ALBERT

Description: While there is a general sense that lakes can act as sentinels of climate change, their efficacy has not been thoroughly analyzed. We identified the key response variables within a lake that act as indicators of the effects of climate change on both the lake and the catchment. These variables reflect a wide range of physical, chemical, and biological responses to climate. However, the efficacy of the different indicators is affected by regional response to climate change, characteristics of the catchment, and lake mixing regimes. Thus, particular indicators or combinations of indicators are more effective for different lake types and geographic regions. The extraction of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication or acidification, and the equivalent reverse phenomena, in addition to other land-use influences. In many cases, however, confounding factors can be addressed through analytical tools such as detrending or filtering. Lakes are effective sentinels for climate change because they are sensitive to climate, respond rapidly to change, and integrate information about changes in the catchment.

Description: Life history responses are expected to accompany climate warming, yet little is known how long-term effects of climate and environmental change affect the seasonal dynamics of planktonic organisms. We used an historical data set from Lake Washington (U.S.A.) to quantify population responses of a calanoid copepod (Leptodiaptomus ashlandi) to long-term changes in temperature and resource availability and explore potential mechanisms for the responses. Increasing water temperatures (annual mean increase of 1.5 degrees C in the upper 10-m water volume) and longer stratification periods (about 4 weeks) were observed between 1962 and 2005, coincident with a pronounced decline in Leptodiaptomus densities. However, production was maintained because of an increase in the production to biomass ratio and a life cycle shift in Leptodiaptomus from an annual to a 6-month cycle. Cross-wavelet analyses demonstrated that the annual thermal forcing of copepod recruitment observed during the first two decades of the study weakened substantially, leading to more stochastic population dynamics during the past two decades. This shift from one to two generations per year was most likely produced by a longer and warmer growing period combined with changing fluctuations in resource (phytoplankton) availability. Climate change can lead to higher-frequency voltinism in ectothermic organisms and to temporal reorganization of their population dynamics.

Description: Thousands of moribund thaliacean carcasses (Pyrosoma atlanticum) were deposited between February and March 2006 at the seafloor in the Ivory Coast area (West Africa). Remotely operated vehicle surveys were conducted in a continuous depth gradient between 20 and 1275 m along an oil pipeline. Video and still photography were used to estimate the carcass distribution, density, and size on the seabed, as well as recording the local megafauna interactions with the gelatinous material. Large patches of dead pyrosomids covered extensive areas on the continental slope, whereas minor aggregations were found on the shelf. The carcasses were in many instances trapped along the pipelines, accumulating extensively in troughs and furrows in the slope, and especially in soft sediment channels. Pyrosoma atlanticum dried samples were used to calculate the carbon content, enabling the extrapolation to the densities and sizes recorded in the video surveys. The average standing stock of organic carbon associated with the carcasses was 〉5 g C m-2 in the whole slope and canyon, with values as high as 22 g C m-2 in certain areas. Eight megafaunal species from three different phyla were observed 63 times directly feeding on the decomposing carcasses. The gelatinous carbon may have contributed substantially to the detrital food web including microbes at the seabed, and certainly to the diet of larger benthic organisms. The organism-level carbon measurements and documented fate of pyrosomid organic carbon is new evidence of the importance of gelatinous material in large-scale processes and elemental cycling.

Description: To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions.

Description: We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and projected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4-1.8-fold and 1.2-2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales.

Description: Nutrient addition bioassay experiments were performed in the low-nutrient, low-chlorophyll oligotrophic subtropical North Atlantic Ocean to investigate the influence of nitrogen (N), phosphorus (P), and/or iron (Fe) on phytoplankton physiology and the limitation of primary productivity or picophytoplankton biomass. Additions of N alone resulted in 1.5-2 fold increases in primary productivity and chlorophyll after 48 h, with larger (~threefold) increases observed for the addition of P in combination with N (NP). Measurements of cellular chlorophyll contents permitted evaluation of the physiological response of the photosynthetic apparatus to N and P additions in three picophytoplankton groups. In both Prochlorococcus and the picoeukaryotes, cellular chlorophyll increased by similar amounts in N and NP treatments relative to all other treatments, suggesting that pigment synthesis was N limited. In contrast, the increase of cellular chlorophyll was greater in NP than in N treatments in Synechococcus, suggestive of NP co-limitation. Relative increases in cellular nucleic acid were also only observed in Synechococcus for NP treatments, indicating co-limitation of net nucleic acid synthesis. A lack of response to relief of nutrient stress for the efficiency of photosystem II photochemistry, Fv :Fm, suggests that the low nutrient supply to this region resulted in a condition of balanced nutrient limited growth, rather than starvation. N thus appears to be the proximal (i.e. direct physiological) limiting nutrient in the oligotrophic sub-tropical North Atlantic. In addition, some major picophytoplankton groups, as well as overall autotrophic community biomass, appears to be co-limited by N and P.

