April-May 2005 (Figure 3). The transect began in the oligotrophic waters of the SPSG, passed through the nutrient enrich upwelled waters of the PEQD (-167° W), the chlorophyll enhanced waters of the PNEC, and ended in the oligotrophic waters of the NPTG coinciding with the HOT Station
ALOHA. The major nutrients (N+N, SRP and Si(OH)4'2) needed for phytoplankton growth increased in concentration in the surface waters of the equatorial Pacific between
5° S and the equator. Surface nutrient concentrations in this region exhibited rapid transitions associated with the strong westerly SEC and the region of equatorial divergence where nutrients are being upwelled.
Since phytoplankton require nutrients for their growth, nutrient stoichiometry and deviations from the Redfield ratio can limit phytoplankton primary production, and affect
59 phytoplankton biomass, species composition, and consequently food web dynamics. In the upper ocean, recycling ofN+N and phosphate are generally considered to be tightly coupled. Variations in nutrient supply, export or cellular incorporation can result in both local and regional changes in the N+N : SRP ratios. N+N: SRP stoichiometry across the transect never exceeded the Redfield molar ratio of 16N : IP and therefore imply a possible nitrogen limitation. The mean N+N : SRP ratio in the SPSG, PNEC and NPTG provinces was extremely low (mean = 0.08. n = 16). These results are similar to those found in the surface waters at Station ALOHA located in the NTPG province (Karl et m.,
2001b). However, measurements ofN+N and SRP by inorganic analysis alone could lead to erroneous conclusions about nutrient limitation (Downing, 1997; Karl et al.,
2001 b). Dissolved organic nutrients such as dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are also produced locally and consumed during microbial metabolism and when included, have been shown to influence the N:P ratio
(Karl et aI., 2001b). Therefore, ifthe community was actually nitrogen limited, and no other source of fixed nitrogen were available (e.g. NH/ or DON) there would be a strong selection for the growth of diazotrophic microorganisms including the marine cyanobacteria Trichodesmium spp., the diatom endosymbiont Richelia and other unidentified N2-fixing bacteria capable of utilizing the nearly inexhaustible supply of nitrogen gas (Nz) in seawater and contributing to new production (Carpenter and
Ramons, 1991; Karl et al., 1992; 1997; Letelier and Karl, 1996; Capone et al., 1997; Zehr et al., 1998; 2001). In the PEQD province the N+N : SRP ratio associated with equatorial upwelling is closer to Redfield having a mean value of 7.6. With a N+N : SRP
60 ratio higher than that of oligotrophic regions one could expect a change in phytoplankton community structure, perhaps selecting against N2-fixing microorganisms.
While N+N and SRP are used to form soft tissues of organisms, Si(OH)4-2 is essential for the growth of the outer tests and frustules of plankton such as diatoms, silicoflagellates and radiolarians. The dissolved Si(OH)4-2 : N+N ratio in the surface waters throughout the BEACH-BASH transect almost always exceeded the I: I ratio typical for diatom growth (Brzezinski, 1985). However, the ratio ofSi(OH)4-2 : N+N in equatorial upwelled source waters was less than the I: I ratio, which complements findings by
Wilkerson and Dugdale (1996) for this region. Assuming nutrients are constantly upwelled, an elevated Si(OH)4-2: N+N ratio > 1:1 would be indicative ofN-limitation, while a depressed ratio would be indicative ofSi(OH)4-2Iimitation (Conley and Malone.
1992). One explanation for the occurrence ofa Si(OH)4-2: N+N ratio < 1:1 in the equatorial region is a process called the "silicate pump" described in Dugdale et al.
(1995). The silicate pump works to export Si(OH)4-2 to deeper waters more efficiently than N+N. due to the faster regeneration rates ofN+N in surface waters relative to
Si(0H>4-2, and to the enhanced export ofSi(OH)4-2 that can occur when frustules are repackaged into fecal pellets, thus driving the system to Si-limitation. Si(OH)4-2 uptake by diatoms depends on light, temperature, the supply of nutrients and trace metals which brings us to another possible explanation. It has been suggested that phytoplankton growth in the equatorial Pacific is limited by iron (Fe) aVailability (Coale et al., 1996).
