Diel vertical migration (DVM) of krill was studied throughout 36 h at a fixed station (46.05°S, 66.19°W; 98-m depth) located in the center of the San Jorge Gulf, Southern Patagonia area, during February 2014. Using an echosounder system, combined with an autonomous Video Plankton Recorder (Auto-VPR) and Jacknet samplings, we describe the migration pattern, the associated biomass and the macrozooplankton species involved. The net sampling and the Auto-VPR images allowed us to identify the krill species detected in the echosounder signals, which corresponded to Euphausia lucens, Euphausia vallentini and Nematoscelis megalops. The krill community followed a “normal pattern” of DVM, ascending at dusk (~18:30 h) and descending at dawn (~06:30 h), forming a dense layer near the bottom during the day. Krill vertical migration speed was estimated from the echogram data at ~ 1 cm s⁻¹ (1 body length per s for 1-cm-long animal), and the integrated mean biomass was 57.8 g m⁻². This study provides a description of temporal and spatial patterns of krill vertical distribution, which should be taken into account when studying the complexity of the SJG ecosystem dynamics and carbon flux.

Detailed in situ vertical and temporal distribution of appendicularians, marine snow, fecal pellets, nano- and microplankton were recorded simultaneously with environmental data in the San Jorge Gulf, Argentinean Patagonia (45°–47°S). Data were taken at a fixed station over 36 h in February 2014 with an autonomous Video Plankton Recorder and a FlowCAM®. The water column was thermally stratified with a pycnocline at ~ 40 m. Appendicularians dominated in the upper 65 m with a condensed pattern above the pycnocline at high chlorophyll a concentrations, matching the subsurface chlorophyll maximum layer at ~ 20 m. Our results suggest the absence of vertical migration of appendicularians. Marine snow, strongly correlated with appendicularians, showed high concentrations above the pycnocline, whereas fecal pellets from krill were distributed throughout the water column. Discarded houses of appendicularians or their mucus fragments were the main components of marine snow aggregates, with phytoplankton, detritus and krill pellets also contributing. Nanoplankton dominated over microplankton, with vertical distribution patterns that might depend on local grazing pressure and advective processes. Our study, the first one in the region using underwater imagery, emphasizes the leading contribution of appendicularians to marine snow aggregates in the San Jorge Gulf and their potential implications in the bentho-pelagic coupling.

Increasing CO ₂ emissions have led to the development of CO ₂ removal strategies to counteract ocean acidification. Among these, ocean alkalinity enhancement techniques, particularly ocean liming, may represent a promising approach to restore seawater pH and boost CO ₂ sequestration. Yet, the impact of liming on plankton communities remains underexplored. In the framework of a mesocosm experiment, we conducted three dilution incubations to assess natural plankton response (abundance, composition, growth, grazing, diet, and food selectivity) to liming, achieved with calcium hydroxide (Ca(OH) ₂ ) additions. Experiments included two liming treatments (low concentration, and high concentration) and a control treatment without Ca(OH) ₂ . The community was dominated by small‐sized plankton (bacteria, Synechococcus and pigmented picoflagellates), outnumbering larger diatoms, dinoflagellates, and ciliates. While chlorophyll a , heterotrophic bacteria, and pigmented picoflagellates remained stable across treatments, the abundance of Synechococcus and dinoflagellates increased, whereas diatoms and ciliates declined particularly under high Ca(OH) ₂ . Growth and grazing rates were largely unaffected by alkalinity, except for increased growth in pigmented picoflagellates upon liming. Microzooplankton showed low ingestion of Synechococcus and pigmented picoflagellates and higher intake of diatoms, dinoflagellates, and ciliates. Food selectivity was unresponsive to liming, as the grazers selected prey based on size, regardless of Ca(OH) ₂ concentrations. Increased alkalinity and pH, and the parallel effect of trophic cascades might have driven nutrient fluctuations and shaped downstream trophic interactions. Despite positive responses highlighted in this study, further research is needed to explore liming potentiality on a wider range of food‐web components and larger scales in the framework of ocean alkalinity enhancement research.