Abstract
A marked increase in global temperature over the last century was confirmed by the second Assessment Report of the Intergovernmental Panel on Climate Change1. Here we report significant positive and negative linear trends from 1948 to 1995 in phytoplankton measured by the Continuous Plankton Recorder survey in the northeast Atlantic and North Sea that might reflect a response to changing climate on a timescale of decades. Spreading of unusually cold waters2 from the Arctic might have contributed to the decline in phytoplankton north of 59o N. Further south, phytoplankton season length and abundance seem to have increased.
Main
The Continuous Plankton Recorder (CPR) survey3 is the only biological monitoring programme that operates on an ocean basin scale. Merchant ships voluntarily tow CPRs, at a depth of ∼10 m, on their normal routes of passage4. The survey is unique in that the methodology and plankton analysis procedures for ∼ 400 taxa and Phytoplankton Colour have been maintained with little change for more than 50 years.
Phytoplankton Colour is a visual index of chlorophyll based on the intensity of the green coloration of the CPR filtering silk, which is assigned numerical values in four categories. Calculated annual primary production averaged for seven areas of the North Sea5 is statistically comparable to mapped Phytoplankton Colour averaged for the same areas (r2=0.9, p<0.01), suggesting that the colour index might also reflect variation in plant production in the sea.
Results averaged for each month during 1948-95 for three areas of the North Sea and northeast Atlantic (Fig. 1) were analysed. An increasing trend is evident in the North Sea and the Atlantic between 52o and 58o N (Fig. 2), with evidence for a stepwise increase after the mid-1980s. The results for these areas show similar changes in trend and increased season length over the years 1981-91 to a satellite-derived vegetation index for the Northern Hemisphere above 50o N (ref. 6). An inverse pattern of change is seen in Phytoplankton Colour for the northern northeast Atlantic in an area that has tended to experience colder surface temperatures.
The eastward spread of negative-anomaly sea-surface temperatures towards the Faeroes in the northern northeast Atlantic2 is most likely to be related to progressive changes in regional atmospheric forcing. Changes in the North Atlantic Oscillation have altered the centre of deep-water convection from the Greenland Sea to the Labrador Sea after 1988 (ref. 7). The likely sources of these colder waters are stronger westerlies, increased formation of Arctic surface water in the Greenland Sea and/or a larger export of fresh water from melted ice8 and permafrost9 in and around the Arctic Sea as a response to high positive-temperature anomalies in northern Eurasia and Alaska.
Our observations provide the first marine surface time-series evidence for a vegetation response to what seems to be climatic forcing. This has implications for CO2 fluxes and the productivity of the North Atlantic. The results draw attention to the possible importance of shelf phytoplankton growth to the global carbon budget, and highlight the paucity of comparable information that exists as time series for other shelf regions and for the Southern Hemisphere.
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Reid, P., Edwards, M., Hunt, H. et al. Phytoplankton change in the North Atlantic. Nature 391, 546 (1998). https://doi.org/10.1038/35290
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DOI: https://doi.org/10.1038/35290
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