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Lake Waahi and Lake Puketirini are neighboring lakes situated near the township of Huntly, New Zealand. Lake Waahi is a small, shallow lake with a maximum depth of 5 m and a surface area of 5 km2. It is located in an agricultural catchment covering 93 km2 primarily used for sheep and cattle production. Lake Waahi originated behind levee banks of pumice carried downstream by the Waikato River and received wastes from a coal carbonisation factory through inflow from the Awaroa Stream.
In contrast, Lake Puketirini is a relatively young lake.
The Puketirini area was drained and used for coal mining between 1929 and 1993. Subsequently, a project was initiated to rehabilitate the disused mine into a lake. Lake Puketirini has an approximate depth of 64 m and a surface area of 0.5 km2. Its catchment is significantly smaller than that of Lake Waahi, and it took seven years to fill. This study employs a limnological approach to investigate the biological, chemical, and physical features of both lakes.
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The goal is to provide a comprehensive account of these features to determine the trophic states of each lake and identify necessary management strategies to improve or maintain water quality. Given the distinct histories and physical characteristics of the two lakes, significant differences in several studied features are expected.
Samples were collected on 4th August 2018 from Lake Puketirini and Lake Waahi, located outside the township of Huntly, New Zealand. In addition to sample collection, several on-site and laboratory experiments were conducted to gather various data about the lakes.
Water clarity was assessed using a Secchi disc.
The depth at which the disc disappeared was recorded. The depth of the eutrophic zone (Zeu), where light is 1% of the surface, was calculated using the following equation:
Zeu = 2.5 x Secchi
Between 25 to 50 mL of lake water was filtered using syringe filters. The water was collected, and the filter was removed from the syringe, folded in half, wrapped in aluminum foil, labeled, and placed in a plastic container. In the laboratory, the water and the filter were frozen for subsequent analysis of nutrient and chlorophyll α concentrations. Nutrient concentrations were determined using flow injection analysis, while chlorophyll α concentrations were determined through the extraction of chlorophyll pigment from the filters using acetone, followed by spectrofluorometric analysis.
Fish species were collected using both fyke and seine nets. Fyke nets, which were set a day before collection, were retrieved, and fish were transferred to a plastic tub. Fish were sedated, identified, counted, and then returned to their respective lakes. Seine nets were used to collect smaller fish species from the littoral (near-shore) zone. Similarly, fish were transferred to plastic tubs, identified, counted, and then returned to the lakes.
A phytoplankton net (mesh size 40 µm) was horizontally dragged through the water column for 2-3 meters. Samples collected by the net were transferred to honey bottles and preserved with 4-5 drops of Lugol’s iodine. In the laboratory, water from the phytoplankton samples was examined using a light microscope. Dominant phytoplankton species were determined using taxonomic guides, and drawings were made for one of the species.
A zooplankton net (mesh size 90 µm) was horizontally dragged through the water column for 2-3 meters, and samples were transferred to honey bottles. This process was repeated three times. Samples were preserved by filling the bottles with 70% ethanol. In the laboratory, water from the zooplankton samples was transferred onto a glass vessel and examined using a dissecting microscope. Dominant classes and genera of zooplankton were identified using taxonomic guides, and drawings were made for one of the species.
A variety of submerged and emergent macrophytes were collected. In the laboratory, collected samples were identified using guides and recorded. The proportional cover of the lakeshore with marginal vegetation was estimated.
Hand nets were employed to collect macroinvertebrates from the bottom sediment and around macrophytes. Samples were transferred to shallow plastic trays, and macroinvertebrates were picked out and placed into honey bottles. These were preserved by filling the containers with 70% ethanol. In the laboratory, macroinvertebrates were identified using low-magnification microscopy and taxonomic guides.
Physical measurements, including water temperature, dissolved oxygen, conductivity, and pH, were also taken using the YSI Sonde, a multi-parameter water quality probe.
