Plankton, jellyfish and climate in the North-East Atlantic

Abstract

Extensive changes in plankton ecosystems around the British Isles over the last 60 years, including production, biodiversity and species distributions, have had effects on fisheries production and other marine life. This has been mainly driven by climate variability and ocean warming. These changes include:
• Extensive changes in the planktonic ecosystem in terms of plankton production, biodiversity, species distribution which have effects on fisheries production and other marine life (e.g. seabirds). • In the North Sea, the population of the previously dominant and important zooplankton species, (the cold-water species Calanus finmarchicus) has declined in biomass by 70% since the 1960s. Species with warmer-water affinities (e.g. Calanus helgolandicus) are moving northwards to replace the species, but are not as numerically abundant.
• There has been a shift in the distribution of many plankton and fish species around the planet. For example, during the last 50 years there has been a northerly movement of some warmer water plankton by 10° latitude in the Northeast Atlantic and a similar retreat of colder water plankton northwards (a mean poleward movement of between 200-250 km per decade).
• The seasonal timing of some plankton production has also altered in response to recent climate changes. This has consequences for plankton predator species, including fish, whose life cycles are timed in order to make use of seasonal production of particular prey species.
• The decline of the European cod stocks due to overfishing may have been exacerbated by climate warming and climate-induced changes in plankton production (Beaugrand et al., 2003). It is hypothesised that the survival of young cod in the North Sea depends on the abundance, seasonal timing and size composition of their planktonic prey. As the stocks declined, they have become more-sensitive to the effects of regional climate warming due to shrinkage of the age distribution and geographic extent.
• Future warming is likely to alter the geographical distribution of
primary and secondary pelagic production, affecting ecosystem
services such as oxygen production, carbon sequestration and
biogeochemical cycling. These changes may place additional stress
on already depleted fish stocks, as well as have consequences for
mammal and seabird populations. Additionally, melting of Arctic
waters may increase the likelihood of trans-Arctic migrations of
species between the Pacific and Atlantic oceans.

Full text

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Plankton, jellyfish and climate in the
North-East Atlantic
M. Edwards 1,2, A. Atkinson 3, E. Bresnan 4, P. Helaouet 1, A. McQuatters-
Gollop 2, C. Ostle 1, S. Pitois 5 and C. Widdicombe 3
1 CPR Survey, Marine Biological Association, Plymouth, UK
2 School of Biological and Marine Sciences, University of Plymouth, Drake Circus,
Plymouth, UK
3 Plymouth Marine Laboratory, West Hoe, Plymouth, PL1 3DH, UK
4 Marine Scotland Science, 375 Victoria Road, Aberdeen, UK
5 Centre for Environment Fisheries and Aquaculture Science, Lowestoft, Suffolk, UK
EXECUTIVE SUMMARY
Extensive changes in plankton ecosystems around the British Isles over the
last 60 years, including production, biodiversity and species distributions,
have had effects on fisheries production and other marine life. This has been
mainly driven by climate variability and ocean warming. These changes
include:
• Extensive changes in the planktonic ecosystem in terms of plankton
production, biodiversity, species distribution which have effects on
fisheries production and other marine life (e.g. seabirds).
• In the North Sea, the population of the previously dominant and
important zooplankton species, (the cold-water species Calanus
finmarchicus) has declined in biomass by 70% since the 1960s.
Species with warmer-water affinities (e.g. Calanus helgolandicus) are
moving northwards to replace the species, but are not as numerically
abundant.
• There has been a shift in the distribution of many plankton and fish
species around the planet. For example, during the last 50 years there
has been a northerly movement of some warmer water plankton by
10° latitude in the North-east Atlantic and a similar retreat of colder
water plankton northwards (a mean poleward movement of between
200–250 km per decade).
• The seasonal timing of some plankton production has also altered in
response to recent climate changes. This has consequences for
plankton predator species, including fish, whose life cycles are timed
in order to make use of seasonal production of particular prey species.
• The decline of the European cod stocks due to overfishing may have
been exacerbated by climate warming and climate-induced changes in
plankton production (Beaugrand et al., 2003). It is hypothesised that
the survival of young cod in the North Sea depends on the abundance,
seasonal timing and size composition of their planktonic prey. As the
Citation: Edwards, M.,
Atkinson, A., Bresnan, E.,
Helaouet, P., McQuatters-
Gollup, A., Ostle, C., Pitois, S.
and Widdicombe, C. (2020)
Plankton, jellyfish and climate
in the North-East Atlantic.
MCCIP Science Review 2020,
322–353.
doi: 10.14465/2020.arc15.plk
Submitted: 07 2019
Published online: 15th January
2020.

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stocks declined, they have become more-sensitive to the effects of
regional climate warming due to shrinkage of the age distribution and
geographic extent.
• Future warming is likely to alter the geographical distribution of
primary and secondary pelagic production, affecting ecosystem
services such as oxygen production, carbon sequestration and
biogeochemical cycling. These changes may place additional stress
on already depleted fish stocks, as well as have consequences for
mammal and seabird populations. Additionally, melting of Arctic
waters may increase the likelihood of trans-Arctic migrations of
species between the Pacific and Atlantic oceans.
1. PLANKTON AND CLIMATE CHANGE IMPACTS
Plankton includes both the free-floating photosynthesising life of the oceans,
as well as marine microscopic animals. Algal phytoplankton, bacteria and
other photosynthesising protists produce c. 50% of net global primary
production (Field et al., 1998). They export carbon to the deep ocean and as
the base of the marine food-web provide food for the animal plankton
(zooplankton) which in turn provides food for many other marine lifeforms
ranging from microscopic organisms to baleen whales. The carrying capacity
of pelagic ecosystems in terms of the size of fish resources and recruitment
to individual stocks as well as the abundance of marine wildlife (e.g. seabirds
and marine mammals) is highly dependent on variations in the abundance,
seasonal timing and composition of this plankton.
In marine environments, the main drivers of change include climate warming,
point-source eutrophication, deoxygenation and unsustainable fishing
(Edwards, 2016). Furthermore, unique to the marine environment,
anthropogenic CO2 is also associated with Ocean Acidification (OA). OA has
the potential to affect the process of calcification and therefore certain
planktonic organisms dependent on calcium carbonate for shells and
skeletons (e.g. coccolithophores, foraminifera, pelagic molluscs,
echinoderms) may be particularly vulnerable to increasing CO2 emissions
(Edwards, 2016). It is also worth noting that while pelagic systems are
undergoing large changes caused by climate change, they have also been
identified as a form of mitigation of climate change through possible human
manipulation of these systems through geoengineering. It has been shown that
at small scales the addition of iron to certain oceanic environments (ocean
fertilisation) can increase productivity and net export of carbon to the deep
ocean. However, this approach is still controversial with largely unknown
long-term ramifications for marine ecosystems at the large scale. For
example, it could lead to negative effects such as the stimulation of Harmful
Algal Blooms (HABs) or hypoxia, but further investigations are needed
(Güssow et al., 2010). While there are a myriad of pressures and intertwined