Description: We investigated potential connections between climate and the population size of gelatinous carnivores in the northwestern Mediterranean. The interannual variability of large- and meso-scale climate factors was related to gelatinous carnivore dynamics by means of a multivariate regression model. Gelatinous carnivore population dynamics integrate the climate-related changes in the northwestern Mediterranean with close correlations between climate and the population size of the Calycophoran siphonophores Chelophyes appendiculata and Abylopsis tetragona and the Hydromedusae Rhopalonema velatum and Solmundella bitentaculata. We show that the sensitivity of particular gelatinous carnivore species to environmental forcing relates to their seasonal appearance and peak of abundance. Our results advocate the occurrence of short time windows, during which gelatinous carnivores appear more sensitive to environmental conditions, and favorable conditions may therefore substantially enhance their annual peaks. Furthermore, there were threshold values from which climate effects on gelatinous carnivores become noticeable, indicating that the climate-gelatinous carnivore relationship intensifies according to the strength of climate forcing. The possibility of using the North Atlantic climate variability for assessing and predicting interannual abundance changes of these organisms in the northwestern Mediterranean is considered.

Description: Identification of the proximal nutrient limiting primary production is a necessary first step toward evaluating the physiological state of phytoplankton communities and the biogeochemical constraints on the current oceanic carbon cycle. We conducted 48-h nutrient addition bioassay experiments to evaluate nitrogen, phosphorus, and iron limitation of primary productivity, net chlorophyll synthesis, and net increase in cell numbers of the dominant picophytoplankton from the tropical North Atlantic. Our results indicate that N was the proximal limiting factor for primary production during the autumn of 2002, followed by P and then Fe. Net chlorophyll synthesis was significantly stimulated by addition of N alone and further stimulated by addition of P. Analysis of picophytoplankton populations by analytical flow cytometry revealed a more complex response. Cellular red fluorescence, an index of cell chlorophyll content, increased in Prochlorococcus, Synechococcus, and picoeukaryotes in response to addition of NH4NO3 but was not affected by single or combined additions of P and Fe. In contrast, cell abundance in these picophytoplankton populations increased only after combined N and P (63% of comparisons) or N and Fe (21% of comparisons) additions. Thus, our experiments revealed that chlorophyll synthesis and primary production were limited by the availability of nitrogen alone, while net increase in cell abundance was colimited by N and P or N and Fe in the majority of these picophytoplankton populations.

Description: Bacterial productivity and biomass are thought to be limited by dissolved organic carbon (DOC) in much of the world’s oceans. However, the mixed layer of oligotrophic oceans is often depleted in dissolved inorganic nitrogen and phosphate, raising the possibility that macronutrients may also limit heterotrophic bacterial growth. We used nutrient bioassay experiments to determine whether inorganic nutrients (N, P, Fe) and/or DOC could limit bacterial productivity and biomass in the central North Atlantic during the spring of 2004 (Mar–Apr). We observed that both heterotrophic bacterial productivity and biomass were co-limited by N and P in the oligotrophic North Atlantic, and additions of labile DOC (glucose) provided no stimulation unless N and P were also added. Flow cytometry results indicated that only a small subset of large cells high in nucleic acid content were responsible for the increased productivity in the combined NP amendments. In contrast, nutrient additions elicited no net change on the dominant component of the bacterial population, composed of small cells with relatively low nucleic acid content. In the combined NP treatments the relative increase in bacterial production was greater than that measured when phytoplankton productivity was relieved of nitrogen limitation. These results suggest that N and P co-limitation in the bacterial community results in increased competition between the heterotrophic and autotrophic components of the surface communities in the Central North Atlantic Ocean, and potentially impacts the cycling of organic matter by the bacterioplankton.

Description: Calcification of the cosmopolitan coccolithophore species Emiliania huxleyi was investigated in relation to the cell division cycle with the use of batch cultures. With a 12 : 12 h light : dark cycle, the population was synchronised to undergo division as a cohort, simultaneously passing through the G1 (assimilation), S (DNA replication), and G2+M (cell division and mitosis) phases. Cell division was followed with the use of quantitative DNA staining and flow cytometry. Simultaneously, carbon-14 (14C) assimilation in organic and inorganic carbon as well as cell abundance, size, and organic nitrogen content were measured at 2-h intervals. In additional experiments, changes in calcification and cell cycle stages were investigated in nitrogen-, phosphorus-, and light-limited cultures. Calcification occurred only during the G1 cell cycle phase, as seen by the very tight correlation between the percentage of cells in G1 and calcification during the dark period. When growth was limited by nitrogen, cells decreased in size, remained in the G1 phase, and showed a moderate increase in the cell-specific calcite content. Limitation of growth by phosphorus, however, caused a significant increase in cell size and a dramatic increase in cellular calcite. Light limitation, by slowing the growth rate, prolonged the time cells spent in the G1 phase with a corresponding increase in the cellular calcite content. These results help explain the differing responses of coccolithophorid growth to nitrogen, phosphorus, and light limitation.

