Phytoplankton Colour Index Research Articles

The fifth shade of green: A novel approach to phytoplankton color index assessment in an oligotrophic system

AbstractMeasurements of phytoplankton color index (PCI), an indicator of phytoplankton biomass, represent one of the largest datasets acquired through the continuous plankton recorder (CPR) survey. PCI has been used to describe changes in the spatial and temporal distribution of phytoplankton, especially in the North Atlantic, where the methodology was developed and primarily applied. The method functions by grouping samples, based on their color shade, into four categories. Each category is assigned a numerical value to aid spatial and temporal averaging. Recently a CPR has been operated in the Eastern Mediterranean, one of the most oligotrophic areas of the global oceans. To capture the specific characteristics of the area, an additional greenness grading is introduced. In this study, we use satellite‐derived chlorophyll a (Chl a) data to examine whether PCI categories can be distinguished within the low and narrow range of Chl a values found in the study area and to calculate the range of Chl a concentrations expressed by each category. Furthermore, a newly calculated numerical system for the general use of PCI in oligotrophic areas and previously calculated PCI numerical value systems utilizing a variety of in situ, laboratory and remote sensing experiments, were tested for their applicability in the area. Present results indicate the feasibility of employing PCI measurements to provide an estimate of Chl a concentration in the Eastern Mediterranean. This provides confidence in the implementation of a new category system for PCI, as well as the use of the new PCI numerical system for the assessment of phytoplankton in oligotrophic waters.

Biogeography of key mesozooplankton species in the North Atlantic and egg production of <i>Calanus finmarchicus</i>

Abstract. Here we present a new, pan-North-Atlantic compilation of data on key mesozooplankton species, including the most important copepod, Calanus finmarchicus. Distributional data of eight representative zooplankton taxa, from recent (2000–2009) Continuous Plankton Recorder data, are presented, along with basin-scale data of the phytoplankton colour index. Then we present a compilation of data on C. finmarchicus, including observations of abundance, demography, egg production and female size, with accompanying data on temperature and chlorophyll. This is a contribution by Canadian, European and US scientists and their institutions: http://doi.pangaea.de/10.1594/PANGAEA.820732, http://doi.pangaea.de/10.1594/PANGAEA.824423, http://doi.pangaea.de/10.1594/PANGAEA.828393 (please also see Melle et al., 2013; Castellani and Licandro, 2013; Jónasdóttir et al., 2014).

Evidencing a regime shift in the North Sea using early-warning signals as indicators of critical transitions

One of the longest marine monitoring programs in the North Sea has been the spatiotemporal surveying of subsurface plankton since 1931. During this period a regime shift was detected in the late 1980s culminating in marked changes in phytoplankton, zooplankton and in the fisheries of horse mackerel. Here we used the phytoplankton colour index, a visual biomass estimate, from 1948 to 2010 and total diatom abundance from 1958 to 2010 to test whether the well-documented regime shift could have been anticipated by the recently developed Early-Warning Signals for critical transitions (EWS). We estimated EWS, namely autocorrelation and standard deviation, within moving windows along the time series prior to the regime shift. We found that both statistics increased revealing that the North Sea ecosystem was becoming progressively unstable prior to the regime shift. Moreover, this high-resolution time series permitted us to test for robustness, error and significance of the EWS. We did that by dividing the time series into independent blocks and estimating EWS after bootstrapping and randomising the blocks. This alternative approach confirmed the robustness of the EWS with limited associated errors. In particular, we found that the warning was significantly evident years before the onset of the regime shift. We conclude that EWS may provide robust and timely warning for upcoming regime shifts depending on the quality and quantity of recorded data in marine ecosystems.

Understanding Long-Term Changes in Species Abundance Using a Niche-Based Approach

One of the major challenges to understanding population changes in ecology for assessment purposes is the difficulty in evaluating the suitability of an area for a given species. Here we used a new simple approach able to faithfully predict through time the abundance of two key zooplanktonic species by focusing on the relationship between the species’ environmental preferences and their observed abundances. The approach is applied to the marine copepods Calanus finmarchicus and C. helgolandicus as a case study characterising the multidecadal dynamics of the North Sea ecosystem. We removed all North Sea data from the Continuous Plankton Recorder (CPR) dataset and described for both species a simplified ecological niche using Sea Surface Temperature (SST) and CPR Phytoplankton Colour Index (PCI). We then modelled the dynamics of each species by associating the North Sea’s environmental parameters to the species’ ecological niches, thus creating a method to assess the suitability of this area. By using both C. finmarchicus and C. helgolandicus as indicators, the procedure reproduces the documented switches from cold to warm temperate states observed in the North Sea.

