Fluctuations of Phytoplankton Community in the Coastal Waters of Caspian Sea in 2006

Problem statement: The Caspian Sea ecosystem has been suffered with m any problems since 1980s. Aanthropogenic pollution from heavy me tals, hydrocarbons, pesticides, changes in the quantity of nutrient inputs by rivers, are signific ant threats to biodiversity and biological resource s such as plankton structure in the Caspian Sea. Acco rding to the significant of phytoplankton community in marine system. The state of the fluctu ations of phytoplankton communities of the southwestern Caspian Sea was investigated and compa red with the findings of before 2006. Approach: Phytoplankton abundance and species composition of the Caspian Sea were evaluated by using samples collected at 12 stations along three transects. Samplings were conducted seasonal in 2006 at 5, 10, 20 and 50 m depth were fixed for eac h transect in the southwestern Caspian Sea. Results: A total of 39 species phytoplankton species were d istinguished during 2006, the annual phytoplankton abundance were calculated as 57, 300± 15,550 cells.l , which ranged from 89, 250±35, 062 cells.l −1 in September to 16, 200±6, 664 cells.l −1 in February. The diatoms formed more than half of the total abundance (61%) while cyanophytes were the second important group in view of contribution to total phytoplankton (26%) in 2006. The study showed that diatoms Thalassionema nitzschioides, Cyclotella meneghiniana and cyanophyte Osillatoria sp. numerically dominated in this area. Conclusion: The study revealed that diatoms were higher than o ther groups of phytoplankton in 2006. The hydrology variation, increased fresh wate r inflow via rivers and a rise in nutrients concentrations have played important roles in bloom ing of phytoplankton species, e.g., the diatoms in this study, which is also known from other marines. Similar studies on determination of the effects of environmental degradation on phytoplankton and hydr ological processes should be taken into account in near future.


INRTODUCTION
The Caspian Sea is the largest inland water body on earth; it is located at the far end of southeastern Europe, bordering Asia (Kosarev and Yablonskaya, 1994). Approximately, 130 rivers with various sizes drain into the Caspian Sea with an average annual input of about 300 km 3 . The most important river is Volga and provides about 80.0% of the total fresh water input (Dumont, 1998). The southwestren of the Caspian Sea receives 80 rivers; (the Sefidrood is the largest river with a 67,000 Km 2 catchment area and discharge of 4,037 million m 3 ; Lahijani et al., 2008). The Anzali wetland is the other freshwater source, which with a catchment area of 3,740 km 2 , contributes about 2 million m 3 of fresh water per year, this wetland has a passage to the Sea with the width of 426 m and 11 tributary rivers flow into the Anzali wetland (Sharifi, 2006). The salinity of the Caspian Sea ranges from 0.10-13.50 ppt from north to south. There is also a slight increase in salinity with depth (Kosarev and Yablonskaya, 1994). In the northern Caspian Sea, inorganic phosphate levels are on average 0.12-0.80 µM. Nitrogen is largely present in organic form (10.0-250.0µg.l −1 ). Nitrate reaches up to 0.50µM in spring and summer and 10.0µM in winter. Silica shows a strong seasonal cycle and decreases from 60.0µM in winter to <20.0 µM in summer, when diatoms bloom (Dumont, 1998).
In an early study of phytoplankton in the Caspian, the total number of phytoplankton species found from 1962-1974 was 449 (Kosarev and Yablonskaya, 1994). These species consisted of 163 diatoms, 139 chlorophytes, 102 cyanophytes, 39 dinoflagellates, 5 euglenophytes and 1 chrysophyte. In addition, the species number was found to decrease from the north (414 species) to the middle (225 species) and the southern area (71 species) mainly due to the disappearance of fresh water forms towards the south (Dumont, 1998). Recently, Kideys et al. (2005; reported there was a significant increase in phytoplankton abundance and noxious visibility in the Caspian Sea. Moreover, Nasrollahzadeh et al. (2008a) and Bagheri et al. (2010) observed an increase in phytoplankton abundance in 2001-2002 and 2005 as compared to previous years. According to Khodaparast (2006) and Makaremi et al. (2007) cyanophytes Nodularia spumigena and dinoflagellates Heterocapsa sp. produced two anomalous algal blooms for the first time in the southwestern Caspian in September 2005 and October 2006. The increased nutrient load into the southwestern Caspian Sea caused an increase in primary productivity which was reflected by high chlorophyll a levels (2.71-35.25µg.dm −3 ) in 2006 and the levels were 0.56-1.34µg.dm −3 in 1994 (Khodaparast, 2006;CEP, 2006;Jamshidi et al., 2009). In the Caspian Sea, the fauna that have developed there are largely endemic and are therefore particularly susceptible to external influences (Dumont, 1998). There are also major anthropogenic impacts on the system originated from domestic pollutants (e.g., phosphorous-containing detergents), industrial pollutants (e.g., heavy metals and other industrial byproducts) and agricultural pollutants (e.g., nitrogen-containing fertilizers and pesticides). Furthermore, development of oil and gas fields creates stress on the ecosystem and its biological producers especially fish species (Salmanov, 1999;Aladin and Plotnikov, 2003). Stone (2002) described most of the acute problems in the Caspian Sea: reduction of the river run-off; the unstable water level; the various sources of pollution. Therefore, varied hydrological regimes and nutrient levels input by the Anzali wetland and the Sefidrood river can impact the phytoplankton structure in the coasts of southwestern Caspian Sea.
A few phytoplankton studies have been conducted on the south Caspian Sea in recent years (Nasrollahzadeh et al., 2008a(Nasrollahzadeh et al., , 2008b(Nasrollahzadeh et al., , 2008cRoohi et al., 2010;Ganjian et al., 2010). They documented the annual and seasonal fluctuation of phytoplankton communities and nutrient concentrations in the southern Caspian from 1996-2005. The authors concluded that the comb jellyfish, ctenophores have played an important role in increasing in nutrients levels and phytoplankton populations in the Caspian after 2000, at present there is only a survey on the phytoplankton community in the southwestern Caspian Sea during 2001-2002 by Bagheri et al. (2010) and a few reports and local publications for this region. In order to investigate the situation that has developed since 2005, a new survey was undertaken in 2006. In this survey, the state of the composition of phytoplankton communities of the southwestern Caspian Sea was investigated and compared with the findings of before 2006.

