Aim Invasive species are of increasing global concern. Nevertheless, the mechanisms driving further distribution after the initial establishment of non‐native species remain largely unresolved, especially in marine systems. Ocean currents can be a major driver governing range occupancy, but this has not been accounted for in most invasion ecology studies so far. We investigate how well initial establishment areas are interconnected to later occupancy regions to test for the potential role of ocean currents driving secondary spread dynamics in order to infer invasion corridors and the source–sink dynamics of a non‐native holoplanktonic biological probe species on a continental scale. Location Western Eurasia. Time period 1980s–2016. Major taxa studied ‘Comb jelly’ Mnemiopsis leidyi. Methods Based on 12,400 geo‐referenced occurrence data, we reconstruct the invasion history of M. leidyi in western Eurasia. We model ocean currents and calculate their stability to match the temporal and spatial spread dynamics with large‐scale connectivity patterns via ocean currents. Additionally, genetic markers are used to test the predicted connectivity between subpopulations. Results Ocean currents can explain secondary spread dynamics, matching observed range expansions and the timing of first occurrence of our holoplanktonic non‐native biological probe species, leading to invasion corridors in western Eurasia. In northern Europe, regional extinctions after cold winters were followed by rapid recolonizations at a speed of up to 2,000 km per season. Source areas hosting year‐round populations in highly interconnected regions can re‐seed genotypes over large distances after local extinctions. Main conclusions Although the release of ballast water from container ships may contribute to the dispersal of non‐native species, our results highlight the importance of ocean currents driving secondary spread dynamics. Highly interconnected areas hosting invasive species are crucial for secondary spread dynamics on a continental scale. Invasion risk assessments should consider large‐scale connectivity patterns and the potential source regions of non‐native marine species.
The impact of the invasive ctenophore Mnemiopsis leidyi on the zooplankton community of the Caspian Sea was quantified according to food consumption and other major physiological activities (i.e. respiration and reproduction), coupled with field data on population structure. The adverse effects of M. leidyi on the zooplankton community during the first years of the invasion were tremendous for the Caspian Sea compared to other regions affected by this ctenophore. The impact was highest in summer, due to high water temperatures and a population size structure in which juvenile ctenophores with mean lengths of 2 to 5 mm accounted for most of the population. During winter/spring, these ctenophores could consume the available stock of zooplankton in 3 to 8 d, whereas in summer consumption took only 1 d. The computed critical ctenophore biomass that does not affect (decrease) the abundance of mesozooplankton in the Caspian Sea is about 4 g m-3 (or 120 g m-2 , assuming most of the ctenophores occur in the upper 30 m layer). As is clear from the monitoring data, the M. leidyi biomass in summer in different regions of the Caspian Sea is far in excess of this value. Such a high abundance of ctenophores, if maintained, would constantly keep the nongelatinous zooplankton biomass at very low levels, and, as a consequence, no recovery could be expected in the pelagic fishery.
This study focuses on spatial and temporal distribution and species composition of phytoplankton in the south-western Caspian Sea between July 2009 and March 2010. Samples were collected from 11 stations along three transects: Lisar, Anzali and Sefidrood. Among 44 identified phytoplankton species, diatoms (70.2%) and cyanophytes (25.0%) were dominant. The average phytoplankton abundance was calculated as 1.085E + 05 cells l−1. Among the phytoplankton groups, diatom density was higher (70.2%) than two out of three of total abundance in density of 1.085E + 05 cells l−1. Cyanophytes were the second important group (25.0%) contributing to total phytoplankton. Dinoflagellates, chlorophytes and euglenoids were other contributors to total phytoplankton. Diatoms Dactyliosolen fragilissimus and Skeletonema costatum and cyanophyte Oscillatoria sp. numerically dominated in the system. There were major changes in phytoplankton composition and average phytoplankton density was higher than those documented in 1996–1997 and 2005. The average concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate were 14.5 ± 6.32, 1.14 ± 0.44 and 5.10 ± 3.98 μM, respectively, and these concentrations were strikingly high. Increases in DIN and DIP concentrations were more than twofold compared to recorded values during the last 2 decades due to the eutrophication. Fluctuations in nutrients played an important role in the variation of phytoplankton composition and abundance. Chlorophyll-a concentrations varied between 3.22 and 16.1 μg l−1 and there was a significant increase in chlorophyll-a (8.13 ± 5.72 μg l−1) compared to the values in 1996–1997 (1.44 ± 1.48 μg l−1), 2001 (2.62 ± 1.48 μg l−1) and 2005 (2.14 ±1.94 μg l−1).
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