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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.

Cave

A Perspective on Teaching Cave CCR

Veteran Irish cave and CCR instructor cum sports psychologist Matt Jevon explains how he teaches divers to become competent underground rebreather divers who “err safely” and thus are likely to return home at the end of the dive.

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by Matt Jevon
Header image courtesy of Marissa Eckert

“To err is human” Alexander Pope

In his “Essay on Criticism,” Alexander Pope wrote “To err is human, to forgive divine.” However, if you are not prepared to err safely in cave or rebreather diving, you will come face to face with your preferred divine being, begging for forgiveness.

Stratis Kas’s book, Close Calls, a compilation of stories from a roll call of “who’s who” in diving, attests to the fact that the very best of us can and do make mistakes, or err. That they are still here to share these lessons with us affirms the huge amount of training, preparation, and experience required—and, as many will admit, no small amount of luck.

Gareth Lock, author of Under Pressure, is fond of the phrase “fail safely,” and with good cause. As he puts it, and I paraphrase; the human in the machine is at the heart of likely outcomes. In my own experience as a psychologist with expertise in human performance, the best systems, processes, and technologies are often outwitted by an unwitting fool or an arrogant wise man.

The ultimate dream of becoming a cave diver accessible for all? Photo by Marissa Eckert.

Today there is a surge of divers wishing to become cave divers, perhaps because it is perceived by some as the pinnacle of diving—in skill and status—or perhaps because it is seen as more accessible. Certainly, social media has given access to the incredible and beautiful environments that were once the playground of a select few. Divers are discovering that cave and modern diving practices, equipment, and training are making it a much safer environment until they start exploring virgin caves. Closed circuit rebreathers (CCR) are now mainstream and in wide use by many divers. In cave and deep dives, I would say they have become the primary tool; the limitations of open circuit scuba are seen as making it inappropriate for most “big” dives.

By the time a diver reaches their CCR cave course, they will, or should be, a knowledgeable, skillful, and competent diver on a CCR. Perhaps the odd one will find a shortcut, but it is the exception rather than the rule. In addition, the majority will already have some open circuit cave training, at least to intro level if not to full cave. The pathway from zero to hero in the cave is much longer and more difficult to shortcut than, say, open water to instructor status. Starting cave diving on CCR from cavern to full cave is, and should be, a much longer route.

[Ed.note: There are arguments against allowing a student to pursue any form of diving before gaining open circuit experience. Some argue that one should first become competent on open circuit in the relevant environment and THEN train in that environment on RB/CCR. This argument asserts that RB failures will find a diver on open circuit, requiring them to be proficient on this equipment in the relevant environment. These factors may be progressively more relevant with more complex environments.]

So, the CCR cave instructor is not dealing with an inexperienced CCR diver; nor, if they are as careful in their acceptance of students as most are, will they be dealing with an adrenaline seeking-junkie. See “Why We Cave Dive” (video) for reasons why some divers seek out the karst realm, as well as examples of divers we hope to encourage into the sport and those we prefer to avoid it.

How do we handle tricky situations? Photo by Mirsoslav Dvoracek

The job of a cave CCR instructor is not to prevent all errors or mistakes. It would be both arrogant and foolish to believe that instructors can overcome human nature and the situational factors found in closed circuit cave diving. The instructor’s role is to lessen the frequency and severity and to mitigate the consequences of those errors as, and when, they occur. The instructor must do this in the course, ideally exposing students to likely errors or challenges in controlled conditions and embedding appropriate solutions. Students should acquire appropriate and controlled emotional, cognitive, and behavioural responses.

Being a cave instructor has a few significant differences from being a deep technical rebreather instructor. Here are a few:

Kit

Cave diving demands a greater equipment load. The number of backups can be summarised as “Three is two, and one is none.” So, three sources of light sufficient to complete an exit, three cutting devices, reel/spools, markers, breathing sources, and more. Before entering any overhead environment, the instructor must help students configure, become familiar with, and master accessing and manipulating their configuration. For this reason, cave divers opt for simple, easy solutions that are robust and definitely not prone to failure. This applies to their primary gear (CCR choice) and to every single piece of backup gear.

