Increased carbon dioxide availability alters phytoplankton stoichiometry and affects carbon cycling and growth of a marine planktonic herbivore

Rising levels of CO2 in the atmosphere have led to increased CO2 concentrations in the oceans. This enhanced carbon availability to the marine primary producers has the potential to change their nutrient stoichiometry, and higher carbon to nutrient ratios are expected. As a result, the quality of the primary producers as food for herbivores may change. Here, we present experimental work showing the effect of feeding Rhodomonas salina grown under different pCO2 (200, 400 and 800 µatm) on the copepod Acartia tonsa. The rate of development of copepodites decreased with increasing CO2 availability to the algae. The surplus carbon in the algae was excreted by the copepods, with younger stages (copepodites) excreting most of their surplus carbon through respiration, and adult copepods excreting surplus carbon mostly as DOC. We consider the possible consequences of different excretory pathways for the ecosystem. A continued increase in the CO2 availability for primary production, together with changes in the nutrient loading of coastal ecosystems, may cause changes in the trophic links between primary producers and herbivores.

(Table 1) Composition of stomach and gut content of the amphipod Onisimus litoralis, and algal availability near Barrow, Alaska

Sea ice algae have been widely discussed as a potential food source for pelagic and benthic animals in ice-covered waters, specifically in the light of current substantial changes in the Arctic ice regime. Stomach and gut contents of the Arctic nearshore lysianassid amphipod Onisimus litoralis sampled from February to May 2003 indicate that Arctic ice algae were dominant food no earlier than the onset of ice melt. Crustaceans, common prey in a previous study, were absent in stomachs and guts during the survey period. Our data support the concept that sea ice-derived organic carbon is of specific relevance for Arctic plankton and benthos during the period of ice melt.

Decreased light availability can amplify negative impacts of ocean acidification on calcifying coral reef organisms

Coral reef organisms are increasingly and simultaneously affected by global and local stressors such as ocean acidification (OA) and reduced light availability. However, knowledge of the interplay between OA and light availability is scarce. We exposed 2 calcifying coral reef species (the scleractinian coral Acropora millepora and the green alga Halimeda opuntia) to combinations of ambient and increased pCO2 (427 and 1073 µatm, respectively), and 2 light intensities (35 and 150 µmol photons/m**2/s) for 16 d. We evaluated the individual and combined effects of these 2 stressors on weight increase, calcification rates, O2 fluxes and chlorophyll a content for the species investigated. Weight increase of A. millepora was significantly reduced by OA (48%) and low light intensity (96%) compared to controls. While OA did not affect coral calcification in the light, it decreased calcification in the dark by 155%, leading to dissolution of the skeleton. H. opuntia weight increase was not affected by OA, but decreased (40%) at low light. OA did not affect algae calcification in the light, but decreased calcification in the dark by 164%, leading to dissolution. Low light significantly reduced gross photosynthesis (56 and 57%), net photosynthesis (62 and 60%) and respiration (43 and 48%) of A. millepora and H. opuntia, respectively. In contrast to A. millepora, H. opuntia significantly increased chlorophyll content by 15% over the course of the experiment. No interactive effects of OA and low light intensity were found on any response variable for either organism. However, A. millepora exhibited additive effects of OA and low light, while H. opuntia was only affected by low light. Thus, this study suggests that negative effects of low light and OA are additive on corals, which may have implications for management of river discharge into coastal coral reefs.

Impacts of food availability and pCO2 on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral Balanophyllia elegans

Ocean acidification, the assimilation of atmospheric CO2 by the oceans that decreases the pH and CaCO3 saturation state (Omega) of seawater, is projected to have severe adverse consequences for calcifying organisms. While strong evidence suggests calcification by tropical reef-building corals containing algal symbionts (zooxanthellae) will decline over the next century, likely responses of azooxanthellate corals to ocean acidification are less well understood. Because azooxanthellate corals do not obtain photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on energy available for calcification. The solitary azooxanthellate orange cup coral Balanophyllia elegans often lives in low-pH, upwelled waters along the California coast. In an 8-month factorial experiment, we measured the effects of three pCO2 treatments (410, 770, and 1220 µatm) and two feeding frequencies (3-day and 21-day intervals) on "planulation" (larval release) by adult B. elegans, and on the survival, skeletal growth, and calcification of newly settled juveniles. Planulation rates were affected by food level but not pCO2. Juvenile mortality was highest under high pCO2 (1220 µatm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO2. While net calcification was positive even at 1220 µatm (~3 times current atmospheric pCO2), overall calcification declined by ~25-45%, and skeletal density declined by ~35-45% as pCO2 increased from 410 to 1220 µatm. Aragonite crystal morphology changed at high pCO2, becoming significantly shorter but not wider at 1220 µatm. We conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food.

Nutrient availability affects the response of juvenile corals and the endosymbionts to ocean acidification

The interactive effects of nutrient availability and ocean acidification on coral calcification were investigated using post-settlement juvenile corals of Acropora digitifera cultured in nutrient-sufficient or nutrient-depleted seawater for 4 d and then exposed to seawater with different partial pressure of carbon dioxide () conditions (38.8 or 92.5 Pa) for 10 d. After the nutrient pretreatment, corals in the high nutrient condition (HN corals) had a significantly higher abundance of endosymbiotic algae than did those in the low nutrient condition (LN corals). The high abundance of endosymbionts in HN corals was reduced as a result of subsequent seawater acidification, and the chlorophyll a per algal cell increased. The photosynthetic oxygen production rate by endosymbionts was enhanced by the acidified seawater regardless of the nutrient treatment, indicating that the reduction in endosymbiont density in HN corals due to acidification was compensated for by the increase in chlorophyll a per cell. Though the photosynthetic rate increased in the acidified conditions for both LN and HN corals, the calcification rate significantly decreased for LN corals but not for HN corals. The acquisition of nutrients from seawater, rather than the increase in alkalinity caused by photosynthesis, might effectively alleviate the negative response of coral calcification to seawater acidification, suggesting that the response of corals and their endosymbionts to ocean acidification can be influenced by nutrient conditions.