fbpx
Connect with us

Community

Uncovering A Mass Extinction Event: The Lake Rototoa Mussel Survey

Water quality scientist Ebi Hussain led a Project Baseline citizen science team to determine what was happening to New Zealand’s Lake Rototoa mussel population. Crying cockles and mussels, alive, alive, No!! Here are the deets.

Published

on

By Ebrahim (Ebi) Hussain
Header photo courtesy of Oliver Horschig.

Lake Rototoa, a cold, monomictic1 dune lake in a rural area northwest of Auckland, New Zealand, is in peril. With a maximum depth of 26m/85 ft, Rototoa is the largest and deepest of a series of sand dune lakes along the country’s western coastline. Known for its increasingly rare, diverse population of native submerged macrophytes i.e., aquatic plants, and large, freshwater mussel beds, this lake is under increasing threat from a deteriorated water quality. Although the exact cause of this deterioration is unclear, the likely culprit is a combination of factors: eutrophication, land use activities, pest invasion, and climate change.

In late 2019, the Project Baseline Aotearoa Lakes team noted signs of a freshwater mussel population collapse as well as other evidence of environmental degradation. This was alarming, as freshwater mussels are rapidly declining in New Zealand, and globally, with 70 percent of the species considered at risk or threatened.

Many people are unaware that freshwater mussels are an important part of a lake ecosystem; as biofilters and bioturbators, they filter out nutrients, algae, bacteria, and fine organic material which helps purify the water. The loss of these keystone  species has likely contributed to the decline in water quality seen at Lake Rototoa.

The team’s observations prompted the design of a collaborative project between Project Baseline Aotearoa Lakes and the Auckland Council Biodiversity Team. This project is the first of its kind in New Zealand; it aims to fill critical knowledge gaps and, for the first time, quantify mussel populations in Lake Rototoa in a scientific manner. 

Ebrahim Hussain installing the transect. This peg marks the beginning of the first transect across the mussel bed. Photo by Oliver Horschig.

This project is the first of its kind in New Zealand; it aims to fill critical knowledge gaps and, for the first time, quantify mussel populations in Lake Rototoa in a scientific manner. 

The first objective was to assess the mussel population statistics, including species composition, abundance, size class, and recruitment success. The second objective was to determine habitat preferences, bed locations, and bed limiting factors. In order to satisfy the project objectives, the team designed a bespoke survey methodology to collect all the required information in a standardized way.

Digging Into The Data

The initial series of dives focused on habitat mapping and collecting bed scale survey information. The team has mapped almost 5 km2/3.1 mi2 of lakebed and 2.2 km2/1.4 mi2 of mussel bed so far. This information provided critical insight into mussel bed formation and habitat preferences which the team used to inform the site selection for the more detailed follow up surveys. 

The first phase of surveys has been completed and the results are frightening. A total of 1604 mussels (Echyridella menziesii) were counted. The combined density across all three survey sites was 41.4 mussels per m2/3.8 mussels/ft2. Out of the 1604 mussels found, 1320 (82.3%) were dead and only 284 (17.7%) were alive. The dead mussel shells were in a similar condition to the live individuals indicating that they may have all died during a recent mass extinction event.

No juveniles were seen during the surveys and all the mussels were larger than 51 mm/2 in. The surveyed population is composed entirely of mature adults, 64.1% of live mussels were larger than 70 mm/2.8 in in length, 30.6% were between 61 to 70 mm/2.4 to 2.8 in and the remaining 5.3% were in the 51 to 60 mm/2 to 2.4 in size class.

Measuring an adult Echyridella menziesii, this individual is 78mm in length and approximately 15-20 years old. Photo by Oliver Horschig.

Individual dead mussels were not measured but were placed into approximate size classes, all dead mussels were larger than 51 mm/2 in with the majority of them being placed in the 61 to 70 mm and >70 mm size classes. The average age of the mussels surveyed was estimated to be between 20 and 30 years old based on their size. Some larger individuals were 80 to 100 mm long and were estimated to be around 50 years old.  

