# Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

Pierre Cadeau, Didier Jézéquel, Alexis Groleau, Andrea Di Muro, Magali Ader

► To cite this version:

Pierre Cadeau, Didier Jézéquel, Alexis Groleau, Andrea Di Muro, Magali Ader. Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake. Comptes Rendus. Géoscience, 2022, 354 (S2), pp.1-18. 10.5802/crgeos.172. hal-03936479

HAL Id: hal-03936479

https://hal.science/hal-03936479

Submitted on 12 Jan 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Comptes Rendus
## Géoscience
### Sciences de la Planète

Pierre Cadeau, Didier Jézéquel, Alexis Groleau, Andrea Di Muro and Magali Ader

**Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake**

Published online: 9 December 2022

https://doi.org/10.5802/crgeos.172

**Part of Special Issue:** The Mayotte seismo-volcanic crisis of 2018-2021 in the Comoros archipelago (Mozambique channel)

**Guest editors:** Jérôme Van der Woerd (Institut Terre Environnement de Strasbourg, UMR 7063 CNRS / Université de Strasbourg, 67084 Strasbourg, France), Vincent Famin (Laboratoire Géosciences Réunion, Université de La Réunion - IPGP, 97744 Saint-Denis, France) and Eric Humler (Professeur Université de Nantes, Laboratoire de Planétologie et Géosciences, UMR 6112, Faculté des Sciences et Techniques, Nantes Université, 44322 Nante, France)

<figure><img src="image_2.png" /></figure>

This article is licensed under the
CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL LICENSE.
http://creativecommons.org/licenses/by/4.0/

<figure><img src="image_3.png" /></figure>

Les Comptes Rendus. Géoscience — Sciences de la Planète sont membres du
Centre Mersenne pour l'édition scientifique ouverte

www.centre-mersenne.org

e-ISSN : 1778-7025
the Comoros Archipelago were known for their dif-
fuse and moderate seismicity with few historically
felt events. The ongoing seismic sequence is thus un-
expected and remarkable with regard to its duration
and intensity (i.e. earthquakes of magnitude 5.9 on
15 May 2018). After the first two months of intense
seismicity at the beginning of the crisis, seismicity
still persisted a long time with numerous small earth-
quakes [Bertil et al., 2019], and ground deformation
was observed [Lemoine et al., 2020]. A new subma-
rine volcano was discovered at 50 km east of Petite
Terre [Feuillet et al., 2019, 2021], and significant subs-
dence of Mayotte Island (up to 20 cm) was attrib-
uted to the drainage of a deep magma chamber near
the coast [Foix et al., 2021]. This long-term seismic
sequence and the associated new active volcano off-
shore Mayotte led the scientific community to inves-
tigate further the seismo-volcanic hazard in this area
[Feuillet et al., 2019, Cesca et al., 2020, Darnet et al.,
2020, Lemoine et al., 2020, Berthod et al., 2021, Foix
et al., 2021, Saurel et al., 2022]. This seismo-volcanic
crisis has been monitored since the beginning of May
2018; numerous studies were performed and many
others are in progress to gain further information
about the deep volcanic system, to better constrain
the seismo-volcanic hazard for Mayotte and its pop-
ulation (e.g. seismicity, slope instability, tsunami, is-
land subsidence or even new volcanic activity on-
shore).

On the Petite Terre Island in Mayotte, one of the
latest eruptive events proposed at around 7 and 4 ky
is probably at the origin of the Dziani Dzaha Lake for-
mation [Zinke et al., 2003]. For the last decade, this
atypical lacustrine system has been extensively stud-
ied [Leboulanger et al., 2017, Cellamare et al., 2018,
Gérard et al., 2018, Milesi et al., 2019, Aucher et al.,
2020, Jovovic et al., 2020, Sala et al., 2022]. Based on
its water chemistry, the lake was most likely initially
filled with seawater [Sarazin et al., 2021], and has
since evolved as a closed system. Before the seismo-
volcanic crisis, the physical, chemical and biolog-
cal features of this ecosystem were far from those
observed in a modern oceanic or lacustrine envi-
ronment (details in Study site section). The ongoing
seismo-volcanic crisis offshore Mayotte could have
a significant impact on the stability and the func-
tioning of this ecosystem through the reactivation
of fractures in the crater/basement, the infiltration
of fresh/sea water and/or the increase in magmatic

bubbling and degassing through its waters [Liuzzo
et al., 2021].

