M J Atkinson

Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef

The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will reduce the concentration of CO32− of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage. Using the large 2650 m3 coral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemical variables (temperature, salinity, light, nutrients, Ca2+,pCO2, TCO2, and total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcification in response to our manipulations of the saturation state. We show that the net community calcification rate responds to manipulations in the concentrations of both Ca2+ and CO32− and that the rate is well described as a linear function of the ion concentration product, [Ca2+]0.69[CO32−]. This suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification to short-term (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification between the years 1880 and 2065 A.D. based on our long-term results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduction of 14-30%. This much longer, community-scale study suggests that the impact on coral reefs may be greater than previously suspected. In the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.

Effect of elevated pCO2 on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment

[1]xa0An investigation was conducted to determine the effects of elevated pCO2 on the net production and calcification of an assemblage of corals maintained under near-natural conditions of temperature, light, nutrient, and flow. Experiments were performed in summer and winter to explore possible interactions between seasonal change in temperature and irradiance and the effect of elevated pCO2. Particular attention was paid to interactions between net production and calcification because these two processes are thought to compete for the same internal supply of dissolved inorganic carbon (DIC). A nutrient enrichment experiment was performed because it has been shown to induce a competitive interaction between photosynthesis and calcification that may serve as an analog to the effect of elevated pCO2. Net carbon production, NPC, increased with increased pCO2 at the rate of 3 ± 2% (μmol CO2aq kg−1)−1. Seasonal change of the slope NPC-[CO2aq] relationship was not significant. Calcification (G) was strongly related to the aragonite saturation state Ωa. Seasonal change of the G-Ωa relationship was not significant. The first-order saturation state model gave a good fit to the pooled summer and winter data: G = (8 ± 1 mmol CaCO3 m−2 h−1)(Ωa − 1), r2 = 0.87, P = 0.0001. Both nutrient and CO2 enrichment resulted in an increase in NPC and a decrease in G, giving support to the hypothesis that the cellular mechanism underlying the decrease in calcification in response to increased pCO2 could be competition between photosynthesis and calcification for a limited supply of DIC.

Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths.

[1]xa0Eight-month-old blocks of the coral Porites lobata colonized by natural Hawaiian euendolithic and epilithic communities were experimentally exposed to two different aqueous pCO2 treatments, 400 ppmv and 750 ppmv, for 3 months. The chlorophyte Ostreobium quekettii dominated communities at the start and at the end of the experiment (65–90%). There were no significant differences in the relative abundance of euendolithic species, nor were there any differences in bioeroded area at the surface of blocks (27%) between pCO2 treatments. The depth of penetration of filaments of O. quekettii was, however, significantly higher under 750 ppmv (1.4 mm) than under 400 ppmv (1 mm). Consequently, rates of carbonate dissolution measured under elevated pCO2 were 48% higher than under ambient pCO2 (0.46 kg CaCO3 dissolved m−2 a−1 versus 0.31 kg m−2 a−1). Thus, biogenic dissolution of carbonates by euendoliths in coral reefs may be a dominant mechanism of carbonate dissolution in a more acidic ocean.

Endolithic microflora are major primary producers in dead carbonate substrates of Hawaiian coral reefs

To quantify the contribution of endolithic phototrophs to primary production of dead carbonate substrates, experimental blocks of cleaned Porites lobata Dana skeleton were placed at three different sites in Kaneohe Bay, Hawaii: inshore, lagoonal, and oceanic. After 6 months of exposure, experimental blocks were colonized by communities characteristic of their estuarine (inshore, lagoonal) and oceanic (ocean) environments. Blocks were sub‐sampled; net photosynthesis (NP) and chl a concentrations of the whole blocks (epi‐ and endoliths) and scrapped blocks (only endoliths) were quantified. Green turf algae colonized predominantly inshore and lagoonal blocks, while encrusting corallines were the dominant epiliths colonizing oceanic blocks. Four main species of endolithic phototrophs were identified in all blocks: Mastigocoleus testarum Lagerheim, Plectonema terebrans Bornet and Flahault (cyanobacteria), Phaeophila dendroides Crouan and Crouan, and Ostreobium quekettii Bornet and Flahault (Chlorophytes). While epiliths were very different between sites, NP rates and chl a concentration of endoliths did not vary significantly and were positively correlated (191±25u2003mmol C·m−2·day−1 and 590±150u2003mg chl a·m−2 of reef, respectively). Assimilation numbers for whole communities, including both epilithic and endolithic communities, were similar to those measured for endolithic communities alone (average of 0.3u2003g C·g chl a·h−1). Under experimental conditions, the contribution of endolithic phototrophs to community NP rates of blocks ranged from 56% to 81%, and under natural conditions, we estimated that this contribution ranged between 32% and 46%. Thus, we showed that the endolithic phototrophs are one of the major primary producers in dead coral substrates in a wide range of coral reef environments.

The Biosphere 2 coral reef biome

Abstract The Biosphere 2 coral reef biome is a large tank of living coral reef organisms (water volume of 2650 m 3 , water surface area of 711 m 2 and 590 m 2 of reef benthos). The water of the biome is characteristically very low in dissolved nutrients and phytoplankton. The present community of organisms is largely comprised of macroalgae, including 11 genera of green algae, eight genera of red algae, two genera of brown algae, and some blue-green algae. There are 25 genera of coral and two genera of sponges, but they do not dominate the benthos. Fish comprise 16 genera, with seven genera of echinoids and three genera of crustaceans. The coral reef biome water is presently monitored continuously for temperature, salinity, light, O 2 , and pCO 2 , and monitored daily to weekly for alkalinity, ΣCO 2 , pH, nutrients and δ 13 C DIC and δ 18 O water values. There are a number of filtration devices, pumps and aerators which have been used in the past to manipulate water movement and composition, but at present the community has come to steady-state without this machinery. Diel changes in O 2 and CO 2 allow measurements of community metabolism under different experimental conditions of water chemistry, water motion, seasonal light changes, and temperature. Typical values for community metabolic parameters under steady state conditions are: gross production ( P ), ∼290 mmol C m −2 d −1 , respiration ( R ), ∼270 mmol C m −2 d −1 , P / R , ∼1.1 and community calcification, ∼23 mmol CaCO 3 m −2 d −1 , or only 8% of gross production. Calcification rate has been altered, 0–140 mmol CaCO 3 m −2 d −1, , and is positively correlated to saturation state or CO 3 −2 concentration. The community metabolism values are about half of a natural tropical algal/coral reef flat, but typical of high latitude, shallow, coral reef lagoonal environments. Even though there are some peculiar characteristics of the Biosphere coral reef, the coral reef biome functions as a recognizable coral reef community. The Biosphere 2 coral reef system offers an excellent opportunity to test questions of how environmental factors influence processes at community and organismal scales.