Vertical distribution

To clarify the central hypothesis whether the vertical zooplankton composition in the Bornholm Sea changes with different hydrographic parameters, it was necessary to deter-mine whether samples from different layers can be distinguished by their species composi-tion. To achieve this goal multivariate statistical analyses and conventional techniques were used. The results show that the characteristic stratification of the Bornholm Sea di-vides the water column into zones, differently inhabited by zooplankton species. The physical outlines of these zones are defined by different combinations of temperature and salinity. Consequently, two zones are present during winter, when the halocline separates the high saline bottom water from the lower saline intermediate winter water above. Three zones are found after the formation of the thermocline during summer. The intermediate winter water is then compressed by the seasonal summer water. The inhomogeneous utili-sation of the zones indicates that they are of heterogeneous ecological importance and can be conceived as distinct habitats. To survive under these hydrographic conditions species are impelled to utilise one or several of these habitats according to their ecophysiological tolerances (Paper Z1-Z3). None of the investigated zooplankton species and developmental stages utilised all available habitats. No direct evidence was found, that the inflow waters differed significantly from the stagnant haline waters in species composition. The limited number of habitats results in five categorical utilisation modes (Figure 5.1) that group simi-lar strategies of utilisation. Every utilisation mode (mode I-V) is characterised by at least one dominant species:

I. The first mode includes species which are most abundant during the warm season and mainly restricted to the area above the thermocline (Figure 5.1a). Although they can become an eminent part of the zooplankton community they are only seasonally pre-sent. The endemic and due to parthenogenesis fast reproducing cladoceran Bosmina coregoni maritima is the dominant species of mode I. Furthermore, the cladoceran Podon intermedius and all developmental stages of Eurytemora sp. are most

abun-dant in the warm surface layer, but more or less completely absent dur-ing the rest of the year.

II. Species assigned to the second mode are abundant the whole year round and seasonally take advantage of the warm surface layer. They access the volume above the thermocline during summer and reside in the winter wa-ter for the rest of the year (Figure 5.1b). The copepod Acartia bifilosa is the dominant species of this mode.

All developmental stages of A. bi-filosa showed this behaviour. Also Keratella spp. and the C1-C3 stages of Centropages hamatus and Temora longicornis utilise the warm surface water when present.

III. The third mode includes species pre-ferring the lower temperatures be-tween the thermo- and halocline.

They reside mainly in the intermedi-ate winter wintermedi-ater and are excluded from the surface during summer (Figure 5.1c). All developmental stages of the copepod Acartia longiremis are characteristic for this mode. A. longiremis is abundant throughout the year, but avoids the

Figure 5.1: The five different utilisation modes of zooplankton in relation to hydrographic parame-ters as revealed by Multivariate Discriminant Function Analysis and traditional approaches. The modes include species a) that are only present when a warm surface layer is established and dwell mainly in this layer, b) that utilise the warm surface layer but are also present prior to and after the formation of this seasonally appearing habitat, c) that are present the whole year, but reside in the layer of the intermediate winter water between thermo- and halocline during summer, d) that

warm surface layer during summer. This was also found for Synchaeta spp. and the C4-C6 stages of Centropages hamatus and Temora longicornis.

IV. Mode four includes species that utilise the winter water and the deep haline waters.

Characteristic for this mode is the appendicularian Fritillaria borealis. It mainly re-sides in the winter water before the thermocline develops. With the formation of the thermocline it migrates below the halocline. The water above the thermocline in summer is the only unused layer (Figure 5.1d).

V. Mode five includes the species living mainly below the halocline (Figure 5.1e).

Oithona similis is a signature species of this utilisation mode. Also Oikopleura dioica and the C4-C6 stages of Pseudocalanus sp. were mainly found in the high saline wa-ters.

Most of the investigated zooplankton taxa could be clearly assigned to one of these five modes. Different results between analyses of one station (Paper Z1) and the complete data set (nine stations covering the Bornholm Basin, Paper Z3) were obtained only for the taxa Bivalve larvae and Pseudocalanus sp. C1-C3 and the cladocerans Evadne nordmanni and Podon leuckarti. This might be explained by assignment problems of mean values for tem-perature and salinity across clines. Every net that samples through a cline contains species of two different habitats, while the algorithmic classification assigns the sample to the cate-gory where physical parameters had the greatest influence. Most of these four taxa belong to categories that usually inhabit the upper layers, but show an indifferent behaviour com-pared to the algorithmic definition of the layers. Bivalve larvae utilise the warm surface water, but seem to avoid the warmest and uppermost layers. E. nordmanni and P. leuckarti are most abundant when the thermocline develops or disappears. Therefore it seems appro-priate to assign these taxa to mode II. Further the grouping of copepodids to only two dif-ferent groups might impact the analysis, as several species show ontogenetic vertical dis-tributions or sex differences. This applies at least to the C1-C3 stages of Pseudocalanus sp.

(Renz & Hirche 2005). The data suggest that they have to be assigned to mode IV. As the samples used for the analyses were taken regardless of daytime a possible diel vertical mi-gration (DVM) might also have influenced the results. However, the DVM is reduced in the Baltic Sea (e.g. Titelman & Fiksen 2004, Schmidt 2006) and has recently been reported to occur only in Acartia longiremis and Temora longicornis (Schmidt 2006). During sum-mer the depth centroid of T. longicornis was found at approximately 20 m at night and 60

cline (Schmidt 2006). Both species showed lower abundances in the uppermost layer, when temperatures were highest. Therefore the assignment to vertical distribution modes defined by the ambient temperature and salinity may be a simplification and not necessar-ily the primordial causal connection. This can be seen in Fritillaria borealis, a presumptive glacial relict species, which is capable of utilising the low saline surface layer. During summer F. borealis protrudes down into the haline waters, although a layer of the cool intermediate winter water is still present (Paper Z2). This may be due to the fact that in the density gradient of the halocline organisms and particles of size classes sufficient for its feeding can accumulate (Lande & Wood 1987, MacIntyre et al. 1995, Vallin & Nissling 2000). The period during which Keratella spp. showed high abundances was found in a time frame that started a few weeks prior and lasted a few weeks longer than the persis-tence of the algorithmic defined SUMMER category. The temperature threshold of 8°C for this category was chosen for proper identification of the thermal stratification. The begin-ning of this stratification was already observed below the chosen temperature threshold.

Thus, it can be suggested that Keratella spp. should be seen as a summer species and as-signed to mode I.