Regime shifts in the breeding of an Atlantic puffin population

Timing of breeding is a key factor determining the reproductive success in bird populations and known to be affected by climate fluctuations. We investigated the long‐term (1978–2002) relationship between climate and hatching date within a population of Atlantic puffin Fratercula arctica at Røst in the Norwegian Sea. The timing of puffin breeding was found to be influenced by the North Atlantic Oscillation winter index (NAO). We isolated two temporal regimes, one where NAO had a significant effect on hatching date (1978–1986 and 1995–2002) and one where these variables were independent (1987–1994). Hatching date could be modelled using, in addition to NAO, hatching date and food abundance in the preceding breeding season (possibly proxies of parental effort). The models remained significant for regime 1 but not for regime 2. NAO differed between the two regimes suggesting that the shifts were induced by climate change, possibly via its effect on the availability of prey in the preceding year. The novelty of our study is the identification of temporal regimes in the effects of climate within one population.     

Scale-dependent climate signals drive breeding phenology of three seabird species

Breeding at the right time is essential for animals in seasonal climates in order to ensure that the energy demands of reproduction, particularly the nutritional requirements of growing young, coincide with peak food availability. Global climate change is likely to cause shifts in the timing of peak food availability, and in order to adapt successfully to current and future climate change, animals need to be able to adjust the time at which they initiate breeding. Many animals use environmental cues available before the breeding season to predict the seasonal peak in food availability and adjust their phenology accordingly. We tested the hypothesis that regulation of breeding onset should reflect the scale at which organisms perceive their environment by comparing phenology of three seabird species at a North Sea colony. As predicted, the phenology of two dispersive species, black-legged kittiwake ( Rissa tridactyla ) and common guillemot ( Uria aalge ), correlated with a large-scale environmental cue (the North Atlantic Oscillation), whereas a resident species, European shag ( Phalacrocorax aristotelis ), was more affected by local conditions (sea surface temperature) around the colony. Annual mean breeding success was lower in late years for European shags, but not for the other two species. Since correlations among climate patterns at different scales are likely to change in the future, these findings have important implications for how migratory animals can respond to future climate change.     

Relation between climatic factors,diet and reproductive parameters of Little Terns over a decade

We used 10 years of data on clutch size, egg size and diet, and 8 years of data on timing of laying on Little Terns (Sternula albifrons) breeding in Ria Formosa lagoon system, Algarve, Portugal to assess whether diet acts as an important intermediary between climatic conditions and breeding parameters. We used Generalized Linear Models to relate (1) the relative occurrence and size of the main prey species, sand smelts (Atherina spp.), with environmental variables, a large-scale climate variable, the North Atlantic Oscillation (NAO) index, and a local scale variable, the sea-surface temperature (SST), and (2) the respective effects of sand smelts relative occurrence, NAO index and SST on Little Tern breeding parameters. The diet of Little Terns was dominated by sand smelts, with a frequency occurrence of over 60% in all years. The winter SST (February) was negatively associated with the relative occurrence of sand smelts in the diet of Little Terns during the breeding season which, in turn, was positively associated with Little Tern clutch size. Our results suggest that negative NAO conditions in the Atlantic Ocean, often associated with rougher sea conditions (greater vertical mixing, stronger winds and lower SST) were related with earlier breeding, and lower SST in the surroundings of the colony during winter–spring favour the abundance of prey fish for Little Terns as well as their reproductive parameters. Climate patterns at both large and local scales are likely to change in the future, which may have important implications for estuarine seabirds in Southern Europe.     

Climate warming decreases the survival of the little auk (Alle alle), a high Arctic avian predator

Delayed maturity, low fecundity, and high adult survival are traits typical for species with a long‐life expectancy. For such species, even a small change in adult survival can strongly affect the population dynamics and viability. We examined the effects of both regional and local climatic variability on adult survival of the little auk, a long‐lived and numerous Arctic seabird species. We conducted a mark‐resighting study for a period of 8 years (2006‐2013) simultaneously at three little auk breeding sites that are influenced by the West Spitsbergen Current, which is the main carrier of warm, Atlantic water into the Arctic. We found that the survival of adult little auks was negatively correlated with both the North Atlantic Oscillation (NAO) index and local summer sea surface temperature (SST), with a time lag of 2 and 1 year, respectively. The effects of NAO and SST were likely mediated through a change in food quality and/or availability: (1) reproduction, growth, and development of Arctic Calanus copepods, the main prey of little auks, are negatively influenced by a reduction in sea ice, reduced ice algal production, and an earlier but shorter lasting spring bloom, all of which result from an increased NAO; (2) a high sea surface temperature shortens the reproductive period of Arctic Calanus, decreasing the number of eggs produced. A synchronous variation in survival rates at the different colonies indicates that climatic forcing was similar throughout the study area. Our findings suggest that a predicted warmer climate in the Arctic will negatively affect the population dynamics of the little auk, a high Arctic avian predator.     