Multidecadal Atlantic climate variability and its impact on marine pelagic communities

A large scale analysis of sea surface temperature (SST) and climate variability over the North Atlantic and its interactions with plankton over the North East Atlantic was carried out to better understand what drives both temperature and species abundance. The spatio-temporal pattern of SST was found to correspond to known climate indices, namely the Atlantic Multidecadal Oscillation (AMO), the East Atlantic Pattern (EAP) and the North Atlantic Oscillation (NAO). The spatial influence of these indices is heterogeneous. Although the AMO is present across all regions, it is most strongly represented in the SST signal in the subpolar gyre region. The NAO instead is strongly weighted in the North Sea and the pattern of its influence is oscillatory in space with a wavelength of approximately 6000km. Natural oscillations might obscure the influence of climate change effects, making it difficult to determine how much of the variation is attributable to longer term trends. In order to separate the influences of different climate signals the SST signals were decomposed in to spatial and temporal components using principal component analysis (PCA). A similar analysis is carried out on various indicator species of plankton: Calanus finmarchicus, Phytoplankton Colour Index and total copepod abundance, as well as phytoplankton and zooplankton communities. By comparing the two outputs it is apparent that the dominant driver is the recent warming trend, which has a negative influence on C. finmarchicus and total copepods, but has a positive one on phytoplankton colour. However natural oscillations also influence the abundance of plankton, in particular the AMO is a driver of diatom abundance. Fourier principal component analysis, an approach which is novel in terms of the ecological data, was used to analyse the behaviour of various communities averaged over space. The zooplankton community is found to be primarily influenced by climate warming trends. The analysis provides compelling evidence for the hypothesis that cold water species are gradually being replaced by more temperate species in the North Atlantic. This may have detrimental effects for the entire marine ecosystem, by affecting on organisms such as fish larva for example. The second group, a phytoplankton subset consisting primarily of diatom species, is primarily influenced by the AMO rather than the average temperature trend. This result highlights the importance of natural oscillations to certain functional groups, in particular those subgroups which are less directly metabolically affected by changes in temperature.

The response of phytoplankton to climate variability associated with the North Atlantic Oscillation

Inter-annual variability in the timing of phytoplankton spring bloom and phytoplankton community structure in the central North Atlantic Ocean was quantified using ocean color data and continuous plankton recorder (CPR) data. This variability was related to the North Atlantic Oscillation using correlation analysis and multivariate auto-regression models. The initiation of the spring bloom derived from CPR phytoplankton color index data is similar to that derived from satellite chlorophyll, and exhibits a nominal correlation with the sea surface temperature (SST) and the North Atlantic Oscillation (NAO). The extrapolated spring bloom timing suggested later initiation of blooms in the mid-1980s and earlier initiation of blooms in the 1990s. The climatological phytoplankton community structure in the central North Atlantic is dominated by diatoms, except for a shift in community composition favoring dinoflagellates in August. The ratio of diatoms to total phytoplankton abundance and the ratio of dinoflagellates to total phytoplankton abundance are both closely correlated with the NAO and SST. The extended time series of phytoplankton community structure between 1985 and 2009, deduced from the time series of SST and NAO over the same interval, showed a decadal shift away from diatoms towards dinoflagellates. The linkages between the NAO, and changes in stratification and phytoplankton processes occur over a larger scale than previously observed.

A 60-year ocean colour data set from the continuous plankton recorder

The phytoplankton colour index (PCI) of the Continuous Plankton Recorder (CPR) survey is an in situ measure of ocean colour, which is considered a proxy of the phytoplankton biomass. PCI has been extensively used to describe the major spatiotemporal patterns of phytoplankton in the North Atlantic Ocean and North Sea since 1931. Regardless of its wide application, the lack of an adequate evaluation to test the PCI's quantitative nature is an important limitation. To address this concern, a field trial over the main production season has been undertaken to assess the numerical values assigned by previous investigations for each category of the greenness of the PCI. CPRs were towed across the English Channel from Roscoff to Plymouth consecutively for each of 8 months producing 76 standard CPR samples, each representing 10 nautical miles of tow. The results of this experiment test and update the PCI methodology, and confirm the validity of this long-term in situ ocean colour data set. In addition, using a 60-year time series of the PCI of the western English Channel, a comparison is made between the previous and the current revised experimental calculations of PCI.

