Kandidatuppsats
Biologi - naturvård och artmångfald
Effects of Ocean Acidification on Species Composition and Biodiversity in the Region of Kongsfjord (Svalbard)
Examensarbete 15 hp
Halmstad 2020-07-10
Nathalia Grandon
Effects of Ocean Acidification on Species Composition and Biodiversity in the Region of Kongsfjord (Svalbard)
Nathalia Grandon
Bachelor Thesis in Biology (Conservation and Biodiversity) 2020
Abstract
Ocean acidification is a great threat to marine ecosystems and the oceans around the poles are first in line to be affected by the increase in CO2 concentration, effectively damaging the living conditions of marine life present here. This paper analysis and discusses whether studies on the effects of ocean acidification on single species can be used to predict future changes in food web dynamics. The results indicate that by looking at effects on different physiological functions on single species, predictions on how the species in question will cope with a more acidic environment can be made. These predictions can in turn hint on possible changes in the food web. The increase in DOC as well as the shift to smaller and more toxic species of phytoplankton will prove advantageous to bacterioplankton who will play a bigger role in future carbon cycling. The toxic HAB’s could present a hazardous future for primary consumers, but especially secondary consumers. If not affected by the toxicity of the more frequent algal blooms, primary consumers will face negative consequences to the larval recruitment of copepods and shell formation of pteropods by ocean acidification. This would mean a decrease in abundance causing eutrophication and smaller amounts of food sources for fish higher up the trophic cascades, such as the endemic polar cod and the Atlantic cod.
Regardless of the general scarcity of data available from these areas, all studies point to a shift in species composition. A clear indication of a future of lower diversity and ultimately an Arctic Ocean with lower ecological resilience.
Sammanfattning
Rådande havsförsurning utgör ett stort hot mot marina ekosystem och dess vitala biologiska mångfald. Polerna står först på tur av att påverkas av förhöjda nivåer av koldioxid i haven, en process som skulle medföra stora konsekvenser för det liv det bereder plats åt. Denna studie avser att besvara huruvida det går att förutse några indirekta konsekvenser gällande
havsförsurning på det marina ekosystemet i Arktis genom att analysera data samlad i studier som fokuserar på enskilda arters påverkan av lägre pH. Resultaten antyder att
havsförsurningen påverkar särskilda arter direkt, medan andra arter i sin tur påverkas indirekt genom förändringar i näringsväven. Resultaten visar en ökning av DOC samt ett skifte i artsammansättning, eftersom mindre toxiska algarter trivs i högre koldioxidhalter. Detta i sin tur leder till att bakterier frodas då mer DOC blir tillgänglig för tillväxt. Den toxiska
algblomningen är giftig för arter i de högre nivåerna av näringsväven, men kan därtill även visa sig vara farlig för primära konsumenter. Även om dessa inte skulle påverkas negativt av dessa algblomningar så visar data på att copepodernas rekrytering och pteropodernas
skalformation kan komma att påverkas negativt av havsförsurning. Denna potentiella minskning i antalet primära konsumenter skulle därför resultera i eutrofiering. Detta skulle följaktligen även innebära en reducerad mängd tillgänglig föda för sekundära konsumenter.
Trots den uppenbara bristen på omfattande data från de arktiska haven så signalerar den
tillgängliga datan om ett skifte i artkomposition och därmed en framtid med lägre biologisk mångfald. Detta i sin tur skulle vara förödande för det arktiska ekosystemet som skulle få lägre ekologisk motståndskraft.
Introduction
Ocean acidification is one of the threats facing the marine environment today. Due to
emissions from fossil fuels, levels of carbon dioxide (CO2) in the atmosphere have increased to a present level of 413 ppm, compared to the level of 280 ppm measured prior to the
industrialized revolution (NOAA, 2020). The rise of atmospheric CO2 is expected to continue to levels of 800 ppm by the end of the century (Feely et al., 2009). The ocean takes up
approximately half of the CO2 released into the atmosphere, something which in turn alters the chemical composition of the ocean by decreasing its pH-unit – i.e. ocean acidification.
The value of pH in the ocean during the pre-industrial era was 8.2, and currently the pH-value is at 8.1 (EPA, 2020). This means that the amount of CO2 in the ocean has risen by 25% since the start of the industrial revolution. Different marine habitats can have different responses to an increase in acidity. Since the solubility of CO2 increases in water with lower temperatures, regions around the poles are areas with naturally higher CO2 (Sabine et al., 2004). The higher concentration of CO2 in combination with ocean mixing patterns result in a naturally low carbonate ion concentration as well (NOAA, 2020). These factors combined make the polar oceans highly vulnerable to ocean acidification and projection models show that these areas will be first in experiencing the consequences of it, with undersaturation of aragonite (i.e most common naturally occurring crystal forms of calcium carbonate (CaCO3) precipitated by marine and freshwater organisms) having detrimental effects in the Arctic marine ecosystem (Steinacher et al., 2009).
Ocean acidification poses a major threat to shell-forming and calcifying organisms (Kroeker et al., 2010; Hendriks et al., 2009). Its effect on different taxa can vary, some being highly sensitive while some showing mixed responses (Kroeker et al., 2009; Hofmann et al., 2010).