The effects of climate change on harp seals (Pagophilus groenlandicus)

Harp seals (Pagophilus groenlandicus) have evolved life history strategies to exploit seasonal sea ice as a breeding platform. As such, individuals are prepared to deal with fluctuations in the quantity and quality of ice in their breeding areas. It remains unclear, however, how shifts in climate may affect seal populations. The present study assesses the effects of climate change on harp seals through three linked analyses. First, we tested the effects of short-term climate variability on young-of-the year harp seal mortality using a linear regression of sea ice cover in the Gulf of St. Lawrence against stranding rates of dead harp seals in the region during 1992 to 2010. A similar regression of stranding rates and North Atlantic Oscillation (NAO) index values was also conducted. These analyses revealed negative correlations between both ice cover and NAO conditions and seal mortality, indicating that lighter ice cover and lower NAO values result in higher mortality. A retrospective cross-correlation analysis of NAO conditions and sea ice cover from 1978 to 2011 revealed that NAO-related changes in sea ice may have contributed to the depletion of seals on the east coast of Canada during 1950 to 1972, and to their recovery during 1973 to 2000. This historical retrospective also reveals opposite links between neonatal mortality in harp seals in the Northeast Atlantic and NAO phase. Finally, an assessment of the long-term trends in sea ice cover in the breeding regions of harp seals across the entire North Atlantic during 1979 through 2011 using multiple linear regression models and mixed effects linear regression models revealed that sea ice cover in all harp seal breeding regions has been declining by as much as 6 percent per decade over the time series of available satellite data.     

Climate change and breeding parameters of great and blue tits throughout the western Palaearctic

Increasing evidence suggests that climate change has consequences on avian breeding phenology. Here, variations in laying date and clutch size of great tit Parus major and blue tit Parus caeruleus within and between breeding populations through the western Palaearctic are examined in relation to climatic fluctuations, measured by the winter North Atlantic Oscillation (NAO) index. Within and across breeding sites, laying date was related to winter‐NAO index such that great and blue tit females lay earlier after warmer, moister winters (positive values of winter NAO‐index). The present study shows that for most populations there is an advancement of laying date, but the rate of change with respect to NAO significantly differed geographically across the western Palaearctic and did not differ between species. However, clutch size of great and blue tits was not affected by climatic fluctuations, presumably because the whole season is being shifted, but not in relation to food supplies. These combined analyses for the two species controlled for potentially confounding variables such as latitude, longitude, elevation and habitat of each study site.     

Long-term trends in the timing of breeding and brood size in the Red-Backed Shrike Lanius collurio in the Czech Republic, 1964–2004

Climate change has affected breeding dates and clutch sizes in many bird species. To date, most of the studies aimed at assessing the effects of climate change on these phenological events in birds have been on hole-nesting species and the changes linked either to local climate variation or to some large-scale composite variables, such as the North Atlantic Oscillation (NAO). Relatively less is known about the climate responses of open-nesting birds and on the relative roles of climate variables at different scales. Using bird ringing records covering a time span of 41 years, we documented shifts in the timing of breeding and brood size in a long-distance migrant, the Red-Backed Shrike (Lanius collurio) from a central European population. We found a 3- to 4-day shift towards earlier breeding and an increase in brood size by approximately 0.3 nestlings since 1964. The Red-Backed Shrikes start to breed in May and rear the first nestlings in June. During the period 1964–2004, temperatures in May significantly increased, while the increase in June temperatures was not significant. Simultaneous tests on the influence of local climate variables and the NAO index revealed a better performance of local climate. The increasing temperature in May was positively associated with the advancement of breeding. Similarly, at a local scale, higher May temperatures were followed by larger brood sizes, while a high amount of rainfall had a strong negative effect.     

