Physico‐chemical features of lake Ototoa,a sand‐dune lake in northern New Zealand

Lake Ototoa is a warm monomictic lake at 36° 31’ S, 174° 14'E. During a year's study (March 1969‐March 1970), the lake became thermally stratified in November, the metalimnion being between depths of 12 m and 16 m. Surface temperatures ranged between 10.2°c (in August) and 25.2°c (in late January), and bottom temperatures between 9.7°c and 17.5°c. The annual heat budget was calculated to be 642 354 KJ.m^‐2 (15 500 cal.cm^‐2) and the work of the wind in distributing the heat income 1.730 KJ.m^‐2 (1766 g.cm.cm^‐2). Secchi disc transparencies ranged between 5 m and 9.2 m (mean 7.07 m) and were greatest in the summer. Light transmission per metre was also high, ranging between 61% and 87%. Surface waters were normally supersaturated with oxygen, but during summer stratification oxygen concentrations in the bottom waters dropped to a minimum of 2.3 mg.litre^‐2 and a positive heterograde distribution of oxygen with depth was found. The oxygen deficit was 0.015 mg.cm^‐2.day^‐1 and showed the lake to be oligotrophic. Mean surface pH was 7.82, and the ionic composition of the waters was similar to that of other small New Zealand and Australian lakes located near the sea. Compared with other New Zealand lakes PO4‐P concentrations (range 1.00–10.20 μg.litre^‐1) were low and NO3—N concentrations (range 0.12–0.60 mg.litre^‐1) high.     

Physico‐chemical features of lake Pupuke,Auckland

Summary

Lake Pupuke, situated in the North Island of New Zealand near Auckland (Latitude 36° 53'S, Longitude 174° 52'E), is a small, circular body of water (maximum width 1.2 km, maximum depth 55 m). From November 1966 to November 1967 observations on its physico‐chemical factors were made twice monthly.

The range of temperatures in the year was 12.0–24.2°c at the surface and 11.3–14.5°c at 40 m. A thermocline farmed between early October and early January, when it became strongly developed at about 17 m and persisted until late April. Between early May and late June the stratification was breaking down and between mid July and mid September the lake was isothermal.

The values for light transmitted per metre through the water were high from mid August to mid December (53–76% per metre), very low from mid December until mid February (23–49% per metre), high in March (75% per metre) and relatively constant over the winter months (53–64% per metre). The amount of light transmitted depended closely on the density of phytoplankton. Changes in the depth at which a Secchi disc disappeared paralleled the light transmission values, and the mean Secchi depth was 2.2 m, range 1.0–5.2 m.

Oxygen concentrations down to 30 m were highest at all depths from mid November to early January (112–134% saturation). From early January to late April oxygen was strongly stratified (epilimnion 55–100% saturation, hypolimnion 24–77% saturation). During winter oxygen values gradually increased at all depths.

The range of pH was 7.8–9.5 units at the surface and 7.8–8.6 units at 30 m. From early January to late April pH values were strongly stratified but from mid July to mid August they were nearly uniform. The range of alkalinity (mainly HCO₃ ^? ions) was 76.0–80.8 ppm CaCO₃ and values showed stratification in summer. Carbonate ion was present from early October to mid June, but free carbon dioxide was only found in trace amounts.

The range of nitrate ion concentrations was 0.05–0.31 ppm; nitrate became obviously depleted in summer. Only one nitrite value was significant, 0.25 ppm. other nitrite values never exceeded 0.10 ppm. The concentration of ammoniacal nitrogen built up to a summer maximum of 0.95 ppm in the hypolimnion (annual range 0–0.95 ppm); values for albuminoid ammonia (0.016–1.03 ppm of nitrogen) varied with the quantity of living matter present.

The annual mean for total ion concentration (3.30 meq./1) was high and the order of ions was Na⁺>Mg²⁺>Ca²⁺>K⁺ and HCO₃ ^?>CI^?>SO₄ ^2?. Such dissolved salts may have arrived as airborne particles, dissolved in rain, or as sea spray.     

