Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

/pdf/anthropogenic-ocean-acidification-over-the-twenty-first-p8m4yiib3x.pdf

Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms

Fabry, V. J., Seibel, B. A., Feely, R. A., and Orr, J. C. 2008. Impacts of ocean acidification on marine fauna and ecosystem processes. - ICES Journal of Marine Science, 65: 414-432.Oceanic uptake of anthropogenic carbon dioxide (CO 2 ) is altering the seawater chemistry of the world’s oceans with consequences for marine biota. Elevated partial pressure of CO 2 (pCO 2 ) is causing the calcium carbonate saturation horizon to shoal in many regions, particularly in high latitudes and regions that intersect with pronounced hypoxic zones. The ability of marine animals, most importantly pteropod molluscs, foraminifera, and some benthic invertebrates, to produce calcareous skeletal structures is directly affected by seawater CO 2 chemistry. CO 2 influences the physiology of marine organisms as well through acid-base imbalance and reduced oxygen transport capacity. The few studies at relevant pCO 2 levels impede our ability to predict future impacts on foodweb dynamics and other ecosystem processes. Here we present new observations, review available data, and identify priorities for future research, based on regions, ecosystems, taxa, and physiological processes believed to be most vulnerable to ocean acidification. We conclude that ocean acidification and the synergistic impacts of other anthropogenic stressors provide great potential for widespread changes to marine ecosystems.

/pdf/impacts-of-ocean-acidification-on-marine-fauna-and-ecosystem-4vqqs5sbg2.pdf

Impacts of ocean acidification on marine fauna and ecosystem processes

We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi-compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid-ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid-ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom-rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.

Production and dissolution of biogenic silica in the ocean: revised global estimates, comparison with regional data and relationship to biogenic sedimentation

The coastal zone is where land, ocean, and atmosphere interact. It exhibits a wide diversity of geomorphological types and ecosystems, each one displaying great variability in terms of physical and biogeochemical forcings. Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles because it receives massive inputs of terrestrial organic matter and nutrients, is among the most geochemically and biologically active areas of the biosphere, and exchanges large amounts of matter and energy with the open ocean. Coastal ecosystems have therefore attracted much attention recently and are the focus of several current national and international research programs (e.g. LOICZ, ELOISE). The primary production, respiration, calcification, carbon burial and exchange with adjacent systems, including the atmosphere, are reviewed for the major coastal ecosystems (estuaries, macrophyte communities, mangroves, coral reefs, and the remaining continental shelf ). All ecosystems

Carbon and carbonate metabolism in coastal aquatic ecosystems

Macroscopic organic aggregates, which are >500 µm and known as marine and lake snow, are important components in the turnover, decomposition and sinking flux of both organic and inorganic matter and elements in aquatic ecosystems. They are composed of various organic and inorganic materials depending largely on the given system and environmental conditions. The sys- tems include the pelagic limnetic, the neritic and oceanic marine region, as well as shallow turbid environments, e.g. rivers, the littoral zone of lakes, estuaries and tidally affected coastal areas with intense turbulence and a high load of suspended matter. Aggregate abundance and size vary greatly among these systems. Macroaggregates are heavily colonized by bacteria and other heterotrophic microbes and greatly enriched in organic and inorganic nutrients as compared to the surrounding water. During the last 15 yr, many studies have been carried out to examine various aspects of the formation of aggregates, their microbial colonization and decomposition, nutrient recycling and their significance for the sinking flux. They have been identified as hot-spots of the microbial decomposi- tion of organic matter. Further, microaggregates, which are <5 to 500 µm in size and stained by different dyes, such as transparent exopolymer particles (TEP) and Coomassie blue-stained particles, have been discovered and shown also to be important in the formation and decomposition of macroaggregates. In this review we give an overview of the present state of the microbial ecology of macro- and microaggregates, including the mentioned points but highlighting in particular the recent findings on the bacterial colonization of aggregates using molecular tools, their microbial decomposition and mineralization, and the significance of protozoans and metazoans for the colo- nization and decomposition of macroaggregates. Today it is evident that not only the aggregates but also their surroundings are sites and hot-spots of microbial processes, with the plume of solutes leak- ing out of the aggregates and greatly extending the volume of the intense decomposition processes. This microheterogeneity has important implications for the spatial and temporal dynamics of the organic-matter field in aquatic ecosystems and for our understanding of how heterotrophic organisms are involved in the decomposition of organic matter. The significance of aggregate-associated micro- bial processes as key processes and also for the overall decomposition and flux of organic matter- varies greatly among the various systems, and is greatly affected by the total amount of suspended particulate matter. A conclusion from the presented studies and results is that the significance of bac- teria for the formation and decomposition of aggregates appears to be much greater than previously estimated. For a better understanding of the functioning of aquatic ecosystems it is of great impor- tance to include aggregate-associated processes in ecosystem modeling approaches.

