| 24 Jul 2026
Physical connectivity and Lagrangian transport patterns in the Bay of Biscay across 30 years
Abstract. Understanding ocean physical connectivity is essential for characterizing and predicting transport patterns, which in turn play a fundamental role in several marine processes. Here, we present an analysis of 30 years of surface water connectivity for the Bay of Biscay, derived from backward Lagrangian particle simulations driven by hourly high-resolution surface velocity reanalysis fields. The Lagrangian simulations are used to characterize transport pathways within and between key subregions across integration times of 7, 30, and 90 days, thereby capturing connectivity from weekly to seasonal scales.
Results show that the obtained seasonal cycle of connectivity reflects the seasonal variability of regional ocean circulation: i) The northward transport along the shelf by the Iberian–Poleward Current, ii) the presence of transport barriers along the continental slope, and iii) the enhanced cross-shelf transport from the Spanish shelf during spring and from the French shelf during summer. Regional maps of particle origins and transit times reveal areas of strong isolation along the French coast and zones of intensive mixing at the French Spanish border. While interannual variability is evident, the simulations also indicate a slight but significant long-term decrease in transport from the Spanish to the French shelf.
Overall, these results provide an overview of the main pathways of transport and retention within the Bay of Biscay at different time scales, offering insights into its potential role in different key ocean processes such as marine litter or plankton dispersal, genetic exchange, and ecosystem functioning.
This study is thorough and well designed, and the manuscript is generally very well written.
I recommend publication after considering the comments and suggestions below.
The first sentence in Introduction reads:
“Ocean connectivity is a fundamental process in the marine system.”
However, connectivity is not a fundamental process in itself, but rather the property or outcome of fundamental processes such as transport/advection. Thus this sentence should be reformulated.
Figure 1:
- For readers without connection to the Bay of Biscay area, it might be a little hard to locate this area. I would suggest to include a little more land to the east and west of the given area, and perhaps also a little to the south and north.
- The legend uses the short names “fr_shelf”, “es_slope” etc, which are also defined in the text. These shortnames make sense in the programming/analysis phase, as names of files and figures etc. However, I am not sure if these shortnames are needed in the manuscript, as it might be clearer to simply write explicitly (also in this legend) as “Frensh shelf” and “Spanish slope” etc. This goes throughout the paper. The exception might be Figures 7-9 where compact representation is needed. The shortnames could be defined and used for these figure labels. I will not insist on this change, but it could be considered.
Section 2.2, line 125:
Ocean model resolution is specified as 1/36 degree of longitude and latitude. It would be useful to also say what this refers to in km, approximately since one degree of longitude depends on the latitude.
It is also a general thing throughout the paper that distances are given in degrees of longitude and latitude. Most places (like here) it would be good to also give the corresponding distance in km. E.g. line 128 says that particles are seeded on 0.08 x 0.08 degree grid - however, this gives different spacing northwards and eastwards. This is not a problem in itself, but should be made more clear by specifying also distances in km.
Section 2.2, line 137. I would suggest to rewrite these to sentences to something like:
“The model configuration included no wind drift or diffusion, and a time step of 15 minutes. Particles hitting the coast were not stranded/removed, but left in place to move ashore once the current has an offshore component.”
Section 2.3, line 156: “preceding two months”
Figure 3: Again it might be a little hard to locate the area, as both land and ocean are white. It could be considered to put a color (black?) on land. But this is hard to know without testing, and thus authors can decide what they think is best.
Figure 4: Lines for 200 and 1000 m isobats are very close, indicating that the shelf is quite steep. This could be commented on somewhere.
Section 3.2, line 258: Although ODI is defined earlier, I would suggest to remind the reader that it means Origin diversity index.
Section 3.2, line 268: within “2-5 degrees” is again a little vague, since a degree of longitude is different from a degree of latitude. This is another example where distances in km would be better.
Section 3.4, line 361: a trend of -0.0006 degrees N is given, corresponding to 67 meters per month. This sounds like a very small trend although stated as significant,, and also I would think that Figure 10a shows rather a decline until ~2012 and thereafter an increase, i.e. a long term oscillation if anything.
However, more prominent on Fig 10a is the apparent oscillation on ~5 yers scale. This is quite interesting, and should be commented in the text. Could it be related to El Nino or other known indices/oscillations?
Section 4, line 383: The paper/simulations uses model surface currents without additional wind drift, which is fine. However, “floating marine litter” is here mentioned, and which is known to need additional wind drift which can be quite significant (typically ~3% of wind). Thus, I would agree that results are valid for microplastics (and plankton etc), but I would comment that connectivity including wind drift could be quite different. E.g. floating plastics is also subject to stranding whenever onshore winds/waves.