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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.

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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.

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Fact or Fiction? Part 2: Interview with World Record Holder Ahmed Gabr

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by Michael Menduno

See Part 1: Fact or Fiction? Revisiting Guinness World Record Deepest Scuba Dive 

The day before this issue of InDepth was to go LIVE with Gabr’s world record story—I had reached out to him to comment but he declined—I received an email from Gabr. He apologized for any previous confusion and told me that he had checked my credentials and legitimacy and was now willing to grant me an interview. An hour later, we were speaking on FaceTime. Gabr was animated and very talkative. He would frequently go off on tangents and I had to keep bringing him back to my questions. At first, he seemed a bit nervous, but he started to relax more as the conversation went on.

After we briefly talked about his military background—note that he made the dive while still a colonel in the Egyptian army—I began by asking Gabr why he thought these allegations were surfacing now, six years after he conducted his world record dive. He said he believed it was because a documentary of his dive had been completed, and his detractors wanted to get back at him. “Their agenda is to discredit me and the movie,” he said. I asked him if he knew who Scuba Sam was, and he told me that he was a former disgruntled member of his team that had fled Egypt under legal suspicion and wanted to get back at him.

Gabr verified that the head cam video included in Scuba Sam’s email was accurate. I asked him why he hadn’t carried a camera and filmed the dive to begin with. He explained at the time he had an 8-year old child and a pregnant wife, and he said it would have been a huge stressor for him to know that they might see the video if he had died. For that reason, he hadn’t videoed his training dives either.

What About The Depth Timer?

I asked Gabr about Scuba Sam’s first allegation, whether his Scubapro 330 m Depth Timer with blacked out corners—that only showed his Maximum Operating Depth—was metric or imperial. He answered by first saying that computers were not required as part of the validation and launched into a detailed explanation of Guinness’s measurement methodology.

Guinness’s procedure for validating the dive was twofold. First, Gabr had to retrieve one of the unique, signed, and wrapped depth tags from the certified descent line. More on procedure surrounding the tags later. In addition, Gabr’s deep support diver, who met him at 90 m/295 ft, instead of 110 m/361 ft, as planned, was supposed to video him giving a special hand sign indication “3-5-0,” which was requested by the Guinness adjudicator shortly before the dive. In addition, he was to display the retrieved tag, which was the 335 (335 m/1094 ft) tag as shown in the video. He said that only the adjudicator, deep support diver, and Gabr knew the sign. Computers were not required. However, Gabr told me that he was so excited that he held out his Depth Timer (DT) which showed a max depth of 330. The question is whether it was feet or meters.

I asked him why he had blackened out the three corners of the Depth Timer so that only the max depth was showing. Note that only the last digit of the current depth reading, and part of the preceding digit, in the upper left-hand corner is showing. He explained that he did that before his last training dive because he was worried that breathing helium beyond 300 meters would give him hallucinations, so he wanted to keep his brain focused by having a single number. Is max depth the best number? His account contradicts the explanation that two of his team members independently gave me. They reported that Gabr told them it was to insure the integrity of the device under pressure as per the manufacturer.

Gabr explained that he made the dive with four measuring devices: two computers, the Depth Timer, and a Swiss military watch which was rated to extreme depth. He told me that he clipped one of the computers on the line at 90 m/295 ft on his way to depth, as he was worried that it would fail and he wanted it to keep track of his overall dive time compared to his runtime table. He later retrieved the computer which is shown in his hand with double enders in the video. Gabr then said that the main computer died (went blank) at 290m/796 ft during his descent.

OMG! How did he monitor his depth with no working computer and a depth timer only showing MOD? Gabr explained that he used the rope as his “depth tool,” and his watch to keep the time?!? Wow. Can you imagine? What if you got blown off the line? The rope was NOT marked in 3 m/10 ft increments. Rather, according to the engineering report, it was marked in 5-meter increments from 320-350m, and 50-meter increments from 300-150 m, then 10m increments from 120-100 m, and the 3m increments from 96-90 m, where he met his first safety diver. In other words, it would be very difficult to follow as a depth guide, particularly if he were suffering confusion. Gabr reported to me that he was mentally confused and feeling drained at depth. 

Note also that Gabr’s first three planned 1-min deep stops (186 m, 171 m, 150 m) were not marked on the rope, nor his stops at 140, 130, 123 (see the partial runtime deco table shown in Part 1). On the video, Gabr gave his first safety diver the OK upon meeting him at 90 m and later reported that the dive went according to plan. No mention of computer problems or the lack of a depth-keeping device at depth.