The study by Kroeker et al. (2009) nonetheless showed that there were differences in response to a higher acidity among calcifying organisms as well. They found that the organisms that were negatively affected by ocean acidification used aragonite and low-magnesium calcite compared to unaffected calcifying organisms that used high-magnesium calcite, such as crustaceans. Furthermore, organisms that calcify in earlier stages in their life cycle, such as echinoderms, corals and crustaceans, show a higher sensitivity to acidification than organisms that calcify as adults (Kurihara, 2008). Echinoderms, e.g., show a delayed development or higher mortality rate when exposed to a more acidic environment (Dupont et al., 2008).
However, the knowledge and understanding of the effect an increased CO2 concentration has on marine organisms are at present limited to single species responses, while the effect on food web dynamics remain largely unknown (Rossoll et al., 2012). Nevertheless, even if the effect of increased acidity appears insignificant to certain marine species directly, there could very well be indirect consequences through changes in food web dynamics. By applying an ecosystem approach, perhaps these indirect consequences could be predicted.
By studying the response of acidification with an ecosystem approach, the organisms can be studied within their functional groups, (i.e. groups of organisms with the same function in the ecosystem in terms of habitat or feeding mechanism) (Dobson & Frid, 2009). Since this paper seeks to examine if and how acidification alters the food web dynamics, the functional groups will be classified by their function of feeding mechanisms. On top are the predatory
organisms, usually larger species, followed by smaller predators. Some smaller species in the mid-trophic levels can be both predators and grazers. All consumers can be numbered to determine their place in the food chain (i.e. primary consumer, secondary consumer, tertiary consumer etc.) with the primary consumer being closest to the primary producers in the food web (Campbell et al., 2015). The primary producers are the photosynthetic organisms, such as phytoplankton, as well as photosynthetic bacteria and microflagellates. The bacteria play an important part in the sequestration of carbon in the ocean (Siu et al., 2014). They quickly act on the DOC (dissolved organic carbon) released by phytoplankton, scavenging on the parts they can readily use for growth (Zweifel et al., 1993).
The Arctic ecosystem is characterized as having a few dominant species contributing to the energy flow from the bottom to the top trophic levels (Murphy et al., 2016). The connections within the food web in the Arctic is largely governed by zooplankton and fish. The main link between primary producers and higher trophic levels is the Calanus copepod and due to the considerably large area of shallow shelf areas, the connection between benthic fauna and zooplankton is also possible through vertical flux (CAFF, 2017). Since there is such a small percentage of species contributing to the energy flow of the Arctic marine food web, the effect of acidification on the dynamics of the food web will be greatly determined by the effect it has on these key species (AMAP, 2013). The term FECs (Focal Ecosystem Components) is used by the CBMP (Circumpolar Biodiversity Monitoring Program) to highlight the species within an ecosystem that provides an important function (CAFF, 2017).
Some of these species play a key role in the energy transfer within the ecosystem, such as the previously mentioned Calanus copepod, as well as the polar cod, Boreogadus saida.
However, as stated above, the smaller the amount of species that dominate the energy transfer of an ecosystem, the more sensitive it is to environmental change. Pelagic species in polar regions generally have a slower growth rate and their life cycles tend to be prolonged (Murphy et al., 2016). The absence of light during long periods of the year contribute to primary production being seasonally based (Frey et al., 2018). These harsh conditions induce a storage of energy in the form of fatty acids and lipids, and metabolic processes can be reduced to the point where it can be completely halted (Falk-Petersen et al., 2008). It also causes a flexibility among species to find food, including migration or an omnivorous diet (Murphy et al., 2016).
Being greatly influenced by different environmental factors, the physical environment of the Arctic is a large contributing factor to the dynamics of the food webs present (Kortsch et al., 2019). The Arctic Ocean is a frozen sea with large basins in the center and approximately 50% continental shelves surrounded by land (Hunt Jr. et al., 2016). Its waters are connected to both the Pacific and the Atlantic and is highly influenced by advection (i.e. the horizontal transport of heat or matter in the atmosphere or the sea). This makes the Arctic ecosystems
vary both spatially and temporally, making interactions in the food web scale dependent.
Additionally, polar regions are difficult to monitor due to large areas being covered with thick layers of ice, which further complicates the research of these areas (CAFF, 2017).
Furthermore, the understanding of how ocean acidification impacts the Arctic ocean is imperative since the region have socio-economic importance. The fishing industry present in the Arctic region accounted for over 10% of the worlds catch of wild fish in 2002 alone (Statistics Norway, 2007) and the aquaculture comprise approximately 50% of the EUs volume (Troell et al., 2017). Aside from socio-economic factors, the Arctic is a region of significant ecological importance. Species that are adapted to the area and that can be found nowhere else on the planet adds to the global biodiversity, making it an important region to manage and conserve. This makes the understanding of not only the direct effect of ocean acidification on single species important, but also the indirect effect it has on food web dynamics, since it is an environment highly vulnerable to change and sustains a relatively large proportion of food and revenue for human societies and contributes to global biodiversity.
