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The Data: Haarlemmermeerse Bos



By Axel Gunderson

We have been collecting temperature, visibility, and photographs from the man-made lake in Haarlemmermeerse bos, a 115-hectare park-like area to the northwest of Hoofddorp in the Netherlands since November of 2011. Our location is a citizen science playground.

The lake is isolated, fairly small (22 hectares) with a maximum depth of 20 meters. The most interesting feature is a water mixing system, which the water authority phased out over a period of three years. It had been operational since 1999 to help minimize cyanobacteria blooms until 2012. In 2013, the mixing system first became inactive because it was written off due to the expense. In June 2014, we still found five varieties of stonewort (Chara), which represents good water quality and is a perfect visible bioindicator. In August 2014, the first cyanobacteria bloom was suppressed by starting the mixing system again, which was kept in standby mode between 2013 and 2015.

As of 2013, the spined loach, a native fish, disappeared. It had been around for years as well as a species of freshwater jellyfish, but the latter can be related to stratification. The jelly fish reappeared in 2015 when the mixing system was active (at 40% power) for the entire season and no cyanobacteria blooms occurred. Stonewort, an aquatic plant species of the genus Chara and a valuable bioindicator, went from five to one species in two years’ time. As of 2017, stonewort had disappeared from the lake. Funny enough, the two main parameters, vertical visibility and temperature, remained rather constant over the span of seven years. The average temperatures are between 11.7 and 13.6 degrees Celsius at three meters depth, and the average vertical visibility is between 5.5 and 6.1 meters.

These two parameters did not seem to correlate with the disappearance of the stonewort. Chemical values of the water did change rather drastically though. Thanks to Harry van Goor, a rise of phosphate levels since 2014 was measured. That is just one of the catalysts for cyanobacteria blooms. This is a fact: Warm, high-phosphate water (upto 1,8 Mg/L) is pumped in surface water of the lake in the summer to maintain a certain water level. A sufficient water level allegedly prevents the influx of saline well water.

In 2018, hydrogen peroxide was applied to kill cyanobacteria for the first time. Two weeks later, another cyanobacteria species became very abundant. We also noticed a high mortality rate amongst older bivalves (freshwater molluscs), while younger specimens were unaffected.

My view based on collected data is that a new water mixing system suppresses cyanobacteria blooms but can’t stop them, and it’s very expensive. But, what if the phosphate source was taken out of the equation? Our goal for 2019 is to monitor saline levels near the bottom in relation to fluctuating water levels. Our Project Baseline initiative reports changes to the water agency, as well as vets ideas and theories about the status of the lake. Check out the data collected in the lake by visiting the Project Baseline’s online spatial database. View more of our data.

The most worrying is this graph of phosphate levels in 2018. Blue bars represent 2017 and red bars represent 2018. Graph by H. van Goor
Average phosphate levels over 5 years. Graph by H. van Goor
Temperature data recorded in the lake by PB: HBOS from 2011 until 2018. Temperature is displayed in Celsius and taken at a depth of 3 meters.
Visibility data was collected at the same place as the temperature from the graph above. The visibility is recorded using a Secchi disk. The Project Baseline: HMBOS
team monitors this station every two weeks, and changes are sometimes very spectacular. The station is on one of the vertical concrete piles that supports a restaurant and stands free in the water at about 13 meters. At 3 meters depth, a simple station was created by using a rope tied around the pile. At this depth, light and temperature are variable during seasonal changes.

Interested in more data? Check out last month’s data highlight.

Axel Gunderson has been diving for 12 years and can’t get enough of it. In 2011 he got his GUE Fundamentals certificate.That soon lead to starting a Project Baseline location in his favorite lake, which lies in the town of Hoofddorp in the Netherlands.

The Project Baseline initiative is driven by data collection. With an online spatial database that hosts the collection efforts of over 100 teams in over 30 countries, anyone can access the temperature, visibility, and images from these aquatic locations.


DAN Europe’s Sustainable Tour




Caring for the Ocean starts by taking the right steps on land

Why CO2 matters

The ocean has the capacity to absorb CO2 from the environment. When excess CO2 dissolves in seawater, it causes an acidity increase. This is known as Ocean Acidification. 

This phenomenon, coupled with other human factors, is destroying marine ecosystems and impacting many ocean species, especially organisms like oysters and corals. We must drastically reduce our CO2 emissions if we want to maintain a healthy ocean and a planet fit for our survival.

To raise awareness, DAN Europe Ambassadors are traveling across Europe and its surroundings in an electric vehicle (recharging it only with renewable energy), meeting with stakeholders and the entire diving community to—literally—drive the change we all need.

25,000 km of new opportunities

From 2021 onwards, DAN Europe has included helping the diving industry to take a more sustainable path, in its own sustainability plan

The Sustainable Tour was born as the result of paying attention to the major threats facing our playground as divers. It’s an environmental education project aimed at dive centres, which seeks to raise awareness about the close relationship between Climate Change and Ocean Acidification.

Last summer, the Sustainable Tour traveled 25,000 km carrying its message through 17 European countries.

These were the results:

  • 70+ meetings with sustainable change leaders
    30+ private companies
    30+ diving centres & clubs
    15+associations working for ocean conservation and sustainable mobility
  • 40+ press mentions In seven different languages
  • 33 blogs 
  • 15 Instagram (IG) Lives
  • 5 lectures: marine conservation
  • 5 Live interviews for the diving industry media
  • 1 Podcast: Hyundai “Are we there yet”

This is no time to stop—we must keep driving for Ocean Conservation!

In 2022, the Sustainable Tour returns, and this time it is the UK’s turn! DAN Europe Ambassadors will tour the entire territory, visiting some 50 dive centres in the area.

The programme will include:

  • Conference: “The link between Climate Change and Ocean Acidification”
  • Special activity with prizes: “DAN Europe by the sea contest”
  • Dedicated Events: Medical Research while Diving Against Debris

The role of DAN Europe in Ocean Protection

DAN Europe exists to assist and protect all divers through the most extensive and reliable network of diving physicians worldwide. We offer the community numerous pioneering services, including the first-ever insurance designed specifically for divers. 

Diving safety is our motto. 

Nowadays, knowing how critically threatened marine ecosystems are, we cannot talk about diving safety if we do not talk about ocean safety too.

If we do nothing, letting CO2 levels continue rising, by 2050 most coral systems in our oceans will be gone. Divers will no longer find any of the wonders shown by Cousteau, and if no one wants to dive any more, even our very existence as an organisation who cares for divers would be pointless.

We recognise the unique beauty and fragility of the Blue Planet and act as ambassadors for more sustainable practices within the diving community.

“We do not inherit the Earth from our ancestors; we borrow it from our children.”

—Native American proverb

We recently asked a group of children what they wanted to be when they grew up. Their answers astonished us. 

We remember that when we were little, whenever someone asked us this typical question, we would immediately go through the repertoire of known professions in the world and choose the one we thought was the most fun at the time, maybe a pediatrician or a teacher.

This does not seem to be the case with the children of this new generation. 

Most of the children immediately responded: “When I grow up, I want to save the planet and the ocean.”

Future generations deserve more from us. We need to act now!

Join us along the way as we meet stakeholders and the whole diving community to boost the change we all need: / IG @daneurope / Twitter @DAN_Europe / FB DAN Europe (Divers Alert Network Europe) / LinkedIn @DAN Europe

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