Pseudocalanus Minutus Research Articles

Seasonal Variability in the Zooplankton Community Structure in a Sub-Arctic Fjord as Revealed by Morphological and Molecular Approaches

Phyto- and zooplankton in Arctic and sub-Arctic seas show very strong seasonal changes in diversity and biomass. Here we document the seasonal variability in the mesozooplankton community structure in a sub-Arctic fjord in Northern Norway based on monthly sampling between November 2018 and February 2020. We combined traditional morphological zooplankton identification with DNA metabarcoding of a 313 base pair fragment of the COI gene. This approach allowed us to provide the most detailed mesozooplankton species list known for this region across an entire year, including both holo- and meroplankton. The zooplankton community was dominated by small copepods throughout the sampling period both in terms of abundance and relative sequence counts. However, meroplankton was the most diverse group, especially within the phylum polychaeta. We identified four distinct periods based on the seasonal analysis of the zooplankton community composition. The pre-spring bloom period (February–March) was characterized by low abundance and biomass of zooplankton. The spring bloom (April) was characterized by the presence of Calanus young stages, cirripedia and krill eggs. The spring-summer period (May–August) was characterized by a succession of meroplankton and a relatively high abundance of copepods of the genus Calanus spp. Finally, the autumn-winter period (September–December) was characterized by a high copepod diversity and a peak in abundance of small copepods (e.g., Oithona similis, Acartia longiremis, Pseudocalanus acuspes, Pseudocalanus elongatus, Pseudocalanus moultoni, Pseudocalanus minutus). During this period, we also observed an influx of boreal warm-water species which were notably absent during the rest of the year. Both the traditional community analysis and metabarcoding were highly complementary and with a few exceptions showed similar trends in the seasonal changes of the zooplankton community structure.

Lipid turnover reflects life-cycle strategies of small-sized Arctic copepods

This study aimed at understanding how life-cycle strategies of the primarily herbivorous Pseudocalanus minutus and the omnivorous Oithona similis are reflected by their lipid carbon turnover capacities. The copepods were collected in Billefjorden, Svalbard, and fed with 13C labeled flagellates and diatoms during 3 weeks. Fatty acid (FA) and fatty alcohol compositions were determined by gas chromatography, 13C incorporation was monitored using isotope ratio mass spectrometry. Maximum lipid turnover occurred in P. minutus, which exchanged 54.4% of total lipid, whereas 9.4% were exchanged in O. similis. In P. minutus, the diatom markers 16:1(n-7), 16:2(n-4) and 16:3(n-4) were almost completely renewed from the diet within 21 days, while 15% of the flagellate markers 18:2(n-6), 18:3(n-3) and 18:4(n-3) were exchanged. In O. similis, 15% of both flagellate and diatom markers were renewed. P. minutus exhibited typical physiological adaptations of herbivorous copepod species, with a very high lipid turnover rate and the ability to integrate FAs more rapidly from diatoms than from flagellates. O. similis depended much less on lipid reserves and had a lower lipid turnover rate, but was able to ingest and/or assimilate lipids with the same intensity from various food sources, to sustain shorter periods of food shortage.

Seasonal dynamics of mesozooplankton in the Arctic Kongsfjord (Svalbard) during year-round observations from August 1998 to July 1999

We conducted a year-round mesozooplankton study in the Arctic Kongsfjord from August 1998 until July 1999 to investigate seasonal abundance and vertical as well as stage distributions of the prevalent taxa. It is the first investigation in Kongsfjord that covers the Arctic winter season and provides reasonable estimates also of small-sized copepod species. Abundant smaller copepods comprised Oithona similis, Pseudocalanus minutus, Microcalanus spp., Triconia borealis and Acartia longiremis. Among the larger copepods, Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa dominated. The thecosome pteropod Limacina helicina was also an important component. Abundance maxima occurred in November (988,669 ind. m−2) with one to two orders of magnitude higher numbers as compared to all other months (39,832–200,067 ind. m−2). The summers of 1998 and 1999 were characterized by intrusions of Atlantic water, but the community was not entirely dominated by advected boreal species. During winter, the majority of the mesozooplankton occurred below 100 m. Advection is the most likely reason for the accumulation of zooplankton at depth in winter, but local production may also contribute to high overwintering numbers. Much lower abundances of most species in spring suggest high winter mortality and emphasize the importance of sufficient reproductive success during the previous summer to ensure enough winter survivors as seed stock for the coming reproductive season. This study was conducted prior to the recent warming trend in the Arctic. Therefore, it provides valuable baseline data and allows comparing present and future states of the zooplankton community in Kongsfjord.

