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Latitudinal Variation in Plankton Size Spectra along the Atlantic Ocean

(master's thesis)

Abstract

Abundance-size distributions of organisms within a community reflect fundamental properties underlying population dynamics. These include characteristics such as predator to prey biomass ratios, given the relationships that exist between body mass and metabolic activity, and between body mass and the ecological regulation of population density. In this way, plankton size has an important role in structuring the rates and pathways of material transfer in the marine pelagic food web, and consequently the oceanic carbon cycle. The transfer of energy between trophic levels can be inferred from regular patterns in population size structure, where plots of abundance within size classes, also known as plankton size spectra, typically show a power-law dependence on size. Metabolic theory, based on such size relations, has provided the basis for using an allometric approach to investigate the metabolic balance of the Atlantic Ocean and to identify the main drivers of trophic status in the plankton community.

Samples were collected during three Atlantic Meridional Transect (AMT) cruises with further samples from a Marine Productivity (MarProd) cruise in the Irminger Sea. Three image analysis instruments were used to obtain plankton size spectra in the pico- to mesozooplankton size range. Data from a decadal time series at a coastal station off Plymouth, UK additionally enabled seasonal trends in plankton size spectra to be interpreted. Allometric relationships were also derived from physiological rates of individual plankton and scaled from organisms to ecosystems using community size structure data that were obtained from six earlier AMT cruises.

Contrary to common perception, the transfer efficiency between phytoplankton and mesozooplankton in the Atlantic was not related to ecosystem productivity in oceanic and coastal systems. The flow of carbon up the food web was controlled by how quickly the consumers are able to respond to a resource pulse. These findings have fundamental implications for upper ocean carbon flux and suggest that global carbon flux models should reconsider the differences in carbon transfer efficiency between productive and oligotrophic areas of the world's ocean.

The allometric models of microbial community respiration and production provide a complementary method for understanding the metabolic balance of the upper ocean. Respiration exceeded photosynthesis in large areas of the Atlantic Ocean, suggesting that planktonic communities act as potential net sources of CO2. Large-sized phytoplankton are suggested as the main drivers of the balance between net autotrophy and heterotrophy.

CONTENTS

CHAPTER 1: INTRODUCTION

1.1 Atlantic Meridional Transect (AMT) Programme

1.2 Community size structure

1.2.1 Why size matters

1.2.2 Scaling in biology

Energetic equivalence rule

1.2.3 Community size spectra

Theory of plankton size spectra

Limitations to size spectra theory

1.2.4 Factors shaping plankton size spectra

Ecosystem stability

Pelagic food web

Flux of organic matter

Phytoplankton productivity

Metabolic balance in the oceans

Mesozooplankton of the Atlantic Ocean

1.3 Hypothesis and objectives

CHAPTER 2: METHODS

2.1 Sample collection

2.1.1 AMT cruise programme

2.1.2 CTD sampling

2.1.3 Net sampling

2.2 Sample analysis

2.2.1 Carbon and Nitrogen analysis

2.2.2 Size structure analysis

Nanoplankton and microplankton

Mesozooplankton

Direct measurements versus estimates of carbon biomass

Picoplankton

2.2.3 Marine Productivity research programme

2.3 Data processing

2.3.1 L4 plankton monitoring programme

2.3.2 Quantification of plankton size structure

i. Size classes

ii. Normalised biomass-size spectra

iii. Mean organism size

2.4 Scaling the metabolic balance of the oceans

2.4.1 Error propagation

CHAPTER 3: LATITUDINAL VARIATION IN PLANKTON SIZE SPECTRA ALONG THE ATLANTIC

3.1 Introduction

3.1.1 Atlantic Ocean Circulation

3.1.2 Oceanic provinces

3.2 Large scale changes in community size structure

3.2.1 Vertical structure

Nano- to microplankton NB-S spectra

3.2.2 Latitudinal variation

Community NB-S spectra

Mesozooplankton size structure

Total biomass and abundance

3.3 Comparison between cruises

3.3.1 AMT 12 and 14

3.3.2 AMT 13

Seasonal/spatial variability

3.4 Discussion

3.4.1 Latitude

3.4.2 Vertical structure

3.4.3 Seasonality

3.5 Conclusions

CHAPTER 4: CAN THE SLOPE OF A PLANKTON SIZE SPECTRUM BE USED AS AN INDICATOR OF CARBON FLUX?