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multiple drivers on the marine environment and on plankton ecosystems,
some of which are synergistic (for example, the interaction of temperature,
ocean acidification and hypoxia) in this report, the focus is on the effects of
climate change impacts on planktonic communities.
1.1 Plankton in the policy context
As understanding of the ecological role of plankton in marine systems has
developed, plankton have become increasingly used as indicators of
environmental status to support marine management under European
management mechanisms, which are implemented in UK waters. The UK’s
Marine Strategy and the EU Marine Strategy Framework Directive (MSFD)
(European Commission, 2008) seek to achieve ‘Good Environmental Status’
(GES) for European Seas. As the base of the marine pelagic ecosystem,
indicators of plankton community structure are used to assess the ‘pelagic
habitat’ component of UK and European marine ecosystems under the
Directive and Marine Strategy, and are representative of broader pelagic
ecosystem status. A suite of plankton indicators recently developed for these
policy drivers captures aspects of pelagic diversity (McQuatters-Gollop et al.,
2019; OSPAR, 2017e), functioning (OSPAR, 2017c) and productivity
(OSPAR, 2017b, d) in the North-East Atlantic and are included in the
biodiversity and food-webs state assessments under the MSFD at the OSPAR
level and the Marine Strategy at the UK level, while chlorophyll a is used as
a eutrophication indicator (OSPAR, 2017a). A key challenge under these
policy drivers is separating plankton change driven by climate change from
change in plankton caused by directly manageable pressures, such as nutrients
and fishing (McQuatters-Gollop, 2012). This information is required so that
management efforts can be effectively focused.
Complementarily to the MSFD and UK Marine Strategy that focus on
regional- and national-scale management of marine waters, nearshore
phytoplankton are integral to the EU Water Framework Directive (European
Commission, 2000) which aims to achieve GES of European waters within
one nautical mile of shore; the UK remains committed to this goal post-
Brexit. Plankton indicators under the Water Framework Directive (WFD) are
primarily linked to eutrophication pressures (Devlin et al., 2009). The EU
Control of Products of Animal Origin Regulation mandates the monitoring of
potential toxin-producing phytoplankton species in shellfish production areas
as part of a statutory monitoring programme to protect human health from
algal toxins (European Commission, 2017).
Each of these legislative examples demonstrates the importance of plankton
monitoring programmes for managing marine resources, informing
conservation measures, and protecting human health.

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1.2 Plankton and climate change impacts: the North Atlantic wide
context
Large-scale trends in plankton and climate variability
The Continuous Plankton Recorder (CPR) survey is a long-term, sub-surface,
marine plankton monitoring programme consisting of a network of CPR
transects towed monthly across the major geographical regions of the North
Atlantic. It provides excellent spatial and temporal coverage around the
British Isles. It has been operating in the North Sea since 1931 with some
standard routes existing with a virtually unbroken monthly coverage back to
1946. Figure 1 and Table 1 show the distribution of CPR samples and the
number of CPR samples in the eight UK ecoregional areas in the North-East
Atlantic. The CPR covers most ecoregional areas very well with the
exception of the Inner Hebrides of Western Scotland (Region 6) where
sampling was not good enough to provide multidecadal time–series data and
trends. To summarise the long-term trends in plankton at the large-scale, we
used a number of indices of plankton from the CPR survey that included the
sum of the abundance of all counted diatoms (number of taxa: 125) and all
counted dinoflagellates (number of taxa: 79) and total copepod numbers
(number of taxa: 196) for these eight ecoregional areas (Figures 2, 3 and 4).
Using bulk indices like this is less sensitive to environmental change and will
quite often mask the subtleties that individual species will provide; however,
it is thought that these bulk indices represent the general functional group
response of plankton to the changing environment an approach that has been
adopted for the assessment of GES for the MSFD.
In the North Atlantic, at the ocean-basin scale and over multidecadal periods,
changes in plankton species and communities have been associated with
Northern Hemisphere Temperature (NHT) trends and natural climate
variability such as the Atlantic Multidecadal Oscillation (AMO); the East
Atlantic Pattern (EAP) and variations in the North Atlantic Oscillation (NAO)
index (Edwards et al., 2013a). These have included changes in species
distributions and abundance, the occurrence of sub-tropical species in
temperate waters, changes in overall plankton biomass and seasonal length,
changes in the ecosystem functioning and productivity of the North Atlantic
(Beaugrand et al., 2003; Edwards et al., 2001; Edwards et al., 2002; Edwards
and Richardson, 2004; Reid and Edwards, 2001).

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Figure 1: Distribution of CPR samples in UK sea regions (top) and monthly sampling effort
from 1958–2016 (bottom; see Table 1). Based on the Charting Progress 2 assessment
which sub-divides the UK sea area into eight regions. Total sampling effort based on
monthly sampling in UK regional seas, highest percentile in red and lowest percentile in
blue.

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Table 1: UK regional areas and number of CPR samples per region
Area number
Area name
Number of samples
Colour on map
1
Northern North Sea
21 824
Dark blue
2
Southern North Sea
10 152
Blue
3
Eastern Channel
1669
Light blue
4
Western Channel and
Celtic Sea
10 568
Green
5
Irish Sea
3498
Yellow
6
Minches and Western
Scotland
144
Orange
7
Scottish Continental
Shelf
5724
Light red
8
Atlantic North West
Approaches and
Faroe–Shetland
Channel and Rockall
Trough and Bank
6180
Dark red
Contemporary observations over a 10-year period of satellite in-situ blended
ocean chlorophyll records indicate that global ocean net primary production
has declined over the last decade, particularly in the oligotrophic gyres of the
world’s oceans (Behrenfeld et al., 2006). However, over the whole temperate
North-East Atlantic, there has been an increase in phytoplankton biomass
with increasing temperatures but a decrease in phytoplankton biomass in
warmer regions to the south (Richardson and Schoeman, 2004). These
changes have been linked to changes in the climate and temperature of the
North Atlantic over the last 50 years.
It must be noted, however, that climate variability has a spatially
heterogeneous impact on plankton in the North Atlantic and around the
British Isles and not all ecoregional areas are correlated to the same climatic
index. For example, trends in the AMO are particularly prevalent in the
oceanic regions and in the sub-polar gyre of the North Atlantic and the NAO
has a bigger impact in the shallower southern North Sea (Harris et al., 2014).
This is also apparent with respect to the Northern Hemisphere Temperature
where the response is also spatially heterogeneous with areas of the North-
East Atlantic and shelf areas of the North-West Atlantic warming faster than
the North Atlantic average and some areas like the sub-polar gyre actually
cooling. Similarly, abrupt ecosystem shifts do not always occur in the same
region or at the same time. The major shift that occurred in plankton in the
late 1980s was particularly prevalent in the North Sea and was not seen in
oceanic regions of the North Atlantic. However, a similar ecosystem shift
occurred in the plankton abundance 10 years later in the Icelandic Basin and
in oceanic regions west of the British Isles. The different timing and differing
regional responses to ecological shifts have been associated with the

