V ERTICAL DISTRIBUTION OF ZOOPLANKTON

1.1.1. Biological oceanography

The presence of discontinuity layers is a ubiquitous and recurrent feature in the world oceans. Of eminent biological importance are inhomogeneities in temperature, salinity and oxygen. These environmental parameters have a profound impact on the physiological conditions of zooplankton communities. Differences in zooplankton distribution often co-incide with different water masses characterised by these three physical parameters (e.g.

Fager & McGowan 1963, Owen 1989, Geller et al. 1992, Roemmich & McGowan 1995).

In contrast to changes in oxygen levels, alterations in temperature or salinity influence the density of a body of water. Surface layers, heated by solar radiation, become lower in den-sity and stratify over cooler and denser water layers. A thermocline marks the transition zone between layers of different temperatures and the stratification becomes more stable when the thermal gradient increases (e.g. Lalli & Parsons 1997). Removal or addition of fresh water changes the salinity and in turn also the density of seawater (e.g. Brown et al.

1998). As evaporation and freezing remove fresh water, the increasing concentration of dissolved salt in the remaining water volume increases its density. Conversely, precipita-tion, melting and river run-off decrease the salinity and cause lower densities. An area in which salinity changes rapidly with depth is termed halocline and separates waters of dif-ferent densities vertically (e.g. Lalli & Parsons 1997). Stable clines generally separate wa-ter masses of different temperature and salinity combinations and can form stacked vol-umes of diverse ecological conditions on scales from centimetres to several hundreds of metres. Such clines often restrict mixing of adjacent layers and impact vertical processes of physical and biological exchange (Brainerd & Gregg 1995) and interrupt exchange proc-esses between the euphotic zone and the mesopelagic (e.g. Humboldt Current: Escribano et al. 2004; California Current: Alldredge et al. 1984, Roemmich & McGowan 1995; Black Sea: Vinogradov 1993). Also in estuaries strong stratifications can reduce vertical ex-change (Denman & Gargett 1988) and impact both primary and secondary production in of yet not completely understood ways (Owen 1989, Cowles et al. 1998). When water masses with a higher density are layered over ones of lower density, the stratification becomes

unstable and the above layer sinks down or admixes with the underlying one. In this case inherent species may be dislocated or face changing environmental conditions.

As many zooplankton species are poikilothermal and have a limited osmoregulatory capa-bility, their distribution patterns are determined by the physiological tolerance for ambient parameters. In addition to physiological demands (Saito & Hattori 1997), the availability of food resources (Hattori & Saito 1997) and ontogenetic migrations (Renz & Hirche 2006) are often important factors for habitat selection. The influence of small scale turbulence on feeding strategies (Maar et al. 2006), histo-geographic traits (Ojaveer et al. 1998, Renz &

Hirche 2006) and predator avoidance (Bollens & Frost 1989, Titelman & Fiksen 2004) further affect vertical distribution (Ohman 1988) and can constrain distribution to fringed layers (Gallager et al. 2004). Species adapted to cool waters show submergence towards greater depth at lower latitudes because of higher surface temperatures (Jespersen 1940).

As light attenuation restricts primary production to a shallow layer, expelled species need to adapt to alternative food resources to prevent starvation. Small scale zooplankton patchiness and aggregation are effects of the physico-chemical structure of the water col-umn and important features of the environment of planktivorous predators that impact both bottom-up and top-down processes (Owen 1989). As trophodynamic relationships in pe-lagic systems depend on temporal as well as spatial overlap, the understanding of mecha-nisms that lead to different vertical distributions is essential (Banse 1964). However, re-garding zooplankton ecology, variability in the vertical plane is probably more important than in the horizontal plane (e.g. Dagg 1977, Longhurst & Harrison 1989). The match-mismatch theory (Cushing 1975, Cushing 1990) and most successors dealing with food requirement of fish larvae just consider temporal aspects and neglect heterogeneities in vertical distribution.

1.1.2. The Baltic Sea

The semi-enclosed Baltic Sea (Figure 1.1) is characterised by a strong fresh water influ-ence. It covers an area of approximately 412.560 km² and is the largest brackish water sys-tem of the world (Fonselius 1970). Freshwater supply from river run-off and precipitation from the riparian states dilute the surface layer (Brogmus 1952, Fonselius 1970). Water exchange with the North Sea takes place via the connection of Skagerrak and Kattegatt, the only gateway for saline waters to enter this marginal sea. Dense saline waters are captured in the troughs and replaced during inflow events from the North Sea with highly saline and

oxygenised waters (Matthäus & Schinke 1994). Sills separate several consecutive deep basins and constrain the exchange of high saline waters in the deep (Matthäus 1995). They also restrict propagation of haline waters from the North Sea towards the innermost basins.