The aim of this paper is to examine whether studies on the effects of ocean acidification on single species can be used to predict future changes in food web dynamics, and furthermore to see if these changes would impact the species composition and biodiversity of the Arctic Ocean, namely Kongsfjord, Svalbard and ultimately affect the resilience of its ecosystem.
This has previously never been done before and may be a new way of studying ecology using studies that look at single species and single biological functions. This could provide a greater understanding of the effects of ocean acidification on an ecosystem level and perhaps aid in management of these highly sensitive and unique marine habitats.
The specific questions that are desired to answer are:
1. Can studies looking at effects of ocean acidification on single species help foresee changes in an ecosystem level?
2. Can these single species studies predict future changes in food web dynamics that ultimately causes trophic cascades?
3. Will the effects of ocean acidification result in changes in species composition and reduced biodiversity – effectively reducing its ecosystem resilience?
4. Can the method of using studies on single species provide a new method to study marine ecology and changes on an ecosystem level?
Materials and methods Data collection
A literature search was conducted in order to select studies reporting the effects of ocean acidification on biological functions of different taxa found in the Arctic Ocean. The search for data was implementedat three databases; PANGAEA, SCOPUS and Google Scholar. The first search was through PANGAEA and SCOPUS on January 15th with the keywords
acidification AND biological AND response AND polar. The search gave 527 and 25 results respectively. Among these, 27 articles were retrieved from PANGAEA and 3 from SCOPUS.
The second search occurred the following day, January 16th. PANGAEA was searched again, but this time with the keyword Arctic instead of polar. This gave 773 results and among these 3 new articles were retrieved. Articles referring to the biological responses of fish were in a minority among the articles retrieved so another search was conducted in Google Scholar with the keyword’s acidification AND biological AND response AND Arctic AND fish. This resulted in 376 datasets and out of these 11 articles were retrieved. Further articles containing general information on the Arctic Ocean, its ecosystem and food web dynamics were also retrieved from other articles reference lists.
Processing of data
Among the 44 articles retrieved, 21 could be used as they deal with the effect ocean acidification has on biological functions which is the focus of this literature study. The articles are similar in terms of sampling design and experiment design, i.e. exposing the organisms to the same or similar level of pCO2 (partial pressure of CO2), ranging all the way from controlled values of 185 to 1420 μatm (micro-atmospheres), which corresponds to pH- levels of 8.32 to 7.51. A majority of studies were conducted with animal samples taken from the region of the Norwegian Sea. Since this paper intends to look at changes within a food web it did not seem appropriate to use data from the Beaufort Sea or the Bering Sea were some occasional studies were conducted. Considering that these areas are remote from each other and the aforementioned variability within the Arctic Ocean, it is not likely that these species interact within the same food web. All articles collected for this literature study had samples that were collected in Kongsfjord, Svalbard. Among the 21 articles that were used, 5 were related to bacterioplankton, 5 to phytoplankton, 8 to zooplankton, 2 articles wererelated to fish and 1 to benthic fauna. In this paper the organisms will be categorized into the
functional groups’ primary producers (phytoplankton and bacterioplankton), primary
consumers (zooplankton) and secondary consumers (benthic fauna and fish). By analyzing the effectsof ocean acidification on the different biological functions of these functional groups above, a change in food web dynamics, effectively bringing changes in species composition and ultimately the biodiversity of the region could be predicted.
Figure 1. Map of Arctic Ocean highlighting the Svalbard archipelago with the scale 1:250000. The blue star shows the area of Kongsfjord where all the samples of data for this paper were collected. (Norwegian Polar Institute, 2020).
Results and discussion
Among the 21 articles reviewed, 8 concluded no significant response to acidification (p >
0.05), 8 studies found a significant difference (p < 0.05) and 5 showed mixed responses, with some studies showing significant results for some species in their experiments and some not.
Thus, a small majority of the studies (13 out of 21 studies) demonstrated an overall significant response to acidification, with some significant differences shown in studies with mixed responses. The results of each functional group will be presented and discussed below, with each group being presented separately, followed by a methodological discussion. The implications of these results on the food web dynamics and its indirect effect on species composition and ultimately the biodiversity of the area will consequently be discussed after the results of each functional group.
Results for primary producers
Five studies were reviewed on the effect of higher CO2 concentrations on phytoplankton and another five on bacterioplankton. The factors studied among phytoplankton differed, with three studies looking at the effect acidification had on primary production, one study looking at effects on fatty acid composition and one on dynamics in phytoplankton assemblage. All three studies that focused on primary production saw a significant decrease in NCP (i.e. net carbon production) in high CO2 concentrations. In the first study this was due to an increase in POC (particulate organic carbon) and DOC (Engel et al., 2013). The second study saw a weak correlation showing a decrease in NCP with increased pCO2 (Tanaka et al., 2013). The third study saw a similar cumulative NCP, but it was lowest at the highest pCO2 level
(Silyakov et al., 2013). Levels of pCO2 in all three studies ranged from 185 to 1420 μatm (approximately pH 8.32-7.51) and indicates that NCP would decrease at future projections of ocean acidification. Microbial communities of phytoplankton were also studied to see if their community dynamics were influenced by a change in pCO2 levels. One study by Brussaard et al. (2013) showed that ocean acidification resulted in a shift towards smaller phytoplankton (<3µm) that thrived in higher CO2 concentrations. They were also more sensitive to viral lysis (i.e. the breaking down of the membrane of a cell by a virus), which would, besides from grazing, further increase the DOC.
