Cm Turley

Responses by benthic organisms to inputs of organic material to the ocean floor: a review

Most of the photosynthetically produced organic material reaching the ocean-floor is transported as settling particles, among which larger particles such as faecal pellets and macroaggregates (marine snow) are particularly important. Recent studies in the northeastern Atlantic have demonstrated that macroaggregates originating from the euphotic zone settle at a rate of approximately 100-150 m d-1 to form a deposit (phytodetritus) on the sediment surface. Bacteria and protozoa (flagellates and foraminifers) rapidly colonize and multiply on phytodetritus, while large deposit feeding animals ingest it. Other inputs, for example Sargassum, wood and vertebrate carcasses, also evoke a rapid response by benthic organisms. However, the taxa that respond depend on the form of the organic material. The intermittent or seasonally pulsed nature of phytodetritus and many other inputs regulate the population dynamics and reproductive cycles of some responding species. These are often opportunists that are able to utilize ephemeral food resources and, therefore, undergo rapid fluctuations in population density. In addition, the patchy distribution of much of the organic material deposited on the ocean-floor probably plays a major role in structuring deep-sea benthic ecosystems.

Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios

Carbon emissions and their ocean impacts Anthropogenic CO2 emissions directly affect atmospheric chemistry but also have a strong influence on the oceans. Gattuso et al. review how the physics, chemistry, and ecology of the oceans might be affected based on two CO2 emission trajectories: one business as usual and one with aggressive reductions. Ocean warming, acidification, sea-level rise, and the expansion of oxygen minimum zones will continue to have distinct impacts on marine communities and ecosystems. The path that humanity takes regarding CO2 emissions will largely determine the severity of these phenomena. Science, this issue 10.1126/science.aac4722 The amount and pace of our carbon dioxide emissions will determine how the oceans respond. BACKGROUND Although the ocean moderates anthropogenic climate change, this has great impacts on its fundamental physics and chemistry, with important consequences for ecosystems and people. Yet, despite the ocean’s critical role in regulating climate—and providing food security and livelihoods for millions of people—international climate negotiations have only minimally considered impacts on the ocean. Here, we evaluate changes to the ocean and its ecosystems, as well as to the goods and services they provide, under two contrasting CO2 scenarios: the current high-emissions trajectory (Representative Concentration Pathway 8.5, RCP8.5) and a stringent emissions scenario (RCP2.6) consistent with the Copenhagen Accord of keeping mean global temperature increase below 2°C in the 21st century. To do this, we draw on the consensus science in the latest assessment report of the Intergovernmental Panel on Climate Change and papers published since the assessment. ADVANCES Warming and acidification of surface ocean waters will increase proportionately with cumulative CO2 emissions (see figure). Warm-water corals have already been affected, as have mid-latitude seagrass, high-latitude pteropods and krill, mid-latitude bivalves, and fin fishes. Even under the stringent emissions scenario (RCP2.6), warm-water corals and mid-latitude bivalves will be at high risk by 2100. Under our current rate of emissions, most marine organisms evaluated will have very high risk of impacts by 2100 and many by 2050. These results—derived from experiments, field observations, and modeling—are consistent with evidence from high-CO2 periods in the paleorecord. Impacts to the ocean’s ecosystem services follow a parallel trajectory. Services such as coastal protection and capture fisheries are already affected by ocean warming and acidification. The risks of impacts to these services increase with continued emissions: They are predicted to remain moderate for the next 85 years for most services under stringent emission reductions, but the business-as-usual scenario (RCP8.5) would put all ecosystem services we considered at high or very high risk over the same time frame. These impacts will be cumulative or synergistic with other human impacts, such as overexploitation of living resources, habitat destruction, and pollution. Fin fisheries at low latitudes, which are a key source of protein and income for millions of people, will be at high risk. OUTLOOK Four key messages emerge. First, the ocean strongly influences the climate system and provides important services to humans. Second, impacts on key marine and coastal organisms, ecosystems, and services are already detectable, and several will face high risk of impacts well before 2100, even under the low-emissions scenario (RCP2.6). These impacts will occur across all latitudes, making this a global concern beyond the north/south divide. Third, immediate and substantial reduction of CO2 emissions is required to prevent the massive and mostly irreversible impacts on ocean ecosystems and their services that are projected with emissions greater than those in RCP2.6. Limiting emissions to this level is necessary to meet stated objectives of the United Nations Framework Convention on Climate Change; a substantially different ocean would result from any less-stringent emissions scenario. Fourth, as atmospheric CO2 increases, protection, adaptation, and repair options for the ocean become fewer and less effective. The ocean provides compelling arguments for rapid reductions in CO2 emissions and eventually atmospheric CO2 drawdown. Hence, any new global climate agreement that does not minimize the impacts on the ocean will be inadequate. Changes in ocean physics and chemistry and impacts on organisms and ecosystem services according to stringent (RCP2.6) and high business-as-usual (RCP8.5) CO2 emissions scenarios. Changes in temperature (∆T) and pH (∆pH) in 2090 to 2099 are relative to preindustrial (1870 to 1899). Sea level rise (SLR) in 2100 is relative to 1901. RCP2.6 is much more favorable to the ocean, although important ecosystems, goods, and services remain vulnerable, and allows more-efficient management options. l, m, h: low, mid-, and high latitudes, respectively. The ocean moderates anthropogenic climate change at the cost of profound alterations of its physics, chemistry, ecology, and services. Here, we evaluate and compare the risks of impacts on marine and coastal ecosystems—and the goods and services they provide—for growing cumulative carbon emissions under two contrasting emissions scenarios. The current emissions trajectory would rapidly and significantly alter many ecosystems and the associated services on which humans heavily depend. A reduced emissions scenario—consistent with the Copenhagen Accord’s goal of a global temperature increase of less than 2°C—is much more favorable to the ocean but still substantially alters important marine ecosystems and associated goods and services. The management options to address ocean impacts narrow as the ocean warms and acidifies. Consequently, any new climate regime that fails to minimize ocean impacts would be incomplete and inadequate.

