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.

Ecological effects of the North Atlantic oscillation

Climatic oscillations as reflected in atmospheric modes such as the North Atlantic Oscillation (NAO) may be seen as a proxy for regulating forces in aquatic and terrestrial ecosystems. Our review highlights the variety of climate processes related to the NAO and the diversity in the type of ecological responses that different biological groups can display. Available evidence suggests that the NAO influences ecological dynamics in both marine and terrestrial systems, and its effects may be seen in variation at the individual, population and community levels. The ecological responses to the NAO encompass changes in timing of reproduction, population dynamics, abundance, spatial distribution and interspecific relationships such as competition and predator-prey relationships. This indicates that local responses to large-scale changes may be more subtle than previously suggested. We propose that the NAO effects may be classified as three types: direct, indirect and integrated. Such a classification will help the design and interpretation of analyses attempting to relate ecological changes to the NAO and, possibly, to climate in general.

Can the subtropical North Atlantic permanent thermocline be observed from space?

The analysis of remotely sensed altimeter data and in situ measurements shows that ERS 2 radar can monitor the ocean permanent thermocline from space. The remotely sensed sea level anomaly data account for similar to 2/3 of the temperature variance or vertical displacement of isotherms at a depth of similar to 550 m in the Subtropical North Atlantic Ocean near 32.5 degree N. This depth corresponds closely to the region of maximum temperature gradient in the permanent thermocline where near semi-annual internal vertical displacements reach 200 to 300 m. The gradient of the altimeter sea level anomaly data correlates well with measured ocean currents to a depth of 750 m. It is shown that observations from space can account for similar to 3/4 of the variance of ocean currents measured in situ in the permanent thermocline over a 2-y period. The magnification of the permanent thermocline displacement with respect to the displacement of the sea surface was determined as - x650 and gives a measure of the ratio of barotropic to baroclinic decay scale of geostrophic current with depth. The overall results are used to interpret an eight year altimeter data tie series in the Subtropical North Atlantic at 32.5 degree N which shows a dominant wave or eddy period near 200 days, rather than semi-annual and increases in energy propagating westward in 1995 (west of 25 degree W). The effects of rapid North Atlantic Oscillation climate change on ocean circulation are discussed. The altimeter data for the Atlantic were Fourier analysed. It is shown how the annual and semi-annual components relate to the seasonal maximum cholorophyll-a SeaWiFS signal in tropical and equatorial regions due to the lifting of the thermocline caused by seasonally varying ocean currents forced by wind stress.

Phytoplankton change in the North Atlantic

A marked increase in global temperature over the last century was confirmed by the second Assessment Report of the Intergovernmental Panel on Climate Change. 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 waters 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.

Interregional Biological Responses in the North Atlantic to Hydrometeorological Forcing

Using data from the CPR survey seven case studies are described that document different spatial and temporal responses in the plankton to hydroclimatic events. Long-term trends in the plankton of the eastern Atlantic and the North Sea over the last five decades are examined. Two of the examples revisit correlations that have been described between copepod abundance in the eastern Atlantic and North Sea and indices of atmospheric variability, the North Atlantic Oscillation index and the Gulf Stream North Wall index. Evidence for an increase in levels of Phytoplankton Colour (a visual index of chlorophyll) on the eastern and western sides of the Atlantic is presented. Changes in three trophic levels and in the hydrodynamics and chemistry of the North Sea circa 1988 are outlined as a regime shift. Two of the case studies emphasise the importance of variability in oceanic advection into shelf seas and the role of western and eastern margin currents at the shelf edge. The plankton appear to be integrating hydrometeorological signals and reflecting basin scale changes in circulation of surface, intermediate and deep waters in part associated with the NAO. The extent to which climatic variability may be contributing to the observed changes in the plankton is discussed with a forecast of potential future ecosystem effects in a climate change scenario.

Phaeocystis colony distribution in the North Atlantic Ocean since 1948, and interpretation of long-term changes in the Phaeocystis hotspot in the North Sea