Table 1. Approximate conversion between pCO2 and pH-levels in an experimental study by Bellerby et al.
(2012).
pCO2 pH
185 8.32
270 8.18
375 8.05
480 7.96
685 7.81
820 7.74
1050 7.64
1420 7.51
The study that looked at effects on fatty acid compositions saw no significant indication that the nutritional value of phytoplankton would be affected in a higher acidic ocean (Leu et al., 2013). However, it did change the taxonomic composition of phytoplankton. They saw that dinoflagellates, of which many are highly toxic, would be favored in lower pH-levels and concluded that the change in species composition would have a greater impact on nutrient availability than the effect on fatty acid composition of the community in general. A similar change was seen in the study that looked at phytoplankton assemblage (Schulz et al., 2013).
They also saw that dinoflagellates would prevail in higher CO2 concentrations together with prasinophytes (chlorophyta) and haptophytes.
Another study by Sperling et al (2013) saw similar results when looking at how ocean acidification would affect the diversity and richness of communities of particle attached (>3μm) and free-living (<3 μm > 0.2 μm) bacteria. They saw that particle attached
communities decreased significantly in pCO2 levels of ∼185-685 μatm (∼pH 8.32-7.81) by 25%, but they increased significantly in higher pCO2 levels (up to 1050 μatm, or pH 7.64) which favored picophytoplankton (0.2 and 2 µm) and enhanced the organic substrates available after viral lysis. This resulted in a higher diversity of the particle attached bacterial
community. They also saw a significantly higher bacterial protein production in higher CO2
concentrations, which resulted in a higher community richness and enhanced carbon cycling by bacteria. However, the results for free-living bacteria saw no significant results.
A third study also implied an indirect consequence on bacteria by ocean acidification rather than a direct consequence (Motegi et al., 2013). They investigated the effect on bacterial production, bacterial respiration and bacterial metabolism, but found no evidence of any pCO2
influence on any of the factors. One study looking at the resilience of major bacterial
phylogenic groups saw no significant effect on the structure of bacterial assemblage (Zhang et al., 2013). However, they did notice a significant reduction in bacterial taxonomic richness and diversity index. The study found that the threshold for this reduction was between pCO2
levels 600-675 μatm (i.e. ∼ pH 7.90-7.81). A second study saw similar results when it came to the assemblage of bacteria (Wang et al., 2015). Their results saw an insignificant effect on bacterial assemblage and saw that the structure of the communities of bacterioplankton was similar in all pCO2 treatments ranging from ∼175 – 1085 μatm (∼ pH 8.0-7.75).
Discussion regarding primary producers
The increase in POC and primarily DOC could mean that primary production remains stable or increases with higher pCO2. The decrease in NCP (i.e. net carbon production) would effectively mean that the increased DOC available prompted higher activity of heterotrophic microorganisms and resulted in lower levels of NCP. The increase in DOC could also be attributed to the increase in smaller phytoplankton species that are more tolerable to higher pH-levels seen by Burssaard et al., (2013). Being more sensitive to viral lysis, the smaller phytoplankton would increase the amount of DOC which would further increase microbial activity. These results concur with the results found by Sperling et al., (2013) which saw that particle attached bacteria increased with higher acidity. The higher concentration of DOC would be advantageous for bacteria that needs a surface to survive. A shift in species composition was observed with the taxonomic richness and diversity index being reduced.
This would suggest a shift in the carbon cycle, allowing bacteria to play a greater role in carbon cycling and could also have consequences for future climate change.
Besides the effect ocean acidification has on the carbon cycle, it also seems to affect the food sources available for higher trophic levels. Even though it does not seem to influence the nutritional value of phytoplankton, it did change the species composition. The species favored by the changes in pH-level were dinoflagellates, prasinophytes (chlorophyta) and
haptophytes, which all present ecological hazards. Some dinoflagellates produce neurotoxins (Wang, 2008), while the clade haptophyte includes Chrysochromulina and Prymnesium, which both produce toxic harmful algal blooms (HAB’s) (Cuvelier et al., 2010). The
prasinophytes includes species that can be parasitic (Joubert & Rijkenberg, 1972) and species that can be pathogenic, such as the green algae Prototheca that can cause the infectious
disease protothecosis in animals (Tartar et al., 2002). These HAB’s could become more frequent in these waters, causing eutrophication and hypoxia (low or depleted levels of oxygen), ultimately harming animals belonging to higher trophic levels (Herbert &
Steffensen, 2005). However, there is also evidence of these toxins harming herbivores, in this case the A. tonsa (Jiang et al., 2009). With increasing cell density of the dinoflagellate
Cochlodinium polykrikoides the nutritional value went from beneficial to toxic, with a 100%
mortality rate 3300-4700 μg. More research is needed to see how these toxins would affect the calanoids of the Arctic Ocean, but with the results showing a decrease in their abundance due to ocean acidification below, it seems likely that the grazing would be reduced, and that eutrophication and possibly hypoxia would ensue.
