1 Chapter 10. Biodiversity and Biogeography of the Lower Trophic fauna of the Pacific Arctic Region

111 Pages·2012·21.32 MB·English

by  Jackie Grebmeier

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Chapter 10.

Biodiversity and Biogeography of the Lower Trophic fauna of the Pacific
Arctic Region – Sensitivities to Climate Change.

R.J. Nelson, C. Ashjian, B. Bluhm, K. Conlan, R. Gradinger, J. Grebmeier, V.
Hill, R. Hopcroft, B. Hunt, H. Joo, D. Kirchman, K. Kosobokova, S. Lee, W.
Li, C. Lovejoy, M. Poulin, E. Sherr, K. Young.

Microbes, zooplankton, and the benthic and sympagic invertebrates, underpin
the marine environment of the Pacific Arctic Region (PAR). Therefore a firm
grasp of their distribution and abundance is necessary to judge how climate
forcing may be affecting the ecology of the region. Recent field observations
indicate that ecosystem responses, including range shifts, and changes in the
relative abundance of particular taxa, have occurred within the last decade. Here
we draw upon historical and new data to provide a region wide survey of viruses,
bacteria, archaea, auto- and heterotrophic protists, as well as metazoans in the
sympagic, pelagic and benthic realms. Our aim is to provide a foundation for the
assessment of the changes within the lower trophic level taxa of the PAR and to
document such change when possible. Sensitivities to the effects of climate
change are also discussed. Our vision is to enable data-based predictions
regarding ecological succession in the PAR under current climate scenarios, and
to deepen our understanding regarding what the future holds for higher trophic
level organisms and the carbon cycle.

10.1 General Introduction
Microbes, zooplankton, and the benthic and sympagic invertebrates, underpin
the ecology of the Pacific Arctic Region (PAR). The ways in which these groups
of lower taxonomic level organisms respond to change in water properties,
currents and sea ice dynamics have important implications for ecosystem
function. Therefore a firm grasp of their distribution, abundance, and community
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composition combined with an understanding of linkages to watermass
properties, is necessary to put such responses in holistic system-wide functional
framework. Responses to habitat change potentially include altered phenology,
abundance, community composition and range shifts. The Pacific Arctic Region
is vulnerable to establishment of southern organisms from both the Atlantic and
Pacific oceans. Future biotic changes may cascade up to higher trophic level
species, some of which are important to the sustenance of local communities.
Recent field observations indicate that ecosystem responses, including range
shifts, and changes in the relative abundance of particular taxa, have occurred
within the last decade (Grebmeier et al. 2006b, Li et al. 2009).
In recent years, increased research effort in the Pacific Arctic Region has led
to greater power to detect and understand ecological change. The life span of
the lower trophic taxa ranges from days (microbes) to decades (benthic infauna).
Those taxa with short life spans and life histories will respond quickly to physical
forcing and can provide a sensitive indication of ecosystem change; on the other
hand, study of these taxa may be confounded by strong seasonality. Benthic
infauna are spatially constrained and have long life spans, and thus integrate
local climate signal over longer periods of time. Study of the benthic infauna has
therefore provided us with some of the strongest indication of ecosystem change
in the PAR to date (Section 10.4.2). Here we draw upon historical and new data
to provide a region wide survey of viruses, bacteria, archaea, auto- and
heterotrophic protists, and metazoans in the ice, pelagic and benthic realms. Our
aim is to provide a data-based foundation for assessment of the response of
lower trophic level taxa in the PAR to climate forcing, and to document change
where sufficient data exists. Sensitivities to the effects of climate forcing are also
discussed. Our vision is to enable data-based predictions regarding ecological
succession in the PAR under current climate scenarios, and to deepen our
understanding regarding what the future holds for higher trophic level organisms
and the carbon cycle.

References:
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Grebmeier JM, Overland JE, Moore SE, Farley EV, Carmack EC, Cooper LW,
Frey KE, Helle JH, McLaughlin FA, McNutt SL (2006) A major ecosystem
shift in the northern Bering Sea. Science 311: 1461-1464.

