Copepods are major secondary producers in the World Ocean. They represent an important link between phytoplankton, microzooplankton and higher trophic levels such as fish. They are an important source of food for many fish species but also a significant producer of detritus. In the terms of the role they play in the marine food web, it is important to know how environmental variability affects the population of copepods.
The study of the zooplankton community in the south-eastern Baltic Sea conducted during a 24-month survey (from January 2010 to November 2011) resulted in the identification of 24 invertebrate species (10 copepods, seven cladocerans, four rotifers, one ctenophore, one larvacean, and one amphipod). Data were collected at two stations located in the open sea waters of the Gulf of Gdansk: the Gdansk Deep (P1) (54°50′N, 19°19′E) and in the western, inner part of the Gulf of Gdansk (P2) (54°32′N, 18°48.2′E). The vertical hauls were carried out with the use of two kinds of plankton nets with a mesh size of 100 µm: a Copenhagen net (in 2010), and a WP-2 net (in 2011).
The paper describes the seasonal changes in the abundance and biomass of copepods, taking into account the main Baltic calanoid copepod taxa (
The environmental conditions of the pelagic habitat change in terms of both depth and distance from the shore. Although the qualitative (taxonomic) structure of zooplankton is almost identical to that of the coastal waters, the quantitative structure (abundance and biomass) changes quite significantly. The maximum values of zooplankton abundance and biomass were observed in the summer season, both in the Gdansk Deep and in the inner part of the Gulf of Gdansk. Copepods dominated in the composition of zooplankton for almost the entire time of the research duration. Quantitative composition of copepods at the P1 Station differed from the one at P2 Station due to the high abundance of
Zooplankton in the marine pelagic food webs plays an important role in the energy transfer between primary producers (phytoplankton) and higher-level consumers, like pelagic fish (
The spatial variation in the species composition of mesozooplankton results primarily from the salinity of the Baltic Sea. The lowest number of species (13–20) occurs in the central region of the Baltic Proper and increases in the marine and freshwater regions. The highest number of species (approx. 28–32) is encountered in the south-western part of the Baltic Proper, which is strongly affected by the North Sea (
Copepods are one of the most important links in the food web. They play an important role in the transmission of energy between producers and consumers of higher orders, being food for many pelagic, planktivorous fish (
The main objective of the study was to describe the seasonal changes in the abundance and biomass of the major Baltic copepod species (
The Baltic is a shallow shelf from the group of internal (intracontinental) seas. It is the youngest European sea and one of the youngest seas of the Atlantic Ocean. It covers an area of approx. 415,000 km2. It is connected with the North Sea through a number of straits: the Danish Straits (Sund, Little Belt and Great Belt), Kattegat, and Skagerrak. The generally accepted division of the Baltic Sea, based on the seabed topography, enables the identification of regions with clearly defined hydrographic parameters (
The Baltic waters are characterized by fluctuations in salinity resulting from, i.a., irregular inflows of fresh and saline waters. The inflows of saline waters occur in the western part of the sea through the Danish Straits, which connect the Baltic Sea and the North Sea. This phenomenon contributes to the two-layer structure of the Baltic waters (
The environment of Gdansk Basin is determined by a varying volume of river runoff; the easy exchange of water with the Baltic Sea, including periodical inflows (infusions) of seawater; and highly variable physicochemical conditions.
Seasonal temperature changes occurring in the upper water layer result from seasonal variability in the meteorological elements. They are affected mainly by vertical processes, in particular convection and wind mixing, as well as the Vistula River water inflows into the Gulf of Gdansk, which raise the water temperature in the spring-summer season and lower it in the autumn-winter season (
The distribution of salinity throughout the year in the surface layer of the Gdansk Basin is affected by the varying volume of river waters reaching the Basin and affecting the anemobaric conditions. Salinity shows a clear seasonal variability in the shallow littoral zone. Differences in the vertical stratification of salinity result from the interactions between the Vistula waters, which reduce salinity, and the deep waters, which increase salinity (
Plankton samples, which are the basis of
The first series consists of the biological material collected aboard the r/v “Oceania”, from the Institute of Oceanology of the Polish Academy of Sciences, during seven cruises in the area of the Gdansk Deep (54°50′N, 19°19′E) (
Gulf of Gdańsk, southern Baltic Sea in 2010—2011. Credit: Anna Tarała, Maritime Institute in Gdansk.
