Helcom : Habitat layer extension and the occurrence of dominant calanoid copepods

Habitat layer extension and the occurrence of dominant calanoid copepods in the Baltic Sea

Author: Lutz Postel, Baltic Sea Research Institute, Germany

Key message

The habitat volume of copepods of marine origin seems to depend on thickness of the layer of higher saline water in the western Baltic Sea. In the Baltic proper, the thickness of the layer of anoxic deep water acts as a second delimiter.

There are calanoid copepods with different salinity preferences and body size. The ratio between the larger Pseudocalanus spp. and the clearly up to slightly smaller Acartia spp. and Temora longicornis respectively depend on salinity in the Baltic Sea.

ASPECT 1: We try to identify the water layers in which the ratio is potentially favourable for Pseudocalanus spp. or for Acartia spp. and Temora longicornis respectively according the salinity.

ASPECT 2: In the deeper layers of the Baltic proper, we show the potential habitat thickness for Pseudocalanus spp. depending on favourable salinity conditions and on sufficient oxygen content. We propose to calculate the potential Pseudocalanus spp. (CI –CV) stock by habitat layer thickness.

Potentials of the method will be briefly discussed.

Background

The system between environmental forcing and food web structure is quite complex. Missing salt water influxes provoke a general prevalence of fresh water influence and an increase of anoxic conditions in deep water layers. This diminishes the habitat layer for heterotrophic organisms in general and those of marine origin, for example copepods, in particular. For example the Pseudocalanus spp. stock decreased in the early eighties because of longer stagnation period since the late seventies (Kononen et al, 1996). Especially the abundance of Acartia species but also this of Temora longicornis increased in return. Because the total of calanoid copepods do not drastically change in the Baltic Sea (Wasmund et al, 2003), the ratio between the larger Pseudocalanus spp. on one hand and the clearly up to slightly smaller Acartia spp. and Temora longicornis on the other may change accordingly.
Applied aspect: These changes of environmental conditions and of community structure of copepods had consequences in higher trophic levels. Herring (Clupea harengus), suffered from declining of its favoured diet, the Pseudocalanus spp. stock, in the northern Baltic proper (c.f. Plikshs et al., 2001; Möllmann and Köster,2002; Möllmann et al., 2003). Consequently herring switched to Acartia species and Temora longicornis in competition against sprat (Sprattus sprattus).
The zooplankton analysis is laborious and time consuming task. Therefore, a zooplankton indicator basing on rapidly available CTD data is an additional motivation for the proposal.
The lower salinity ranges are given by) as follows:

Pseudocalanus spp. (Boeck, 1865) - 7.25 PSU

Acartia longiremis (Lilljeborg,1853) - 6.72 PSU

Temora longicornis (O.F. Müller, 1785) - 6.54 PSU

Acartia bifilosa, Giesbrecht,1881 - 0.30 PSU.

Optimum conditions are to expect at higher values.

The oxygen concentration which allows copepods to survive is quite low. It ranges between 0.2 and 0.04 cm³/dm³ according to Vinogradov and Voronina (1961), Longhurst (1967), Kinzer (1969), Devol (1981), Wishner et al. (1998), and Luo et al. (2000), with a tendency to the lower values. Consequently, the 0 cm³/dm isolines will be used as the appropriate lethal concentration.

Data source

We used salinity and oxygen data sets collected by the Baltic Sea Research Institute (IOW) within the HELCOM monitoring programme. The first were determined by CTD probes the latter by Winkler titration. Calanoid abundances exclusively originate from IOW collections since 1996 and from HELCOM data base for the earlier period. Sampling and analysis were performed according the actual versions of the COMBINE manual (http://www.helcom.fi/combine_manual/anxc7.html) in each case.
We applied copepodit stages, neglecting nauplia and adult calanoids. Nauplia are not quantitatively caught by the nets with 90 and 100 µm mesh size while adults may be affected by grazing pressure of adult herring more than copepodits.