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movement of the 10°C thermal boundary as it moves northwards in the North
Atlantic as a result of climate warming (Edwards et al., 2013).
In examining the long-term trends in the plankton indices, the general pattern
is an increase in phytoplankton biomass for most regions in the North Atlantic
and in the ecoregions around the British Isles, with differing timings for the
main step-wise increase occurring being later in oceanic regions compared to
the North Sea. For the diatoms there is not really a predominant trend for the
North Atlantic Basin as a whole (Figure 2) but some regions show a strong
cyclic behaviour over the multidecadal period. The time signal resembles an
oscillation of about 50 to 60 years featuring a minimum around 1980,
reflecting changes in the AMO signal. Particularly large increases in diatom
abundance over the last few years are seen in the Irish Sea (area 5). For the
dinoflagellates there has been a general increase in abundance in the North-
West Atlantic and a decline in the North-east Atlantic over a multidecadal
period (see Figure 3). In particular, some regions of the North Sea have
experienced a sharp decline over the last decade. This decline has been mainly
caused by the dramatically reduced abundance of the dinoflagellate Tripos
genus (previously Neoceratium and Ceratium Gomez 2013) in the North Sea.
However, Tripos abundance has recovered in the North Sea over the last 5
years. Particularly large decreases in dinoflagellate abundances are seen in
offshore areas to the west of Scotland (area 8).

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Figure 2: Total diatom abundance (standardised) for the eight ecoregions around the British
Isles from 1958–2016. Total diatoms produced using 125 taxa.

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Figure 3: Total dinoflagellate abundance (standardised) for the eight ecoregions around the
British Isles from 1958–2016. Total dinoflagellates produced using 79 taxa.
Trends in copepod abundances (Figure 4: only large copepod species
abundance shown) have been more stable in offshore regions, but the small
species have shown a large decrease in abundance over the last few years,
particularly in the southern North Sea and English Channel (areas 2 and 3).
In summary, while climate warming is a major driver for the overall biomass
of phytoplankton, diatoms are less influenced by temperature and show a
strong correlation with the AMO signal and wind intensity in many regions
(Edwards et al. 2013; Harris et al. 2014).

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Figure 4: Total large copepod abundance >2mm (standardised) for the eight ecoregions
around the British Isles from 1958–2016. Total copepods produced using 196 taxa.
West of the British Isles, the progressive freshening of the Labrador Sea
region, attributed to climate warming, and the increase in freshwater input to
the ocean from melting ice, has resulted in the increasing abundance, blooms
and shifts in seasonal cycles of dinoflagellates due to the increased stability
of the water-column. Similarly, increases in coccolithophore blooms in the
Barents Sea and changes in the distribution of harmful algal bloom species in
the North Sea over a multi decadal scale are associated with negative salinity
anomalies and warmer temperatures leading to increased stratification
(Edwards et al., 2006).

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To the north of the British Isles, the Barents Sea and Arctic sea regions have
been warming at a faster rate than other regional areas of the North Atlantic
(Rahmstorf et al., 2015). Taking the North Atlantic as a whole, of particular
note is the emergence of a coldwater anomaly in the North Atlantic south of
Greenland (sub-polar gyre region) since 2014 (see Figure 5). This area
experienced record cold conditions in 2015 thought to be caused by Atlantic
wide circulation changes and specifically by the slowing down of the Atlantic
Meridional Overturning Circulation (AMOC) (Rahmstorf et al., 2015; Caesar
et al., 2018). The consequences of this anomaly on the climate, Atlantic
circulation and plankton of the North Atlantic will be an ongoing and pressing
investigation.
Figure 5: Maps of Sea Surface Temperature (SST) anomalies for 2014 (left) and 2015 (right)
for the Northern Hemisphere. Anomalies calculated on the mean of the period 1960–2013
for 2014 and 1960–2014 for 2015. Based on GISS data http://data.giss.nasa.gov/gistemp/.
See also SAHFOS Global Marine Ecological Status Report for more information (Edwards
et al., 2016).
In summary, in the North Atlantic, at the ocean-basin scale and over
multidecadal periods, changes in plankton species and communities have
been impacted by climate change with strong correlations with the Northern
Hemisphere Temperature (NHT), the Atlantic Multidecadal Oscillation
(AMO), the East Atlantic Pattern (EAP) and variations in the North Atlantic
Oscillation (NAO) index. It is estimated that 50% of the change is down to
natural climate variability (e.g. AMO and NAO index) and the other due to
forced anthropogenic warming (Harris et al., 2014). These have included
changes in species distributions and abundance, the occurrence of sub-
tropical species in temperate waters, changes in overall plankton biomass and
seasonal length, changes in the ecosystem functioning and productivity of the
North Atlantic (Beaugrand et al., 2003; Edwards et al., 2001; Edwards et al.,
2002; Reid and Edwards, 2001; Edwards and Richardson, 2004). Over the
last five decades there has been a progressive increase in the presence of
warm-water/sub-tropical species into the more temperate areas of the North-