Studies have shown that in oceanic Fe addition experiments, controls were usually dominated by nanoplankton and picoplankton, whereas diatoms bloomed in Fe-enriched
61 incubations (Martin et aI., 1988; Hutchins, 1995; DiTullio et al., 1993). Under Fe limitation, Si(OHU·2 : N+N uptake ratios have been shown to be higher, suggesting diatoms stressed by a lack of iron should therefore deplete surface waters of silicic acid before nitrate, leading to a secondary silicic acid limitation of the phytoplankton community (Hutchins and Bruland, 1998). These results suggest that Fe-limitation may be an important factor driving the silicate pump (Hutchins and Bruland, 1998).
The dissolved inorganic Si(OH)4·2 : SRP ratio in the surface waters throughout the
BEACH-BASH transect almost never exceeded the Redfield ratio 16:1 (Redfield et al.,
1963), except in the extreme north at Station ALOHA with a value of 18. This
Si(OH)4"2 : SRP ratio of < 16: 1 would be indicative of Si-limitation of diatoms and therefore a dominance of either other large phytoplankton or small phytoplankton. The
Si(OH)4"2 : SRP ratio increased north along the transect, due to the increase in Si(OH)4"2 concentration relative to SRP concentrations.
2.4.2 Phytoplankton size structure
The equatorial Pacific. between 5° S - 0° in the PEQD province, despite the presence of excess nutrients in surface waters, is similar to other oligotrophic provinces in the tropical and subtropical open ocean in being dominated by picoplankton (0.2-2 J.1IO). Primary production was not measured on this cruise however, Chavez (1989) showed that cells
< I J.1IO accounted for about 50 - 60 % ofChl a standing stock and 50 % of 14C primary production over a wide expanse of the equatorial Pacific from 5° N to 5° S and from 90° to 180° W. Similarly, PeDa et a/. (1990) showed that the
62 fractions contributed 20 - 51 % and 76 - 87 % respectively to the total Chl a at 135° W, and Bidigare and Ondrusek (1996) showed that during the EqPac studies, 81- 92 % of
Chl a passed through 2-1= filters at 140° W. The fluorometric Chl a results from these studies are in close agreement with our data, in which picoplankton (0.2-2 1=) in the
PEQD province accounted for - 60 % ofChl a standing stock and between 50 - 60 % throughout the transect (14° S - 22° N).
The picoplankton contribution to TChl a increased in the PEQD province but was highest in the nutrient deplete waters of the PNEC province, away from equatorial divergence, and lowest in the oligotrophic waters of the SPSG and NPTG. Despite elevated macro nutrient concentration in the PEQD, phytoplankton chlorophyll concentrations are considerably lower than expected (Thomas, 1979). This inhibition of phytoplankton growth in the equatorial Pacific has been attributed to HNLC conditions (Minas el aJ.,
1986). The dominance of relatively small photosYnthetic cells in waters replete with N, P and Si has been taken as indirect evidence that primary production in this region of upwelling is limited by iron or some other trace nutrient (Chisholm, 1992). One explanation is that smaller cells are better able to use low dissolved Fe concentrations and thus are able to out-compete larger phytoplankton for Fe (Hudson and Morel, 1990). The supply of iron to the central and eastern equatorial Pacific is primarily from the
Equatorial Undercurrent (EUC), in which iron is drawn up to the euphotic zone by upwelling and the upward vertical mixing ofEUC waters (Gordon el aJ., 1997). The increase in TChl a north of the equator (- 6° N) in the PNEC province could be explained by advection of newly upwelled equatorial waters northward. Mechanisms for advection
63 include Ekman transport and the meridional currents associated with TrW's. Chl a standing stock would also be expected to increase if there was a growth-grazing imbalance or an input of the micro-nutrient iron via advection or atmospheric deposition.
Information on what governs phytoplankton community size structure in marine ecosystems is crucial to understanding what governs carbon flux in the ocean. Theories on phytoplankton species composition and size are based on nutrient concentrations and sinking properties (Margalef, 1978; Legendre and Le Fevre, 1989). Large cells, such as diatoms are observed to dominate on a biomass basis in high nutrient areas, while small cells, such as cyanobacteria, dominate in oligotrophic waters of the world's ocean.
