Introduction & Current State

New Zealand has approximately 775 lakes but only 127 of them are regularly monitored. Majority of these lakes are only monitored for water quality trends and almost none of them have any detailed assessment of in-lake biodiversity and ecology.

Water quality parameters can tell you a lot about a water body but is only one piece of a very complex puzzle. Without an ecological context, water quality trends can be misleading as it only describes a subset of ecosystem functions, physicochemical parameters alone are not a suitable proxy for lake ecosystem health.

Lake ecosystems, and the biodiversity they support, are some of the most threatened environments globally. The lack of baseline state data is largely attributed to the complexity and effort involved in obtaining comparable in-lake ecological assessments.

What are we doing about it?

Aotearoa Lakes is an environmental charity, driven by citizen science, working on a variety of lake projects across New Zealand. One of the overarching goals that’s included in every project is establishing an in-lake biodiversity and ecological health assessment for each site. In addition to project specific objectives, we collect a set of standardized observations and measurements from every lake we work in. We are using this information to build a national data base of lake ecosystem health and in-lake biodiversity.

We have used a variety of techniques, including eDNA, light stations and various survey methods, to assess biodiversity values, biosecurity risks, potential impacts, stressors, littoral health, and general ecological functions.

This blog entry will discuss some of our key projects and highlights from the last year of surveys.

Kākahi excreting pseudofeces
Native freshwater mussels (Echyridella menziesii) also known as kākahi excreting pseudofeces. Pseudofeces consists of undigested particles and organic matter which the mussels discard onto the lakebed. Excessive suspended matter can cause mussels to produce large amounts of pseudofeces which put strain on them.

Freshwater Mussel (Kākahi) Restoration Program

This program includes projects from various lakes across the North Island of New Zealand where we have been surveying freshwater mussels (kākahi) for several years now. We have almost completed a comprehensive regional distribution assessment of freshwater mussels across Auckland lakes and are now focusing on in-depth investigations around the key drivers behind the population collapses we are seeing in the region.

Native freshwater mussels (Echyridella menziesii) also known as kākahi filtering. These species can filter up to 1.5 liters of water per hour per mussel and can quickly filter the entire volume of a lake. They filter out suspended particles and contaminants like E. coli, algae, sediment and organic matter. This helps maintain a clear water lake state.

We have set up various projects aimed at studying each part of the mussel life cycle in several lakes to identify where the failure in recruitment stems from. In most cases the low biomass of intermediate fish hosts (largely a result of increased predation pressure from invasive fish species) is likely a limiting factor despite suitable habitat availability and quality. We have been looking at the attachment rate of the parasitic glochidia to the intermediate fish host and have confirmed a high degree of encystment in one of the project lakes. These studies indicate that spawning and attachment to intermediate hosts is occurring successfully. Over the next year we will be focusing on juvenile survival and the development stages once they have detached from the intermediate hosts.

Common Bully – Common bully (Gobiomorphus cotidianus) are the primary intermediate host for native freshwater mussels. These fish are very susceptible to predation pressure from invasive piscivorous fish.
Glochidia on bully – A parasitic glochidia on the dorsal fin of a common bully (Gobiomorphus cotidianus). The glochidia use these fish as an intermediate host. Female mussels excrete glochidia that attach and encyst on the fins and gills of fish, once mature they will drop off onto the lakebed and grow into adult mussels.

We are working with the Auckland Council on an ambitious restoration project where we will be installing large in-lake pens that will exclude invasive fish species and concentrate viable host fish within the area. These pens will help us exclude specific pressures and gauge the corresponding responses from the mussel population, by doing this we will be able to identify the dominant impacts and create a more effective restoration strategy. This work will set an important baseline for the future management of these threatened mussels.

Light Stations Trails

We have been using eDNA and traditional traps to get an overview of fish populations in lakes. Majority of the lakes we work in are dominated by invasive species (perch, tench and gambusia), and in some cases their biomass is so large that they have almost entirely displaced all native fish.

kōura – Freshwater crayfish (Paranephrops planifrons) also know as kōura attracted to the static light station. These crayfish are rarely seen and using light stations is a great way of attracting them. The kōura in this photo is surrounded by native mites.
Mites – These native mites were drawn to the static light stations, and we have never seen them in these numbers before. The light stations are incredibly useful when surveying macroinvertebrates.