Li WKW, McLaughlin FA, Lovejoy C, Carmack EC (2009) Smallest algae thrive
as the Arctic Ocean freshens. Science 326: 539-539

10.2 Phytoplankton in the Pacific Arctic region.

V. Hill, S. Lee, H. Joo, M. Poulin, C. Ashjian, W. Li.

10.2.1 Introduction.

Populations of sub-Arctic and Arctic phytoplankton are controlled by the extreme
seasonality of the polar environment. The phytoplankton bloom occurs when
increases in light, coupled with fresh water and thermally induced stratification,
allows for phytoplankton growth. Intense blooms of diatoms, primarily
Thalassiosira and Chaeroceros spp., (Aizawa et al. 2005; Suzuki et al. 2002) are
observed along the retreating ice edge across shelf and basin regions. Much of
this carbon is sequestered to the benthos through a highly efficient biological
pump (Campbell et al. 2009; Sherr et al. 2009; Wehrmann et al. 2011). This
strong benthic-pelagic coupling supports an extremely high benthic biomass and
upper trophic levels that depend on this biomass, including seabirds, walrus, and
grey whales (e.g., Grebmeier et al. 2006).
Sea surface warming trends in the Arctic Ocean have been most pronounced
in the Pacific region with recent summertime temperature anomalies of + 2.5 oC
(Steele et al. 2008). Summer-time sea ice coverage has deceased across the
Arctic since 1979, with total extent declining by 2.8 to 7.2% per decade per
month (Serreze et al. 2007). An alteration of the food web through the
introduction of temperate species, changes in the location and timing of
phytoplankton blooms forced by changes in SST or ice dynamics may shift
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ecological processes in the Bering and Chukchi Seas. It is postulated that the
timing of phytoplankton growth relative to water temperature plays a critical role
in determining the pathways of reduced carbon biomass. Walsh & McRoy (1986)
hypothesized that blooms occurring in “warm” water would be consumed by
copepods who’s foraging and growth increase in efficiency at higher water
temperatures (Huntley & Lopez 1992). Blooms occurring earlier in cold water
would not be effectively grazed by the copepods, and would fall to the bottom to
support a benthic food web (Hunt et al 2002). Recent observations of warming
and cooling phases in the Bering Sea have indicated the need for revision to this
Oscillating Control Hypothesis (OCH) to incorporate shifts in energy flow through
tropic levels (Coyle et al. 2011). Any modification of benthic communities caused
by changes in sinking of carbon from the photic zone will have profound impacts
on the ability of the system to support those mammals that are considered to be
the signature species of the arctic (i.e. Polar Bears, Grey Whales and Walrus),
and confer the cultural and social identity of the region.
This section describes the biodiversity and biogeography of phytoplankton
within the Pacific Arctic Region, with the intent of understanding and detecting
future changes.

10.2.2 Phytoplankton and Sea Ice Algae: An Overview.

At the pan-Arctic scale a total of 2,106 single-celled eukaryote taxa, including
1,874 phytoplankton and 1,027 sea-ice taxa have been reported (Poulin et al. in
review). The highest diversity of both phytoplankton and sea-ice eukaryotes was
recorded in Canadian waters.

American sector
In the Bering Strait and the Alaskan Beaufort Sea, diatoms and dinoflagellates
account for 74% (phytoplankton) and 17% (sea-ice) of the total number of
unicellular eukaryotes (Table 10.2.1). Landfast and pack ice in the Alaskan
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Beaufort Sea are predominantly colonized by pennate diatoms which account for
77% of all unicellular eukaryotes recorded, with the micro-sized fraction (i.e. 20–
200 µm) representing 96%.

Canadian sector:
Marine phytoplankton in the Beaufort Sea and Canada Basin are predominantly
large celled diatoms and dinoflagellates which account for 58% and 21%,
respectively, of all 555 microscopic forms recorded (Table 10.2.2). Arctic
landfast and pack ice in this sector are principally colonized by pennate diatoms
which accounted for 69% of all 257 microscopic forms recorded, with the micro-
sized fraction (i.e. 20–200 µm) representing 90% of all taxa observed.

Table 10.2.1. Phytoplankton and sea-ice unicellular eukaryotes recorded from
Bering Strait and the Alaskan Beaufort Sea (from Poulin et al. in review).
Taxon # taxa# genera Dominance Dominant taxa
Bacillariophyta 330 91
Centric diatoms 99 29 34 Chaetoceros Attheya septentrionalis >50%
21 ThalassiosiraT halassionema nitzschioides >60%

Pennate diatoms 231 62 Cylindrotheca closterium >65%
10 Entomoneis Fragilariopsis cylindrus >50%
52 Navicula Fragilariopsis oceanica >50%
36 Nitzschia Navicula pelagica >50%
19 Pinnularia Nitzschia frigida 80%
Dinophyceae 74 28 21 Protoperidiniu m
Chlorophyta 13 11
Others 26 36
Total 443 116
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Table 10.2.2. Phytoplankton and Sea-ice Unicellular Eukaryotes recorded from
the
Canadian Beaufort Sea and Canada Basin (from Poulin et al. in review).