The vertical hauls were carried out with the use of two plankton nets with 100 µm mesh size: a Copenhagen net (in 2010) (
The plankton net mesh size was selected to collect the mesozooplankton with the younger developmental stages of copepods, which are the main object of the study. A flow meter was placed at 1/4 of the diameter of the net ring in order to determine the amount of water filtered.
The material was collected in accordance with the HELCOM guidelines (Manual for Marine Monitoring in the COMBINE Programme of HELCOM, annex C-7). The vertical net hauls were carried out in the three layers: the bottom—the upper limit of the halocline (with no halocline being present—75 m), the upper limit of the halocline —thermocline (with no thermocline being present—25 m), and the upper limit of the thermocline—the surface. A total number of 21 samples was collected, both during the daytime and night-time.
The analysed material from the Gdansk Deep was used to determine the composition and seasonal changes in the abundance and biomass in relation to time and space.
The second series of the study material consisted of the monthly zooplankton samples collected in the western part of the Gulf of Gdansk (54°32′N, 18°48.2′E) (
The net hauls were carried out only during the day, using (as in the Gdansk Deep in 2011) a WP-2 closing net with 100 µm mesh size. The flow meter was placed at 1/4 of the net ring in order to determine the amount of water filtered. The collected material was immediately transferred into plastic bottles and treated with 4% solution of formaldehyde to preserve animals for subsequent analysis. A total of 92 samples was analysed to the lowest possible taxonomic level, the copepodite stages of different copepod taxa were identified, and copepod nauplii were assigned to the taxa.
The species abundance represents the sum of all development stages in the entire water column. The data for different vertical layers was calculated as the mean value [m−3] from depth-stratum three (P1 Station) or four (P2 Station).
The biomass was calculated from the abundance of weight standards after
The environmental data at the Gdansk Deep (P1), water temperature and salinity were measured in the whole water column using the CTD-probe. Measurements were performed from the r/v “Oceania” during seven cruises, prior to the biological material collection.
The environmental data on the western, inner part of the Gulf of Gdansk (P2) came from direct measurements made with a portable meter for analysing water parameters, and were carried out on board the k/h “Oceanograf-2” (16 cruises), and on board the “Hestia” (two cruises). The measurements were made for each depth-stratum separately.
Measurements of the hydrometeorological conditions, taken during the biological material sampling (from January 2010 to November 2011), represent an environmental description within a specific time and space frame (
In February 2010 the water temperature at the Gdansk Deep (P1) ranged from 1.8 °C to 9.1 °C on the surface, and the upper limit of the thermocline was determined at a depth of approximately 60 m. While in June (the same year), the measured water temperature ranged from 10.7 °C on the surface to approx. 13 °C in the deepest measured depth (60 m).
The next year, 2011, in March, the water temperature reached the level of 1.4 °C at the surface and remained constant up to the depth of approx. 65 m, where the thermocline began, and where, below that depth the temperature level significantly increased, reaching the value of 6.2 °C at the bottom. In June 2011, the water temperature was measured only to a depth of approx. 50 m. The temperature dropped with the depth increase, from 15.2 °C at the surface to 6.4 °C at a depth of 50 m. November 2011 was characterized by a high surface water temperature, at 11.5 °C, which remained relatively constant up to a depth of approx. 40 m and then rapidly dropped at the greater depths. Due to strong waves and surface-water cooling, the thermocline was at a depth of approx. 40–50 m. The water temperature at the bottom was 5.2 °C.
The salinity of surface waters at P1 Station ranged from 7.5 to 6.9 PSU and gradually increased along the depth gradient, reaching a maximum value of 12.6–10.8 PSU at the bottom. In June 2010, the salinity was measured only to a depth of 60 m; it ranged from 7.3 PSU at the surface to 6.2 PSU in the deepest layer. Such a large decline in salinity was probably caused by an inflow of flash flood waves into the Gulf of Gdansk after a disastrous spring inundation in the Vistula drainage basin.
The temperature of surface water and salinity measured in 2010 and 2011 at P1 Station (based on the example from June) was significantly different during these two years. The runoff of flood waters in May 2010 disturbed the thermohaline system in the Gdansk Deep, which was reflected in the warmer layer of less saline water.
The water temperature at P2 Station in the western part of the Gulf of Gdansk was slightly higher for 2010 when compared to 2011.
From January to March, the surface-water temperature (approx. 1 °C) was lower than the temperature at the bottom. It was gradually increasing from April and was higher at the surface than at the bottom. However, the differences throughout the entire water column were less than 1 °C. From July to October, the differences in water temperature became more apparent: in 2010 it ranged from 3.4 to 14.1 °C, and in 2011 from 0.2 to 7.0 °C. In November a constant temperature level was observed throughout the water column, at an average of 8.6 °C in 2010 and 7.3 °C in 2011. The warmest month in both 2010 and 2011 was August (19.4 °C and 18 °C), whereas the coldest were January and March (ranging from 1 to 2.1 °C).