Description: Sea ice brines were collected from a single floe composed of different ice types in the western Weddell Sea in December 2004. The chemical composition of the brines (temperature: 23.4°C to 22.1°C; salinity: 40–63) was examined on seven occasions over 25 days with measurements of dissolved oxygen, dissolved inorganic macronutrients (nitrate plus nitrite, ammonium, phosphorus [DIP], and silicic acid), pH, total alkalinity (AT), dissolved organic carbon (DOC) and nitrogen (DON), total dissolved inorganic carbon (CT), and the stable isotopic composition of CT (δ13CT). The in situ pH ranged from 8.41-8.82 on the seawater scale, dissolved oxygen from 212-604 µmol kg−1, nitrate from 0.1-3.1 µmol kg−1, ammonium 0.1-2.4 µmol kg−1, DIP 0.4- 2.0 µmol kg−1, silicic acid 4-80 µmol kg−1, AT 2,690-4,620 µeq kg−1, DOC 115-359 µmol kg−1, DON 8-26 µmol kg−1, CT 2,090-3,550 µmol kg−1, and δ13CT +2.9‰ - +6.4‰. Compared with the chemical composition of surface oceanic water (salinity of 34), the brines had elevated pH, reduced concentrations of dissolved inorganic macronutrients (including carbon), especially dissolved inorganic nitrogen, and were mostly supersaturated with dissolved oxygen with respect to equilibrium with air, whereas the CT was considerably enriched in 13C. The chemical composition of the brines was consistent with internal biological productivity, but there was a lack of a distinctive and uniform relationship among the major dissolved inorganic nutrients typically used for describing biological activity. This was interpreted as the result of varying stoichiometry of biological activity within a very small spatial scale. Modification by abiotic processes was a potential contributing factor, such as degassing acting on the dissolved oxygen concentration. Carbonate mineral formation, acting on AT and CT, was not evident in brines from first-year ice but was apparent in brine from second-year ice.

Description: We investigated whether nutrient limitations of primary producers act upward through food webs only in terms of density effects or if there is a second pathway for nutrient limitation signals channelled upward to higher trophic levels. We used tritrophic food chains to assess the effects of nutrient-limited phytoplankters (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the calanoid copepod Acartia tonsa) and finally zooplanktivores (larval herring Clupea harengus) living on the herbivores. The primary producers� food quality had a significant effect on fish condition. Our experimental phosphorus-limited food chain resulted in larval fish with a significantly poorer condition than their counterparts reared under nitrogen-limited or nutrient-sufficient conditions. Our results show that mineral nutrient requirements of consumers have to be satisfied first before fatty acids can promote further growth. This challenges the match/mismatch hypothesis, which links larval fish survival probability solely to prey availability, and could imply that reduced nutrient releases into the environment may affect fish stocks even more severely than previously believed.

Description: To investigate the biogeochemistry of iron in the waters of the European continental margin, we determined the dissolved iron distribution and redox speciation in filtered (〈0.2 μm) open-ocean and shelf waters. Depth profiles were sampled over the shelf slope southeast of the Chapelle Bank area (47.61°N, 4.24°W to 46.00°N, 8.01°W) and a horizontal surface-water transect over the shelf and through the English Channel (la Manche) and the southern North Sea (46°N, 8°W to 52°N, 4°E). An abrupt trace-metal front was found near the shelf slope, indicated by a horizontal gradient of dissolved iron (DFe) and aluminium (DAl), which correlated with changing salinities (r2 = 0.572 and 0.528, respectively, n = 92). Labile Fe(II) concentrations varied from 〈12 pmol L-1 in North Atlantic surface waters to >200 pmol L-1 in the near bottom waters of the shelf break. Labile Fe(II) accounted for ∼5 of the dissolved iron species in surface shelf waters (mean 5.0 ± 2.7), whereas higher Fe(II) fractions (i.e., >8) were observed near the sea bottom on the shelf break and during a midday solar maximum in surface waters in the vicinity of the Scheldt river plume. Benthic processes (resuspension and diagenesis) constituted important sources of Fe(II) and DFe in this region, and photoreduction of Fe(III) species in shelf waters caused enhanced labile Fe(II) concentrations. These processes increased the lability of iron and its potential availability to marine organisms in the shelf ecosystem. © 2007, by the American Society of Limnology and Oceanography, Inc.