Taxon # taxa # genera Dominance Dominant taxa
Bacillariophyta 324 82
Centric diatoms 100 24 Attheya septentrionalis
44Chaetoceros >80%

17Thalassiosira M elosira arctica >75%

Pennate diatoms 224 58
10 Entomoneis Cylindrotheca
closterium 95%
54 Navicula
30 Nitzschia Fragilariopsis cylindrus
16 Pinnularia >75%
N. directa >50%
N. frigida >60%
Dinophyceae 115 34 17 Gymnodinium
18 Gyrodinium
24Protoperidinium
Chlorophyta 13 13
Others 103 71
Total 555 200
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Dominant Arctic taxa are mostly associated with cold waters and the
presence of available sea-ice habitat. Nitzschia frigida can be considered a
sentinel species for the occurrence of first-year sea ice, while Fragilariopsis
cylindrus is often associated with first-year sea ice but is also known to occur in
cold pelagic waters. Melosira arctica is associated with the under surface of the
sea ice forming long threads consisting of huge number of colonies, and Attheya
septentrionalis is correlated to cold pelagic waters. In addition while diatoms
dominate in regions with 50 to 90% ice cover, flagellates are often more
abundant than diatoms under thicker total ice cover (Booth and Horner 1997,
Gosselin et al. 1997). As water temperatures and sea ice dynamics and
characteristics change, we might expect alterations in the distribution and
abundance of these Arctic taxa with unknown consequences for ecosystem
function.