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East Atlantic and a decline of colder-water species. The mass biogeographical
movements are related to climate change and the warming of the North
Atlantic. A particularly interesting feature over the last five years is the
decline in subarctic species to the south-east of Iceland and their movement
to the north and west (Edwards et al., 2016).
Northward shifts and seasonal (phenology) indicators
A useful indicator of the warming trend in the North Sea (a northward shift
indicator) is the percent ratio of the cold-temperate Calanus finmarchicus and
the warm-temperate Calanus helgolandicus copepod species (Figure 6)
(Edwards et al., 2016). Although these species are very similar, they occupy
distinct thermal niches. The thermal boundary for the arctic-boreal distributed
copepod C. finmarchicus in the North-East Atlantic lies between ~10–11°C
isotherm and is a useful indicator of major biogeographical provinces. C.
helgolandicus usually has a northern distributional boundary of 14°C and has
a population optimum lying between 10–20°C. These two species can
therefore overlap in their distributions. When these two species co-occur there
is a tendency for high abundances of C. finmarchicus earlier in the year and
C. helgolandicus later in the year. There is clear evidence of thermal niche
differentiation between these two species as well as successional partitioning
in the North Sea, probably related to cooler temperatures earlier in the year
and warmer temperatures later in the year (Edwards et al., 2016). Over a
decadal period C. helgolandicus has moved northwards from its particular
stronghold in the Celtic Sea to replace C. finmarchicus in most of the
ecoregional areas of the British Isles (see Figure 6). This is a clear sign of
warming waters around the British Isles.
Examination of CPR data up until 2016 revealed the percentage ratio between
C. helgolandicus and C. finmarchicus in 2009 to 2011 was for the first time
in twenty years dominated by C. finmarchicus in spring (Figure 7). This was
a reflection of the particularly cold winter experienced in Northern Europe
caused by a very low winter NAO index during that period.
Uncharacteristically, during this period the NAO has been in a very low
negative phase contributing to the very cold winters experienced in Northern
Europe during 2009/2010 and 2010/2011 reflected in below average SST in
the North-east Atlantic. Similarly, this has had an effect on the timing of
seasonal cycles in the North Sea for many species. The last couple of years
have seen a later seasonal peak of plankton compared to the long-term trend,
which was a trend towards earlier seasonal cycles. While Northern Europe
was experiencing cold weather areas in Greenland, the Canadian Arctic and
the Labrador Sea hit record temperatures in 2010 resulting in extended melt
periods (Edwards et al., 2016). Between the 1960s and the post 1990s, total
Calanus biomass in the northern North Sea has declined by 70% due to
regional warming. This huge reduction in biomass has had important
consequences for other marine wildlife in the North Sea including fish larvae
(Edwards et al., 2016).

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Figure 6: Decadal abundance maps for the cold-water copepod Calanus finmarchicus (top)
and the temperate copepod Calanus helgolandicus (bottom) from 1960–2015. (Data from the
CPR survey.)
In summary, since we are only just beginning to understand the complexity
of ‘bottom-up’ controls on ecosystem structure, our appreciation of their full
ramifications will continue to improve with continued monitoring of the
plankton as the global climate changes. How these ‘bottom-up’ controls of
ecosystem productivity interact with ‘top-down’ effects, such as fishing, will

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be a necessary component of ecosystem management models. Other
components of the plankton community are also changing. Gelatinous
plankton are difficult to sample routinely but data are also showing changes.
For example, long-term monthly changes in the frequency of jellyfish
nematocysts (stinging cells) in CPR samples show an increase in frequency
of gelatinous zooplankton in the North Sea and North-east Atlantic (Attrill et
al., 2007). In many other marine regions worldwide, a proliferation of
jellyfish are seen as an indicator of ecosystem degradation. Since some
jellyfish feed on fish eggs, fish larvae, and zooplankton, they can exert both
top-down and bottom-up control of fish recruitment.
Figure 7: Long-term monthly plots (1960–2016) plots of the percent ratio between Calanus
finmarchicus (cold-water/ blue colour) and Calanus helgolandicus (warm-water/ yellow
colour) copepods in the North Sea. An increase in the warmer C. helgolandicus can be seen
over the last 60 years particularly after the 1980s.

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Trends in jellyfish and other gelatinous plankton around the British Isles
Predatory gelatinous zooplankton (Cnidaria and Ctenophora) populations
play an important role in our coastal and shelf waters. Limited and sporadic
monitoring of these species has hampered understanding of their responses to
climate change and the group warrants further research. Many species of
gelatinous zooplankton are able to rise rapidly in abundance and form
extensive aggregations, commonly known as ‘blooms’, when suitable
environmental conditions arise (e.g. a thermal niche or a high availability of
prey, possibly due to overfishing of planktivores) and as such the group is
potentially an indicator of ecosystem instability (Lynam et al., 2011).
Jellyfish also form the principle prey of exotic species such as sunfish (Mola
mola) and leatherback turtles (Dermochelys coriacea) that migrate to UK and
Irish waters to feed (Houghton et al. 2006). Sporadic sampling has occurred
on scientific surveys that support fisheries assessments and this is typically
the best available information (Bastian et al. 2011; Lynam et al. 2005, 2011).
However, efforts have been undertaken, since 2012, to expand monitoring of
gelatinous plankton, in particular the large cnidaria or ‘true jellyfish’. To this
effect, a cost-effective standard protocol was designed and tested, that can be
applied to any trawl-based fishery survey (Aubert et al., 2018). A harmonised
co-ordinated gelatinous data collection programme will help to fill some of
the knowledge gaps identified above. Furthermore, visual surface counts from
ships of opportunity and long-term information from the CPR can be
particularly useful (Attrill et al., 2007; Bastian et al., 2011), in particular
when integrated with other data sources to evaluate diversity and abundances
(Lincandro et al., 2015).
The frequency at which gelatinous tissues and nematocysts (stinging cells)
are caught in the CPR sampler has been used to map the pattern of gelatinous
zooplankton abundance across the North-East Atlantic Ocean and shelf seas
(Richardson et al., 2009). In oceanic waters, depth >200 m, gelatinous
zooplankton abundance between 1946 and 2005 was linked significantly and
positively to temperature and total copepod abundance. Notably, jellyfish in
the North-East Atlantic show cyclic changes in population sizes (c. 20-year
cycle in oceanic waters and 30-year cycle in shelf seas). However, since 1997
they have been increasing in frequency in CPR samples simultaneously in
shelf and oceanic waters (Attrill et al., 2007; Richardson et al., 2009;
Licandro et al., 2010).
The oceanic scyphozoan, Pelagia noctiluca, was carried into Irish coastal
waters during 2007 and resulted in the mortality of over 150,000 farmed
salmon in Antrim, Northern Ireland. Concern was prompted that this species
is increasing with climate change since this jellyfish is common in the
Mediterranean Sea and considered a warmer-water species (Licandro et al.,
2010). However, historical reports and anecdotal sightings revealed that it has
occurred previously in Irish and UK waters and such events are part of an
intermittent cycle. Nevertheless, P. noctiluca occurs in high abundance in the