Phytodetritus on the deep-sea floor in a central oceanic region of the Northeast Atlantic

In a midoceanic region of the northeast Atlantic, patches of freshly deposited phytodetritus were discovered on the sea floor at a 4500 m depth in July/August 1986. The color of phytodetritus was variable and was obviously related to the degree of degradation. Microscopic analyses showed the presence of planktonic organisms from the euphotic zone, e.g., cyanobacteria, small chlorophytes, diatoms, coccolithophorids, silicoflagellates, dinoflagellates, tintinnids, radiolarians, and foraminifers. Additionally, crustacean exuviae and a great number of small fecal pellets, “minipellets,” were found. Although bacteria were abundant in phytodetritus, their number was not as high as in the sediment. Phytodetrital aggregates also contained a considerable number of benthic organisms such as nematodes and special assemblages of benthic foraminifers. Pigment analyses and the high content of particulate organic carbon indicated that the phytodetritus was relatively undegraded. Concentrations of proteins, carbohydrates, chloroplastic pigments, total adenylates, and bacteria were found to be significantly higher in sediment surface samples when phytodetritus was present than in equivalent samples collected at the same stations in early spring prior to phytodetritus deposition. Only the electron transport system activity showed no significant difference between the two sets of samples, which may be caused by physiological stress during sampling (decompression, warming). The chemical data of phytodetritus samples displayed a great variability indicative of the heterogeneous nature of the detrital material. The gut contents of various megafauna (holothurians, asteroids, sipunculids, and actiniarians) included phytodetritus showing that the detrital material is utilized as a food source by a wide range of benthic organisms. Our data suggest that the detrital material is partly rapidly consumed and remineralized at the sediment surface and partly incorporated into the sediment. Incubations of phytodetritus under simulated in situ conditions and determination of the biological oxygen demand under surface water conditions showed that part of its organic matter can be biologically utilized. Based on the measured standing stock of phytodetritus, it is estimated that 0.3–3% of spring primary production sedimented to the deep-sea floor. Modes of aggregate formation in the surface waters, their sedimentation, and distribution on the seabed are discussed.

