Lake Pupuke

Seasonal Stratification

The temperature throughout the lake is relatively stable from July to September with surface water temperatures less than 3 °C warmer than the bottom water.

In October the lake begins to stratify into three distinct thermal layers: the warm epilimnion on the surface generally reflected the ambient temperatures, the metalimnion/thermocline where the temperature drops rapidly and the hypolimnion which is the colder stable bottom water (this layer remains at approximately 12 °C all year).

Algal Blooms

The formation of the algal blooms coincided with the stratification of the lake. Early signs of algal bloom formation are often seen during September – October and by November the lake is usually in full bloom.

These blooms are a result of eutrophication, lack of competition for nutrients and alterations in natural trophic chains.

When these algal blooms die over winter they settle to the bottom of the lake and decompose. This creates anoxia and releases more nutrients into the water column, further fueling subsequent blooms. As a result, the blooms occur annually and contribute to the wider environmental issues at the lake.

Visual Clarity

The visual clarity changes dramatically with the seasons and at various depths. During the colder months (May - August) the surface visibility (0 - 10 m deep) ranges on average from 4 to 6 m of horizontal visibility, sometimes reaching up to 10 m, depending on the amount of rain in the preceding days. The visibility at depth deteriorates by 1 - 2 m from 10 m deep down to a depth of 18 m, this is largely due to the amount of suspended organic matter in the water. Once you pass 25 m the water clarity increases to 10 m or more with very little suspended matter.

Once the lake stratifies and the algal blooms form, the surface visibility (0 - 5 m deep) ranges from 0 to 0.5 m. The water clarity improves slightly from 6 - 10 m deep and the visibility ranges from 1 - 3 m. Surprisingly, once you pass the thermocline the water clarity increases to 10 m or more however, it is pitch black as the surface algal blooms block out all light.

Anoxia

We have been monitoring dissolved oxygen (DO%) in the lake since the establishment of the project and have found that anoxia (dead zones) occur throughout the lake all year round.

Anoxia is important to track as it effects all aspects of lake health. Without oxygen plants and fish die, anoxia also increases nutrient remobilizations from sediment which further fuels algal blooms.

This lake stratifies over summer and the deep bottom water (hypolimnion) is anoxic for almost 6 months of the year. We have found that the anoxia in the lower hypolimnion persists almost all year long and increases in volume from October to April.

We discovered that there is a mid-water layer of anoxic water that regularly forms over summer. This layer has less than 3% dissolved oxygen and starts at 8 - 12 m deep, it extends to a depth of 14 - 18 m. We assume that this layer is caused by algal decomposition in the metalimnion. The dead algal blooms are trapped above the thermocline as a result of varying densities of water (warm epilimnion and cold hypolimnion) so they decompose mid-water.

We monitored anoxia in the macrophyte beds and at the sediment water interface and found that the DO% in the macrophytes regularly dropped to below 5%, the water just above the sediment was almost fully anoxic in/around the macrophyte beds and in deeper water. These anaerobic conditions are caused by the decomposition of dead plant matter and phytoplankton that settle on the lake bed. The dense plant beds stem flow and create stagnant areas which further promote anoxia. This anoxia encourages the growth of benthic cyanobacteria which is potentially harmful to humans.

We now know that anoxia is a key issue in the lake, and we have identified the areas that are of most concern.

Hydrogen Sulphide Layer

In June 2019 we discovered a mid-water hydrogen sulphide layer across the entire lake that started at 11 m and extended down to approximately 18 m.

This had never been recorded before and is still the only footage that exists of such a layer in NZ.

We tracked this layer for almost 3 months in 2019 and it has not returned in high volumes since. We took various water quality and phytoplankton samples from the layer. Our hypothesis is that as the algal blooms die and settle on top of the thermocline they decompose and create anoxic conditions (we confirmed this through organic matter traps at various depths). Bacteria thrive under these conditions and feed off the dead algae, hydrogen sulphide is a by-product of this anaerobic metabolic activity.

The hydrogen sulphide is trapped above the dense hypolimnion and forms a mid-water layer. This layer is acidic and anoxic which makes it uninhabitable by aquatic life.

The elevated nutrient concentrations, high algal biomass, strong thermal stratification, and anoxic zones all contribute to the formation of this acrid layer.

Sediment

A crucial part of the lake is the bed and sediment. We had done extensive water quality and ecology assessments and the sediment was the last pieces of the puzzle.

We started by identifying areas/depths that were anoxic and had high amounts of organic silt deposition. We took several seasonal sediment cores from various depths and locations and tested them for the following: general sediment chemistry, nutrient concentrations, organic content, heavy metal concentrations and the rate of nutrient remobilizations across a DO% and pH gradient.

We found that the shallower cores (3 - 8 m depth) and the deep cores (30 - 57 m depth) had the highest concentration of organic material and nutrients. The intermediate cores (14 - 20 m depth) had the least amount of nutrients and organic material. This in combination with the suspended matter traps confirmed that organic matter settles in the flat shallow zones and the deep bowl of the lake. The intermediate depths presumably do not accumulate as much organic matter due to their steep slopes.

The sediment released significant concentrations of nutrients under high pH and anoxic conditions. We know that these conditions exist widely across the lake which means that a sizable portion of the dissolved nutrients in the lake is likely being remobilized from the sediment.

This internal loading occurs across the areas in the lake with the largest surface area, shallow euphotic zone and the deep bowl. As a result, we confirmed that eutrophication in the lake is primarily driven by internal nutrient loading rather than inputs from the surrounding catchment.

Macrophytes

The macrophytes in the lake are dominated by invasive species however, these plants still perform a vital ecological function. They provide habitat/shelter for native fish species and assimilate nutrients.

These macrophytes are the only thing in the lake that is competing with the algal blooms for nutrients, if these plants die the algal blooms will grow without restriction.

Unfortunately, we have documented seasonal macrophyte collapse and in some areas a complete loss of plant beds. The summer algal blooms prevent light from reaching the deeper plant beds and cause a significant recession in extent. We installed photosynthetically active radiation (PAR) sensors at various depths over summer to tract the amount of light penetration during the algal bloom formation.

We found that at its peak the algal blooms prevent all usable light from passing deeper than 7 m. This means that all plants growing deeper than 7 m die off during summer. The shading effect also impacts the shallower macrophytes by reducing the light intensity and photosynthesis cycle.

In 2011 plant beds were recorded at depths up to 11 m. Now the maximum depth extent for any plant growth is 6 m, this is a significant loss of plant biomass. We have been tracking seasonal fluctuation in the macrophyte growth and at majority of our sites the plant beds die back to a depth of 3 - 5 m over summer and slowly recover from spring to winter. Some sites have never recovered and continue to reduce each year and other sites have completely died off.

Freshwater Mussels

This lake was once home to large beds of freshwater mussels, but these are now extinct in Lake Pupuke. Both Echyridella menziesii and Echyridella aucklandica were known to exist here and to date we have not found a single live individual.

We have mapped large historic mussel beds up to 150 m2 that are buried in silt. We found very few intact shells and the beds primarily consist of dense clumps of shell hash and thin leaf like fragments. This is a sign that the mussel population in the lake died a long time ago.

The high numbers of perch in the lake have contributed to the extinction of mussels by feeding on the native bullies which the mussels use as an intermediate host. Without the bullies these mussels cannot complete their life cycle. Tench are know to feed on juvenile mussels and they are a likely contributing factor to the lack of mussel recruitment.

Ongoing water quality issues and sedimentation are also known to impact mussels and have contributed to this extinction event.

Pest incursion

This lake is overrun with invasive and pest species that have all contributed to the decline in lake condition.

There are almost no significant native macrophyte populations left and the plant community is dominated by two invasive species, Vallisneria australis and Egeria densa. These plants have out-competed the native species and have contributed to wider environmental health issues.

There are several invasive fish species, and invasive red eared slider turtles, in the lake. We have sighted several perch, tench, koi carp, trout and mosquito fish during our monitoring dives. The perch in particular have caused devastating effects on the lake ecology.

Perch are gape limited and as a result, they feed on almost every level of the trophic chain as they mature. This means they disrupt the natural food web in the lake and contribute to the reoccurring algal blooms by eating the zooplankton that normally keep the phytoplankton population in balance.

We have seen very few native fish species, however there are still small isolated populations of common bullies and a few adult eels. The natives have been out-competed and preyed upon by the invasive fish.

These pest species also reduce the quality of habitat for native benthic organisms. The impact of these pest species has resulted in the loss of native biodiversity in the lake including the extinction of freshwater mussels.