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.

Community

Undergoing PFO Surgery as a Team: Deana & Bert’s Excellent Adventure

People like to give GUE a hard time for their uncompromising focus on team diving. But a pair of divers from GUE Seattle has taken it to a new level: getting their PFOs fixed together. The team that bends together, mends together? Instructor and tech diver James D. Fraser willingly tells the tale.

Published

on

By

by James D. Fraser
Header photo courtesy of Dr. Doug Ebersole

This is the follow-up to the story that ran in InDepth December, 2019: No Fault DCI? The Story of My Wife’s PFO

It has been a year since my wife Deana had a decompression illness (DCI) hit in Bonaire requiring her to do a Table 5 recompression profile in a hyperbaric chamber. At the time of my previous article’s publication, Deana had a Transthoracic Echocardiogram (TTE) bubble study and found out she did have a small to moderate patent foramen ovale (PFO). Two physicians offered similar options for Deana to consider when it came to her diving activities: 

  • Stop diving, as this eliminates any risk of DCI in the future.  
  • Modify her dive profiles to be more conservative: diving only once per day, diving nitrox 32 using air tables, and/or extending her decompression profiles and safety stops.
  • Have the PFO repaired, knowing it is not a guarantee, and continue diving as conservatively as possible. 

Deana had initially decided to wait on doing a PFO closure until after our daughter’s wedding in March 2020, but she realized very quickly that being “conservative” was not in her nature. Deana had already returned to diving within 12 days of her hyperbaric chamber ride. In the 46 days since her treatment, Deana had already done another 15 dives to depths of 90 feet; being conservative really was proving to not be an option for her. Diving was just too much a part of her life. 

Deana with Dr. Ebersole. Photo by D. Ebersole.

In mid-November, Deana reached out to cardiologist and tech diving instructor Dr. Doug Ebersole for a second opinion on the bubble study and his advice about her options. Dr. Ebersole gave Deana the same response as the other physicians; but, knowing Deana and her passion for diving, he suggested that she have her PFO fixed, since her plan was to continue diving. 

Deana also spent time talking to other divers who had been diagnosed with PFOs—some who had them repaired and some who had decided against it—in order to get a more complete picture from both a patient and a doctor point of view. One of the final conversations that pushed Deana to have her PFO repaired was with a coworker who was a nurse practitioner in cardiology with knowledge of PFOs and diving. Her coworker was pretty blunt, stating, “Why are you playing Russian Roulette? You have worked in cardiac and know the risks.” 

Some of these risks include Arterial Gas Embolism (AGE), Venous Gas Embolism (VGE), and cerebral embolism. That was the final “Aha” moment to tip the scale and get Deana to schedule her PFO repair, since “Russian Roulette” was exactly what Deana was doing based on her diving activities following her DCI hit.

Team Approach to Treatment 

Bert Berzicha, one of our GUE Seattle community members, also completed his TTE as a result of having had some symptoms of DCI in the past. The test confirmed the presence of a large PFO. Deana and Bert compared notes initially and discussed diving as a team on future dives using more conservative decompression profiles than other teams, allowing the other teams to get out of the water sooner. Deana, however, related what she had learned from talking with her coworker and changed her mind about diving conservatively and instead decided to get the PFO repaired. 

Bert and Deana with some of the GUE-Seattle Tech crew. Photo by Bert Berzicha.

Deana did not want to take the risk of neurological deficits that could be irreversible. Deana suggested to Bert that he come with her to have his PFO repaired at the same time. Bert continued to research the subject, looked at his work schedule, and decided doing a “team” procedure made sense. Just as a dive team shares a plan, resources, and emergency procedures, a medical procedure shares similar benefits when working as a team. 

It was time to plan a date for both of them to have the procedure. Deana and Bert both live in the Seattle area. Dr. Ebersole lives in Lakeland, Florida, so logistics included time off work, pre- and post-surgical care, flights, hotels, and transportation. Deana arranged to have her sister Jessica fly into Tampa from Dallas, prior to them arriving, so she could pick them up from the airport to make it to the hospital in time for the procedure. I was going to be in Australia on a business trip at the time, so I was not able to be there pre-surgery. I ended up reworking my return trip and flew from Canberra, AU to San Francisco, then on to Tampa, to land just an hour after their surgeries were finished and meet them back at the hotel. 

Even though Deana and Bert could fly home 24 hours after the procedure, they decided to stay the weekend just in case there were any complications and to take it easy. Deana, however, had a different take on “easy.” The morning after surgery, Deana was invited by Dr. Ebersole to watch a procedure that he and his team perform called the “WATCHMAN” procedure (less than 24 hours after post-op). Then we picked up Bert and Jessica, and jumped into the truck to do a 300-mile, five-hour road trip to High Springs, FL, to take a tour of the Halcyon facility and say “Hi” to Orie Braun, Lauren Fanning, and Mark Messersmith; stop in at Global Underwater Explorers (GUE) HQ to buy some swag; and drive down to Ginnie Springs to see where Cave 1 may take place in Deana’s and my near future. Not bad a day after surgery. 

