Ecological modeling on nutrient distribution and phytoplankton diversity in the southern of the Caspian Sea
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Date
2008
Authors
Sarave, Hasan Nasrollahzadeh
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Abstract
This study which was carried out along the Iranian coast of the CS
covered two years of monitoring (Phase I and Phase II) during which time the
southern Caspian Sea faced the introduction of an alien species (Mnemiopsis
leidyt) during Phase II. In addition to the two main sampling exercises,
additional sample collections were carried out during 16 smaller scale sampling
at shallower sites (from1994 to 2005) as a Parallel Study. Results showed that
horizontal distribution of nutrient concentrations (with the exception of dissolved
silicate and ammonium) as well as the number of phytoplankton species and
abundance were significantly higher after the introduction of the ctenophore
(p<0.05) which may also have been caused by vertical mixing. The results on
vertical distribution showed that dissolved inorganic nitrogen/dissolved inorganic
phosphorus (DIN/DIP) ratios in the southern Caspian Sea vary within a very
narrow range (4.47 to 5.78) in the euphotic and aphotic layers and is by one
order of magnitude lower than what have been reported for several other
marine ecosystems. Phytoplankton growth seems to be nitrogen limited while
the levels of P and Si always remain high. The present study also attempted to
test the applicability of the trophic index (TRIX) for assessing trophic status from
data collected between 1994 and 2005 along the Iranian coast of the Caspian
Sea. The results showed that nutrient concentration (DIN, DIP compounds),
oxygen (as absolute %) deviation from saturation (aD%0), chlorophyll a and
also Trophic Index of the Caspian Sea (TRIXCS) increase significantly after the
introduction of the alien species (p<0.05). During Phase I and the Parallel
Study, the phytoplankton community was dominated (based on important
species index) by Tha/assionema nitzschioides, Ske/etonema costatum
(Bacillariophyta) 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 the pre-invasion period and high
trophic level for the post-invasion period. In this study, on the basis of canonical
discriminant function analysis (CDFA) involving physico-chemical parameters
and phytoplankton abundance at the euphotic layers, the southern Caspian Sea
can be discriminated into four groups, which is related the four seasons of the
year. The results also showed that the most significant variables which play the
principal role in the classification comprised physical, biological and nutrient
variables. Results of factor analysis (FA) indicate that the variables within each
factor (VFs) can vary in seasons. However, an overall analysis showed that the
first factor will involve some form of nitrogen compound, suggesting the
importance of this compound for this area during the phytoplankton proliferation
(spring and autumn). During Phase I and II of the study, canonical
correspondence analysis (CCA) suggested that the dominant groups of
phytoplankton have substantial tolerance to different environmental variables.
Description
Keywords
Nutrient distribution , Phytoplankton diversity , Caspian sea