An overview of plankton ecology in the North Sea

The purpose of this report is to give an overview of plankton ecology in the North Sea, and the processes that effect it, as derived from current research. The Sir Alister Hardy Foundation has extensive data for the North Sea area, and other sources have also been used to provide information for this report. Shortfalls in current research have also been highlighted. The information contained herein is to be contributed towards an information base for the Strategic Environmental Assessment. The North Sea is an extension of the North Atlantic that has an area of 574,980 km2. The deepest area is off the coast of Norway (660m), with a number of shallow areas, such as the Dogger Bank (15m). The North Sea represents a large source of hydrocarbons that have been exploited since the early 1970s. The aim of this study is to provide the Department of Trade and Industry with biological data on the planktonic community of the North Sea, as a contribution towards the Strategic Environmental Assessment (SEA 2). An overview of phyto- and zoo- plankton community composition, plankton blooms, Calanus, mero-, pico- and megaplankton, sensitivity to disturbance / contamination, phytodetritus and vertical fluxes and the resting stages of phytoplankton is made using the results of the survey database. Additional published literature has also been used, and gaps in available data have been highlighted. 1.3 The Continuous Plankton Recorder (CPR) survey provides a unique long-term dataset of plankton abundance in the North Atlantic and North Sea (Warner and Hays 1994). The survey has been running for almost 70 years, using ‘ships of opportunity’ to tow CPRs on regular, and incidental routes, sampling at a depth of 10 m. Each sample represents 18 km of tow and approximately 3 m3 of filtered seawater. Over 400 taxa of plankton are routinely identified by a team of taxonomists. The samples are also compared to colour charts to give an indication of ‘greenness’, which provides a visual index of chlorophyll value. CPRs have been towed for over 4 million nautical miles, accumulating almost 200,000 samples. The design of the CPR has remained virtually unchanged since sampling started, thus providing a consistency of sampling that provides good historical comparisons. By systematically monitoring the plankton over a period, changes in abundance and long term trends can be distinguished. From this baseline data, inferences can be made, particularly concerning climate change and potentialanthropogenic impacts.

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.

Arctic boreal plankton species in the Northwest Atlantic

The Continuous Plankton Recorder (CPR) survey has sampled regularly in the Northwest Atlantic since the early 1960s. Over the last decade there has been a dramatic increase in the abundance of a number of arctic boreal plankton species, notably Calanus hyperboreus (Kroyer), Calanus glacialis (Jaschnov), and Ceratium arcticum, and a southerly shift of the copepod C. hyperboreus in the CPR survey. In 1998, C. hyperboreus was recorded at its farthest position south in the survey, 39 degrees N, off the Georges Bank shelf edge. Other studies have reported similar parallel biological responses on three trophic levels. During the late 1990s, production of Labrador Sea Water (LSW) has been at a high, a direct response to the phase of the North Atlantic Oscillation (NAO). The increase in abundance of these species, up to four standard deviations from the long-term mean, is linked to variability in the hydrography of the area and the driving climatic processes of the North Atlantic.

Relating plankton assemblages to environmental variables using instruments towed by ships-of-opportunity

Undulating Oceanographic Recorders (UORs) and Continuous Plankton Recorders (CPRs) equipped with a suite of sensors were towed by merchant vessels in the North Sea between 1988 and 1991, recording a range of environmental variables. These were used to interpret the results of analyses of the plankton taken on CPR tows off the northeast coast of the UK in 1989 and in the Skagerrak and Kattegat in July 1988 and through 1989. Correlations were found between the biota and the environmental variables. The tidal front off the northeast coast of the UK and the front between the low salinity water in the Kattegat and the higher salinity water in the Skagerrak were dominant factors correlating with the distribution of the plankton assemblages. Discontinuities, defining the positions of the fronts, in the values of physical variables (temperature and, where measured, salinity and turbidity) were closely identified with geographical divisions between plankton assemblages. Measures of irradiance were found to be important on several occasions, presumably due to diel migrations of the zooplankton.

Structure of phytoplankton (Continuous Plankton Recorder and SeaWiFS) and impact of climate in the Northwest Atlantic Shelves

All marine organisms are affected to some extent by the movement and thermal properties of oceanic currents. However phytoplankton, because of its small size is most directly coupled to the physical environment. The intense hydrodynamic activity observed in the Northwest Atlantic Shelves Province makes this region especially intriguing from the point of view of physical-biological interactions. In the present work, remote sensed data of Sea Surface Height (SSH) anomalies, Sea-surface chlorophyll a concentrations (SeaWiFS), and Sea Surface Temperature (SST) are used to complement the Continuous Plankton Recorder (CPR) survey that continuously sampled a route between Norfolk (Virginia, USA; 39° N, 71° W) and Argentia (Newfoundland; 47° N, 54° W) over the period 1995–1998. Over this period, we examined physical structures (i.e. SST and SSH) and climatic forcing associated with space-time phytoplankton structure. Along this route, the phytoplankton structures were mainly impacted by the changes in surface flow along the Scotian Shelf rather than significantly influenced by the mesoscale features of the Gulf Stream. These changes in water mass circulation caused a drop in temperature and salinity along the Scotian Shelf that induced changes in phytoplankton and zooplankton abundance.

