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Invercargill, New Zealand

Duggan I.C.,Center for Biodiversity and Ecology Research | White M.A.,Environment Southland
New Zealand Journal of Marine and Freshwater Research | Year: 2010

Barrier bars separating lagoons from oceans are frequently breached as a management tool to prevent flooding of terrestrial ecosystems.The effects of such human-mediated openings on zooplankton have been investigated only in one tropical system.We investigated the temperate Waituna Lagoon, New Zealand, over a 2-year period when the barrier bar was 'artificially' breached on three occasions.Increases in salinity associated with opening of the barrier bars greatly influenced zooplankton community composition, and recovery of communities was dependent on the rate at which salinity returned to pre-disturbance conditions.As such, resilience of zooplankton in coastal lagoons is a function of the lagoon conditions returning to those experienced prior to barrier breach, rather than being a result of the zooplankton community simply recovering from a single defined disturbance event.In contrast to the tropical lagoon studies, temperature in Waituna Lagoon was inferred to explain a significant proportion of the variability in zooplankton community composition, independent of salinity.Appropriate timing for the opening of barrier bars by management authorities in temperate lagoons, which would allow the greatest opportunity for freshwater zooplankton communities to recover rapidly, will rely on determining the best time for rapid barrier bar reformation and high freshwater inflow rates (i.e. the recovery of zooplankton relies on return to initial conditions). However, such an approach is in direct conflict with the opening of barrier bars for management of water levels.

Greer M.J.C.,University of Otago | Greer M.J.C.,NIWA - National Institute of Water and Atmospheric Research | Closs G.P.,University of Otago | Crow S.K.,NIWA - National Institute of Water and Atmospheric Research | Hicks A.S.,Environment Southland
Ecology of Freshwater Fish | Year: 2012

Complete macrophyte removal to maintain drainage performance in lowland streams can have a negative effect on resident fish communities, but few studies have quantified this impact. Moreover, limited research has been carried out exploring alternative approaches for macrophyte removal that minimise the impact on the resident fish community. The aims of this study were (i) to determine how the current practice of removing almost 100% of available macrophyte cover affects native fish populations in lowland New Zealand streams and (ii) to see whether this impact can be reduced by limiting macrophyte removal to alternating 50-m sections of the waterway. Native fish populations were surveyed before and after experimental macrophyte removal for the following three treatments: (i) complete macrophyte removal, (ii) macrophyte removal from alternating 50-m reaches and (iii) control with no macrophyte removal. Radiotelemetry was used to monitor the behavioural response of individual giant kokopu (Galaxias argenteus) to the different treatments. The results of this study suggest that current drain management practices reduce CPUE of fish by 60%. Although limiting macrophyte removal to alternating 50-m sections did not minimise the community impacts of drain clearing, large giant kokopu did benefit from this strategy. All tagged giant kokopu remained in stream reaches partially cleared of macrophytes, while in completely cleared reaches all individuals were displaced. These results demonstrate the threat current drain management practices pose to New Zealand native fish and highlight the value of trialling alternative methods of macrophyte removal. © 2012 John Wiley & Sons A/S.

Sun L.,University of Texas at Dallas | Leybourne M.I.,Laurentian University | Rissmann C.,Environment Southland | Olariu C.,University of Texas at Austin | And 36 more authors.
Geochemistry: Exploration, Environment, Analysis | Year: 2015

Lake Texoma is a large impoundment on the border of Texas and Oklahoma, formed from the confluence of two river systems with different salinities, the Red River (total dissolved salt, TDS, of 2700–11 900 mg/l, average of 4862 mg/l) and the Washita River (TDS of 420–915 mg/l, average of 701.4 mg/l). Systematic analyses for major and trace elements were conducted of water samples collected spatially and with depth in the lake in different seasons. Overall, Lake Texoma waters are characterized by Na-Ca-Cl-SO4-type waters with spatial distribution shifting from Na-Cl type to Ca-SO4 type from the Red River arm to the main lake and to the Washita River arm. In addition, vertical and seasonal variations in major and trace elements concentrations indicate major elements in the lake are mainly controlled by different bedrock weathering from the two river systems. Trace elements that exhibit different distribution patterns to the major species are associated with variable sources such as river inflow, summer stratification effects, biological effects and anthropogenic activities. In Lake Texoma, differential inflow volumes and summer stratification are principal factors controlling the variation and geochemistry of lake waters and mixing dynamics. © 2014 AAG/The Geological Society of London.

