Spatial and temporal distribution of macronutrients and phytoplankton before and after the invasion of the ctenophore,<i>Mnemiopsis leidyi</i>, in the Southern Caspian Sea

Abstract

The spatio-temporal distribution of nutrients and phytoplankton along the Iranian coast of the Southern Caspian Sea (CS) was investigated by comparing two different periods, namely Phase I (1996–1997), representing the period before the introduction of the ctenophore, Mnemiopsis leidyi, into the system, and Phase II (2005), the period after the introduction. The results showed that nutrient concentrations (with the exception of dissolved silicate) in the subsurface water were significantly higher during Phase II (p<0.001), which may be attributed to vertical mixing and the presence of the ctenophores. Long-term data collected from 1994 to 2005 also confirmed these results. As with most other marine ecosystems, the Bacillariophyta (diatoms) was found to be the most dominant phytoplankton group. However, during Phase II (i.e. after the introduction of the ctenophores), a significant increase in the abundance of the Cyanophyta was recorded, especially during summer and autumn. The average abundance of phytoplankton after the introduction of the ctenophores was significantly higher (4.2 fold, p<0.05). Similar trends have been observed in the Narragansett Bay and in the Black, Azov and North Caspian Seas, all of which were related to the predation on zooplankton (the primary consumer of the phytoplankton) by the ctenophores.