/pdf/microbial-ecology-of-organic-aggregates-in-aquatic-1kw4w22q3c.pdf

Microbial ecology of organic aggregates in aquatic ecosystems

Copepod faecal pellets have often been considered as rapid transporters of material out of the euphotic zone. Laboratory experiments on their degradation and sinking rates support this view, but field data on the distribution and flux of pellets through the water colomn present contradictory evidence. We suggest that due to the exclusion of metazoans from previously published degradation experiments, such studies may have little relevance to the natural environments. In 1987/1988 we carried out experiments using adult copepods of mixed species but dominated byCentropages hamatus collected in Kiel Bight (FRG). We have demonstrated that copepods can be highly adept at breaking up their own pellets while ingesting only a small proportion, a behaviour we define as “coprorhexy”. The microbiota is probably unable to cause significant modification to faecal pellets before they are fragmented within a few hours of their production. Thereafter, microbial remineralisation will become important. Many of the “difficult” field data can be readily explained if the process of coprorhexy is taken into account and, indeed, breakage of large particles by crustacean zooplankton may be an important process in modifying material transport in the ocean. Copepods appear to perform coprorhexy by removing the peritrophic membrane with its attached bacterial flora and this may then be ingested. We speculate on the nutritional value of such a behaviour and the possible significance of “ghost” pellets, consisting of a membrane with little or no apparent solid content.

What happens to zooplankton faecal pellets? Implications for material flux

Phytoplankton biomass and composition, primary productivity (in situ simulated and in vitro incubations) and sedimentation rates (measured with free-drifting sediment traps suspended at 100 m depth) were recorded in the Bransfield Strait area of the Antarctic Peninsula during November to December 1980. Three distinct and persistent zones were encountered: low biomass comprising flagellates and diatoms in the Drake Passage and Scotia Sea (zone I): high to moderate biomass of Phaeocystis and diatoms in the northern and central Bransfield Strait (zone II); and moderate biomass (Thalassiosira spp. in the process of forming resting spores) in the vertically homogeneous water on the northern Antarctic Peninsula shelf (zone III). Nutrient concentrations were high throughout; zooplankton grazing relative to phytoplankton biomass and production was heavy in zone I but negligible in the other 2 zones. Rates of primary production in zones I, II, and III averaged 230, 1660 and 830 mg C m−2 d−1, respectively. Assimilation numbers were low throughout (< 1 mg Chl a)−1 h−1) and growth physiology of the zonal phytoplankton assemblages was basically similar. 
 
Sedimentation rates recorded by 2 traps in zone II were low (97 and 138 mg C m−2 d−1) and higher (546 mg C m−2 d−1) in a third trap which collected mostly euphausiid faeces. Sedimentation was heaviest in zone III (450 to 1400 mg C m−2 d−1) where collections of the 3 traps deployed were dominated by intact diatom frustules (Thalassiosira spp.). Spore formation and heavy sedimentation of diatoms thus also occurs at the end of Antarctic blooms in spite of high ambient nutrients. As approximately two-thirds of the diatoms in traps were resting spores, we suggest that sinking of cells represents a seeding strategy which ensures regional persistence of neritic assemblages. Species-specific differences in seeding strategies may well be important in determining spatial and temporal patterns of Antarctic phytoplankton abundance. This aspect of phytoplankton biology is likely to have far-reaching implications, not previously considered, for the structure of Antarctic food webs.

Primary production and sedimentation during spring in the Antarctic Peninsula region

Food web structure and loss rate

Utilization of phytoplankton by copepods in Antarctic waters during spring

The relationships of vertical profiles of phytoplankton photosynthesis to under-water light were studied at 23 stations in the South Scotia Sea and Bransfield Strait. On 3 occasions diurnal photosynthetic patterns were monitored. Chlorophyll-a concentrations varied by a factor of 16.5. Eighty per cent of the variations in light extinction could be explained by variations in chl-a concentration. Accordingly, the euphotic zone (1% surface light level) varied from 15 to 70 m. Photosynthetic profiles were studied in order to assess the production potential of Antarctic phytoplankton. The photosynthetic capacity (photosynthesis per chl-a at optimum light) and maximum quantum yield of photosynthesis (moles carbon dioxide assimilated per mole light quanta absorbed) on the average were smaller by a factor of 7 and 4, respectively, than in phytoplankton at lower latitudes. Diminished low-light photosynthesis suggests that in Antarctic waters temperature-controlled processes take over as rate-limiting steps in otherwise light-limited situations. The utilization efficiency of incident irradiance by phytoplankton at any level of chlorophyll concentration is diminished by both reductions in the photosynthetic capacity and lower light-limited quantum yields. By the combination of both effects, phytoplankton in Antarctic waters can utilize incident light only inefficiently even in situations where biomass accumulation is high.

B. von Bodungen

Papers

What happens to zooplankton faecal pellets? Implications for material flux

Primary production and sedimentation during spring in the Antarctic Peninsula region

Food web structure and loss rate

Utilization of phytoplankton by copepods in Antarctic waters during spring

Light-Dependence of Phytoplankton Photosynthesis in the Antarctic Ocean: Implications for Regulating Productivity