The salinity at P2 Station (depth of 40 m) varied to a small extent, both during the year and throughout the water column. The mean values of water salinity in the western part of the Gulf of Gdansk ranged from 6.7 PSU (in July 2010) to 7.4 PSU (in October 2011). The lowest salinity level was recorded in July 2010 (6.4 PSU), which probably resulted from the inflow of the Vistula flood waters.
According to our environmental studies conducted in 2010/2011, a total of 24 taxa were identified, including 10 Copepoda, four Rotifera, and seven Cladocera, as well as juveniles of unidentified Ctenophora, larvacean
Copepods were usually the main component of zooplankton, in both abundance and biomass (
There were limited possibilities for the monthly collection of biological material, and so data collected in select seasons were interpreted as the average of the entire water column, and different developmental stages were summarized for each species. Nevertheless, they provide a general picture of the situation prevailing at a given time in the pelagic zone.
The average number of copepods during the study period of 2010 was 3,913 ind m−3 (SD 2,572) and their number ranged from 1,184 ind m−3(in winter) to 6,293 ind m−3 (in spring). One year later, the average count of copepods was higher at 11,723 ind m−3 (SD 6,980), and it ranged from 2,351 ind mnd m (in winter) to 18,307 ind m−3 (in summer) (
The maximum number of copepods in spring 2010 in the surface layer (25–0 m) was 12,545 ind m−3, while in spring 2011 the count of copepods in the same layer was 2.5 times higher. In the other months, the highest values of the copepods count were also recorded in the layer between the upper limit of the thermocline and the surface (
The
In 2011, the situation was similar, as
Copepods usually represented the main component of zooplankton. Their average count in 2010 was 29 141 ind m−3 (SD 23315), and ranged from 3,330 ind m−3 (in March) to 67,789 ind m−3 (in May). The average count of copepods in 2011 was much lower at 17,883 ind m−3 (SD 11,407), and ranged from 1,360 ind m−3 (in April) to 39,558 ind m−3 (in May) (
The maximum count of copepods in May 2010 was determined in the 10-0 m layer, at 161,150 ind m−3, while in September 2011 the copepods abundance in the same layer was less than half of that (70,314 ind m−3)
The analysis of seasonal changes in 2010 revealed two peaks in Copepoda abundance: the first in May and the second in September, with an abundance of 67,789 ind m−3 and 57,822 ind m−3, respectively. In 2011, there was a large peak of abundance in September (39,559 ind m−3) (
It appears that the distribution of copepods in the water column is determined by the preferences of a species dominant at a given time and its developmental stage.
In March and June 2010, the largest number of copepods was observed in the 30–20 m layer, while in April this was observed in the 20–10 m layer. During the rest of the year, copepods occurred mainly in the surface layer (10–0 m) (
In 2010, the genus
The contribution of
The abundance of
On the other hand, the contribution of
In 2011, the genus
Similarly to the previous year,
In 2011,
The average biomass of copepods in the zooplankton in 2010 at P1 Station was about 116.68 mg m−3 (SD 37.49) and ranged from 92.19 mg m−3 (in summer) to 159.84 mg m−3 (in spring), while in 2011 the average value was 321.26 mg m−3 (SD 247.418), and ranged from 103.67 mg m−3 (in winter) to 676.20 mg m−3 (in summer) (
The maximum biomass of copepods in spring 2010 was recorded in the surface layer (up to a depth of 25 m), at 83.59 mg m−3, and in summer 2011 in the intermediate layer (from the upper limit of the halocline to the upper limit of the thermocline, i.e., 70–25 m), at 467.07 mg m−3 (
Considering the contribution of individual Copepoda taxa in the zooplankton biomass at P1 Station, one can observe a clear dominance of
In the winter–spring season of 2011,
The average biomass of copepods at station P2 in 2010 was 151.46 mg m−3 (SD 115) and it ranged from 33.87 mg m−3 (in March) to 390.12 mg m−3 (in May). In 2011, the average Copepoda biomass was 95.47 mg m−3 (SD 52) and ranged from 12.40 mg m−3 (in April) to 164.82 mg m−3 (in September) (
The maximum biomass of copepods in May 2010 was recorded in the 10–0 m layer, at 692.12 mg m−3, and at 403.98 mg m−3 in September 2011.