Results for primary consumers
The eight studies surrounding zooplankton was conducted on a variety of species. Five studies looked at copepods, with all of them looking at the native Arctic species of Calanus glacialis.
However, one study also looked at the effects of lower pH on the copepods species Calanus hyperboreus, Oithona similis and nauplii (larvae) of a variety of copepod species (Lewis et al., 2013). This particular study looked at how ocean acidification affected the different life history and behavior of the different species. As previously mentioned, the other studies only looked at C. glacialis. One study looked at the effect on six different naupliar life stages (Bailey et al., 2016). Two studies looked at the effect acidification has on egg production and hatching (Thor et al., 2018; Weydmann et al., 2012) and one looked at the effect on
metabolism and biosynthesis (Thor et al., 2016). The studies that focused on C. glacialis saw no negative effects on the species and concluded that they were resilient to future projections of ocean acidification. The increase in metabolic rate seen in the study by Thor et al. (2016) was attributed to changes in prey concentrations rather than changes in pH and one of the two studies looking at egg hatching only saw a delay in this and could not rule out that more eggs could have hatched after the last day of the experiment. However, the study looking at other species than C. glacialis saw a significantly negative effect on the survival rate of the
remaining species and the same effect was seen on the nauplii. C. hyperboreus, O. similis and large nauplii of other species showed a significantly reduced survival rate in the pCO2
scenario of 700-1000 μatm (~pH 7.81-7.64) and the small nauplii showed a significant reduced survival rate in all treatments, which included 370 μatm (~pH 8.05), besides from 700 and 1000 μatm.
The remaining three studies on calcification as an effect of acidification were done on the pteropod Limacina helicina with one of the studies adding insights to their metabolic activity in a high acidic environment. All three studies saw a significantly negative effect on shell growth with increasing pCO2 levels. A study by Lischka et al. (2011) showed 41% higher shell degradation in elevated pCO2 compared to ambient pCO2. Another study showed the rate of calcification lowered by 28% compared to ambient CO2 concentrations (Comeau et al., 2009).
Discussion regarding primary consumers
A study looking at effects of ocean acidification on different life stages of copepods argued that most studies that focused on these effects looked mainly on adult female copepods (Cripps et al., 2015). The notion that copepods would be resilient against ocean acidification has been stated by previous studies (Weydmann et al. 2012; McConville et al., 2013), but these results misrepresent the overall effect on the different life stages of the copepods. The study by Cripps et al. (2015) looked at the copepod species Acartia tonsa which is mainly found in subarctic oceans. Even if they differ in habitat, the decrease in pH-levels affect all marine habitats, and the Arctic is said to be the most affected. This would mean that the results seen on subarctic species could still be relevant since the acidification will be graver in the polar regions. The studies reviewed on copepods in this paper matches the results seen by Cripps et al. (2015) with negative survival rates in early life stages, which would mean that future generations of copepods could decline in abundance. Being an important food source within the Arctic food web, a decline in copepod biomass could cause trophic cascades and further aggravate the negative effects already predicted for the native B, saida, which mainly feeds on copepods. Since the results show that adult C. glacialis are resilient to acidification compared to other species found in the Arctic sea (C. hyperboreus and O. similis) it could mean a shift in species composition benefiting C. glacialis short-term. However, a negative effect on their larval survival rate could mean a decrease in their population long-term, but such results have not been seen yet.
The metabolic activity of L. helicina seems to be resilient to acidification as shown by Comeau et al., (2010) but still showed a negative effect on calcification. Being an important food source of B. saida, the L. helicina is a major component of the Arctic ecosystem
(Renaud et al., 2012). Commonly known as “sea butterflies”, the L. helicina has an aragonite shell, which is highly soluble. Since the Arctic ocean is projected to experience aragonite- undersaturation, the species is likely to suffer greatly from ocean acidification (Hunt et al., 2010; Yamamoto-Kawai et al., 2009; Bednarsek et al., 2014) This would yet again be detrimental to the B. saida, especially in the Kongsfjord where the pteropod is the second largest food source of the B. saida (Renaud et al., 2012). They also play an important
ecological role as grazers (Hunt et al., 2010) and the loss of the species could cause a decrease in grazing which would effectively cause larger biomass of phytoplankton being left
untouched and ultimately cause eutrophication.
Results for secondary consumers
Only one study was found regarding acidification effects on benthic fauna (included within the trophic level of secondary consumers) in the region of the Norwegian Sea (Harms et al., 2014). It studied the effect acidification had on the gills of the great spider crab Hyas araneus on a molecular level. The study looked for transcriptional changes due to acidification (and different temperatures), due to the gills of marine crustaceans being the major regulatory tissue and defense against disturbances induced by acidification of the hemolymph (Henry &
Wheatly, 1992). They found a significantly negative effect on the gene expression of the gills of H. araneus at a lower pH-level. After a 10-week treatment, 5.3% of the tested transcripts were expressed differentially under the control treatment with the CO2 concentration of 390
μatm (∼ pH 8.10). The largest difference in gene expression was under intermediate CO2
treatment with 1120 μatm (∼ pH 7.60) at 40.0% and the high CO2 treatment of 1960 μatm (∼
pH 7.30)showed a 31.3% difference. The study concluded that the threshold where pCO2
would eventually cause a metabolic depression was ∼ 2000 μatm (∼ pH 7.25), causing changes in their energy budget, hence affecting their growth (Rankin et al., 2019). The hemolymph of crustaceans seems to be able to tolerate lower pH-levels in short periods of time (hours) and keep its ability to maintain the oxygen flow in the body (Whiteley & Taylor, 1992). But long-term effects decrease this ability and compromises the overall survival of the animal. The study by Harms et al. (2014) saw a significantly negative effect on the
transcription in all treatments.