However, as stated before, the HNLC region of the equatorial Pacific waters is dominated by smaller cells typical of oligotrophic regions. While it is difficult to say much about limiting factors based on correlations of size structure and nutrient concentrations, patterns did emerge. Correlation analysis ofTChl a and size-fractionated Chl a (Figures
13a, b and c) indicated that phytoplankton had a significant relationship with some of the nutrient concentrations. TChl a and size-fractionated Chl a concentrations showed a significant correlation to SRP concentrations. This correlation between TChl a, size fractionated Chl a and SRP could indicate SRP plays a role in controlling photosynthetic phytoplankton biomass. A positive correlation between Chl a and SRP, was observed throughout the transect; however, one might expect the opposite (a negative correlation), because phytoplankton growth would increase Chl a coincident with a drawdown in nutrients. However, there may be an imbalance in the time it takes to assimilate nutrients and create chlorophyll which may lead to this positive correlation. N+N and Si(OHk2
64 showed no significant correlation to TChi a, however, Si(OH)4·2 showed significant correlations with nanoplankton of the 2-10 J.I.lll and 10-20 J.I.lll size fraction. This correlation between nanoplankton Chi a and Si(OH)4·2 may be an indication that
Si(OH)4·2 is controlling diatom abundance. Previous studies done by JGOFS, and IronEx
II, revealed an equatorial diatom community dominated by small « 10 J.I.lll), rapidly growing pennate diatoms, some of which were being grazed on by microzooplankton and therefore becoming part of the microbial food web (Latasa et aI., 1997; Landry et aZ.,
2000a; 2000b; 2003). N+N showed no significant correlation with TChi a or size fractionated Chi a, these results were similar to those observed by Chavez (1989).
Phycoerythrin is an accessory pigment which can be used as a biomarker for cyanobacteria (Synechococcus spp. and Trichodesmium spp.) and rhodophytes (Stewart and Farmer, 1984). The cyanobacteria Synechococcus spp. and Trichodesmium spp. that contain phycoerythrin have been found to be important contributors to primary production in the world's oceans (Waterbury et aZ., 1979; Letelier and Karl, 1996).
Surface phycoerythrin concentrations throughout the BEACH-BASH transect were overwhelmingly dominated by picoplankton (0.2-2 J.I.lll) accounting for 75 - 95 % of the total phycoerythrin. This dominance of picoplankton phycoerythrin throughout the transect closely matched Synechococcus spp. abundance and exhibit a significant correlation (r = 0.83). Neveux et al. (1999) also found significant correlations between phycoerythrin and Synechococcus spp. in equatorial and south Pacific waters. Their study also revealed Synechococcus spp. in oligotrophic and eutrophic waters obtained a different phycoerythrin excitation spectra indicating that different Synechococcus spp.
65 may have different ecological requirements. While 75 % of the total phycoerythrin in the
NPTG was attributed to picoplankton, in this province - 20 % of the total phycoerythrin was due to the contribution of organisms in the >20 J.Un and 10-20 J.Un size fraction.
Phycoerythrin associated with these size fractions is probably attributed to large cyanobacterial diazotrophs, which include filamentous and colonial Trichodesmium and diatoms containing endosymbiotic Richelia (Zehr et at., 2001; Karl et at., 2002; Dore et at., 2002).
There are a number of phytoplankton pigment markers like chlorophyll, carotenoids and phycoerythrin which are often used for estimating the presence of individual groups of microorganisms. ATP measurements, on the other hand, can and have been widely used to estimate total microbial biomass (Holm-Hansen and Booth, 1966; Karl, 1980; Karl and
Dobbs, 1998). This holds true, because without exception, all living organisms contain
ATP. Measurements of ATP concentrations are therefore capable of capturing organisms within a community that display a wide variety of nutritional modes such as photoautotrophy, mixotrophy and/or heterotrophy. P-ATP concentrations have been measured extensively in the NPTG province at Station ALOHA (Winn et at., 1995;
Bj6rkman and Karl, 2005). However, to my knowledge P-A TP concentrations have rarely, if ever been measured in the central equatorial Pacific. Throughout the BEACH
BASH transect P-ATP concentrations were highly variable and exhibited a rapid transition starting at the Equatorial Front (-5· S). This variability is most likely attributed to physical processes such as Kelvin waves which modulate the depth of the nutrients in the EUC and by TIW's which are a common phenomenon in the central equatorial
66 Pacific and result from the cyclonic shear between the EVC and the SEC (Luther and
Johnson, 1990).