Description: To date no study exists that directly addresses changes in dynamics of heterotrophic bacteria in surface waters in relation to partial pressure of CO2 (pCO2). Therefore, we studied the effect of changes in pCO2 on bacterial abundance and activities by using mesocosms with different pCO2 levels (~190, ~370, and ~700 ppmV, representing past, present-day, and future atmospheric pCO2, respectively). Abundance of total bacteria did not differ with increasing pCO2 throughout the whole study period, whereas bacterial protein production (BPP) was highest at highest pCO2. This effect was even more pronounced for cell-specific production rates, especially those of attached bacteria, which were up to 25 times higher than those of free bacteria. During the breakdown of the bloom, however, the abundance of both free and attached bacteria was significantly increased with pCO2. Differences in bacterial growth rate (µ) were smaller than those of BPP, but both µ and BPP of attached bacteria were elevated under high pCO2. Averages of total protease as well as α- and α -glucosidase activities were highest at elevated pCO2 levels, but a statistically significant dependence on pCO2 was only evident for protease activity. There is a measurable but indirect effect of changes in pCO2 on bacterial activities that are mainly linked to phytoplankton and presumably particle dynamics

Description: During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms 〉 20 mu m were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mu m) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton 20 mu m) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 mu m) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

Description: We quantify, compare, and generalize responses of experimental nutrient loadings (LN) on planktonic community structure and function in coastal waters. Data were derived from three mesocosm experiments undertaken in Baltic (BAL), Mediterranean (MED), and Norwegian (NOR) coastal waters. A planktonic model with seven functional compartments and 30-32 different carbon flows fit to all three experiments was used as a framework for flow-rate estimation and comparison. Flows were estimated on the basis of time series of measured biomass, some measured flows, and inverse modeling. Biomass and gross uptake rate of carbon of most groups increased linearly with increasing LN in the nutrient input range of 0-1 µmol N L-1 d-1 at all locations. The fate of the gross primary production (GPP) was similar in all systems. Autotrophic biomass varied by two orders of magnitude among locations, with the lowest biomass and response to nutrient addition in MED waters. The variation of GPP among sites was less than one order of magnitude. Mesozooplankton dominated by doliolids (Tunicata), but not those dominated by copepods, presumably exerted efficient control of the autotrophic biomass, thereby buffering responses of autotrophs to high nutrient input. Among the many factors that can modify the responses of autotrophs to nutrients, the time scale over which the enrichment is made and the precise mode of nutrient enrichment are important. We suggest a general concept that may contribute to a scientific basis for understanding and managing coastal eutrophication

Description: We investigated the carbon acquisition of three marine microalgae, Skeletonema costatum, Phaeocystis globosa, and Emiliania huxleyi in response to different light regimes. Rates of photosynthetic O2 evolution and CO2 and HCO3- uptake were measured by membrane inlet mass spectrometry in cells acclimated to cycles of 16 : 8 light : dark (LD; h : h) and 12 : 12 LD and were compared with those obtained under continuous light. In addition, cellular leakage was estimated for different photoperiods and ambient CO2 concentrations during growth. Maximum rates of photosynthesis more or less doubled under LD cycles compared with continuous light. In S. costatum and E. huxleyi, a remarkably higher contribution of HCO3- to the overall carbon uptake was observed under LD cycles. In contrast, P. globosa did not change its CO2 :HCO3- uptake ratio in response to daylength. Half saturation concentrations (K1/2) for O2 evolution and inorganic carbon (Ci) uptake were also influenced by the photoperiod. Under LD cycles K1/2 values for photosynthesis in S. costatum and P. globosa were similar or higher compared with continuous light, whereas they were much lower in E. huxleyi. With the exception of CO2 uptake in E. huxleyi and P. globosa, affinities for Ci decreased under the LD cycles. Cellular leakage was highest for E. huxleyi and lowest for S. costatum and generally decreased with increasing CO2 concentration. Although this study confirms species specific differences in the CO2-concentrating mechanisms (CCMs), the effect of daylength on CO2 and HCO3- uptake has hitherto not been described. We put forward the idea that variations in light condition influence the cellular carbon demand, thereby imposing a stronger control on CCM regulation than the naturally occurring changes in CO2 supply.