In order to investigate the impact of the pertur-
bations related to the growth of the submarine vol-
cano offshore Mayotte on this lacustrine system, we
conducted three field trips in 2020 and 2021 on the
Dziani Dzaha Lake. We documented the evolution
of the physical and chemical properties in the wa-
ter column (e.g. [O₂], pH, salinity, T), as well as
the carbon and nitrogen isotopic signatures of dis-
solved inorganic carbon (DIC) and suspended partic-
ulate matter (SPM)) within it (i.e. δ¹⁵NSPM, δ¹³CDIC,
δ¹³CSPM). The comparison of these results with those
obtained before the seismo-volcanic crisis illustrates
the response of this lacustrine system to this kind of
geological perturbations.

## 2. Study site

The Dziani Dzaha Lake is a shallow maar lake located
on the Petite Terre Island of Mayotte (Figure 1). This
lake is small (i.e. 2.36 × 10⁵ m²), close to sea level and
separated from the nearby seashore by a 220-m-thick
crater wall [Rouwet et al., 2021]. The lake watershed
is restricted to the volcanic crater (i.e. ~1 km in di-
ameter and 50–100 m height). The average depth of
the Dziani Dzaha Lake is about 3 m with a narrow
depression reaching 18 m depth (Figure 1), likely re-
lated to the chimney of the phreatomagmatic erup-
tion at the origin of this lake. Magmatic gases bubble
through the water column in several locations [Milesi
et al., 2020, Liuzzo et al., 2021] mostly in shallow ar-
eas of the lake, with one bubbling site in the central
part for which a strong increase in the bubbling ac-
tivity has been observed since 2020 compared to our
last observations from 2012 to 2017 (Figure 1).

From 2010 to 2017, i.e. before the beginning of
the seismo-volcanic crisis in May 2018, the lake wa-
ter column showed an atypical combination of phys-
ical and chemical features for a modern lacustrine
system [e.g. Sarazin et al., 2021]. Salinity measured
was between 35 and 70 psu (practical salinity unit),
nearly twice the usual salinity of seawater. The alka-
linity was also very high, with values ranging from ca.
0.1 to 0.2 M (nearly 100 times the alkalinity of sea-
water), and pH ranged between 9 and 9.5. The water
column was periodically stratified with a sharp halo-
cline at ca. 2 m depth about half of the year, driven
by the salinity decrease in surface waters as a result
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Location and bathymetric maps of the Dziani Dzaha Lake on Mayotte Islands (Indian Ocean).</figcaption></figure>

(A) Mayotte Islands in Western Indian Ocean, (B) Dziani Dzaha Lake in Petite Terre Island on Mayotte,