An increase in snake pipefish (Entelurus aequoreus) in the northeast Atlantic: possible causes and effects

Climate induced changes in the planktonic community have been reported in the North Atlantic in recent years (Beaugrand et al., 2002), and similar responses has been seen in higher trophic levels (eg fish, Brander et al., 2003). Many of these responses have been identified by the use of the Continuous Plankton Recorder (CPR), and here we discuss recent results from the survey concerning pipefish, numbers of which have increased dramatically around the UK in recent years. This has also been reported in both the scientific and popular press, and anecdotally by many divers. Pipefish are easily recognized, being vermiform with a long slender ‘snout’ and an armoured outer layer, much like an elongated seahorse. This increase has raised many questions, why has it happened and what affects will it have on the ecosystem?

Increasing prevalence of the marine cladoceran Penilia avirostris (Dana, 1852) in the North Sea

Rising sea surface temperatures in the North Sea have had consequential effects on not only indigenous plankton species, but also on the possibility of successful colonisation of the area by invasive plankton species. Previous studies have noted the introduction and integration into the plankton community of various phytoplankton species, but establishment of zooplankton organisms in the North Sea is less well-documented. Examining continuous plankton recorder (CPR) survey data and zooplankton results from the Helgoland Roads study, the autumn of 1999 witnessed the occurrence of the marine cladoceran Penilia avirostris in large numbers in the North Sea. The rapid appearance of the species corresponded with exceptionally warm sea surface temperatures (SSTs). Since 1999, the species has become a regular feature of the autumnal zooplankton community of the North Sea. In 2002 and 2003, the species occurred in greater abundance than recorded before. It is suggested that increased autumn SSTs have proved favourable to P. avirostris, with warmer conditions contributing to the success of the species’ resting eggs and aiding colonisation.

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.

Population-Dynamics And Production Of Euphausiids. IV Euphausia-Krohni Nematoscelis-Megalops And Thysanoessa-Gregaria And 8 Rare Species In The North-Atlantic Ocean

Seasonal changes in the abundance, size and occurrence of furciliae of Euphausia krohni (Brandt), Nematoscelis megalops (G. O. Sars) and Thysanoessa gregaria G. O. Sars are described from samples taken at 10 m depth with the Continuous Plankton Recorder (CPR) over a period of 2 yr (January 1966 to December 1967) in the North Atlantic Ocean. E. krohni and T. gregaria were found to breed through most of the year but N. megalops bred only in spring and summer. Annual mean biomass was calculated directly from the data and production was estimated from published P:B ratios. The seasonal occurrences of E. brevis Hansen, E. hemigibba Hansen, E. mutica Hansen, E. tenera Hansen, Stylocheiron longicorne G. O. Sars, S. maximum Hansen, Thysanopoda acutifrons Holt and Tattershall and T. aequalis Hansen in the samples are described.

Plankton Of The Fladen Ground During Flex-76 .3. Vertical-Distribution Population-Dynamics And Production Of Calanus-Finmarchicus (Crustacea Copepoda)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR), the Longhurst Hardy Plankton Recorder (LHPR) and by our colleagues from other participating Institutes during the Fladen Ground Experiment (FLEX 76) were used to describe the vertical distribution and population dynamics of Calanus finmarchicus (Gunnerus) and to provide estimates of the production and carbon budget of the population from 19 March to 3 June, 1976. Total production of the 19 March to 3 June, 1976. Total production of the nauplii and copepodite stages (including adults), during the exponential growth phase in May, was estimated to be in the range of 0.49 to 0.91 g C m-2 d-1 or 29.0 to 55 g dry wt m-2 (14.5 to 27.8 g C m-2) for the three successive 10 d periods in May. Two gross growth efficiencies (K 1) (20 and 34%), together with the lower value of C. finmarchicus production, were used to calculate the gross ingestion levels of algae as 2.45 and 1.44 g C m-2 d-1 (73.5 and 43.2 g C m-2 over the May period). These ingestion levels, together with the algae ingested by other zooplankton species, are greater than the estimated total phytoplankton production of 45.9 g C m-2 over the FLEX period. A number of factors are discussed which could explain the discrepancies between the production estimates. One suggestion is that the vertical distribution of the development stages of this herbivorous copepod and their diel and ontogenetic migration patterns enable it to efficiently exploit its food source. Data from the FLEX experiment indicated that the depletion of nutrients limited the size of the spring bloom, but that it was the grazing pressure exerted by C. finmarchicus which was responsible for the control and depletion of the phytoplankton in the spring of 1976 in the northern North Sea.