Processing RoxAnn sonar data to improve its categorization of lake bed surficial substrates†

To categorize spawning and nursery habitat for lake trout in Minnesota's near shore waters of Lake Superior, data was collected with a single beam echo sounder coupled with a RoxAnn bottom classification sensor. Test areas representative of the different bottom surficial substrates were sampled. The collected data consisted of acoustic signals which showed both depth and substrate type. The location of the signals was tagged in real-time with a DGPS. All data was imported into a GIS database. To better interpret the output signal from the RoxAnn, several pattern classifiers were developed by multivariate statistical method. From the data a detailed and accurate map of lake bed bathymetry and surficial substrate types was produced. This map will be of great value to fishery and other natural resource managers.     

Numerical analysis of the sea (lake) breeze dynamics and initialization of the model

In this paper, analysed are the effects of synoptic wind, earth-rotation inertia and land surface roughness on sea (lake) breeze process, on the basis of the calculated results of a 2-D primitive equation model with turbulent energy closure. The results show that a moderate background wind field acts as a trigger for sea (lake) breeze onset, and presents prominent effect on the breeze intensity, inland advance rate and structure feature. The effects of Coriolis force not only make the breeze veering round, but also damp the development of the breeze component normal to shore.The paper also discussed the dynamic method of wind field initialization based on one-site radiosonde. Three approaches of initialization have been tested. In comparison, it was found that the approach of one-dimensional dynamic initialization with nudging term was preferable to the others.     

Evidence for seiches and short‐period internal waves in lake Tekapo,South Island,New Zealand

Vertical temperature profiles obtained in April 1971 along the length of Lake Tekapo suggest the presence of an internal seiche. A simple model gives a theoretical period of 48 h. Successive vertical temperature profiles made at 5‐min intervals over 70 min showed in‐phase fluctuations in the isotherms both above and below the thermocline. The fluctuations had an average period of 12.7 min and could have been generated by a surface sieche operating between the southern end of the lake and Motuariki Island, situated about 7.6 km from the southern end.     

Seasonal variation in phytoplankton from lake rotorua and other inland waters,New Zealand, 1966–67

Between, May 1966 and May 1967, 110 preserved samples of the phytoplankton of Lake Rotorua, New Zealand, were examined. Species and cell counts have been compared with data from Lakes Rotoiti (21 samples) and Ohakuri (36 net samples). Brief comparisons have also been made with the flora of two of the Wellington City reservoirs and the oxidation ponds at Mangere, Auckland. In Lake Rotorua the autumn, maximum was more pronounced than the spring maximum. Cell numbers inshore at Kawaha Point were consistently lower than those further out in the open lake. The phytoplankton is classified as a eutrophic formation of the diatom type, with Chlorophyceae as main subordinates. Dominants include Melosira granulata Ralfs (winter, spring, and autumn), Melosira distans (Ehr.) Kütz. (spring, summer, and autumn), and Asterionella formosa Hass. (summer and autumn).     

A temperate,tidal lake‐wetland complex 2. Water quality and implications for zooplankton community structure

Lakes Waihola and Waipori are shallow, coastal, tidal lakes that experience wind‐induced sediment resuspension, saline intrusions, and high inputs of nutrients. To determine the influence of externally‐driven, physical factors on spatial and temporal patterns of water quality in the two lakes, meteorological, hydrological, and water quality data were collected over 1 year. Multivariate analyses indicated that wind energy was driving the main water quality gradient in the lakes, which was primarily related to wind‐induced resuspension of lake sediments. The major, seasonally regulated, non‐tidal freshwater inflow was important in determining nutrient and salinity gradients in the lakes. The main nutrient inputs to the system were identified as the regulated, non‐tidal inflow (the upper Waipori River) and the tidal inflow (lower Waipori River). The impact of water quality gradients on zooplankton community structure in the lakes was assessed by canonical multivariate methods. Salinity gradients, caused by seasonal saltwater intrusions, were strongly related to zooplankton community structure in the lakes. Nutrient gradients (indicative of trophic state) were also related to zooplankton community structure. Although wind‐induced sediment resuspension had the largest impact on water quality in the lakes, it had little impact on the zooplankton community structure in either lake. The relationships between water quality and zooplankton community structure were confounded in Lake Waipori because of its very short hydraulic residence time (annual mean =1.9 days). Zooplankton community structure was resilient to short‐term changes in suspended particulate matter concentrations but not to seasonal changes in salinity. The results of this study support others which have shown impacts of even relatively small variations in salinity on the structure of zooplankton communities. This highlights the vulnerability of zooplankton communities in coastal lakes and wetlands to increasing salinity resulting from sea level rise and global climate change.     

