Abstract
It is a well held concept that the magnitude of red-tide occurrence is dependent on the amount of nutrient supply if the conditions are same for temperature, salinity, light, interspecific competition, etc. However, nutrient sources fueling dinoflagellate red-tides are difficult to identify since red tides usually occur under very low inorganic-nutrient conditions. In this study, we used short-lived Ra isotopes (223Ra and 224Ra) to trace the nutrient sources fueling initiation and spread of Cochlodinium polykrikoides blooms along the coast of Korea during the summers of 2014, 2016, and 2017. Horizontal and vertical distributions of nutrient concentrations correlated well with 224Ra activities in nutrient-source waters. The offshore red-tide areas showed high 224Ra activities and low-inorganic and high-organic nutrient concentrations, which are favorable for blooming C. polykrikoides in competition with diatoms. Based on Ra isotopes, the nutrients fueling red-tide initiation (southern coast of Korea) are found to be transported horizontally from inner-shore waters. However, the nutrients in the spread region (eastern coast of Korea), approximately 200 km from the initiation region, are supplied continuously from the subsurface layer by vertical mixing or upwelling. Our study highlights that short-lived Ra isotopes are excellent tracers of nutrients fueling harmful algal blooms in coastal waters.
Highlights
It is a well held concept that the magnitude of red-tide occurrence is dependent on the amount of nutrient supply if the conditions are same for temperature, salinity, light, interspecific competition, etc
In Korea, C. polykrikoides is a dominant species causing harmful dinoflagellate blooms, and the area affected by red tides has expanded along the coastline[5]
A number of studies have been conducted to determine the mechanisms for the outbreak and spread of red tides occurring along the southern coast of Korea
Summary
It is a well held concept that the magnitude of red-tide occurrence is dependent on the amount of nutrient supply if the conditions are same for temperature, salinity, light, interspecific competition, etc. We used short-lived Ra isotopes (223Ra and 224Ra) to trace the nutrient sources fueling initiation and spread of Cochlodinium polykrikoides blooms along the coast of Korea during the summers of 2014, 2016, and 2017. The magnitude (5800–48000 cells mL−1), duration (29–75 days), and initiation region (eastern or western coasts of Korea) of red tides have varied from 1995 to 2015 During this period, the fish kills caused by red tides have resulted in losses of US$ 1–67 million for Korea’s aquaculture industry[8]. The main source of nutrients fueling red tides in the study region (Tongyeong, the southeastern coast of Korea) is still poorly understood, and the supply of nutrients to the red-tide spread regions in the eastern coast of Korea is unknown. There were no outbreaks of red tides in 2016 and 2017
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