Limnological assessment of Lake Puketirini and Lake Waahi, New Zealand.
IntroductionLake Waahi and Lake Puketirini are located next to each other, 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 lies in an agricultural catchment, which has a surface area of 93 km2 and is used for sheep and cattle production.
Lake Waahi originally formed behind levee banks of pumice carried down by the Waikato River, and received wastes from a coal carbonisation factory through inflow from the Awaroa Stream. Comparatively to Lake Waahi, Lake Puketirini is a relatively young lake. The area comprising Puketirini was drained and used for coal mining between 1929 and 1993, after which a project was instigated 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. It has a much smaller catchment than Lake Waahi and took seven years to fill.
This study employs a limnological approach by investigating the biological, chemical and physical features of both lakes. It aims to provide a detailed account of these features to determine the trophic states of each lake and any management strategies that may be required to improve or maintain water quality. Considering the histories and physical features of both lakes, it is expected that there would be distinct differences between their biological and chemical characteristics.Method Field methods Samples were collected on 4th August 2018 from Lake Puketirini and Lake Waahi (Figure 1), located outside the township of Huntly, New Zealand (Figure 2). Along with the samples collected, a number of on-site and laboratory experiments were carried out to gather a range of data concerning the lakes. Water clarity Water clarity was determined 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). Nutrient and chlorophyll ? concentrations25 to 50 mL of lake water was filtered using syringe filters. The water was contained and the filter removed from the syringe, folded in half with the upper side inner-most, wrapped in aluminium foil, labelled and placed in a plastic container.
In the laboratory, the water and the filter were frozen prior to analysis for nutrient and chlorophyll ? concentrations. Nutrient concentration were determined using flow injection analysis, while chlorophyll ? concentrations were determined through extraction of the chlorophyll pigment from the filters using a solvent (acetone) followed by spectrofluorometric analysis. Collection and identification of fish speciesFish species were collected, using both Fyke and Seine nets. Fyke nets, which were put in place a day prior to collection, were collected and fish transferred to a plastic tub. Fish were sedated, identified and counted before they were 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. Collection and identification of phytoplankton speciesA phytoplankton net (mesh size 40 µm) was dragged horizontally through the water column for 2-3 m. Samples collected by the net were transferred to honey pottles 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 a drawing was made of one of the species.
Collection and identification of zooplankton speciesA zooplankton net (mesh size 90 µm) was dragged horizontally through the water column for 2-3 m and samples were transferred to honey pottles. This process was repeated three times. Samples were preserved by filling the pottles 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 a drawing was made of one of the species. Collection and identification of macrophytes A range of submerged and emergent macrophytes were collected. In the laboratory, collected samples were identified using guides and recorded. The proportional cover of the lake shore with marginal vegetation was estimated. Collection and identification of macroinvertebrates Hand nets were used to collect macroinvertebrates from bottom sediment and around macrophytes. Samples were transferred to shallow plastic trays and macroinvertebrates picked out and placed into honey pottles. 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). ResultsSignificantly different concentrations of a variety of nutrients are recorded for the two lakes. Nutrient and chlorophyll ? concentrations recorded for Lake Waahi samples are all higher than those of Lake Puketirini samples (Table 1).Table 1. Nutrient data from Lake Puketirini and Lake Waahi. Sample site Total phosphorusmg/L Dissolved phosphorusmg/L Total nitrogen mg/L Ammoniummg/L Nitratemg/L Nitritemg/L Chlorophyll ?µg/LLake Puketirine 0.
033 0.005 0.269 0.017 0.015 0.004 2.
63Lake Waahi 0.067 0.011 1.272 0.218 0.026 0.
010 9.37Though physical measurements are relatively similar between the two lakes, there is a noticeable difference in the Secchi disc depth measurement (Table 2). The difference between the depths at which the Secchi disc disappeared in Lake Puketirini compared to Lake Waahi is 1.12 m, indicating a difference in clarity of water. These measurements are supported by the aerial photograph (Figure 1), which shows a distinct difference in colour of the two lakes. Table 2.
Physical measurements recorded for Lake Puketirini and Lake Waahi.Sample site TemperatureoC Dissolved O2% Dissolved O2mg/L pH Specific conductivityµS/cm Secchi disc depthmLake Puketirine 12.40 12.
86 115.0 8.05 259.0 1.
52Lake Waahi 12.23 13.20 126.10 8.29 245.0 0.4Furthermore, a difference is seen in the number and identities of species recorded from lakes Puketirini and Waahi (Table 3).
While the two lakes are home to some of the same species, a number of different species were recorded in only one of the two lakes. This does not imply that these specific species are not found in both lakes, but that they were only recorded in one on the day. When considering fish species, particularly those caught by the Fyke nets, 49 individuals were recorded across three different species in Lake Waahi. In comparison, only 4 individuals across the same three species were recorded from the Fyke nets in Lake Puketirini. Table 3. Species identified in Lake Puketirini and Lake Waahi.