Implementation – ASPECT 1

In order to get an actual impression on the optimal salinity conditions for Pseudocalanus spp. (CI –CV), Acartia spp. (CI –CV) and Temora longicornis (CI –CV) we choose the salinity range between Kiel Bay and the eastern Gotland Basin and exclusively plankton abundances of >10 000 ind/m³, whether for Pseudocalanus spp or for Acartia spp.. We excluded the Temora longicornis case, because it occurred in such concentration at only one occasion. Salinity was averaged over the plankton sampling depths. The results are separately demonstrated in Figure 1 whether for Pseudocalanus spp. (CI –CV) or for Acartia spp. (CI –CV) together with the corresponding abundances of the two other taxa respectively.

Figure 1. Salinity preferences of Pseudocalanus spp. and Acartia spp. in the western Baltic Sea.

The Pseudocalanus spp. case includes results of 13 samples, collected from 1999 to 2003. These large abundances of P. spp. (CI –CV) stem from the entire water column west of Darss Sill and from below the halocline east of it up to the southern Gotland Basin. The Acartia spp. (CI –CV) case includes results of 20 samples collected at the same time. They originate completely from surface layers at stations east of Darss Sill (Arkona Sea, Pomeranian Bay, Bornholm Sea).

The >10 000 ind. /m³ - abundances of Pseudocalanus spp. (CI –CV) correspond with salinities from 8 to 25 PSU, those of Acartia spp. (CI –CV) correspond with a range from 6 to 8 PSU. The single Temora longicornis (CI –CV) concentration of >10 000 ind. /m³ originated from the layer below the halocline of Arkona Sea, collected in May, 1998. It was characterised by 19 680 ind. /m³ at 8.05 PSU, and corresponding 28 320 Acartia spp. (CI –CV) /m³. Pseudocalanus spp. (CI –CV) was missed at this location and time.

The ratio between Pseudocalanus spp. (CI –CV) and Acartia spp. plus Temora longicornis (CI –CV) versus salinity

Next the ratio between Pseudocalanus spp. (CI –CV) and Acartia spp. (CI –CV) plus Temora longicornis (CI –CV) was determined and related it to salinity in order to estimate at which salinity the ratio turns to the one or the other group of calanoid copepodits (Figure 2). For this purpose, we used the abundances of P. spp. (CI –CV) (>10 000 ind. /m³) and divided them by the corresponding A.spp. + T.l. (CI –CV) concentration as well as the opposite case, together. The opposite case means the same quotient but at A. spp. (CI –CV) > 10 000 ind. /m³. Abundance data were the same as in Figure 1.

Figure 2. Ratio between Pseudocalanus spp.- and Acartia spp.+Temora longicornis (CI-CV) abundances (x) in comparison to salinity (y) from Kiel Bight to the central Gotland Basin.

According the formula in Figure 2, the ratio is 1 at the salinity of approximately 12 PSU. Consequently, Pseudocalanus spp. may dominate in contrast to the two other copepod components in habitats of salinities > 12 PSU. Additionally, the formula should allow estimating the salinity which is necessary for a certain ratio. For example, Pseudocalanus spp. potentially outweighs the two other groups by a factor of 10 at 14.6 PSU.

Application

The results should actually allow identifying water layers of favourable life conditions (habitat layers) for the two copepod groups separated by the 12 PSU isohaline taking the salinity profile from the western Baltic Sea to the Farö Deep in Figure 3. The oxicline (oxygen content = 0 cm³/dm³) indicates anaerobic layers without any crustacean life in some deeper parts of the Baltic proper.

Figure 3. Profile of salinity and the oxygen zero concentration from Kiel Bight to Farö Deep in July 2005.

Actual conclusions

In 2005, we expect a positive Pseudocalanus spp. / Acartia spp. and Temora longicornis ratio in the entire water column of the western Baltic Sea and below the halocline east of Darss Sill till the southern Gotland Basin, with some restrictions by anoxia in the deep basins.

