250-500 million USD during this period. M. leidyi did not only affect the quantity of animals but also of
plant organisms, known as phytoplankton. A warning that M. leidyi might also invade the Caspian Sea
had been voiced during the Geneva meeting as well as by Dumont (1995). Unfortunately, at the end of
the 1990s the invasion of M. leidyi in the Caspian Sea was already being reported (Esmaeili et al., 2000;
Ivanov et al., 2000; Roohi, 2000). It must have also been transported in the ballast waters of ships
traveling from the Black Sea (salinity 18 ppt) to the Caspian Sea (max. salinity 13-14 ppt) through the
Volga Don Canal. Investigations in the Caspian Sea showed by September 2000, it was found everywhere
including the northern Caspian where the salinity can be as low as 4 ppt (Shiganova et al., 2001a).
Materials and Methods
During the cruise samples were collected. Afterwards, the laboratory analysis of the samples was
conducted and the complete data on hydro-chemical status of the samples was obtained. Also, the
phytoplankton samples from the scientific cruises were analyzed and their biomass was determined.
Ctenophore samples were collected with an METU (Medalist Technology University) net having a mouth
opening of 0.2 m
2
and a screen with a mesh size of 500 m, from the same depths as the Juday net
(Vinogradov et al., 1989; Kideys et al., 2001). On completion of each tow, the cod end was immediately
passed into a container and ctenophores counted by eye. The body length of each individual with lobes
was measured lying flat (out of water) onboard, and the density of Mnemiopsis leidyi (per m
2
and m
3
)
was calculated from the net diameter and tow depth. The ctenophores were sorted in length groups of
5-mm intervals to determine the abundance of different size groups. Length measurements were
converted to wet weight using an appropriate equation (Kideys et al., 2001). Samples of Mnemiopsis
were collected from 20001 along few semi- transects perpendicular to the Iranian coast of the Caspian
Sea.
Results
The ctenophore Mnemiopsis leidyi was found at all stations from 2001–2009. There was a seasonal
succession of ctenophore densities every year, the maximum being observed in August and September,
and the minimum density in the winter months. A significant correlation was found between the water
temperature and the abundance of Mnemiopsis leidyi (P <0.005) (Fig. 1).
47
Fig. 1. Seasonal distribution of the Caspian population of Mnemiopsis leidyi (ind.m
3
) in 2003 (shiganova
et al., 2004)
The highest summer– autumn average of Mnemiopsis leidyi abundance was observed in 2002 (760 ±
1148 ind.m
3
), although the biomass during this period (23.2 ± 23.3 g.m
3
) was lower than in 2001 (41.5 ±
44.3 g.m
3
). In addition the zooplankton and phytoplankton species were affected by the bloom. The
highest monthly mean phytoplankton abundance and biomass were 396 × 106 ± 299 × 106 cells m-3 in
January 2002 and 1,789 ± 1,761mg m
-3
in May 2002. Minimum abundance and biomass values were
observed in August 2003 (1 × 106 ± 1 × 106 cells m-3 and 7 ± 5 mg m
-3
).
An unprecedented bloom of the toxic cyanophyte Nodularia sp. was observed between the second half
of August and the end of Ecosystems Biodiversity 188 September in 2005. The bloom area covered
˜20,000 km
2
(CEP 2006). According to the sampling on 20 September 2005, in addition to Nodularia sp.,
another cyanophyte Oscillatoria sp. was also high in abundance. Abundance of Nodularia sp. was 18
×106 cells m
-3
at 7 m depth and 1,006 × 106cells m
-3
at 20 m depth. Average cyanophyte abundance and
biomass at 7 and 20 m depths were 582 9 106 cells m
-3
(of which 512 cells m
-3
was Nodularia sp.) and
1,655 mg m
-3
(Fig. 2)
.
48
Fig. 2. Annual variations in the abundance and biomass of phytoplankton, zooplankton and Mnemiopsis
leidyi in the southern Caspian Sea during 2001–2006 (values are depth and station averages). 1996
values are from Hossieni et al. (1996), spring 2001 values are from Kideys et al. (2001).
As known, microorganisms compete with each other for nutrients and oxygen more than natural
situation during a bloom occurrence; since the population of the species has increased some even
struggle to survive. The different group of zooplankton species before and after the M.leidyi invasion has
been depicted in Table 1.
49
Table 1. Species number of zooplankton before and after Mnemiopsis leidyi invasion in the Southern
Caspian Sea (Roohi and Sajadi, 2011).
Discussion
According to the reports of Iranian Fisheries Research Organization, the algal bloom of Nodularia
spumigena was first seen at northwestern waters of Gilan Province and then it moved towards Anzali
and Nowshahr Ports. The observed algal bloom was quite large and covered an area of 150 square
kilometers from Anzali to Nowshahr Ports. The calm sea conditions and appropriate water temperature
(25 degrees Centigrade) contributed to its formation (CEP, 2006).
A review of Mnemiopsis investigations of the Caspian Sea over the last decade (Roohi and Sajadi,
2011).
After Mnemiopsis invasion into the Caspian Sea via the ballast water from the Black Sea and/or the Sea
of Azov in 1999 (Roohi et al., 2008a), some objectives of this alien ctenophore was taken into account in
several local or national projects such as follows:
- Distribution and abundance of Mnemiopsis leidyi in the Caspian Sea (Iran- Russia - Azerbaijan)- in 2001-
2004 and 2009
- Feeding, respiration, reproduction of Mnemiopsis leidyi in the Caspian Sea- in 2001-2009
- Comparative feeding study of Mnemiopsis leidyi and Kilka in the Caspian Sea- in 2003 -2004 and
2008/9
- Zooplankton and phytoplankton changes after ML invasion
The situation in the Black Sea has been one of the most striking examples in marine bioinvasion history.
Due to scale of the problem, UNEP intervened and gathered international experts in Geneva in 1994, for
investigating methods for solving this problem (GESAMP, 1997). The futility of physical and chemical
methods for this problem were noted and therefore, biological control seemed the only workable
remedy. And, based on the literature knowledge of feeding specificity, another ctenophore species
( Beroe ovata) rose as the best candidate for dealing with M. leidyi problem. Indeed, B. ovata reported
feeding only on other ctenophore species (Kremer and Nixon 1976), most notably on M. leidyi. However,
scientists from the Geneva meeting could not stress on using a new predator species for Ecosystems
Biodiversity.
50
Dostları ilə paylaş: |