Marine snow studies in the Northeast Atlantic Ocean: distribution, composition and role as a food source for migrating plankton

During a 25 d Lagrangian study in May and June 1990 in the Northeast Atlantic Ocean, marine snow aggregates were collected using a novel water bottle, and the composition was determined microscopically. The aggregates contained a characteristic signature of a matrix of bacteria, cyanobacteria and autotrophic picoplankton with inter alia inclusions of the tintiniid Dictyocysta elegans and large pennate diatoms. The concentration of bacteria and cyanobacteria was much greater on the aggregates than when free-living by factors of 100 to 6000 and 3000 to 2 500 000, respectively, depending on depth. Various species of crustacean plankton and micronekton were collected, and the faecal pellets produced after capture were examined. These often contained the marine snow signature, indicating that these organisms had been consuming marine snow. In some cases, marine snow material appeared to dominate the diet. This implies a food-chain short cut wherby material, normally too small to be consumed by the mesozooplankton, and considered to constitute the diet of the microplankton can become part of the diet of organisms higher in the food-chain. The micronekton was dominated by the amphipod Themisto compressa, whose pellets also contained the marine snow signature. Shipboard incubation experiments with this species indicated that (1) it does consume marine snow, and (2) its gut-passage time is sufficiently long for material it has eaten in the upper water to be defecated at its day-time depth of several hundred meters. Plankton and micronekton were collected with nets to examine their vertical distribution and diel migration and to put into context the significance of the flux of material in the guts of migrants. “Gut flux” for the T. compressa population was calculated to be up to 2% of the flux measured simultaneously by drifting sediment traps and <5% when all migrants are considered. The in situ abundance and distribution of marine snow aggregates (>0.6 mm) was examined photographically. A sharp concentration peak was usually encountered in the depth range 40 to 80 m which was not associated with peaks of in situ fluorescence or attenuation but was just below or at the base of the upper mixed layer. The feeding behaviour of zooplankton and nekton may influence these concentration gradients to a considerable extent, and hence affect the flux due to passive settling of marine snow aggregates.

The changing Mediterranean Sea — a sensitive ecosystem?

I was asked to present a keynote paper on the socio-economic aspects of oceanographic research in the Mediterranean Sea in the Session on From Oceanographic Science to Policy at the International Conference on Progress in Oceanography of the Mediterranean Sea, Rome November 1997. The session was unique in that it included papers from oceanographers, social scientists and economists. For this reason I have aimed this paper towards, what I consider to be, social and economic issues that may have important oceanographic outcomes and vice versa. I have attempted to express them in a manner that can be understood by economists, social scientists, policy makers and oceanographers alike. The Mediterranean is highly populated and the greatest tourist destination in the world, both of which are predicted by UNEP to rise substantially in the future. Its blue waters, however, include some of the most extreme oligotrophic waters in the world such that it is only capable of supplying 50% of its requirements for fish. The relatively clear, pigment poor surface waters of the Mediterranean have a general increasing oligotrophy eastward with substantially lower phytoplankton, benthic and fish production in the eastern basin. The Mediterranean Sea is highly sensitive to climatic changes; it has high evaporation rates, low land runoff from few rivers and seasonal rains resulting in a deficit in its hydrological balance. This has worsened with the damming of rivers such as the Nile. Nutrient depleted Atlantic water flows into the Mediterranean through the narrow Strait of Gibraltar and exits after circulating the basin with nearly 10% more salt content. This hydrological imbalance may have far-reaching consequences in the Atlantic, producing similar climate changes in Northern Europe, to that seen during the last glaciation, and may be linked to a hydrological deficit in the Mediterranean Sea resulting from a decline in the Nile outflow. The basin-wide circulation, hot-dry and seasonal climate and low land runoff contribute to the low productivity of the sea. Nutrients are a major controlling factor in oceanic productivity and often influence the type and succession of phytoplankton. Changes in river flow and agricultural practice can influence the concentration and ratio of different nutrients flowing into the sea. For example, changing agricultural practices have resulted in higher nitrogen and phosphorus flowing into the Adriatic and lagoons of the Nile which has lead to eutrophication. The predicted population increases, especially along the southern shores, seems likely to result in eutrophication and an increased risk of pollution in other areas unless well managed. A further warning tale from the Black Sea has recently come to light where damming of rivers has resulted in depletion of silica in the seawater. (Humborg, C., Ittekkot, V., Cociasu, A., & Bodungen, B. (1997). Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure. Nature, London, 386, 385–388.) This means that silica-requiring phytoplankton do not have their essential growth nutrient and may explain the unbalanced growth of other toxic forms which do not require silica. Similarly, the Aswan dam holds back massive amounts of silica carried by the Nile from entering the eastern Mediterranean. The future of the Mediterranean ecosystem does not look rosy. If we are to learn from scientific observations, such as those in the Mediterranean Sea, Black Sea and Adriatic, scientists, economists and policy makers, from the 18 countries bordering the Mediterranean, must interface to ensure an adequate and appropriate response.