Two articles were found that related to the response of fish to acidification in the Arctic Ocean. The studies looked at how higher pCO2 affected the behavior due to changes in the central nervous system (Schmidt et al., 2017) and the thermal acclimation due to alterations in cardiac mitochondrial metabolism (Leo et al., 2017) of both polar cod (Boreogadus saida) and Atlantic cod (Gadus morhua). The only significant effect found in either of the studies was that the neuroreceptor functioning of G. morhua was positively affected by acidification at a temperature of 8 °C, ultimately showing that G. morhua was more resilient to a higher CO2 concentration than B. saida. The study looking at the metabolic response saw no
significant effect by acidification. While there were no significant results, the CO2 levels had a synergistic effect with increased temperature on the two species, making the effect of higher temperatures worse. Since B. saida showed a lower adaptability to temperature changes than G. morhua, the projected conditions of acidification at pCO2 levels of1170 μatm (∼ pH 7.60) would favor the later species of cod.
Discussions regarding secondary consumers
Compared to species living in intertidal zones and estuaries, inhabitants of the Arctic Ocean live in a relatively stable environment where physical and chemical factors show small variations. The H. araneus has a lower ability to adapt to environmental changes (Whiteley, 2011). Whiteley states that strong osmoregulating species are expected to endure changes in pH, because these species have physiological mechanisms that compensate for the acid-base disruptions. These are the species inhabiting the variable environments mentioned above. The species that inhabit the polar regions and deeper oceans are exposed to a more stable
environment, with low energy and low metabolic rates, and are therefore poor
osmoregulators. With this in mind it is difficult not to assume that all benthic fauna in the Arctic would be in danger of decreasing in abundance due to ocean acidification, however, only one study was found concerning this area and the number of taxa inhabiting the area are far larger than that (see fig. 2). At present, there is no way of telling which taxa would adapt, or which taxa would disappear. Nonetheless, the results from Harms et al. (2014) mirrors the previous results from similar species with similar regulatory defense, meaning that H. araneus will face a challenging future with survival becoming impossible long-term as the ocean becomes more acidic. Being secondary consumers, they are important in both feeding on zooplankton and being a food source for higher trophic levels. In 2013 crustaceans stood for 8.15% (12.61 million tonnes) of the global seafood production (Ritchie & Roser, 2013). One
could theorize that the loss of poor osmoregulators would result in niches being available for more adaptable species already inhabiting the area or invasive species taking over. Either way it may result in a change in species composition and a loss in diversity.
Figure 2. Model showing the energy pathways of the benthic food web in Kongsfjorden (Hop et al., 2002).
The blue boxes highlight the groups of organisms from which data is available regarding the effect of ocean acidification. The knowledge on the effect of acidification on the rest of the benthic species is currently unknown.
The warming waters of the Arctic has expanded the habitat for the Atlantic cod (G. morhua) and haddock (Melanogrammus aeglefinus) and in 2006 juveniles of both species were found in the Svalbard region (Renaud et al., 2012). They could however not see any evidence of competition between the invasive species and the native B. saida due to their different dietary preferences. While the G. morhua and M. aeglefinus feeds primarily on krill and
appendicularians (i.e. solitary, free-swimming tunicates), the B. saida are mainly
planktivorous, feeding on calanoid copepods (Hop & Gjøsæter, 2013). But other food sources were also observed, with pteropod L. helicina being the second largest food source of B. saida
in Kongsfjorden (Renaud et al., 2012). One can argue that despite there not being any
competition for food resulting in a decrease in the native B. saida, there is still evidence of the invasive G. morhua being more resilient of ocean acidification due to their resilience to higher temperatures (Schmidt et al., 2017). These results were however based on experiments on adult fish. A study on G. morhua during their larval stage was conducted to review the survival rate during exposure to higher CO2 concentrations (Stiansy et al., 2016). They saw a significant reduction in the survival of the larvae and a decrease in recruitment and could not rule out that a more acidic environment could negatively affect the gonadal development of adult G. morhua. This would certainly cause an additional decrease in larval recruitment.
Since the polar regions are predicted to face faster and greater effects of ocean acidification, the invasion of, at least, G. morhua would not necessarily mean that they would take over the habitat on the expense of the native B. saida. The warming ocean temperature in the polar regions could initially expand the habitat range of G. morhua, but ocean acidification could limit their abundance long-term. The change in species composition with an increase in diversity seems to already have happened with Atlantic species being caught in Arctic regions (Renaud et al., 2012), but the long-term survival of any of the species seems to be threatened by ocean acidification. This could ultimately have detrimental effects on the fishing industry as the global demand for seafood increases with the global population increase. As in
previous section above, the number of studies and current data on these specific questions are few and makes it difficult to generate confident predictions, however it does point out any gaps of knowledge that would need to be prioritized for the understanding of the ecological effects on the Arctic marine ecosystem and ultimately the production of seafood in the area.
