| Because of the enormous pressure brought by human activities on marine ecosystems, the extent and scope of coastal eutrophication is gradually increased and expanded in China. Under eutrophic conditions, macroalgae often grow rapidly. And then accumulation of macroalgae such as Ulva prolifera in the coastal area may cause large green-algae blooms, which is also called"green tide". Large green algae blooms not only have harmful effects on marine ecosystems, but do harm to the development of the local aquaculture industry, fisheries and tourism.Tth effects of different forms, types and concentration nutrients on the growth of Ulva prolifera were investigated by combining mesocosm experiment with lab experiment.The absorption characteristics of two different macroalgae Ulva prolifera and Ulva linza were compared. Additionally, the nutrient kinetic characteristic of Ulvaprolifera was preliminary discussed.The results were as follows:1. The experiment on the influence of inorganic nutrients on the growth of Ulva prolifera in the laboratory showed that the daily average relative growth rate of Ulva prolifera was 3.85%/d for the control sample group without any nutrients added. The growth rate of the group with 10μmol·L-1 concentration of nitrogen (N) addition is 6.31%/d and the rate of the group with 50μmol·L-1 concentration of N added is 6.32%/d. The growth rate of the group with 1μmol·L-1 phosphorus (P) added was 5.79%/d, and lower than any of the N-added groups. There was unsiganificant difference in the rate of the experimental group between both 20μmol·L-1 of N and 1 μmol·L-1 of P added and only N added. If N, P, and other tracers are all added, the growth rate reached 6.84%/d, and was the highest among those groups. The rate can be further higher once the nutrients solution was renewed everyday and it is 7.75%/d. The maximum rate was observed on the third day. It could reach as high as 27.57%/d under the optimum conditions. Our results suggested that N and tracer metals had positive effects on the growth of Ulva prolifera.2. The experiments of the effect on the growth of Ulva prolifera and Ulva linza under different nitrogen (N) and phosphorus (P) nutrients proved that there were two optimal groups of N and P for Ulva prolifera, which were nitrate (NO3-N) or carbamide (CO(NH2)2-N) accompanied with phosphate under the higher concentration of N 128μmol?L-1 and P 8μmol?L-1, whose daily average relative growth rate of biomass was 10.57% and 10.35%, respectively; the two optimal groups of N and P for Ulva linza were very different from that of Ulva prolifera: one group was ammonium (NH4-N) and glycerophosphate (DOP) with the lower concentration of N 64μmol?L-1 and P 4μmol?L-1, and the other group was NH4-N and PO4-P with higher concentration. Their daily average relative growth rates of biomass were 15.49% and 15.34% respectively. The maximum relative growth rate was found on the 5th day in the experimental groups of Ulva prolifera and the maximum relative growth rate of Ulva linza was found on the 3rd day. Except each control group, the average level of maximum relative growth rate could reach 24.87% and 22.66%, respectively. At 15°C, the lifecycle of Ulva prolifera was 15 days which was longer than that of Ulva linza whose lifecycle was 8 days. The maximum biomass of Ulva linza was merely about 61% of that of the other one, however, its capacity of absorpting nutrients was stronger than the other one.3. Mesocosm experiment of the effect of nutrients on the growth of E. prolifera showed that wet weight biomass of E. prolifera in mesocosms with nutrients added were evidently higher than those without added nutrients. The growth rates of the groups with f/2 one-off added and with nitrogen (N) one-off added were up to 10% and 9%, respectively; however, the rate of the sea area group was 4% only. A large number of nutrients could be rapidly absorbed by E. prolifera within a relative short-term. In the mesocosm with f/2 one-off added, the absorption rate of N became 9.25μmol·g-1·h-1 in the first 24 h and the absorption rate of phosphorus (P) was 0.63μmol·g-1·h-1. The results showed that nutrients addition would facilitate the growth of E. prolifera rapidly. The type and concentration of nutrients showed significant effects on the growth of E. prolifera. Compare to NH4-N and PO4-P, sufficient NO3-N was an important factor which was related to the fast-growing of E. prolifera. |
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