Significant differences in the concentrations of various nutrients were recorded for the two lakes. Nutrient and chlorophyll α concentrations obtained from Lake Waahi samples were consistently higher than those from Lake Puketirini samples (see Table 1).
| Sample Site | Total Phosphorus (mg/L) | Dissolved Phosphorus (mg/L) | Total Nitrogen (mg/L) | Ammonium (mg/L) | Nitrate (mg/L) | Nitrite (mg/L) | Chlorophyll α (µg/L) |
|---|---|---|---|---|---|---|---|
| Lake Puketirini | 0.033 | 0.005 | 0.269 | 0.017 | 0.015 | 0.004 | 2.63 |
| Lake Waahi | 0.067 | 0.011 | 1.272 | 0.218 | 0.026 | 0.010 | 9.37 |
Physical measurements between the two lakes were relatively similar, except for the Secchi disc depth measurement. Lake Puketirini exhibited a Secchi disc depth 1.12 meters deeper than Lake Waahi, indicating differences in water clarity. Aerial photographs (see Figure 1) further corroborate these findings by revealing distinct color variations between the two lakes.
| Sample Site | Temperature (°C) | Dissolved O2 (%) | Dissolved O2 (mg/L) | pH | Specific Conductivity (µS/cm) | Secchi Disc Depth (m) |
|---|---|---|---|---|---|---|
| Lake Puketirini | 12.40 | 12.86 | 115.0 | 8.05 | 259.0 | 1.52 |
| Lake Waahi | 12.23 | 13.20 | 126.10 | 8.29 | 245.0 | 0.4 |
Furthermore, a discrepancy exists in the number and identities of species recorded in Lakes Puketirini and Waahi. Although both lakes share some species, several different species were recorded in only one of the two lakes. This does not necessarily indicate that these specific species do not exist in both lakes but rather that they were only recorded in one lake on the day of sampling. When examining fish species, particularly those captured by the Fyke nets, 49 individuals belonging to three different species were recorded in Lake Waahi. In contrast, only 4 individuals from the same three species were recorded from the Fyke nets in Lake Puketirini.
| Species | Lake Puketirini | Lake Waahi |
|---|---|---|
| Phytoplankton | Ceratium Sp. | Ceratium Sp. |
| Mykophyceae | uhroococcus | |
| Chaetophora Sp. | ||
| Dinophyta peridinium | ||
| Zooplankton | Daphnia galeata | Daphnia galeata |
| Cyclopoiid copepod | Cyclopoiid copepod | |
| Calanoid copepod | Bosmina Sp. | |
| Keratella Sp. | ||
| Macro-invertebrates | Gastropoda potamopyrgus | Gastropoda potamopyrgus |
| Gastropoda physa | Gastropoda physa | |
| Gastropoda gyraulus | ||
| Odonata Zygoptera | Odonata anisoptera | |
| Amphipoda paracalliope | ||
| Macrophytes | Ceratophyllum demersum | Typha orientalis |
| Elodea canadensis | Potamogeton Sp. | |
| Egeria densa | ||
| Baumea articulate | ||
| Eleocharis sphacelate |
| Species | Lake Puketirini | Lake Waahi |
|---|---|---|
| Fish- Seine nets | Gobiomorphus coditianus (8) | Gobiomorphus coditianus (7) |
| Gambusia affinis (1) | ||
| Fish- Fyke nets | Perca fluviatilis (1) | Perca fluviatilis (22) |
| Anguilla australis (1) | Anguilla australis (25) | |
| Anguilla dieffenbachia (2) | Anguilla dieffenbachia (2) |
Table 4 displays total phosphorus, chlor ophyll α, and Secchi disc depth data for Lake Taupo, Lake Puketirini, and Lake Waahi, retrieved from the Waikato District Council (2009).
| Lake | Total Phosphorus (mg/m3) | Chlorophyll α (mg/m3) | Secchi Disc Depth (m) |
|---|---|---|---|
| Lake Taupo | 6 | 1 | 15 |
| Lake Puketirini | 12 | 3.5 | 4 |
| Lake Waahi | 80 | 74 | 0.4 |
Considerable variability exists among the biological, physical, and chemical features of Lake Puketirini and Lake Waahi. Lake Puketirini is classified as mesotrophic, indicating an intermediate level of productivity supported by a moderate amount of dissolved nutrients (Balvert, 2006). Nutrient concentrations in Lake Puketirini are higher than those in oligotrophic Lake Taupo but lower than those in supertrophic Lake Waahi (Table 4). Lake Waahi exhibits elevated productivity, evident from the large number of fish caught in the Fyke nets (Table 3), and is sustained by substantial dissolved nutrient levels.
