| 30 Jun 2026
How do fronts affect phytoplankton communities over oceanic regions? Contrasting responses across trophic regimes, seasons and front diagnostics
Abstract. Oceanic fronts are key features structuring marine ecosystems, yet their influence on phytoplankton communities remains difficult to quantify at regional scale. In particular, how phytoplankton responses to fronts vary across environmental conditions is still poorly understood, and may depend on the way fronts are detected. Here, we investigate how fronts affect phytoplankton biomass and community composition across the Mediterranean Sea, a region characterized by strong seasonal variability and contrasted trophic regimes. Using 23 years (1998–2021) of satellite-derived chlorophyll-a and pigment-derived phytoplankton groups data, we quantify phytoplankton responses to fronts across seasons and biogeochemical provinces, from productive northwestern regions to ultra-oligotrophic eastern basins. Fronts are identified using either the measure of an heterogeneity index (HI) or of Finite-Size Lyapunov Exponents (FSLE), applied, in both cases, to different data sources (satellite data or high-resolution fields reconstructed from machine-learning). This approach allows us to assess the sensibility of observed ecological responses to the choice of front detection method. We show that fronts are consistently associated with enhanced phytoplankton biomass by 4 % to 35 % and reshaped community composition across the Mediterranean Sea, with the strongest effects observed in oligotrophic regimes and during stratified periods. In particular, fronts are associated with an enrichment in diatoms by up to 20 % and a relative decrease in prokaryotes, suggesting enhanced nutrient supply. However, the magnitude and consistency of these responses vary depending on the diagnostic used, with weaker to no signals detected using FSLEs. These results demonstrate that phytoplankton responses to fronts are both environmentally controlled and method-dependent. They highlight the key role of fronts in structuring ecosystems in nutrient-limited regions, and support the idea that changes in oceanic fronts should be considered when investigating ecological reorganization under future climate change scenarios, particularly in climate change hotspots such as the Mediterranean Sea.
The role of the physical circulation, especially fronts and eddies, in controlling phytoplankton abundance and community structure is one of wide interest, making overlap with ecology, biodiversity, remote sensing, non-linear dynamics and biological-physical interactions. This paper therefore is likely to be widely read. It takes a variety of methods for locating fronts and uses them to highlight front related anomalies, which themselves vary across a region with strong large scale biogeochemical gradients (the Mediterranean). It is clearly written and provides both a good technical basis and, based on the results, some interesting lines of future study for application to other ocean regions. I should say at the outset that I do not have detailed knowledge of the methods used but, while I think the main results are well argued and clearly presented, I feel that there needs to be more discussion about assumptions and limitations before I can recommend publication.
It is a strength of the paper that the authors deliberately use multiple methods to detect and quantify frontal activity, acknowledging that there is still no consensus on the optimal one to use, should such an optimal one exist. While I understand that the paper should not go into too much detail describing those methods, I think that it is important that the method by which the high-resolution salinity field was constructed needs to be described and discussed. I was surprised that it was possible to re-construct a salinity field at the high resolution used but, as I said, this is not my field so I looked into the cited papers. They in turn refer to Olmedo E. et al. Improving SMOS Sea Surface Salinity in the Western Mediterranean Sea through Multivariate and Multifractal Analysis. Remote Sensing. 2018; 10(3):485. https://doi.org/10.3390/rs10030485. Section 3.4 of that paper suggests that a linear relationship between salinity and temperature is used to increase the resolution of the product (their Equation 8). There is also no apparent validation of the performance of the product at the high resolution scales in that paper. Particularly if salinity is to be combined with temperature to create density, as in the paper under review, this is a major assumption. Two questions arise from this. First, if salinity is being reconstructed using temperature at small scales, how well is the component of salinity independent of temperature (which leads to temperature not being perfectly correlated with density) reconstructed? Second, to what extent does the frontal map using density differ from that using temperature not because of small-scale salinity but because of differences between the two different sources of high-resolution temperature data that are used and which are unlikely to match perfectly at these small scales? I am not going to suggest that this needs to somehow be corrected but I do think it is something that needs to be acknowledged and the possible consequences for the robustness of the results to be discussed.
There are other places where more detail is needed too. While they are more minor, they are still significant for those who wish to apply the approach themselves:
Another area of discussion needed is the uncertainty on the results that quantify the anomalies of Chl-a and community structure associated with fronts. This applies for both (HI and FSLE) methods of front detection. Some of the differences are small (a few percent) but where is the threshold to regard a signal as significant? I accept that calculating this might not be straightforward but it requires at least an acknowledgement and ideally a suggestion for how future work might address this. For the HI approach, there is already an aspect that can be better described. The justification for and sensitivity of results to the choice of threshold in Heterogeneity Index to decide whether a pixel is in a front are not given (lines 177-178) and should be provided.
I also have a few suggestions for the discussion.
There are also a few minor presentation issues: