Research into the light climate of a large, shallow, hypereutrophic lake in North Waikato (June 2005)
Group or Institution
University of Waikato
Author(s)
Alex Hopkins
Abstract
Introduction
Many lakes in the Waikato region are suffering the effects of cultural eutrophication. An increase in nutrients from the land catchment ultimately leads to a decline in aquatic plants that impact on the aesthetic and cultural values of the lakes. Lake Waikare, near Te Kauwhata in North Waikato, is a large, shallow lake, affected in this way. With the loss of aquatic plants, sediments are stirred from the lake bottom during wind events and become suspended in the water column. Such sediments reduce the amount of light able to penetrate through the water column, preventing the re-establishment of plant growth that would stabilize the lake bed and reduce re-suspension. This study will focus on examining potential methods for reducing the re-suspension of lake-bottom sediments in Lake Waikare, with the ultimate aim of being a potential method for improving the lakes’ light climate, water quality, and allowing aquatic plant regrowth.
Methods
Large, shallow eutrophic lakes require novel approaches to facilitate their sustainable restoration. Lake-water drawdown to consolidate these sediments has been deemed unsuitable in this lake without the use of other complimentary methods. I will therefore examine alternative methods. I will examine the potential use of alum (Aluminium Sulphate) to flocculate (bind) the sediments in the water column together. By increasing the sediment particle sizes through flocculation, these sediments should settle from the water column more quickly, and subsequent re-suspension of particles should require greater force (i.e., higher winds). Doing so will allow a greater quantity and quality of light to pass through the water column which is required for plant growth. This hypothesis will be tested. I will also investigate whether bio-manipulation, an altering of the current fish species abundance, will be required for plants to re-establish. A mix of native and introduced fish currently resides in Lake Waikare that may both disturb, or consume plant growth. Results of a study from Lake Rotoroa (Hamilton Lake), where fish were unable to access plants housed inside cages, have shown fish exclusion will allow plants to grow in algae-dominated lake.
The framework used by this research will be the phenomenon of alternate stable states and lake restoration. Large shallow lakes can assume one of two alternate stable states: either 1) algae and sediment-, or; 2) plant-dominated. Both states may be extremely stable over time. As such, great effort is required to return lakes to a state which is plant dominated with high water quality, if it has been changed to an alternative state. Algal dominated lakes (e.g., Lake Waikare) are characterized by poor water quality due to high concentrations of nutrients, plankton (algae) and sediments. Aquatic plant growth depends on sunlight reaching bottom waters for growth. Sediments and algae in the water column reduce the amount of light that can reach the aquatic plants, leading to their decline. With the loss of aquatic plants, sediments are stirred from the lake bottom during wind events and become suspended in the water column. Such sediments reduce the amount of light able to penetrate through the water column, preventing the re-establishment of plant growth that would stabilize the lake bed and reduce re-suspension. The current high nutrients, sediment and algae dominated nature of Lake Waikare has been due to human influences in the catchments beginning in the 1940’s, culminating in a collapse of the aquatic plants in the late 1970’s. This study will focus on examining potential methods for reducing the re-suspension of lake-bottom sediments in Lake Waikare, with the ultimate aim of being a potential method for improving the lakes’ light climate, water quality, and allowing aquatic plant growth.
Experimental Design
The exact nature of the experiments for Alum flocculation trials are yet to be determined. However, initial small scale laboratory experiments will be run on suspended sediments collected from Lake Waikare to determine the concentrations of Alum required to effectively settle sediments from the water column. Once this has been determined, larger scale replicated experiments will be run to compare the settlement rates of sediments in large tubs that have or have not been treated with Alum. Sediments in the tubs will be suspended by stirring prior to Alum addition. Sediment concentrations in the water column will be determined over time by removing known volume water samples from different depths in the tubs, which will be passed through fine glass fibre filters. These filters will be oven dried and weighed to compare sediment loads treatments and different depths. Light concentrations will also be compared down the water column using automated meters and penetration using a Secchi disk. Specifically, greater sediment settlement rates in tubs with Alum will be tested for. Sediments in tubs will be resuspended at several subsequent time intervals to assess the longevity of the effectiveness of Alum treatment, and to assess if a single dosage is adequate or whether re-applications will be required. Sediment and light conditions in tubs will be compared using t-tests or ANOVA depending on the number of treatments.
The effects of fish on plant re-establishment will be carried out using similar methods as used by de Winton et al. (2002) for Lake Rotoroa (Hamilton Lake). Native aquatic plants (charophytes) will be germinated in pots and grown in the laboratory. Plants will be selected and placed in the pots (in Lake Waikare) at two sites that differ in their exposure to the wind (an exposed site and one in a sheltered embayment). At each site, pots will be placed inside and outside mesh cages designed to exclude fish at depths of approximately 1 m. After three weeks, plants will be retrieved from the exclosures in the lake and biomass (dry weight) will be compared between the control (outside) and caged treatments. If biomass in the uncaged treatments is significantly lower than those within the caged treatments, this will indicate that local fish populations are responsible for preventing plant re-establishment rather than light climate. Exclosures will be cleared of debris and algae through the three week duration of experiments to ensure light reaches plants inside and outside of plants similarly. Similar experiments will be run on un-germinated oospores (resting stages) of charophytes, to determine whether the proportion of plants germinating differs in the absence or presence of fish. All experiments will be run in different seasons.