New Zealand Journal of Marine and Freshwater Research, 1983, Vol. 11 : 185-204 185 0028-8330/83/1702-0185S2.50/0 © Crown copyright 1983 Bottom sediments of Lake Rotoma CAMPBELL S. NELSON INTRODUCTION Department of Earth Sciences Although much is known of the chemistry and University of Waikato biology of lakes in central North Island, New Private Bag Zealand (e.g., Fish 1970; McColl 1972; Jolly & Hamilton, New Zealand Brown 1975; Hint 1977), little information is yet available on the texture and composition of the bottom sediments or the patterns and processes of Abstract Lake Rotoma is a deep (70-80 m), sedimentation in these lakes. Because of the oligotrophic, warm monomictic lake of volcanic influence of sediment substrate on such diverse origin with insignificant stream inflow and no clearly characteristics as sublacustrine slope stability (e.g., defined outflow. For at least 60 years up to 1972 the Irwin 1975), the nature and density of aquatic lake level fluctuated markedly about an overall vegetation (e.g., Brown 1975), the geochemical rising trend of some 6-10 m. Nearshore profiles are environment at the lake floor (e.g.. Glasby 1973), related to the prevailing wave climate superimposed and ultimately the trophic status of waters (e.g., upon the overall rising lake level, shelves being McColl 1977; White et al. 1978), a sampling and wider, less steep, and deeper about the more analysis programme of bottom sediments of central exposed eastern and southern shorelines. The outer North Island lakes has been started at the University portions of shelves extending well below modern of Waikato. To date, sediment characteristics are storm wave base into waters as deep as 15-25 m are available for Lake Taupo (Lister 1978), Lake relict features from lower lake level stands. Matahina (Phillips & Nelson 1981), Lake Rotoiti Sediments fine from sand-gravel mixtures nearshore (Craig & Nelson 1981), and Lake Rotoma, reported to silts in basinal areas. Their composition reflects a here. composite provenance involving the lavas and tephras about the lake, as well as intralake diatom Lake Rotoma (Fig. 1) is the easternmost of 16 frustules and organic matter. The distribution lakes of primarily volcanic origin forming the pattern of surficial bottom sediments is an interplay Rotorua lakes system in the Rotorua district. between grains of both biological and terrigenous However, unlike most Rotorua lakes, which are origin, supplied presently and in the past by a eutrophic, it remains essentially oligotrophic variety of processes, that have been dispersed either because of a combination of minimal inflows from by the modern hydrodynamic regime or by former hot springs and surface streams, the relatively small ones associated with lower lake levels. These area (38%) of drainage catchment in pasture, and interrelationships are structured by erecting 5 the existence of uniformly deep (70-80 m) basins process-age sediment classes in the lake, namely (McColl 1974). neoteric, amphoteric, proteric, palimpsest, and relict sediments, analogous to categories postulated for sediments on oceanic continental shelves. Short- METHODS core stratigraphy includes the Kaharoa (A.D. A working field sheet was constructed from Irwin's -1020) and Tarawera (A.D. 1886) tephras. The (1967) bathymetric map of Lake Rotoma. Bottom rates of sedimentation of diatomaceous silts in samples were collected in winter 1975 using a 5-L- basinal areas have more than doubled since the capacity Ekman-Berge grab sampler and a Model Tarawera eruption, indicating an overall increase in 202 Alpine piston corer with 4-cm-diameter PVC the fertility level of lake waters associated, perhaps, liner. Beach samples were scooped by trowel from with recent farm development in the catchment. the upper 2 cm of the shoreface. Sediment pH and Eh were measured immediately onboard using a probe-type combination pH/Eh meter calibrated Keywords Lake Rotoma; lakes; limnology; against standard solutions. Sample stations (Fig. 2) sediments; sedimentation; stratigraphy; morphol- were positioned by intersection of several compass ogy; sediment texture; sediment collections; sedi- bearings to prominent features about the shoreline, ment sampling; sediment transport; sediment and supplemented by bathymetric control using a distribution; bottom topography Marlin DIR60 echo sounder. Structure within the top few metres of lake sediment was recorded using Received 16 July 1981; accepted 18 November 1982 a 7 kHz Raytheon seismic profiler in 3 across-lake 186 New Zealand Journal of Marine and Freshwater Research, 1983, Vol. 17 transects (Fig. 2). Secchi disc measurements were show that the depth of the shelf-slope break varies made at most stations, and a single temperature from 3 m to over 13 m, with a lake-wide average of profile was measured on 7 July 1975 with a reversing about 8 m. Shelves bordering the more exposed thermometer in the deepest (83 m) part of the lake, eastern half of the lake (Fig. 1, wind-rose) extend near Stn 58. into deeper water (commonly over 8 m and locally In the laboratory, samples were split, digested in to about 20 m), are wider (often exceed 100 m), and 10% H2O2 to remove and determine the percent by have a more gentle gradient (less than 1 in 10) than weight of organic matter, wet-sieved through 2 mm their counterparts along the western shoreline (Fig. ( —1<{>) and 0.06 mm (4<j>) mesh screens to obtain 3). The depth of the slope-basin break is 50-70 m in their percent by weight of gravel, sand, and mud, the main basins and, like the shelves, slopes are and classified using the textural scheme of Folk wider, of lower gradient, and extend deepest (1968). Grain-size statistics for 25 samples rep- adjacent to shores facing the prevailing winds. resentative of different textural classes were There are several lagoons, separated from the calculated by computer (Kamp 1979), after sieving lake by vegetated bars of mixed sand and gravel of the sand and gravel fractions at i<J> intervals and (Fig. 1). The bars are 50-200 m wide, and show a hydrophotometer analysis (Jordan et al. 1971) of the series of beach ridges parallel to the shoreline, built silt fraction at i<j> intervals and of the total clay up by storms during higher lake levels. The bar fraction (<0.004 mm; 8<f>). Statistics determined forming the Otumarokura Lagoon in Matutu Basin included the first percentile (C$) and median (Md<|>) was largely submerged and being eroded, with an of Passega (1964), and the mean (Mz<j>), sorting (cr,<|>), skewness (Skj), and kurtosis (KQ) values of Folk & Ward (1957). Tabulated results of textural, pH, and Eh values are available from the author. Bulk sediment composition was determined by X- Table 1 Morphometric and hydrologic data for Lake Rotoma (adapted mainly from McColl (1972, 1975), Jolly ray diffraction analyses, by microscopic observation & Irwin (1975), and Clayton (1978)). The maximum depth of the light and heavy mineralogy of sand fractions and the secchi disc depth in parentheses are those after sedimentation through tetrabromoethane (s.g. measured in this survey. 2.94), and by examination of scanning electron images of the less than 2 |xm, 2-16 (Am, and 16-63 Morphometry jj,m centrifuged separates prepared using Lister's Altitude (m) 315 Area (km2) 11 (1978) technique. Volume (106m3) 458.9 Maximum length (km) 5.5 Maximum breadth (km) 3.5 RESULTS AND DISCUSSION Maximum depth (m; July 1975) 83 Mean depth (m) 37 Morphology and geology Shore line (km) 28.7 Shore line development 2.4 Morphometric and other data for Lake Rotoma and 2 Drainage area (km ) 16 its catchment are given in Table 1. The lake % area in grass 38 comprises 3 main basins: North Basin (max. depth % area in scrub 5 83 m), South Basin (73.5 m), and Matutu Basin (38 % area in native forest 53 m) (Fig. 1). The basins possibly formed as explosion % area in housing 4 craters and are separated by sediment-veneered Hydrology bed-rock sills (Grange 1937). Their original shapes Stratification Summer (Nov-Jul) have been modified by extrusion of rhyolite flows on Circulation Monomictic (Jul-Nov) the west side of the lake and by spit and bar Secchi disc depth (m) 10.5-11.1 (13.7) Thermocline depth (m): mean 24 development along parts of the shoreline. Tephra range 12-45 thicknesses suggest that the Matutu Basin formed Surface temperature (°C):mean 16.2 during eruption of Rotoma Ash, about 7300 years range 10.8-24.0 ago (Healy 1975). South Basin contains a prominent Bottom temperature (°C):mean 11.0 range 10.2-11.4 central submerged pinnacle, the Otangiwai Bank, 2 formed by a steep-sided rhyolite plug. In pre- Annual heat budget (cal cm" ) 19432 Surface water dissolved oxygen (%)100 European time this bank was an island with a village Bottom water dissolved oxygen (%)69 (Healy 1975), but now its summit lies beneath about Surface water pH 7.6-7.7 6 m of water. Similar, but smaller, submerged peaks Bottom water pH 7.0-7.4 are defined by the irregular form of depth contours Water type Chloride waters elsewhere in South and North Basins (Irwin 1967). Major anion species (mg L~') C1>HCO3»SO4 Major cation species (mgL"1) Na:s>Si>K>Ca3Mg Echo-sounding profiles, along with the detailed 1 shore-normal transects measured by Clayton (1978), Salinity (mg L" ) 100.8 Nelson—Bottom sediments of Lake Rotoma 187 174 '178 E Lake Rotoma Otumarokura Lagoon North Island Parata Point Otumarokura Point Whakarewarewa \Spring- fed Lagoon / ^.Onewhero \. J Lagoon Okopua Southwest Point Inlet Hikataua Point 0 10 20 30 Otangiwai Frequency (% Bank Vx? ^ Matahi Otangiwa Lagoon Point South Basin Anaputa Point Fig. 1 Locality map for Lake Rotoma showing generalised bathymetry (m; after Irwin (1967)) and wind-rose for nearby Rotoehu Forest (N.Z.