Implementation – ASPECT 2

The two outlier in Figure 2 (ratio of 24 and 21 at 8.7 and 9.9 PSU respectively) originate from deeper layers below the halocline in the southern Gotland Basin. It indicates a certain adaptation to lower salinities or the occurrence of second species with another salinity preference in the eastern central Baltic proper. The Figure 4 underlines this possibility.

Figure 4 includes ratios between Pseudocalanus spp.- and Acartia spp.+Temora longicornis (CI-CV) abundances in comparison to salinity in the eastern, central Gotland Basin, using two “boundary abundances” at > 10 000 ind./m³ (left) and >5.000 ind./m³ (right). It was produced according to Figure 2 using the data between 1979 and 2002. Both approaches result in salinities of about 8 PSU at a ratio of 1.

Figure 4. Ratio between Pseudocalanus spp.- and Acartia spp.+Temora longicornis (CI-CV) abundances (x) in comparison to salinity (y) in the eastern, central Gotland Basin, using two “boundary” abundances at > 10 000 ind./m³ (left) and >5.000 ind./m³ (right).

According to Figure 5, the percentage of Pseudocalanus spp. (CI – CV) does not exceed these of Acartia spp.- and Temora longicornis (CI - CV) in the upper layer, but below of it. This finding corresponds with the depth of the 8 PSU isohaline in Figure 6, upper panel and underlines the results in Figure 4 indirectly.

Figure 5. Average percentage (dominance) of Pseudocalanus spp.- and Acartia spp.+Temora longicornis (CI - CV) abundances in average depth layers east of Gotland Island. The averages base on 392 data for Pseudocalanus spp. (maximum = 56648 ind./m³), 350 data for Acartia spp. (maximum =10446 ind./m³), and 334 data for Temora longicornis (maximum=13858 ind./m³), obtained between 1979 and 2002. The averages of depth layers base on 158, 105, 126, 60 samples in the order from top do down.

Figure 6. Vertical profile of salinity and position of the oxicline (oxygen content = 0 cm³/dm³) east of Gotland Island from 1977 until mid of 2005 (upper panel) in comparison with the abundance of Pseudocalanus spp.(CI - CV) at the same location from beginning of 1980 to end of 2004 (lower panel).

Taking the findings of Figures 4 and 5, the course of the 8 PSU isohaline and of the oxicline very probably delimits the habitat layer off predominating Pseudocalanus spp. (CI - CV) in the central Gotland Basin, east of Gotland Island. It steadily declined from the beginning up to the stronger salt water influx in 1993 (Matthäus and Nausch, 2003) by both the decline in salinity of the surface layer and the increasing thickness of the anoxic deep water layer. Deep water renewals in 1993 and 2003 (Feistel et al., 2003) interrupted this development. The minimum habitat layer thickness amounts about 80 m at the end of the eighties and from 2000 to 2002.

According the lower panel in Figure 6, there is an agreement between a narrower habitat layer and the concentration of Pseudocalanus spp. (CI - CV) of >10,000 ind/m³. These “top concentrations” were realised in the eighties before the longer stagnation period and just after the deep water renewals in 1993 and 2003. Figure 7 illustrates the habitat layer thickness.

Figure 7. The difference between the depth of the 8 PSU isolines and of the oxicline as habitat layer thickness east of Gotland Island from 1977 until mid of 2005.

The difference between the depth of the 8 PSU isolines and of the oxicline was finally compared with the annual and vertical mean of the Pseudocalanus spp. (CI - CV) abundance [ind./m³] in Figure 8.

The annual averages of Pseudocalanus spp. (CI –CV) abundance were linearly correlated to the thickness of habitat layer. The averages were calculated from 5 cruises per year in maximum and 4 depth layers. Annual maximum abundances were also correlated but weaker. For the trend line computation, outliers between 1994 and 1998, and in 2003 and 2004 had been rejected (see discussion).