Size-fractionated P-ATP measurements also exhibited regional variability. In the SPSG, picoplankton dominated total biomass, accounting for - 45 % of the total P-ATP, consistent with increased abundance ofheterotropbic bacteria in this province (Figure
17a). Since size-fractionated Chl a and phycoerythrin data suggests the dominance of photosynthetic picoplankton throughout the transect one would expect similar trends in
P-ATP. However, in PEQD, PNEC and NPTG provinces P-ATP concentrations are mainly dominated by organisms in the 2-10 f.UIl and >20 f.UIl size fraction, suggesting that larger heterotropbic organisms, not accounted for in Chl a measurements may be playing an important role in size structure and therefore food web dynamics. Hagstr6m et al.
(1988) suggested that although microplankton can effectively graze picoplankton populations, it is the nanoplankton protists that are responsible for the majority of picoplankton clearance. VIMS et aI. (1995) showed that during the EqPac studies the heterotrophic protistan communities were dominated by nanoplankton flagellates, such as dinoflagellates, ciliates, choanoflagellates as well as naked flagellates such as amoebae.
Brown et aI. (2003) also found that nearly all of the larger heterotrophic protistan ~ 8 f.UIl grazers were dinoflagellates.
2.4.3 Community structure and taxonomic composition
Picoplankton are now widely recognized as dominating biomass, production and metabolic activity in the open oceans (Williams, 1981; Azam et al., 1983; Li et al., 1983;
67 Takahashi and Bienfang, 1983; Murphy and Haugen, 1985; Iturriaga and Mitchel, 1986).
The composition of the microbial community in the equatorial Pacific has been characterized as being dominated by small cells and deficient in large bloom-forming diatoms (Le Bouteiller et al., 1992; Mackey et aI., 2002; Landry and Kirchman, 2002;
Brown et aZ., 2003). Flow cytometry has been instrwnental in demonstrating the structure of the picoplankton community in the equatorial Pacific. It has shown that the equatorial Pacific is dominated by heterotrophic bacteria, two populations of cyanobacteria (Prochlorococcus spp. and Synechococcus spp.) and a mixed population of picoeukaryotes. However, less well understood is the community iesponse to varying physical forces and the relationship between microbial abundances and nutrient concentrations.
The picoplankton of the tropical Pacific have been described as the background over which the dynamic of the larger organisms are imprinted (Landry and Kirchman, 2002).
The abundance of heterotrophic bacteria during the BEACH-BASH transect was highest in the PEQD province, just south of the equator in nutrient replete waters and generally decreased from equatorial to subtropical waters. These results are similar to those found by Landry and Kirchman (2002) which compared heterotrophic abundances from three different equatorial Pacific cruises (EqPac TIOII, EqPac TI007 and EBENE). Landry and Kirchman (2002) found that among the three cruises heterotrophic abundances were tightly constrained south of the equator and among the three cruises no systematic differences were observed. In contrast, north of the equator heterotrophic bacterial abundance differed greatly among the three cruises. Our data most closely resembled the
68 La Nifia EqPac cross equatorial transect (120 N - 120 S, 1400 W) cruise ITOll with relatively high abundances of heterotrophic bacteria to the north. Patterns in the abundance of heterotrophic bacteria and picopiankton in surface samples along the
o2 transect aligned with some of the prominent features in N+N, SRP and Si(OH)4
(Figure 18).
Throughout the BEACH-BASH transect Prochlorococcus spp. was by far the most nwnerica1ly dominant photosynthetic picoplankton. Prochlorococcus spp. exhibited a maximwn abundance occurring at 60 N, and a minimwn abundance in the SPSG, NPTG and in the region of equatorial upwelling in the PEQD. Thus, Prochlorococcus spp. probably accounted for most of the Chi a observed in the smallest (0.2-2 ~) size fraction. While previous studies also found Prochlorococcus spp. the most dominant autotrophic group, abundances observed in this study were a factor offour lower (Binder et 01., 1996; Blanchot et al., 200 I; Mackey et 01., 2002; Landry and Kirchman, 2002;
Brown et 01., 2003). Together, these past studies showed an average surface
Prochlorococcus spp. abundance of approximately 20 x 104 cells mrl whereas, average abundances during this study were approximately 5 x 104 cells ml°l. These differences in abundances could be attributed to seasonal or climatic variations (Landry et al., 1996).