4.1 Introduction

4.1.1. Ocean carbon cycle

4.1.2 Trophic transfer efficiency

4.2 Large scale spatial variability

4.2.1 Latitudinal distributions of temperature

4.2.2 Spatial structure of nitrate

4.2.3 Chlorophyll a

4.3 Factors influencing plankton size spectra

4.3.1 Temperature

4.3.2 Nutrients

4.4 Consumer versus resource control of the pelagic community

4.4.1 Chlorophyll a

4.4.2 Primary Production

4.4.3 Community interactions

4.5 Other carbon flux descriptors

4.5.1 Metabolic balance

4.5.2 Thorium estimates of C export

4.6 Discussion

4.6.1 Environment

Temperature

Nutrients

4.6.2 Turnover of material

4.6.3 Trophic status

4.6.4 Carbon export

4.6.5 Trophic transfer efficiency

4.6.6 Some limitations to the size spectra approach

4.7 Conclusions

CHAPTER 5: TEMPORAL VARIABILITY OF PLANKTON SIZE SPECTRA

5.1 Introduction

5.1.1 The continental shelf

5.2 Community structure

5.2.1 Microbial community

5.2.2 Mesozooplankton community

5.3 Plankton size spectra

5.3.1 Average size spectrum

5.3.2 Seasonality

Across trophic levels

Within trophic levels

5.3.3 Interannual variability

5.4 Discussion

5.4.1 Between trophic levels

5.4.2 Within trophic levels

5.5 Conclusions

CHAPTER 6: SCALING THE METABOLIC BALANCE IN THE OCEANS

6.1 Introduction

6.1.1 Allometry

6.1.2 Abundance-size structure

6.2 Empirical allometric models

6.2.1 Respiration

6.2.2 Primary production

6.3 Comparison between allometric estimates with traditional in situ measurements

6.3.1 Respiration

6.3.2 Primary production

6.4 Metabolic balance in the Atlantic

6.5 Discussion

Implications to the global CO₂ budget

Traditional incubations versus metabolic theory

Further caveats

Energetic equivalence rule

6.6 Conclusions

CHAPTER 7: DISCUSSION

LIST OF TABLES

Table 2.1. Summary of cruises.

Table 2.2. Plankton size categories.

Table 2.3. Results of the least-squares (model 1) regression of size fractionated biomass estimates derived from CHN and PVA analyses.

Table 3.1. Methods used to measure the community size structure.

Table 3.2. Mean parameters of the least-squares (model 1) regression of the NB-S model for each % light level.

Table 3.3. Mean parameters of depth-integrated community NB-S slopes for each oceanic province on the AMT (50-0 m) and the MarProd cruises (120-0 m).

Table 3.4. Mean 50-0 m depth-integrated mesozooplankton and nano-/microplankton biomass, abundance and size for each oceanic province.

Table 4.1. List of data obtained to compare to plankton NB-S slopes.

Table 4.2. Regression analysis of the least squares (model 1) regression between the community slope and biomass.

Table 5.1. Data available for community and within trophic level spectra analysis at L4.

Table 5.2. Regression analyses of the least-squares (model 1) regression between NB-S slopes and biomass.

Table 5.3 Least-squares (model 1) regression of within trophic level spectra.

Table 6.1. Summary statistics for the metabolic scaling of the individual physiological rates of marine plankton. 25

LIST OF FIGURES

Figure 1.1. SeaWIFS image of mean surface chlorophyll concentration in the Atlantic Ocean with AMT cruise tracks shown.

Figure 1.2. A graphical model to explain the variance of biomass as a function of body size of pelagic organisms in ocean and lake ecosystems a) within trophic levels and b) across trophic levels.

Figure 1.3. A schematic of the NB-S spectrum (Platt and Denman 1978).