Is there a decline in marine phytoplankton?

Arising from D. G. Boyce, M. R. Lewis & B. Worm , 591–596 (2010)10.1038/nature09268 ; Boyce et al. reply Phytoplankton account for approximately 50% of global primary production, form the trophic base of nearly all marine ecosystems, are fundamental in trophic energy transfer and have key roles in climate regulation, carbon sequestration and oxygen production. Boyce et al.1 compiled a chlorophyll index by combining in situ chlorophyll and Secchi disk depth measurements that spanned a more than 100-year time period and showed a decrease in marine phytoplankton biomass of approximately 1% of the global median per year over the past century. Eight decades of data on phytoplankton biomass collected in the North Atlantic by the Continuous Plankton Recorder (CPR) survey2, however, show an increase in an index of chlorophyll (Phytoplankton Colour Index) in both the Northeast and Northwest Atlantic basins3,4,5,6,7 (Fig. 1), and other long-term time series, including the Hawaii Ocean Time-series (HOT)8, the Bermuda Atlantic Time Series (BATS)8 and the California Cooperative Oceanic Fisheries Investigations (CalCOFI)9 also indicate increased phytoplankton biomass over the last 20–50 years. These findings, which were not discussed by Boyce et al.1, are not in accordance with their conclusions and illustrate the importance of using consistent observations when estimating long-term trends.

Have jellyfish in the Irish Sea benefited from climate change and overfishing?

AbstractClimate change and overfishing may lead to ecosystem instability and may benefit nonexploited organisms such as jellyfish. In the Irish Sea, an increase in jellyfish abundance was evident (r2=0.29, P=0.03) in a 16‐year time‐series (1994–2009) collected during juvenile fish surveys. Jellyfish abundance correlated positively with sea surface temperature (SST) over the preceding 18 months (r=0.65, pACF<0.001) and copepod biomass in the previous year (r=0.56, pACF=0.03) and negatively with spring (February–May) precipitation (r=−0.57, pACF=0.02). Principal components regression indicated that climatic indices explained 68% of the interannual variability in jellyfish abundance (P=0.003), where the components were based on the North Atlantic Oscillation Index, SST and precipitation. The frequency of cnidarian material present in Continuous Plankton Recorder (CPR) samples has also increased since 1970, with a period of frequent outbreaks between 1982 and 1991. Before this period, the herring stock in the northern Irish Sea declined rapidly to a low level, potentially stimulating structural change in the ecosystem. In 1985, there was a step decrease in CPR copepod biomass and in 1989, a step increase in the phytoplankton colour index, suggesting a cascading regime shift during the 1980s. Subsequent overexploitation of gadids, coupled with warm temperatures and the poor recruitment of cod, led to the rapid decline in cod biomass from 1990. While the biomass of sprat has decreased in the last decade, the herring stock has recovered partially. Reductions in demersal fishing pressure since 2000, intended to stimulate cod recovery, appear to have facilitated further rises in haddock biomass. Since the 1980s regime shift, sea temperatures have increased, the fish community has altered and jellyfish abundance has risen such that jellyfish and haddock may now play an increasingly important role in the ecosystem.

Monitoring changes in phytoplankton abundance and composition in the Northwest Atlantic: a comparison of results obtained by continuous plankton recorder sampling and colour satellite imagery

Phytoplankton abundance in the NW Atlantic was measured by continuous plankton recorder (CPR) sampling along tracks between Iceland and the western Scotian Shelf from 1998 to 2006, when sea-surface chlorophyll (SSChl) measurements were also being made by ocean colour satellite imagery using the SeaWiFS sensor. Seasonal and inter-annual changes in phytoplankton abundance were examined using data collected by both techniques, averaged over each of four shelf regions and four deep ocean regions. CPR sampling had gaps (missing months) in all regions and in the four deep ocean regions satellite observations were too sparse between November and February to be of use. Average seasonal cycles of SSChl were similar to those of total diatom abundance in seven regions, to those of the phytoplankton colour index in six regions, but were not similar to those of total dinoflagellate abundance anywhere. Large inter-annual changes in spring bloom dynamics were captured by both samplers in shelf regions. Changes in annual (or 8 months) averages of SSChl did not generally follow those of the CPR indices within regions and multi-year averages of SSChl, and the three CPR indices were generally higher in shelf than in deep ocean regions. Remote sensing and CPR sampling provide complementary ways of monitoring phytoplankton in the ocean: the former has superior temporal and spatial coverage and temporal resolution, and the latter provides better taxonomic information.