Monitoring of Phaeocystis since 1948 during the Continuous Plankton Recorder survey indicates that over the last 5.5 decades the distribution of its colonies in the North Atlantic Ocean was not restricted to neritic waters: occurrence was also recorded in the open Atlantic regions sampled, most frequently in the spring. Apparently, environmental conditions in open ocean waters, also those far oVshore, are suitable for complete lifecycle development of colonies (the only stage recorded in the survey). In the North Sea the frequency of occurrence was also highest in spring. Its southeastern part was the Phaeocystis abundance hotspot of the whole area covered by the survey. Frequency was especially high before the 1960s and after the 1980s, i.e., in the periods when anthropogenic nutrient enrichment was relatively low. Changes in eutrophication have obviously not been a major cause of long-term Phaeocystis variation in the southeastern North Sea, where total phytoplankton biomass was related signiWcantly to river discharge. Evidence is presented for the suggestion that Phaeocystis abundance in the southern North Sea is to a large extent determined by the amount of Atlantic Ocean water Xushed in through the Dover Strait. Since Phaeocystis plays a key role in element Xuxes relevant to climate the results presented here have implications for biogeochemical models of cycling of carbon and sulphur. Sea-to-air exchange of CO2 and dimethyl sulphide (DMS) has been calculated on the basis of measurements during single-year cruises. The considerable annual variation in phytoplankton and in its Phaeocystis component reported here does not warrant extrapolation of such figures.

Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas

Global climate change is expected to modify the spatial distribution of marine organisms. However, projections of future changes should be based on robust information on the ecological niche of species. This paper presents a macroecological study of the environmental tolerance and ecological niche (sensu Hutchinson 1957, i.e. the field of tolerance of a species to the principal factors of its environment) of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas. Biological data were collected by the Continuous Plankton Recorder (CPR) Survey, which samples plankton in the North Atlantic and adjacent seas at a standard depth of 7 m. Eleven parameters were chosen including bathymetry, temperature, salinity, nutrients, mixed-layer depth and an index of turbulence compiled from wind data and chlorophyll a concentrations (used herein as an index of available food). The environmental window and the optimum level were determined for both species and for each abiotic factor and chlorophyll concentration. The most important parameters that influenced abundance and spatial distribution were temperature and its correlates such as oxygen and nutrients. Bathymetry and other water-column-related parameters also played an important role. The ecological niche of C. finmarchicus was larger than that of C. helgolandicus and both niches were significantly separated. Our results have important implications in the context of global climate change. As temperature (and to some extent stratification) is predicted to continue to rise in the North Atlantic sector, changes in the spatial distribution of these 2 Calanus species can be expected. Application of this approach to the 1980s North Sea regime shift provides evidence that changes in sea temperature alone could have triggered the substantial and rapid changes identified in the dynamic regimes of these ecosystems. C. finmarchicus appears to be a good indicator of the Atlantic Polar Biome (mainly the Atlantic Subarctic and Arctic provinces) while C. helgolandicus is an indicator of more temperate waters (Atlantic Westerly Winds Biome) in regions characterised by more pronounced spatial changes in bathymetry.

Coccolithophore surface distributions in the North Atlantic and their modulation of the air-sea flux of CO2 from 10 years of satellite Earth observation data

Coccolithophores are the primary oceanic phytoplankton responsible for the production of calcium carbonate (CaCO3). These climatically important plankton play a key role in the oceanic carbon cycle as a major contributor of carbon to the open ocean carbonate pump (similar to 50 %) and their calcification can affect the atmosphere-to-ocean (air-sea) uptake of carbon dioxide (CO2) through increasing the seawater partial pressure of CO2 (pCO(2)). Here we document variations in the areal extent of surface blooms of the globally important coccolithophore, Emiliania huxleyi, in the North Atlantic over a 10-year period (1998-2007), using Earth observation data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). We calculate the annual mean sea surface areal coverage of E. huxleyi in the North Atlantic to be 474 000 +/- 104 000 km(2), which results in a net CaCO3 carbon (CaCO3-C) production of 0.14-1.71 Tg CaCO3-C per year. However, this surface coverage (and, thus, net production) can fluctuate inter-annually by -54/+81% about the mean value and is strongly correlated with the El Nino/Southern Oscillation (ENSO) climate oscillation index (r = 0.75, p < 0.02). Our analysis evaluates the spatial extent over which the E. huxleyi blooms in the North Atlantic can increase the pCO(2) and, thus, decrease the localised air-sea flux of atmospheric CO2. In regions where the blooms are prevalent, the average reduction in the monthly air-sea CO2 flux can reach 55%. The maximum reduction of the monthly air-sea CO2 flux in the time series is 155 %. This work suggests that the high variability, frequency and distribution of these calcifying plankton and their impact on pCO(2) should be considered if we are to fully understand the variability of the North Atlantic air-to-sea flux of CO2. We estimate that these blooms can reduce the annual N. Atlantic net sink atmospheric CO2 by between 3-28 %.

