ALBERT

Description: In the North Patagonian fjord region, the cold-water coral (CWC) Desmophyllum dianthus occurs in high densities, in spite of low pH and aragonite saturation. If and how these conditions affect the energy demand of the corals is so far unknown. In a laboratory experiment, we investigated the carbon and nitrogen (C, N) budget of D. dianthus from Comau Fjord under three feeding scenarios: (1) live fjord zooplankton (100 2,300 mm), (2) live fjord zooplankton plus krill (〉7 mm), and (3) four-day food deprivation. In closed incubations, C and N budgets were derived from the difference between C and N uptake during feeding and subsequent C and N loss through respiration, ammonium excretion, release of particulate organic carbon and nitrogen (POC, PON). Additional feeding with krill significantly increased coral respiration (35%), excretion (131%), and POC release (67%) compared to feeding on zooplankton only. Nevertheless, the higher C and N losses were overcompensated by the threefold higher C and N uptake, indicating a high assimilation and growth efficiency for the krill plus zooplankton diet. In contrast, short food deprivation caused a substantial reduction in respiration (59%), excretion (54%), release of POC (73%) and PON (87%) compared to feeding on zooplankton, suggesting a high potential to acclimatize to food scarcity (e.g., in winter). Notwithstanding, unfed corals `lost' 2% of their tissue-C and 1.2% of their tissue-N per day in terms of metabolism and released particulate organic matter (likely mucus). To balance the C (N) losses, each D. dianthus polyp has to consume around 700 (400) zooplankters per day. The capture of a single, large krill individual, however, provides enough C and N to compensate daily C and N losses and grow tissue reserves, suggesting that krill plays an important nutritional role for the fjord corals. Efficient krill and zooplankton capture, as well as dietary and metabolic flexibility, may enable D. dianthus to thrive under adverse environmental conditions in its fjord habitat; however, it is not known how combined anthropogenic warming, acidification and eutrophication jeopardize the energy balance of this important habitat-building species.

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) can be found throughout a wide range of latitudes (79°N–78°S). Since they lack the photosymbiosis known for most of their tropical counterparts, they may thrive below the euphotic zone. Consequently, their growth predominantly depends on the prevalent environmental conditions, such as general food availability, seawater chemistry, currents, and temperature. Most CWC communities live in regions that will face CaCO3 undersaturation by the end of the century and are thus predicted to be threatened by ocean acidification (OA). This scenario is especially true for species inhabiting the Chilean fjord system, where present-day carbonate water chemistry already reaches values predicted for the end of the century. To understand the effect of the prevailing environmental conditions on the biomineralization of the CWC Tethocyathus endesa, a solitary scleractinian widely distributed in the Chilean Comau Fjord, a 12-month in situ experiment was conducted. The in situ skeletal growth of the test corals was assessed at two sites using the buoyant weight method. Sites were chosen to cover the naturally present carbonate chemistry gradient, with pH levels ranging between 7.90 ± 0.01 (mean ± SD) and 7.70 ± 0.02, and an aragonite saturation (Ωarag) between 1.47 ± 0.03 and 0.98 ± 0.05. The findings of this study provide one of the first in situ growth assessments of a solitary CWC species, with a skeletal mass increase of 46 ± 28 mg per year and individual, at a rate of 0.03 ± 0.02% day. They also indicate that, although the local seawater chemistry can be assumed to be unfavorable for calcification, growth rates of T. endesa are comparable to other cold-water scleractinians in less corrosive waters (e.g., Lophelia pertusa in the Mediterranean Sea).

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.

Description: Cold-water corals (CWC) are exposed to multiple environmental stressors in a changing ocean. Several laboratory experiments have shown that adult CWC can survive at low pH and elevated temperature, but the effects on early life stages are largely unknown, let alone the interactive effects of changing pH, temperature and food availability. We conducted a six-month aquarium experiment to investigate the physiological responses of early juveniles, late juveniles and adults of the CWC Caryophyllia huinayensis to multiple environmental stressors by measuring key coral traits (survival, growth and respiration). We examined the single and interactive effects of pH (7.5 and 8.0), temperature (11 and 15 °C) and food availability (low and high) on the three life stages. The treatment levels reflect current conditions in the natural habitat of C. huinayensis in Comau Fjord, Chile. All life stages of C. huinayensis were more affected by warming than by acidification. At elevated temperature and in the combined treatment of elevated temperature and reduced pH, growth rates of all three life stages decreased after three months. After six months, mortality was highest in early juveniles and adults in these treatments. High feeding did not compensate for the negative effect of elevated temperature on growth and respiration rates of all life stages, but increased the growth rates of early and late juveniles at ambient and low pH conditions compared to low feeding. We identified ontogenetic shifts in resilience to future environmental conditions and highlight the importance of increased food availability for CWC resistance to ocean acidification. Our findings underscore the need to consider potential ontogenetic differences when addressing CWC responses to climate change.