No abstract

The invasive ctenophore Mnemiopsis leidyi (Agassiz), which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001–2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (four Copepoda and one Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of Mnemiopsis leidyi into the Caspian Sea. Cladocera species seemed to be highly affected by the invasion of Mnemiopsis leidyi; only one species, Podon polyphemoides, remained in the study area, whereas 24 Cladocera species were found in the study carried out in 1996. Whereas among the Copepoda Eurytemora minor, Eurytemora grimmi, Calanipeda aquae dulcis and Acartia tonsa that were abundant before the Mnemiopsis leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maximum in zooplankton abundance (22,088 ± 24,840 ind·m−3) and biomass (64.1 ± 56.8 mg·m−3) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001–2006 was in the range of 3361–8940 ind·m−3; this was two‐ to five‐fold less than the zooplankton abundance in 1996. During 2001–2006, the highest abundance and biomass of Mnemiopsis leidyi were observed during summer‐autumn months coincident with warm temperatures and generally when the abundance of other zooplankton organisms was low.

A significant correlation was observed between satellite derived chlorophyll a (Chl a) concentrations and the biomass of the invasive comb jellyfish Mnemiopsis leidyi in the southern Caspian Sea. By consuming the herbivorous zooplankton, the predatory ctenophore M. leidyi may have caused levels of Chl a to rise to very high values (∼9 mg m −3 ) in the southern Caspian Sea. There might also be several other factors concurrent with predation effects of M. leidyi influencing Chl a levels in this region, such as eutrophication and climatic changes which play major roles in nutrient, phytoplankton, and zooplankton variations. The decrease in pelagic fishes due to overfishing, natural, and anthropogenic impacts might have provided a suitable environment for M. leidyi to spread throughout this enclosed basin. Copyright © 2008 Ahmet E. Kideys et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.The Caspian Sea is the largest inland water body in the world sustaining large stocks of small commercially important zooplanktivorous, pelagic fish. In such ecosystems, a consistent, significant decrease in numbers of grazing zooplankton would be expected to result in a decrease in pelagic fish stocks and their predators.Mnemiopsis leidyi is a highly fecund comb jelly feeding extensively on zooplankton. The main diet of this ctenophore in the southwestern Caspian Sea was found to be copepods (∼45%) during May 2000-March 2001, as found previously in the Black Sea [2]. Predation impacts of M. leidyi on zooplanktic prey organisms have been previously demonstrated in its native waters, the western Atlantic [3], and in introduced regions [4,5]. Furthermore, a recent study based on feeding experiments in the Caspian Sea suggested that the predation pressure of M. leidyi alone would be sufficient to suppress available stocks of zooplankton within a short period (1 day in summer and 3-8 days during winter/spring) [6] and thus would allow phytoplankton biomass to increase.In the late 1990s, M. leidyi was transported into the Caspian Sea [5], possibly in ballast water [5] and spread throughout the Caspian Sea within a few years [1,5,7,10]. Overfishing, eutrophication, and climatic changes (such as global warming) have been suggested as triggering factors of the blooms of jellyfish in both native and introduced waters [11][12][13][14]. Native predators (e.g., goby species [15]) of M. leidyi in the Caspian Sea did not appear to be as efficient as B. ovata, which feeds almost exclusively on M. leidyi [16], in the Black Sea, in consuming M. leidyi biomass [17].At the end of the 1991-2000 period, in which relatively good recruitment and high spawning-stock biomass of anchovy kilka were recorded, fishing mortality (1.8 y Despite the substantial decreases in zooplanktivorous fish and still available phytoplankton biomass (inferred from Chl a levels), sharp declines in the zooplan...

No abstract

No abstract