Phylogeography and connectivity of the Pseudocalanus (Copepoda: Calanoida) species complex in the eastern North Pacific and the Pacific Arctic Region.

The genus Pseudocalanus (Copepoda, Calanoida) is among the most numerically dominant copepods in eastern North Pacific and Pacific-Arctic waters. We compared population connectivity and phylogeography based on DNA sequence variation for a portion of the mitochondrial cytochrome oxidase I gene for four Pseudocalanus species with differing biogeographical ranges within these ocean regions. Genetic analyses were linked to characterization of biological and physical environmental variables for each sampled region. Haplotype diversity was higher for the temperate species (Pseudocalanus mimus and Pseudocalanus newmani) than for the Arctic species (Pseudocalanus acuspes and Pseudocalanus minutus). Genetic differentiation among populations at regional scales was observed for all species, except P. minutus. The program Migrate-N tested the likelihood of alternative models of directional gene flow between sampled populations in relation to oceanographic features. Model results estimated predominantly northward gene flow from the Gulf of Alaska to the Beaufort Sea for P. newmani. Model scenarios that allowed bidirectional gene flow between sampled populations gave the best Bayesian predictions for P. acuspes, P. mimus and P. minutus. Under current warming trends, biogeographical boundaries and barriers for Pseudocalanus species may shift, allowing habitat range expansion or contraction and resulting in altered population connectivity between Arctic and sub-Arctic populations.

Species composition and abundance of copepods in the morphologically cryptic genus Pseudocalanus in the Bering Sea

Copepods in the genus Pseudocalanus are abundant in temperate-boreal waters of the Northern Hemisphere. However, their relative distributions are not well known because many species are morphologically cryptic. Here, Pseudocalanus species abundance and distribution in the southeastern Bering Sea from March to June, 2010, were determined using samples spanning the middle (50–100m depth) and outer (>100m depth) shelf domains. Based on analysis of mitochondrial COI sequences of 637 adult females, four species were identified; two are temperate (Pseudocalanus mimus, Pseudocalanus newmani) and two arctic (Pseudocalanus minutus, Pseudocalanus acuspes). Average genetic divergence among species was 14.7±4%. Species abundance and distribution differed significantly between the outer and middle shelf domains. For example, the temperate species P. mimus was more abundant in the outer shelf domain of the SE Bering Sea, while the arctic species P. acuspes was more abundant in the middle shelf domain. Genetic diversity within species varied. Haplotype and nucleotide diversities were higher in temperate than arctic species. No genetic structure was observed in the temperate species P. newmani or the arctic P. minutus across the outer and middle shelf domains, suggesting high levels of gene flow and connectivity across the shelf. Differences in the abundance, distribution and genetics among Pseudocalanus species in the Bering Sea highlight their unique ecological dynamics and evolutionary histories and suggest that they may have distinct responses to future environmental change.

