The analysis of the meteorological data in the period 1998–2005 showed an increase in temperature oscillations since 2003, characterised by colder winters and warmer summers. The coldest mean daily air temperatures were recorded from mid-December to February 2003, with a minimum of −4.56 °C on 8 January 2003. The warmest mean daily air temperature of the 8-y time series occurred from 3–12 August 2003 (Fig. 1A). This heat wave showed two peaks: the first peak on 4–5 August with mean daily air temperature ∼25 °C associated with north-easterly winds (∼75°), and a more intense second peak on 10–11 August with mean daily air temperature ∼28 °C peak associated with south-easterly winds (∼120°). The warmest temperatures were recorded on 11 August with 35 °C. Our sampling region, located near the Straits of Dover, is characterised by strong winds. The summer 2003 European heat wave was correlated with a strong decrease in wind stress and an exceptional incidence of easterly winds in August when compared to other years (Fig. 1D, E).

The coldest month of February in the 8-y time series was recorded in 2005 and was associated with a marked change of the wind direction (Fig. 1B, C). The mean air temperature fell below 0 °C on 3 March, which was the latest subzero temperature in the 8-y time series. In contrast, warm temperatures arrived unusually early in 2005, up to 25.32 °C in early June, and continued to late autumn (Fig. 1A). The warmest years, based on the number of days with mean daily air temperatures higher than 22 °C, were 2003 (8 days) and 2005 (6 days). These data reveal the extreme variability in air temperatures for the years 2003 and 2005 when compared to the other years of the 8-y time series.

The Seine River is the main supplier of freshwater into the English Channel and has a significant contribution to the nutrient load of this area. These nutrient inputs are transported into our sampling stations, mainly the inshore station, through the northward ‘coastal flow’ [17,18]. The river discharges at the Poses Dam showed a peak in winter, with the exception of 2005. The river discharges showed a minimum in the summer of 2003 (Fig. 1F).

Within the context of global climate change, warm events typically receive greater attention as they may allow the evaluation of the ecosystem response to future warming scenarios. The exceptionally warm conditions in August 2003 favoured the development of the dinoflagellates Akashiwo sanguinea (Hirasaka) G. Hansen et Moestrup and Ceratium fusus (Ehrenberg) Dujardin. During the 8-y time series, A. sanguinea were restricted to sporadic single records in summer and autumn. Exceptionally in late summer 2003, Akashiwo sanguinea showed a marked peak of 9600 cells L⁻¹ in the surface waters of the neritic station (Fig. 2A–D). Ceratium fusus also reached an exceptional abundance after the heat wave, with the highest abundances in the surface offshore waters (320 cells L⁻¹) (Fig. 2E–H). The genus Ceratium was also represented by sporadic records of C. lineatum (Ehrenberg) Cleve and C. furca (Ehrenberg) Claparède et Lachmann. Other dinoflagellates such as Karenia mikimotoi (Miyake et Kominami ex Oda) G. Hansen et Moestrup were observed after the heat wave, although the abundance was lower than in other years.

The blooms of Phaeocystis globosa are a recurrent phenomenon in the northern English Channel and the southern North Sea. In our sampling stations the abundance during the blooms ranged from 30–50×106cellsL−1 in onshore waters, whereas in offshore waters (8 km off) the abundance rarely exceeded 20×106cellsL−1. The blooms began in February and extended until late May or early June, with the highest values usually in April. The highest value (101×106cellsL−1) was recorded in March 1999. The most relevant feature during this 8-y time series was the absence of the Phaeocystis bloom in spring 2005 (Fig. 2I–L). Single cells and a low number of Phaeocystis colonies were observed in January–February of 2005. However, in spring 2005 the proliferation of Phaeocystis was negligible compared to previous years. A slight proliferation of Phaeocystis was observed in October 2005 (Fig. 2J).

As a general trend, the phytoplankton abundance in December–January was very low and the dominant species were tychoplanktonic diatoms such as Paralia sulcata (Ehrenberg) Cleve or Raphoneis amphiceros (Ehrenberg) Ehrenberg. Skeletonema cf. marinoi Sarno et Zingone and Asterionellopsis glacialis (Castracane) Round were usually present, being replaced along the month of February by the spring blooming species: Phaeocystis and the diatoms Guinardia delicatula (Cleve) Hasle, Rhizosolenia imbricata var. shrubsolei (Cleve) Schröder, Guinardia striata (Stolterfoth) Hasle and Guinardia flaccida (Castracane) H. Peragallo. Anomalously in 2005, the tychoplanktonic diatoms dominated until March, and Skeletonema reached high abundances from February to the middle of March (up to 116×103cellsL−1). Microscopy observations revealed a high concentration of particulate matter, probably associated with the suspended sediment due to the increase of water column mixing. These particles, together with organic compounds from the excretion or decomposition of Skeletonema, are expected to contribute to the reduction of underwater light availability in late winter 2005 when compared to other years.

The summer of 2003 was characterized by exceptionally warm weather in Europe, with the mean temperature exceeding that of any summer for the previous 500 years [6]. Thermal stratification is a major factor affecting the reduction of water column mixing [30]. In the case of the sampling stations located near the Straits of Dover the winds reach a high magnitude [31]. The winds showed low intensity in August and September 2003. In August 2003 when compared with the same months in other years, the easterlies showed an unusual predominance (Fig. 1C).

