2.1. Key Findings and the Importance of a Multidisciplinary Approach¶
This study illustrates the value of integrating satellite, in situ, and geochemical data to assess complex oceanographic phenomena triggered by volcanic activity. The preliminary results underscore how a multidisciplinary approach is essential for disentangling the physical, chemical, and biological responses of the ocean to the Hunga Tonga eruption.
2.2. Insights on the Potential Phytoplankton Bloom¶
The Hunga Tonga volcano is mainly submarine and has not formed a large island yet in December 2021. Hence, we can clearly highlight anomalies in both BBP and CHl that are not linked to “island-processes”, as can be seen at the neighboring Tonga islands.
We have a weak, but remarkable, excess (several pixels) in the very near-source on 20 December 2021 in the BBP (and also the CHL, even if weaker). This anomaly coincides with the first ash emissions associated with the submarine volcanic activity, released on 20 December. This supports the conclusion that both BBP and CHL excess signals do not result from the submarine volcanic activity but are associated with ash particles, either suspended in the atmosphere, or deposited onto the surface of the ocean.
On 22 December 2021, a higher BBP excess signal was detected near the source, colocated in space with a CHL excess (compared to background values, the CHL excess is weaker than the BBP signal though). In the exact same area, we observe the next day (23 December 2021) an increase in the CHL signal, while the BBP seems stable, or a bit weaker (we do not observe anymore the near source peak observed on 22 Dec). Could this support a boost in the phytoplankton activity, which takes some time, explaining a time delay in the CHL increase.
Between 23 and 24 December 2021, CHL abundance decreases while the BBP signal stays intense and spreads geographically. Could there be again ash emissions that impact first BBP, before CHL?
On 28 December 2021, again the BBP signal is more intense than the CHL one.
After the major largest explosion taking place on 15 January 2022, the comparison of 16 January versus 19 January 2022 indicates again a remarkable delay in the CHL increase, compared to BBP (reaching higher values than in December 2021). On 16, 19, 20 January 2022, signals are broadly correlated in space but with much higher variability in the CHL signal: could it be the result of a biological activation? On 18 January 2022, a strong CHL excess signal was observed, while many data gaps were recorded in the BBP signal.
To summarise, this set of data could shows that ash particles (either suspended in the atmosphere or deposited onto the ocean) impact immediately the BBP signal, whereas a time delay is required to trigger an increase in the CHL excess abundance, supporting an increase in the phytoplankton activity that requires some time to be produced.
2.3. Open Questions and Areas for Further Investigation¶
Several questions remain to be addressed to fully interpret the satellite signals and validate the hypothesized bloom mechanisms:
Timing Accuracy of Satellite Products¶
The image metadata shows differing acquisition times for BBP (00:06:40) and CHL (00:53:20) products. Are these timestamps indicative of actual observation times, or are they part of a daily composite? Understanding this is crucial for aligning satellite data with atmospheric plume events.
Potential Misclassification of Ash as Cloud¶
In areas marked as “no data” in the ocean colour products, it is unclear whether these gaps are due to meteorological cloud cover or to volcanic ash plumes misclassified by cloud filters. Such confusion is common in aerosol-rich scenes and may hinder the detection of surface anomalies.
Differential Spatial Distributions of BBP and CHL¶
Although BBP and CHL signals are often co-located, they differ in spatial intensity and structure. For instance, on 22 December, BBP exhibited a clear peak near the eruption site, while CHL showed a broader but less intense signal. What mechanisms or algorithmic factors account for these differences? Could the BBP signal be less sensitive to certain ash deposition types than CHL, or vice versa?
Time Lag and Bloom Dynamics¶
Is the observed time delay between BBP and CHL increases consistent with phytoplankton bloom response times to particle or nutrient deposition, as reported in the literature? Further comparison with known bloom dynamics may help to constrain the interpretation.