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Mediterranean following warm, dry periods and occurrences of this species
in northern waters might be expected to become more frequent following
climate change (Licandro et al., 2010). Data from the Irish bottom trawl
surveys in 2009 indicated that P. noctiluca was present on the Malin Shelf, to
the north of Ireland, particularly in subsurface temperatures >13.2 °C (Bastian
et al., 2011a).
The most common medusae in UK and Irish waters are the scyphozoans
Aurelia aurita and Cyanea spp. and data from plankton nets has shown that
these taxa have increased in abundance in the Irish Sea between 1994 and
2009 (Lynam et al., 2011). Statistical analyses of these data indicated that
catch rates of jellyfish were high following warm and dry periods. Notably,
the frequency of cnidarian occurrence from CPR samples in the Irish Sea
correlated significantly and positively with the catch rates from the plankton
nets (Lynam et al., 2011). In this area, CPR data reach back to 1970. Not only
does the cnidarian index indicate higher frequencies of occurrence in the
period 1992–2010 relative to 1970–1981, but the data also suggest a period
of frequent and extensive outbreaks between 1982 and 1991. Given that the
period 1982–1991 was not dominated by warm-dry years, it is interesting to
note that these outbreaks occurred during structural change in the
phytoplankton- and copepod-community and that this followed a period of
overexploitation of planktivorous herring (Lynam et al., 2011).
The North Sea has suffered from limited data in relation to gelatinous
zooplankton since the end of the International pelagic trawl survey for young
gadoid fish (1971–1986; Hay et al. 1990). These historical trawl data
indicated great fluctuations in jellyfish abundance linked to variability in the
NAO and increases over time in Cyanea capillata abundances in the northern
and eastern North Sea (Lynam et al., 2005). In contrast to the Irish Sea data,
the CPR cnidarian occurrence index does not correlate with scyphozoan
abundances in the North Sea (Lynam et al., 2010). However, the CPR data do
indicate an overall increase in the occurrence of gelatinous zooplankton in the
North Sea since the early 1980s coincident with a change from a cold to a
warm hydroclimatic regime (Licandro et al., 2010).
The non-native ctenophore Mnemiopsis leidyi was first reported in the North
Sea and Baltic Sea in 2005 (Faasse and Bayha, 2006; Antajan et al., 2014).
There has been cause for concern regarding the spread of this species given
the previous invasions of the Black and Caspian Seas (Shiganova et al.,
2001). In parallel with the rise of this predator, fish eggs and larvae from
already depleted overfished stocks collapsed in this region (Daskalov and
Mamedov, 2007).

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Plankton and climate change impacts: ecoregional summaries
Northern North Sea (Region 1)
The northern North Sea was, until recently, a cold-boreal province. However,
after the late 1980s regime shift, the northern North Sea is now considered a
temperate province. Plankton in the northern North Sea generally comprise
Atlantic and offshore species as these waters are stratified during summer
months. Copepods such as Calanus finmarchicus and Metridia lucens are
typically found in this region. Larger-sized phytoplankton measured at the
Dove Time-Series (DTS) station have undergone a significant change in
biodiversity (species numbers) roughly centred on 1988–1990. Prior to this,
biodiversity had steadily declined since the start of the time series in 1971
before beginning a general increase from 1990 to the present day. Preliminary
analyses indicate that the pre-1990 phase of the time series was more strongly
influenced by the monthly NAO index, while post 1990 biodiversity patterns
appear to have been more influenced by local SST. This is interpreted as a
shift from basin scale driving of biodiversity to an emergence of local climate
as the most important environmental factor. The change in biodiversity
coincides with an intrusion of warmer, more saline water into the North Sea
in the late 1980s (Beaugrand, 2003) that appears to have persisted since then,
reducing thermohaline stratification and the definition of frontal regions
(Beare et al., 2002).
Data from the Marine Scotland Scottish Coastal Observatory (SCObs)
monitoring site at Stonehaven in the coastal north-western North Sea, show
large interannual variation in the abundance of plankton. Due to the length of
this time-series it is difficult to attribute climate change directly to these
observed changes. However, some of these more-localised changes do show
similarities with the offshore data collected by the CPR survey. A low diatom
abundance was observed from 2001–2004 particularly during the spring
bloom period (Bresnan et al., 2009, 2015a). Diatom cell densities
subsequently increased for a period since 2005 with Skeletonema becoming
more abundant during some years. A decrease in the abundance of the
summer thecate dinoflagellate Tripos has been observed since 2000, but has
latterly begun to recover (Bresnan et al., 2016). This is in line with patterns
observed in the open northern North Sea by the CPR. A study examining the
diversity of the ecologically important diatom Pseudo-nitzschia in the North
Sea has highlighted diversity differences in the community at a species level
between the monitoring site at Stonehaven and that in Helgoland in the
southern North Sea (Bresnan et al., 2016).
A strong seasonal signal typical of northern latitudes has been observed in the
zooplankton community at Stonehaven (Fanjul et al., 2017, 2018) with timing
of the spring diatom bloom having a strong influence in shaping the seasonal
signal (Fanjul et al., 2017). The two Calanus copepod species, Calanus
finmarchicus and Calanus helgolandicus, show different seasonal dynamics:
C. finmarchicus is carried into the area in the spring as late-stage copepodites.

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These produce one, sometimes two, generations of offspring none of which
survive locally through the winter. C. helgolandicus is present at very low
levels during the winter and there is a small spring increase in abundance
during April–May. These copepodite stages then decline rapidly so that the
overwintering numbers are very low (Bresnan et al., 2016).
Considerable interannual variability has been observed in the abundance of
both C. finmarchicus and C. heloglandicus at the Stonehaven monitoring site.
Numbers of C. finmarchicus have been generally low, but the annual average
increased since monitoring began in 1997 until 2013. A sharp spike in
springtime abundance was observed in 2008 and 2009. An extension of the
C. helgolandicus growing season into the earlier summer months has been
observed from the beginning of the monitoring period in 1997 until 2008. In
2009, due to a combination of high C. finmarchicus and low C. helgolandicus
abundances, C. finmarchicus became more dominant at this site than C.
helgolandicus for the first time since 1997. In 2010 extremely low numbers
of both species were recorded, and C. helgolandicus was again the dominant
of the two species. This is similar to patterns observed in the CPR time–
series.
Calanoid copepods are an important food source for the ecologically
important sandeel (Ammnodytes marinus). The synchrony between egg
production of C. helgolandicus and sandeel hatching has been observed to be
an important factor in early larval development which influences year-class
strength in this region (Regnier et al. 2017). Sandeel abundance in turn
influences kittiwakes breeding success however analysis of a 12-year time-
series study from this region shows there are some years where this
relationship does not hold (Eerkes-Medrano et al., 2017).
Southern North Sea (Region 2)
The plankton community of the southern North Sea primarily consists of
coastal species which are well-suited to the mixed waters of this region.
Decapod larvae, along with copepod species such as Centropages hamatus
and Calanus helgolandicus, are commonly found in the southern North Sea.
Phytoplankton biomass is greater here than in the northern North Sea, and has
been increasing since the 1988 ecological shift. Although some localised
coastal areas in this region may be affected by eutrophication, this is primarily
a problem in inshore coastal waters. For the most part changes in plankton in
the southern North Sea are driven by climatic variability. Over the last few
decades, climate warming in the southern North Sea has been noticeably
faster than in the northern North Sea (mainly due to being shallower)
(Edwards et al., 2016). This is reflected in the biological response of
planktonic organisms; for example, phenological cycles observed in the
southern North Sea have moved further forward in time than in the northern
North Sea (Edwards and Richardson 2004). There has been a general decline
in small copepods in this region over the last few years.