Deana and Bert after surgery. Photo by James Fraser

It was at Ginnie Springs where Bert came to Deana and stated he thought he had active bleeding. We all paused and turned pale, knowing we were not in a great location for this to be happening, but after being assessed by Deana it turned out to be post-op bruising from the surgery. This did, however, make us stop and think, “We just drove 300 miles away from the hospital we had decided to be close to in case of complications.” I am sure Gareth Lock would find a really good human factors story in there somewhere. 



Deana’s PFO adventure Timeline

2019

  • OCT 8: 47 m/153 ft technical dive resulting in a DCI episode requiring recompression.
  • OCT 20: First dive post-chamber ride to 16 m/52 ft
  • OCT 29: TTE Bubble Study; “Deana has a small to moderate PFO”
  • NOV 17: Dr. Ebersole receives Deana’s TTE study for a second opinion 
  • NOV 21: Deanna dives now to 28 m/90 ft
  • DEC 4: Last dive before PFO repair. In the 46 days since her hyperbaric treatment Deana made 15 dives: “Conservative Not”
  • DEC 12: Deana and Bert have PFO procedure
  • DEC 13: Lakeland to High Springs road trip

2020

  • JAN 27: First dive post closure—15 m/49 ft and spaced dives 2-3 days apart
  • FEB 15: Started doing multiple dives daily no greater 15 m/50 ft
  • MAR 8: PFO follow-up; OFFICIALLY cleared by Dr Ebersole to dive
  • MAY 7: Dives now pushing 30 m/100 ft
  • MAY 31: First Tec dive to 33 m/110 ft 

Since May, Deana has done 120 dives in 2020 with a max depth of 52 m/170ft, which she did on September 12. Deana has gone back to no-restriction diving and has completed 16 technical dives since this summer. Some of these have been assisting with photogrammetry dives.

  • 46 m/150 ft to 52 m/170 ft: 3 dives 
  • 40 m/130 ft to 46 m/150 ft: 5 dives 
  • 30 m/100 ft to 40 m/130 ft: 8 dives 

Getting Personal With PFOs

COVID-19 has prevented us from doing a dive trip this year, which is the one main test we still have yet to do: repeat the scenario that always led to her getting DCI, which was three consecutive days of recreation and tech dives, to see if she experiences any recurrence of DCI symptoms. 2021 will hopefully open up this opportunity, or by that time Deana will already be training for GUE’s Tech 2 course. In either case, Deana and Bert are both very happy to have had their PFOs repaired; both have seen improvements in their health in other areas such as endurance, no longer being easily winded, and, in Bert’s case, less headaches, which he had prior to the PFO closure. 

James, Deana and Bert. Photo courtesy of James Fraser.

To get a PFO repaired is a personal choice, and no one should ever take surgery lightly as it has its own risks. Divers with PFOs need to do their own research and consult an interventional cardiologist, such as Dr. Ebersole, who understands diving. Only then can they make an informed choice based on their own unique situation whether or not a PFO closure is right for them. This article is meant to show the process and outcome of two very experienced and ambitious divers who made the choice to have their PFO repaired and the results of that decision.  

(PFO) Free At Last! Deana after completing her first dive to 52 m/170 ft in Lake Crescent, WA diving the Warren car. Photo by Bert Berzicha

Additional Resources

Diving and Hyperbaric Medicine: The effectiveness of risk mitigation interventions in divers with persistent (patent) foramen ovale  by George Anderson, Douglas Ebersole, Derek Covington and Petar J Denoble. 2019 Jun 30.

Alert DiverPFO Study Update by Petar J Denoble

Alert Diver: Cases studies of divers who had their PFOs closed with transcatheter-applied occluders: Divers with Holes in their Hearts by Petar J Denoble 2010

This image has an empty alt attribute; its file name is image.png

James D. Fraser is a GUE Fundamentals and Rec 1/2 Instructor, PADI MSDT, and NAUI Scuba Instructor, and has been diving in the Pacific Northwest for over 30 years. James currently lives in Seattle, WA, with his wife and dive teammate Deana Fraser. As a member of the GUE Seattle Board of Directors, James is able to share his experiences and work with Deana at growing the local diving community sharing their passion with all who are interested. James recently embraced technical diving, becoming certified as a Technical 1 diver with GUE. James and Deana have had opportunities to travel all over the world to experience their passion in amazing places such as Egypt and the Maldives. James currently works as a Cyber Security Director with a Fortune 500 Defense Contractor and has been a residential construction business owner and Emergency Medical Technician (EMT). 


Continue Reading

Subscribe

Submerge yourself in our content by signing up for our monthly newsletters. Stay up to date and on top of your diving.

Thank You to Our Sponsors

Education