Study On Gracilaria Lemaneaformis's Ability To Treat Mariculture Discharge

Abstract This article compared the nutrient removal capacity (NRC) of two common species of marcoalgae: Gracilaria lemaneaformis and Uiva pertusa. It also discussed the feasibility of using polyculture system of Gracilaria lemaneaformis and Crassostrea gigas to remove nutrients generated by cultured fish through a series of experiments. The results of these experiments are listed below:1) Comparing to Uiva pertusa, Gracilaria lemaneaformis is a better choice for algae-mollusk polycuture, since it can remove 90.32% of ammonia nitrogen and 60.45% of nitrate. While for Uiva pertusa, the rates are only 75.86% and 55.81% respectively.2) Gracilaria lemaneaformis's removal capacity of nitrogen and phosphorus is dependent upon the concentration of nutrients and environmental factors such as illumination intensity. At the temperature of 22℃, Gracilaria lemaneaformis's reaches its highest nitrogen removal capacity when the concentration of dissolved inorganic nitrogen (DIN) is 50umol/l. At the temperature of 25℃, Gracilaria lemaneaformis's reaches its highest phosphorus removal capacity when the concentration of dissolved inorganic phosphorus (DIP) is 3.1umol/l. Higher water temperature and nutrient concentration than the ideal ones will decrease Gracilaria lemaneaformis's NRC. Gracilaria lemaneaformis's NRC also changes with illumination intensity. Its NRC of nitrogen peaks when the level of illumination is at 3000lx, and its NRC of phosphorus reaches maximum at 2000lx. Illumination intensity higher than the above mentioned optimal numbers will decrease Gracilaria lemaneaformis's NRC.3) The concentration of nutrients in mariculture discharge can affect Gracilaria lemaneaformis's growth. For nitrogen, Gracilaria lemaneaformis grows well when the concentration of DIN ranges from 20 to 50umol/l; its growth rate peaks when the concentration of DIN is 30umol/l. Gracilaria lemaneaformis's grow slower after DIN reaches 150umol/l.When DIN is higher than 250umol/l, the growth of Gracilaria lemaneaformis is hardly detectable. In the case of phosphorus, Gracilaria lemaneaformis grows well when DIP is ranging from 1 to 10umol/l; its growth rate slows down after total inorganic phosphorus (TIP) reaches 20umol/l.4) The optimal stocking density of the polyculture system of Gracilaria lemaneaformis and oyster Crassostrea gigas is 3 oysters and 50 gram of Gracilaria lemaneaformis per square meter. This system works well at 25℃, but when temperature exceeds 28℃,it fails to prevent the water body from entering into the stage of eutrophication.5) The polyculture of Gracilaria lemaneaformis and oyster can effectively remove dissolved nutrients and suspending particles from the mariculture discharge. It has been proved by experiments that this polyculutre system can protect the environment by upgrading the quality of mariculture discharge to 2nd Class (according to National Sea Water Quality Standard). The facts that both of the two species are readily available and are of high commercial value make this approach promising to be widely adopted.One notable problem of the polyculture system is its susceptibility to the change of environment. For example, once water temperature or illumination intensity gets too high, Gracilaria lemaneaformis's nutrient removal capacity and rate of photosynthesis will both decrease, leaving the water body with more wastes, but less dissolved oxygen. This will debilitate the oyster. More research is needed to test whether the above mentioned polyculture system can be improved to be able to operate all the year round, especially during the summer time.