ALBERT

Description: Comau Fjord is a stratified Chilean Patagonian Fjord characterized by a shallow brackish surface layer and a 〉400 m layer of aragonite-depleted subsurface waters. Despite the energetic burden of low aragonite saturation levels to calcification, Comau Fjord harbours dense populations of cold-water corals (CWC). While this paradox has been attributed to a rich supply of zooplankton, supporting abundance and biomass data are so far lacking. In this study, we investigated the seasonal and diel changes of the zooplankton community over the entire water column. We used a Nansen net (100 mm mesh) to take stratified vertical hauls between the surface and the bottom (0-50-100-200-300-400-450 m). Samples were scanned with a ZooScan, and abundance, biovolume and biomass were determined for 41 taxa identified on the web-based platform EcoTaxa 2.0. Zooplankton biomass was the highest in summer (209 g dry massm  2) and the lowest in winter (61 g dry massm  2). Abundance, however, peaked in spring, suggesting a close correspondence between reproduction and phytoplankton spring blooms (Chl a max. 50.86 mgm  3, 3mdepth). Overall, copepods were the most important group of the total zooplankton community, both in abundance (64 81%) and biovolume (20 70%) followed by mysids and chaetognaths (in terms of biovolume and biomass), and nauplii and Appendicularia (in terms of abundance). Throughout the year, diel changes in the vertical distribution of biomass were found with a daytime maximum in the 100 200 m depth layer and a nighttime maximum in surface waters (0 50 m), associated with the diel vertical migration of the calanoid copepod family Metridinidae. Diel differences in integrated zooplankton abundance, biovolume and biomass were probably due to a high zooplankton patchiness driven by biological processes (e.g., diel vertical migration or predation avoidance), and oceanographic processes (estuarine circulation, tidal mixing or water column stratification). Those factors are considered to be the main drivers of the zooplankton vertical distribution in Comau Fjord.

Description: Cold-water corals (CWC) face an uncertain future under climate change. They seem to grow successfully under low pH conditions but physiological mechanisms and the role of energy efficiency in sustaining metabolic rates are largely unknown. The solitary, pseudo-colonial CWC Desmophyllum dianthus thrives in Comau Fjord (Northern Patagonia, Chile) despite low levels of aragonite saturation (Ωarag). To examine the seasonal growth and metabolism in relation with food availability and the physico-chemical environment of the fjord, we carried out an in situ reciprocal transplantation-experiment between (1) fjord mouth (20 m: Ωarag 〉 1, high seasonality) and fjord head (20 m: Ωarag 〉 1 in winter, Ωarag 〈 1 in summer, high seasonality) and (2) two depths at the middle of the fjord (20 m: Ωarag 〉 1, high seasonality; 300 m: Ωarag 〈 1, low seasonality). D. dianthus showed highest calcification and respiration rates in 300 m depth with maximum growth rates in winter. This applied to in situ control corals and those transplanted from 20 m to 300 m. The lower plankton availability at depth and in winter suggests seasonal differences in energy gain and allocation among growth, basal metabolism and other processes (e.g. reproduction). Despite Ωarag 〈 1 the lack of seasonal variation in the physico-chemical environment in deep waters may be beneficial for growth in contrast to seasonal fluctuations in shallow waters which may require a recurrent energy expenditure of acclimation. In 20 m depth, calcification and respiration rates at the fjord mouth were similar between seasons and always higher than at the fjord head. No significant differences were detected between control and transplanted corals from the fjord head to the mouth. A high aragonite saturation (Ωarag 〉 1) and plankton supply during summer combined with a well-functioning redistribution of energy reserves within D. dianthus in winter can explain this picture. At the fjord head growth changed seasonally in control and transplanted corals with higher rates during summer indicating good growth conditions despite Ωarag 〈 1 due to sufficient food availability but a less effective energy allocation during low food supply in winter. The present results emphasize the effect of seasonal environmental changes on CWC to conceive both the extent of natural variability where these corals live in and their acclimation potential to deal with it.

Description: The stratified Chilean Comau Fjord sustains a dense population of the cold-water coral (CWC) Desmophyllum dianthus in aragonite supersaturated shallow and aragonite under- saturated deep water. This provides a rare opportunity to evaluate CWC fitness trade-offs in response to physico-chemical drivers and their variability. Here, we combined year-long reciprocal transplantation experiments along natural oceanographic gradients with an in situ assessment of CWC fitness. Following transplantation, corals acclimated fast to the novel environment with no discernible difference between native and novel (i.e. cross-transplanted) corals, demonstrating high phenotypic plasticity. Surprisingly, corals exposed to lowest ara- gonite saturation (Ωarag 〈 1) and temperature (T 〈 12.0 °C), but stable environmental condi- tions, at the deep station grew fastest and expressed the fittest phenotype. We found an inverse relationship between CWC fitness and environmental variability and propose to consider the high frequency fluctuations of abiotic and biotic factors to better predict the future of CWCs in a changing ocean.