(C) Picture of the lake taken in October 2014, and (D) Bathymetric map of the lake, with blue circle representing the water column sampling location and the red circles representing the identified bubbling areas at the water-air interface (modified from Leboulanger et al. [2017] and Cadeau et al. [2022]).
of increasing precipitation during the rainy season. A permanent chemocline was also present at ca. 14 m depth in the narrow depression [Sarazin et al., 2021]. Importantly, the lake was permanently anoxic below ca. 1.5 m depth despite seasonal variations in its water column structure, and was strongly euxinic (sulfides $H_2S/HS^−$ concentration ranging from 2 to 6 mM) below 2 m depth when the halocline is present (during the stratified period), or only below a 14 m depth when it is not [during the non-stratified period, Sarazin et al., 2021]. Details on the evolution of the physical and chemical parameters according to stratification are available in previous works [e.g. Leboulanger et al., 2017, Cadeau et al., 2020, Sarazin et al., 2021]. All our field trips done since the beginning of the crisis were conducted during non-stratified periods (i.e. from August to December). Before the crisis, these non-stratified periods were characterized by the absence of halocline (except at ca. 14 m depth). As a result, most physical, chemical and biological parameters were constant with depth down to the deep chemocline at 14 m depth, except for the dissolved oxygen that was only present down to a maximum of 1.5 m depth depending on the photosynthetic activity. The fact that $O_2$ saturation values fluctuate widely (from 0 to 350%) is related to large nycthemeral fluctuations in the balance between the activities of photosynthesis (which produces $O_2$) and respiration (which consumes it) in this biologically very active lacustrine system [e.g. Leboulanger et al., 2017, Cadeau et al., 2020]. The salinity was close to 60 psu, the alkalinity close to 0.14 M, the pH close to 9.2, the $[SO_4^{2-}]$ content close to 3 mM, $[H_2S]$ content close to 0.1 mM, and a relatively homogeneous diversity of cyanobacteria, bacteria, and archaea was observed along the water column [Hugoni et al., 2018]. Below the 14 m deep chemocline, the salinity and alkalinity increased to 70 psu and 0.2 M, respectively, the pH decreased to a value close to 9, no $SO_4^{2-}$ was observed while $H_2S$ accumulated up to 6 mM. Our previous work has demonstrated that the characteristics of non-stratified seasons have remained stable since 2010 [e.g. Leboulanger et al., 2017, Hugoni et al., 2018, Bernard et al., 2019, Sarazin et al., 2021].

## 3. Materials and methods

The isotope signatures in this study were acquired on samples from surveys conducted in November 2020,

September 2021 and December 2021. A nomenclature consistent with previous publications was used to identify each sample taken during the different surveys. For example, for the sample "DZ21-9 2m," "DZ" indicates the Dziani Dzaha Lake, "21-9" indicates the year (2021) and month (September) of the survey, "2m" refers to sampling depth of the water column station called "18m" as shown on Figure 1.

Water samples were collected using a horizontal 1.2 L Niskin bottle along a vertical profile at the 18 m depth station for each survey. For each sampling depth, samples were taken in 500 mL plastic bottles. Each sample was then filtered onto pre-combusted Whatman GF/F glass fiber filters (0.7 µm) and quartz Millipore AQFA (size cut-off ca. 3 µm) for $\delta^{13}C_{POC}$ and $\delta^{15}N_{PON}$ analysis, and a 12 mL Labco Exetainer glass tube was filled with the filtrate and fixed with 0.2 mL of saturated $HgCl_2$ solution for DIC and $\delta^{13}C_{DIC}$ analyses. The GF/F filters with SPM were decarbonated by exposure to concentrated HCl fumes in a desiccator for 12 h to avoid loss of material.

### 3.1. *Physical and chemical in situ measurements*

*In situ* profiling were performed with a CTD multiparameter EXO2 probe (temperature, conductivity/salinity, pH, ORP, $O_2$, turbidity, chlorophyll, phycocyanine (PC) and fDOM) and a turbidity AQUalogger 210 TYPT Aquatec probe. The pH electrode was calibrated using pH 4.00, 7.01 and 10.00 NBS-NIST buffers; $O_2$ optode was calibrated at 100% saturation in wet air; salinity sensor was calibrated with a 34.271 psu seawater. Other sensors have not been calibrated and the corresponding parameters are used as relative values; nevertheless, most of these sensors offer quite stable response (especially T, turb., chl., PC, fDOM), at the exception of oxidation-reduction potential (ORP) electrode which can only be used as relative values. In addition, two $O_2$ NKE SDOT optodes were deployed at 0.25 m depth from 10/09 to 13/12 2021 near the central gas bubbling zone (at ca. 1–3 m from the outlet, respectively), and a water level probe (OTT Orpheus Mini) was installed in September 2021 in the NO part of the lake.