Lake Puketirine Lake WaahiPhytoplankton Ceratium sp.Mykophyceae chroococcus Ceratium sp.Chaetophora sp.Peridinium sp.Zooplankton Daphnia galeata Copepoda: Cyclopoida CalanoidaKeratella sp. Daphnia galeata Copepoda: CyclopoidaBosmina sp.
Macro-invertebrates Gastropoda: Potamopyrgus sp. Physa sp. Gyraulus sp.Odonata: Zygoptera sp.Amphipoda: Paracalliope sp. Gastropoda: Potamopyrgus sp. Physa sp.
Odonata: Zygoptera sp. Anisoptera sp.Macrophytes Ceratophyllum demersumElodea canadensisTypha orientalisPotamogeton sp.Egeria densa Baumea articulataEleocharis sphacelate Ceratophyllum demersumElodea canadensisTypha orientalisPotamogeton sp.
Fish- Siene nets Gobiomorphus coditianus (8) Gobiomorphus coditianus (7)Gambusia affinis (1)Fish- Fyke nets Perca fluviatilis (1)Anguilla australis (1)Anguilla dieffenbachi (2) Perca fluviatilis (22)Anguilla australis (25)Anguilla dieffenbachi (2)Table 4. Total phosphorus, chlorophyll ? and secchi depth within Lake Taupo, Lake Puketirini and Lake Waahi. Retrieved from Waikato District Council (2009). Lake Taupo Lake Puketirini Lake WaahiTotal phosphorus (mg/m3) 6 12 80Chlorophyll ? (mg/m3) 1 3.5 74Secchi disc depth (m) 15 4 0.4DiscussionGreat variability exists between the biological, physical and chemical features of Lake Puketirini and Lake Waahi. Lake Puketirini is classified as mesotrophic, having an intermediate level of productivity, supported by a moderate amount of dissolved nutrients (Balvert, 2006).
The concentration of a variety of nutrients in Lake Puketirini is higher than that of Lake Taupo, which is oligotrophic and lower than that of Lake Waahi, which is supertrophic (Table 4). Compared to Lake Puketirini, Lake Waahi shows elevated levels of productivity, represented by the large number of fish caught in the Fyke net (Table 3), which is sustained by great amounts of dissolved nutrients. Lakes such as Waahi, which are classified as supertrophic, are saturated in phosphorus and nitrogen and often associated with poor water clarity and quality. Supertrophic lakes typically have poor ecosystems due to a decreased level of dissolved oxygen, which does not support great biodiversity. Severe oxygen depletion in the bottom water of such lakes largely confines benthic and weed-dwelling invertebrates to the littoral zone. Furthermore, excessive phytoplankton growth is known to occur in these lakes, especially during summer time. Chapman (1980) reports on the summer limnology of Lake Waahi, describing an algal blood that reduced Secchi transparencies to as little as 20 cm. Algal blooms are the result of excess nutrients, often caused by the runoff of fertilizer from agricultural land.
During an algal bloom, the decay process consumes dissolved oxygen, often leading to a large number of deaths in both plant and animal species. Chapman (1980) notes that Lake Waahi is not an isolated case, but rather a very typical example of New Zealand’s major eutrophic problems. She highlights the main cause of Lake Waahi’s eutrophication as the agricultural development of its catchment. Along with algal blooms, this eutrophication has resulted in the rampant growth of exotic macrophytes, depriving locals of a significant recreational resource.
Comparatively, Lake Puketirini is unique among Waikato lakes due to its high water quality. Increased secchi transparency compared to Lake Waahi indicates a higher water clarity, which is due to its relatively low concentrations of phosphorus and nitrogen (Table 2). The extreme depth of the lake compared to its small surface area restricts mixing of the water column. Consequently, bottom waters (below 20 m) may be semi-permanently devoid of dissolved oxygen, restricting plant and animal life from this zone.
The Waikato District Council (2009) comments on the probability of water quality declining as the lake comes to equilibrium and contaminants from the bottom sediments are released into overlaying water. Management projects are a vital part of maintaining the water quality of Lake Puketirini and improving that of Lake Waahi. The Waikato District Council (2009) notes that limiting the inflow of nutrients into Lake Puketirini is a primary task in the management of its water quality. The council emphasises that nutrient inflow, particularly non-point run-off from land use within the lake’s catchment area, must to be minimised. If not managed, elevated levels of nutrients will increase the level of algae within the lake, potentially decreasing the suitability of the lake for contact recreational activities. Hence, the district council describes a management plan with a vision to protect the health and well-being of Lake Puketirini and to develop Lake Puketirini for community use by working in partnership with landowners around the lake area on matters that may affect the lake health. The district council also emphasises the control and elimination of pest fish species, such as Cyprinus carpio (koi carp), which destroy lake vegetation and release nutrients into the waters.
The absence of this species in our data may be an indication of their control in recent years (Table 3). Finally, the council also suggests the creation of suitable habitats for wildlife through the development of wetland areas and a general increase in vegetation around Lake Puketirini. Lake Waahi, along with the 71 other ‘shallow lakes’ (max depth of