Biodiversity as a dynamic variable in the Gulf of Maine continuous plankton recorder transect

Studies relating biodiversity to ecosystem processes typically do not take into account changes in biodiversity through time. Marine systems are highly dynamic, with biodiversity changing at diel, seasonal and inter-decadal timescales. We examined the dynamics of biodiversity in the Gulf of Maine pelagic zooplankton community. Taxonomic data came from the Gulf of Maine continuous plankton recorder (CPR) transect, spanning the years 1961–2006. The CPR transect also contains coincident information on temperature and phytoplankton biomass (measured by the phytoplankton color index). Taxonomic richness varied at all timescales considered. The relationships between temperature and richness, and between phytoplankton and richness, also depended on temporal scale. The temperature–richness relationship was monotonic at the multi-decadal scale, and tended to be hump-shaped at finer scales; the productivity–richness relationship was hump-shaped at the multi-decadal scale, and tended to be monotonic at finer scales. Seasonal biodiversity dynamics were linked to temperature; inter-decadal biodiversity dynamics were linked to phytoplankton.

Decadal variability in biogeochemical models: Comparison with a 50‐year ocean colour dataset

Assessing the skill of biogeochemical models to hindcast past variability is challenging, yet vital in order to assess their ability to predict biogeochemical change. However, the validation of decadal variability is limited by the sparsity of consistent, long‐term biological datasets. The Phytoplankton Colour Index (PCI) product from the Continuous Plankton Recorder survey, which has been sampling the North Atlantic since 1948, is an example of such a dataset. Converting the PCI to chlorophyll values using SeaWiFS data allows a direct comparison with model output. Here we validate decadal variability in chlorophyll from the GFDL TOPAZ model. The model demonstrates skill at reproducing interannual variability, but cannot simulate the regime shifts evident in the PCI data. Comparison of the model output, data and climate indices highlights under‐represented processes that it may be necessary to include in future biogeochemical models in order to accurately simulate decadal variability in ocean ecosystems.

Decadal changes in climate and ecosystems in the North Atlantic Ocean and adjacent seas

Climate change is unambiguous and its effects are clearly detected in all functional units of the Earth system. This study presents new analyses of sea-surface temperature changes and show that climate change is affecting ecosystems of the North Atlantic. Changes are seen from phytoplankton to zooplankton to fish and are modifying the dominance of species and the structure, the diversity and the functioning of marine ecosystems. Changes also range from phenological to biogeographical shifts and have involved in some regions of the Atlantic abrupt ecosystem shifts. These alterations reflect a response of pelagic ecosystems to a warmer temperature regime. Mechanisms are complex because they are nonlinear exhibiting tipping points and varying in space and time. Sensitivity of organisms to temperature changes is high, implicating that a small temperature modification can have sustained ecosystem effects. Implications of these changes for biogeochemical cycles are discussed. Two observed changes detected in the North Sea that could have opposite effects on carbon cycle are discussed. Increase in phytoplankton, as inferred from the phytoplankton colour index derived from the Continuous Plankton Recorder (CPR) survey, has been detected in the North Sea. This pattern has been accompanied by a reduction in the abundance of the herbivorous species Calanus finmarchicus. This might have reduced the grazing pressure and increase diatomaceous ‘fluff’, therefore carbon export in the North Sea. Therefore, it could be argued that the biological carbon pump might increase in this region with sea warming. In the meantime, however, the mean size of organisms (calanoid copepods) has dropped. Such changes have implications for the turnover time of biogenic carbon in plankton organisms and the mean residence time of particulate carbon they produce. The system characterising the warmer period is more based on recycling and less on export. The increase in the minimum turnover time indicates an increase in the ecosystem metabolism, which can be considered as a response of the pelagic ecosystems to climate warming. This phenomenon could reduce carbon export. These two opposite patterns of change are examples of the diversity of mechanisms and pathways the ecosystems may exhibit with climate change. Oversimplification of current biogeochemical models, often due to lack of data and biological understanding, could lead to wrong projection on the direction ecosystems and therefore some biogeochemical cycles might take in a warmer world.