Rapid biogeographical plankton shifts in the North Atlantic Ocean

Large-scale biogeographical changes in the biodiversity of a key zooplankton group (calanoid copepods) were detected in the north-eastern part of the North Atlantic Ocean and its adjacent seas over the period 1960–1999. These findings provided key empirical evidence for climate change impacts on marine ecosystems at the regional to oceanic scale. Since 1999, global temperatures have continued to rise in the region. Here, we extend the analysis to the period 1958–2005 using all calanoid copepod species assemblages (nine species assemblages based on an analysis including a total of 108 calanoid species or taxa) and show that this phenomenon has been reinforced in all regions. Our study reveals that the biodiversity of calanoid copepods are responding quickly to sea surface temperature (SST) rise by moving geographically northward at a rapid rate up to about 23.16 km yr−1. Our analysis suggests that nearly half of the increase in sea temperature in the northeast Atlantic and adjacent seas is related to global temperature rises (46.35% of the total variance of temperature) while changes in both natural modes of atmospheric and oceanic circulation explain 26.45% of the total variance of temperature. Although some SST isotherms have moved northwards by an average rate of up to 21.75 km yr−1 (e.g. the North Sea), their movement cannot fully quantify all species assemblage shifts. Furthermore, the observed rates of biogeographical movements are far greater than those observed in the terrestrial realm. Here, we discuss the processes that may explain such a discrepancy and suggest that the differences are mainly explained by the fluid nature of the pelagic domain, the life cycle of the zooplankton and the lesser anthropogenic influence (e.g. exploitation, habitat fragmentation) on these organisms. We also hypothesize that despite changes in the path and intensity of the oceanic currents that may modify quickly and greatly pelagic zooplankton species, these organisms may reflect better the current impact of climate warming on ecosystems as terrestrial organisms are likely to significantly lag the current impact of climate change.

Relationships between North Atlantic salmon, plankton and hydroclimatic change in the Northeast Atlantic

The abundance of wild salmon (Salmo salar) in the North Atlantic has declined markedly since the late 1980s as a result of increased marine mortality that coincided with a marked rise in sea temperature in oceanic foraging areas. There is substantial evidence to show that temperature governs the growth, survival, and maturation of salmon during their marine migrations through either direct or indirect effects. In an earlier study (2003), long-term changes in three trophic levels (salmon, zooplankton, and phytoplankton) were shown to be correlated significantly with sea surface temperature (SST) and northern hemisphere temperature (NHT). A sequence of trophic changes ending with a stepwise decline in the total nominal catch of North Atlantic salmon (regime shift in ∼1986/1987) was superimposed on a trend to a warmer dynamic regime. Here, the earlier study is updated with catch and abundance data to 2010, confirming earlier results and detecting a new abrupt shift in ∼1996/1997. Although correlations between changes in salmon, plankton, and temperature are reinforced, the significance of the correlations is reduced because the temporal autocorrelation of time-series substantially increased due to a monotonic trend in the time-series, probably related to global warming. This effect may complicate future detection of effects of climate change on natural systems.

Temporal and spatial variability of Oithona spp. abundance and phenology in the North Atlantic, North Pacific and Southern Ocean using the CPR data

The genus Oithona is considered the most ubiquitous and abundant copepod group in the world oceans. Although they generally make-up a lower proportion of the total copepod biomass, because of their high numerical abundance, preferential feeding for microzooplankton and motile preys, Oithona spp. plays an important role in microbial food webs and can provide a food source for other planktonic organisms. Thus, changes in Oithona spp. overall abundance and the timing of their annual maximum (i.e. phenology) can have important consequences for both energy flow within marine food webs and secondary production. Using the long term data (1954-2005) collected by the Continuous Plankton Recorder (CPR), the present study, investigates whether global climate warming my have affected the long term trends in Oithona spp. population abundance and phenology in relation to biotic and abiotic variables and over a wide latitudinal range and diverse oceanographic regions in the Atlantic, Pacific and Southern Ocean.

Macrophysiology of Calanus finmarchicus in the North Atlantic Ocean

Copepods represent the major part of the dry weight of the mesozooplankton in pelagic ecosystems and therefore have a central role in the secondary production of the North Atlantic Ocean. The calanoid copepod species Calanus finmarchicus is the main large copepod in subarctic waters of the North Atlantic, dominating the dry weight of the mesozooplankton in regions such as the northern North Sea and the Norwegian Sea. The objective of this work was to investigate the relationships between both the fundamental and realised niches of C. finmarchicus in order to better understand the future influence of global climate change on the abundance, the spatial distribution and the phenology of this key-structural species. Based on standardised Principal Component Analyses (PCAs), a macroecological approach was applied to determine factors affecting the spatial distribution of C. finmarchicus and to characterise its realised niche. Second, an ecophysiological model was used to calculate the Potential Egg Production Rate (PEPR) of C. finmarchicus and the centre of its fundamental niche. Relationships between the two niches were then investigated by correlation analysis. We found a close relationship between the fundamental and realised niches of C. finmarchicus at spatial, monthly and decadal scales. While the species is at the centre of its niche in the subarctic gyre, our joint macroecological and macrophysiological analyses show that it is at the edge of its niche in the North Sea, making the species in this region more vulnerable to temperature changes.