Figure 8. The thickness of habitat layer [m] in comparison with the annual averages of Pseudocalanus spp.(CI –CV) abundance [ind./m³] in the central Gotland Basin between 1980 and 2004. 6 outliers were neglected (see text).

Neglecting the outliers, a multiple regression model was constructed using Statistica software (StatSoft, Inc.) in order to get a formula which should allow calculating the potential annual and vertical average of Pseudocalanus spp. (CI –CV) abundance by the depths [m] of the 8 PSU isoline (z _8PSU) and of the oxicline (z_oxicline) only. The statistics and the equations are summarised as follows (including the expected standard error):

Correlation coefficients:

(1) (2) (3)

Depth of 8 PSU isohaline (1) 1,000000 -0,577223 -0,788054

Depth of oxicline (2) -0,577223 1,000000 0,657284

P.spp. average abundance (3) -0,788054 0,657284 1,000000

Number of data couples N=18, degree of freedom dF =16,

Regression summary for depended variable: ”P.spp. average abundance”

Coefficient of correlation R= 0.82611421 coefficient of determination R²=0.68246469

Test number Fisher distribution F(2,15)=16.119

p< 0.00018, standard error of estimation: 1035.9 ind./m³

Regression formula:

P. spp. [ind./m³] = - 45,0576 + (- 92,2933)*(z_8PSU [m]) + 31,3406 * (z_oxicline [m])

Next, the model results were compared with the observations (Figure 9). There was a fairly good correlation except in the years of the already mentioned outliers between 1994 and 1998, and in 2003 and 2004. These deviations could be of special interest (see “Potentials of the method”, paragraphs 4 and 5). The Figure 9 includes also the estimation for 2005.

Figure 9. The annual averages of Pseudocalanus spp. (CI –CV) abundance [ind./m³] in the central Gotland Basin -Model results (1977-2005) in comparison with the observations (1980-2004).

Actual conclusion

There is an expectation of annual mean Pseudocalanus spp. (CI –CV) abundance of about 4000 ind./m³ +/- 1000 ind./m³ in the entire water column in the central Gotland Basin in 2005, which would be in the magnitude of the abundance of the nineteen eighties.

Potentials of the methods (Aspects 1 and 2)

The courses of the 12 PSU isohaline in a vertical profile of salinity separates the water layers in which the copepodit ratio is potentially favourable for Pseudocalanus spp. or for Acartia spp. and Temora longicornis (CI-CV), especially in the western Baltic Sea. The oxicline depth is additionally of interest for the habitat layer thickness of Pseudocalanus spp. (CI-CV).
In the central Gotland Basin the 8 PSU isoline is the salinity at which the conditions become favourable whether for the one or the other mentioned copepod group. The annual average abundance of Pseudocalanus spp. (CI-CV) is to predict from the depths of the 8 PSU isohaline and the oxicline respectively using the given formula.
The different salinity preferences of Pseudocalanus spp. (CI-CV) in the western Baltic Sea and in the central Gotland basin could be the result of two different species (Pseudocalanus acuspes (Giesbrecht, 1881) - in the western Baltic Sea and Pseudocalanus minutus elongatus - in the central and northern Baltic proper). Nevertheless, such multiple regression formulas could be developed in dependency on the Baltic Sea areas.
There are deviations in the comparison of model results and the observations in the nineties and the beginning of the two thousands. It could be that warm summers in 1997 and in 2003 may additionally delimit the habitat layer of Pseudocalanus spp. (CI-CV) because of an increase of the warm surface layer which is known to be avoided by Pseudocalanus spp. (CI-CV).
If these deviations are the result of a top down control of the Pseudocalanus spp. (CI-CV) stock by herring as discussed by Möllmann and Köster (1999), we would be able to mark such “top down control” - periods by the results in Figure 7.

References

Devol, A.H., 1981. Vertical distribution of zooplankton respiration in relation to the intense oxygen minimum zones in two British Columbia Fjords. Journal of Plankton Research 3(4), 593-602.