Figure 3. Model showing the plausible trophic cascades that could occur with the future projection of ocean acidification in the Arctic Ocean (Model made by author, 2020). Figure (a) shows the Arctic marine ecosystem prior to the effects of acidification and (b) shows the reduction of consumers as well as a shift in phytoplankton to smaller dinoflagellates and an increase in bacterioplankton, due to an increase in DOC.
Method and material discussion
Lack of data from the Arctic Ocean is more common than any other ocean in the world (perhaps the Antarctic Ocean being an exception) (NOAA, 2002). This is partly due to the harsh conditions in these areas with half of the year being devoid of sunlight. The thick ice
cover makes sampling of species difficult, verging on impossible. It became increasingly more obvious that the areas where more sampling has been taken is around the Svalbard archipelago, with a majority of samples taken specifically from the Kongsfjord. This automatically creates a sampling bias since the Arctic Ocean covers a lot more space than a small fjord of one island. However, this paper aim to look at how food web dynamics potentially could change due to ocean acidification and impact the diversity of species.
Hence, it did seem rational to focus this paper on a smaller area, due to the fact that it was of relevant interest to look at species that interact with each other. However, it is naturally of great importance to take this into account when analyzing this paper’s results and conclusion.
Results from other parts of the Arctic Ocean could differ, but the gap of knowledge from these parts are large. Certainly, concerning the effects of ocean acidification.
It should be noted that some articles found that certain biological functions seem to be adapted to higher concentrations of CO2, such as the metabolism of both the Arctic and the Atlantic cod (Leo, et al., 2017). It should also be noted that in many of these cases, further research and more data is needed to draw accurate conclusions to whether the effects observed are in fact true or if the samples taken was misleading due to the huge gap of knowledge in this area. Certainly, when it comes to the results of organisms belonging to the functional group “secondary consumers”. The number of articles (2 regarding fish and 1 regarding benthic fauna) is not nearly enough to make certain conclusions and more data is left to be desired so as to get a clearer picture of the true effect of ocean acidification on these organisms. As prementioned, only one study on the effects of ocean acidification was made on benthic fauna. This can only give a hint as to other osmoregulators, similar to the Hyas araneus would react, but these hints are only speculations and a broader view of how different species cope to acidification is needed to draw conclusions on how these organisms in turn would affect organisms in other trophic levels. Further study could perhaps reveal knowledge on species that has learned to adapt to the environmental changes facing the Arctic Ocean and aid researchers in finding possible shifts in species composition among benthic fauna. The remaining functional groups had more data available to analyze, which made it easier to draw conclusions on possible trophic cascades. The studies on the Calanus copepods should
however include all life stages as the results for the Calanus glacialis indicated a resilience against acidification, but only in the adult stages. Their nauplii stages showed a reduced ability to survive a more acidic environment and this could ultimately result in fewer
individuals reaching their adult stage. If single species studies are to be used to aid in marine ecology and be of use when looking at the effect ocean acidification has on an ecosystem level, more underlying data will be needed to get a clearer picture.
Conclusion
Even though some of the conclusions presented here are based on small quantities of data, the data still presents results comparable to relevant studies conducted elsewhere. The increase in phytoplankton abundance and shift towards a more prominent microbial carbon pump appears as a plausible, imaginable future for the Arctic Ocean and could trigger damaging climate feedback loops due to the shift of the carbon cycle. The shift in species towards
phytoplankton that creates toxic algal blooms could potentially harm primary consumers, and certainly consumers higher up the food chain. It will undoubtably cause eutrophication since the calcifying grazers would not survive future projections of pH-levels and even non- calcifying grazers show evidence of reduced abundance. Some primary consumers (the copepods primarily) show more resilience towards lower pH-levels, and could cause a shift towards lower genetic diversity, but the question remains if they could survive the toxicity of the HAB’s and if their reduced larval recruitment would ultimately lead to a bottleneck effect.
More conclusive evidence regarding the effect of ocean acidification on different life stages of the copepods could perhaps answer that question. A shift in species composition and reduced diversity of secondary consumers due to acidification further reduces their resilience towards the likely eutrophic sea bottoms causing hypoxia. Furthermore, the reduction of food sources could also prove damaging not only to the species in question but also to the fishing industry in the area as well.
It is evident that more research is needed in regard to the effect of ocean acidification on single species of the Arctic Ocean (and probably elsewhere). and any conclusions presented in this paper on how these effects would in turn effect the ecosystem as a whole should be taken with caution. Future, more extensive data on any marine ecosystem could potentially dismiss or defy any conclusions presented here. Furthermore, the effects of ocean acidification are already present and will never be isolated from rising temperatures due to global warming.
The two factors go hand in hand and more often than not have a synergistic effect on the environment. The Arctic has already been projected to be hit by climate change not only first but the hardest, which makes filling the gap of knowledge more imperative than ever. This paper additionally highlights the current lack of knowledge that is needed in order to understand and mitigate future issues. Having an ecosystem approach to these issues could perhaps mitigate the impact and sustain whatever ecosystem resilience is needed in this area.
