Chemistry of sediments in relation to trophic condition of eight Rotorua Lakes

Abstract

Chemical parameters (pH, Eh, carbon, Kjeldahl nitrogen, total phosphorus, 0.5M H 2 SO 4 -extractable phosphorus, organic phosphorus, and water-soluble phosphorus) were measured in the surface layers of sediments collected from various depths in Lakes Rotowhero, Okaro, Ngapouri, Rotokakahi, Okareka, Tikitapu, Okataina, and. Rotoma during October 1972. The sediments of the productive geothermal lake, Rotowhero, were markedly different from those of the cold-water lakes: they had relatively low pH values, high carbon (mean 8.5%) and organic phosphorus (mean 4160 µg.g -1 ) concentrations, and very high total phosphorus concentrations (mean 4770 µg.g -1 ), probably as a result of enrichment by hot springs.The mean concentrations in the sediments of the cold-water lakes were carbon 3.2-7.9%, Kjeldahl nitrogen 3380-8310 µg.g -1 and phosphorus 690-1780 µg.g -1 . These concentrations are within the ranges for New Zealand terrestrial topsoils, but the lake sediments appear enriched in phosphorus relative to local topsoils. Total carbon, nitrogen, and phosphorus concentrations of sediments tended to be highest in the eutrophic lakes (Okaro, Ngapouri) although the deep oligotrophic lakes (Okataina, Rotoma) had relatively high total phosphorus concentrations (means 1400, 1510 µg.g -1 ). Overall, the carbon, nitrogen, and phosphorus concentrations of the sediments showed little relationship to the trophic state of the lake.Organic phosphorus concentrations of the surface layers of sediments were similar in all the cold-water lakes (mean 319 µg.g -1 ). The proportion of the total phosphorus apparently 'fixed' in mineral material was minimal (0-1%) in sediments from the eutrophic and mesotrophic lakes, but in the oligotrophic lakes was similar to that in New Zealand topsoils (9-14%). Reducing conditions may cause solution of a high proportion of the 'fixed' phosphorus in the eutrophic lakes.The water-soluble phosphorus concentrations in the sediments of the five shallow cold-water lakes (Okaro, Ngapouri, Rotokakahi, Okareka, Tikitapu) correlated positively with trophic state and with concentrations of dissolved phosphorus in the lake waters.Carbon, nitrogen, and phosphorus concentrations in the sediments tended to vary with overlying water depth. This should be considered when comparisons are made between lakes.

New Zealand lakes research, 1967–91

“…During the: late 1950s and early 1960s, Jolly (1968) andFish (1969b, 1970 measured oxygen levels at depth in several Rotorua lakes providing, thereby, valuable baselines against which to assess subsequent changes. By comparing dissolved oxygen profiles of seven Rotorua lakes in 1970-71 with earlier data, McColl (1972) showed that oxygen had decreased in at least six of the seven lakes during the previous 15 years and that the decreases were correlated with increased agricultural, forestry, or urban development in the catchments (Fig. 2).…”

“…The initial approach in the 1970s was to classify New Zealand lakes according to trophic state based on indices of eutrophication that had been developed in the Northern Hemisphere. For example, measurements of chlorophyll a (as an index of phytoplankton biomass), transparency (Secchi depth), various forms of phosphorus and nitrogen, and dissolved oxygen revealed a wide range of trophic states among lakes in the Rotorua area (McColl 1972). This study is notable in being the first to document the relation, in New Zealand, between increased land development 1966-70 1971-75 1976-80 1981-85 3O-i 1966-70 1971-75 1976-80 1981-85 1966-70 1971-75 1976-SO 1981 …”

“…Nutrient limitation and predictive models A major finding during the 1960s and early 1970s was that, by world standards, New Zealand lakes are generally low in total nitrogen (e.g., Jolly 1968;McColl 1972). Levels of inorganic nitrogen (NO3-N) and ratios of nitrogen (N) and phosphorus (P) are generally much lower in New Zealand lakes than in most northern hemisphere lakes (White 1982(White ,1983Fig.…”

“…The availability of both P and N from these sources, assessed by McColl (1975a)and McColl etal. (1977, showed that agricultural catchments can contribute significant amounts of nitrate and phosphate to lakes compared with forested catchments.…”

Natural and anthropogenic lead in sediments of the Rotorua lakes, New Zealand

“…The Rotorua lakes vary widely in trophic status (oligotrophic to highly eutrophic) and have two predominant mixing regimes; monomictic and polymictic (Table 1) (McColl, 1977;Burns et al, 1997;Trolle et al, 2008). Lake trophic status and silica availability are important controls on sedimentation rates (mainly diatom frustules where there is sufficient silica) and the extent of chemical reduction within the sediments (Trolle et al, 2008).…”