Feeding by Pseudocalanus copepods in the Bering Sea: Trophic linkages and a potential mechanism of niche partitioning

Pseudocalanus copepods are small, abundant zooplankton in the Bering Sea ecosystem that play an important role in transferring primary production to fish and other higher trophic-level predators. Four morphologically cryptic species, the primarily arctic Pseudocalanus minutus and Pseudocalanus acuspes, and the more temperate Pseudocalanus newmani and Pseudocalanus mimus, are found within the Bering Sea. Pseudocalanus are generally considered phytoplanktivores. However, their feeding is poorly known, despite their importance to the ecosystem. In situ feeding by the three most abundant Pseudocalanus congeners, P. minutus, P. newmani, and P. acuspes, was investigated by sequencing partial 18S rDNA (ribosomal Deoxyribonucleic Acid) of gut contents from 225 individuals sampled from 8 stations across the Bering Sea in May and June of 2010. The 28,456 prey 18S rDNA sequences obtained clustered into 138 distinct prey items with a 97% similarity cut-off, and included diatoms, dinoflagellates, microzooplankton, mesozooplankton, and vascular plants. Pseudocalanus diets reflected variations in the environment, with phytoplankton sequences relatively more abundant in copepods from stations with higher water-column chlorophyll a concentrations. Feeding differences were observed between species. P. acuspes diet contained relatively more heterotrophic dinoflagellate sequences, and was significantly different from that of P. minutus and P. newmani, both of which contained relatively more diatom sequences, and between which no significant difference was observed. Feeding differences between the two primarily arctic species may be a mechanism of niche partitioning between these spatially co-located congeners and may have implications for the effects of climate change on the success of these abundant zooplankters and their many predators in this ecosystem.

Integral values of biomass and stock of zooplankton in the epipelagic layer of the area 71 in the North Pacific, including the Bering and Okhotsk Seas, and patterns of distribution for mass species

The Okhotsk and Bering Seas and the subarctic zone of the North Pacific are surveyed well by the expeditions conducted by Pacific Fish. Res. Center (TINRO) in the last three decades (115 surveys in 1984-2013), with sampling of zooplankton for monitoring of feeding resources of commercial species. Mean total zooplankton biomass and biomasses of size fractions of zooplankton, its taxonomic groups and mass species are calculated, by biostatistical areas. General patterns of the biomass distribution between the biostatistical areas are determined and presented as the charts of the biomass averaged by the areas. The most abundant species of zooplankton with similar systematic and trophic status are spatially separated that allows them to reduce competition for food, for example the pairs of small-sized, medium-sized and large-sized Copepoda species ( Pseudocalanus minutus - Oithona similis, Metridia pacifica - M. okhotensis , Calanus glacialis - Neocalanus plumchrus ), the pair of Hyperiida species ( Themisto pacifica - T. libellula ) and four Euphausia species ( Thysanoessa raschii - Th. inermis - Th. longipes - Euphausia pacifica ) in the Okhotsk Sea. Detailed tables on biomass and stock of the main components of zooplankton are presented, by regions and biostatistical areas.

Dynamics of production parameters for zooplankton as the main component of forage base for nekton in the western Bering Sea

Production parameters of zooplankton are calculated and analyzed on the data of plankton surveys in the framework of BASIS-1 and BASIS-2 research projects conducted in the western Bering Sea in 2002-2013. Top-ten species forming the bulk of non-predatory zooplankton production are the copepods Calanus glacialis, Eucalanus bungii, Neocalanus plumchrus, Pseudocalanus minutus, Metridia pacifica, Oithona similis, and Neocalanus cristatus and euphausiids Thysanoessa inermis, Th. raschii , and Th. longipes . The arrowworm Sagitta elegans produces the main part of predatory zooplankton production. Functional characteristics of trophic groups and production of the whole zooplankton community depend on ratio of these mass species. During the surveyed period, the non-predatory zooplankton production was the highest in the northern part of the western Bering Sea (2531-3160 mg/m 3 yr) and the lowest in its deep-water part (944 mg/m 3 yr), whereas the predatory zooplankton production was the highest in the deep-water part of the Sea and over the western shelf. The portion of zooplankton production available for grazing by fish is estimated, and the areas with possible problems for planktivorous consumers are determined: all of them are located in the deep-water part of the western Bering Sea.