MATERIALS AND METHODS
Phytoplankton abundance and species composition of the Caspian Sea were evaluated by using samples collected at 12 stations along three transects (Lisar, Anzali and Sefidrood) in the western Iranian coasts of the Caspian Sea. Samplings were conducted in 2006 (February, September, October, December), four stations located at 5 m (L1, A1, and S1), 10 m (L2, A2, and S2), 20 m (L3, A3 and S3) and 50 m (L4, A4, and S4) were fixed for each transect in the southwestern Caspian Sea (Fig. 1). Water samples were collected by using of Nansen water sampler 1.71 liter (Hydro-Bios, Germany, TPN; Transparent Plastic Nansen water sampler, No: 436201), water temperature level of the seawater at 5, 10, 20 and 50 m was measured in situ by using a reverse thermometer (Hydro-Bios, TPN) and salinity was estimated salinometer (Beckman; RS-7B, U.S. Patent, No: 2542057). Water transparency was determined with a Secchi disk depth. Water samples were deep frozen for analyses of inorganic nutrients. Dissolved Inorganic Phosphorus (DIP = P-PO 4 ), Dissolved Inorganic Nitrogen (DIN = N-NO 2 , N-NO 3 , N-NH 4 ) and dissolved Silicate (DSi = Si-Sio 2 ) were determined with a spectrophotometer system using standard methods (Clesceri et al., 2005).
Phytoplankton samples were collected from different depths with a Nansen water sampler. The samples were kept in 500 mL bottles and preserved using buffered formaldehyde 4%. The samples were let to settle for at least 10 days following which the water was siphoned off from the top layer to a volume of approximately 250 mL. The samples were then centrifuged (ALC-PK131R; Germany, No: 30206372) for 5 min with 3000 rpm and further siphoning off to a volume of 30ml, phytoplankton present in a subsample of 0.1ml was counted using a Sedgewick-Rafter cell under a binocular microscope (cover slip 24×24 mm and with magnifications of 10×, 20× and 40×) (Prescott, 1962;Newell and Newell, 1977;Sournia, 1978;Clesceri et al., 2005). Phytoplankton taxonomic classification was performed based on Tiffany and Britton (1971). Statistical comparisons between months were made using statistical software SPSS version 13 for Windows. Analysis of variance comparisons (One-way ANOVA) for water parameters and nonparametric test (Kruskal-Wallis) for phytoplankton number were used to identify the importance of variables between different months. Spearman rank correlation coefficients (r) were used to evaluate the relationships between phytoplankton abundance and environment parameters.

