Reconstructing Ecological Change, Catchment Disturbance, and Anthropogenic Impact over the last 3000 years at Lake Pounui, Wairarapa, New Zealand

<p>Increased eutrophication of freshwater lakes has been attributed to an intensification in agriculture, with global warming projected to further compound the problem. Determining pre-human conditions of a lake has major conservation implications for water quality management, such as setting evidence-based restoration targets. New Zealand’s historical monitoring records of lake water quality are short and typically only begin after the onset of deterioration. Paleolimnology offers a complementary means of evaluating historical trends that predate human impact. This thesis investigates the declining water quality of Lake Pounui. Lake Pounui possesses high ecological integrity, though the lake is experiencing an increased frequency of severe algal blooms. The primary aim of this thesis is to reconstruct the past environment of Lake Pounui using paleolimnological methods to extend the historical monitoring data beyond human arrival. The reconstruction is used to address whether algal blooms are a natural feature of the lake, and examine anthropogenic impact. This study then attempts to identify reference conditions and critical transitions within the lake environment. Using this information possible targets for lake health restoration are discussed. Based on elevated charcoal influx, palynological evidence, and catchment disturbance indicators, such as organic content (Loss-on-Ignition (LOI)), grain size, and micro-X-ray fluorescence (µ-XRF), Māori land clearance was identified at ~450-350 calendar years before present (cal yr BP) (95% confidence interval (CI): 515.2-202.3 cal yr BP). The appearance of Pinus pollen and the diatom Asterionella formosa placed European arrival at ~150 cal yr BP (95% CI: 243-39 cal yr BP). Pre- and post-human environments of Lake Pounui were characterised using diatom analysis, bacterial DNA analysis, and supporting evidence from µ-XRF data. It appears that the lake existed in both a higher nutrient (3000-2100 cal yr BP, 95% CI: 3210-1977 cal yr BP) and lower nutrient (1600-450 cal yr BP, 95% CI: 1737- 389 cal yr BP) state, separated by a period of natural disturbance, which could relate to a combination of earthquakes and increased storminess. During Māori occupation (450-150 cal yr BP, 95% CI: 515-57 cal yr BP) the quality of the lake remained relatively high; however from 150 cal yr BP, the lake appears to become more nutrient enriched, and the cyanobacteria responsible for today’s blooms (Dolichospermum and Phormidium) become abundant. Paleolimnological analysis identified that the decline in water quality seen at Lake Pounui is a trend that has occurred over the last 200 years. Dissimilarity and critical transition analysis support this finding and suggest that rapid decline began just prior to 1950 AD. Based on dissimilarity analysis, the period of Māori occupation provides the most realistic restoration target. Planting native vegetation, ceasing stock grazing, and removing perch should be investigated to control the phytoplankton biomass, at the owner’s discretion.</p>

Reconstructing Ecological Change, Catchment Disturbance, and Anthropogenic Impact over the last 3000 years at Lake Pounui, Wairarapa, New Zealand