10.2.3 Latitudinal variation of Phytoplankton Biodiversity and Community
Composition in the Western Arctic Ocean

Temporal succession in phytoplankton species is observed from the dark cold
winter into the warm, stratified summer. During winter, biomass is extremely low,
and it is postulated that some Arctic species overwinter in the sea ice, seeding
the water column in the spring as the ice melts. The primary spring bloom is
dominated by large rapidly growing species. Typically, large taxa of the early
bloom are followed by smaller celled taxa which rely on recycled nutrients, with
increases in cell abundance occurring at the nutricline (Hill et al. 2005). As this
pattern follows the retreat of sea ice in the region, a single transect from the
Bering Sea to the Canada Basin taken as a snapshot, will reveal a gradient of
phytoplankton structure from post bloom conditions backwards towards the
winter structure.
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The sea ice melt, which stimulates phytoplankton growth, occurs first in
the Bering Sea, which is also the best studied region within the Pacific Arctic
Region (Shirshov 1982). Smaller phytoplankton are generally characteristic of the
Bering Sea, although larger diatoms can contribute substantially to the biomass
(Shiomoto 1999). Aizawa et al. (2005) found that phytoplankton communities
associated with the spring bloom on the shelf were dominated by centric diatoms
in the form of Thalassiosira spp. (among others by T. gravida Cleve, T. trifulta
Fryxell, and T. conferta Hasle), Chaetoceros debilis and C. contortus Schütt.
Several pennate diatoms were also constituent members of such blooms (for
example Pseudo-nitzschia seriata, Fragilariopsis cylindriformis (Hasle) Hasle,
Fragilariopsis oceanica (Cleve) Hasle; Broerse et al. 2003). In summer,
Chaetoceros spp. and elongate centric diatoms such as Leptocylindrus spp.,
Proboscia spp., Guinardia spp., Rhizosolenia spp. were abundant as well
(Sukhanova et al. 1999). In the surface waters overlying the deeper basins,
dominant diatoms were Chaetoceros spp. (both sections, Chaetoceros spp. and
Hyalochaete spp.) and Thalassiosira spp. in spring, whereas smaller diatoms
Nitzschia seminae and Fragilariopsis cylindriformis are more abundant in
summer (Aizawa et al. 2005). Evidence exists for phytoplankton community
structure shifts in the Bering Sea, with the recent appearance of coccolithophorid
blooms and increased contributions of Phaeocystis spp. during the summer
(Merico et al., 2003).
The shelf assemblage of the adjoining Aleutian Sea was characterized by
Chaetoceros contortus and C. debilis, and Thalassiosira spp., (for example by T.
conferta, T. gravida, and T. trifulta (Aizawa et al. 2005). Although low in
abundance, C. diadema was generally found over the shelf or close to the
Aleutian Islands. The Aleutian Basin assemblage was dominated by pennate
diatoms such as Fragilariopsis pseudonana, Nitzschia seminae, and Pseudo-
nitzschia spp. Results from a long-term sediment trap study in the Aleutian Basin
showed that Nitzschia seminae represented 25–90% of the vertical diatom flux,
with resting spores of Chaetoceros spp. becoming very abundant (Kurihara and
Takahashi 2002).
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Microalgal populations on the Chukchi Sea Shelf follow a similar pattern to
the Bering Sea. Communities associated with the seasonal ice zone were
characterized by a mixture of small (<5 µm) taxa, including prasinophyes and
chrysophytes and larger phytoplankton (> 5 µm) identified as diatoms and
haptophytes (Hill at al 2005). Chlorophyll concentrations characteristic of bloom
conditions were associated with an overwhelming dominance by diatoms.
Smaller sized phytoplankton, most likely prasinophytes were found over the
slope and basin areas where ice cover was still continuous. Post bloom “bust”
conditions on the Chukchi Shelf are associated with low nutrients and biomass in
the surface layer along with smaller celled prasinophytes, haptophytes and
diatoms, while diatoms dominate the subsurface chlorophyll maximum. On the
shelf break of the Chukchi Sea (70 oN to 72oN), large centric diatoms (>10 um)
dominate the pelagic environment, with pennate diatoms prevalent in the ice
bottom algae (Booth and Horner 1997, Gosselin et al. 1997).
A transect from the Bering Sea, through the Chukchi Sea to the Canadian
Basin in the summer of 2008 showed that larger phytoplankton were present in
the seasonal ice zone, while the most abundant species were nano-pico sized
species at the surface in open waters and also at the subsurface chlorophyll
maximum. The second most dominant species were variable but most commonly
Thalassiosira sp., Chaetoceros sp. and unidentified nano-pico phytoplankton
such as Dinobryon belgica and Cryptomonas sp (Figure 10.2.1).
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Figure 10.2.1. Relative abundances of phytoplankton assemblages in the North
Pacific Ocean and the western Arctic seas (A: surface, B: SCM depth). The pie
charts show relative abundance shares of the major taxa: Dinophyceae,
Bacillariophyceae, Cryptophyceae, Chrysophyceae, Prasinophyceae,
Prymneosiophyceae, unidentified nano-sized phytoplankton, unidentified pico-
sized phytoplankton (Lee and Joo unpublished).
10.2.4 Synechococcus.

One of the more intriguing distribution patterns for Western Arctic
autotrophs is the occasional presence of the coccoid cyanobacteria
Synechococcus. Synechococcus was earlier thought to be intolerant of cold
water (Murphy and Haugen 1985), to be found only at temperate and tropical
latitudes (e.g., Waterbury et al. 1986), and not grow at temperatures less than
~5°C (Waterbury et al.,1986). However, extensive subsequent explorations have
shown that Synechococcus can be found in waters of lower temperatures at
concentrations of about 10,000 to 100,000 cells per liter (Li 2009a, b), for
example in the northern subpolar waters of the Labrador Sea shelf (Harrison et
al. 2011), and the southern subpolar waters adjacent to the Antarctic polar front
in the southern Atlantic Ocean (Doolittle et al. 2008).
In the western Arctic, Synechococcus has been observed at locations
where Pacific Water is or has been present (Cottrell and Kirchman 2009; Ashjian
et al. 2010; Sherr, Sherr, and Ashjian, unpubl.). Water from the northern Bering
Sea enters the Arctic through Bering Strait and is believed to flow northward to
the Arctic primarily along three routes; (1) along the western coast of Alaska and
through Barrow Canyon as the Alaska Coastal Current, (2) along the eastern
coast of the Kamchatka Peninsula and Siberia and through Herald Canyon and
Valley, and (3) as a broader flow through the Central Chukchi across Hannah
Shoal (Weingartner et al. 2005). These flows of Pacific Water can introduce
Synechococcus to much of the Chukchi Sea. The relative quantity of Pacific
Water at locations in the Chukchi and Beaufort Seas varies annually. Virtually no