Phytoplankton produce half of the world’s oxygen, comprise the base of the marine food web, and play an important role in carbon cycling and climate regulation. Changes in phytoplankton communities impact climate processes and all trophic levels of marine ecosystems, from zooplankton to fish to whales.

In 2010 research scientist Daniel Boyce and colleagues at Dalhousie University created an index of phytoplankton biomass by combining records of water transparency with in situ chlorophyll concentrations to create a dataset spanning a >100 year time period. This new time-series showed a worrying decline in global marine phytoplankton biomass of approximately 1% per year over the last century.

Boyce’s results, however, were not in agreement with research using data from the Continuous Plankton Recorder (CPR) survey—the world’s largest marine macroecological dataset (Figure 1). The CPR survey, coordinated by the Sir Alister Hardy Foundation for Ocean Science (SAHFOS) in Plymouth, UK, has sampled the surface waters of the North Atlantic since 1931, collecting plankton with a high-speed recorder. In addition to identifying and counting ~500 phyto- and zooplankton taxa, CPR analysts also produce an index of chlorophyll: the Phytoplankton Colour Index (PCI), a visual estimate of phytoplankton biomass (see Box). More than five decades of PCI data show a clear increase in phytoplankton biomass in both the north-east and north-west Atlantic basins (Figure 2). The 1980s saw a rapid increase in phytoplankton biomass in the North-east Atlantic. We now know this dramatic change was part of a regime shift—a climate-driven stepwise change in the structure and functioning of the north-east Atlantic marine ecosystem. Cuts in funding resulted in the loss of CPR routes in the north-west Atlantic in the 1980s but they were resumed in the 1990s revealing that, as in the north-east Atlantic, PCI had increased. Post- 2000 most North Atlantic regions have higher PCI than in past decades, the open ocean included.

Fig. 1. The Continuous Plankton Recorder (CPR) was developed by Sir Alister Hardy (right) as a way to collect information on food for fish stocks. The CPR’s current design is virtually the same as the model used in 1931, which makes the CPR dataset one of the world’s longest and most spatially extensive macroecological time-series, with over 6 million nautical miles of ocean sampled (above—each orange dot is one sample). Uniquely, CPR data are collected using ships of opportunity, such as cargo vessels and ferries, rather than expensive research vessels. The ships’ crews tow the CPRs as a greatly-appreciated gesture of goodwill which has enabled the survey to cost-effectively collect data for the past eight decades.

The above CPR findings contradict the decline in phytoplankton biomass described by Boyce et al. this could be due to differences in the consistency of the phytoplankton biomass datasets used to estimate the long-term trends. For the first 50 years of the Boyce et al. time-series most of the phytoplankton biomass estimates were derived from measurements of water transparency collected using a Secchi disc; later, chlorophyll sampling became a standard oceanographic procedure and after 1980 most of the data were from chlorophyll measurements. The ‘mixed’ dataset of Boyce et al. does not take into account that the relationship between water transparency and chlorophyll concentration may vary geographically or temporally, and may therefore be biased. Additionally, throughout the world’s oceans, even in regions with low productivity, water transparency is influenced by sediment and other non-living suspended particles and by dissolved organic matter, not only by phytoplankton chlorophyll. Therefore, water transparency measurements may not accurately reflect the amount of phytoplankton biomass in the water. In contrast, since 1931 the PCI has been derived for more than 6 million nautical miles of ocean (> 250,000 analysed samples) which have been directly sampled by ships of opportunity towing the CPR. The virtually unchanged methodology and consistent long-term time-series makes the CPR survey a robust source of plankton data.

(Right) Fig. 2. Since the 1950s the Phytoplankton Colour Index from the Continuous Plankton Recorder survey has shown a clear increase in phytoplankton biomass throughout much of the North Atlantic basin.

The increase in phytoplankton biomass observed by the CPR is supported by data from other long-term time-series, including the Hawaii Ocean Time-series (HOT), the Bermuda Atlantic Time-series (BATS), and the California Cooperative Oceanic Fisheries Investigations (CalCOFI) which also show increased phytoplankton biomass during the last 20–50 years. This considerable body of data contrasts with the results presented by Boyce et al., indicating that there is no strong evidence for a marked decline in global marine phytoplankton. Additionally,remote sensing data suggest that changes in phytoplankton biomass are not globally uniform, with biomass increasing in some marine regions while remaining stable or decreasing in others. More work exploring regional changes in phytoplankton biomass, and the drivers behind these changes, is clearly needed. Investigation into which components of the phytoplankton are driving the regional increases or decreases in phytoplankton biomass could provide information about future responses to climate change or food web alterations. Continuous long-term time-series of plankton community composition are rare, but the CPR survey’s extensive 80 year dataset can be used for analysis of North Atlantic phytoplankton community dynamics, including changes in individual taxa and functional groups, which may offer insight into observed changes in phytoplankton biomass. Long-term ecological time-series such as the ones mentioned here are crucial for filling scientific knowledge gaps about changes in our seas and for providing robust evidence to support decisions regarding the management of the marine environment.