Historic developments in macrozoobenthos of the Rhine–Meuse estuary: From a tidal inlet to a freshwater lake

Water works during the 1960s and 1970s changed the northern part of the Rhine–Meuse estuary in the south-west of the Netherlands into a freshwater lake, from west to east divided into three basins called the Haringvliet, the Hollands Diep and the Biesbosch. Concurrently water quality parameters (e.g. nutrients and pollutants) changed drastically during the last 50 years. This study combines macrozoobenthic monitoring data from the region from 1960 to 2001 with trends in abiotic parameters to evaluate historic developments of the communities, including densities, species numbers and diversity, and assess future developments as a first step to a rehabilitation of the estuary as planned for January 01, 2008. During the 1960s, the macrozoobenthic densities of Oligochaeta and/or Polychaeta dominated communities increased with a gradual decrease of saltwater intrusion and salinity variability. The first years after the basins became stagnant, the species numbers per sample and the Shannon diversity were high due to the coexistence of salt and freshwater species. An increase in nutrient and pollutant loads led to a decrease in the macrozoobenthos densities. As water and sediment quality gradually improved, nowadays the former estuary contains high diversity and high density macrozoobenthic communities, whereas Oligochaeta and/or Polychaeta were dominant in the 1960s, and Bivalvia and Gastropoda were more abundant during the 1970s. Macrozoobenthic communities moved from the east to west with a time-lag, which may primarily be attributed to changing salinities, salinity variances and oxygen levels. Therefore, the current communities of the Haringvliet show similarities with the communities that occurred already during the 1960s in the Biesbosch. This study shows the value of macrozoobenthos monitoring data over longer periods. The possible impact of a new saltwater inlet in the west of the Haringvliet, allowing in the near future saltwater to enter 11.5 km eastward, yet alternated by frequent flushing with freshwater to ascertain that the salt intrusion does not reach further, on the development of the macrozoobentic communities is discussed.     

Seasonal variance of salinity tolerance of a copepod Drepanopus bispinosus in an Antarctic littoral saline lake (Burton Lake)

In Burton Lake (an Antarctic littoral saline lake), as one of the overwinter species, the female Drepanopus bispinosus reach adult form in autumn. and early winter.For the subsequent life period of more than eight months, the animals experience such changing environment as increasing salinities from approximately 33 (in May) to 39 (in November). There is a considerable difference of salinity tolerance of female adults between summer and winter populations.Winter collected copepods survived lower salinities than summer collected copepods in this experiment. The upward shift in their salinity tolerance range is related to the development of field acclimation to salinity. Respiration rates of the summer animals showed a visible increase over those of winter copepods in simitar salinity and temperature conditions, thus supporting the above conclusion.     

Effect of hypersaline and low lake conditions on ecological functioning of St Lucia estuarine system,South Africa: An overview 2002–2008

Lake St Lucia, the largest estuarine lake in Africa has been subjected to hypersaline conditions and low lake levels over the past eight years following the closure of its mouth due to drought in the region. This paper documents the physical changes through which the lake has passed and summarises the main findings of research undertaken on the three major biotic components that have been subjected to these conditions. A review of the anthropogenic impacts which have affected the system is provided. These indicate that in combination with drought conditions greater pressure is placed on the system that was the case in the historical past. Available data indicate that the current situation is not only impacting on the lake and its fauna but also on the adjacent nearshore marine environment, It is considered that the Meta area is potentially also under threat. Medium to long term relief possibilities that are under consideration are discussed in relation to the restructuring of something resembling the historically combined uMfolozi–St Lucia ecosystem that existed in the past. It is concluded that in the short term only two options are available to potentially provide relief for the system, the first is to breach the connection between the mouth and the sea. The second is to re-establish some form of more permanent connection, between uMfolozi and St Lucia.