This aging population and lack of younger individuals indicates limited-to-no viable recruitment in the surveyed area for more than a decade. Considering that most of the live mussels were at the upper end of their life expectancy and that there was no evidence of recent recruitment, the long-term viability of the surveyed population is low. 

While the exact reasons for this population collapse are not known, recent lake surveys (fish, water quality, and macrophytes)  provide some indication of possible causes. Recent fish surveys indicate a significant drop in the number of  the primary intermediate host species. Both galaxiid and bully species are declining due to predation by pest fish species. Without these native fish, the mussels cannot effectively complete their life cycle.

The declining water quality of the lake is also a contributing factor. The lake’s change from an oligotrophic state, which is low in plant nutrients and high oxygen at depth, to a mesotrophic state with moderate nutrients, subjected it to increased eutrophication.  

Eutrophication causes an increase in bioavailable nutrients which stimulates algal growth and in turn causes high organic silting. This silt settles on the lakebed and decomposes creating areas of low dissolved oxygen, which can cause animal die offs. 

Some studies suggest that these mussels cannot survive at dissolved oxygen concentrations below 5mg/L and it is possible that the lake undergoes prolonged periods of low-dissolved oxygen during seasonal stratification. The wide scale coverage of benthic blue-green algal mats further points to periods of anoxia, or absence of oxygen, and general eutrophication. 



Mussel Mania

Due to the low nutrient concentrations and the filtration capacity of the extensive mussel population, Lake Rototoa historically had good water clarity. Mussel filtration rates generally match their food ingestion rate, but once they reach their food ingestion rate, no further filtration will occur. If there is a high concentration of food (phytoplankton and zooplankton) in the water, the filtration rate is likely to be low. This means that as the lake becomes more eutrophic, the algal biomass increases, and the mussel’s filtration rate will continue to decrease. 

An Echyridella menziesii bed, showing a mixture of live and dead adult sized mussels. Photo by Ebrahim Hussain.

This decrease in filtration rates will contribute to the declining visual clarity. The significant loss of mussel biomass and ultimately the loss of mussels in Lake Rototoa exacerbated the situation and may have facilitated a higher rate of eutrophication.

Sediment is also known to affect mussel populations, and there are signs of increased sedimentation; however, no clear evidence of smothering or suffocating was observed. The combination of the organic silt, sediment, and benthic algal growth can clog the mussel gills, so there are likely to be some sediment-induced population stressors.

In terms of bed extent and bed limiting factors, the team made several key observations. The mussels tended to prefer gentle slopes and did not occur in great densities on steep faced slopes/shelves. Water level, riparian vegetation extent, and wind/wave-induced disturbance appeared to dictate the upper extent. Mussel beds were generally established at a depth just below the permanent water line a short distance away from the end of the riparian edge. Fewer mussels were observed in shallow, exposed areas with visible signs of wind/wave-induced substrate disturbance. 

The establishment of aquatic plants, changes in substrate, thermoclines, and potentially anoxia limited the lower bed extent. Mussels were commonly found in lower numbers in amandaphyte stands within the wider bed area and were not found at all within dense charophyte meadows. Mussels tended to establish around isolated macrophyte stands rather than in them. The lower extent of the bed mirrored the start of the deeper charophyte meadows. The littoral zone had clearly defined sections of mussels in the shallower areas (1.5 to 5 m/5 to 16 ft ) and dense macrophyte dominated areas in the deeper portion (6 to 10 m), which were relatively devoid of mussels. 

Ebrahim Hussain counting and size classing mussels in a 0.25m 2 quadrate. Photo by Oliver Horschig.

In the absence of aquatic plants, the thermocline separating the warmer epilimnion above from the colder hypolimnion below appeared to dictate the lower bed extent. Almost no mussels were found past the thermocline, which was between 6 and 7 m/20 to 23 ft deep during the survey period. Since mussel bed establishment is not known to be thermally regulated, the limiting factor here may be anoxic conditions, commonly associated with hypolimnetic water. This assumption has not been validated, and a more detailed investigation of stratification profiles are planned for this upcoming year.