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Eastern Channel (Region 3)
The Eastern English Channel is characterised by strong tidal currents and
shallow bathymetry, leading to well-mixed water columns. There is also
substantial freshwater influence, particularly from rivers emanating from the
French coast. These influences contribute to the relatively high proportion of
benthic larvae in the plankton. Delavenne et al. (2013) produced a pelagic
habitat typology for the Eastern Channel, producing seven water masses for
each of the four seasons of the year. These classifications, based on physical
variables, phytoplankton, zooplankton and pelagic fish, reflected the relative
stability of the French waters and the central part of the Eastern English
Channel, relative to the waters of the Dover strait and the English coastal
waters. While regular inshore sampling to monitor phytoplankton and
nutrients exists along the Eastern Channel coasts (e.g. Hernández-Fariñas et
al., 2014), few fixed point, long time–series data are available from which to
determine the responses of plankton to multidecadal environmental change.
One such suitable time–series is the inshore time series at Gravelines, located
near the Western Port of Dunkirk Sampling with a standard methodology
shows sub-decadal cyclicity but no clear trends in plankton abundance
possibly due to the short length of the time-series study.
Climate change may also modulate the spread and establishment of resident
population of harmful invasive species. For example, Antajan et al. (2014)
reported the establishment of the comb jellyfish Mnemiopsis leidyii along
ports along the French Channel coast that dated from 2005. This species has
yet to become established along the northern side of the English Channel and
the anthropogenic and climate change and influences on the spread of this
predator is an active area of research (Jaspers et al., 2018).
Western Channel and Celtic Sea (Region 4)
These waters are more substantially influenced by oceanic waters than the
eastern Channel and are typically seasonally stratified, although regions of
relatively low surface temperatures in summer indicate areas of enhanced
tidal mixing, for example south of Ireland, north of Brittany and west of
Cornwall. Nutrient concentrations are depleted substantially during the
growth season and indeed recent cruises to the Celtic sea have shown that iron
concentrations can approach limiting levels in the Celtic Sea (Birchill et al.,
2017).
Long-term, full-depth observations of plankton exist at a series of monitoring
sites collectively known as the ‘Western Channel Observatory’
(https://www.westernchannelobservatory.org.uk/) south of Plymouth and
since the Edwards et al. (2013) MCCIP report these have exhibited a
continued increase in surface- and near-seabed-water temperature,
punctuated by the relatively cold period around 2010. This general warming
mirrors the long timescale (over ~30 year) warming trend observed across the
North Atlantic (O’Brien et al., 2017). Several long-term datasets from
Plymouth stations L5 and E1 have been used to consider long-term climate

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related changes in zooplankton and fish larvae populations. McManus et al.
(2016) examined a multidecadal cycle in multiple components of the pelagic
food web known as ‘the Russell Cycle’ (Cushing and Dickson, 1976) in
relation to the Atlantic Multidecadal Oscillation (AMO) and the North
Atlantic Oscillation (NAO). Likewise, Blackett et al (2014) examined long-
term data for two relatively warm water siphonophore species and found that
establishment of resident populations dated from the late 1960s. These studies
provide further evidence for the climatic warming of water temperature as a
key factor in the biogeographical scale redistribution of species.
In addition to biogeographical adjustments to climatic warming, phenological
adjustments (changes in seasonal timing) can be envisaged as a mechanism
by which species can adjust to changing temperatures in situ (Beaugrand et
al. 2014). However, trophic levels may respond to different timing cues, for
example spring phytoplankton blooms may be triggered by changes in
photoperiod and underwater light (Ji et al., 2010, Wiltshire et al., 2008),
whereas their zooplankton grazers may respond more closely to temperature
(Mackas et al. 2012). This has led to concerns that a warming climate may
lead to differential timing shifts between trophic levels and a de-
synchronisation of the food web (Richardson, 2008). The weekly resolution
data at Plymouth L4 was used to examine this in a stratifying coastal shelf
site where weekly resolution sampling was possible (Atkinson et al., 2015).
Only a minority of species showed strong shifts in phenology and even for
these, mismatches with food increases did not clearly penalise the grazer.
Two factors appeared to provide resilience to this mismatching. First, food
concentrations were relatively high throughout the year, enabling species
with diverse diets to be partially immune from timing shifts. Second,
mortality strongly shaped the phenology of species (Cornwell et al. 2017),
obscuring the phenology shifts observed.
Zooplankton populations can be modified both directly through temperature
and indirectly through climatic effects on their phytoplankton food sources.
At the Plymouth L4 site the phytoplankton community displays large inter-
annual variability and strong seasonal patterns (Widdicombe et al., 2010).
Twenty three years (1993–2015) of data show diatoms have increased during
the autumn and declined significantly during the winter while annual averages
do not show significant change. Conversely, coccolithophores have also
increased significantly during the autumn, but not at other times or between
years. Phaeocystis was routinely found, albeit in low numbers, during the
autumn until 2002 when it disappeared until reappearance in 2013. These
changes suggest a recent revival of the autumn bloom in the western English
Channel. Dinoflagellate numbers have not changed significantly and are
highly variable between seasons and years. Despite this, a slight increase in
the overall proportion of diatoms relative to dinoflagellates is in line with
findings based on CPR data across the North-East Atlantic and North Sea
(Hinder et al. 2012). However, an almost 10-fold change in the mean biomass
of both diatoms and dinoflagellates that can occur between successive years

