We measured short-lived radium isotopes ( 223 Ra and 224 Ra), dissolved inorganic and organic nutrients, and photosynthetic pigments during the summers of 2006 and 2007 in the southern sea of Korea, where harmful dinoflagellate blooms occur every year. The Ra tracer measurements reveal that coastal groundwater, rather than other sources previous suggested (i.e., Yangtze River diluted water or Kuroshio currents), is the main source of nutrients that fuel red tides in this region. Although inorganic-nutrient levels are different for different regions and different years, either dissolved inorganic nitrogen or phosphorus is depleted in the red-tide region. This depletion is accompanied by highly elevated levels of dissolved organic nutrients, transformed from groundwaterborne dissolved inorganic nutrients either inside Yeoja Bay or in offshore red-tide areas, thereby creating favorable conditions for the growth of dinoflagellates in competition with diatoms. The intensity of red tides correlates well with the activity of 224 Ra (half life 5 3.66 d) in seawater over daily or yearly time scales. Because the chemically conservative 224 Ra can trace groundwater-borne nutrients, which are utilized by marine biota in this red-tide region, the intensity of red tides seems to be related to the amount of nutrient-enriched groundwater supplied to the offshore red-tide region.

Addition of the increased anthropogenic nitrogen (NO x and NH y) emitted from northeast Asian countries to the Yellow and East China seas and coastal waters around Korea has resulted in an unparalleled increase in the nitrate (N) concentration relative to the phosphate (P) and silicate (Si) concentrations in the upper ocean. We found that for the Yellow Sea the increase in N over P was largely explained by increased atmospheric nitrogen deposition, whereas for the northern East China Sea, downstream of the Changjiang River plume, the trend in N increase relative to P was more associated with a change in the combined input of nutrients from atmospheric deposition and riverine discharges. In contrast, the dynamics of the N to P relationship in the southern East China Sea was largely controlled by a change in the intrusion intensity of the Kuroshio Current, which has a low N : P ratio. The disproportionate and persistent input of nutrients to the marine waters of this region over the past four decades has transformed extensive areas from being N deficient to being P deficient, and has concurrently decreased the concentration of Si relative to N. In coastal waters around Korea in particular, these shifts in the nutrient regime have been accompanied by a change from diatom-dominated to dinoflagellatedominated blooms. Given the complexity of coastal ecosystems, the associations between changes in nutrient regimes and biological changes need to be investigated in other coastal areas receiving increasing loads of anthropogenic nitrogen.

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The marine dinoflagellate Prorocentrum minimum is the cause of harmful algal blooms and may grow in association with co-occurring bacteria as ectosymbiotic, endosymbiotic and free-living forms. In the present study we investigated the bacterial community composition of both free-living bacteria (FLB) and particle-associated bacteria (PAB) in the lag, exponential and stationary growth stages of P. minimum using pyrosequencing. Metagenomics, hierarchical cluster and non-metric multidimensional scaling analyses revealed that FLB and PAB had significantly different bacterial community compositions. The PAB community had greater taxonomic richness and diversity than the FLB community. In addition, the shared bacteria identified were clearly dominant in both the FLB (≥98.2%) and PAB (≥89.9%) communities. Among shared bacteria, the genera Seohaeicola (P. minimum operational taxonomic unit (OTU) #1) and Roseovarius (P. minimum OTU #6), belonging to the Roseobacter clade, were predominant in FLB (42–57%) and PAB (11–14%) communities respectively. In the PAB community, the Marinobacter clade (P. minimum OTU #13 and #15) was also a dominant taxon. Interestingly, in response to the growth of P. minimum, the proportion of the Roseobacter clade increased gradually, whereas the genus Marinobacter decreased in both the FLB and PAB communities. These results suggest that Roseobacter and Marinobacter clades are intimately associated with host dinoflagellate.

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