Linkages between common wheat yields and climate in Morocco (1982–2008)

In Morocco, wheat production shows a high inter-annual variability due to uncertain rainfall. In view of the importance of this resource to the country’s economy, it is important to gain a better understanding of the natural large-scale climate oscillation governing this variability. In this study, we analyzed de-trended (1) time series of common wheat yields (1983–2008) from 11 agricultural provinces that account for 80 % of national wheat production; (2) monthly rainfall and 10-day temperature from ten meteorological stations; (3) 10-day normalized difference vegetation index (NDVI) from the AVHRR sensor; (4) monthly atmospheric climate indices [North Atlantic Oscillation (NAO) and Scandinavian Pattern (SCA)] and monthly 500 hPa geopotentials fields; and (5) monthly sea surface temperature (SST) fields and indices (NIÑO3, Tropical North Atlantic and Tropical South Atlantic). The relationship between rainfall and temperature during tillering in early winter and grain filling in early spring and wheat yields already observed at the plot scale was also found to be significant at the provincial scale. The linkages between wheat yields and large scale climate have been analyzed for the first time over Morocco. In agreement with previous studies, results show a complex and competing influence of different climate phenomena. The NAO is found to be significantly related to yields during the early stage of wheat growth in December, whereas the SCA correlates with yields later in the season, in January and February. Interesting lagged correlations with higher lead time are also highlighted, with the leading modes of SST variability in the equatorial Atlantic during October (the “Atlantic Niño” mode) and in the North Atlantic (the “Atlantic tripole” mode) in February. Our conclusion is that regional climate indices and variables represent valuable information with which to increase lead time and skill regarding wheat yield predictions in Morocco.     

Large-scale effect of climate change on breeding parameters of pied flycatchers in Western Europe

Variations in laying date and clutch size of pied flycatchers Ficedula hypoleuca across populations throughout western Europe are examined in relation to climatic fluctuations, measured by the winter North Atlantic Oscillation (NAO) index. Across breeding sites, the winter-NAO index affected laying date such that females lay earlier after warmer and moister winters (positive values of winter NAO-index). Female pied flycatchers breed progressively earlier because presumably the whole breeding season is being shifted, as a direct result of the positive values of winter NAO-index. Moreover, clutch size of pied flycatchers across populations was negatively related to winter NAO-index during the last 50 yr. These analyses controlled for potentially confounding variables such as latitude, longitude, elevation and habitat of each study site. The present study conclude that pied flycatchers across western Europe are breeding earlier and laying smaller clutch sizes and that the most likely cause is a long-term increase in spring temperature. On the other hand, this study shows that climate change may not act uniformly between breeding populations in Western Europe. From those results, this study concludes that northern pied flycatcher populations are more sensitive to climate change than southern populations breeding in montane habitats.     

Influences of the El Niño/Southern Oscillation and the North Atlantic Oscillation on avian productivity in forests of the Pacific Northwest of North America

To model the effects of global climate phenomena on avian population dynamics, we must identify and quantify the spatial and temporal relationships between climate, weather and bird populations. Previous studies show that in Europe, the North Atlantic Oscillation (NAO) influences winter and spring weather that in turn affects resident and migratory landbird species. Similarly, in North America, the El Niño/Southern Oscillation (ENSO) of the Pacific Ocean reportedly drives weather patterns that affect prey availability and population dynamics of landbird species which winter in the Caribbean. Here we show that ENSO‐ and NAO‐induced seasonal weather conditions differentially affect neotropical‐ and temperate‐wintering landbird species that breed in Pacific North‐west forests of North America. For neotropical species wintering in western Mexico, El Niño conditions correlate with cooler, wetter conditions prior to spring migration, and with high reproductive success the following summer. For temperate wintering species, springtime NAO indices correlate strongly with levels of forest defoliation by the larvae of two moth species and also with annual reproductive success, especially among species known to prey upon those larvae. Generalized linear models incorporating NAO indices and ENSO precipitation indices explain 50–90% of the annual variation in productivity reported for 10 landbird species. These results represent an important step towards spatially explicit modelling of avian population dynamics at regional scales.     

