Phaeocystis colony distribution in the North Atlantic Ocean since 1948 and interpretation of longterm changes in the Phaeocystis hotspot in the North

W. W. C. Gieskes • S. C. Leterme • H. Peletier • M. Edwards • P. C. Reid

Received: 13 October 2005 / Accepted: 12 June 2006 / Published online: 10 March 2007 © Springer Science+Business Media B.V. 2007

Abstract 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 offshore, 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

Department of Ocean Ecosystems,

University of Groningen, Haren, The Netherlands e-mail: [email protected]

M.B.E.R.C., School University of Plymouth, Plymouth, PL4 8AA, UK

H. Peletier

R.I.K.Z.-Rijkswaterstaat, Haren, The Netherlands

S. C. Leterme (&) • M. Edwards • P. C. Reid S.A.H.F.O.S., Citadel Hill, Plymouth, PL1 2PB, UK e-mail: [email protected]

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 phyto-plankton biomass was related significantly 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 flushed in through the Dover Strait.

Since Phaeocystis plays a key role in element fluxes 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.

Keywords Annual variation • North Sea hotspot • North Atlantic-wide • Phaeocystis

Introduction

In the North Atlantic and the North Sea seasonal variations in marine phytoplankton composition are well known, but annual variations in abundance have hardly been described, with only a few exceptions. Monitoring has been maintained for several decades close to British (Russell et al. 1971), Dutch (Cadee and Hegeman 1991), German (Hickel et al. 1996) research institutes, off Belgium between 1988 and 2000 (Breton et al. 2006). Observed changes in plankton have been related to shifts in ocean current patterns (Russell 1935, 1973; Lindley et al. 1990; Taylor et al. 1998; Reid et al. 2003) that are assumed to be governed by large-scale meteorological phenomena (Beaugrand et al. 2002; Edwards et al. 2002; Reid and Edwards 2001; Drinkwater et al. 2003; Leterme et al. 2005; Breton etal. 2006), to climate (Radach 1984; Seuront and Souissi 2001; Reid et al. 1998), or to increased coastal eutrophication (Richardson 1989, 1997; Greve et al. 1996), but the response of plankton to anthropogenic interference (Cadee and Hegeman 2002; Beaugrand 2004) has not always been demonstrated in a convincing way.

The Continuous Plankton Recorder survey is the longest-running monitoring programme in the North Atlantic and the North Sea, with a remarkably wide coverage (Fig. 1a), based on one standard sampling method (Glover 1967; Colebrook 1975; Warner and Hays 1995). Since the time series spans several decades, it has been tempting to interpret striking changes in abundance of the plankton caught on the nets ('silks') of Hardy's recorders as the consequence of long-term environmental change because variations often took place simultaneously over very large areas (Colebrook and Robinson 1964; Edwards et al. 2002). Actually, it has often been taken for granted that hydroclimatic forcing in relation to global warming must have controlled the long- and short-term plankton variability. The northward shift in subtropical and temperate zooplankton groups and the gradual disappearance of cold-adapted copepod species in the eastern North Atlantic (Beaugrand et al. 2002) has indeed been quite conspicuous. However, trends in ocean water temperature, in the Atlantic multidecadal oscillation, or in the state of the North Atlantic oscillation and related hydrographical features (e.g., vertical stratification: Martin and Hall 1975; Fromentin and Planque 1996, Reid et al. 2001) could often only be related to the abundance of plankton components and transitions in taxonomic composition after sophisticated statistical treatment (Beaugrand et al. 2000, 2003; Beaugrand 2004) or on assumptions involving differential lags in the response of ecosystem components (Drinkwater et al. 2003; Kane 2005).

The Continuous Plankton Recorder survey has provided a tremendous data set on the occurrence of Phaeocystis sp. Phaeocystis was already known to vary as much as any other group of plankton in the North Atlantic (Owens et al. 1989) and the North Sea, in a way apparently independent of the phytoplankton in general or of other microal-gal species groups such as dinoflagellates and diatoms (Gieskes and Kraay 1977a), for reasons not known at the time. We present here an overview of the occurrence of Phaeocystis throughout the North Atlantic. In our discussion we will focus on its presence in the North Sea, where it is most abundant. We offer an attempt to link occurrence to the pattern of long-term change in hydro-graphic events in the eastern North Atlantic. We end the presentation of our analysis by highlighting the biogeochemical implications of the long-term changes in the abundance of this species, which is known to affect profoundly the cycling and sea-to-air exchange of climate-relevant elements, especially carbon and sulphur, components in the greenhouse gases CO2 and dimethyl sulphide (DMS; see review by Schoemann et al. 2005).

Materials and methods

The Continuous Plankton Recorder

The Continuous Plankton Recorder (CPR) survey consists of a dense network of transects across the North Atlantic and the North Sea. Plankton recorders are towed on a monthly basis at a depth of 8-10 m from ships-of-oppportunity that travel at 10-18 knots. The sampling mechanism inside the recorders consists of a narrow band of filtering silk (mesh 270 ^m) that is driven by an impeller at the rear of the recorder at a speed adjusted according to the speed of the ship. The silk catches particles entering the 12 mm2 aperture while it passes (at a rate of 10 cm per 10 nautical miles, 18.5 km) through the end of a wide tunnel behind the narrow opening in front. About 3 m3

of seawater is filtered every 18.5 km. Clogging of silks is an exception, but it happens.

Phaeocystis colonies are recorded visually as present or absent on lengths of 10 cm of silk (a sample). From this information a time series from all the CPR samples for a given region can be used to create maps of the frequency of occurrence (the percentage of samples with Phaeocystis presence). 0% means that for a given month not a single sample contained Phaeocystis, 100% that it was present (no matter how much) in every single sample taken in that month. Each section covered by the CPR constitutes a variable number of samples. Total coverage of a section is determined as an average of all percentages.