Inflow events are related to atmospheric circulation patterns and often separated by long stagnation periods (Matthäus & Franck 1992, Matthäus & Schinke 1994, Matthäus 1995, Lehmann et al. 2002). During these periods conditions below the halocline deteriorate due to decomposition processes of organic matter and lead to hypoxic or anoxic conditions in the deep, and only a narrow zone of oxygenated water remains below the halocline (Fon-selius 1970). The decrease in salinity from the Skagerrak towards the eastern end is ac-companied by distinct floral and faunal alterations (Bonsdorff 2006). In addition to a halo-cline, a seasonal thermocline establishes in spring and persists until fall. It separates the warm surface layer from the intermediate winter-water and forms a three-storeyed eco-system of water masses with different thermal and haline combinations (Figure 1.2). The

Figure 1.1: The Baltic Sea.

to investigate the relationship between hydrographic structures and the zonation of zoo-plankton. From a geological view the Baltic is a young sea, with biotic colonisation closely related to events after the last glacial period (Voipio 1981, Andrén et al. 2000, Andrén et al. 2002, Bonsdorff 2006). Consequently, few endemic species are found in this brackish environment (Ackefors 1969) and the characteristic low diversity of brackish systems (Remane & Schlieper 1971) results in unoccupied ecological niches (Elmgren 1984). The zooplankton community in the Baltic Sea consists of freshwater, brackish and marine spe-cies (e.g. Ackefors 1969, Remane & Schlieper 1971, Hernroth & Ackefors 1979, Ojaveer et al. 1998). Both horizontal and vertical distribution of zooplankton species is influenced by temperature and salinity gradients (Ackefors 1969, Hernroth & Ackefors 1979, Hansen et al. 2004) which also affect population dynamics (Viitasalo et al. 1995a, Viitasalo et al.

1995b, Vuorinen et al. 1998, Ojaveer et al. 1998, Dippner et al. 2000, Möllmann et al.

2000). While stenoecious species are expected to inhabit a distinct layer with certain hy-drographic characteristics, euryoecious species may reside in different strata. Zooplankton investigations by Ackefors (1969) and Hernroth & Ackefors (1979) give a general

over-Figure 1.2: Schematic overview of the seasonal hydrographic situations in the Baltic Sea. During summer the seasonal thermocline compresses the continuously present winter water to a narrow layer between the thermo- and halocline. Due to its higher density the deep haline waters are re-stricted to troughs defined by local topography. The local topography that different layers interact with is independent from the time scale.

view on the vertical distribution of different species in the central Baltic Sea, but their low spatial and temporal sampling resolution is not sufficient to determine the main residence layers and life strategies. As several zooplankton species live at the lower limit of their physiological tolerances, small hydrographic changes can alter the structure of the pelagic community significantly (Hernroth & Ackefors 1979). Time series analyses have shown that zooplankton species are strongly affected by episodic inflow events from the North Sea. The decreasing frequency of inflows since the 1980’s is responsible for observed shifts in the ecosystem structure due to changes in the hydrographic regime (e.g. Ojaveer et al. 1998, Möllmann et al. 2000, Möllmann et al. 2002).

1.1.3. Trophodynamic interactions

In the Baltic Sea a few key-species dominate the zooplankton community. These are major contributors structuring the ecosystem and hence crucial for the successful recruitment of higher trophic levels (Alheit et al. 2005). The commercially important fish species sprat (Sprattus sprattus L.), herring (Clupea harengus L.) and cod (Gadus morhua L.) are at least in early life stages planktivorous (e.g. Last 1980). The vertical distribution of their larvae and juveniles is subjected to hydrographic and environmental parameters and there-fore their feeding sites are located at different depths (Voss et al. 2002, Voss et al. 2003).

As many zooplankton species have potentially a similar role in the food web, inhomogene-ous aggregation patterns would result in different prey fields among the layers. The feeding behaviour of fish is often triggered by parameters such as size, visibility, pigmentation, encounter rate or detectionable hydrodynamic cues created by the prey (e.g. Flinkman et al.

1993, Viitasalo et al. 2001 and references therein). In the case of different vertical zoo-plankton distributions an inhomogeneous access to this resource needs also to be consid-ered. To investigate vertical distribution patterns requires a parallel evaluation of the abun-dance of dominant zooplankton species in different depths. Distinguishable groups and alterations between layers may impact eminent structures of the food web in terms of re-gime shift scenarios. Thus, a hydrographic segregation of different types of prey would create functional groups of accessibility.