Effects Of Irradiance On Growth Of Phytoplankton In The Changjiang Estuary And Adjacent Coastal Waters

Harmful algal blooms (HAB) has been one of the important environmental problems worry the coastal countries. Aimed at the HAB mechanisms study and in order to expatiate effects of irradiance on HAB occurrences in the Changjiang Estuary and Adjacent Coastal Waters, with field culture experiments and model calculations, effects of irradiance on growth of 2 species of importance harmful algae (Skeletonema Costatum Cleve and Prorocentrum donghaiense Lu) were studied. Firstly, the optimal light intensity (Iopt) of 7 species of harmful algae and the relative temperature effects were measured with field culture experiments. Then, a method analyzing the effects of irradiance on phytoplankton growth was improved on based on the light-growth model of phytoplankton. Using this method, the relationship between irradiance and vertical and horizontal distribution of Skeletonema Costatum Cleve and Prorocentrum donghaiense Lu were analyzed. Finally, the relationship between irradiance and phytoplankton biomass during the last nearly 50 years was discussed by use of long-term series data. The main results are as the following:1."Phytoplankton growth light-effect analysis method"is modified based on phytoplankton growth-light model, which combines phytoplankton growth characteristics and irradiance characteristics under sea water. With this modified method, light effects on horizontal distribution of phytoplankton can be measured using the so-called"phytoplankton-growth light-effect water-column-accumulation index"; and light effects on vertical distribution of phytoplankton can be measured using the index"optimal light-effect depth".2. It is confirmed that irradiance in sea water, without the limiting of nutrition, temperature, and other environmental factors, is the key factor to prevent red tides from extending more to the inshore area in the Changjiang Estuary and Adjacent Coastal Waters.By the phytoplankton growth light-effect analysis method, with growth parameters obtained from field culture experiments, light effects on phytoplankton horizontal distribution were measured. It is indicated that irradiance in sea water, without the limiting of nutrition, temperature, and other environmental factors, is the key factor to prevent red tides from extending more to the inshore area in the Changjiang Estuary and Adjacent Coastal Waters, by comparing the calculated growth light-effect, which is that"phytoplankton-growth light-effect water-column-accumulation index"increases from inshore to offshore, with the vessel investigation results carried out in this sea area. In fact, it is the trade-off of light and nutrient fitness that results in blooms in the so called red tide area.3. Light-optimum characteristics in surface and subsurface water is an important factor for S. Costatum and P. donghaiense to reproduce so rapidly in surface and subsurface water, respectively, as to blooms occurring.By the phytoplankton growth light-effect analysis method, with growth parameters obtained from field culture experiments, light effects on phytoplankton vertical distribution were measured. The calculated results show that the optimal light-effect depth for S. Costatum is about 0.4m in the surface water and 4m or so (in spring and summer) in the subsurface water for P. donghaiense, which is consistent with the vessel investigation results that S. Costatum usually blooms in the surface water and P. donghaiense in the subsurface water. The consistency of the calculated light-effect with the investigated data indicates that light-optimum characteristics in surface and subsurface water, without the limitation of nutrition, temperature, and other environmental factors, is an important factor for S. Costatum and P. donghaiense to reproduce rapidly and bloom in surface and subsurface water, respectively.4. From late 1950s to early 2000s, irradiance above sea surface was correlated in some degree with annual phytoplankton biomass in the Changjiang Estuary and Adjacent Coastal Waters.By collecting and analyzing phytoplankton investigation data from late 1950s, especially from 1980s, to early 2000s in this sea area, it is found that phytoplankton biomass (CDNet-PPT, Chl-aSur, Chl-aEu and PP) fluctuated as an"N shape"with a cycle of about 20a, which were correlated with irradiance above sea surface, especially, Chl-aSur, Chl-aEu and PP were closely correlated (p<0.01, p<0.05 and p<0.05, respectively)with irradiance.The author believes that the modified"phytoplankton growth light-effect analysis method"can be applied in marine and lake phytoplankton ecosystem, the above results are of significance for understanding the mechanisms of HAB occurrence in the Changjiang Estuary and Adjacent Coastal Waters, as well as for the prevention and control of HABs in this sea area.