Schallenberg M.,University of Otago | Larned S.T.,NIWA - National Institute of Water and Atmospheric Research | Hayward S.,Environment Canterbury | Arbuckle C.,Environment Southland
Estuarine, Coastal and Shelf Science | Year: 2010

Intermittently closed and open lakes and lagoons (ICOLLs) are shallow barrier lakes which are intermittently connected to the sea and experience saline intrusions. Many ICOLLs are mechanically opened to prevent flooding of surrounding agricultural and urban land and to flush water of poor quality. In this study, the effects of modified opening regimes (frequency and duration of barrier openings and closures) on water quality and phytoplankton in two New Zealand ICOLLs were investigated over a number of opening/closure cycles. Water quality in Lake Ellesmere (Te Waihora) responded weakly to both opening and closing events, indicating that sea-ICOLL exchange did not markedly improve water quality. Conversely, water quality in Waituna Lagoon responded rapidly to barrier openings; water level decreased to near sea level within days of opening and subsequent seawater exchange resulted in rapid decreases in nitrate and chlorophyll a concentrations. The closure of Waituna Lagoon resulted in rapid rise in water level and a pulse of nitrate and phosphorus in the water column and phytoplankton chlorophyll a concentrations increased with increasing closed-period duration. Based on data on the underwater light climate and nutrient dynamics, phytoplankton in Lake Ellesmere was probably light-limited, whereas phytoplankton in Waituna Lagoon was rarely light-limited, and appeared to be predominately P-limited. The marked differences in responses of Lake Ellesmere and Waituna Lagoon to barrier openings and closures reflected differences in ICOLL water levels and morphological characteristics, which dictated the degree of tidal flushing when the barriers were open. The inter-ICOLL differences observed in this study indicate that unless the effects of ICOLL openings/closures on phytoplankton and nutrient dynamics are understood, changes to ICOLL opening regimes may have unintended consequences for the water quality and ecology of these systems. © 2009 Elsevier Ltd. All rights reserved.

Sun L.,University of Texas at Dallas | Leybourne M.I.,Laurentian University | Rissmann C.F.W.,Environment Southland | Brikowski T.,University of Texas at Dallas
Geochemistry: Exploration, Environment, Analysis | Year: 2016

Depletion of oxygen in lakes and reservoirs due to summer stratification has significant effects on many aspects of water chemistry, including lake productivity, elemental cycling and water quality.We have studied the redox conditions of Lake Texoma, a large impoundment lake on the border of Texas and Oklahoma, USA, formed from the confluence of the Red and Washita rivers, in order to understand the impact of summer anoxia on metal distribution. In summer, dissolved oxygen decreases with depth (from c. 7.0 to 0.1 mg/L), whereas dissolved (<0.45 μm) Fe (and Fe2+), Mn and HS− concentrations show complementary increases in the hypolimnion. Summer anoxia is, to a large degree, responsible for vertical variations in Fe and Mn concentrations, and Fe speciation is controlled by Fe-oxyhydroxide reduction and subsequent pyrite precipitation in sulphide-rich bottom waters. Summer anoxia is also responsible for vertical variations in the concentration of other metals including Ba, Pb and Ni in the deepest portion (main lake) of the lake. Lake Texoma and its two river arms showrelatively minor variation in δ34SCDT (from +11.5 to +13.4‰), mirroring variation in Permian/Cretaceous marine gypsum and anhydrite deposits (δ34SCDT +10 to +15‰) in the headwater regions of the catchment. Increasing δ34SCDT with depth (>16 m) in the main lake is consistent with fractionation associated with sulphate reduction in anoxic bottom waters. δ13CPDB values of dissolved inorganic carbon (DIC) become more negative (from -2.5 to -8.2 ‰) with depth in summer, due to bacterial oxidation of organic matter linked to sulphate reduction. Summer anoxia may induce temporal degradation of water quality in the central three zones of the lake with elevated Fe and Mn concentrations owing to breakdown of oxyhydroxides and release of adsorbed heavy metals, such as Pb and Ni. However, complete turnover of the water column in the autumn lowers dissolved Fe, Mn and heavy metal concentrations by oxidation and formation of oxyhydroxides. The characterization of anoxia in Lake Texoma provides the background for further water quality research and management for the rapidly increasing population of North Texas and improves our understanding of redox cycling and metal mobility in reservoirs. © 2016 The Author(s).

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