Occurrence of red tides in Lake Nakaumi [Japan] related to annual variation in water quality and C:N:P ratio of phytoplankton

• Published in: Japanese Journal of Limnology (Rikusuigaku Zasshi) Vol. 65; no. 2; pp. 69 - 82
• Main Authors: Kato, K. (Shimane-ken. Inst. for Public Health and Environmental Science, Matsue (Japan)), Godo, T, Kageyama, A, Ashiya, R, Mishima, K, Kamiya, H, Zhu, G, Ohtani, S, Ishitobi, Y
• Format: Journal Article
• Language: Japanese
• Published: The Japanese Society of Limnology 2004
• Subjects: Brackish; C:N:P ratio; CARBON; COASTAL LAGOONS; JAPAN; Lake Nakaumi; NITROGEN; P.minimum; PHOSPHORUS; PHYTOPLANKTON; phytoplankton bloom; PLANKTON BLOOMS; Prorocentrum minimum; red tide; WATER QUALITY; Japan, Honshu, Shimane Prefecture, Nakaumi L
• ISSN: 0021-5104; 1882-4897
• DOI: 10.3739/rikusui.65.69

Frequent water quality monitoring, identification of phytoplankton species, and measurement of C:N:P ratios were conducted to clarify the time series in the occurrence of a red tide by a dinoflagellate, Prorocentrum minimum, in Lake Nakaumi, a coastal lagoon with a two-layered structure. In late autumn 1996, when vertical mixing by seasonal winds started, a red tide from P. minimum occurred after a supply of NO3-N due to a freshet in an upper layer where PO4-P had excessively accumulated. That red tide continued throughout the winter, strengthened in spring, and disappeared in mid-May 1997 when fluxes of inorganic nitrogen and phosphorus to the upper layer decreased. During the period from autumn 1996 to spring 1997, the C:N:P ratio of suspended material gradually increased from 130:14:1 to 300:30:1. Compared with the early 1980s, the concentration of PO4-P in the central lake increased in the lower layer during the warmer seasons. Consequently, water quality in the upper layer changed from P to N deficiency in late autumn. P. minimum was dominant enough to absorb the supplied NO3-N under excess phosphorus conditions. The change toward N deficiency in the water quality was considered to be the reason why almost all red tides recently formed were due to P. minimum rather than to the diatom, Skeletonema costatum.