Traditional fish surveys did not provide much information on juvenile fish numbers (native and exotic) which is a critical knowledge gap when trying to manage invasive species. We experimented with using passive light stations to draw out juvenile fish and get estimated counts. The light stations worked amazingly and within minutes we started seeing juvenile fish and invertebrates.

Dwarf inanga (Galaxias gracilis) in Lake Rototoa were once seen in large open water schools but are now limited to a very small number that hide amongst the emergent riparian vegetation to avoid predation from invasive perch.

We saw dwarf inanga (Galaxias gracilis) in one of our project lakes which was surprising as this native fish that was thought to have gone extinct in this lake. Kōura (native freshwater crayfish) were also drawn to the light stations, these species were once abundant in most lake but are rarely seen now. The passive light stations proved to be great way to detect cryptic species that occur in low numbers.

The light stations were also a useful way of assessing the macroinvertebrate populations. Macroinvertebrates are a food source for many species but also provide valuable ecosystem functions like processing organic matter. Currently there is no effective method for surveying macroinvertebrates in deep margined lakes with dense emergent vegetation, passive light stations coupled with eDNA show great potential and have given us a much better understanding of the macroinvertebrate diversity and biomass.

Juvenile perch (Perca fluviatilis) are positively phototaxic and are readily attracted to static light stations. This allows us to do static fish counts and get an estimate of their abundance within the shallow littoral zone.
Native mites swarming a static light station. These mites perform critical ecosystem functions and serve as a prey species for fish. Macroinvertebrates don’t emerge in large numbers during the day but light stations at night serve as a beacon that attracts them in large numbers. This is one of the best ways to assess macroinvertebrate biodiversity in Lake Rototoa.

Invasive Macrophyte Control Methods

Over the past year we have done a lot of work on macrophytes across Auckland and Hawke’s Bay. One of the highlights for the team was the discovery of native macrophytes in Lake Tomarata, this lake was considered devoid of vegetation since 2012. We found several native species (Chara australis and Nitella leonhardii) still growing in pockets around the lake, we were amazed to see healthy growth and evidence that the macrophyte cover is increasing despite a decline in water quality.

Fruiting head of Nitella leonhardii
Mucus enclosing the fruiting bodies (female oogonia and male antheridia) of Nitella leonhardii.

We have been focusing on assessing the impacts of invasive macrophyte species across several lakes, in particular Hornwort (Ceratophyllum demersum), Oxygen weed (Egeria densa) and Canadian pond weed (Elodea canadensis). These invasive species are causing significant damage and are altering the ecosystem functions across all our project lakes.

Left photo – A dense wall of Hornwort (Ceratophyllum demersum) towering over a low growing native charophyte bed. Hornwort grows rapidly and pushes native species into deeper water where they can no longer survive. They alter the littoral zone so that charophytes can no longer establish.
Right photo – Masses of decomposing Hornwort on the lakebed creates anoxic conditions, sulphur precipitate and promotes sediment phosphorus remobilization. These conditions are unsuitable for most native species and cause dead zones on the lakebed.

Over the next year we will be trailing various invasive macrophyte control techniques that can be done by communities and citizen scientists without the use of herbicides. We have mapped isolated stands of invasive Egeria densa in Lake kowhai and will be using a staged manual removal plan with the hope of fully eradicating it from this lake within 3 – 5 years. If we can successfully remove all the Egeria from this lake, it will be the only lake in the Auckland region that has no significant invasive macrophyte biomass. Considering the high ecological value of this, removing this pest species will help preserve one of the best examples of native charophyte meadows left in the region.

We are also working with the local Iwi (tribe) in the Hawke’s Bay region to develop new macrophyte control techniques that use cultural knowledge and practices in combination with western science. One of these techniques is the use of traditional woven mats as ground covers to smother invasive macrophytes. 

Take Home Message

We have collected a huge amount of data from several lakes, and we are seeing some concerning trends. In general, we are seeing a displacement of native biodiversity by invasive species, loss of key stone species, increased eutrophication, and a collapse of littoral ecosystem function.

Our observations paint a dire picture for our lakes and highlight the importance of underwater observations and the contribution of citizen science to sustainable lake health management. Over the next year we will continue to build our baseline data sets, expand our research programs and emphasize the importance in maintaining littoral zone health and native biodiversity.

Aotearoa Lakes is located in New Zealand. Their team consists of Ebrahim (Ebi) Hussain, Maddison Jones, Louise Greenshields, Russell Hughes, Mark Long.
Article written by Ebrahim (Ebi) Hussain