A clear limiting factor is the change in substrate seen past the 7 to 10 m/23 to 33 ft depth contour. The substrate changes from sand with a surficial layer of silt to a semi liquid silt/soft mud. No mussels or macrophytes were found in these areas, and the substrate does not appear to support bed establishment. Benthic algal mats covered the lower extent of some beds but did not clearly limit their establishment; since these mussels are mobile, presumably they will move if they are being smothered.

In Conclusion

Despite the concerning results, this project is a landmark event as it is the first study of its kind in New Zealand and the first detailed survey of the mussel population in Lake Rototoa. This project highlighted the pressures faced by our aquatic environments and exposed the ugly truth of what is going on below the surface. We have uncovered a mass extinction event that is currently occurring in our back yard that no one even knew was happening. 

We have uncovered a mass extinction event that is currently occurring in our back yard that no one even knew was happening.

Now more than ever, projects like this are critical. Our environments are under increasing pressure, and it is up to all of us to take action to ensure that we preserve these ecosystems for future generations.

The follow-up phases of this project are planned to be carried out this summer. The data we have collected thus far has enabled the Auckland Council to make informed decisions on how best to manage these threatened species and preserve native biodiversity. We hope that our continued efforts at this lake will contribute to preserving this ecosystem and prevent the complete extinction of these threatened species.

Footnotes:

  1. Cold monomictic lakes are lakes that are covered by ice throughout much of the year. During their brief “summer”, the surface waters remain at or below 4°C. The ice prevents these lakes from mixing in winter. During summer, these lakes lack significant thermal stratification, and they mix thoroughly from top to bottom. These lakes are typical of cold-climate regions. 

Additional Resources:

InDepth V 1.6: Bringing Citizen Science To Lake Pupuke by Ebrahim Hussain

Learn more about Project Baseline
View their online database
Get involved 


Ebrahim (Ebi) Hussain is a water quality scientist who grew up in South Africa. As far back as he can remember he has always wanted to scuba dive and explore the underwater world. He began diving when he was 12 years old and he has never looked back. Diving opened up a new world for him and he quickly developed a passion for aquatic ecosystems and how they work. The complexity of all the abiotic and biotic interactions fascinates him and has inspired Ebi to pursue a career in this field.
He studied aquatic ecotoxicology and zoology at university, and it was clear that Ebi wanted to spend his life studying these subsurface ecosystems and the anthropogenic stressors that impact them. After traveling to New Zealand, Ebi decided to move to this amazing country. The natural beauty drew him in, and even though there were signs of environmental degradation, there was still hope. Ebi founded Project Baseline Aotearoa Lakes with the goal of contributing to preserving and enhancing this natural beauty as well as encouraging others to get involved in actively monitoring their natural surroundings.

Cave

No Direction Home: A Slovenia Cave Diving Adventure

Suffering from Covid lockdown, young, poetic Italian explorer, instructor, and gear-maker, Andrea Murdock Alpini, decided to take social distancing to the max! He packed his specially designed cave-van and set out on a three-week solo road trip to dive the water-filled caves lying beneath the Slovenian soil. His report and must-see video log, dubbed, “No Direction Home”—an homage to Martin Scorsese’s Bob Dylan docu—will likely satisfy those deeper urges for adventure. Did I mention the killer soundtrack? Kids don’t try this at home!

Published

on

By

Text: Andrea Murdock Alpini

Photo & Video: Andrea Murdock Alpini

Ecco la storia originale così com’è stata scritta in italiano

Author’s note: I do not encourage other divers to conduct solo diving. The trip and the dives described in this article were conducted after significant training and experience.

Ed.Note: Global Underwater Explorers does not sanction solo diving.

Freedom!

That was the feeling I had last June 2020 when I left my home to begin a journey alone. Caves, abandoned mines, alpine lakes, and a few wrecks—that was my plan for a great adventure.  

The first COVID-19 lockdown had been in place for a couple of weeks, and I was afraid of going out and meeting people. Social distancing left an open wound. I loaded my wreck-van with plenty of stuff to survive alone for a long month traveling amongst rivers, lakes, mountains, and forests, and I was ready to practice scuba diving. 

