The main objectives of this study were to; (i) Determine how the 𝜹¹³C of autochthonous production in fresh water lakes varies with productivity and trophic state. (ii) Determine if such variations can be used for the investigation of lake palaeoenvironment. (iii) Quantify the effect of anthropogenic activity on the productivity of Hamilton Basin lakes. The viability of this project was assessed by monitoring the dissolved inorganic carbon chemistry of an eutrophic pond during a period of high insolation and algal productivity. These observations revealed that during the day, the Pc0₂ in the euphotic zone was reduced to levels low enough to affect. photosynthetic carbon isotopic fractionation, and hence plankton 𝜹¹³C values. The enrichment of ¹³C in the dissolved inorganic carbon (DIC) pool during periods of high photosynthetic productivity also affects plankton 𝜹¹³C values. Analysis of diurnal changes in 𝜹¹³C and DIC concentration using the Rayleigh equation suggested that photosynthetic fractionation factors changed in response to changes in ambient P𝒸ₒ₂ levels. The effect of lake trophic state on phytoplankton photosynthetic carbon isotopic fractionation was further investigated using closed system batch cultures of zooplankton-free natural algal populations collected from lakes of differing trophic state. The results indicated that; (i) There are considerable differences between algal communities, photosynthetic production and apparent CO₂ compensation points for cultures of algae from lakes of differing trophic state. (ii) Plankton from a eutrophic lake were able to photosynthesise down to a low and had a variable photosynthetic carbon isotopic fractionation factor, (ep), which varied as a function of the P𝒸ₒ₂ . (iii) Plankton from a mesotrophic lake were unable to reduce the Pc0₂ below 300ppm and exhibited a constant ∈p . (iv) Carbon dioxide availability could be limiting plankton photosynthesis in productive eutrophic lakes, and may result in HCO₃⁻ use. Phytoplankton 𝜹¹³C values and photosynthetic fractionation factors have the potential to be used to detect changes in plankton productivity and substrate use. A study was made of the isotopic chemistry of the DIC and particulate organic matter, (POM), in six Hamilton Basin lakes with trophic states ranging from hypertrophic to oligotrophic-mesotrophic and dystrophic. From this study, it was concluded that; (i) The isotopic chemistry of the DIC is a function of: - catchment composition - the relative amounts and 𝜹¹³C of inorganic carbon supplied from the atmosphere and biogenic sources. - the mixing regime of the lake. (ii) 𝜹¹³Cₚₒₘ varies seasonally in concert with changes in algal biomass. (iii) Because variations in 𝜹¹³Cₚₒₘ result primarily from changes in the photosynthetic fractionation factor, ∆¹³C₍ₚₒₘ₋𝒸ₒ₂₍ₐq₎₎they can be used to detect changes in the productivity of lakes. (i v) 𝜹¹³Cₚₒₘ values could not be used to rank lakes in order of increasing productivity because of the effect of systematic differences in the 𝜹¹³C 𝒸ₒ₂₍ₐq₎of different lakes. (v) POM produced in the euphotic zone is incorporated in the sediment with only a small (-1%) change in its 𝜹¹³C, thus enabling sediment 𝜹¹³C values to be used to source organic carbon in the sediment and to investigate lake palaeoproductivity. Large systematic variations were observed in sediment 𝜹¹³C values during the development of lakes. By assuming that the 𝜹¹³C 𝒸ₒ₂₍ₐq₎ had remained constant during lake development, an attempt was made to determine the palaeoenvironment of these lakes from sediment 𝜹¹³C values. It was concluded that the productivity of these lakes had undergone considerable changes during their development in response to climatically and anthropologically induced changes in the catchment vegetation. Low forest cover results in high or increased lake productivity, whilst dense forest cover led to low or reduced lake productivity. Lakes formed in the late glacial times had a high productivity, (low forest cover and high nutrient loading), which reduced as the forest cover developed. Peat development also affected some of the lakes. An attempt was made to assess the effect of anthropogenic activity in the Hamilton Basin on the metabolism of the lakes from changes in the recent sediment 𝜹¹³C values and a comparison of present day 𝜹¹³Cₚₒₘ values with recent sediment 𝜹¹³C values. The results indicate that; (i) Anthropogenic activity had a marked affect on lake metabolism, resulting in increased terrigenous inputs and/or increased productivity. This was particularly evident in Lake Hakanoa, where deforestation of the area by early Polynesian settlers resulted in a large increase in lake productivity. (ii) Several of the lakes have very recently undergone a marked increase in their productivity, a result of European activity in the area. Thus, plankton productivity has a systematic affect on the 𝜹¹³C of the POM incorporated in lake sediments, which has the potential to be used in the investigation of lake palaeoproductivity, and in assessing the effect of anthropogenic activity on lake productivity.