The Effect Of Solar Radiation On HAB Algae Growth In High Frequency HAB Occurrence Areas In ECS

In recent years, with the extending factitious disturbance, the water quality along China coastal area have become worse and worse and harmful algal blooms (HAB) broke out frequently. Especially over the last three decades, HAB tripled every ten years. The area nearby the Changjiang (Yangtze) River Estuary, Hangzhou Bay and the Zhoushan fishery area has become one of the areas suffered frequently from serious HAB damages and was named high frequency HAB occurrence areas in East China Sea (ECS). The HAB breaking out period in this area is from April to September and the HAB species in this area show a clear seasonal feature: the dominating algae in blooms from spring to early summer are mainly dinoflagellates such as Prorocentrum, Gymnodinium and Noctiluca scintillans, while from middle summer to autumn are diatoms such as Skeletonema costatum and Pseudo-nitzschia pungens. The seasonal feature of HAB species may related not only to nutrient conditions such as nitrate, phosphate, silicate concentrations, N/P ratio, eutrophication, but also to climate conditions such as temperature, precipitation, wind and especially solar insolation. In fact, in the ECS, the HABs mainly caused by the N. scintillans and Prorocentrum donghaiense Lu were observed in cloudy days in spring, but S. costatum blooms in sunny days in summer and autumn. Till now the research of the effect of solar radiation on phytoplankton was deficient, and such report in this area was little. Based on former research, through field survey combining in-site light-dependent batch culture, this thesis discusses the effect of solar radiation on the HAB algae growth and phytoplankton succession in this area, in order to lay a solid theoretical foundation for future research on HAB outbreak mechanism in East China Sea. The main research work and conclusions list as follows:1. In the field survey from spring 2002 to spring 2003, primary discussion of the effect from temperature, macronutrients and insolation on the changes of Chlorophyll-a (Chla) was conducted in high frequency HAB occurrence areas in ECS.1.1 Except the temperature in winter in this area was as low as 10.6℃ and may limit the growth of the algae, the temperature in summer was as high as 27.6℃ and in springand autumn were nearby 18°C, which could meet the algae' growth.1.2 The eutrophication condition in this area was serious. The macronutrient concentrations were higher in autumn and winter, and were lower in spring and summer. At the same time, inshore concentrations were higher than offshore ones. Furthermore in the area between 20-50m isobaths with prolific phytoplankton, the macronutrients were abundant.1.3 Solar radiation of different seasons in this area was measured and the insolation in water was calculated for the first time. Influenced by weather, the highest solar insolation occurred in summer 2002 and spring 2003 (>20 MJ-m^-d'1) and the lowest one in spring 2002 (<10 MJ-m^-d"1), while the solar insolation in autumn 2002 and winter 2003 was intermediate (about 15 MJ-m"2-d"'). The total suspended particles (TSP) in water were lower in rainy seasons of spring and summer 2002, and were higher in dry seasons of autumn and winter. However, the TSP in spring 2003 was also high which might be caused by the change of the survey region and the outbreak of algae bloom. Insolation in water decreases by exponential rule with depth. From spring 2002, insolation in water first rose to the strongest point in summer, then dropped to the lowest point in winter, and began to rise again in spring 2003.1.4 Chla concentration in sea water rose from spring 2002 and reached to a higher point in summer, then sharply fell in autumn and winter, and rose to the highest point in spring 2003. On the other hand, Chla of surface layer had the highest concentration in the area between 20-50m isobaths.1.5 From the survey data and sections data, the influence of temperature, macronutrients and insolation in water to Chla was analysed. With suitable temperature condition, in high frequency HAB occurrence areas in ECS, especially in the middle part of survey area, insolation and PO4-P were two of the important factors that could influence the seasonal changes of alga.2. In-site light-dependent batch culture experiments were conducted for the first time during four surveys of autumn 2002, winter 2003, spring 2003 and 2004 in high frequency HAB occurrence areas in ECS. The study aimed at finding the influence of solar radiation on harmful algae growth among seasons in this area.2.1 The dominating algae belonged to diatom in autumn 2002 culture and there was no obvious algae succession. Solar radiation was the controlling factor in experiment. The results of light-dependent Bf, umax, vpp and vPN showed that the ambient solar insolation of 12.8MJ-m~ -d" couldn't restrain the growth of 5. costatum and Coscinodiscus sp. when they were the single dominating algae, but suppressed the growth of Thalassiothrix frauenfeldii and Coscinodiscus sp. when they grew together and the saturated insolation of these two algae was T.TMJ-m^-d"1 (60% of daylight).2.2 The dominating algae were Coscinodiscus sp. in