Bb0021 Assignment

Abstract

The Baltic Sea is the largest brackish water area of the world. On the basis of the data from 16 cruises, we show the seasonal and vertical distribution patterns of the appendicularians Fritillaria borealis, Oikopleura dioica and the cyclopoid copepod Oithona similis, in the highly stratified Bornholm Basin. These species live at least temporarily below the permanent halocline and use different life strategies to cope with the brackish environment. The cold-water species F. borealis is abundant in the upper layers of the water column before the thermocline develops. With the formation of the thermocline abundance decreases and the specimens outlast higher temperatures below the halocline. Distribution and strategy suggest that F. borealis might be a glacial relict species in the Baltic Sea. Although Oikopleura dioica is only abundant during summer, O. similis is present all year round. Both species have in common that their vertical distribution is restricted to the waters below the halocline, most likely due to their requirements of higher salinities. We argue that the observed strategies are determined by ecophysiological constraints and life history traits. These species share an omnivorous feeding behaviour and the capability to utilise a spectra of small particles as food. As phytoplankton concentration is negligible below the halocline, we suggest that these species feed on organic material and heterotrophic organisms that accumulate in the density gradient of the halocline. Therefore, the deep haline waters in the Baltic Sea represent a habitat providing shelter from predation and food supply for adapted species that allows them to gather sufficient resources and to maintain populations.

INTRODUCTION

The distribution patterns of zooplankton species in the water column often coincide with discriminable conditions of temperature, salinity and oxygen (e.g. Fager and McGowan, 1963; Owen, 1989; Geller et al., 1992; Roemmich and McGowan, 1995). The physical layering can limit species dispersal and constrain appearance to fringed layers (Gallager et al., 2004). Especially in regions with strong fresh water influence, the decreased vertical exchange between different water masses (Denman and Gargett, 1988) impacts both primary and secondary production in as yet not completely understood ways (Owen, 1989; Cowles et al., 1998).

There is an ongoing discussion on how zooplankton preferences for different layers are affected by hydrography. Ecophysiological demands (Saito and Hattori, 1997), the availability of food resources (Hattori and Saito, 1997) and predator avoidance (Bollens and Frost, 1989; Titelman and Fiksen, 2004) are significant factors for habitat selection. Ontogenetic migrations (Renz and Hirche, 2006), histo-geographic traits (Ojaveer et al., 1998; Renz and Hirche, 2006) and the impact of turbulence on feeding (Maar et al., 2006) further impact the optimum depth (Ohman, 1988).

The Baltic Sea is the largest brackish water area in the world and is characterised by strong thermal and saline stratifications (Fonselius, 1970). During summer, a thermocline forms a steep temperature gradient in the upper 20–30 m. A permanent halocline separates the low saline surface water from a denser layer with higher salinity in ∼60 m depth (Fonselius, 1970; Hernroth and Ackefors, 1979). Shallower areas lack these high saline bottom waters, whereas sills constrain water exchange between the deep basins. Degradation processes result in low oxygen concentrations below the halocline, whereas ventilation depends on inflows from the North Sea (Vallin and Nissling, 2000), which communicates with the Baltic Sea via the Belt Sea and the Danish Sound (Brogmus, 1952). From a geological view, the Baltic is a young sea (Andrén et al., 2000; Andrén et al., 2002), with biotic immigration closely related to the events after the last glacial period. Consequently, few endemic species are present in this brackish environment (Ackefors, 1969) and many species live at the lower range of their osmotic tolerance limit (Hernroth and Ackefors, 1979). This results in the typical low diversity of brackish systems (Remane and Schlieper, 1971) and unoccupied ecological niches (Elmgren, 1984). Newly inserted species, arriving with inflow waters from the North Sea, have the possibility to establish in this evolutionary proving ground and are frequently observed with varying success (cf. Postel, 1996). Although the role of the thermo- and halocline is not completely understood in this system, the physical layering separates the water column into different habitats and impacts vertical zooplankton community assemblages (Schulz et al., 2006). This results in characteristic assemblage patterns in certain depths and impacts interactions with different trophic levels. Successful species either require a euryoecious capacity or are forced to develop a suitable niche and survival strategy.

In the Bornholm Basin, one of the target areas of (GLOBEC-Germany, 2002), the appendicularians Fritillaria borealis, Oikopleura dioica and the cyclopoid copepod Oithona similis are members of the zooplankton community below the halocline (Hansen et al., 2004; Schulz et al., 2006). On the basis of three stations with different depths and hence differently developed stratifications, we present a high-resolution dataset obtained during 16 cruises. The behaviour of these three marine species in relation to the changing physical parameters temperature, salinity and oxygen was investigated. Although the annual abundance of Oikopleura dioica contributes with only 0.02% to the community composition, F. borealis shows a proportion of ∼8.2% and O. similis of 4.6% (Schulz, 2006). Little is known about their life strategies, although these species represent an important contribution to the total biomass of zooplankton at certain times of the year (Hernroth and Ackefors, 1979; Hansen et al., 2004). Investigations by Ackefors (Ackefors, 1969) and Hernroth and Ackefors (Hernroth and Ackefors, 1979) give a general overview on the vertical distribution of different zooplankton species in the Baltic Sea, but low vertical and temporal sampling resolution does not allow determining the main residence layers and life cycles. The three species are good examples for successful immigration and allow studying the effect of hydrography on the vertical distribution. The overall knowledge of appendicularian vertical distribution, as well as their diet is quite limited (Fenaux, 1968; Bone, 1998; Calbet, 2001) and little information is available for the Baltic. Although O. similis is widely distributed its vertical abundance pattern and role in the food web is not fully understood (Turner, 2004). Recent investigations have shown that in the Baltic Sea, O. similis dwells mainly in the halocline (Hansen et al., 2004).