Again,  I asked him about the Depth Timer, whether it was the imperial (feet) version as alleged, or the metric version. Specifically, if it were the metric unit, why wasn’t the decimal showing before the last digit of current depth? [See the analysis in Part 1] Note that the Scubapro metric unit shows the current depth shallower than 100 m in three digits XX.X, to the tenth of a meter. Gabr said that the device “wasn’t in feet.” 

What happened, he explained, was that the depth timer began to flood on ascent, and as a result, the decimal point indicating that it was a metric unit and the temp reading had disappeared by the time he reached 90 m/295 ft. He said that eventually the face of the depth timer went blank. Again, Gabr pressed the point, “Why do I need to fake something that is not even required [for the record]?” 

One source told me that Gabr had been wearing his metric depth timer on his previous training dives with a standard strap mounting. However, the depth timer in the video has bungee cords. Was the metric device swapped out for an imperial version for this dive?

What About the Gas?

I asked Gabr about arriving at his 90 m/295 ft stop, where he met his deep safety diver, roughly eight-minutes (21 meters shallower) earlier than the plan. He explained that it wasn’t an issue and was within his calculated safety limit. He said that he had reached the 335 m/1099 ft depth tag between 12-14 minutes, and in the absence of a working depth monitor, he ended up making his first stop from depth at 150 m/492 ft using the rope markings. The plan called for slowing his ascent rates as he ascended through various depth zones. 

I next asked him about the video which shows him breathing his back gas (trimix 4/85) as he is met by his safety diver at approximately 90 m: “Why were you breathing your bottom mix?” Gabr answered immediately, “I ran out of deco gas.” He then launched into an explanation about Isobaric Counter Diffusion (ICD), and why it wasn’t a big concern for him to switch back to his back gas because the differences in nitrogen were minor. But what about the oxygen?!?

Note also, that none of the tanks had SPGs. I asked him about this. “It’s a weak point for failure,” he explained. However team members told me that all cylinders were triple checked, analyzed, and marked for proper contents prior to the dive.

Deco plan for the dive. Photo courtesy of DeeperBlue.com

In the video, the safety diver descends to Gabr, asks in sign, Are you Ok? Gabr, who appears very relaxed, responds with an OK sign. No mention of an out-of-gas issue. Gabr then displays his retrieved 335 tag, shows his Depth Timer reading 330, and makes the sign requested by the Guinness adjudicator 3-5-0. “The first thing I wanted to do was secure the record, and do the whole video process,” Gabr explained. He next secured the tag to his support diver to prepare it for its trip topside. “My nightmare was that my deep support would lose the tag so I zip-tied it to his D-ring.” 

After the Guinness protocol was accomplished, the safety diver switched out Gabr’s empty stages and passed him his stage bottle with trimix 12/75 deco gas, that was supposed to be breathed from 120 m up to 90 m. Gabr then went back on his deco gas. The support diver also sent up a status slate to the surface indicating Gabr was OK. The 90 m safety diver met them soon after, and facilitated Gabr’s gas switch to trimix 16/69.

There were several things that were a bit inexplicable. First, if you had just completed a 330+ meter diver, lost a computer, and then ran out of deco gas prior to your 90 m stop while you were facing another 14-hours of decompression, would you give your safety diver, who was carrying back-up deco gas the OK sign? Again, according to Guinness and other news reports, Gabr said that the dive went as planned. Second, were Gabr’s priorities. Wouldn’t you switch to your deco gas and then do the record protocol?

Finally, most troubling, Gabr had four 12L cylinders of 12/75: three staged on the line and one in a carried stage bottle. According to the plan, Gabr was to breathe 12/75 from 120 m to 90 m, for a period of 15 min at an average depth of 110 m or 12 atm. You do the math. For example, with a high 20L surface consumption rate for example, he should have burned through 20L/min x 12 ATM x 15 min=3600 liters. That’s roughly 1.5 12L cylinders, not four. Was he in that depth range longer? Gabr acknowledged that he had burned through lots of gas between those depths and was crazily coughing and had some chest pain. He thinks he had mild pulmonary edema. 

Lines, Tags and Fitness

The fourth allegation concerns the angle of the descent line. The video, 50-minutes into the dive, shows the line at what is estimated to be 25º angle, which performing a trigonometric analysis, would mean that the 335m tag would be at approximately 303 m/989 ft short of the record. Gabr explained to me that the Guinness adjudicator and the calibration engineer who was on board measured the angle of the descent line at 15 minutes into the dive. Accordingly they subtracted 2.65 meters to arrive at the record of 332.35 m/1094 ft and 4.5 inches.