Description: In recent studies the Sr/Ca ratio of coccolithophore calcite was used as a proxy for past coccolithophore growth and calcification rates. Since Sr and Ca concentrations in seawater have not remained constant through time, interpretation of Sr/Ca data from the coccolith-dominated sedimentary record requires knowledge about the incorporation of seawater Sr into coccolith calcite during coccolithogenesis. Here we show that Sr/Ca of Emiliania huxleyi coccoliths is linearly related to seawater Sr/Ca, meaning that the Sr exchange coefficient does not change with changing seawater Sr/Ca. The exchange coefficient for Sr in this study, 0.39, is close to values presented in the literature and is high compared with values obtained by inorganic precipitation experiments. This suggests a strong effect of cell physiology on biogenic calcite precipitation in coccolithophores. We present a conceptual model, based on the transmembrane transport of Sr and Ca, which explains the offset.

Description: Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy-forming algae such as Fucus spp. with annual, bloom-forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae�s susceptible recruits. We examine these generalizations across large scales. We use replicated ��bioassay�� experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalis propagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10-fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant (;80%). Comparative field surveys showed that perennial algae decreased by 30-60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24-46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity.

Description: We investigated the effect of seasonal environmental changes on the rate and distribution of anaerobic oxidation of methane (AOM) in Eckernforde Bay sediments (German Baltic Sea) and identified organisms that are likely to be involved in the process. Surface sediments were sampled during September and March. Field rates of AOM and sulfate reduction (SR) were measured with radiotracer methods. Additional parameters were determined that potentially influence AOM, i.e., temperature, salinity, methane, sulfate, and chlorophyll a. Methanogenesis as well as potential rates of AOM and aerobic oxidation of methane were measured in vitro. AOM changed seasonally within the upper 20 cm of the sediment, with rates being between 1 and 14 nmol cm(-3) d(-1). Its distribution is suggested to be controlled by oxygen and sulfate penetration, temperature, as well as methane supply, leading to a shallow AOM zone during the warm productive season and to a slightly deeper AOM zone during the cold winter season. Rising methane bubbles apparently fed AOM above the sulfate-methane transition. Methanosarcinales-related anaerobic methanotrophs (ANME-2), identified with fluorescence in situ hybridization, is suggested to mediate AOM in Eckerntorde Bay. These archaea are known also from other marine methane-rich locations. However, they were not directly associated with sulfate-reducing bacteria. AOM is possibly mediated solely by these archaea that show a mesophilic physiology according to the seasonal temperature changes in Eckernforde Bay.

Description: We studied the direct effects of CO2 Engel et al. in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (190 parts per million by volume (ppmV) CO2 organisms in a mesocosm experiment. In nine outdoor enclosures (11 m3 present (410 ppmV CO2 and high (710 ppmV CO2 pCO2 and related changes in seawater carbonate chemistry on marine planktonic each), the partial pressure of CO2 (pCO2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2 within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO2 This broad influence of CO2 -related effects. Normalization of production rates . on the E. huxleyi bloom suggests that changes in CO2 physiology with likely effects on the marine biogeochemistry.

Description: At Hook Ridge hydrothermal vent, a new species of Sclerolinum (Monilifera, Siboglinidae) was found at a water depth of 1,045 m. On the basis of investigations of multicores and gravity cores, the species habitat is characterized. Sclerolinum does not occur in sediments that are most strongly influenced by hydrothermal fluids, probably because of high temperature (up to 49°C) and precipitation of siliceous crusts. About 800 individuals m-2 occur in sediments that are only weakly exposed to hydrothermal flow and have the following characteristics: 20°C (15 cm sediment depth) to 21.5°C (bottom water), 18-40 cm yr-1 advection rates, pH 5.5, 〈25 µmol L-1 methane, 〈170 µmol L-1 sulfide, and 〈0.0054 mol m-2 yr-1 sulfide flux. Comparison with geochemical data from other reducing sediments indicates that the two groups of Siboglinidae, Monilifera and Frenulata, occur in sediments with low sulfide concentration and flux. In contrast, sulfurbased chemosynthetic organisms that typically occur at hydrothermal vents and cold seeps (e.g., Vestimentifera, vesicomyid clams, and bacterial mats) occur in sediments with higher sulfide availability; threshold values are around 500 µmol L-1 sulfide and 0.1 mol m-2 yr-1 sulfide fluxes. We did not find typical hydrothermal vent species at Hook Ridge hydrothermal vent, which might be explained by the unfavorable physicochemical habitat: At sites inhabited by Sclerolinum, sulfide availability appears to be too low, whereas at sites with higher sulfide availability, the temperatures might be too high, siliceous crust precipitation could preclude their occurrence, or both.