| Marine phytoplankton is the primary producer in ocean and plays an important role in energy flow and nutrient cycling of marine ecosystems. Dust deposition into ocean could provide the essential nutrient elements (such as N, P and Si) and trace elements (such as Fe) for growth of marine phytoplankton, enhancing photosynthesis and promoting marine primary productivity. However, effects of Dust deposition on marine ecosystems were reported to be highly variable in various oceans. The Yellow Sea is characterized by high concentrations of nutrients and chlorophyll-a while the reverse is true in the northwest Pacific Ocean. Thus, a comparative analysis between the two regions would provide a new insight on relationship between Dust deposition and marine ecosystems. In this study, two incubation experiments were did in the Yellow Sea (36°1′40″N, 124°2′27″E) during the period from 26 March to 15 April, 2009 and in the NW Pacific ocean (29°59′80″N,136°59′23″E) during the period from22 May to 3 June, 2010, respectively. The influence of Asian Dust input on the growth of Marine Phytoplankton in the Yellow Sea and NW Pacific Ocean was investigated in terms of promotion of chlorophyll-a and change of phytoplankton community.The in-situ incubation experiment results in the Yellow Sea showed that the concentration of chlorophyll-a in the control group and the dust-added group (the added dust was collected from Qingdao in 28 May 2008, named A) started to markedly increase after 4 days, and reached the maximum on the 7th day. The concentrations of chlorophyll-a in the A1 group (the dust concentration added in the experimental bottle was 150mg/L) and A2 group (the dust concentration added in the experimental bottle was 300mg/L) were about 1.3 times of the control group on the 7th day. The concentration of chlorophyll-a in the control group and the A1 group began to decrease after 8 days. But the concentration of chlorophyll-a in A2 group kept at a high level between 7-12th days, and decreased after 13th days. The results showed that the dust added could promote the growth of phytoplankton in the Yellow Sea, and the more dust added could maintain primary productivity at the high level for a longer time. In the sea-water collected for the experiment, the dominant species of phytoplankton include Cryptomonads, skelotonema costatum (diatoms) and Dinoflagellate. The relative abundance were 36.6%, 29.5% and 14.1%, respectively, however, after 9 days incubation, diatoms was the only dominant species of phytoplankton in the dust-added group, mainly including skelotonema costatum, Chaetoceros spp and Thalassiosira spp. In the A1 group, the cell density of the three dominant species was respectively 66 times, 488 times and 60 times of that in the control group. The relative abundances were 56.5%, 19.6% and 22.5%, respectively. In the A2 group, the cell density of three dominant species was 251 times, 3060 times and 350 times of that in the control group, and the relative abundances were 45.4%, 25.8% and 27.6%. Compared to the control group, there were a significant increase in the cell density of skelotonema costatum,Chaetoceros spp and Thalassiosira spp, while cell density of other species decreased or changed a little.The in-situ incubation experiment results of the NW Pacific Ocean showed that the concentration of chlorophyll-a in the control group changed a little, less than 0.10μg/L, in the whole incubation process. But the chlorophyll-a concentration in the dust-added group (the added dusts were collected from Qingdao in 28 May 2008 and 20 March 2010, respectively, which were named A and B) initially increased after 2 days, and reached the maximum on the 3th or 4th day, then decreased. The maximum concentration of chlorophyll-a in the A1 group (the dust concentration added in the experimental bottle was 150mg/L) was 81 times of that in the control group. The maximum concentration of chlorophyll-a in the B1 group (the dust concentration added in the experimental bottle was 150mg/L) and B2 group (the dust concentration added the experimental bottle was 300mg/L) were 149 times and 292 times of that in the control group. In the nitrate and urea added groups, the growth rate of phytoplankton was much lower than that in the dust added group even if the initial concentration of nitrogen was 10 times than that in the dust-added group. After 5 days incubation, the chlorophyll-a concentration in the nitrogen added groups initially increase. The maximum occurred on the 9th day, which was about 10 times of that in the control group. In the group with both dust and nitrate being added, the total yield of the chlorophyll-a was 3 times higher than that the sum of the total yield in the dust added group and in the nitrate added group. In the group with both dust and urea being added, the total yield of the chlorophyll-a was 1.6 times higher than that the sum of the total yield in the dust added group and in the urea added group. The results showed that the dust added could significantly promote the growth of phytoplankton. However, when the chemical composition of dust changed, the effect in promoting the growth of phytoplankton was different. The nitrate and urea added to the sea-water collected in the NW Pacific could also promote the growth of phytoplankton. But the growth rates of phytoplankton were much lower than that in the corresponding dust-added group. Thus, we inferred that nitrogen is a control factor of the growth of phytoplankton in the NW Pacific while other nutrient elements provided by the added dust are also control factors. The synergy of dust and nitrogen could better promote the growth of marine primary productivity in the NW Pacific. In addition, the structure of phytoplankton community greatly changed during the incubation experiment regardless of the control group and other groups. For example, the dominant species of phytoplankton in the collected sea-water included Nitzschia spp (diatoms), Chaetoceros spp (diatoms), Dinoflagellate (dinoflagellate) and Cryptomonas. On the 4th day, the maximum concentration of chlorophyll-a arrived. The dominant species of phytoplankton in the control group were Nitzschia spp (diatoms), Chaetoceros spp (diatoms) and Lepeocylindrus danicus (diatoms). The relative abundance were 55.6%,31.1% and 13.3%, respectively. However, the dominant specie of phytoplankton in the A1 group was only Nitzschia spp, its relative abundance was 99.8%, and its cell density was 242 times of that in the control group. The dominant specie of phytoplankton in the B1 group was only Chaetoceros sp, its relative abundance was 96.6%, and its cell density was 900 times of that in the control group. When the experimental results between the Yellow Sea and the northwest Pacific Ocean were compared to each other, the same amount of dust added to the sea-water collected in the NW Pacific increased more chlorophyll-a in concentration. In addition, the Asian dust added could affect the community structure of phytoplankton in the both of Yellow Sea and NW Pacific Ocean and chemical composition of dust added should play an important role as well as other parameters. |
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