<p>Increased eutrophication of freshwater lakes has been attributed to an intensification in agriculture, with global warming projected to further compound the problem. Determining pre-human conditions of a lake has major conservation implications for water quality management, such as setting evidence-based restoration targets. New Zealand’s historical monitoring records of lake water quality are short and typically only begin after the onset of deterioration. Paleolimnology offers a complementary means of evaluating historical trends that predate human impact. This thesis investigates the declining water quality of Lake Pounui. Lake Pounui possesses high ecological integrity, though the lake is experiencing an increased frequency of severe algal blooms. The primary aim of this thesis is to reconstruct the past environment of Lake Pounui using paleolimnological methods to extend the historical monitoring data beyond human arrival. The reconstruction is used to address whether algal blooms are a natural feature of the lake, and examine anthropogenic impact. This study then attempts to identify reference conditions and critical transitions within the lake environment. Using this information possible targets for lake health restoration are discussed. Based on elevated charcoal influx, palynological evidence, and catchment disturbance indicators, such as organic content (Loss-on-Ignition (LOI)), grain size, and micro-X-ray fluorescence (µ-XRF), Māori land clearance was identified at ~450-350 calendar years before present (cal yr BP) (95% confidence interval (CI): 515.2-202.3 cal yr BP). The appearance of Pinus pollen and the diatom Asterionella formosa placed European arrival at ~150 cal yr BP (95% CI: 243-39 cal yr BP). Pre- and post-human environments of Lake Pounui were characterised using diatom analysis, bacterial DNA analysis, and supporting evidence from µ-XRF data. It appears that the lake existed in both a higher nutrient (3000-2100 cal yr BP, 95% CI: 3210-1977 cal yr BP) and lower nutrient (1600-450 cal yr BP, 95% CI: 1737- 389 cal yr BP) state, separated by a period of natural disturbance, which could relate to a combination of earthquakes and increased storminess. During Māori occupation (450-150 cal yr BP, 95% CI: 515-57 cal yr BP) the quality of the lake remained relatively high; however from 150 cal yr BP, the lake appears to become more nutrient enriched, and the cyanobacteria responsible for today’s blooms (Dolichospermum and Phormidium) become abundant. Paleolimnological analysis identified that the decline in water quality seen at Lake Pounui is a trend that has occurred over the last 200 years. Dissimilarity and critical transition analysis support this finding and suggest that rapid decline began just prior to 1950 AD. Based on dissimilarity analysis, the period of Māori occupation provides the most realistic restoration target. Planting native vegetation, ceasing stock grazing, and removing perch should be investigated to control the phytoplankton biomass, at the owner’s discretion.</p>

Early land use and centennial scale changes in lake-water organic carbon prior to contemporary monitoring

Organic carbon concentrations have increased in surface waters across parts of Europe and North America during the past decades, but the main drivers causing this phenomenon are still debated. A lack of observations beyond the last few decades inhibits a better mechanistic understanding of this process and thus a reliable prediction of future changes. Here we present past lake-water organic carbon trends inferred from sediment records across central Sweden that allow us to assess the observed increase on a centennial to millennial time scale. Our data show the recent increase in lake-water carbon but also that this increase was preceded by a landscape-wide, long-term decrease beginning already A.D. 1450–1600. Geochemical and biological proxies reveal that these dynamics coincided with an intensification of human catchment disturbance that decreased over the past century. Catchment disturbance was driven by the expansion and later cessation of widespread summer forest grazing and farming across central Scandinavia. Our findings demonstrate that early land use strongly affected past organic carbon dynamics and suggest that the influence of historical landscape utilization on contemporary changes in lake-water carbon levels has thus far been underestimated. We propose that past changes in land use are also a strong contributing factor in ongoing organic carbon trends in other regions that underwent similar comprehensive changes due to early cultivation and grazing over centuries to millennia.

A chironomid-based reconstruction of summer temperatures in NW Iceland since AD 1650

Few studies currently exist that aim to validate a proxy chironomid-temperature reconstruction with instrumental temperature measurements. We used a reconstruction from a chironomid percentage abundance data set to produce quantitative summer temperature estimates since AD 1650 for NW Iceland through a transfer function approach, and validated the record against instrumental temperature measurements from Stykkishólmur in western Iceland. The core was dated through Pb-210, Cs-137 and tephra analyses (Hekla 1693) which produced a well-constrained dating model across the whole study period. Little catchment disturbance, as shown through geochemical (Itrax) and loss-on-ignition data, throughout the period further reinforce the premise that the chironomids were responding to temperature and not other catchment or within-lake variables. Particularly cold phases were identified between AD 1683–1710, AD 1765–1780 and AD 1890–1917, with relative drops in summer temperatures in the order of 1.5–2°C. The timing of these cold phases agree well with other evidence of cooler temperatures, notably increased extent of Little Ice Age (LIA) glaciers. Our evidence suggests that the magnitude of summer temperature cooling (1.5–2°C) was enough to force LIA Icelandic glaciers into their maximum Holocene extent, which is in accordance with previous modelling experiments for an Icelandic ice cap (Langjökull).