Preparing the kit. Photo by Petr Chmel.

My own choices are primarily sidemount-based in the cave; the Liberty rebreather; Divesoft computers, primary reel and markers; plus O’Three 90ninety shell suit; and Apeks regulators and spools, all based on a Razor Sidemount System. In backmount I use a JJ-CCR, but I am now using the Liberty Sidemount rebreather as a bailout system. All simple, proven, tough, and each piece having substantial built-in redundancy/failure management options.

Safety

The instructor’s primary role—despite what many believe to the contrary—is, in any diving, to ensure that the students are safe and that they go home unharmed medically, physically, or mentally. Secondary to this is teaching skills, having fun, and awesome and epic dives. What a big ask in cave diving!

Progression in open water diving is more straightforward, especially using mixed gas. In the absence of narcosis, divers can build up deco time gradually and have a pre-rehearsed familiar exit/ascent permanently above them. Although not different in terms of time to exit, an open water deco ceiling somehow seems, to most, to be less of a psychological threat than several hundred tons of rock.

Penetration stress can occur within only a few meters. Photo by Kathrine Livins.

I have seen cave divers suddenly go from a point of being perfectly happy to being very unsettled and distressed within a few meters. There is actually a term for this: penetration stress. Penetration should be slowly built up over time with confidence in the linear distance built through many dives—some, but not all, including stressful exits (blind, bailed out, manual control, or touch contact). 

To do this, an instructor needs considerable empathy. Some instructors may shy away from this and instead use a pseudo militaristic approach by battering, bullying, or belittling the student, constantly tearing off masks, shutting down gas, or more. (We are talking personality types here, not problem solving training.) Stay away from these people at all costs.

Instructors cope with a high task load. Not only do they have to monitor the group’s penetration distance, navigation, and teamwork, but they also need to monitor the students and their own PO2, decompression obligations and time to surface (TTL), bailout supply and limits, on board gas supply, scrubber durations, as well as to teach. In order to do this, a few tricks are employed. Some of these may be useful when diving in any CCR team:

  1. PO2 monitoring. HUDS can easily be seen reflected in students’ masks. It’s much easier than trying to read someone’s handset.
  2. Instructor Ghost Mode. Not just used for sneaky (pre-warned and planned) drills, the lights off/blackout ghost mode is often accompanied by pull and glide along ceilings or cave walls where no damage to the environment is possible. Instructors, especially on quiet CCRs, can get within a few centimeters of a student without their knowing, or they can shoot ahead. It is a bad practice to turn off (as opposed to cover) one’s primarily in a cave. The on-off button/switch is a weak point, especially at depth—sufficient working backups are required.
  1. Buddy lights on CCRs are brilliant for instructor/team monitoring, Divesoft’s show up well and Sentinels almost too well. When I was a student, my instructor found ghost mode difficult to fully pull off, since I saw this green light above me every time he tried it!
Lights are an integral part of safely experiencing a cave. Photo by Kathrine Livine.

Teamwork

As an instructor, you want the students to develop their own robust team dynamic. If you are part of this, too often, students will always defer to your authority and default to you for leadership and solutions. So, if you do join the team to make up numbers, always be number 2, the weakest member, and play the part. Students don’t need to see how clever or skilled you are, they need to develop their own skills.

Navigation: Know the cave you are teaching in. For students’ first dives where I may not know them or their capabilities, I like to be in caves where a lost line would not be an issue for me in terms of exiting. Take Ressell in France for instance: A quick glance at the ceiling and a look at the scallop shapes in the rock, and I know which way is out.

Know where you are and where you’re going. Photo by Miroslav Dvoracek.