### 3.2. *Carbon and nitrogen isotope measurements*

DIC and $\delta^{13}C_{DIC}$ were measured using an AP2003 mass spectrometer. Phosphoric acid (100% $H_3PO_4$)
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Vertical profiles of physical and chemical parameters recorded during each survey including salinity (psu), temperature (°C), pH and dissolved oxygen (%). The grey profiles correspond to previous data obtained before 2018 from Cadeau et al. [2022].</figcaption></figure>

was injected into 12 mL Labco Exetainer tubes that were then flushed with helium. For each sample, 0.1 mL water was sub-sampled and transferred into the acidified Labco Exetainer tubes. The samples were agitated for 15–24 h so that CO₂ in the water was in equilibrium with the CO₂ in the headspace. The CO₂ in the headspace gas was then separated by gas chromatography with helium as a carrier gas and analyzed using the AP2003 mass spectrometer. Solutions presenting a range of DIC were prepared using sodium hydrogen carbonate for DIC calibration. Three internal carbonate standards with known isotopic compositions (Across, Merck and Rennes II) were used for isotopic composition calibration. The standard carbonate powder was loaded into 12 mL Labco Exetainer tubes with 0.1 mL distilled water before being flushed with helium and then analyzed in the same way as DIC. DIC values are expressed in mol·L⁻¹ with a reproducibility of ±0.01 mol·L⁻¹, and δ¹³C_DIC values are expressed in *h* relative to Vienna Pee Dee Belemnite (VPDB) with a reproducibility of ±0.1*h* (1σ).

The δ¹³C~POC~ and δ¹⁵N~PON~ of the decarbonated fil-
ters were measured using a Flash EA1112 elemen-
tal analyzer coupled to a Thermo Finnigan Delta-
plus XP mass spectrometer via a Conflo IV interface
(Thermo Fisher Scientific, Waltham, MA, USA). The
decarbonated samples were loaded into tin capsules

and heated to 1200 °C in a combustion tube with
a mixture of chromium oxide and silver cobalt ox-
ides. The combustion gases were carried by helium
through a reduction column and a gas chromatog-
raphy column to separate CO₂ from N₂ and from
the other gases. CO₂ and N₂ were successively in-
jected into the mass spectrometer for isotopic anal-
ysis. For the isotope calibration, four internal stan-
dards of organic-rich soil or sediment were analyzed
in the same way. δ¹³C~POC~ values are expressed as *h*
relative to VPDB with a reproducibility of ±0.2*h* (1σ)
and δ¹⁵N~PON~ are expressed as *h* relative to air with a
reproducibility of ±0.3*h* (1σ).