In the southern English Channel the magnitude of the winds is markedly lower than on the northern side. The summer stratification favours the proliferation of motile phytoplankton such as the dinoflagellate Karenia mikimotoi and coccolithophorids [32]. In June and July 2003 prior to the exceptional August heat wave, temperatures higher than the average values in Europe were also recorded [33]. A Karenia mikimotoi bloom was observed in June–July 2003 in the southern English Channel [34]. The heat wave of August 2003 may favour the development of K. mikimotoi or other dinoflagellates. However, it can be hypothesised that the previous nutrient depletion may limit an extension of the Karenia bloom into August despite the favourable physical conditions.

Global climate change is expected to alter the species composition of local planktonic communities as well increasing the risk of invasion by exotic species [35]. However, the summer 2003 European heat wave favoured the development of two autochthonous species. Akashiwo sanguinea is a widespread species described as Gymnodinium splendens Lebour in the English Channel off Plymouth [36], and Ceratium fusus is also a widespread species known in European waters. The elevated temperatures may favour exotic warm-water species. However, if ‘seed populations’ of these species have a low abundance, a short event such as a two-week heat wave may be insufficient to trigger proliferations.

The spring blooms of Phaeocystis which have been documented since the early phytoplankton studies [19–21], are a recurrent phenomenon in the northern English Channel and North Sea. The magnitude of the blooms in the region has increased since the middle of the twentieth century, with the greater bloom durations in the 1990s [14]. Coastal eutrophication is generally perceived as a shift from diatom towards Phaeocystis dominance due to an over-enrichment of nitrogen and phosphorus with respect to silicic acid [15]. In our sampling location, the Phaeocystis blooms reached 101×106cellsL−1 in March 1999. An abundance of 200×106cellsL−1 has been reported along the Dutch coast in May 1985 [22]. In the last decade the magnitude of the coastal North Sea blooms has decreased. This appears to be associated with the ‘de-eutrophication’ of the North Sea [14]. In the northern English Channel, the nutrient loads exported by the Seine River as well as other rivers in the region have decreased [23]. The general trend towards ‘de-eutrophication’ may be primarily responsible for the reduction in the magnitude of the Phaeocystis blooms.

This study has focused on the absence in 2005, for the first time, of the Phaeocystis spring bloom in the north-eastern English Channel. In the North Sea, Cadée and Hegeman reported that currently Phaeocystis cell numbers and bloom durations are decreasing, and in 2002 the spring peak was absent [14]. These authors reported that since 1975 the beginning of the Phaeocystis bloom (>1×106cellsL−1) has changed from ∼16 April to ∼27 March, coinciding with the progressive increase of spring temperatures. It may be expected that the weather features in February and early March will now have more effects on the development of the Phaeocystis bloom than in the past.

Beyond the long-term decrease of the nutrient loads into the northern English Channel, the absence of the Phaeocystis bloom may also be associated with unfavourable meteorological conditions in a particular year. It is generally accepted that phytoplankton production in late winter and early spring is limited by light availability, because under the prevailing non-stratified conditions individual algal cells spend most of the time below the euphotic zone. Relatively low temperatures and high wind speeds during winter and early spring promote a late onset of stratification [3,24]. The mean daily air temperature in winter 2005 fell below 0 °C on 3 March, which was the latest occurring subzero temperature in the 8-y time series (Fig. 1A). The usual winter peak of river discharges (nutrient loads) was absent in 2005 (Fig. 1F). In addition to nutrients, the development of the flagellate Phaeocystis requires an increase of the stratification and light [25,26]. The anomalous cold conditions in February and early March extended the mixing period, with two effects in underwater light availability: directly through the reduction of irradiance due to cloudiness, and indirectly due to high turbidity from sediment resuspension. The extension of these anomalous weather features until early March favoured the development of Skeletonema cf. marinoi. This is a common diatom in high turbidity environments such as the Seine Estuary [27] and in 2005 the proliferation even dominated in our sampling stations located 2 and 8 km offshore. The development of Skeletonema and other diatoms may increase particle concentration due to the excretion of organic compounds [28] and cell debris. The organic particles in suspension may intercept light more effectively than the inorganic matter [29]. In addition, it can be hypothesized that these diatoms partially consume the winter stock of nutrients required to trigger the Phaeocystis spring bloom. It is not easy to establish the key factor(s) that hindered the proliferation of Phaeocystis. If global warming is associated with the earlier appearance of the spring phytoplankton bloom, this increases the probability of a cold wave event that may affect the development of the dominant species. Our results show that climate variability during the pre-bloom period (February and early March) was essential for the development of the main primary producer.

This study describes the response of the phytoplankton community to extreme weather features. The summer 2003 European heat wave occurred in mid-summer that constitutes the normal period for the occurrence of heat waves. In mid-summer, the spring phytoplankton bloom had already declined and the phytoplankton biomass showed the lowest annual values. Any change at that period is expected to have little influence on total annual primary production and carbon flux to higher trophic levels. The same conclusion can be applied for the cold waves. The first half of winter 2003 was the coldest of the 8-y time series. Phytoplankton abundance was low and restricted to tychoplanktonic species in December and January. A cold wave at these months, beyond the increase of the winter nutrient stock, is expected to have little influence on the further development of the spring phytoplankton bloom. In contrast to other regions where diatoms dominate the spring bloom, in the northern English Channel and the southern North Sea diatoms and the flagellate Phaeocystis compete with each other. The massive blooms of Phaeocystis favoured by the over-enrichment of nitrate and phosphate versus silicic acid (use of artificial fertilizers) are progressively weaker due to the ‘de-eutrophication’, consequently being more vulnerable to anomalous climatic events. Cold events in the pre-bloom period, February and early March, seem to negatively affect Phaeocystis in its competition with the diatoms. This alteration is expected to have important consequences for higher trophic levels that need to be evaluated.