Impact of climate change on plant phenology in Mediterranean ecosystems

Plant phenology is strongly controlled by climate and has consequently become one of the most reliable bioindicators of ongoing climate change. We used a dataset of more than 200 000 records for six phenological events of 29 perennial plant species monitored from 1943 to 2003 for a comprehensive assessment of plant phenological responses to climate change in the Mediterranean region. Temperature, precipitation and North Atlantic Oscillation (NAO) were studied together during a complete annual cycle before phenological events to determine their relative importance and potential seasonal carry‐over effects. Warm and dry springs under a positive phase of NAO advance flowering, leaf unfolding and fruiting dates and lengthen the growing season. Spatial variability of dates (range among sites) was also reduced during warm and dry years, especially for spring events. Climate during previous weeks to phenophases occurrence had the greatest impact on plants, although all events were also affected by climate conditions several months before. Immediate along with delayed climate effects suggest dual triggers in plant phenology. Climatic models accounted for more than 80% of variability in flowering and leaf unfolding dates, and in length of the growing season, but for lower proportions in fruiting and leaf falling. Most part of year‐to‐year changes in dates was accounted for temperature, while precipitation and NAO accounted for <10% of dates' variability. In the case of flowering, insect‐pollinated species were better modelled by climate than wind‐pollinated species. Differences in temporal responses of plant phenology to recent climate change are due to differences in the sensitivity to climate among events and species. Spring events are changing more than autumn events as they are more sensitive to climate and are also undergoing the greatest alterations of climate relative to other seasons. In conclusion, climate change has shifted plant phenology in the Mediterranean region.     

Distribution-wide effects of climate on population densities of a declining migratory landbird

1. Increases in global temperatures have created concern about effects of climatic variability on populations, and climate has been shown to affect population dynamics in an increasing number of species. Testing for effects of climate on population densities across a species' distribution allows for elucidation of effects of climate that would not be apparent at smaller spatial scales. 2. Using autoregressive population models, we tested for effects of the North Atlantic Oscillation (NAO) and the El Ni?o Southern Oscillation (ENSO) on annual population densities of a North American migratory landbird, the yellow-billed cuckoo Coccyzus americanus, across the species' breeding distribution over a 37-year period (1966-2002). 3. Our results indicate that both the NAO and ENSO have affected population densities of C. americanus across much of the species' breeding range, with the strongest effects of climate in regions in which these climate systems have the strongest effects on local temperatures. Analyses also indicate that the strength of the effect of local temperatures on C. americanus populations was predictive of long-term population decline, with populations that were more negatively affected by warm temperatures experiencing steeper declines. 4. Results of this study highlight the importance of distribution-wide analyses of climatic effects and demonstrate that increases in global temperatures have the potential to lead to additional population declines.     

Winter climate affects subsequent breeding success of common eiders

The phenology of spring migration depends on the severity of the preceding winter and approaching spring. This severity can be quantified using the North Atlantic Oscillation (NAO) index; positive values indicate mild winters. Although milder winters are correlated with earlier migration in many birds in temperate regions, few studies have addressed how climate‐induced variation in spring arrival relates to breeding success. In northern Europe, the NAO‐index correlates with ice cover and timing of ice break‐up of the Baltic Sea. Ice cover plays an important role for breeding waterfowl, since the timing of ice break‐up constrains both spring arrival and onset of breeding. We studied the effects of the winter‐NAO‐index and timing of ice break‐up on spring migration, laying date, clutch size, female body condition at hatching and fledging success of a short‐distance migrant common eider (Somateria mollissima) population from SW Finland, the Baltic Sea, 1991–2004 (migration data 1979–2004). We also examined the correlation between the NAO‐index and the proportion of juvenile eiders in the Danish hunting bag, which reflects the breeding success on a larger spatial scale. The body condition of breeding females and proportion of juveniles in the hunting bag showed significant positive correlations with the NAO, whereas arrival dates showed positive correlations and clutch size and fledging success showed negative correlations with the timing of ice break‐up. The results suggest that climate, which also affects ice conditions, has an important effect on the fledging success of eiders. Outbreaks of duckling disease epidemics may be the primary mechanism underlying this effect. Eider females are in poorer condition after severe winters and cannot allocate as much resources to breeding, which may impair the immune defense of ducklings. Global climate warming is expected to increase the future breeding success of eiders in our study population.     

Are responses of herbivores to environmental variability spatially consistent in alpine ecosystems?