Pre-Dive Checks

These assume a whole other level of importance in cave and rebreather diving. A checklist is useful but only if you properly check everything on it. Turn backup lights on and off, breathe bailout regs at least 4-5 breaths. Fill and dump wings and drysuits. Prevention will ensure survival. It will also give students confidence, which means you are less likely to have issues, you’ll get a better response if you do, and you can actually enjoy the dive. [Ed.—Check out GUE’s Pre-Dive Sequence here]

Here are a few tricks I also like to instill:

  1. Link routines. For example, PO2 check and back reference. I use a hand mirror, so looking back is easy, and a quick over the shoulder is not difficult. Every time I check PO2, I look behind me. Caves often look very different on the way out and if I can, I will mentally imprint landmarks that I will see on exit. Some caves have distance markers every 100-150 m/328-492 ft on the main line, especially training caves. I’m not a huge fan of these for my own diving, as it’s a bit like graffiti; but, for trainees, PO2 plus back reference anytime you pass any navigational marker is a good routine.
  2. Wetnotes use. A good habit in a new cave is to make a note of time, distance, gas, and the navigation marking/direction in your wetnotes at any substantive navigation. On some dives, this will be two or three notes. Do this in some Mexican caves and you will get about 300 m/0.2 miles from the entrance and need a new Wetnotes book, so be sensible!


Finally, students will learn a lot of new skills, from what to do when you lose teammates, lose or become entangled in the line, encounter a broken line, have light and equipment failures, and more. Many of these will be done with blindfolds or blacked-out masks (mine say, “Use the force” on the front). On an open circuit, these situations can be challenging. On CCR, doing blackout drills while controlling loop content and volume, handling multi bailouts, and more, requires time both to learn and to embed. Don’t do it until you get it right, do it until you can’t get it wrong. Sometimes the lost line drill will provide unique challenges to get it right. If conducted correctly, you will probably get it wrong half the time!

Ultimately, graduating a new CCR cave diver is a moment to enjoy for the instructor—one with a need for appropriate gravitas and consideration. I have certified divers who were less proficient than other divers that I failed or asked to repeat. That was because a student’s attitude, mental strength, and sound decision-making ensured that they would likely go home safely from each dive. As the sign posted at the entrance of almost every cave reads, “Nothing in this cave is worth dying for.” There is an awful lot of cave diving worth living for, and I have been privileged to see some spectacular caves.

References:

 Barnson S.C. (2014) The Authentic Coaching Model: A Grounded Theory of Coaching. Human kinetics, Champaign, Il.

Troy A. Moles, Alex D. Auerbach & Trent A. Petrie (2017) Grit Happens: Moderating Effects on Motivational Feedback and Sport Performance, Journal of Applied Sport Psychology, 29:4, 418-433, DOI: 10.1080/10413200.2017.1306729

Swann, C., Crust, L., Jackman, P., Vella, S. A., Allen, M. S. & Keegan, R. (2017). Performing under pressure: Exploring the psychological state underlying clutch performance in sport. Journal of Sports Sciences, 35 (23),2272-2280.

Additional Resources:

The Darkness Beckons by Martyn Farr

Basic Cave Diving a Blueprint for Survival by Sheck Exley (Freedownload)

Facebook Pages/Groups:

Psychological Skills for Diving @PSTforDIVING

Diving Educators – Teaching and Learning Techniques (Group)

Human Factors in Diving

Web pages:

www.mattjevon.com

www.swt.ie

www.thehumandiver.com


Matt Jevon, M.Sc. F.IoD, is a Full Expedition level Trimix and Cave instructor on OC and CCR with TDI and ANDI. He is a JJ-CCR and Divesoft Liberty Sidemount instructor and dealer for Ireland. Matt’s personal diving has included cave exploration in the Philippines and wreck projects in Croatia and Ireland, and he was one of the inaugural Dirty Dozen in Truk! Matt has held accreditations as an interdisciplinary sports scientist, sports psychologist with the British Association of Sport and Exercise Sciences (BASES), and was a British Olympic Registered Strength and Conditioning Coach and invitee on the Olympic Psychology Advisory Group. Matt works in the high performance business as a board advisor and non-exec, high performance sport, and expeditionary level diving as a partner in South West Technical Diving in Ireland (), and hosts the Facebook page “Psychological Skills for Diving.”

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