Population-Dynamics And Production Of Euphausiids .2. Thysanoessa-Inermis And Thysanoessa-Raschi In The North-Sea And American Coastal Waters

Results from the Continuous Plankton Recorder (CPR) survey for 1966 and 1967 are used to describe seasonal changes in abundance, size and aspects of the population structure of Thysanoessa inermis (Krøyer) and T. raschi (M. Sars) at a depth of 10 m in the North Sea and in American coastal waters from the Grand Banks to the Gulf of Maine. Production and dry weight were estimated from these data. Two year-groups were usually present in the breeding population, the proportion surviving into a second year being higher in American waters than in the North Sea. Annual production for each species was within the range 0.69 to 4.66 mg m-3 and the ratio between production and biomass (P:B) was between 1.3 and 4.2; values outside these ranges were obtained only for American coastal waters in 1967, when the frequency of sampling was low.

Plankton Of The Fladen Ground During Flex-76 .1. Spring Development Of The Plankton Community

Results from plankton sampling in the northern North Sea with the Continuous Plankton Recorder (CPR) and the Undulating Oceanographic Recorder (UOR) during the Fladen Ground Experiment in 1976 (FLEX 76) are summarised. The first evidence of the spring outbreak of phytoplankton was on 19 April, the day after the first signs of vertical stability of the water column were observed. This was followed by spawning of the euphausiid Thysanoessa inermis and rapid increase in the numbers of Calanus finmarchicus. C. finmarchicus was the most abundant species over the FLEX period (19 March to 3 June) and, together with T. inermis, accounted for over 80% of the dry weight of the zooplankton standing stock. By early June the standing crop of phytoplankton had been depleted and nutrients levels were reduced to very low concentrations in the upper 50 m.

Plankton Of The Fladen Ground During Flex-76 .2. Population-Dynamics And Production Of Thysanoessa-Inermis (Crustacea Euphausiacea)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR) and the Longhurst-Hardy Plankton Recorder (LHPR) during the Fladen Ground Experiment in 1976 (FLEX 76) are used to describe the vertical distribution and population dynamics of Thysanoessa inermis (Krøyer) and to provide estimates of the production and carbon budget of the population from 19th March to 3 June 1976. Spawning occurred in late April and early May, in near synchronisation with the start of the spring bloom of phytoplankton. Eggs, nauplii and calyptopes reached maximum abundance in succession, and furciliae were numerous when sampling ceased in early June. Adults increased in length from a mean of 12.1 mm in mid-March to 17.5 mm in early June and the estimated production was 2.40 mg m-3 over the 74 d period. Total carbon ingested by the population of T. inermis was estimated to be 10 mg C m-2 d-1 in the upper 100m which was only 1.5% of the daily primary production of 0.68 gC m-2 measured over the FLEX period 26 March to 4 June 1976. The grazing by T. inermis on the phytoplankton population was assumed to have little effect on the control and depletion of the spring phytoplankton bloom during FLEX 77.

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.

On the roles of cell size and trophic strategy in North Atlantic diatom and dinoflagellate communities

We have examined the inter- and intra-group seasonal succession of 113 diatom and dinoflagellate taxa, as surveyed by the Continuous Plankton Recorder (CPR) in the North Atlantic, by grouping taxa according to two key functional traits: cell size (mg C cell21) and trophic strategy (photoautotrophy, mixotrophy, or heterotrophy). Mixotrophic dinoflagellates follow photoautotrophic diatoms but precede their obligate heterotrophic counterparts in the succession because of the relative advantages afforded by photosynthesizing when light and nutrients are available in spring. The mean cell size of the sampled diatoms is smallest in the summer, likely because of the higher specific nutrient affinity of smaller relative to larger cells. Contrastingly, we hypothesize that mixotrophy diminishes the size selection based on nutrient limitation and accounts for the lack of a seasonal size shift among surveyed dinoflagellates. Relatively small, heterotrophic dinoflagellates (mg C cell21 , 1023) peak after other, larger dinoflagellates, in part because of the increased abundance of their small prey during nutrientdeplete summer months. The largest surveyed diatoms (mg C cell21 . 1022) bloom later than others, and we hypothesize that this may be because of their relatively slow maximum potential growth rates and high internal nutrient storage, as well as to the slower predation of these larger cells. The new trait database and analysis presented here helps translate the taxonomic information of the CPR survey into metrics that can be directly compared with trait-based models.