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) (see Communication arising in Nature). 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.
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.
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