Driving-forces for Greenland Offshore Groundfish Assemblages: Interplay of Climate, Ocean Productivity and Fisheries

Principal component analysis of German survey data from 1981 to 2006 yielded four main trend components accounting for 60% of variance of the demersal fish assemblage on the Greenland shelf. The leading principal component (prin1) accounted for 22.5% of total variance of abundance and was positively correlated with deep-sea and golden redfish >17 cm (Sebastes mentella and S. marinus, respectively). Prin2 accounted for 16.7% of total variance of abundance and was correlated to juvenile redfish <17 cm. Prin3 and prin4 accounted for 12.9% and 7.5% of total variance, respectively. American plaice (Hippoglossoides platessoides) was attributed to prin3. Atlantic cod (Gadus morhua) was almost equally assigned to prin3 and prin4. Prin1 and prin4 were significantly correlated with air temperature anomalies from Nuuk (p <0.01), representing the climate effect on the demersal assemblage. Prin2 and prin3 were significantly correlated with the July–August phytoplankton colour index for East Greenland representing ocean productivity and fishing mortality from cod VPA analysis, the latter indicating fishing pressure effects on the ecosystem. About 30% of total variance can be explained by climate. For East Greenland, strong bottom-up forcing (climate, productivity) and effects of fisheries were identified, whereas for West Greenland both bottom-up (climate) and top-down forcing (community interactions) were effective. With respect to Atlantic cod, fisheries in the case of prin3 and climate in the case of prin4 were equally important to explain long-term dynamics.

A long‐term chlorophyll dataset reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient levels

During the 1980s, a rapid increase in the Phytoplankton Colour Index (PCI), a semiquantitative visual estimate of algal biomass, was observed in the North Sea as part of a regionwide regime shift. Two new data sets created from the relationship between the PCI and SeaWiFS chlorophyll a (Chl a) quantify differences in the previous and current regimes for both the anthropogenically affected coastal North Sea and the comparatively unaffected open North Sea. The new regime maintains a 13% higher Chl a concentration in the open North Sea and a 21% higher concentration in coastal North Sea waters. However, the current regime has lower total nitrogen and total phosphorus concentrations than the previous regime, although the molar N: P ratio in coastal waters is now well above the Redfield ratio and continually increasing. Besides becoming warmer, North Sea waters are also becoming clearer (i.e., less turbid), thereby allowing the normally light‐limited coastal phytoplankton to more effectively utilize lower concentrations of nutrients. Linear regression analyses indicate that winter Secchi depth and sea surface temperature are the most important predictors of coastal Chl a, while Atlantic inflow is the best predictor of open Chl a; nutrient concentrations are not a significant predictor in either model. Thus, despite decreasing nutrient concentrations, Chl a continues to increase, suggesting that climatic variability and water transparency may be more important than nutrient concentrations to phytoplankton production at the scale of this study.

Differential contribution of diatoms and dinoflagellates to phytoplankton biomass in the NE Atlantic Ocean and the North Sea

Sampling by the continuous plankton recorder (CPR) survey over the North Atlantic Ocean and the North Sea has enabled long-term studies of phytoplankton biomass. Analysis of an index of phytoplankton biomass, the phytoplankton colour index (PCI), has previously shown an increase in phytoplankton biomass in the NE Atlantic. In the current study, further investigations were conducted to determine the contribution of diatom and dinoflagellate cell counts to the PCI, their fluctuations over the last 45 yr and their geographical variations in the eastern North Atlantic and the North Sea. An increased contribution of dinoflagellates to the PCI was revealed over the south NE Atlantic and the northern North Sea. In contrast, the contribution of diatoms decreased in the north NE Atlantic and the northern North Sea. No discernible trends were found in the other regions of the North Sea. The relative contributions of diatoms and dinoflagellates to the PCI led to the identification of 3 geographically distinct dynamic regimes in the diatom/dinoflagellate dynamics in the NE Atlantic and the North Sea. Finally, it is stressed that the discrepancy observed in the pat- terns of PCI and diatom and dinoflagellate cell counts suggests that changes in PCI do not reflect changes in the community structure and that the exclusive use of PCI is not adequate to investigate the long-term trends in the trophic link between phytoplankton and herbivorous zooplankton.

