Temporal distribution and composition of phytoplankton in the Southern part of Caspian Sea in Iranian waters from 1994 to 2007
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Date
2011
Authors
Khenari, Ali Ganjian
Journal Title
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Abstract
Caspian Sea is the largest inner water body on the earth underwent sharp changes
in its ecosystem during the recent decades of the twenty century. They were caused by
natural eg. transgression of the sea level, climate change, increase of fresh water inflow
and anthropogenic factors eg. pollution, invasion of ctenophore Mnemiopsis leidyi and
illegal fishing for sturgeons at sea. All that led to significant change in hydrological
regime; modify the food base, abundance and biomass of biological resources and bioproductivity
of the sea. Long-term qualitative and quantitative characteristics of
phytoplankton community structure and biomass were investigated in from 1994 to2007.
During the study period, 4556 samples of phytoplankton were collected during 33
cruises in the southern part of the Caspian Sea (SCS). A total of 334 species (88 genera)
of phytoplankton were jdentified in the SCS, which comprised of Bacillariophyta,
Chlorophyta, Cyanophyta, Pyrrophyta, Euglenophyta, Chrysophyta, Xanthophyta and
Cryptophyta. Bacillariophyta and Pyrrophyta were present in all ecological groups. The
phytoplankton ecological groups were fresh water species (34%), fresh - brackish -
water species (18%), brackish-water species (14%), brackish-marine-water species (4%),
marine species (8%) and other species (22%). The dominant groups of phytoplankton
from 1994 to 2007 were Bacillariophyta which constituted the major cell abundance and
biomass (37 % and 52%, respectively). The highest annual mean phytoplankton cell
abundance and biomass were 249.1x106 ± 22.4x106 cells/m3 and 1034.17±117.81
mg/m3
, respectively; and higher population was due to the presence of Pyrrophyta,
Bacillariophyta and Cyanophyta between 2001-2002. The cell abundance and biomass
of phytoplankton population showed significant difference in years 1994-2007
(p<0.001). The maximum cell abundance (9.7 x106 cells I m3
) and biomass (83.8
mg/m3
) were observed in brackish-water and marine water forms, respectively. Longterm
changes of cell abundance and biomass in five ecological groups and dominant
species were observed in different years. In 2001-2002, the dominant phytoplankton
were Exuviella cordata and Rhizosolenia fragilissima; and high density was due to
Bacillariophyta and Pyrrophyta in winter. In 1999-2007, after the invasion of ctenophore
Mnemiopsis leidyi, higher cell abundance was no longer recorded in spring but in winter
and autumn; while higher biomass shifted from summer to winter and spring. High
diversity index of phytoplankton was associated with high level of transparency and
salinity. Phytoplankton population aggregated at the depth of 0 - 20 m with more than
87% of total cell abundance due to some favorable conditions such as higher water
temperature, light penetration, dissolved oxygen and nutrient concentrations. Vertical
distribution of phytopl~nkton cell abundance and bio~ass decreased with increasing
depth. Water quality in SCS was classified as moderately and highly organically
polluted. In the present study, algal genus and species pollution indices (API) were due
to the oscillation in the number of phytoplankton species composition. The
eutrophication index (EI) showed an increasing trend from oligotrophic to eutrophic,
with its lower value was observed for the typical oligotrophic (0.21) in 1994 and the
highest in 2005-06 (1.22). Long term EI ranged from 0.26 to 1.22 in 1994/1996 and
1996/1997 were grouped in the typical oligotrophic (0) and 1999/2000 were grouped as
the standard mesotrophic (M) and 2003/2004, 2004/2005, 2005/2006 and 2006/2007
were grouped with the typical eutrophic (E). Cyanophyta were represented mainly by
Aphanothece elabens, Microcystis spp., Oscillatoria limosa, Spint!ina laxissma and
Anabaena spp. throughout the years. The highest cell abundance and biomass were
observed in summer and autumn. Long term biomonitoring in the SCS is required to
manage its sustainable resources and to prevent environmental degradation.
Description
Keywords
Phytoplankton , Caspian sea