Figure 1.4. A simplified diagram of the main trophic pathways in the planktonic food web.

Figure 2.1. AMT cruise tracks.

Figure 2.2. Comparison between CHN and PVA estimates of mesozooplankton a) total biomass and b) size fractionated biomass estimates.

Fig. 2.3. Location of L4 coastal station.

Figure 2.4. 50-0 m depth-integrated NB-S spectra of the pico-, nano- to microplankton and mesozooplankton community at a) 1?N on AMT 12, b) 35?N on AMT 13, c) 41?S on AMT 13 and d) 24?S on AMT 14.

Figure 3.1. Major surface currents and upwelling zones of the Atlantic Ocean.

Figure 3.2. Location of the 82 AMT and 13 MarProd stations sampled and the oceanic provinces that were crossed.

Figure 3.3. Cluster analysis of the pigment distribution on AMT 12.

Figure 3.4. Latitudinal variation in i) the vertical structure of discrete nano-/microplankton NB-S slopes with superimposed temperature contours in black and ii) the depth of the mixed layer along AMT 12-14.

Figure 3.5. All the depth profiles from AMT 12-14 of discrete nano-/microplankton NB-S slopes plotted against percentage light level.

Figure 3.6. Regression of nano-/microplankton abundance against the y-intercepts of the linear fits to discrete NB-S spectra.

Figure 3.7. The latitudinal pattern of AMT (50-0 m) and MarProd (120-0 m) depth-integrated community NB-S slopes in each oceanic province.

Figure 3.8. Relationship between latitude and depth-integrated community NB-S slopes.

Figure 3.9. MDS ordination of size fractionated plankton biomass integrated from 50-0 m along AMT 12-14 with superimposed a) total biomass b) oceanic provinces and c) cruise tracks.

Figure 3.10. Percentage of total mesozooplankton abundance in each size fraction in 200-0 and 120-0 m for AMT 12-14 and MarProd cruises respectively. a) >1000, b) 500-1000 and c) 200-500 мm.

Figure 3.11. Scanned images from a) the Mauritenean upwelling station at 21?N on AMT 13 and b) the arctic station in the Irminger Sea at 61?N on the MarProd cruise.

Figure 3.12. Latitudinal variation in mean mesozooplankton equivalent spherical diameter (ESD) in the upper 50 m along AMT 12-14 and upper 120 m on MarProd.

Figure 3.13. The variation in total mesozooplankton carbon from a) 200-0 m and b) 50-0 m nets along the AMT.

Figure 3.14. Scanned images from the northern temperate stations on AMT 14 at a) 42?N and b) 49?N, which were analysed using the Plankton Visual Analyser (PVA).

Figure 3.15. Latitudinal variation in a) estimated biomass and b) abundance of (i) mesozooplankton and (ii) nano-/microplankton integrated over the upper 50 m and 120 m along AMT 12-14 and MarProd respectively.

Figure 3.16. Relationship between nano-/microplankton biomass and NB-S slopes of depth-integrated community over the upper 50 m and 120 m along AMT 12-14 and MarProd respectively.

Figure 3.17. Regression between mean nano-/microplankton ESD (мm) and discrete nano-/microplankton NB-S slopes.

Figure 4.1. The oceanic carbon cycle.

Figure 4.2. Spatial distribution of temperature along AMT 12-14.

Figure 4.3. Spatial distribution of the concentration of nitrate from discrete bottle data during AMT 12-14.

Figure 4.4. Latitudinal distribution of chlorophyll a concentration during AMT 12-14.

Figure 4.5. Relationship between discrete in situ temperature and the discrete nano-/microplankton NB-S slope.

Figure 4.6. Effect of sea surface temperature (SST) on 50-0 m depth-integrated NB-S slopes of a) the nano- to mesozooplankton community and b) the pico- to mesozooplankton community.

Figure 4.7. Relationship between the concentration of in situ nitrate and the NB-S slope of the discrete nano-/microplankton community size spectrum.

Figure 4.8. Regression between maximum nitrate concentrations in 300-0 m and NB-S slopes of the nano- to mesozooplankton and pico- to mesozooplankton community.