Relationship Between Nutrients And The Occurrence Of Macroalgal Blooms In The Yellow Sea

During 2007 to 2014, the world’s largest trans-regional green tide bloom have broken out for eight consecutive years along the coast of the Yellow Sea (YS) in China, originating from the coastline of Jiangsu province in mid-April every year, floating north under the action of wind and current and eventually depositing onshore in Qingdao in August around. Such massive green tide disasters have broke the balance of coastal marine ecosystems, caused huge economic loss to the local government, especially to the Shangdong and Jiangsu provinces. For example, in order to ensure the normal hosting of the Olympic sailing regatta in Qingdao, costs for cleanup macroalgae during the 2008 green tide were assessed at about 200 million RMB (-30.8 million US dollars).Nutrients, including inorganic and organic forms, maybe play an crucial role for sustaining the occurrence of macroalgal blooms in the YS. Based on our field data and historical data, the relationship between nutrients and the occurrence of macroaigal blooms in the YS was investigated in this study, thus long-time chemical oceanography of nutrients in Subei coastal area and southern Yellow Sea (SYS), the spatial and temporal distributions of dissolved inorganic nutrients (DIN, PO4-P and SiO3-Si) and dissolved organic nutrients (DON, DOP), especially in the Subei coastal area during green tide occurrences were investigated, respectively. And the differential growth response of Ulva prolifera to inorganic and organic nutrients were also analyzed under laboratory conditions in the present study. The main results are as follows:1. An increasing trend presented in yearly dissolved inorganic nitrogen (DIN) values along the Jiangsu coasts over the past thirty years, and a significant correlation occurred between DIN and U. prolifera biomass (r=0.84, p<0.01), implying that nitrogen may be one of the most important contributors to the macroalgal bloom in this area. And the annual N/P ratios also increased across the 30 years. And the nutrient input from Guanhe River, Sheyang River and marine animal aquaculture in the Jiangsu province, all contributed substantive nitrogen and phosphorus to the macroalgal blooms. Additionally, compared with non-green tide area, higher temperature, lower salinity and more abundant nutrients were contained in green tide zone along the Jiangsu coasts before the harmful macroalgae event occurrence, satisfying the macroalgae growth and the hydrodynamic transport of green tides out to sea in the Subei coastal area.2. The annual DIN concentration in the SYS generally increased steadily from the 1980s to 2008; however, the DIN concentration increased sharply in the SYS in 2008, which suggested that the intense influence of terrestrial discharge to the SYS. And the high nitrogen levels may be one of the most important contributors to green algal blooms in this area. Moreover, seasonal concentrations of nutrients (DIN, PO4-P and SiO3-Si) generally peaked during the winter (or early spring), whereas the lowest value usually occurred in mid-summer, presenting a "double peak". During late spring or summer, phytoplankton grow more vigorously than other seasons during one year, especially the macroalgae blooms commonly occur from late April to early July in the SYS. The relatively high phytoplankton photosynthetic rates associated with high growth rates and high nutrient uptake rates led to nutrient depletion from the study area, eventually resulting to the lowest nutrient level over the whole year. Additionally, based on our calculation of nutrient fluxes from diverse sources into the SYS, nutrient contribution from the atmosphere was second next to river input, the exchange between sediments and overlying seawater was the third, and marine animal aquaculture was the last, all above significantly sustaining the primary productivity in this area.3. The horizontal distributions of nutrient concentrations generally decreased from inshore to offshore areas in the SYS from 2008 to 2012. And nutrient concentrations were relatively higher in southern coastal areas than those of northern areas of the SYS. especially in estuaries influenced by terrigenous input, such as the Sheyang River and the Guanhe River, implying that abundant nutrients seem to be a major factor in large-scale U. prolifera blooms. And nutrient values (DIN, PO4-P. SiO3-Si, DON and DOP) in surface layers all exhibited degressive trends with the hydrodynamic transport of U. prolifera from south to north in the SYS, especially the DIN concentrations. Furthermore, both the inorganic and organic forms of nutrients descended significantly during the entire course of green tide events in the SYS, and the time of DON and DOP concentrations descent was clearly lag to that of inorganic nutrients, thus we could presume that the assimilation capability of U. prolifera to inorganic nitrogen was obviously greater than to organic types.4. The growth response of U. prolifera exposed to different sources of nitrogen and phosphorus was examined by two laboratory experiments in this study, and its uptake capability to inorganic and organic nutrients was also investigated. The results showed that the Vmax and Ks values of inorganic N and P uptake by U. prolifera were both higher than those of organic forms. This indicated that the assimilation capability of U. prolifera to inorganic nutrients was obviously greater than to organic types. However, when ambient nutrient in low concentrations, the appetency of macroalgae to organic types was higher than inorganic nutrients. The main results are as follows: (1) Compared with control group (no N source addition), an enhanced and hyperbolic growth response of U. prolifera was monitored daily throughout the 14-day culture that was induced by different inorganic and organic nitrogen enrichment. The fronds that were enriched with urea and glycin produced a more denser biomass and brighter green color than NO3- and NH4+ groups. And compared with organic nitrogen, U. prolifera fronds could absorb dissolved inorganic forms preferentially. The Vmax and Ks values of DIN uptake by U. prolifera were higher than those of DON (NH4+> NO3-> NO2-> glycin> urea). This indicated that the assimilation capability of U. prolifera to inorganic nitrogen was obviously greater than to organic types. However, when ambient nutrient in low concentrations, the appetency of macroalgae to organic nitrogen was higher than inorganic nutrients. Therefore, we could find that organic nitrogen availability in the medium contributed more to the growth and expansion of U. prolifera than the inorganic nitrate-enriched medium, especially in the mid-late time of the cultivation.(2) To examine the growth response and assimilation rate of U. prolifera exposed to different sources of phosphorus, the inorganic (PO43") and organic forms (ATP and G-6-P) P-enrichment experiment were also performed on the macroalgae by the 20-day culture. The resulted showed that both inorganic and organic P nutrients could enhance the U. prolifera growth, especially the ATP addition. And Vmax and Km values of PO43- were both exceeded those of two organic types (ATP and G-6-P), implying that inorganic forms phosphate was the preferred P source for U. prolifera during the incubations. Furthermore, the Km values of two organic P were significantly lower than that of PO43-, thus the appetency of U. prolifera to organic P was higher than PO43- at low nutrient concentration conditions.