The present study attempted to test the applicability of the trophic index (TRIX) for assessing trophic
status along the Iranian coast of the Caspian Sea (CS). In order to increase the sensitivity of the TRIX for
this area, we defined the range (lower and upper limits) from data collected between 1994 and 2005
which have been used as a reference. Several biological and chemical water quality parameters were
determined and compared with the TRIX in order to describe the water quality status of the area.
Comparisons were also made on two temporarily and spatially varied trophic status at the study site.
Sampling was carried out at 36 stations during Phase I (1996–1997: before the introduction of an alien
species Mnemiopsis leidyi, as a background data) while 24 stations were sampled during Phase II in 2005
(after the introduction of the alien species). A Parallel Study (as supplementary data) from 16 smaller
scale sampling at shallower sites was also included in the discussion (1994–2005 on 18 transects). The
results show that nutrient concentration (DIN, DIP compounds), oxygen (as absolute %) deviation from
saturation (aD%O), chlorophyll a and also the Caspian Sea Trophic Index (TRIXCS) increase significantly
after the introduction of an alien species (po0.01). During Phase I and the Parallel Study, the
phytoplankton community was dominated (based on important species index) by Thalassionema
nitzschioides, Skeletonema costatum (Chrysophyta) year round but during Phase II, Spirulina laxissma
(Cyanophyta) dominated annually and in autumn, coinciding with the minimum Shannon–Weaver
diversity and Evenness indices recorded. Several trophic status indices and indicators were applied and
an overall analysis suggested that the area has low trophic level during Phase I and high trophic level
during Phase II. During the Parallel Study, low trophic level was recorded during the pre-invasion period
and high trophic level for the post-invasion period.