At that time, tourist travel was impossible in Italy or abroad—anywhere  in Europe—because the coronavirus had locked the borders. I asked an editor in chief from a magazine—one whom I am used to sending articles to—to prepare a couple of official invitation letters for customs. For my trip, I converted my wreck van into a cave van. It was fully equipped with a 300-bar air compressor, helium, oxygen, deco cylinders, twinsets of different sizes, gas booster, fins, mountain boots, tent, camp burner, and brand-new dry suits, as well as thermal underwear to be tested for my company PHY Diving Equipment. 

I remember the day well. I was thrilled as I crossed the border between Italy and Slovenia. I had been restricted to nothing but a 200 m/650 ft walk from my house because of the pandemic restrictions, but with an eight-hour drive, I was free to enjoy walking into wild nature all alone.

The mental switch was awesome, and unexpected. I did make just one phone call from abroad. I talked to an incredible Russian who was the first guy I met in a small rural village in Slovenia. He had emigrated some years ago, and now he welcomed travelers by sharing his farmstead. 

However, once I arrived on site, I was not very welcomed by  the weather; instead, I was met by heavy rain. After the storm passed, I went out walking and filming with my phone. I had decided to record all of the trip. As luck would have it,  the rain returned again, and it never left me for the entire duration of my trip (almost a month).

My tour was articulated throughout Slovenia, Garda Lake (Italy), Austria, and South Tirol’s Alps, Tuscany’s caves, and finally I reached the central part of Italy—Appenini mountains and their peaks. I planned to reach two mines, but heavy rains stopped my dream. Excluding Slovenia, where I slept in a traditional bed, I passed all my time living in my tent. Cold weather and storms were my constant companions. 

I managed to see  a ray of light for just a few hours, I never had any chance to dry my equipment, and I warmed up inside my van. Every night I slept only a few hours because of loud wind noise or strong rain storms. Day-by-day I grew tireder and more feeble. One day, three weeks after I left home, I was in South Tirol descending a mountain when I decided to conclude my trip, and I returned home safe.

The goal of my trip was to tell scuba adventures from the surface point of view where the water is only a part of the context and not the objective. I made a mini-series film composed of three chapters. Each one brings you inside the scene. What  follows here is the first episode of the trip.

Social Distancing Beneath The Slovenian Soil

The first day of cave diving in Slovenia was very tricky and full of adventures. I had no idea how the second day would go. 

I left my accommodations around 6 a.m., after a good breakfast of cereal, dark chocolate with black coffee, dried fruit, and tasty Italian Parmesan cheese. I could not see anything from my window because what had fallen was not simply rain; it appeared to be an awesome flood. My plan for that day had been delayed.

I think that most parts of dry caves are condemned for hundreds of kilometers. So, I decided to check the weather forecast and water level conditions in caves close to the Croatian border. It would mean driving about four hours to see for myself whether scuba diving was allowed. I didn’t have to remind myself, I was alone here.



Wheels were on the road and local conditions seemed quite good. I had checked the weather on my laptop and understood the risk. If I was lucky, I could dive; if not, I would have to drive back. I drove through Slovenia forest meeting no one. With less than an hour left to my destination, I came across an abandoned farm village, completely empty.

The dive inside Bilpa Jama was breathtaking. Now I was seated beside the cave shore preparing soup to warm myself. After a stunning solo dive, I was cold and wanted only to taste the peace of this magnificent place. While I was dipping  the spoon in my soup cup, I heard a faraway voice, a police woman calling me and asking me to stop eating and come quickly to her. 

After I did as I was asked, she started examining my passport, documents, and permissions. A few minutes later, a huge National Army truck reached us. The soldier had  an abnormal body shape, a man the size of a walking mountain in an Army uniform. Can you imagine how I was feeling in those moments?!

Well, in the end, everything went really well, and I now have a story to tell my grandchildren. 

Once the passport control was over, and they had checked that I did not cross  the border from Croatia to Slovenia illegally (customs was only a few hundred meters  from us), I had the chance to get back to my soup, which by then had turned cold. I warmed it up again, and I spent half an hour seated on a slippery stone covered with moss and lichens watching the beauty of the forest surrounding me.

On the way back to my accomodations in my cave van, I  played a new playlist. 