Heterotrophic flagellates and other protists associated with oceanic detritus throughout the water column in the mid North Atlantic

Heterotrophic protists, mostly flagellates, encountered in association with marine detritus from various collections in the mid North Atlantic are described. About 40 species have been identified and are reported. Taxa reported here for the first time are: Caecitellus gen. nov. (Protista incertae sedis) and Ministeria marisola gen. nov., sp. nov. (Protista incertae sedis). The flagellates form a subset of the community of heterotrophic marine flagellates encountered in more productive marine sites. Most species are bacterivorous and small. The community extends to the ocean floor but the diversity is reduced in samples taken from greater depths. The decline in species diversity is linked also to a decline in numbers of individuals. We discuss these changes in relation to food supply and pressure effects.

Microbial response to the input of fresh detritus to the deep-sea bed

Abstract In order to assess the response of a deep-sea microbial population from the N.E. Atlantic to simulated falls of detrital aggregates we added sterile detritus to deep-sea microbial communities and incubated them under high pressure and low temperature. Rapid colonization, growth, and decomposition rates indicate that the deep-sea benthic microbial community can react quickly to such inputs of organic carbon to the sea bed. Microbial decomposition and transformation of sedimented detrital aggregates may be important in material flow in the deep-sea and may influence the local seawater chemistry.

Corals in deep-water: will the unseen hand of ocean acidification destroy cold-water ecosystems?

Scleractinian cold-water corals, sometimes referred to as deep-water or deep-sea corals, form perhaps the most vulnerable marine ecosystems to the human dependence on burning fossil fuels (Guinotte et al. 2006). While coldwater corals were discovered two centuries ago, their significance in habitat formation is only just emerging with the deployment of manned and unmanned submersibles and the development of advanced acoustics to map their distribution (Hovland et al. 2002; Roberts et al. 2005; Grasmueck et al. 2006; Fig. 1a). They are found throughout the world oceans, usually between approximately 200– 1,000+ m depth, and unlike many warm-water corals do not contain photosynthetic symbiotic algae (Freiwald 2002 and see papers within Freiwald and Roberts 2005). They are long-lived (several 100 s of years old), form reef frameworks that persist for millennia and are thought to experience relatively little environmental variability (reviewed by Roberts et al. 2006). Reef-like structures can be sizable (e.g., the Rost Lophelia Reef off northern Norway is 100 km with some parts reaching 30 m off the seabed) and may cover a similar or even greater proportion of the oceans as warm-water coral reefs (Mortensen et al. 2001; Freiwald and Roberts 2005; Guinotte et al. 2006; Fig. 1c). Whilst only around 6 out of the 700 known species act as reef framework-forming species in deep waters, these deep-water reef structures are biodiversity hotspots and play an important role as a refuge, feeding ground and nursery for deep-sea organisms, including commercial fish (Rogers 1999; Fossa et al. 2002; Husebo et al. 2002). Little is known about the feeding behaviour of cold-water corals, but they are thought to depend on zooplankton and organic matter that sinks from the productive euphotic zone or organic matter laterally transported by currents for their nutritional requirements (Duineveld et al. 2004; Kiriakoulakis et al. 2004). They are therefore sensitive to changes in currents, surface ocean productivity and the strength of the biological pump of particles to deeper waters. Their slow growth and limited ability to recover make them particularly vulnerable to anthropogenic activities such as bottom trawling, seabed mining, cable and pipe laying, and oil and gas exploration. Some NE Atlantic deep-water reefs have now been severely damaged by bottom trawling (Rogers 1999; Roberts et al. 2000; Fossa et al. 2002; Hall-Spencer et al. 2002; Reed 2002; Freiwald et al. 2004; Wheeler et al. 2005). High atmospheric carbon dioxide concentrations caused by emissions from fossil fuel burning are now recognised to be the major cause of global warming, but these emissions are also acidifying our oceans (IPCC 2007). The oceans are a massive reservoir for CO2 and there is a flux Communicated by Editor in Chief B.E. Brown.