Update on the relationship between the North Atlantic Oscillation and Calanus finmarchicus

The relationship between climate, represented by the North Atlantic Oscillation (NAO), and the calanoid copepod Calanus finmarchicus has been extensively studied. The correlation between NAO and C. finmarchicus has broken down (post-1995). In the present study, we revisit the relationship between C. finmarchicus and the NAO. Our reanalysis shows that previous treatment of this data did not take into account 2 aspects of both the C. finmarchicus and NAO index time-series: (1) the presence of significant trends and (2) significant autocorrelation. Our analysis suggests that previously reported relationships between NAO and C. finmarchicus abundance can be explained largely by the trends in both data series. Removing the trend from both time-series resulted in a decrease in the amount of C. finmarchicus abundance variability explained by the NAO. Trend removal eliminated the autocorrelation from the NAO time-series, but not from the C. finmarchicus time-series. Partial autocorrelation analysis showed that the autocorrelation present in the C. finmarchicus time-series is only found at a lag of 1 yr, suggesting strong, year-to-year connectivity in this population. We included the lagged C. finmarchicus abundance into a regression with the NAO and found that C. finmarchicus variability is explained by the previous year’s abundance and, to a much smaller extent, by NAO variability. Limiting the time-series to the most recent 22 yr period (1981 to 2002) showed that the NAO is no longer correlated to C. finmarchicus abundance, and the autocorrelation in the C. finmarchicus abundance series also appears to be weakening.

High dispersal potential has maintained long-term population stability in the North Atlantic copepod Calanus finmarchicus

The cool-water copepod Calanus finmarchicus is a key species in North Atlantic marine ecosystems since it represents an important food resource for the developmental stages of several fish of major economic value. Over the last 40 years, however, data from the Continuous Plankton Recorder survey have highlighted a 70 per cent reduction in C. finmarchicus biomass, coupled with a gradual northward shift in the species's distribution, which have both been linked with climate change. To determine the potential for C. finmarchicus to track changes in habitat availability and maintain stable effective population sizes, we have assessed levels of gene flow and dispersal in current populations, as well as using a coalescent approach together with palaeodistribution modelling to elucidate the historical population demography of the species over previous changes in Earth's climate. Our findings indicate high levels of dispersal and a constant effective population size over the period 359 000–566 000 BP and suggest that C. finmarchicus possesses the capacity to track changes in available habitat, a feature that may be of crucial importance to the species's ability to cope with the current period of global climate change.

A biological consequence of reducing Arctic ice cover: arrival of the Pacific diatom Neodenticula seminae in the North Atlantic for the first time in 800,000 years

The Continuous Plankton Recorder survey has monitored plankton in the Northwest Atlantic at monthly intervals since 1962, with an interegnum between 1978 and 1990. In May 1999, large numbers of the Pacific diatom Neodenticula seminae were found in Continuous Plankton Recorder (CPR) samples in the Labrador Sea as the first record in the North Atlantic for more than 800 000 years. The event coincided with modifications in Arctic hydrography and circulation, increased flows of Pacific water into the Northwest Atlantic and in the previous year the exceptional occurrence of extensive ice-free water to the North of Canada. These observations indicate that N. seminae was carried in a pulse of Pacific water in 1998/early 1999 via the Canadian Arctic Archipelago and/or Fram Strait. The species occurred previously in the North Atlantic during the Pleistocene from similar to 1.2 to similar to 0.8 Ma as recorded in deep sea sediment cores. The reappearance of N. seminae in the North Atlantic is an indicator of the scale and speed of changes that are taking place in the Arctic and North Atlantic oceans as a consequence of regional climate warming. Because of the unusual nature of the event it appears that a threshold has been passed, marking a change in the circulation between the North Pacific and North Atlantic Oceans via the Arctic. Trans-Arctic migrations from the Pacific into the Atlantic are likely to occur increasingly over the next 100 years as Arctic ice continues to melt affecting Atlantic biodiversity and the biological pump with consequent feedbacks to the carbon cycle.