Four hours later, I approached my country lodge. I was really exhausted,  but I had to refill tanks and plan the next scuba diving days. Once I finished, I watched the forecast again. Unfortunately, it was growing worse, so I decided not to dive and instead get a surface break. Tomorrow I would drive, search, and catch info and GPS coordinates of caves. My tomorrow plans had turned into a sketching and surveying day. 

The Road To Suha Dolca 

I drove and walked for hours and hours, up and down the forest or on lonely roads in search of caves where I could return in winter or perhaps next year. During the last survey of the day, I watched a talented young guy playing a traditional concertina and thought, what a lovely atmosphere and a fitting way to close my hard-working day!


I decided to give a last gaze to Suha Dolca cave, my favorite one, on the way home. This was the third consecutive day I had arrived back at this spot. Observing it day-by-day, I tried to find the best moment to dive this cave.

Until now, it was inaccessible due to the strong flow. I wanted to dive here before leaving Slovenia. Tired and  driving slowly, I parked my van away from my accommodation. Since I had no lunch, I started feeling very hungry. A simple dinner was quickly served: dried fruits and a cup of hot noodle soup.

My ‘NO DIRECTION HOME’ trip was now at its peak. I had become a wanderer. I was alone in a wild country with, yes, an internet connection for historical research and checking the weather. That was the only technology I used. Aside from that, I lived simply. I walked, dived, wrote, and filmed my experience all with my mobile phone.

Rain was tougher than expected. I had hoped to stop for one day, not the two that it took. Following the surveys, the next day I started fixing my video equipment and saving photos and videos I had made on my hard drive.

I had too many ideas, no one clear till the end, and too many cave sketches and GPS points to reorganize; I needed a day to regroup. I just went out for a few hours to check Suha Dolca’s Cave conditions. On this day it seemed that the flow was getting more stable, and general water conditions were growing better. I had to be patient and wait one or two days more for the right conditions. I tried and failed to find a solution on my own, but the water always showed me the way. She told me to wait and to go back to where I came from. Step-by-step I walked the path again.  

The third video chapter of Slovenia Solo Cave Diving is the one I prefer, because I remember the indecision I felt, to stay or to leave. Solo trips are strictly linked to life’s decision.


The last day I was in Slovenia I left the accommodations and asked a new farmer, close to a different cave, if I could sleep inside his barn and dive the river hole on the following day. I was at the same place where I had dived the first day. He told me I could not stay in the barn due to the high risk of bears who live in the surrounding area. I jumped in my van again and I drove to the lake beside Suha Dolca’s Cave. 

I descended the path several times and brought all my scuba gear piece-by-piece. I decided to give myself a chance to dive my dream cave in the late afternoon. I had no other choice. Once I was inside the cave it was unbelievable, and I had a very nice dive even though I was really tired, and again I broke my light arms and camera housing. I resurfaced after the dive into a reed’s lake, which made me feel like a beaver.

I had conflicting feelings as I left Slovenia that same night after making a tricky and stunning dive. Bears, awesome forests, and rural areas were now all behind me. The cave-van played a new disc, I needed to shake off these feelings and look forward to my new goals: Garda Lake’s wrecks, South Tyrol’s stunning lakes, and finally Austria. In the country of green and wide grazing land I wish to dive surrounded by the amazing scenario of beautiful Alps mountains. 

At 9:30 PM I crossed the border again, and  Italy was straight ahead.


Andrea Murdock Alpini is a TDI and CMAS technical trimix and advanced wreck-overhead instructor based in Italy. He is fascinated by deep wrecks, historical research, decompression studies, caves, filming, and writing. He holds a Master’s degree in Architecture and an MBA in Economics for The Arts. Andrea is also the founder of Phy Diving Equipment. His life revolves around teaching open circuit scuba diving, conducting expeditions, developing gear, and writing essays about his philosophy of wreck and cave diving. Recently he published his first book entitled, Deep Blue: storie di relitti e luoghi insoliti.

Continue Reading

Thank You to Our Sponsors

Subscribe

Education, Conservation, and Exploration articles for the diving obsessed. Subscribe to our monthly blog and get our latest stories and content delivered to your inbox every Thursday.

Latest Features