Population dynamics of Pseudocalanus minutus elongatus in the Gulf of Gdansk (southern Baltic Sea) – experimental and numerical results

The seasonal and spatial distribution of the pelagic calanoid copepod Pseudocalanus minutus elongatus was studied in the Gulf of Gdansk (southern Baltic Sea) between January 2006 and December 2007 based on experimental data (from six stations) and numerical simulations. Numerical calculations were performed using a structured zooplankton population model that was adapted to this species and coupled to the ecosystem model. Spatiotemporal distribution of this species depends on salinity as a masking factor. Highest abundances at station J23, an open sea site, were observed in July 2006 and March 2007 (10,050 and 11,000 ind. m−3, respectively) and resulted from the large numbers of earlier copepodites C1–C3 in 2006 and nauplii in 2007. In shallow stations abundances declined. Our results show that only one complete generation of this copepod species developed during the year in this region. Differences in development between the experimental and numerical data are analysed.

Spatiotemporal distribution of copepod populations in the Gulf of Gdansk (southern Baltic Sea)

In the present study, we used a 3D Coupled Ecosystem Model of Baltic Sea version 1 (3D CEMBSv1) coupled with a copepod model to examine the spatiotemporal distribution of two representative copepod populations in the Gulf of Gdansk (southern Baltic Sea) including Acartia spp. and Pseudocalanus minutus elongatus. The annual cycles simulated for 2000 under realistic weather and hydrographic conditions were studied with the three-dimensional version of the coupled ecosystem-copepod model in the south-eastern Baltic Sea. The paper presents the comparison of simulated and observed copepod development at two stations in the Gulf of Gdansk. A validation of influential state variables gives confidence that the model is able to calculate reliably the stage development of dominant species in the southern Baltic Sea. The number of generations was one for P. m. elongatus and 3–5 for Acartia spp.. A mean of five generations for the latter species per year were estimated in the coastal region and ca. three generations at the Gdansk Deep (in the open sea). Food concentration and temperature as the main factors controlling the development of the investigated copepods as well as salinity as a masking factor (i.e. salinity modifies the rate of their development) in the case of Pseudocalanus minutus elongatus are included in the present study.

Morphometric differentiation ofPseudocalanus minutuspopulations in the Barents Sea

Dvoretsky VG. and Dvoretsky AG. 2011. Morphometric differentiation of Pseudocalanus minutus populations in the Barents Sea. —Acta Zoologica (Stockholm) 00: 1–12. We investigated spatial variations in the morphometric characteristics (total length of body, lengths of cephalothorax, abdomen and antennules, and their relative proportions) of Pseudocalanus minutus, an abundant copepod species across the Barents Sea in August–September 2007. Females were found to have higher values for the measured parameters than males. The average absolute morphometric characters of both sexes increased from the south to the north. In most cases, parameters were similar in the southern, central, and eastern regions delineated by cluster analyses of oceanographic variables. The morphometric characteristics were strongly correlated with environmental variables in both males and females. Multiple regression analysis showed that temperature together with salinity explained 72–85% (in females) and 38–91% (in males) of the total variations in the log10-transformed morphometric parameters. According to principal component analysis and discriminant function analysis, two distinct groups could be separated in the Barents Sea. The first group included the copepods from the northern region; the other included populations from the southern, central, and eastern regions. The observed morphological variation can be interpreted as geographical variation connected with hydrological variability.

Checklist of fauna found in zooplankton samples from the Barents Sea

We present a list of zooplankton taxa occurring in the Barents Sea based on our analysis of 360 samples from the upper 100 m of the water column as well as on reports from published sources. Moreover, we provide information on adult size, ecological and geographical distribution, habitat preference and frequency of occurrence of each taxon. The list encompasses a total of 374 species and higher taxa. Most are mesozooplankton, with Copepoda prevailing (153 species), and belong to the holoplankton. However, pseudoplankton, organisms only circumstantially occurring in the water column, also comprise a significant part of the fauna (86 taxa). The coastal zooplankton (264 neritic species) was richer than the fauna of the open sea. In our samples, the majority of the zooplankton taxa were rarely found (94 of 157 taxa, equivalent of 60%), 46 taxa (29%) were moderate and only 17 taxa (11%) were common (found in >80% of the samples). Among these, the copepods Calanus finmarchicus, C. glacialis, Oithona similis and Pseudocalanus minutus were most frequent.