A barophilic flagellate isolated from 4500 m in the mid-North Atlantic

A bacterivorous, barophilic, kidney-shaped flagellate, 3.5–6 μm in length, was isolated from 4500 m in the mid-Atlantic by enrichment of water directly overlying the sediment with sterilized phytodetritus collected from the English Channel during the spring and summer. The light microscopical and electron microscopical appearances of the cell are described and used to identify it to the genus Bodo (Protozoa, Bodoninae). Bodo sp. did not grow at 1 atm and 2°C. It, however, grew under 450 atm and 2°C, with a mean relative growth rate over the exponential growth phase of 0.33 day−1, equivalent to a doubling time of 2.11 days. The flagellate was bacterivorous and had an estimated carbon conversion efficiency of bacterial carbon into flagellate carbon of 17–25%. The step in the microbial food web from bacteria to flagellate could be an important site for remineralization. Bacterial density in deep-sea phytodetritus (1–34 × 107 ml−1) and sediment (5–54 × 109 ml−1) must be sufficient to support a flagellate population. The evidence reported here of a rapidly developing bacterial and flagellate population under simulated deep-sea conditions may indicate that a microbial decomposer pathway similar to that described in shallow waters may be significant in the decomposition of sedimented biogenic particles and energy flow in the deep sea.

The societal challenge of ocean acidification.

The carbonate chemistry of the world’s oceans, including their pH, has been remarkably constant for hundreds of thousands of years (Pearson and Palmer, 2000), with typical surface ocean variations between ice ages and warm phases of no more than 0.2 pH units ([Sanyal et al., 1995], [Honisch and Hemming, 2005] and [Foster, 2008]). However, since the beginning of the industrial revolution, the oceans have taken up approximately 30% of the CO2 produced from fossil fuel burning, cement manufacture and land use changes (Sabine et al., 2004). While the invasion of CO2 into the ocean removes this greenhouse gas from the atmosphere and thereby dampens global warming, it forms carbonic acid in seawater and lowers ambient surface ocean pH (Broecker and Peng, 1982). Ocean acidification is the direct consequence of the excessive addition of CO2 to seawater (Broecker and Takahashi, 1977) and is therefore inherently more predictable than temperature and precipitation changes due to rising CO2 in the atmosphere. Changes are already measurable today ([Bates, 2001], [Bates et al., 2002], [Takahashi et al., 2003], [Keeling et al., 2004] and [Santana-Casiano et al., 2007]) and will become more pronounced as humankind emits more CO2 into the atmosphere, with surface ocean pH expected to decline by a further 0.3 pH units by the end of the century, corresponding to an approximately 100% increase in ocean acidity (hydrogen ion concentration [H+]), on top of the not, vert, similar0.1 pH unit decline to date ([Caldeira and Wickett, 2003], [Orr et al., 2005] and Solomon et al., 2007 In: S. Solomon et al., Editors, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Fourth Assessment Report of the IPCC, Cambridge University Press, Cambridge (2007).[Solomon et al., 2007]) (Fig. 1). Such a rapid change in ocean pH has very likely not happened since the time the dinosaurs went extinct 65 million years ago ([van der Burgh et al., 1993], [Pearson and Palmer, 2000] and [Pagani et al., 2005]). While the dissolution of carbonate sediments on the bottom of the ocean and the weathering of rocks on land coupled with mixing of surface and deeper waters will eventually restore ocean pH to its pre-industrial state, this process will take up to a million years to complete ([Archer, 2005] and [Ridgwell and Zeebe, 2005]).