RESULTS
No difference was noted in the spatial distribution of phytoplankton (non-parametric test; Kruskal-Wallis) and water parameters (one-way analyses of variance; ANOVA) between the three transects of Lisar, Anzali, and Sefidrood. Therefore, the data of the three transects were combined per months.  Table. 1. The surface temperature ranged between 8.83 and 25.74°C due to monthly variations in weather temperature throughout year. Monthly temperature variations were significant (ANOVA, p<0.01). The drastic thermocline was observed in September and October which formed in nearly 30 m depth. The average of water temperature varied between 25.7, 9.7°C, respectively in above and below of thermocline layers. In February and December, the temperature of the water column was nearly uniform, varying between 10-15.8°C at the surface and 8-14.9°C at the bottom, respectively (Fig. 2). The variation of surface salinity was recorded as 9.32 and 12.25 ppt in the southwestern Caspian Sea (Table 1). Based on the ANOVA findings, the salinity differences was not meaningful monthly (p>0.05) although a decreasing trend in the average salinity values was observed during February (Table 1). The gradient of salinity from surface to bottom is presented in Fig. 2 in which the salinity increase from 10.43 to 12.85 ppt, between surface and deep water (50 m depth) in the southwestern Caspian Sea. Temporal variations of Secchi disk depths in the period of February and December is presented in Table. 1. The Secchi disk depth, an indicator of water turbidity, the average Secchi disk depth was measured 5.0 m and changed between 3.8 and 6.4 m, respectively in September and February during the study period. Statistical variance analysis (ANOVA) showed that Secchi disk depths were not significantly different between the months (p>0.05). Furthermore, the occurrence of Secchi disk depths was negatively correlated with phytoplankton abundance in this study (r = -0.870, p<0.05).

Quantitative phytoplankton composition:
The contributions of different phytoplankton groups to the total phytoplankton abundance during different month of 2006 are figured in Fig. 3 and 4. In this study, average number of phytoplankton were 57,300±15,550 cells.l −1 . Among the phytoplankton groups, diatoms formed almost half of the total abundance (61%). Cyanophytes were the second important group contributing to total phytoplankton (26.0%). Dinoflagellates (12.0%), chlorophytes and euglenoids (1.0 and 0.0%, respectively) were other contributors (Fig. 3).