Light-dependent behavior of abundant zooplankton species in the White Sea

Light-dependent behavior of the abundant zooplankton species inhabiting the White Sea were studied experimentally during: (i) the spring equinox (March); (ii) the polar day (late May to June), (iii) August, 17/7 h day-night light cycle, (iv) the fall equinox (October). Behavioral patterns were investigated for eight species of Copepoda (Metridia longa, Calanus glacialis, Pseudocalanus minutus, Oithona similis, Oncaea borealis, Temora longicornis, Centropages hamatus, Acartia spp.), one Cladocera species (Evadne nordmanni) and Polychaeta larvae. The hypothesis was tested that attraction to (or repulsion from) light is the primary mechanism involved in the vertical migration of zooplankton with different trophic characteristics in relation to phytoplankton-rich upper water layer. The impact of red (680 nm), yellow (560 nm) and UV (280 nm) light was tested. The animals were acclimated to two food conditions: natural seawater (satiated) and filtered (1 μm) seawater (hungry). The positive light response of predominantly herbivorous and omnivorous copepods and cladocerans inhabiting the photic water layer corresponds with their distribution and their food vertical distribution. Hungry animals display the strongest responses to light. Light effects on behavior were weak in deep-dwelling 0. borealis. We suggest that red and yellow light is an indicator of the photic layer (high food concentration) to zooplankton groups that feed on phytoplankton. In contrast, diapausing (e.g. non-feeding) copepods totally avoid light, especially when they hibernate in the aphotic layer. We hypothesize that there is a relationship between the light response of the zooplankton, their trophic characteristics, migration behavior (diel and ontogenetic) and the water layer occupied.

Species composition, distribution, and specific features of feeding of Fish Larvae, and fry in the Sea of Okhotsk from October to December 2007

Ichthyoplankton of the Sea of Okhotsk from October to December 2007 comprised larvae and fry of eight fish species belonging to five families. The larvae of lord Hemilepidotus gilberti, white-spotted greenling Hexagrammos stelleri, and Atka mackerel Pleurogrammus monopterygius were the most widespread and numerous in the study period. Fish larvae from October to December 2007 were seined over the vast water area of the Sea of Okhotsk, maximum catches were made in waters of western Kamchatka. The food spectrum of larvae of five fish species (H. gilberti, Hexagrammos octogrammus, H. stelleri, P. monopterygius, and Bathymaster signatus) included approximately 20 plankters. Irrespective of species belonging, copepods Pseudocalanus minutus and Oithona similis, eggs and nauplii of copepods, as well as juveniles of pteropods were common food items for all fish. Larvae of all considered fish species in the Sea of Okhotsk fed mainly during the light hours of the day.

Distribution and egg production of Pseudocalanus species in the Chukchi Sea

The distribution and egg production of Pseudocalanus species was studied in the Chukchi Sea during August 2004. Three species were identified throughout the study area, Pseudocalanus minutus, Pseudocalanus acuspes and Pseudocalanus newmani. The more subpolar/temperate P. newmani-dominated stations where water was warmer and likely influenced by Alaska coastal waters, and it penetrated northward into the south-eastern corner of Herald Valley. The Arctic P. acuspes dominated the colder waters. The number of eggs produced by individuals and populations differed among species, primarily because of differences in body size, but mass-specific egg production was similar among species, and was on the order of 15% per day. Future increases in the prevalence of P. newmani within the Chukchi Sea might be anticipated, and would result in a decreased average size of the copepod community, with potential implication for higher trophic levels.