4. Results

4.1. *Physical and chemical features of the water column*

In September 2020, salinity was constant down to 14 m depth with an average value close to 55.6 psu and increased below this depth up to 59.7 psu (Figure 2, Table 1). pH was constant down to 14 m depth with an average value of 9.1 and decreased below this depth down to 8.6. Temperature decreased in surface waters from 34 °C to 30.3 °C at 1.5 m depth, and remained constant with depth down to the lake bottom. In September 2021, salinity, pH and temperature significantly decreased in the water column.
<table><caption>**Table 1.** Salinity (psu), temperature (°C), pH and dissolved oxygen (%) measured in November 2020, September 2021 and December 2021 in the Dziani Dzaha Lake</caption><thead><tr><th>Sample (name)</th><th>Depth (cm)</th><th>Temperature (°C)</th><th>pH</th><th>[O<sub>2</sub>] (%)</th><th>Salinity (psu)</th></tr></thead><tbody><tr><td>DZ20-11</td><td>0.25</td><td>34</td><td>9.07</td><td></td><td>55.6</td></tr><tr><td>DZ20-11</td><td>1.5</td><td>30.3</td><td>9.09</td><td></td><td>55.6</td></tr><tr><td>DZ20-11</td><td>2.5</td><td>30.5</td><td>9.095</td><td></td><td>55.6</td></tr><tr><td>DZ20-11</td><td>5</td><td>30.4</td><td>9.093</td><td></td><td>55.6</td></tr><tr><td>DZ20-11</td><td>11</td><td>30.5</td><td>9.083</td><td></td><td>55.8</td></tr><tr><td>DZ20-11</td><td>14</td><td>30.3</td><td>9.038</td><td></td><td>57.3</td></tr><tr><td>DZ20-11</td><td>16</td><td>30.7</td><td>9.009</td><td>0.2</td><td>58.1</td></tr><tr><td>DZ20-11</td><td>17</td><td>29.8</td><td>8.610</td><td></td><td>59.7</td></tr><tr><td>DZ21-9</td><td>0.25</td><td>32.2</td><td>8.6</td><td>327</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>0.50</td><td>30.4</td><td>8.6</td><td>212</td><td>55.8</td></tr><tr><td>DZ21-9</td><td>0.75</td><td>29.5</td><td>8.6</td><td>149</td><td>55.6</td></tr><tr><td>DZ21-9</td><td>1.01</td><td>29.1</td><td>8.6</td><td>128</td><td>55.3</td></tr><tr><td>DZ21-9</td><td>1.25</td><td>29.0</td><td>8.6</td><td>116</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>1.50</td><td>28.9</td><td>8.6</td><td>100</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>1.75</td><td>28.8</td><td>8.6</td><td>93</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>2.01</td><td>28.8</td><td>8.6</td><td>91</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>2.26</td><td>28.7</td><td>8.6</td><td>84</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>2.51</td><td>28.7</td><td>8.6</td><td>80</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>2.76</td><td>28.7</td><td>8.6</td><td>73</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>3.01</td><td>28.6</td><td>8.6</td><td>70</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>3.26</td><td>28.6</td><td>8.6</td><td>67</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>3.51</td><td>28.6</td><td>8.6</td><td>65</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>3.76</td><td>28.6</td><td>8.6</td><td>64</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>4.00</td><td>28.6</td><td>8.5</td><td>61</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>4.50</td><td>28.5</td><td>8.5</td><td>53</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>5.00</td><td>28.5</td><td>8.5</td><td>51</td><td>55.1</td></tr><tr><td>DZ21-9</td><td>5.51</td><td>28.5</td><td>8.5</td><td>48</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>6.61</td><td>28.5</td><td>8.5</td><td>47</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>7.01</td><td>28.5</td><td>8.5</td><td>45</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>7.47</td><td>28.5</td><td>8.5</td><td>43</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>8.18</td><td>28.5</td><td>8.5</td><td>43</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>8.50</td><td>28.5</td><td>8.5</td><td>43</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>8.86</td><td>28.5</td><td>8.5</td><td>42</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>9.58</td><td>28.5</td><td>8.5</td><td>42</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>10.00</td><td>28.5</td><td>8.5</td><td>42</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>11.04</td><td>28.5</td><td>8.5</td><td>41</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>11.51</td><td>28.5</td><td>8.5</td><td>40</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>12.01</td><td>28.5</td><td>8.5</td><td>39</td><td>55.2</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1.