Over the 15 years of records of Prochlorococcus spp. abundances in the surface waters of
Station ALOHA (Figure 23a) there were periods of relatively high (1995-2000) and relatively low Prochlorococcus sppo abundances (2000-2005). Karl et 01. (2001a) hypothesized that at Station ALOHA a possible "domain shift" occurred leading to an ecosystem dominated by prokaryotes in response to climatic variations. Perhaps these
69 40.------~------~----~ (a)
x E 30
a. a. (J) 20
~ '"tl 8 Qe 10 1i Qe
88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 Year 30 .------, (b) 20
10
o o (/) -10
-20
-30 _ SOl --- 5 month weighted mean ~O+--,_,--r-,-_r_.--,_,__r_,--,__r_._,r_.__._,--~ 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 Year
Figure 23. (a) Temporal di stribution of Prochlorococcus spp. abundance at Station ALOHA between 1991-2005 (integrated from 0-10 m) . (b) Southern Oscillation Index (SOl) between 1988-2007.
70 differences observed at Station ALOHA can be seen throughout the entire Pacific basin and are a reflection of climate forcing and the Southern Oscillation Index (SOl) (Figure
23b).
While it is difficult to say much about limiting factors based on correlations of plankton abundan~s and nutrient concentrations, patterns did emerge. Prochlorococcus spp. showed a slightly negative correlation with N+N and no correlation with SRP and
Si(0H)4·2. These results were similar to those found in the Atlantic by Cavender-Bares et al. (2001). Cavender-Bares et al. (2001) found that temperature was an important regulating factor for Prochlorococcus spp.; however, no relationship between temperature and Prochlorococcus spp. abundance was observed in this study (data not shown). Recent laboratory studies have shown that cultures of Prochlorococcus spp. cannot utilize nitrate for growth (Rippka et al., 2000; Moore et al., 2002), this could possibly explain the low abundances of Prochlorococcus spp. found in the N+N replete waters of the equatorial Pacific. Another possible explanation is perhaps
Prochlorococcus spp. are being out-competed when nutrients are abundant because they are either unavailable to them (e.g., nitrate) or their maximum growth rate is not as fast as that of other species such as Synechococcus spp. (Moore et al., 1995).
Surface Synechococcus spp. abundances throughout the BEACH-BASH transect
exhibited a local minimum in abundance centered about the equator (- 2° N) with higher abundance to the north (6° N) and to the south (S° S) in the N+N replete waters. The
lowest abundances of Synechococcus spp. were observed in the oligotrophic waters of the 71 SPSG and NPTG provinces. These results closely resemble those found during the
EqPac cross equatorial transect (120 N - 120 S. 1400 W) cruises IT007 and IT011 in
February-March 1992 and August-September 1992, respectively (Landry et al., 1996).
During these cruises, Landry et aI. (1996) found that climatic events such as El Nillo
(IT007) generally increased Synechococcus spp. abundances compared to La Nifta
(ITO 1 I) conditions. The BEACH-BASH transect took place during normal conditions a few months after a slight warming event and exhibited similar surface distributions to the
La Nifta EqPac cruise IT011. Surface Synechococcus spp. abundances along the transect
in contrast to Prochlorococcus spp. abundances correlated well with surface SRP and
N+N concentrations. Tbis may be because Synechococcus spp. can utilize nitrate as a nitrogen source (Waterbury et aI., 1986) and therefore possibly out compete
Prochlorococcus spp. which showed lower abundances in the N+N replete waters of the equatorial Pacific.