Feistel, R., Nausch, G., Matthäus, W. and Hagen,E.,2003. Temporal and spatial evolution of the Baltic deep water renewal in spring 2003. Oceanologia 45: 623-642

Kinzer, J., 1969. On the quantitative distribution of zooplankton in deep scattering layers. Deep-Sea Research Part I 16, 117-125.

Kononen, K., H. Kuosa, J.-M. Leppänen, R. Olsonen, J. Kuparinen, L. Postel and G. Behrends, 1996. Overall assessment: Pelagic biology. Third periodic assessment of the state of the marine environment of the Baltic Sea, 1989-93: background document. Helsinki: Helsinki Commission - Baltic Marine Environment Protection Commission. (Baltic Sea Environment Proceedings; 64B): 215-222 (http://www.helcom.fi/Monas/BSEP82B.pdf).

Longhurst, A. R.,1967. Vertical distribution of zooplankton in relation to the eastern Pacific oxygen minimum. Deep-Sea Research Part I 14, 51-63.

Luo, J., Ortner, P.B., Forcucci, D., Cummings, S.R., 2000. Diel vertical migration of zooplankton and mesopelagic fish in the Arabian Sea. Deep-Sea Research Part II, 47 (7-8), 1451-1473.

Matthäus, W. Nausch, G. ,2003). Hydrographic-hydrochemical variability in the Baltic Sea during the 1990s in relation to changes during the 20th century. ICES mar. sci. symp. 219: 132-143 Möllmann, C., & Köster, F. W. (1999). Food consumption by clupeids in the central baltic: Evidence for top-down control? ICES Journal of Marine Science, 56(a), 100-113.

Möllmann, C., Köster, F. W., 2002. Population dynamics of calanoid copepods and the implications of their predation by clupeid fish in the central baltic sea. Journal of Plankton Research, 24(10), 959-978.

Möllmann, C., Kornilovs, G., Fetter, M., Köster, F., & Hinrichsen, H. (2003). The marine copepod, pseudocalanus elongatus, as a mediator between climate variability and fisheries in the central baltic sea. Fisheries Oceanography, 12(4-5), 360-368.

Plikshs, M., Jula, E., Fetter, M.,Uzars, D., Kornilovs, G., Schvetsov, F., Makarchouk, A.,2001. Long-term changes of oceanographic regime in the Gotland Basin of the Baltic Sea: influence on fish species composition and fishery. Theme Session on Ecosystem Change in the Baltic. ICES CM 2001/U:11

Sewell, R.B., 1948. The free-swimming planktonic Copepoda, Systematic account. Sc. Rep. John Murray Exp. (Brit. Mus.Nat.Hist.) :1-303.

Vinogradov, M.E., Voronina, N.M.,1961. Influence of the oxygen deficit on the distribution of plankton in the Arabian Sea. Okeanologia 1(4), 670-678.

Wasmund, N., F. Pollehne, L. Postel, H. Siegel and M. L. Zettler 2004. Biologische Zustandseinschätzung der Ostsee im Jahre 2003. Meereswiss.Ber. Warnemünde 60: 89 S. (http://www.io-warnemuende.de/documents/mebe60_2003-zustand-bio.pdf).

Wishner, K. F., Gowing, M.M., Gelfman, C., 1998. Mesozooplankton biomass in the upper 1000 m in the Arabian Sea: overall seasonal and geographic patterns, and relationship to oxygen gradients. Deep-Sea Resarch Part II 45, 2405-2432.

Acknowledgment

Many thanks for numerous people who were engaged in collecting zooplankton and CTD – oxygen data, who were involved in zooplankton analysis and in data compilation and validation over the years. In the last time, Anneli Postel, Annett Grüttmüller, Steffen Bock and Jan Donath (Baltic Sea Research Institute, Warnemünde, Germany) were especially engaged. The task was funded by the Federal Maritime and Hydrographic Agency of Germany (BSH).

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated 25 Nov 2005.