** (continued)</caption><thead><tr><th>Sample (name)</th><th>Depth (cm)</th><th>Temperature (°C)</th><th>pH</th><th>[O<sub>2</sub>] (%)</th><th>Salinity (psu)</th></tr></thead><tbody><tr><td>DZ21-9</td><td>13.01</td><td>28.4</td><td>8.5</td><td>39</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>13.51</td><td>28.4</td><td>8.5</td><td>37</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>14.51</td><td>28.4</td><td>8.5</td><td>36</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>15.00</td><td>28.4</td><td>8.5</td><td>35</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>16.00</td><td>28.4</td><td>8.5</td><td>29</td><td>55.2</td></tr><tr><td>DZ21-9</td><td>17.01</td><td>28.4</td><td>8.5</td><td>23</td><td>55.2</td></tr><tr><td>DZ21-12</td><td>0.25</td><td>31.4</td><td>8.6</td><td>152</td><td>57.9</td></tr><tr><td>DZ21-12</td><td>0.50</td><td>30.4</td><td>8.6</td><td>58</td><td>58.2</td></tr><tr><td>DZ21-12</td><td>0.75</td><td>30.3</td><td>8.6</td><td>22</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>1.00</td><td>30.3</td><td>8.5</td><td>7</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>1.25</td><td>30.2</td><td>8.5</td><td>5</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>1.50</td><td>30.2</td><td>8.5</td><td>4</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>1.75</td><td>30.2</td><td>8.5</td><td>2</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>2.00</td><td>30.2</td><td>8.5</td><td>2</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>2.25</td><td>30.2</td><td>8.5</td><td>2</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>2.50</td><td>30.2</td><td>8.5</td><td>2</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>2.75</td><td>30.2</td><td>8.5</td><td>2</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>3.00</td><td>30.2</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>3.25</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>3.50</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>3.75</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>4.01</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>4.50</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>4.99</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>5.51</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>6.00</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>6.50</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>7.01</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>7.50</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>8.01</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>8.51</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>9.00</td><td>30.0</td><td>8.5</td><td>1</td><td>58.0</td></tr><tr><td>DZ21-12</td><td>9.50</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>10.00</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>10.49</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>11.01</td><td>30.0</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>11.50</td><td>30.1</td><td>8.5</td><td>1</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>12.01</td><td>30.1</td><td>8.5</td><td>0</td><td>58.1</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1.** (continued)</caption><thead><tr><th>Sample (name)</th><th>Depth (cm)</th><th>Temperature (°C)</th><th>pH</th><th>[O<sub>2</sub>] (%)</th><th>Salinity (psu)</th></tr></thead><tbody><tr><td>DZ21-12</td><td>12.49</td><td>30.1</td><td>8.5</td><td>0</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>13.01</td><td>30.1</td><td>8.5</td><td>0</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>13.50</td><td>30.1</td><td>8.5</td><td>0</td><td>58.1</td></tr><tr><td>DZ21-12</td><td>14.01</td><td>30.3</td><td>8.5</td><td>0</td><td>58.2</td></tr><tr><td>DZ21-12</td><td>14.50</td><td>30.4</td><td>8.5</td><td>0</td><td>58.7</td></tr><tr><td>DZ21-12</td><td>15.02</td><td>30.4</td><td>8.4</td><td>0</td><td>59.5</td></tr><tr><td>DZ21-12</td><td>15.50</td><td>30.4</td><td>8.4</td><td>0</td><td>59.8</td></tr><tr><td>DZ21-12</td><td>16.00</td><td>30.5</td><td>8.4</td><td>0</td><td>60.0</td></tr><tr><td>DZ21-12</td><td>16.50</td><td>30.5</td><td>8.4</td><td>0</td><td>60.2</td></tr><tr><td>DZ21-12</td><td>17.00</td><td>30.4</td><td>8.4</td><td>0</td><td>60.2</td></tr></tbody></table>

Salinity and pH values were still constant with depth throughout the whole water column but with average values close to 55 psu and 8.5, respectively. Temperature decreased from 32 °C in surface water to 28.2 °C at about 2 m depth, and then remained constant with depth down to the lake bottom. Dissolved oxygen was present throughout the whole water column. Surface waters showed very high O₂ saturation values (i.e. up to 327%, Figure 2) that are related to the high photosynthetic activity similar to before the volcanic crisis. Surprisingly however, the deeper part
<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /></figure>