The fifth allegation is that there was a broken chain of custody with the tags, meaning that they could have been altered or replaced prior to the dive. Gabr disputed the claims. Though he wasn’t present for all of it, he said that the tags were under the control of the adjudicator the whole time, who even had his own locks to secure the tagged line the night before the dive. That is in Guinness’ bailiwick to determine. Gabr said that the adjudicator watched him dress and searched him prior to the dive. Eye witnesses reported that Gabr and his lead support diver were not searched thoroughly for tags before entering the water, and that the diver who escorted Gabr to depth was not searched.

Gabr showing off his record to his friends. Photo courtesy of DeeperBlue.com.

Finally, I asked Gabr about his reported high energy level following the dive, which contrasted sharply with Gomes’ experience who had to be helped to walk (See Part 1). Gabr, who is reportedly very fit, explained that, unlike Gomes, he removed his quads at 21 m/70 ft and so only wore them for 4.5 hours. Gabr said that he also hydrated and ate every hour during his shallower deco stops, which he asserted Gomes’ did not. He also pointed out that at 41-years old at the time of the dive, he was 12-years younger than Gomes when he set his record. Ouch! I later checked with Gomes’ who said that he also hydrated and ate. As discussed in Part 1, Gomes insists the quads were not an issue for him. We ended the post-dive discussion with Gabr pointing out that its difficult to compare one person to another.

What now?

I asked Gabr what he intended to do. To my surprise he launched into a story about a sustainability project he was involved in with the first Egyptian to climb Mt. Everest. He then told me about a shipwreck exploration project in the Mediterranean in late September that he planned to do a pilot film about. I finally interrupted him. “But what about the allegations?” 

“I have no idea,” he said. “What do you think?” “It’s up to Guinness,” I offered. “No, Guinness is done with it. It’s over,” he said. I asked Gabr if he had been in touch with or heard from Guinness or his project leader. He said he hadn’t, adding that, “They have a bullet-proof system.” He went on. “I am confident in what I did. But it bothers me that my reputation has been targeted. Why do I have to justify myself? I am confident, but I’m being hurt,” he said. 

He went on. “Why are they doing this now, after six years,” he asked rhetorically. “Usually people try to discredit a record at the time it is made. But why six years later?”

“Do you think it’s because of the documentary film?” I asked. “I’m sure about that,” Gabr replied. “It’s the same person who was with the team. He was trying to sell helium illegally in my name and got me into trouble. He wants to discredit my whole record and the movie,” he said.

I asked if Netflix was going to release the documentary, which several people had told me about. Gabr said, no, that the documentary, which had investors, had not been sold yet. “Netflix didn’t buy it?” I asked. “No,” he said. I asked him several times if he had a financial stake in the film but did not get a clear answer, though he acknowledged he was under contract. 

Gabr then said about his anonymous accusers. “They are not searching for the truth. They only mention half the truth. There was no peer review from professionals. My record would be discredited if I had relied on one of the computers. For sure. But that was not the measurement methodology.”

I asked Gabr if he would be willing to have a peer group review all the information and possibly clear him. Gabr said that he was open to anything. He said that he has been using the same video in his talks as presented in the emails and is always willing to sit and discuss things at every dive show he attends. “I don’t mind talking about my dive,” he said.

Conclusions?

There is obviously a lot of information to unpack, and numerous additional details, as well as information protected by confidentiality that I have omitted here. It’s clear to me that I am not in a position to render a judgement, nor is it my job. My job has been to ask questions, lay out what appear to be the facts as best and fairly as I can and point out inconsistencies as I see them. 

As I said before in Part 1, it’s up to Guinness to decide if they want to investigate the allegations or not, and on that basis to determine if Gabr completed a record dive or not. However, given the widespread distribution of allegations about the record, and what seem like surprising discrepancies, it’s likely that his record will remain under a shadow until the full truth is known, whether by Guinness or perhaps, as Gabr suggests, through a peer review by professionals. 


Michael Menduno is InDepth’s editor-in-chief and an award-winning reporter and technologist who has written about diving and diving technology for 30 years. He coined the term “technical diving.” His magazine aquaCORPS: The Journal for Technical Diving (1990-1996), helped usher tech diving into mainstream sports diving. He also produced the first Tek, EUROTek, and ASIATek conferences, and organized Rebreather Forums 1.0 and 2.0. Michael received the OZTEKMedia Excellence Award in 2011, the EUROTek Lifetime Achievement Award in 2012, and the TEKDive USA Media Award in 2018. In addition to his responsibilities at InDepth, Menduno is a contributing editor for DAN Europe’s Alert Diver magazine and X-Ray Magazine, a staff writer for DeeperBlue.com, and is on the board of the Historical Diving Society (USA).

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