Animal responses to global climate variation might be spatially inconsistent. This may arise from spatial variation in factors limiting populations' growth or from differences in the links between global climate patterns and ecologically relevant local climate variation. For example, the North Atlantic Oscillation (NAO) has a spatially consistent relation to temperature, but inconsistent spatial relation to snow depth in Scandinavia. Furthermore, there are multiple mechanistic ways by which climate may limit animal populations, involving both direct effects through thermoregulation and indirect pathways through trophic interactions. It is conceptually appealing to directly model the predicted mechanistic links. This includes the use of climate variables mimicking such interactions, for example, to use growing degree days (GDD) as a proxy for plant growth rather than average monthly temperature. Using a unique database of autumn body mass of 83331 domestic lambs from the period 1992–2007 in four alpine ranges in Norway, we demonstrate the utility of hierarchical, mechanistic path models fitted using a Bayesian approach to analyse explicitly predicted relationships among environmental variables and between lamb body mass and the environmental variables. We found large spatial variation in strength of responses of autumn lamb body mass to the NAO, to a proxy for plant growth in spring (the Normalized Difference Vegetation Index, NDVI) and effects even differed in direction to local summer climate. Average local temperature outperformed GDD as a predictor of the NDVI, whereas the NAO index in two areas outperformed local weather variables as a predictor of lamb body mass, despite the weaker mechanistic link. Our study highlights that spatial variation in strength of herbivore responses may arise from several processes. Furthermore, mechanistically more appealing measures do not always increase predictive power due to scale of measurement and since global measures may provide more relevant “weather packages” for larger scales.     

Climate and body size influence nest survival in a fish with parental care

1. The current study examined the effect of broad-scale climate and individual-specific covariates on nest survival in smallmouth bass over a 20-year period. 2. Large-scale climate indices [winter North Atlantic Oscillation (NAO) and winter El Ni?o/Southern Oscillation (ENSO)] and body size of parental males were important covariates in nest survival along with nest age and a quadratic trend in survival. 3. We did not find an effect due to a habitat covariate (total effective fetch) or a phenology covariate (degree-days at start of nesting) on nest survival. 4. Male size in the second half of the nesting season was a more influential covariate on nest success than male size in the first half or throughout the nesting period. 5. We present evidence showing that winter NAO/ENSO indices establish limnological conditions the following spring that influence thermal stability of the lake during the nesting period. 6. The combined climate and body size covariates point to nest survival as a function of lagged climate-scale influences on limnology and the individual-scale influence of bioenergetics on the duration of parental care and nest success.     

Effects of extrinsic and intrinsic factors on breeding success in a long lived seabird

There is growing concern over the impacts of climate change on animal species. Many studies have demonstrated impacts of climate change at the population level, and density dependent effects of climate are frequently reported. However, there is an increasing recognition of the differential impact of such factors on individuals since there is marked variation in individual performance. We investigated the relationships between breeding success and environmental conditions (winter NAO and one year lagged winter NAO) and intrinsic effects (colony size, pair bond duration, past breeding success rate) in the northern fulmar Fulmarus glacialis , using data from a long-term study commenced in 1950. There was a negative trend in breeding success over time, and a negative relationship with winter NAO and lagged winter NAO, which themselves had shown positive increases over the study period. The effects of lagged winter NAO remained after accounting for the linear trend. There was no evidence of density dependence, with breeding success positively related to colony size. We found strong evidence that breeding success was negatively related to pair bond duration but positively related to past breeding success rate. There was also an interaction between these two intrinsic effects such that those pairs that had historically been successful maintained success with increasing pair bond duration, whereas less successful pairs showed a decline. The prediction that there would be a differential impact of extrinsic factors among pairs was supported by an interaction between past breeding success rate and winter NAO, such that pairs with low past success rate exhibited a sharp decline in breeding success with increasing winter NAO, whereas more successful pairs did not. It is critically important to understand interactions between extrinsic factors and individual heterogeneity since a differential impact on individuals will affect population structure, and hence population dynamics.     

The North Atlantic Oscillation and regional phenology prediction over Europe

We present an integrated modeling study designed to investigate changes in ecosystem level phenology over Europe associated with changes in climate pattern, by the North Atlantic Oscillation (NAO). We derived onset dates from processed NDVI data sets and used growing degree day (GDD) summations from the NCEP re‐analysis to calibrate and validate a phenology model to predict the onset of the growing season over Europe. In a cross‐validation hindcast, the model (PHENOM) is able to explain 63% of the variance in onset date for grid cells containing at least 50% mixed and boreal forest. Using a model developed from previous work we performed climate change scenarios, generating synthetic temperature and GDD distributions under a hypothetically increasing NAO. These new distributions were used to drive PHENOM and project changes in the timing of onset for forested cells over Europe. Results from the climate change scenarios indicate that, if the current trend in the NAO continues, there is the potential for a continued advance to the start of the growing season by as much as 13 days in some areas.     