DISCUSSION
Hydrophysical characteristics: Many researchers (Dumont, 1998;Kideys and Moghim, 2003;Bagheri and Kideys, 2003;Roohi et al., 2008;Nasrollahzadeh et al., 2008a;Bagheri et al., 2010) reported that temperature variations in surface water in the southern Caspian Sea varied between 7.00°C (in winter) and 29.0°C (in summer). However, during this study, temperature variation in surface water varied between 8.83°C (in Febuary) and 25.74°C (in September) during 2006 (Table 1). The annual surface water temperature was lower than in 2006 (annual average: 18.43°C; Table 1) as compared to -2002(Nasrollahzadeh et al., 2008aBagheri et al., 2010;annual average: 19.83-21.14°C). In this study thermocline were observed in September and October which occured in nearly 30 m depth and the stratification started to break up in December and February (Fig. 2). Kideys and Moghim (2003) and Zaker et al. (2007) noted, a strong thermocline located between 30m and 50m depths in the beginning of autumn with 15 o C temperature, the thickness of the thermocline was located between 30 m and 45 m depths in the southern Caspian Sea, as was observed in our study, they reported the depth of the mixed layer was not the same as in the Caspian. In additions, we believed the stratification of water layers could be related to meteorological monthly fluctuations in the Caspian Sea.
In 2006, the average annual surface salinity was low (11.45 ppt; Table 1) as compared to -2002(Nasrollahzadeh et al., 2008bBagheri et al., 2010;annual average: 12.54-12.20 ppt). The salinity varied between 9.32 ppt and 12.25 ppt, there was a decreasing trend in the average salinity values of the surface water in February 2006 (Table 1), also salinity was increased from surface water to deep layer (50 m depth; Fig. 2). These trends could be related to fresh water inputs from the Anzali wetland and the Sefidrood river in this month. According to Bagheri et al. (2010) and GWRO (2010), there was a strongly negative correlated between salinity and freshwater discharge in the southwestern Caspian Sea during 2001-2002. In additions, our findings were similar to the previous findings reported by ;Dumont (1998); Kideys and Moghim (2003); Bagheri and Kideys (2003); Zaker et al. (2007);Nasrollahzadeh et al. (2008aNasrollahzadeh et al. ( , 2008b in the Caspian. In this study, average Secchi disk depth was recorded 5.0m in 2006 in the southwestern Caspian Sea ( Table 1). Increase of anthropogenic inputs from Lisar and Sefidrood rivers and Anzali wetlands, has caused an accumulation of suspended inorganic and organic materials in the southwestern Caspian Sea. Nasrollahzade et al. (2008b) reported that the mean values of the Secchi depth were 6.65 m in 1996-97. The reduced Secchi disk depth in 2006 could be related to the increase of phytoplankton occurring during our study (57,300 cells.l −1 ; Fig. 3) as compared to 1996-1997(Nasrollahzadeh et al., 2008bannual average: 13,000 cells.l −1 ) and the beginning of eutrophication in coastal of marine ecosystem (Yunev et al., 2005). Unfortunately, we could not talk strongly regarding this relationship, as there are no phytoplankton biomass data during 2006.    Nasrollahzadeh et al. (2008a) reported, the process of eutrophication is accompanied by a shift in the existing qualitative and quantitative relationship between the major phytoplankton groups. According to    Fig. 3 and 4).
Diatoms require silicate for their shells in addition to these nutrients and about 90% of the silicate input to the global marine is estimated to come from rivers (Sommer, 1994;Eker and Kideys, 2003;Humborg et al., 2004). In addition, Bagheri et al. (2010) documented, only 20.0% of the phytoplankton abundance were made up of diatoms Thalassionema nitzschioides and Cyclotella meneghiniana in the southwestern Caspian in 2001. In our study, increase diatoms T. nitzschioides and C. meneghiniana abundance (61.0%) in 2006 could be related to increased freshwater inflow (the Sefidrood river discharge was estimated 33 and 42 million m 3 .year −1 , respectively during -2002GWRO, 2010) and silicate levels (7.85 µM.dm −3 ; Table 1) as compare to -2002Bagheri et al., 2010). Nasrollahzadeh et al. (2008b) reported with decreasing DSi:DIP ratio from 25 to 11, respectively in 1996-1997 and 2005 the abundance of cyanophytes increased from 4 to 25% in the south of Caspian Sea. In addition, Khodaparast (2006) noted in the Caspian Sea, cyanophytes bloom was observed during periods of decline in nutrients ratios. In our study, the cyanophytes number were increased in September-October (Fig. 4), It could be related to decreased of nutrients ratios in these months as compared to February and December in 2006 (Table 1). Some studies linked drastic changes in the phytoplankton community with comb jellyfish invasion in the Caspian Sea after the year 2000 (Nasrollahzadeh et al., 2008a;Ganjian et al., 2010;Roohi et al., 2010). We could not estimate the impact of comb jellyfish on the fluctuation of phytoplankton and dominant taxa such as, diatoms and cyanophytes in 2006. According to Bagheri et al. (2010), the abundance of phytoplankton was not correlated with the number of comb jellyfish in the southwestern Caspian during 2001-2002. Accordingly, it was not possible to determine to what extent the fluctuation of the phytoplankton is due to the impact of comb jellyfish. Recent observations in other seas indicated that the changing phytoplankton community can be related to climatic variability (Polonsky et al., 2004;Bilio and Niermann, 2004). Furthermore, the fluctuations of the phytoplankton community's relationship to environmental parameters (pollutions) and nutrient upwelling were not extensively investigated up to now Bagheri et al., 2010). Since the southern Caspian Sea is influenced to a high extent by fresh water inflow with a heavy load of artificial nutrients (Dumont, 1995;Salmanov, 1999;Yunev et al., 2005;Sharifi, 2006;CEP, 2006;Stolberg et al., 2006), it is important to assess to which extent the increased eutrophication affects the phytoplankton abundance and species composition in the coasts of Caspian Sea.

CONCLUSION
Our survey documented the temporal distribution of the phytoplankton in the southwestern Caspian Sea in 2006. The study showed that diatoms such as Thalassionema nitzschioides and Cyclotella meneghiniana and cyanophyte Oscillatoria sp. numerically dominated in the southwestern Caspian Sea. We believe that hydrology variation, increased fresh water inflow via rivers, a rise in nutrient concentrations have played important roles in blooming of phytoplankton species in the Caspian Sea, which is also known from other marines. Similar studies on determination of the effects of environmental degradation on phytoplankton communiy must be taken into account in near future.