Lakes classified as supertrophic, like Lake Waahi, are saturated with phosphorus and nitrogen, often leading to poor water quality and clarity. These lakes typically suffer from reduced dissolved oxygen levels, which hinders biodiversity. Severe oxygen depletion in the bottom water restricts benthic and weed-dwelling invertebrates to the littoral zone. Additionally, excessive phytoplankton growth is a common issue in supertrophic lakes, especially during summer. Chapman (1980) reports on the summer limnology of Waahi, describing algal blooms that reduced Secchi transparencies to as little as 20 cm. Algal blooms result from excess nutrients, often originating from runoff of agricultural fertilizers. During an algal bloom, the decay process consumes dissolved oxygen, leading to mass mortality among plant and animal species. Chapman (1980) notes that Waahi's eutrophication is a typical example of New Zealand's major eutrophic problems, with agricultural development of its catchment being the main cause.
Conversely, Lake Puketirini stands out among Waikato lakes for its high water quality. Its increased secchi transparency compared to Lake Waahi indicates superior water clarity, attributed to its relatively lower concentrations of phosphorus and nitrogen (Table 2). The lake's significant depth compared to its surface area limits vertical mixing of the water column. Consequently, bottom waters (below 20 m) may lack dissolved oxygen, restricting plant and animal life in that zone. The Waikato District Council (2009) highlights the possibility of declining water quality as the lake reaches equilibrium and contaminants from bottom sediments are released into the overlying water.
Management projects are crucial for maintaining the water quality of Lake Puketirini and improving that of Lake Waahi. The Waikato District Council (2009) emphasizes the importance of limiting nutrient inflow into Lake Puketirini. Controlling nutrient inflow, especially non-point runoff from land within the lake's catchment area, is vital. Unmanaged, elevated nutrient levels can increase algae within the lake, potentially diminishing its suitability for recreational activities. Thus, the district council outlines a management plan with the vision of protecting Lake Puketirini's health and well-being while developing it for community use through collaboration with landowners around the lake area.
The council also stresses the importance of controlling and eliminating pest fish species, such as Cyprinus carpio (koi carp), which damage lake vegetation and release nutrients into the waters. The absence of this species in our data may indicate recent control efforts (Table 3). Additionally, the council recommends creating suitable wildlife habitats through the development of wetland areas and increasing vegetation around Lake Puketirini.
Lake Waahi, along with 71 other "shallow lakes" in the Waikato region, faces similar challenges in maintaining water quality and ecological balance. Given the significant differences between Lake Puketirini and Lake Waahi in terms of nutrient levels, trophic state, and biodiversity, management strategies are likely to differ for each lake. Nonetheless, a common strategy highlighted by regional and district councils is the reduction of nutrient inflow. The high nutrient concentration in Lake Waahi poses a significant threat to Lake Puketirini if not managed effectively. The presence of numerous privately owned properties surrounding the lake emphasizes the need for collaboration between government authorities and landowners to implement and execute management plans.
The Waikato region in New Zealand boasts numerous lakes with varying histories and characteristics. These lakes' water quality and clarity are influenced by various human activities, including agriculture and the introduction of invasive species. Lake Puketirini and Lake Waahi, located near Huntly, exhibit significant differences in appearance, as well as biological, chemical, and physical features. While Lake Puketirini demonstrates superior water quality compared to Lake Waahi, both lakes require management strategies to ensure their continued well-being.
Due to the distinct physical and chemical differences between the two lakes, management approaches are likely to vary. However, a shared emphasis on reducing nutrient inflow is essential. The high nutrient concentration in Lake Waahi poses a substantial risk to Lake Puketirini, making effective management crucial. Collaboration between government entities and landowners is essential to implement and execute these management plans successfully.
Limnological Assessment of Lake Puketirini and Lake Waahi, New Zealand. (2024, Jan 02). Retrieved from https://studymoose.com/document/limnological-assessment-of-lake-puketirini-and-lake-waahi-new-zealand
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