Distribution of the under-ice mesozooplankton in the Kara Sea in February 2002

Mesozooplankton distribution was investigated under the sea ice in the Kara Sea at five stations in February of 2002 by Juday net hauls. Copepods dominated the mesozooplankton community, accounting for 46–88% of the total abundance and 68–99% of the biomass. Oithona similis was the most abundant species in Yenisei Bay, being present with all age stages (including egg-carrying females). For the first time, Oithona atlantica (CIII–CV copepodites, females and males) were found in the southeastern Kara Sea. In the southern part, Copepoda nauplii prevailed in terms of total abundance while the mesozooplankton in the northwestern part was entirely dominated by older stages of Pseudocalanus minutus. The mesozooplankton structure appears to be determined by available food resources and increased water temperature due to a strong influence of warm Atlantic waters.

Summer-autumn ichthyoplankton of the Sea of Okhotsk and the Sea of Japan and special traits of feeding of fish larvae and fry in 2003–2004

In summer-autumn of 2003–2004, the ichthyoplankton of the Sea of Okhotsk comprised 35 species. In this period the most widely distributed and numerous were larvae of the lord Hemilepidotus gilberti, the Pacific stout sand lance Ammodytes hexapterus, and the Sakhalin dab Limanda sakhalinensis. The maximum catches of fish larvae were attributed to coastal waters off eastern Sakhalin and to the shelf of the northern part of the Sea of Okhotsk. In November of 2003, the ichthyoplankton of the Sea of Japan was represented by fish larvae belonging mainly to the boreal ichthyocomplex. The catches consisted predominantly of larvae of the arabesque greenling Pleurogrammus azonus, the ronquil Bathymaster derjugini, and the rockfish Sebastes owstoni. Fish larvae and fry in the northwestern part of the Sea of Japan were caught principally within 43°–45° N and 137°–139° E above the depth 1500–2000 m. The food spectrum of fish larvae in the Sea of Okhotsk and the Sea of Japan comprised over 20 plankters of various size belonging to seven taxa. Irrespective of fish species, the food items common of all fish were copepods Pseudocalanus minutus and Oithona similis. The daily rations were calculated for mass species (Hemilepidotus gilberti, Ammodytes hexapterus, Hexagrammos stelleri, Pleurogrammus azonus, Bathymaster derjugini, and Sebastes owstoni). The larvae of all considered species in the Sea of Japan and in the Sea of Okhotsk fed predominantly in the light period of the day.

Seasonal changes in digestive enzyme (trypsin) activity of the copepods Pseudocalanus minutus (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

Seasonal activities of the digestive enzyme trypsin were measured between August 1998 and May 1999 to study different nutritional strategies of the two copepods Pseudocalanus minutus and Oithona similis in the Arctic Kongsfjorden (Svalbard) using a highly sensitive fluorescence technique. Stage-, depth- and season-specific characteristics of digestive activity were reflected in the trypsin activity. P. minutus females and stage V copepodids (C) had highest trypsin activities in spring during reproduction (197.5 and 145.7 nmol min−1 ng C−1, respectively). In summer stages CIII–V and in autumn stages CIV and V had high activities (80–116 nmol min−1 ng C−1) in the shallow layer ( 100 m) in summer and autumn were low in stages CIII and CIV (29–60 nmol min−1 ng C−1) and in winter in all stages in both layers (20–43 nmol min−1 ng C−1). Based on low trypsin activity, males most likely did not feed. In O. similis, the spring phytoplankton bloom did not significantly affect trypsin activity as compared to the other seasons. O. similis CV and females had high trypsin activities in summer in the deep stratum (304.5 nmol min−1 ng C−1), which was concomitant with reproductive processes and energy storage for overwintering. In autumn, stage CV and female O. similis had significantly higher activities than stage CIV (130–152 versus 78 nmol min−1 ng C−1), which is in accordance with still ongoing developmental and reproductive processes in CVs and females. Comparisons of both species revealed different depth-related responses emphasizing different nutritional preferences: the mainly herbivorous P. minutus is more actively feeding in the shallow layer, where primary production occurs, whereas the omnivorous O. similis is not as much restricted to a certain depth layer, when searching for food. P. minutus had lower levels of trypsin activity during all seasons. In contrast to P. minutus, higher enzyme activities in males of O. similis suggest that they continue to feed and survive after fertilization of females.

