Rawlence: Post-Tarawera eruption (A.D. 1 . . .

New Zealand Journal of Marine and Freshwater Research abstracts

Post-Tarawera eruption (A.D. 1886) diatom stratigraphy of Lake Rotoiti, North Island, New Zealand

DAVID J. RAWLENCE

Department of Biology University of New Brunswick P. O. Box 5050, Saint John New Brunswick Canada E2L 4L5

Abstract The recent sedimentary diatom succession and chlorophyll distribution were determined in two 40 cm cores obtained from 60 m depth on 25.1.73 from Lake Rotoiti, North Island, New Zealand. The sequence contained basaltic ejecta from the A.D. 1886 Mt. Tarawera eruption from 39-25 cm below the sediment surface. Chlorophyll levels increased gradually until c. 1916, when a slight decline occurred, and then increased to a major peak c. 1959. The diatom component of the phytoplankton exhibited a similar trend, with a minimum c. 1926 and a maximum c. 1952. The flora has been dominated by Melosira granulata (Ehr.) Ralfs. throughout most of the post-Tarawera period. Cyclotella stelligera Cl. u. Grun. and Aster-ionella formosa Hassall formed a small component of the flora during this time, and Melosira distans (Ehr.) Kutz. formed a major part of the c. 1952 maximum. Until this time, M. distans remained a comparatively minor part of the flora. No major increase in pigment or diatoms followed the Tarawera eruption. The lack of coincidence between the subsurface diatom and pigment peaks suggests that other plant groups, probably non-siliceous phytoplankton, also contributed to the pigment peak. The blooming of M. distans possibly reflects a change in nutrient status of surface waters, a change which proved favourable for the growth of other groups of phytoplankton. A change in catchment land use could account for these changes in the sediment.

Keywords volcanism; sediments; diatoms; pigments; eutrophication; Lake Rotoiti; core; Mt. Tarawera

New Zealand Journal of Marine and Freshwater Research, 1985, Vol. 19: 391-397Received 1 September 1983; accepted 31 January 1985

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