Scyphozoa in the Bornholm Basin (central Baltic Sea)–The role of advection

Abstract

The usual absence of ephyrae and late appearance of medusae of the Scyphozoa Aurelia aurita and Cyanea capillata in the Bornholm Basin (BB; central Baltic Sea) indicate that these species are not strobilating in the region and their presence depends on advection. To study their potential origin we compared drift from historically known strobilation areas derived from a circulation model with spatial distributions observed during 19 cruises in the BB during 2002 and 2003. The model results are in good accordance with the field observations. According to the model results inter-annual differences in the timing of first appearance and life stage at appearance of A. aurita were clearly related to differences in the hydrodynamic regime during the investigation periods. During the stagnation regime in 2002 young medusae occurred first in June in the BB. In contrast, in 2003 fast transport due to several inflow events advected ephyrae released between January and March in the western Baltic already in April to the BB. Although the Gullmar Fjord (western Sweden) is the nearest known strobilation area for C. capillata, the model did not support advection from there in numbers explaining the occurrence of this species in the BB in 2002 and 2003. If the model works adequately in this regions we have to assume that the Gullmar Fjord is not a main source region of C. capillata in the BB, but other strobilation areas in the Kattegat or the North Sea appear more important.

Our results imply that advection and inflow events are critical for the occurrence and distribution of early stages of jellyfish in the central Baltic Sea. They demonstrate the potential of circulation models as tools to study the effect of long-range transport on the spatial composition of these organisms.

Introduction

The abundance of scyphomedusae is increasing in many marine pelagic ecosystems, perhaps as a result of regime shifts (Brodeur et al., 1999, Brierley et al., 2001, Mills, 2001, Lynam et al., 2004). It has been shown that medusae can reduce the stocks of mesozooplankton communities considerably in years of high abundance (e.g. Möller, 1980a, Matsakis and Conover, 1991, Purcell, 1992, Olesen, 1995, Omori et al., 1995, Lucas et al., 1997, Schneider and Behrends, 1998). They compete for zooplankton with commercially important planktivorous fish species, but they may also prey on fish eggs and larvae and thus directly affect their recruitment.

The deep basins in the Baltic Sea, including the Bornholm Basin (BB, central Baltic), are significant spawning grounds for cod (Gadus morhua) (Bagge et al., 1994, McKenzie et al., 2000) and sprat (Sprattus sprattus) (Köster et al., 2001). Therefore, the patterns of medusae abundance and occurrence and the factors regulating them are of special interest to assess, for instance, the impact of medusae on the populations of zooplankton and fish.

In the Baltic Sea only the scyphozoan medusae Aurelia aurita and Cyanea capillata are found. They are observed regularly in the BB, but are not present all year round (Janas and Witek, 1993, Barz and Hirche, 2005). Both species perform a typical cnidarian life cycle, including a planula which develops into a polyp. The polyp asexually produces medusae by strobilation, which in turn reproduce sexually (metagenesis). However, it is unclear if the species complete their whole life cycle in the BB. The facts that ephyrae are not found regularly and medusae appear later and were smaller than in adjacent seas suggest that A. aurita does not strobilate in this area (Janas and Witek, 1993, Barz and Hirche, 2005). C. capillata is usually much less abundant in the central Baltic than A. aurita (Janas and Witek, 1993, Margonski and Horbowa, 1995, Barz and Hirche, 2005). The absence of ephyrae and generally low abundance suggest that there is no strobilation area either (Barz and Hirche, 2005). Therefore, the question arises where the scyphomedusae appearing in the BB originate.

Large A. aurita polyp populations are reported from Kiel Bight (western Baltic) (Kändler, 1961, Thiel, 1962, Möller, 1979, Möller, 1980a, Möller, 1980b, Schneider and Behrends, 1998), Kerteminde Fjord (Belt Sea, Denmark) (Olesen et al., 1994) and Gullmar Fjord (Skagerrak, Sweden) (Hernroth and Gröndahl, 1983, Gröndahl, 1988). In older publications, the archipelago of S. W. Finland is also mentioned as a strobilation area (Wikström, 1932, Palmen, 1953). The Gullmar Fjord is the nearest known production area of C. capillata, although the low abundance of ephyrae there was interpreted as an indication for marginal living conditions by Gröndahl and Hernroth (1987) and Gröndahl (1988). The vast majority of this species is probably advected into the Baltic from the North Sea, which accordingly makes it an indicator of North Sea water masses in the Baltic Sea (Möller, 1980c, Gröndahl and Hernroth, 1987).

During an intensive field study in the BB, within the framework of German GLOBEC, which focused on the interactions between zooplankton and fish under the influence of physical processes, the seasonal and spatial occurrence patterns of A. aurita and C. capillata were investigated. Detailed data on medusa abundance and distribution in 2002 are already published (Barz and Hirche, 2005). Combined with new data from 2003 they allowed an inter-annual comparison under different hydrographic conditions. Thus, a Major Baltic Inflow event was recorded in the beginning of 2003 (Feistel et al., 2003), which may have affected the occurrence and distribution patterns of the medusae. Johnson et al. (2001) have shown how inter-annual variability of current patterns can produce remarkable fluctuations in jellyfish populations.

In order to study the advection of medusae from the known strobilation areas we used a three-dimensional eddy resolving baroclinic circulation model of the Baltic Sea. The modelled arrival of medusae in the BB was then compared with our observations in 2002 and 2003. The major aim was to identify the potential sources of the scyphomedusae in the central Baltic Sea, and to evaluate the processes controlling their occurrence and distribution in the area.