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at L4 (Atkinson et al. 2015) means that evidence of a long-term change is not
conclusive, especially when based on annual averages as subtle changes can
occur seasonally. For example, the dinoflagellate Tripos has been increasing
at Station L4, which is in contrast with observations from the Stonehaven
time–series in the north western North Sea where Tripos populations are
recently recovering after a 10-year decline since 2000.
Unusual species have been recorded in the western English Channel in recent
years. A raphid diatom Plagiolemma distortum (Nézan et al., 2018), was first
observed off the southern coast of Brittany in December 2014 and later in the
Celtic Sea and at Station L4 during winter and spring (Kraberg et al., 2018).
Subsequent investigations found further records dating back to 1992 in the
southern North Sea and eastern English Channel. To date, P. distortum has
not been recorded in the northern North Sea suggesting this taxa is thus far
restricted in its distribution. Asteromphalus sarcophagus and A. flabellatus
were also found during winter months at Station L4 and the effects of climate
warming and increased turbulence through winter storms could influence
phytoplankton diversity in the future.
Preliminary analysis of 30-year zooplankton trends at Station L4 suggests a
decline in several major small copepod species and increases in benthic
larvae. This is in line with the wider scale trends in declining small copepods
(Edwards et al., 2013). The benthic larvae increase to partially replace
copepods at L4 parallels the general pattern found in the North Sea by Kirby
et al. (2008). Gelatinous and semi-gelatinous predators have increased
slightly at L4 over the last three decades, such that the mean carbon content
(carbon mass as a percentage of wet mass) in recent years is around 5%, as
compared to about 7% three decades ago (McConville, 2017). Again, this is
in line with the notion of multidecadal cycles for larger jellyfish species based
on data from fisheries surveys in the Irish and North Seas (Lynam, 2011).
Irish Sea (Region 5)
The plankton community in the Irish Sea contains warm-temperate Atlantic
and offshore species and its composition is influenced by the region’s
hydrological regime (mixed in the winter and stratified during summer). Like
the North-East Atlantic as a whole, Irish Sea plankton are primarily regulated
by the sea’s hydroclimatic regime. However, some coastal regions of the Irish
Sea, such as Liverpool Bay, have elevated phytoplankton biomass levels that
have been attributed to nutrient enrichment (Gowen et al. (2000)). Although
nutrient concentrations in some localised areas are elevated, for the most part
the Irish Sea has not shown signs of eutrophication such as: (a) trends in the
frequency of Phaeocystis spp. blooms and occurrence of toxin producing
algae; (b) changes in the dominant life form of pelagic primary producers and
(c) oxygen depletion in nearshore and open waters of the Irish Sea (except the
seasonally isolated western Irish Sea bottom water). This suggests that
widespread anthropogenic eutrophication has not impacted the Irish Sea at a
regional scale (Gowen et al., 2000).

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The main zooplankton grazers comprise small zooplankton species
(Pseudocalanus, Temora and Acarita) with Oithona peaking in the summer
months. Species of Calanus recorded are believed to be as a result of oceanic
intrusions (Scherer et al., 2016). Increases in the number of jellyfish were
observed from 1994–2009 (Lynam et al., 2011).
A comprehensive report of the ecosystem of the Clyde sea area in Region 5
has also been produced (McIntyre et al., 2012). Data from the MSS SCObs
monitoring station in Millport has been included in this report. The
phytoplankton community was monitored at this site between 2005 and 2013.
While the phytoplankton community in this area showed regional differences
in the timing and composition of the spring diatom bloom compared with
other monitoring sites, the time-series study is too short to identify climate-
related signals (Bresnan et al., 2016). In contrast to sites in region 6, the
autumn diatom bloom is largely absent from this site. The development and
positioning of frontal systems have been seen to influence the spatial
distribution of some phytoplankton species in this regional sea (Paterson et
al., 2017).
Minches and western Scotland (Region 6)
The Minches and western Scotland region consists of transitional waters
which, like the Irish Sea, are mixed during winter and stratified during
summer. In addition, the region receives freshwater runoff from the
Highlands of Scotland via the many fjords along the mainland coast and
islands. In general, the plankton community in this region consists of cold-
temperate boreal species. Apart from regular Harmful Algal Bloom (HAB)
monitoring in coastal areas, this region as a whole is poorly monitored (also
see Figure 1 for CPR sampling). Investigation of the phytoplankton
community in Loch Creran has shown changes in the microplankton
community including a decrease in the abundance of diatoms during the
spring bloom period. This is believed to be due to changes in rainfall patterns
and intensity in area since the 1980s (Whyte et al., 2017). Marine Scotland
has been operating a SCObs monitoring site since 2003 in Loch Ewe Some
of the phytoplankton community changes observed on the east coast have also
been observed at Loch Ewe. For example, Skeletonema has also become more
abundant at this site during 2005, and a similar pattern of decrease and
subsequent recovery of the thecate dinoflagellate Tripos has also been
observed (Bresnan et al., 2015b, 2016). In general, due to the length of this
time–series in this region, it is difficult to attribute climate change directly to
these observed changes.
Scottish Continental Shelf (Region 7)
Like the Minches and western Scotland region, the Scottish Continental Shelf
consists of transitional waters which are mixed during winter and stratified
during summer. In general, the plankton community in this region consists of
cold-temperate boreal species and includes Atlantic and offshore species as
well as some shelf species. Marine Scotland has operated a SCObs monitoring

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sites at Scapa Bay in the Orkney Isles and Scalloway in the Shetland Isles
since 2001. The phytoplankton community structure at the Orkney site has a
similar composition to that observed at Stonehaven (Region 1) and Loch Ewe
(Region 6). In comparison, some differences can be observed in the
composition of the phytoplankton community at Scalloway. The
dinoflagellate Tripos is infrequently observed during the summer months.
Instead, the dinoflagellate community is dominated by genera such as
Gonyaulax and Alexandrium. An increase in the abundance of the diatom
Skeletonema has been observed at both of these sites since 2005. At the
Orkney site, a decrease in the abundance and subsequent recovery of the
dinoflagellate Tripos has also been recorded. Similarly to Region 6, due to
the length of this time-series in this region it is difficult to attribute climate
change directly to these observed changes. However, the CPR survey also
monitors offshore regions in this area which does show changes to the
plankton community are related to climate-change impacts. This region is a
particularly productive shelf system, especially around the Orkney and
Shetland Islands.
Atlantic North-west approaches, Rockall Bank and Trough and Faroe–
Shetland Channel (Region 8)
The Rockall Bank and Trough area is oceanic in nature and the plankton
consist of both warm-temperate oceanic species as well as cold-boreal
species. As this region is on the cusp of the warm-temperate and cold-boreal
marine provinces, biogeographical shifts have occurred more rapidly here
than in any other region due to advective processes (Beaugrand et al., 2009).
This region is highly biodiverse because of the higher proportion of warm-
temperate species and occasional sub-tropical incursions. The Rockall Bank
and Trough region is also characterised by high primary productivity and high
zooplankton biomass. It is thought that mesoscale eddies within this region
play an important role in maintaining high productivity. The offshore oceanic
region is characterised by high productivity, particularly along the continental
shelf edge. The shelf edge current and North Atlantic current extend into this
region bringing more southerly distributed species to the area. Plankton
studies in this area are scant and mostly as a result of one-off cruises.
Differences in the phytoplankton community composition and abundance has
been observed on and off the shelf edge (Fehling et al., 2012, Siemering et
al., 2016).
The Faroe–Shetland area is more complex. The upper 500 m of the water
column has its origins in the Rockall Trough and poleward flowing North
Atlantic Current, and this is reflected in the plankton community. However,
below 600 m depth in the Faroe-Shetland Channel and Faroe-Bank Channel,
there is a counter-flow of cold, less saline water from the deep Norwegian
Sea into the Atlantic. This water has its origins in the Arctic and temperatures
decline to below 0ºC. Here, the plankton community is entirely different.
Zooplankton are scarce at these depths during the summer and few diel
migrating species enter these waters. But, in the winter abundance of