This study contributes to the understanding of the halocline as an environmental characteristic of the Bornholm Basin that impacts vertical zooplankton community assemblages. We discuss strategies how the three species cope with the challenging conditions of this highly stratified basin and trophodynamic implications for the food web.

METHODS

Hydrography

Data come from 16 cruises between March 2002 and May 2003 with an almost monthly coverage. On seven stations in the Bornholm Basin, vertical profiles of temperature, salinity and oxygen were obtained by using a Seabird CTD probe (Fig. 1). The stations were located on a west to east transect, perpendicular to 55.292°N, representing a section through the Basin, the communicating water masses from the North Sea successively pass (Fig. 2). Spatial and seasonal charts were created with Surfer (Golden Software Inc., Surfer version 8) using kriging as the gridding method. Temperature was visualised as greyscale background with a contour plot overlay rendered from salinity data. The 1 mL L−1 oxygen isopleth was additionally included. Bathymetric maps were generated with Ocean Data View (Schlitzer, 2004).

Fig. 1.

Sampling site in the Bornholm Basin (central Baltic Sea) with the three stations at which zooplankton samples and CTD casts were obtained (white circles). Additional CTD casts were performed on four further stations along a transect through the Bornholm Basin (black dots).

Fig. 1.

Sampling site in the Bornholm Basin (central Baltic Sea) with the three stations at which zooplankton samples and CTD casts were obtained (white circles). Additional CTD casts were performed on four further stations along a transect through the Bornholm Basin (black dots).

Fig. 2.

Hydrographic transects through the Bornholm Basin covering the sampled stations. (a) April 2002, (b) July 2002, (c) October 2002, (d) November 2002 and (e) February 2003. Temperature is shown by grey scale background and salinity by solid isolines. The 1 mL L−1 oxycline is displayed by the dashed line. In February 2003, oxygen concentration was never found to be below 3 mL L−1.

Fig. 2.

Hydrographic transects through the Bornholm Basin covering the sampled stations. (a) April 2002, (b) July 2002, (c) October 2002, (d) November 2002 and (e) February 2003. Temperature is shown by grey scale background and salinity by solid isolines. The 1 mL L−1 oxycline is displayed by the dashed line. In February 2003, oxygen concentration was never found to be below 3 mL L−1.

Zooplankton data

Zooplankton sampling was performed with a multinet (50 µm mesh size, 0.25 m2 mouth opening, HydroBios, Kiel, Germany) on three of the seven stations (Fig. 1). Samples were taken from bottom to surface in stacked 10 m intervals. The time lag between sampling the three stations on each cruise was normally less than 2 days (Table I). Samples were taken regardless of daytime and preserved immediately aboard in a borax-buffered 4% formalin-seawater solution. Subsamples of at least 500 individuals were enumerated and the number of individuals extrapolated to individuals per cubic meter (n m−3) in each depth stratum. Data of F. borealis, Oikopleura dioica and O. similis were used for the analysis. Copepod stages were assigned to three groups: the binned copepodite stages C1–C5, female and male individuals. Nauplii data were not used for the analysis.

Table I:

Sampling sites and dates in the Bornholm Basin (central Baltic Sea)

Station 
BB0021 BB0023 BB0026 
Latitude (deg, dec) 55.292 55.292 55.292 
Longitude (deg, dec) 15.283 15.750 16.500 
Bottom depth (m) 90 95 62 
Sampling dates 
 March 2002 18 18 – 
 April 2002 05 05 06 
 April 2002 21 21 19 
 May 2002 08 08 08 
 May 2002 25 26 26 
 June 2002 14 14 16 
 July 2002 25 25 27 
 August 2002 16 16 14 
 September 2002 06 07 07 
 October 2002 06 06 05 
 November 2002 16 16 14 
 January 2003 16 19 18 
 February 2003 13 14 16 
 March 2003 12 13 11 
 April 2003 20 18 23 
 May 2003 – 28 29 
Station 
BB0021 BB0023 BB0026 
Latitude (deg, dec) 55.292 55.292 55.292 
Longitude (deg, dec) 15.283 15.750 16.500 
Bottom depth (m) 90 95 62 
Sampling dates 
 March 2002 18 18 – 
 April 2002 05 05 06 
 April 2002 21 21 19 
 May 2002 08 08 08 
 May 2002 25 26 26 
 June 2002 14 14 16 
 July 2002 25 25 27 
 August 2002 16 16 14 
 September 2002 06 07 07 
 October 2002 06 06 05 
 November 2002 16 16 14 
 January 2003 16 19 18 
 February 2003 13 14 16 
 March 2003 12 13 11 
 April 2003 20 18 23 
 May 2003 – 28 29 

View Large

As an index of the vertical orientation of the standing stocks, the weighted mean depths (WMDs) were calculated. With respect to the covered depth range of a single net in a haul, the abundance data were also used to compute the number of individuals per square meter (SQM, n m−2): where d is the mid-depth of the range covered by the respective net, i the index of the species or stage, j the station, k the index of the net on the jth station, l the total number of nets on a station, m the number of individuals per cubic meter and r the depth range of the kth net. For semi-automated processing, the formulas were coded as algorithms, implemented into Ocean Sneaker’s Tool (Schulz, 2005

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