When looking into the seasonal changes in the Copepoda biomass in 2010, it appears that a significant peak occurred in May, at 390.12 mg m−3, and that two smaller peaks occurred both in September (186.73 mg m−3) and November (114.36 mg m−3). In 2011, biomass levelled from June to September, reaching its maximum in September (164.82 mg m−3) (
In March and June 2010, the largest number of copepods was observed in the 30–20 m layer, while in April 2010 it was in the 20–10 m layer. In the other months, the highest values of biomass were determined in the surface layer (10–0 m) (
In 2011, the biomass values had a similar pattern, except for January and October when the values were slightly higher at the bottom (40–30 m) (
The species from the genus
As in the case of abundance, a significant percentage of
In 2011, the genus
The maximum biomass of
In the terms of biomass and abundance, Copepoda are the most important zooplankton taxa in the southern Baltic, and they are mainly represented by e.g.,
Copepods represent one of the largest groups of secondary producers in the World Ocean. They are an important link between phytoplankton, microzooplankton and higher trophic levels such as fish (
The study presents an analysis of 92 zooplankton samples from the Gdansk Deep (Gdansk Basin) and from the western part of the Gulf of Gdansk in the terms of composition, abundance and biomass of zooplankton, with particular emphasis on copepods, as well as on the structure of populations of species occurring in large numbers in the southern Baltic, i.e.,
Taxa occurring in the samples occasionally or in small numbers (
The taxonomic structure of zooplankton observed during the research period was quite typical for the southern Baltic (
The average count of zooplankton in the Gdansk Deep (P1 Station) during the conducted studies was 10,685 ind per m−3 (SD 12,027), whereas in 2011 it was 14,607 ind per. m−3 (SD 9565). The highest mean values of abundance in the water column were recorded in the summer seasons of 2010 and 2011, at 24,238 ind m−3 and 23,659 ind m−3, respectively. The minimum values were observed in the winter–spring season (1,283 ind m−3 and 2,807 ind m−3) (
The average count of zooplankton in the western part of the Gulf of Gdansk (at P2 Station) in 2010 was 87,122 ind m−3 (SD 104,836), and in 2011 it was 31,649 ind m−3 (SD 20,487). In 2010, the maximum average count of zooplankton in the water column was recorded in July, whereas in 2011 in September it was 282,166 ind m−3 and 56,657 ind m−3, respectively. The minimum values were recorded in March 2010 (3,617 ind m−3) and April 2011 (7,249 ind m−3) (
The zooplankton at P1 Station varied, depending on the seasons, although not as much as in the shallow regions of the Gulf of Gdansk. In the two-year cycle of the scientific studies, copepods were the main component of zooplankton, representing from 69% of the total zooplankton in spring 2011 to 96% in spring 2010 (except for the summer in 2010, approx. 18%)
In 2010 and 2011, copepods occurred at P2 Station throughout the study period and for most of the months they were the main component of zooplankton, with the contribution ranging from approx. 67% (in September) to 92% (in March) in 2010 and from 47% (in June) to 93% (in January) in 2011, except for May (24%) and July (over 9%), when rotifers dominated in the zooplankton. In August, the contribution of Copepoda was similar to Cladocera and Rotifera and amounted to approx. 40%. In 2011, the exceptions were April and July, when pelagic fauna was dominated by meroplankton, mainly veligers of bivalves
Copepods were the main component of the zooplankton biomass at P1 Station for the whole study duration, with the contribution ranging from 55.3% in summer 2010 to 99.2% in winter 2010.
The situation was different at P2 Station. In March, April and June, as well as in September, October and November 2010, Copepoda accounted for the main part of the zooplankton biomass, from 67.6% in October to approx. 94.6% in March. In May, July and August, as a result of the seasonal zooplankton components occurring during these months (e.g., Cladocera), the proportion of copepods significantly decreased and ranged from 24.2 to 36.7%. In 2011, copepods dominated at P2 Station, and their contribution in the total biomass ranged from 31.7% (in April) to 96.7% (in January). In April, juvenile stages of the benthic fauna dominated in the zooplankton biomass at 64.35%, while in the following months their contribution dropped to 7.03%, and then increased again in July, to 34.95%
The coastal region of the Gulf of Gdansk is wide open towards the Gdansk Deep, which is part of the Gdansk Basin, the southernmost part of the Gotland Basin, which is the largest and the deepest basin of the Baltic Sea.
