Human activities have greatly facilitated the range expansion of aquatic organisms (Carlton 1985; Carlton & Geller 1993; Konopacka 2004), for example Ponto-Caspian gammarids (Crustacea: Pontogammaridae, Amphipoda):
At present, the issues related to migrations of gammarids are well recognized (Bij de Vaate et al. 2002; Panov et al. 2009). Although the gammarid species examined in this study come from habitats with a wide temporal or spatial salinity range, they are mainly known as invaders of freshwater ecosystems in non-native areas (Dodd et al. 2014; Guerlet et al. 2008; Kinzler et al. 2009; Dobrzycka-Krahel et al. 2013; Jermacz et al. 2015), where they are successful colonizers (Grabowski et al. 2007; Jermacz et al. 2015). There are numerous publications on the distribution, biology and ecology of invasive gammarid species in freshwater reservoirs (e.g. Dodd et al. 2014; Grabowski et al. 2009; Kinzler et al. 2009), but their status in coastal brackish waters is less known. Based on the previous laboratory studies (Dobrzycka-Krahel & Surowiec 2011; Dobrzycka-Krahel & Graca 2014; Dobrzycka-Krahel & Graca 2018), we can conclude that low salinities below 5 PSU are suboptimal for these species. Results of these experimental studies on
The Vistula estuary (Fig. 1) is one of the largest estuaries in the southern Baltic Sea. Sometimes referred to as Vistula Bay or Vistula Gulf, the estuary is unusual in the sense that it mostly takes the form of a lagoon, an arm-shaped, elongated body of water.
The Vistula Lagoon (VL) is a broad, elongated, brackish water body, separated from the Baltic Sea by the narrow Vistula Spit. The salinity of VL is affected by the connection with the brackish Baltic Sea with an average annual salinity of up to 3.5 PSU (Chubarenko & Tchepikova 2001).
The Vistula Delta (VD) is an arm-shaped body of water. It has a number of branches. The western branch, the Dead Vistula (Martwa Wisła), carries brackish waters into the Baltic Sea using two connections. One passes through the port of Gdansk, the other, known as the Brave Vistula (Wisła Smiała), is responsible for the elevated salinity of the Dead Vistula. An artificial channel and the mouth of the Vistula is called Przekop (Klekot 1972, 1973). The mean salinity at the Vistula Delta is only 0.3 PSU (Pruszak et al. 2005).
The studies were carried out in the Vistula estuary: the Vistula Lagoon and the Vistula Delta (the transitional zone with salinity below 5 PSU). The studied gammarids were recorded for the first time in this area in 1998–2000 (Jazdzewski & Konopacka 2000; Jazdzewski et al. 2002; Jazdzewski et al. 2004).
Material for the study was collected from two localities: VL and VD in warm months (26 May 2009, 22 June 2009, 7 May 2010, 1 June 2010, 2 June 2010, 30 July 2010, 20 May 2011, 31 May 2011), because the activity of the species peaks in such periods. The samples were collected from the sampling sites once a year. Water salinity and temperature were measured for all sites during each sampling event. Gammarids were sampled at each site by two people for 45 min using a 1 mm mesh size hand net (Jazdzewski et al. 2004).
The material was preserved in 4% formaldehyde, in which the biomass of animals is similar to that obtained using fresh animals (Leuven et al. 1985; Miyasaka et al. 2008).
The composition of gammarids was determined according to taxonomic features described by Mordukhaj-Boltovskoj et al. (1969) and Konopacka (2004). Wet weight was determined after the animals were blotted on filter paper, using a Mettler Toledo XS 205 scale. Length was also determined – each individual was measured from the tip of the head to the telson base, using a binocular Nikon SMZ 800 with a measuring ocular. To determine the size frequency distribution, gammarids were divided into body classes at 1 mm intervals.
The total length and wet weight of the examined individuals were presented as means and standard deviations for each species separately. Total length– wet weight relationships for each gammarid species were calculated according to the exponential equation y = axb, (Ww = aLtb), where: y – wet weight in mg, x – total length in mm, a – intercept, b – slope. The correlation coefficient
Water salinity (S) at sampling sites varied from 0.2 to 4.8, water temperature (T) – from 12.6 to 25.8°C. Mean water salinity was 1.13 PSU (3.6±SD) and mean water temperature was 1.06°C (20.75±SD).
