Last ca 2500 Yr history of the harmful algal blooms in the South China reconstructed based on the sediment record of the organic-walled dinoflagellate cysts

Harmful algal bloom (HAB) is a known negative phenomenon that can result in a series of deleterious effects, such as beach fouling, paralytic shellfish poisoning, mass mortality of marine species and a threat to human health, especially if toxins pollute drinking water or occur near by the public resorts. In the South China the problem of HABs has an ultimately important meaning. For this study, we used a 1.5 m sediment core LX-2018-2 collected from the Zhanjiang Mangrove National Nature Reserve (109°03´E, 20°30´N), Guangdong Province, South China. High-resolution coastal environment reconstruction with a specific focus on the HABs history during the last ca 2500 yrs was attempted. Age control was performed with the five radiocarbon dates obtained from benthic foraminifera. A total number of 71 dinoflagellate cyst types was registered. The most common types found consistently throughout the sediment sequence were autotrophic Spiniferites spp., Spiniferites hyperacanthus and S. mirabilis, S. ramosus, Operculodinium centrocarpum sensu Wall and Dale 1966, Polysphaeridium zoharyi, and heterotrophic Brigantedinium ssp., cyst of Gymnodinium catenatum and cysts mixture of Protoperidinium. Three local dinoflagellate zones LX-1 to LX-3 were established based on the results of the constrained cluster analysis and data ordination; additionally, the middle zone LX-2 was derived into two subzones, LX-2a and LX-2b, based on the dynamics of toxic and heterotrophic cysts as well as on the significant changes (probability, P=0.89) in percentages of eutrophic indicators. The total cyst count varied from 106 to 410 cysts per slide with 177 cyst types on average. Dinocyst assemblages were characterized by the high values of the post-depositional degradation index (kt) that varies between 3.6 and 7.6 (averaging 5.4) which is relatively high and typical for the areas with selective dinoflagellate cyst preservation that is related to bottom-water oxygen concentrations.