Picoeukaryote abundances were the lowest among the groups of photosynthetic picoplankton studied, exhibiting a similar distribution to Prochlorococcus spp. through out the transect. These results are similar to those found by Landry et al. (1996) where high surface abundances of picoeukaryotes occurred on the northern side of the transect during the EqPac cruise IT007. In contrast to previous comparisons in which
Synechococcus spp. and heterotrophic bacteria abundances closely matched the La Nifta
EqPac cruise IT01l, picoeukaryotes abundances matched the El Nillo EqPac cruise
IT007. Since the timing of the BEACH-BASH cruise took place during a normal ENSO phase, perhaps the picoplankton community response to environmental forcing reflects 72 ENSO phases. Picoeukaryote abundance correlated best with SRP and Si(OH)4-2 concentrations.
The distributions of chlorophyll and carotenoid pigments throughout the BEACH-BASH transect allowed us to examine the spatial variations in phytoplankton pigments, and taxonomic composition in the Pacific. TChl b [MVChl b + DVChl b] was a relatively abundant accessory chlorophyll in the equatorial and tropical Pacific. Potential sources for MVChl b detected in surface waters are not always clear (Neveux et al., 2003).
However, groups that typically contain MVChl b include chlorophytes and prasinophytes and DVChl b is a light harvesting pigment found exclusively in Prochlorococcus spp.
(Bidigare and Ondrusek, 1996). Concentrations of carotenoids relative to chlorophytes and prasinophytes were low and variable throughout the transect, these results matched those ofBidigare and Ondrusek (1996). This along with the work ofPartenskey et aZ.
(1993) which suggests that some strains of Prochlorococcus spp. are indeed able to synthesize MVChl b and DVChl b at high growth irradiances lead them and others
(Landry et al., 2000b and 2003) to conclude that most of the MVChl b and DVChl b at the surface in the enriched equatorial area probably belonged to Prochlorococcus spp.
TChl c concentrations were also an abundant accessory chlorophyll with highest concentrations occurring in the PEQD. Some of the primary organisms that contain
TChl c include prymnesiophytes, pelagophytes and diatoms (Jeffrey et al., 1997).
The ability to quantify reliably MVChl a, a pigment found in all phytoplankton (except
Prochlorococcus spp.) and DVChl a, a pigment marker found in Prochlorococcus spp.
73 allows us to look at the distribution of this prochlorophyte and to assess its contribution to the total plankton biomass «DVChl a I (MVChl a +DVChl a +MVChld a))*100 %).
Overall, DVChl a accounted for 50 - 60 % of the TChl a throughout the BEACH-BASH transect and agreed well with our flow cytometry data. This contribution ofDVChl a to
TChl a reinforces the importance of these picoplankton (i.e. Prochlorococcus spp.) to the phytobiomass over extensive oceanic regions. The latitudinal distribution shows a decrease in the contribution of DVChl a to TChl a in the PEQD province. Since
DVChl a, is a major pigment marker for Prochlorococcus spp. we could potentially attribute this drop in DVChl a to a possible nutrient limitation, such as Fe, or as
mentioned earlier to the enhancement ofN+N in surface waters. This is confirmed by
looking at the contribution of diagnostic pigments to chromophyte microalgae
(ChromoDP; i.e. prymnesiophytes, pelagophytes and diatoms) and to cyanobacteria and
prochlorophytes (CyanoDP; i.e. Prochlorococcus spp. and Synechococcus spp.).
Throughout the transect the concentrations ofCyanoDP dominated. These results
complement our size-fractionated and flow cytometry data obtained during this study.
However, in the PEQD province the proportion of CyanoDP decreased as ChromoDP
increased, suggesting that prymnesiophytes. pelagophytes and diatom abundances
increased in the nutrient rich waters of the equatorial Pacific although they did not
dominate. These high concentrations ofChromoDP (Hex, But, Fuco and TChl c) are
consistent with previous studies which report relatively high abundances of some if not
all of the these diagnostic pigments in the equatorial Pacific (Chavez et aZ., 1990;
Bidigare and Ondrusek, 1996; Mackey et al., 1998; Higgans and Mackey, 2000; Mackey
et al., 2002; Brown et aZ., 2003). 74 2.4.4 Implications for the biological pump
Currently, the conceptual view of carbon export from the oceanic surface layer is controlled by the biological transfo~ons that occur within the food web. Small photosynthetic cyanobacteria and heterotrophic bacteria which are thought to dominate oligotrophic open ocean ecosystems are believed to contribute relatively little to carbon export from the surface layers to the deep because of their small sizes, slow sinking rates and there efficient utilization of recycled organic carbon in the microbial loop (Legendre and Le Fevre, 1989; Michaels and Silver, 1998). Larger, faster growing phytoplankton such as diatoms are thought to dominate during increased nutrient availability and are believed to be more efficient in transferring carbon from the surface layers to the deep because of their large sizes, rapid sinking rates and their availability for predation by higher trophic levels (i.e. zooplankton and fish larvae; Legendre and Le Fevre, 1989;
Michaels and Silver, 1998) and for export through sedimentation (Ki0rboe, 1993).
