Palaeolimnological studies on sediments from Lake Grasmere, South Island, New Zealand, with special reference to the chironomidae (Diptera)
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Palaeolimnological studies were carried out on a 326 cm long sediment core from Lake Grasmere. The core covered the last 6000 years of the developmental history of the lake. A pollen analysis revealed that Nothofagus pollen was dominant from the base to 30 cm. Above that level an increase in grass pollen indicated the presence of Polynesian people. The European period was represented in the core by the uppermost 6 cm, indicated by the presence of introduced Pinus pollen. Results of a fossil diatom analysis showed that this fossil flora was dominated by small epiphytic Fragilaria species, the facultative planktic Cyclotella stelligera, and in the youngest sediments by the planktic Diatoma elongatum. The distribution of these taxa in the core indicated that the lake was initially shallow. The water level increased at the 240 cm horizon in the core, fluctuated in the subsequent sediment layers, and remained high above the 50 cm level, except for one possible short-term fluctuation, and increased to the present depth in the youngest sediments. The fossil Chironomidae fauna of the Lake Grasmere core was analysed, and 35 taxa were recognised. Tanytarsini were the dominant component of the fauna with Corynocera sp. as the most abundant species during pre-Polynesian times (before ca. 1000 yr B.P.). During the Polynesian period Orthocladiinae sp. IX was dominant and in the youngest sediments, the European period, Chironomus became the dominant fossil chironomid. The abundance and composition of the fossil Chironomidae fluctuated markedly over the last 6000 years. These fluctuations could be partly correlated to changes in the stratigraphy of the sediments in the core. Layers of highly minerogenic sediment contained the lowest numbers of remains and fossil taxa, whereas high abundances and diversities were found in the sections of the core with higher proportions of organic matter. Some of the pronounced shifts in the structure of the Chironomidae community were caused mainly by changes in the hydrology and the inflows to the lake, and by variations in the rate and type of sedimentation. These variations induced alterations in the substrate type which appeared to have changed the dominance pattern of the fossil chironomid fauna. Changes in lake productivity influenced the composition of the fauna to a lesser extent. In the fossil Cladocera fauna of the core, 14 species were distinguished. Two species were planktic and ten species belonged to the benthic Chydoridae. Several factors influenced the composition of the fossil Cladocera fauna, including increased input of minerogenous matter, changes in water depth, and lake productivity. Concentrations of fossil pigments, chlorophyll derivatives, total carotenoids, and the ratio of chlorophyll derivatives to carotenoids exhibited frequent and distinct fluctuations during the past 6000 years of Lake Grasmere's history indicating alternating periods of low and higher lake productivity. The recently deposited chironomid remains in surficial sediments from 32 lakes in the South Island were studied to establish relationships between faunal assemblages and limnological conditions of the lakes studied. A classification analysis based on the relative abundance of 19 chironomid taxa separated three main groupings of lakes and identified four lakes as outliers. These lake groups had distinctly different conditions which could be related to the composition of the chironomid fauna. The stratigraphy of the fossil Chironomidae from Blue Lake, Mt. Kosciusko National Park, Australia, is presented for comparison. The fossil fauna comprised 28 taxa, but was dominated by Podonomus in the Late-Pleistocene sediments, by nr. Eukiefferiella in the early Holocene and by Chironomus in the younger sediments. Changes in temperature and substrate may have been mainly responsible for the changes in the composition of the fossil chironomid fauna during the last 13,000 years. In correlating the stratigraphies of the different parameters studied in the Lake Grasmere core (organic matter content, Chironomidae, Cladocera, plant pigments) it can be shown that the lake has been influenced strongly by a geologically active and variable environment. Allochthonous minerogenous input was relatively high during the last 6000 years, but environmental disturbances of different degrees of magnitude caused pulses of increased amounts of mineral matter input. The response of the organic remains to these events was a decline in abundance and diversity. Two short periods (in the oldest and younger sediments) with higher lake productivity were indicated during which the lake was shallow and macrophytes were important. The longest stable period (from ca. 3000-1700 yr B.P.) was characterized by higher lake levels with a relatively great proportion of planktic diatoms and Cladocera. Lake waters were probably very clear and the sediments contained a large amount of benthic algae. The composition of the fauna in surficial sediments from several lakes could be used as analogues for fossil assemblages in the core. The fossil chironomid fauna from Lake Grasmere differed markedly from that in Blue Lake. Abrupt changes in the composition of the fauna and numbers of remains (as was found in Lake Grasmere) were restricted to the part of the Blue Lake core deposited during the late-Pleistocene, whereas changes in the fauna during the Holocene occurred gradually.