Decadal basin-scale changes in diatoms, dinoflagellates, and phytoplankton color across the North Atlantic

The Continuous Plankton Recorder (CPR) survey has been used to characterize phytoplankton and zooplankton space-time dynamics in the North Sea since 1931 and in the North Atlantic since 1939. Phytoplankton biomass is assessed from these samples by visual assessment of the green color of the silk mesh, the Phytoplankton Color Index (PCI), and the total count of diatoms and dinoflagellates. Species with a frequency of occurrence greater than 1% in the samples are used as indicator species of the community. We investigated (1) long-term fluctuations of phytoplankton biomass, total diatoms, and total dinoflagellates; (2) geographical variation of patterns; (3) the relationship between phytoplankton and climate forcing in the North Atlantic CPR samples; (4) the relative contribution of diatoms and dinoflagellates to the PCI; and (5) the fluctuations of the dominant species over the period of survey to provide more information on the processes linking climate to changes in the phytoplankton community. As a result of the differences in microscopic analysis methods prior to 1958, our analyses were conducted for the period ranging from 1958 to 2002. The North Atlantic was divided into six regions identified through bathymetric criteria and separated along a North-South axis. Based on 12 monthly time series, we demonstrate increasing trends in PCI and total dinoflagellates and a decrease in total diatoms.

Extending the SeaWiFS chlorophyll data set back 50 years in the northeast Atlantic

Phytoplankton play a key role in biogeochemical cycling and climate processes. Precise quantitative measurements of chlorophyll‐a (Chl‐a), a measure of phytoplankton biomass, have only been available globally since 1997 from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS). In the North Atlantic, semi‐quantitative measurements of chlorophyll (Phytoplankton Color Index, PCI) for &gt;50 years have been collected by the Continuous Plankton Recorder. Here we demonstrate a significant correlation between PCI and SeaWiFS Chl‐a from 1997–2002. Combining both time series allows quantification of the stepwise increase in biomass in the mid‐1980s; this regime shift corresponded to a 60% increase in Chl‐a. This was a result of an 80% increase in Chl‐a during winter, alongside a smaller summer increase. This new high‐resolution data set on the monthly variation of Chl‐a in the North Atlantic since 1948 is now available for the development and validation of climate models, and for interpretation of ecological changes related to climate.

Phytoplankton biomass from continuous plankton recorder data: an assessment of the phytoplankton colour index

A 15 month time-series of simultaneous fluorometric measurements of chlorophyll a, and phytoplankton cell abundance and a phytoplankton colour index (PCI) from continuous plankton recorder (CPR) samples are compared for the Iberian margin, western Europe. The relationships between fluorometrically determined chlorophyll, cell abundance and PCI are all significant, with PCI showing a better relationship to chlorophyll than cell abundance. This supports previous research which suggests that the PCI is a useful index of phytoplankton standing stock and, furthermore, may be a measure of cells that are not preserved or counted in the routine microscopic processing of CPR samples. The relationship between PCI and chlorophyll appears to vary seasonally, although more extensive data are needed to quantify this variability. The study also compares the PCI and fluorometer data with chlorophyll values obtained from satellite imagery (SeaWiFS) for the Iberian margin. Significant correlations exist during the 15 month time-series between these three estimates of phytoplankton concentration. This result demonstrates that the ~50 year time-series of PCI for the North Atlantic can provide valuable information on changes in phytoplankton standing stock.

Increased blooms of a dinoflagellate in the NW Atlantic

Sampling by the Continuous Plankton Recorder (CPR) over the NW Atlantic from 1960 to 2000 has enabled long-term studies of the larger components of the phytoplankton community, highlighting various changes, particularly during the 1990s. Analysis of an index of phytoplankton biomass, the Phytoplankton Colour Index (PCI) has revealed an increase over the past decade, most marked during the winter (December to February) months. Examination of the structure of the community using multiple linear-regression models indicates that the winter phytoplankton community composition has changed markedly in the 1990s compared to the 1960s. One phytoplankter, the dinoflagellate Ceratium arcticum (Cleve), has undergone dramatic changes in abundance during this period, with pronounced large winter blooms and decreased autumnal levels, and its contribution to the Phytoplankton Colour index values has increased significantly. Other dominant species in the phytoplankton community, both diatoms and dinoflagellates, did not show the same variations over the examined time period. It is suggested that the response of C. arcticum is probably a result of previously reported changes in stratification in the NW Atlantic, due to dynamic hydro-climatic (freshening and cooling) events.