Seasonal dynamics of <i>Pseudocalanus minutus elongatus</i> and <i>Acartia</i> spp. in the southern Baltic Sea (Gdańsk Deep) – numerical simulations

Abstract. A population dynamics model for copepods is presented, describing the seasonal dynamics of Pseudocalanus minutus elongatus and Acartia spp. in the southern Baltic Sea (Gdańsk Deep). The copepod model was coupled with a one-dimensional physical and biological upper layer model for nutrients (total inorganic nitrogen, phosphate), phytoplankton, microzooplankton, and an early juvenile of herring as a predator. In this model, mesozooplankton (herbivorous copepods) has been introduced as an animal having definite patterns of growth in successive stages, reproduction and mortality. The populations are represented by 6 cohorts in different developmental stages, thus assuming that recruitment of the next generation occurs after a fixed period of adult life. The copepod model links trophic processes and population dynamics, and simulates individual growth within cohorts and the changes in biomass between cohorts. The simulations of annual cycles of copepods contain one complete generation of Pseudocalanus and two generations of Acartia in the whole column water, and indicate the importance of growth in the older stages of 6 cohorts of each species, to arrive at a total population biomass. The peaks of copepods' biomass are larger at the turn of June and July for Pseudocalanus and smaller in July for Acartia, lagging that of phytoplankton by ca. two mouths, due to the growth of cohorts in successive stages and egg production by females. The numerical results show that the investigated species could not be the main factor limiting the spring phytoplankton bloom in the Gdańsk Deep, because the initial development was slow for Acartia and faster for Pseudocalanus, but the main development formed after the bloom, in both cases. The phytoplankton bloom is very important in the diet of the adults of the copepods, but it is not particularly important for the youngest part of new generation (early nauplii). However, the simulated microzooplankton biomass was enough high to conclude, in our opinion, that, in this case, it was a major cause in limiting phytoplankton bloom. The model presented here is a next step in understanding how the population dynamics of a dominant species in the southern Baltic Sea interact with the environment.

Seasonal dynamics of sub-ice fauna below pack ice in the Arctic (Fram Strait)

Pronounced seasonality is a characteristic feature of polar ecosystems, but seasonal studies in the high-Arctic pack-ice zone are still scarce because of logistical constraints. During six expeditions (1994–2003) to the Fram Strait area between Greenland and Svalbard in winter, spring, early summer, late summer and autumn, the sub-ice habitat and fauna below the pack ice (0–1 m depth) were analyzed for seasonal patterns. Both environmental variables such as ice cover, temperature, salinity and chlorophyll a (chl a), as well as species composition, abundance and biomass of the sub-ice fauna showed distinct seasonal dynamics. Most species of the sub-ice fauna were found in early summer, followed by autumn, spring and late summer; the lowest number occurred in winter. The sub-ice fauna was dominated by copepod nauplii during all seasons. Next numerous was the small pelagic copepod Oithona similis, followed by occassional swarms of Pseudocalanus minutus and Calanus spp. Abundances of the sympagic fauna in the sub-ice water layer were much lower, with ectinosomatid copepods being usually the most numerous sympagic group. In the course of the year, total abundances of the sub-ice fauna showed a steep increase from the earliest sampling dates towards the end of winter/beginning of spring reaching maximum numbers then, and a decrease to minimum numbers in early summer. A second peak occurred in late summer, followed by a decrease towards autumn. This significant trend was due to the abundances of copepod nauplii and Oithona similis. Sympagic species were virtually absent during winter, and increased significantly in spring and early and late summer. A factor analysis revealed the variables ice cover and thickness, water temperature and salinity, as well as chl a as the major controlling factors for the seasonal patterns in different groups and species of the sub-ice fauna. Because of the special environmental conditions in the sub-ice habitat, and the unique species composition characterized by small taxa, young stages, and sympagic species, the seasonal dynamics of the Arctic sub-ice fauna differ substantially from those of the epipelagic zooplankton community in the Arctic Ocean.