winter 2003 culture and there was no obvious algae succession. Solar radiation was also the controlling factor in experiment. The light-dependent Bf, |xmax, vPP and vPN data showed that the ambient solar insolation of 11.5MJ-m"2-d4 restrained the growth of Coscinodiscus sp. and the saturated insolation was about 7MJ-m" -d" . Meanwhile, the algae's growth was also limited by the low temperature, which made the growth rates and the nitrate, phosphate nutrients uptake rates of winter much lower than those of autumn.2.3 Large-scale P. donghaiense Lu bloom broke out during the voyages of spring 2004 and the dominating algae was P. donghaiense Lu in culture. Because of the high preponderance, there was no obvious algae succession in HAB stations, but succession trace could be seen in station outside HAB area. Solar radiation was also the controlling factor in experiment. The light-dependent Bf, u.max, vpp and vPN data demonstrated that the ambient solar insolation of n-lSMJ-m^-d"1 restrained the growth of P. donghaienseLu and the saturated insolation was about 10-llMJ-m^-d"1.2.4 During 2004 spring voyages, large-scale P. donghaiense Lu bloom of a little lower preponderance broke out. During the experiment the dominating algae was P. donghaiense Lu, but obvious algae succession occurred in station outside HAB area. Solar radiation was also the controlling factor in experiment. The light-dependent Bf, p,max, vpp and vPN value displayed that the ambient solar restrained the growth of the phytoplankton community and the saturated insolation was about 4-8MJ-m"2-d'1.3. In spring, HABs in this area were dinoflagellates blooms, while in summer and autumn were diatoms blooms. Furthermore in-site light-dependent culture experiments showed the same regulation: the dominant culture algae were dinoflagellates in spring anddiatoms in autumn and winter. Therefore the phytoplankion in high frequency HAB occurrence areas in ECS was divided into diatoms and dinoflagellates for further analysis.3.1 The densities of diatoms in surface layer in summer and autumn surpassed 104cells-dm"3, in spring and winter were about 103cellsdm"3. From spring 2002, diatoms first rose to the top in summer, then kept a dropping trend.3.2 The variation trend of surface diatoms was consistent with insolation in water; it seemed that insolation influenced the seasonal changing of diatom more than temperature and macronutrients. Diatoms grew better under the ambient insolation than under weaker insolation.3.3 The density of dinoflagellates in surface layer in springs were over 104cells-dm"3, and in summer, autumn and winter was between 10 and 10 cells-dm' . From spring 2002, the dinoflagellates dropped down and reached the rock bottom in winter, then rose again in spring.3.4 None of the change of temperature, macronutrients and insolation could match the change of dinoflagellates in surface layer. The seasonal distribution of dinoflagellates was the synthetical results of temperature, macronutrients, insolation and competition with diatoms. When insolation was weaker and macronutrients level was lower, dinoflagellates outvied diatoms.4. Similarly, the algae in in-site light-dependent culture were divided into diatoms and dinoflagellates to analyze the effect of radiation on algal succession.4.1 In the in-site light-dependent culture experiment, the dominating culture algae in autumn and winter were diatoms, such as S. costatum and Coscinodiscus sp., in spring were dinoflagellates, such as P. donghaiense Lu and Alexandrium tamarense. This accorded with the situation of voyage survey.4.2 The dominating culture algae were important HAB algae in high frequency HAB occurrence areas in ECS. For example, blooms arose by S. costatum of diatom or P. donghaiense Lu of dinoflagellate were the largest and most harmful ones in this area.4.3 S. costatum of diatom preferred to grow under ambient solar radiation and both spring and autumn in-site radiation couldn't restrain its growth; T. frauenfeldii and Coscinodiscus sp. of diatom were suppressed by the ambient radiation of autumn andwinter, the suitable insolation of these two algae was about 7MJ-m"2-d"'. P. donghaiense Lu and A. tamarense of dinoflagellate preferred to grown under weaker solar radiation, the ambient radiation of spring restrained their growth, the suitable insolation was about4.4 There was no visible algae succession in autumn and winter culture. In spring, algae succession didn't occurred in HAB stations for the high preponderance of dominating alga, while obvious algae succession was observed in station outside HAB area. Diatoms in this area contributed more and more in density and growth rate of the phytoplankton community along with the increase of spring radiation, while dinoflagellates just opposite. Accordingly, dominating alga changed from P. donghaiense Lu to S. costatum. These phenomena was consistent with the fact that from spring to summer, HAB in this area changed from dinoflagellate blooms to diatom blooms, which indicated that solar insolation played an important role in this process.To summarize, solar radiation plays an important role in the succession of the dinoflagellate blooms in spring and early summer to diatom blooms in mid-summer and autumn.