The Phytoplankton Colour Index (PCI) is a visual estimate of phytoplankton biomass derived from Continuous Plankton Recorder (CPR) samples; PCI is essentially a measure of silk ‘greenness’. Chlorophyll in phytoplankton cells colour the silk, with high biomass samples stained dark green while samples with low biomass are pale in colour. Each CPR sample is compared to a standard colour chart with four values ranging from ‘No Green’ to ‘Green’. PCI is semi-quantitative and has been successfully intercalibrated with measurements of both flourometric and satellite chlorophyll. Acetone extraction experiments revealed that the PCI colour categories equate to a ratio scale, with Pale Green silks containing twice the amount of chlorophyll as Very Pale Green silks, while Green silks are 6.5 times richer in chlorophyll than Very Pale Green silks. Though the CPR’s mesh size of 270 µm is considered large when it comes to plankton sampling, the device consistently collects small cells, such as coccolithophores, on the silk with recent work suggesting that the relative contribution of smaller size phytoplankton to the PCI is increasing in some regions. The PCI also accounts for fragile, broken and fragmented cells that contribute to phytoplankton biomass but are not morphologically identifiable.

Further Reading

Boyce, D. G., Lewis, M. R. & Worm, B. (2010) Global phytoplankton decline over the past century. Nature 466, 591-596.

Reid, P. C., Colebrook, J. M., Matthews, J. B. L. & Aiken, J. (2003) The Continuous Plankton Recorder: concepts and history, from plankton indicator to undulating recorders. Prog. Oceanogr. 58, 117-173.

Edwards, M., Reid, P. C. & Planque, B. (2001) Long-term and regional variability of phytoplankton biomass in the Northeast Atlantic (1960-1995). ICES Journal of Marine Science 58, 39-49.

Head, E. J. H. & Pepin, P. Spatial (2010) and inter-decadal variability in plankton abundance and composition in the Northwest Atlantic (1958-2006). Journal of Plankton Research 32, 1633-1648.

Reid, P. C., Edwards, M., Hunt, H. G. & Warner, A. J. (1998) Phytoplankton change in the North Atlantic. Nature 391, 546.

Raitsos, D. E., Reid, P. C., Lavender, S. J., Edwards, M. & Richardson, A. J. (2005) Extending the SeaWiFS chlorophyll data set back 50 years in the northeast Atlantic. Geophysical Research Letters 32, L06603.

McQuatters-Gollop, A. et al. (2007) A long-term chlorophyll dataset reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient levels. Limnology and Oceanography 52, 635-648.

Beaugrand, G. (2004) The North Sea regime shift: Evidence, causes, mechanisms and consequences. --> 60, 245-262.

McQuatters-Gollop, A. et al. (2009) How well do ecosystem indicators communicate the effects of anthropogenic eutrophication? Estuarine, Coastal and Shelf Science 82, 583–596.

Saba, V. S. et al. (2010) Challenges of modeling depth-integrated marine primary productivity over multiple decades: A case study at BATS and HOT. Global Change Biology 24, in press, DOI:10.1029/2009GB003655.

Kahru, M., Kudela, R., ManzanoSarabia, M. & Mitchell, B. G. (2009) Trends in primary production in the California Current detected with satellite data. Geophysical Research Letters 114, C02004, DOI:10.1029/2008JC004979.

Gregg, W. W. & Conkright, M. E. (2002) Decadal changes in global ocean chlorophyll. Geophysical Research Letters 29, 1730, DOI:10.1029/2002GL014689.

Batten, S. D. et al. (2003) CPR sampling: the technical background, materials and methods, consistency and comparability. Prog. Oceanogr. 58, 193-215.

Leterme, S. C., Seuront, L. & Edwards, M. (2006) Differential contribution of diatoms and dinoflagellates to phytoplankton biomass in the NE Atlantic Ocean and the North Sea. Marine Ecology Progress Series 312, 57-65.

Author

Abigail McQuatters-Gollop

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