Individuals of Ponto-Caspian gammarid species:
Descriptive statistics of body length (L) and wet weight (W) of gammarid species from the Vistula Lagoon and the Vistula Delta in 2009–2011
Species | N | % | L (mm) ± SD | W (g) ± SD |
---|---|---|---|---|
374 | 24 | 8.55 ± 4.02 | 0.019 ± 0.047 | |
617 | 41 | 5.93 ± 1.82 | 0.003 ± 0.004 | |
530 | 35 | 7.73 ± 2.65 | 0.012 ± 0.011 |
The data show variations in length of individuals of each examined species. Despite similarly small lengths of the examined alien gammarids, the ranges of length are different for each species in the study area and varied from 2.9 to 21.4 mm for
The relationships between wet weight and total length of individuals representing
Similarities in body length and weight of the analyzed invasive gammarid species were observed. The results also show a high proportion of juveniles (characterized by a fast growth rate) in the case of
In general, our results show that the environment of the Vistula estuary is favorable for the examined species and the relationships demonstrate good fitness of gammarids and the high body weight growth rate in relation to the length growth rate.
If we consider the importance of the body condition of organisms, we can mention examples of previous research. Many authors have examined the relationships between length and weight (e.g. Siegel 1982; 1989; Morris et al. 1988), because species differ in body weight growth in relation to length growth (Lindqvist & Lahti 1983). Factors such as resource availability (Basset & Glazier 1995) and competitive interactions among species (Basset & Rossi 1990) are known to affect the body size and body condition of individuals. The relationships between the total body length and wet weight of species may be used to assess the size and mass of species consumed by predators (Janas 2005). The Ponto-Caspian gammarid species in question may be important food components for such fishes as
The length–weight relationships may also indicate morphological differences in various areas (Färber-Lorda 1994). To our knowledge, data on the condition of the examined gammarid species from their original area have not yet been published. However, we have calculated intercepts (a) and slopes (b) of Ponto-Caspian gammarids from the Gulf of Gdansk (salinity above 5 PSU) (Dobrzycka-Krahel et al. 2016). The values of the intercepts (a) and slopes (b) calculated for
Calculated intercepts (a) and slopes (b) in the total length–wet weight relationship regression models
Species | Area of occurrence | intercepts (a) | slopes (b) | individuals (n) | Reference |
---|---|---|---|---|---|
Vistula Lagoon and Delta Europe: Poland (brackish environment) | 0.0107 | 3.1317 | 374 | current study | |
0.0041 | 3.5461 | 617 | |||
0.141 | 2.0896 | 530 | |||
Gulf of Gdańsk Europe: Poland (brackish environment) | 0.0367 | 2.852 | 72 | Dobrzycka-Krahel et al. 2016 | |
0.0069 | 3.3477 | 75 | |||
0.0033 | 3.7855 | 225 | |||
0.0674 | 2.852 | 117 | |||
Black Sea (brackish environment) | 0.0016 | 2.87 | 20 | Rosati et al. 2012 | |
North America (freshwater environment) | 0.012 | 2.74 | Basset & Glazier 1995 | ||
0.0049 | 3.001 | 37 | Marchant & Hynes 1981 | ||
Gammaridae | Mediterranean (transiti onal and waters) Black Sea | 0.0046 | 1.2 | 20 | Rosati et al. 2012 |
On the one hand, the good condition of Ponto-Caspian gammarids (
According to Calow & Forbes (1998), fitness traits help not only to define stress, but more generally to understand how environmental variables affect the distribution and abundance of organisms. Good fitness corresponds to normal responses (Jacobsen & Forbes 1997). On the other hand, good condition of alien species affects the process of invasion or colonization. Fitness of invaders increases its ability to displace residents (Peacor et al. 2006). When invasive species strays from its optimal strategy, it can no longer compete with native species (Gross 2006). The optimal strategy of invasive alien gammarids may help them maintain strong competitive position in the environment as well as affect the colonization process of the Vistula Lagoon and the Vistula Delta. Such studies may be used in aquatic habitat management as they indicate the status of invasive alien species.