Seawater carbonate chemistry and skeletal development, size, weight, total lipid and symbiont density of coral Favia fragum in a laboratory experiment

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO3) production of framework-building corals thus reducing the amount of CaCO3 the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 µatm) or significantly elevated (1,311 µatm) CO2 conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO2 condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO2 level, fed corals accreted more CaCO3 than unfed corals, and fed corals reared under 1,311 µatm CO2 accreted as much CaCO3 as unfed corals reared under ambient CO2. However, feeding did not alter the sensitivity of calcification to increased CO2; Delta calcification/Delta Omega was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO3 production under OA than those in nutritionally limited environments.

(Table 1) Reproductive performance of brown skua pairs with and without feeding territories at Potter Peninsula/King George Island from 1998/1999 to 2000/2001

In the maritime Antarctic, brown skuas (Catharacta antarctica lonnbergi) show two foraging strategies: some pairs occupy feeding territories in penguin colonies, while others can only feed in unoccupied areas of a penguin colony without defending a feeding territory. One-third of the studied breeding skua population in the South Shetlands occupied territories of varying size (48 to >3,000 penguin nests) and monopolised 93% of all penguin nests in sub-colonies. Skuas without feeding territories foraged in only 7% of penguin sub-colonies and in part of the main colony. Females owning feeding territories were larger in body size than females without feeding territories; no differences in size were found in males. Territory holders permanently controlled their resources but defence power diminished towards the end of the reproductive season. Territory ownership guaranteed sufficient food supply and led to a 5.5 days earlier egg-laying and chick-hatching. Short distances between nest and foraging site allowed territorial pairs a higher nest-attendance rate such that their chicks survived better (71%) than chicks from skua pairs without feeding territories (45%). Due to lower hatching success in territorial pairs, no difference in breeding success of pairs with and without feeding territories was found in 3 years. We conclude that skuas owning feeding territories in penguin colonies benefit from the predictable and stable food resource by an earlier termination of the annual breeding cycle and higher offspring survivorship.

Effects of feeding and light intensity on the response of the coral Porites rus to ocean acidification

Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Increased feeding and nutrient excretion of adult antarctic krill, Euphausia superba, exposed to enhanced carbon dioxide (CO2)

Ocean acidification has a wide-ranging potential for impacting the physiology and metabolism of zooplankton. Sufficiently elevated CO2 concentrations can alter internal acid-base balance, compromising homeostatic regulation and disrupting internal systems ranging from oxygen transport to ion balance. We assessed feeding and nutrient excretion rates in natural populations of the keystone species Euphausia superba (Antarctic krill) by conducting a CO2 perturbation experiment at ambient and elevated atmospheric CO2 levels in January 2011 along the West Antarctic Peninsula (WAP). Under elevated CO2 conditions (~672 ppm), ingestion rates of krill averaged 78 µg C/individual/d and were 3.5 times higher than krill ingestion rates at ambient, present day CO2 concentrations. Additionally, rates of ammonium, phosphate, and dissolved organic carbon (DOC) excretion by krill were 1.5, 1.5, and 3.0 times higher, respectively, in the high CO2 treatment than at ambient CO2 concentrations. Excretion of urea, however, was ~17% lower in the high CO2 treatment, suggesting differences in catabolic processes of krill between treatments. Activities of key metabolic enzymes, malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), were consistently higher in the high CO2 treatment. The observed shifts in metabolism are consistent with increased physiological costs associated with regulating internal acid-base equilibria. This represents an additional stress that may hamper growth and reproduction, which would negatively impact an already declining krill population along the WAP.

Threatened Caribbean coral is able to mitigate the adverse effects of ocean acidification on calcification by increasing feeding rate

Global climate change threatens coral growth and reef ecosystem health via ocean warming and ocean acidification (OA). Whereas the negative impacts of these stressors are increasingly well-documented, studies identifying pathways to resilience are still poorly understood. Heterotrophy has been shown to help corals experiencing decreases in growth due to either thermal or OA stress; however, the mechanism by which it mitigates these decreases remains unclear. This study tested the ability of coral heterotrophy to mitigate reductions in growth due to climate change stress in the critically endangered Caribbean coral Acropora cervicornis via changes in feeding rate and lipid content. Corals were either fed or unfed and exposed to elevated temperature (30°C), enriched pCO2 (800 ppm), or both (30°C/800 ppm) as compared to a control (26°C/390 ppm) for 8 weeks. Feeding rate and lipid content both increased in corals experiencing OA vs. present-day conditions, and were significantly correlated. Fed corals were able to maintain ambient growth rates at both elevated temperature and elevated CO2, while unfed corals experienced significant decreases in growth with respect to fed conspecifics. Our results show for the first time that a threatened coral species can buffer OA-reduced calcification by increasing feeding rates and lipid content.

Juvenile sea stars exposed to acidification decrease feeding and growth with no acclimation potential

Ocean acidification has the potential to affect growth and calcification of benthic marine invertebrates, particularly during their early life history. We exposed field-collected juveniles of Asterias rubens from Kiel Fjord (western Baltic Sea) to 3 seawater CO2 partial pressure (pCO2) levels (ranging from around 650 to 3500 µatm) in a long-term (39 wk) and a short-term (6 wk) experiment. In both experiments, survival and calcification were not affected by elevated pCO2. However, feeding rates decreased strongly with increasing pCO2, while aerobic metabolism and NH4+ excretion were not significantly affected by CO2 exposure. Consequently, high pCO2 reduced the scope for growth in A. rubens. Growth rates decreased substantially with increasing pCO2 and were reduced even at pCO2 levels occurring in the habitat today (e.g. during upwelling events). Sea stars were not able to acclimate to higher pCO2, and growth performance did not recover during the long-term experiment. Therefore, the top-down control exerted by this keystone species may be diminished during periods of high environmental pCO2 that already occur occasionally and will be even higher in the future. However, some individuals were able to grow at high rates even at high pCO2, indicating potential for rapid adaption. The selection of adapted specimens of A. rubens in this seasonally acidified habitat may lead to higher CO2 tolerance in adult sea stars of this population compared to the juvenile stage. Future studies need to address the synergistic effects of multiple stressors such as acidification, warming and reduced salinity, which will simultaneously impact the performance of sea stars in this habitat.