The vertical profile of waters in the Gulf of Gdansk can be divided into two layers. The surface layer in the coastal area reaches the bottom. In the deeper part, it is separated from the lower layer by the intermediate waters, up to 60–80 m in depth. The surface layer is subject to seasonal changes in temperature, caused by meteorological factors, convection, wind mixing and the impact of the Vistula River water, which causes warming in spring and summer and cooling in autumn and winter seasons. The impact of the Vistula River varies during the year: in spring and summer seasons it covers almost the entire gulf while in November it is limited to estuaries. This is due to the force and the direction of winds. There is a difference in the vertical distribution between the coastal and the deep-sea regions. The coastal areas have higher temperatures in summer compared to the surrounding waters, while in winter they are cooler. The annual report shows that the salinity in the Gulf of Gdansk is lower in winter than in summer. A key factor affecting the salinity of the surface waters are fresh waters from the Vistula River.
The environmental conditions of the pelagic habitat change with both depth and distance from the shore. Although the qualitative (taxonomic) structure of zooplankton is almost identical with that of the coastal waters, the quantitative structure (abundance and biomass) changes quite significantly. The maximum values of zooplankton abundance and biomass were observed in the summer season, both in the Gdansk Deep and in the inner part of the Gulf of Gdansk. Copepods dominated in the composition of zooplankton for almost the entire duration of the research. Quantitative composition of copepods at P1 Station differed from that at P2 Station due to the high abundance of
In the open part of the Gulf of Gdansk the concentration of
In both 2010 and 2011,
The analysis of the variation in the Copepoda taxonomic structure in the inner part of the Gulf of Gdansk at P2 Station indicates that
Taking into account the two periods—2006/2007 (
Data from the inner Gulf of Gdańsk for 2006 and 2007 (
Year | Max ind m−3 (month and layer) | Average ind m−3 (month) |
---|---|---|
2006 | 57,500 (July in 40–30 m ) | 25,600 (June) |
2007 | 127,000 (July in 20–10 m) | 83,500 (July) |
2010 | 161,150 (May in 10–0 m) | 67,790 (May) |
2011 | 70,300 (Sept. in 10–0 m) | 39,560 (Sept.) |
2014 ∗ | 40,317 (July in 25–0 m) | 36,320 (July) |
2014 ∗∗ | 43,912 (July in 25–0 m) | 21,327 (July) |
In general, the maximum contribution (%) of
Data from the inner Gulf of Gdańsk for 2006 and 2007 (
2006 (P2) | 86 (Sept.) | 57 (Nov.) | 25 (Feb.) |
2007 (P2) | 82 (Aug. and Sept.) | 51 (June) | 25 (March) |
2010 (P2) | 90 (June, July and Sept.) | 45 (Nov.) | 29 (April) |
2011 (P2) | 85 (Aug) | 77 (May) | 23 (April) |
2010 (P1) | 40 (June) | 33 (June) | 53 (April) |
2011 (P1) | 33 (March) | 45 (June) | 62 (May) |
2014∗ | [57](43) (July) | [66] (47) (July) | [39] (13) (April) |
2014∗∗ | [59] (39) (April) | [71] (56) (July) | [69] (37) (April) |
The taxon
At P1 Station (The Gdansk Deep) in 2010 and 2011, the maximum contribution (%) of
The percentage contribution which has been observed for this species in the Gulf of Gdansk (P1) was similar to that which has been observed in the Lithuanian Baltic Sea on the open sea stations (B5–B9): for
The taxonomic composition of zooplankton in the Gulf of Gdansk appears to be stable. An additional difficulty in comparing the data from different years results mainly from the various sampling methods, especially the mesh size of the sampling nets used. It appears that the high similarity of zooplankton composition between the Gdansk Deep and the more coastal part of the inner Gdansk Gulf confirms that the latter is highly influenced by the open sea waters of the Baltic proper. This makes it different from the other large Baltic gulfs, the Gulfs of Bothnia, Finland and Riga, which differ from the Baltic proper by their biotic and abiotic characteristics and are often categorized as their own autonomous subunits (
Thorough knowledge of the species composition, the dominance of particular taxa, density and biomass—in combination with abiotic—makes it easier to assess changes which take place in the ecosystem. In combination with the simulation models, such knowledge provides hypothetical forecasts for the future, leading to anticipation of positive or negative effects of environmental changes.
We express our gratefulness to the anonymous reviewers for their valuable comments on the earlier versions of the manuscript.
The authors declare there are no competing interests.
The following information was supplied regarding data availability:
The raw data are provided in