Anna Mikhina, Ekaterina Draganova, Olga Kanishcheva, Ekaterina Evseeva, Ksenia Zaytseva Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia Abstract
The most abundant macroplankton groups in the Barents Sea are Chaetognatha, Euphausiacea, Hyperiidea and Pteropoda. The paper considers distribution, species structure and abundance of the taxa, as well as impact of the main factors: water temperature and predation in 2000s. The researched period in the Barents Sea was warm and anomalously warm, which affected the distribution and abundance of macrozooplankton. Abundance of predatory chaetognaths (predominant species Sagitta elegans) increased throughout the researched period and had an adverse impact on the communities of mesoplankton. Species structure of euphausiids was typical for warm years, Thysanoessa. Inermis predominated. Portion, abundance and distribution area of the brought species T. longicaudata and Meganyctiphanes norvegica also increased. Abundance of hyperiids decreased due to the reduction of abundance and distribution area of Arctic T. libellula.
Abundance decrease of pteropods was also revealed, they are mainly represented by cold-water Clione limacine.
Keywords: macrozooplankton, Chaetognatha, Euphausiids, Hyperiidea, Pteropoda, distribution, abundance, biomass, Barents Sea
Introduction
Chaetognatha, Euphausiacea, Hyperiidea and Pteropoda are the most abundant macroplankton groups in the Barents Sea. Distribution, species structure, abundance of these taxa, and influence of water temperature in 2000s as the main factor were studied under the present research. In the Barents Sea, the investigated period conditions were characterized as warm and abnormally warm, this factor had impacted the abundance and distribution of macrozooplankton.
Material and methods
Data obtained during the annual macroplankton assessment survey conducted in October-December 2001-2015 within the survey of demersal fish were used in the research. Samples were collected using a bottom trawl net (mesh size 564 micron, 50 cm mouth opening diameter) which was attached to the middle of the bottom trawl headline and captured euphausiids in the near-bottom layer (6-10 meters above the bottom). Abundance of various species and groups was expressed as number of individuals per 1000 m3 and biomass was expressed as g wet weight per 1000 m3.
Results and discussion
Chaetognatha
Average abundance of predatory chaetognaths with Sagitta elegans dominating was generally tending towards increase during the investigated period, impacting negatively the mezoplankton community. Long-term average abundance in 2001-2015 was 940 ind./1000 m3. The lowest average abundance was in 2004 (369 ind./1000 m3) and the highest was in 2010 (1601 ind./1000 m3). We have noted a trend of decrease in average abundance at temperature fluctuations and of its increase at stable temperatures (when the heat content of waters was at the “warm years” level) (Figures 1 and 2).
Figure 1. Average abundance of Chaetognata in the Barents Sea in 2001-2015.
Figure 2. Distribution of Chaetognata aggregations in the Barents Sea in “normal” year (2003) and “abnormally warm”
Euphausiacea
Structure of euphausiids species was typical for warm years with local Barents Sea species dominating, particularly Thysanoessa inermis. Average abundance of this species amounted to 1061 ind./1000 m3 in the assessment period with 59% mean portion (Figures 3, 4, and 5). Moreover, distribution of euphausiids was typical for warm years with significant increase in distribution of dense euphausiids aggregations in abnormally warm years (Figure 6).
Figure 3. Proportion of average abundance of different euphausiids species in the Barents Sea in “normal” year (2003) and “abnormally warm” year (2015).
Figure 4. Proportion of average biomass of different euphausiids species in the Barents Sea in “normal” year (2003) and “abnormally warm” year (2015).
At the same time, we noted an increase of abundance, portion and distribution of transferred species T. longicaudata and Meganyctiphanes norvegica (Figures 5, 7, and 8). As M. norvegica is the largest species among euphausiids (up to 44 mm), its increased transfer in the area of research was essential. In 2015 the M. norvegica biomass comprised 9.73 g/1000 m3 (22 % from total biomass), despite the fact that average abundance of this species was only 114 ind./1000 m3 (6% from total euphausiids abundance in the Barents Sea).
Distribution of different euphausiids species followed the consistent pattern (Figures 7 and 8).
Despite the fact that T. inermis was widespread, its main aggregations were found in the north-western areas of the Barents Sea. The densest aggregations of T. raschii traditionally spread in the eastern areas. Transferred species T. longicaudata and M. norvegica were found within the whole area, however their aggregations were recorded in the south-eastern Barents Sea (at the Norwegian coast of and in the western areas), along the cost of Norway and the Kola Peninsula and in certain years due to its accumulation in the remoted border areas of the Barents Sea (the eastern part and areas around Spitsbergen).
Figure 5. Abundance (А) and biomass (Б) proportions of local and transferred euphausiids species in the Barents Sea in 2001-2015 (blue color is for local species, orange is for transferred species).
Figure 6. Distribution of euphausiids aggregations in the Barents Sea in “normal” year (2003) and “abnormally warm” year (2015).
Hyperiidea
Abundance of hyperiids had been decreased in 2000s and only in 2015 its growth was recorded (Figure 9). Average abundance of arctic T. libellula in 2004-2015 comprised 4.6 ind./1000 m3 in the north-western Barents Sea and 0.6 ind./1000 m3 in the southern part. The highest abundance of this species was recorded in the north-western part in 2015 and comprised 34 ind./1000 m3, density of its aggregations reached 934 ind./1000 m3 on the Great Bank.
Figure 7. Distribution of average abundance of different euphausiids species in 2012 (abnormally warm year), ind./1000 m3.
Figure 8. Distribution of average biomass of different euphausiids species in 2012 (abnormally warm year), g/1000 m3.
Average abundance of boreal T. abyssorum was higher than of T. libellula and comprised 34.4 ind./1000 m3 in the north-western Barents Sea and 10.4 ind./1000 m3 in the southern part. T.
abyssorum spread within almost the whole Barents Sea area and T. libellula was found mainly in the northern part (Figure 10).
Figure 9. Average abundance of Т. abyssorum and T. libellula in the Barents Sea in October-December 2004-2015;
ind./1000 m3.
Figure 10. Distribution of Т. abyssorum (A) and T. libellula (B) in the Barents Sea in October-December 2015;
Pteropoda
Aggregations of the most abundant pteropods species Clione limacina were not dense. In 2013, this species was smoothly distributed in the southern part Barents Sea and only in 2014-2015 some dense aggregations of C. limacina were recorded in several areas (Figure 11). Average abundance of this species was low and varied from 1 ind./1000 m3 in 2015 to 4 ind./1000 m3 in 2013 (Figure 12). Dense concentration of C. limacina in the Kopytov area (up to 302 ind./1000 m3) resulted in high average abundance of this species in north-western areas in 2014.
Figure 11. Distribution of C. limacina in the Barents Sea in October-December 2009-2015, ind./1000 m3.
Figure 12. Average abundance of C. limacina in the Barents Sea in October-December 2009-2015, ind./1000 m3.
Conclusions
To sum up, during the current warm period in the Barents Sea (2000-2015) conditions for the warm-water macroplankton species were characterized as good, particularly for euphausiids, the abundance of which had been close to the long-term average level (since 1950s) or higher.
Abundance of predatory chaetognaths had increased during the assessment period as well.
However, distribution area and abundance of hyperiids and pteropods in the Barents Sea had decreased.