However, here I have shown that in the equatorial Pacific, despite elevated macronutrient concentrations, surface waters are similar to oligotrophic provinces in being dominsted by small picoplankton (0.2-2 IIID). This therefore suggests that a strong and stable microbial food web exists in the equatorial Pacific, which is thought to derive from a
close coupling of growth, grazing and reminera1ization (Landry et al., 1991; 2000a;
2000b; Landry and Kirchman, 2002). Estimates of new production using stable nitrogen
isotopes have also found low f -ratios, which indicated highly efficient recycling of
nitrogen (Murray et aI., 1989; Price et al., 1991; Pella et al., 1992; Dugdale et al., 1992;
McCarthy et al., 1996).
15 A recent study by Richardson and Jackson (2007) which used an inverse model approach to trace carbon fluxes through food webs suggests that the contributions of picoplankton to carbon flux from the surface waters to the deep have been overlooked and that in fact picoplankton contribute to export in the form of aggregates, which create larger particles that can sink faster and contribute to export either directly though sinking or indirectly through consumption. The works of Smith et al. (1996) during the EqPac studies have reported abyssal accumulations of phytodetritus in the central equatorial Pacific. Another recent study by Salihoglu and Hofinann (2007) which used a one-dimensional multi component lower trophic level ecosystem model that included detailed algal physiology was used to investigate the response of phytoplankton communities and carbon production and export to variations in physical and biochemical processes in the equatorial Pacific. Their results show that high-frequency variability in vertical advection and temperature is an important mechanism driving carbon export. For example, times of increased stratification can result in decreased iron concentrations and therefore reduced vertical velocities which would lead to decreased carbon export. From these studies it is clear that both biological and physical processes contribute to the efficiency of the biological carbon pump and the resulting magnitude of carbon export to the deep sea.
76 CHAPTER 3. CONCLUSION
Investigations of nutrient inventory and dynamics, phytoplankton size structure, community composition and succession, and rates of primary productivity are key variables in marine ecological research. These processes depend on a variety of time and space scales and together determine the ocean's ability to sequester carbon dioxide from the atmosphere. The BEACH-BASH transect was a single cruise, or "snapshot in time", focusing on phytoplankton size structure and community composition across biogeochemical gradients in the Pacific Ocean. The grouping of stations into biogeochemical provinces as defined by Longhurst (1998), allowed me to compare data over an extensive spatial scale.
The observations made during this and previous studies have contributed to the knowledge of phytoplankton dynamics in the equatorial Pacific Ocean. Observed variations found during this and other studies were most likely caused by a combination of physical (Kelvin waves, TIWs and advection), chemical (iron limitation). biological
(growth-grazing imbalances) and climatic (EI Niiio or La Nifta ENSO phase) processes.
However, it is clear that the finer details of the observed variability in phytoplankton size-structure, abundances and community composition will only be resolved when more samples are collected with simultaneous measurements on the appropriate time and space scales. For example, moorings with sensors that can produce high resolution measurements on chemical and biological properties such as nitrate, oxygen and fluorescence would complement the Tropical Atmosphere Ocean (TAO) arrays which 77 measure physical and atmospheric properties throughout the equatorial Pacific.
Comparisons to Station ALOHA throughout this study bring about the importance of time-series measurements. Repeated oceanographic measurements are crucial to understanding natural habitat variability as well as climate influences. Therefore, conducting repeat transects in the future would facilitate a better understanding on the controlling factors of phytoplankton dynamics over wide spatial scales in the Pacific
Ocean and significantly improve our understanding of variability in the system. Lastly, the incorporation of data into models will aid in understanding and predicting the complex interaction between biological and chemical processes.
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