The threshold that needs to be crossed before resilience is lost is a question that remains unanswered. The effects of ocean acidification on the species presented in this paper are already observed, and surely more species in the Arctic Ocean suffers the same or other damages due to it. However, the aim of this paper was to evaluate whether studies on the effect of ocean acidification on single species could help foresee any ecological changes, which was something this paper confirms. With the results from the 21 articles reviewed, following changes to the food web dynamics and trophic cascades can be predicted:
1. Increase in DOC, due to smaller phytoplankton becoming more abundant and viral lysis causing further DOC results in a higher microbial activity. This could result in a shift towards a greater role played by microbes in the carbon cycle, which in turn could have consequences to future climate change.
2. The reduced abundance in shell forming organisms and shift in species composition and genetic diversity of copepods would reduce the number of grazers present to feed on phytoplankton, ultimately causing eutrophication. The smaller species of
phytoplankton, which are favored in a more acidic environment would cause toxic algal blooms, which in turn would prove hazardous to organisms higher up the food chain.
3. The reduced abundance of primary consumers causes a reduction in food supply for secondary consumers. Besides from the neuroreceptor functioning being affected by acidification, the lack of food and the toxic HAB’s would further threaten the survival of these organisms.
In conclusion, the answer to the questions in the aim of this paper is that you can foresee changes in an ecosystem by studying the effect of acidification on single species. By
understanding the life history and niche of each species in the ecosystem as well as how they are affected by ocean acidification, you could very likely foresee future damage due to indirect changes in the food web dynamics that would cause trophic cascades and ultimately an ecosystem collapse. In this case both bottom-up and top-down effects can be predicted with both shifts in species composition and reduced biodiversity – ultimately resulting in a lower ecosystem resilience. This in turn would have a detrimental effect on the polar ocean ecosystem services that provide food and resources for millions of people. This paper, however, shows a possible way forward for the study of marine ecology. By combining and sharing data and knowledge between different fields this could be a way to understand the future changes facing the Arctic Ocean.
Acknowledgments
I wish to thank everyone who helped, encouraged and supported me during the thesis process.
Specially to Lars-Erik Widahl for all his aid, information and topic suggestion.
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Appendix
Appendix 1.
Dictionary and definitions
Aragonite - a carbonate mineral that is one of the three most common naturally occurring crystal forms of calcium carbonate (CaCO3). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
CBMP (abbr. Circumpolar Biodiversity Monitoring Program). An international network of scientists, governments, Indigenous organizations and conservation groups working to harmonize and integrate efforts to monitor the Arctic's living resources.
CO2 – Chemical formula for carbon dioxide.
DOC (abbr. Dissolved Organic Carbon). Is the fraction of total organic carbon operationally defined as that which can pass through a filter size that typically ranges in size from 0.22 and 0.7 micrometers.
EU (abbr. The European Union).
FECs (abbr. Focal Ecosystem Components). The CBMP Monitoring Plans identify key elements for each ecosystem, known as Focal Ecosystem Components, where changes in FEC status likely indicates changes in the overall environment and therefore should be monitored.
FL-Bacteria (abbr. free-living bacteria). They don't need to create symbiotic relationships with phytoplankton to survive and replicate.
HAB’S (abbr. Harmful Algal Blooms). Is highly toxic and can severely lower oxygen levels in natural waters, killing marine life.
Magnesium-calcite - a carbonate mineral carbonate mineral and the most stable form of calcium carbonate (CaCO3). Mg content in the ocean results in the crystallization of magnesium calcite.
NCP (abbr. Net Carbon Production). Organic carbon produced through photosynthesis that is not lost through RA (autotrophic respiration) or RH (heterotrophic respiration).
NOAA (abbr. The National Oceanic and Atmospheric Administration). An American scientific agency within the United States Department of Commerce that focuses on the conditions of the oceans, major waterways, and the atmosphere.
PA-Bacteria (abbr. Particle-Attached Bacteria). They live in symbiotic relationships with phytoplankton to survive and replicate.
pCO2 (abbr. Partial Pressure of Carbon Dioxide). Often used in reference to blood but also used in oceanography to describe the partial pressure of CO2 in the ocean and in life support systems engineering and underwater diving to describe the partial pressure in a breathing gas.
pH (abbr. power of hydrogenor potential for hydrogen). Is a scale used to specify how acidic or basic a water-based solution is.
POC (abbr. Particulate Organic Carbon) The fraction remaining on the filter mentioned in the definition of DOC.
ppm (abbr. parts per million). Can also be expressed as milligrams per liter (mg/L). This measurement is the mass of a chemical or contaminate per unit volume of water.
Protothecosis - a disease found in dogs, cats, cattle and humans caused by a type of green algae known as Prototheca that lacks chlorofyll.
Viral lysis – the breaking down of a membrane of a cell by a virus.
Appendix 2.
Table showing the species or groups of organisms sampled and which functional groups they correspond to. The biological functions that were studied under the effect of acidification is also featured with the authors of the 21 references reviewed for this thesis.
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