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zooplankton is high, comprising mainly overwintering stages of the
ecologically important copepod Calanus finmarchicus, and the Arctic
copepod Calanus hyperboreus (Heath et al., 1999). Few fish or euphausiids
enter these cold, deep waters, so the overwintering copepods are effectively
in a refuge from predation. The overwintering C. finmarchicus in the Faroe-
Shetland Channel are thought to be an important seeding area for productive
summer populations in the northern North Sea, since a proportion are carried
onto the North-West Scottish shelf when they migrate back to the surface
waters in the spring. Similarly, to Regions 6 and 7, due to the length of this
time-series in this region, it is difficult to attribute climate change directly to
these observed changes. However, the CPR survey also monitors offshore
regions in this area which does show changes to the plankton community are
related to climate change impacts (see ‘Plankton and climate change impacts:
the North Atlantic wide context’, above).
Marine Microbes
The very small component of the marine plankton community (nano plankton
2–20 µm diameter, picoplankton 0.2–2 µm diameter, marine bacteria and
viruses) are poorly monitored. Their very small size and rapid generation
times mean that they have the potential to act as early indicators of climate
change. Routine monitoring of this component of the plankton community in
the UK is scant with the most sustained measurements being made at L4
(Tarran and Bruun, 2015 revealing a distinct seasonality. New technologies
are allowing previously unexplored components of the marine ecosystem to
be explored on a broader scale, e.g. genetic diversity of marine viruses (Garin-
Fernandez et al., 2018), fungi (Stern et al., 2015, Taylor and Cunliffe, 2014)
etc. This knowledge gap needs to be filled if the impacts of climate change
on the marine environment are to be fully understood.
2. WHAT IS ALREADY HAPPENING?
In summary, in the North Atlantic, at the ocean basin scale and over
multidecadal periods, changes in plankton species and communities have
been impacted by climate change with strong correlations with the Northern
Hemisphere Temperature (NHT), the Atlantic Multidecadal Oscillation
(AMO), the East Atlantic Pattern (EAP) and variations in the North Atlantic
Oscillation (NAO) index. It is estimated that 50% of the change is down to
natural climate variability (e.g. AMO and NAO index) and the other due to
forced anthropogenic warming (Harris et al., 2014). These have included
changes in species distributions and abundance, the occurrence of sub-
tropical species in temperate waters, changes in overall plankton biomass and
seasonal length, changes in the ecosystem functioning and productivity of the
North Atlantic (Beaugrand et al., 2009; Edwards et al., 2010; Edwards et al.,
2013; Beaugrand et al., 2019). More recently, these changes have also has
included trans-Arctic migration of species from the Pacific to the Atlantic, a

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change in biodiversity at the ocean basin scale and a move towards smaller
sized community composition (Reid et al., 2007; Beaugrand et al., 2010).
What is already happening
X
3. WHAT COULD HAPPEN IN THE FUTURE?
Regional climate warming and hydro-climatic variability has had and is
continuing to have a major effect on the plankton in Northern European seas.
Future warming is likely to alter the geographical distribution of primary and
secondary plankton production (0–5 years), affecting ecosystem services such
as oxygen production, carbon sequestration and biogeochemical cycling (20–
50 years). These changes may place additional stress on already-depleted fish
stocks as well as have consequences for mammal and seabird populations.
Currently the distributions of plankton organisms are moving northwards at
an average rate of ~23 km per year, although the rates of individual species
vary substantially (Beaugrand et al., 2009).
Ocean acidification may become a problem in the future (20–100 years) and
has the potential to affect the process of calcification. Therefore, certain taxa
such as molluscs and other calcifying organisms of the plankton may be
particularly vulnerable to CO2 emissions. Potential chemical changes to the
oceans and their effects on the marine biology could reduce the ocean’s ability
to absorb additional CO2 from the atmosphere, which in turn could affect the
rate and scale of global warming.
Warming and potentially acidification will increase the risks to natural
carbon stores and carbon sequestration. As temperature increases, the
geographical distribution of primary and secondary plankton production is
likely to be impacted, effecting ecosystem services such as oxygen
production, carbon sequestration and biogeochemical cycling (10–100 years).
Changes in phenology and biogeographical changes in plankton community
composition leading to whole ecosystem shifts are likely to result.
High
Medium
Low
Amount of evidence
Level of
agreement/consensus
H
M
L
H
M
L

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Increased length of stratification period is expected to affect phytoplankton
community composition through physical processes as well as through
changes in nutrient cycling as flagellates are generally better suited than
diatoms for the predicted stratified nutrient-depleted conditions. There is also
recent evidence from the CPR survey that warming temperatures decrease the
size of the plankton community (for both phytoplankton and zooplankton);
this may also eventually lead to a decrease in size of fish species (Beaugrand
et al., 2010). A smaller-sized community will lead to more regeneration of
carbon within the surface layers and it is presumed carbon sequestration to
the deep ocean will be less efficient. In summary, these changes in the
plankton community may place additional stress on already-depleted fish
stocks as well as having consequences for mammal and seabird populations.
Risks to species and habitats and opportunities for new organisms to
become established. Recent evidence from the CPR survey also suggests an
increase in pathogenic organisms such as Vibrio in UK waters related to
warming temperatures (Vezzulli et al., 2011). Continued warming will allow
new species colonisations as well as potential new pathogens and Harmful
Algal Bloom species.
What could happen in the future?
X
Observational evidence is high, however, the ability of models to predict
changes in the future are still quite low.
4. KEY CHALLENGES AND EMERGING ISSUES
Priority challenges
• Mechanistic links between climate warming, plankton and
fisheries (and other higher trophic levels such as seabirds) to form
a predictive capacity.
High
Medium
Low
Amount of evidence
Level of
agreement/consensus
H
M
L
H
M
L

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• Identifying species and habitats particularly vulnerable or resilient
to climate change impacts and separating the impacts of climate
from other anthropogenic pressures such as nutrients.
• Understanding the risks to species and habitats and the potential
opportunities for new species colonisations as well as potential
new pathogens and Harmful Algal Bloom species.
Emerging issues
• Understanding the risks caused by warming temperatures and
acidification on native marine organisms.
• Understanding the processes involved in the biological pump
(plankton draw-down of atmospheric CO2) and understanding
future changes (risks to natural carbon stores and carbon
sequestration).
• Understanding the rate of genetic adaptation to climate change
impacts.
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