Effects of climate variation on timing of nesting,reproductive success,and offspring sex ratios of red-winged blackbirds

Predicting ecological consequences of climate change will be improved by understanding how species are affected by contemporary climate variation, particularly if analyses involve more than single ecological variables and focus on large-scale climate phenomena. I used 18 years of data from red-winged blackbirds (Agelaius phoeniceus) studied over a 25-year period in eastern Ontario to explore chronological and climate-related patterns of reproduction. Although blackbirds started nesting earlier in years with warmer springs, associated with low winter values of the North Atlantic Oscillation Index (NAOI), there was no advance in laying dates over the study. Nesting ended progressively later and the breeding season lasted longer over the study, however, associated with higher spring values of NAOI. As the length of the nesting season increased, offspring sex ratios became more female biased, apparently as a result of females adjusting the sex of the eggs they laid, rather than from sex-biased nestling mortality. Clutch size did not vary systematically over the study or with climate. Opposing trends of declining nest success and increasing productivity of successful nests over the study resulted in no chronological change in productivity per female. Higher productivity of successful nests was associated with higher winter NAOI values, possibly because synchrony between nesting and food availability was higher in years with high NAOI values. Other than the association between the start of nesting and spring temperatures, local weather (e.g., temperature, rainfall) patterns that linked NAOI with reproduction were not identified, suggesting that weather patterns may be complex. Because climate affected most aspects of red-winged blackbird reproduction examined, focusing on associations between climate and single variables (e.g., first-egg dates) will have limited value in predicting how future climates will affect populations.     

The influence of atmospheric wave dynamics on interannual variation in the surface temperature of lakes in the English Lake District

Surface water temperatures in four lakes of the English Lake District (T_L) are shown to be sensitive to climate change and a large‐scale atmospheric phenomenon known as tropospheric Rossby wave breaking (RWB). RWB occurs frequently near the English Lake District, bringing warm and moist air, or cool and dry air, from distant sources. RWB case examples and composites are used to show three dimensional circulations and anomalies of near‐surface temperature and humidity associated with the two types of RWB (anticyclonic and cyclonic). Statistical models of lake surface temperature are developed for each season using objectively identified variability patterns of anticyclonic and cyclonic RWB, along with an index of Northern Hemisphere annual mean surface temperature (T_NH) to account for climate change. The statistical models, depending on season, account for 54–69% of T_L variance. RWB alone contributes significantly during each season, accounting for 37–52% of T_L variance after the effect of T_NH is removed. RWB is a key physical mechanism underlying the North Atlantic Oscillation (NAO), a regional‐scale weather‐pattern that is frequently related to coherent lake properties. RWB may therefore be a more fundamental driver than the NAO in controlling interannual variation in the properties of lakes such as ice duration, metabolic rates, phenology, species composition and, via effects on stratification, underwater light‐climate, nutrient‐cycling and oxygen‐depletion. Variation in other meteorological features that are linked to RWB, such as precipitation, may have additional effects. RWB is also likely to influence terrestrial and marine environments.     

Warmer,deeper, and greener mixed layers in the North Atlantic subpolar gyre over the last 50 years

Shifts in global climate resonate in plankton dynamics, biogeochemical cycles, and marine food webs. We studied these linkages in the North Atlantic subpolar gyre (NASG), which hosts extensive phytoplankton blooms. We show that phytoplankton abundance increased since the 1960s in parallel to a deepening of the mixed layer and a strengthening of winds and heat losses from the ocean, as driven by the low frequency of the North Atlantic Oscillation (NAO). In parallel to these bottom‐up processes, the top‐down control of phytoplankton by copepods decreased over the same time period in the western NASG, following sea surface temperature changes typical of the Atlantic Multi‐decadal Oscillation (AMO). While previous studies have hypothesized that climate‐driven warming would facilitate seasonal stratification of surface waters and long‐term phytoplankton increase in subpolar regions, here we show that deeper mixed layers in the NASG can be warmer and host a higher phytoplankton biomass. These results emphasize that different modes of climate variability regulate bottom‐up (NAO control) and top‐down (AMO control) forcing on phytoplankton at decadal timescales. As a consequence, different relationships between phytoplankton, zooplankton, and their physical environment appear subject to the disparate temporal scale of the observations (seasonal, interannual, or decadal). The prediction of phytoplankton response to climate change should be built upon what is learnt from observations at the longest timescales.