| 06 Aug 2026
Phytoplankton size structure in the northwestern Weddell Sea
Abstract. The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- <2 μm, nano- 2–20 μm, and microphytoplankton >20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
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The manuscript presents an analysis of ship-based data that are used to evaluate the contribution of different phytoplankton size classes to total chlorophyll concentration in the northwestern Weddell Sea region of the Southern Ocean. The work is well-written and thorough and the authors have considered a variety of methods in their modeling of different phytoplankton size classes, and total chlorophyll concentration from reflectance measurements. The manuscript is an effective contribution to the body of knowledge on phytoplankton ecosystems in a specific region of the Southern Ocean.
Main comments:
As a reader I am curious how the regional-specific methods to separate size classes from satellite data would be relevant to other regions of the Southern Ocean; recognizing that the data are specific to the northwestern Weddell Sea, but it does beg the question of whether the differences observed between AMT and PICCOLO/Rothera also apply to other parts of the high latitude Southern Ocean? Maybe the authors could comment on the recommended use of the size class equations (i.e., should application of the Table 2 to satellite OC data be limited to the geographic region shown in Figure 6?). Relatedly, it could be worth commenting on, if not showing, how different the satellite maps looks when using the tuned PICCOLO coefficients vs previously published coefficients (or the standard Chl-a algorithm for total Chl-a).
The methods and Table 1 indicate that samples from a range of depths were used in the analyses – perhaps it could be worth testing a re-calculation of the parameters in Table 2 using only near-surface data, for application to satellite OC data. Or at least testing the surface vs. full water column values and commenting on whether they are significantly different (i.e., do the PICCOLO curves in Figure 5g-i change noticeably if only near-surface samples are used?).
Minor/Technical comments:
Figure 1 – Suggest including an inset map of location of the peninsula.
L169 – should be ‘to derive’
L245 should be ‘a commonly-used’
L333 should be ‘picophytoplankton’
L275 Re: “Depth matching was performed using the nearest available sampling depth.” Perhaps include statistics on this - how far apart are the matches at most?