Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake. Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m. Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers. The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment. Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.

To gain an understanding of the transfer of nutrients and trace elements from sediment pore waters to surface waters of eutrophic Lake Rotorua and the early diagenetic processes controlling the transfer, pore water chemistry in the sediments of Lake Rotorua was investigated over a one year period in 2006 by collection of sediment cores on three occasions and deployment of pore water equilibrators on two occasions. Pore water concentrations of Fe2+, Mn2+, S, PO4, NH4, As, Cd, and Pb were analysed. Phosphate and ammonium fluxes to the water column from the sediments were calculated from measured concentration gradients by Fick's law of diffusion. Gas present in the sediments was analysed for composition, and source, and its ebullition rate measured. Anaerobic oxidation of organic matter is indicated by negative Eh values. Sulfate reduction was indicated near the sediment-water interface and releases of Fe2+, Mn2+, PO4 and NH4 into the pore water from particulate material were associated with the reducing conditions. Peaks in concentration of nutrients and elements occurred at the sediment surface over summer and deeper in the pore water profile over the cooler months of May and September. Sampling with peepers at fine scales immediately above the sediment-water interface indicated the presence of a nepheloid layer where elements are actively being recycled. Sulfate reduction appears to occur in the layer above the sediment-water interface, indicating that dissolved oxygen has already been reduced. Phosphorus is possibly being removed by iron and manganese oxide/hydroxide precipitation 5 to 15 cm above the sediment-water interface. Pore water saturation calculations indicate that sulfides may be controlling concentrations of iron and possibly other metals in the pore water by formation of pyrite in the zone of sulfate reduction. Below the zone of sulfate reduction, siderite and vivianite may be precipitating and acting as an additional sink for iron and phosphorus. ii Nutrient release rates based on Fick's law of diffusion indicated 430 tonnes of dissolved phosphorus and 1150 tonnes of ammonium were released to Lake Rotorua's water column in 2006, suggesting nutrient release from the sediments is the dominant flux of nutrients to the water column of Lake Rotorua. Methanogenesis, from acetate fermentation, occurs below the zone of sulfate reduction, where it becomes the dominant process in organic matter degradation. Ebullition of gas was measured at 126 ml m-2 d-1 and this gas was comprised dominantly of methane. Possible remediation techniques that could reduce the internal load of nutrients released from the lake sediments include sediment removal by dredging or capping the sediments with an adsorbent or sealing layer. Capping the sediments could be compromised by ebullition of gas that would disrupt the capped layer, opening up pathways that allow more readily for exchange between pore water nutrients and the water column. Dredging is likely to stimulate the ebullition of most of the trapped gas and result in a rapid efflux of much of the nutrient rich pore water into the lake, however dredging the top 10 to 20 cm of the sediments may partially reduce phosphorus in the pore waters but would not substantially reduce ammonium and fluxes would remain similar to current levels. Improving redox conditions in the sediments could reduce pyrite formation improving phosphorus binding with iron.

A palaeolimnological study was carried out on a high-resolution, 7.62 m-long core (RU188-07) from northern Lake Rotorua, North Island. The core consists predominantly of olive diatomaceous ooze, laminated in places, and contains five tephras including Tarawera (1886 A.D.), Kaharoa (c. 1314 A.D.), Taupo (c. 233 A.D.) and Whakatane (c. 5500 cal. years B.P.). The core terminated in Whakatane Tephra giving the sediment a maximum age of 5530 60 cal. years B.P. An age model for the sediment was developed using tephrochronology. Radiocarbon dates obtained on the sediment returned ages too old because of contamination by old CO2 or CH4, or both. Investigations carried out on the core included spectrophotometric, sedimentological and geochemical analyses, and diatom identifications, which provided a number of proxies from which inferences were made about lake history, catchment development, and palaeoclimate since c. 5500 cal. years B.P. The laminations, evident only in the upper, post-Kaharoa Tephra part of the record, comprise alternations of thin, dark, detrital deposits and pale, relatively fine-grained diatom assemblages. Sediment geochemistry indicates that the Rotorua catchment has undergone several changes since c. 5500 cal. years B.P., alternating between periods of variable and stable environmental conditions. Following the Whakatane and Waimihia eruptions and up to approximately 3000 cal. years B.P., the catchment surrounding Lake Rotorua was rather unstable. Fluctuations in many of the proxies during this period are likely to be associated with a variable climate with periods of storminess, coinciding with the establishment of ENSO conditions in New Zealand. A notable feature of the record is two phases of stability, the first following the Taupo eruption (from c. 1700 cal. years B.P. to c. 630 cal. years B.P.) and the second from c. 580 cal. years B.P. to c. 300 cal. years B.P. The latest, most significant event in the catchment history of Lake Rotorua was the settlement by Polynesians. M.S. McGlone implied from pollen profiles (from Holden's Bay) that initial settlement took place around the time of the Kaharoa eruption (c. 630 cal years B.P.; c. 1314 A.D.), but the sediment chemistry and erosion profiles obtained here, from the northern part of Lake Rotorua, indicate that although there may have been some early clearing in the northern catchment for tracks or buildings, large-scale clearing in the area probably did not occur until considerably later, c. 300 cal. years B.P. Also contained within the sediments are three layers of reworked tephric material that probably originate from the transfer of coarse grained tephra from shallow to deeper water during large storms at c. 1300 cal. years B.P, c. 520 cal. years B.P, and c. 220 cal. years B.P. Each event coincides with storm events inferred from records from Lake Tutira in eastern North Island. Because of Lake Rotorua's inland position, these inferred storm events probably represent only the largest cyclonic events (e.g. ex-tropical cyclones).