Details of the sampling procedure have been described by Warner and Hays (1995). Total phy-toplankton biomas' is assessed in four categories by comparing the colouration of the silks (mostly caused by algal pigments) with a colour chart. The colour values [Phytoplankton Colour index (PCI)] are related to chlorophyll concentration on the silks and in the field (Gieskes and Kraay 1977a; Batten et al. 2003).

Standard regions of the CPR survey are shown in Fig. 1a; each consists of smaller rectangles; rectangle 'Ow', located in the southeastern part of the Southern Bight of the North Sea just off The Netherlands where major rivers of the continent (Rhine, Scheldt) discharge, has received particular attention in the present study.

Rijkswaterstaat-RIKZ survey

The chlorophyll concentrations of the monitoring programme run by Rijkswaterstaat-RIKZ off The

Fig. 1 (a) Standard areas of the continuous plankton recorder survey. (b)Distribution of Phaeocystis colonies over the North Atlantic and the North Sea over the years 1948-2003. Annual variations: see Fig. 2

Fig. 1 (a) Standard areas of the continuous plankton recorder survey. (b)Distribution of Phaeocystis colonies over the North Atlantic and the North Sea over the years 1948-2003. Annual variations: see Fig. 2

Phaeocystis

Netherlands since the early 1970s and analysed here (Fig. 4) have been measured by fluorometry and later by high-performance liquid chromatography (HPLC). These data, also those of Rhine river discharge, are available free of charge to the public on the internet because Rijkswaterstaat is a government agency funded by taxes; waterbase is the keyword for access. Briefly, samples are taken near the surface; they are considered to be representative for the whole water column because normally Dutch coastal waters are mixed from the surface to bottom by tides and wind. Information on the frequency of sampling, the station network, numbers of samples taken, and other details of the sampling procedure are implicit in the data set that can be extracted by using waterbase. We have chosen section Noo-rdwijk (N) of this programme because it starts just north of the Rhine outflow and samples taken there are likely to be influenced most by this river. The numbers (N2, N10, etc.) refer to the number of kilometers from the coast.

Data presentation

The monthly mean phytoplankton and Phaeocys-tis plots presented here (months versus years, 1948-2003) are so-called Hovmoller diagrams, two-dimensional (2D) plots are often used to display large amounts of data in a readily understandable way (Hovmoller 1949); the computing and statistical analysis has been presented by Leterme et al. (2005). In order to reveal anomalies in the trends in the North Sea more clearly, the Phytoplankton Colour index (PCI) (chlorophyll) and Phaeocystis colony percent frequency-of-occurrence data were also standardized to zero mean and unit variance in the North Sea survey regions D1, D2, C1 and C2 together (refered to herein as 'the North Sea'), the cumulative sums method (Ibanez et al. 1993). The calculation consists of subtracting a reference value (here the mean of the series) from the data. The residuals are then successively added, forming a cumulative function. Cumulative sum plots have been introduced into the SAHFOS data set interpretation (Beaugrand et al. 2000) to summarise major changes and identify transitional periods in phy-toplankton time series. The cumulative function results in the smoothing of high-frequency inter-annual variability and highlights changes in mean values along the time series.

Results

The distribution of Phaeocystis colonies over the North Sea and the North Atlantic (Fig. 1b) seems to support the notion of an ocean-wide species with peak abundance in spring everywhere (Fig. 2). The centre of abundance over the study period, spanning five decades, was restricted nearly entirely to the North Sea, with the southeastern part clearly being a hotspot. In the open and central Atlantic regions the long record suggests low (cf. Owens et al. 1989) but persistent occurrence (Fig. 2). Abundance was also low (but equally persistent) in the northern North Sea (regions C1 and C2) where Atlantic water flows in from the northwest and dominates the water budget. Low colony frequencies were seen (only until 1985) in the front between the North Atlantic current in the Irminger Sea and the East Greenland current, south of Greenland (Figs. 1b and 2); the season of highest occurrence was spring also there.

Annual variation of Phaeocystis colony abundance did not at all resemble the variation in phy-toplankton biomass [the chlorophyll-related Phytoplankton Colour index (PCI)] anywhere, certainly not in the bloom period, i.e. the spring (see Fig. 3). Year-month plots of true chlorophyll concentrations, derived from the Dutch monitoring programme, in CPR rectangle 'Ow' (see Fig. 1b) are presented in Fig. 4. Notice the gradually earlier chlorophyll spring bloom over the monitoring period at all stations (2, 10, 20 and 70 km off the coast of Noordwijk, The Netherlands).

The Phytoplankton Colour (PCI) cumulative sum plot showed a (single) shift (Fig. 5) in the mid 1980s. This was very different from the two shifts in the Phaeocystis plot (mid 1960s and early 1990s, see Fig. 5), which shows again that Phaeocystis does not reflect the variation observed in phytoplankton biomass. Between 1970 and 1985, low phytoplankton abundance (PCI) in the North Sea coincided with a late spring bloom. This

Fig. 2 Hovmoller diagrams (years 1948-2003 versus months) of Phaeocystis frequency in different regions of the North Sea and the Atlantic Ocean (see Fig. 1b for the location of the regions). Rectangle 'Ow' is the southeastern part of the Southern Bight of the North Sea, directly off the

Netherlands. South and West Greenland corresponds to the regions A8, B7 and B8 (see Fig. 1). Western Atlantic regrouped regions E9, E10 and F10. Central Atlantic corresponds to regions E5 to E8 and D5 to D8

Years I

0 0

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