Showing papers in "Limnology and Oceanography in 2011"

Flow-induced reconfiguration of buoyant and flexible aquatic vegetation

Abstract: Plant posture can play a key role in the health of aquatic vegetation, by setting drag, controlling light availability, and mediating the exchange of nutrients and oxygen. We study the flow-induced reconfiguration of buoyant, flexible aquatic vegetation through a combination of laboratory flume experiments and theoretical modeling. The laboratory experiments measure drag and posture for model blades that span the natural range for seagrass stiffness and buoyancy. The theoretical model calculates plant posture based on a force balance that includes posture-dependent drag and the restoring forces due to vegetation stiffness and buoyancy. When the hydrodynamic forcing is small compared to the restoring forces, the model blades remain upright and the quadratic law, Fx 3 U2, predicts the drag well (Fx is drag, U is velocity). When the hydrodynamic forcing exceeds the restoring forces, the blades are pushed over by the flow, and the quadratic drag law no longer applies. The model successfully predicts when this transition occurs. The model also predicts that when the dominant restoring mechanism is blade stiffness, reconfiguration leads to the scaling Fx 3 U4/3. When the dominant restoring mechanism is blade buoyancy, reconfiguration can lead to a sub-linear increase in drag with velocity, i.e., Fx 3 Ua with a , 1. Laboratory measurements confirm both these predictions. The model also predicts drag and posture successfully for natural systems ranging from seagrasses to marine macroalgae of more complex morphology. The most obvious hydrodynamic effect of aquatic vegetation is that it provides resistance to flow. In the past, this has led to aquatic vegetation being removed from river channels to increase conveyance capacity and reduce

Shutdown of turbulent convection as a new criterion for the onset of spring phytoplankton blooms

Abstract: The onset of phytoplankton blooms in late winter, early spring has been traditionally associated with the shoaling of the mixed layer above a critical depth. Here we show that the onset of a bloom can also be triggered by a reduction in air–sea fluxes at the end of winter. When net cooling subsides at the end of winter, turbulent mixing becomes weak, thereby increasing the residence time of phytoplankton cells in the euphotic layer and allowing a bloom to develop. The necessary change in the air–sea flux generally precedes mixed-layer shoaling, and may provide a better indicator for the onset of the spring bloom than the mixed-layer depth alone. Our hypothesis is supported by numerical simulations and remote sensing data. The annual cycle of phytoplankton growth in many parts of the ocean is dominated by a dramatic population increase known as the spring bloom. High levels of primary production during the spring bloom, and the subsequent sinking of organic material, contribute significantly to the carbon flux to the deep ocean (Smetacek et al. 1978; Townsend et al. 1994). The spring bloom occurs globally in coastal seas, lakes, in the Mediterranean and Black Seas, and most famously in the North Atlantic, where the associated change in ocean color can be seen from space. Although it has been the focus of research for decades, there is an ongoing debate as to what conditions prompt the onset of the spring bloom (Townsend et al. 1994; Huisman et al. 1999; Behrenfeld 2010). The onset of phytoplankton blooms is affected by many factors including turbulent mixing and light exposure (the physical controls), respiration and predation rates (the biological controls), and the availability of essential nutrients (the chemical controls) as described for example in Miller (2004). Here, we focus on identifying the most important physical controls. The depth of the mixed layer, where density is nearly homogeneous in the vertical, has been traditionally identified as the crucial physical control for the onset of blooms. According to the ‘critical depth’ hypothesis (Gran and Braarud 1935; Riley 1946; Sverdrup 1953), strong wind and buoyancy forcing during winter lead to deep mixed layers, rich in nutrients entrained from the underlying thermocline. However, at this time the mixed layer is typically deeper than a critical depth, HC, and primary production is limited by light availability despite the abundance of nutrients. A bloom then develops in spring when the mixed layer becomes shallower than the critical depth and phytoplankton cells are exposed to sufficient light to support net population growth. An expression for the critical depth, HC, was derived by Sverdrup (1953):

Geochemical evidence for iron-mediated anaerobic oxidation of methane

Abstract: Anaerobic oxidation of methane (AOM) by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere. We show geochemical evidence for AOM in deep lake sediments and demonstrate that AOM is likely driven by iron (Fe) reduction. Pore-water profiles from Lake Kinneret (Sea of Galilee, Israel) show that this sink for methane is located below the 20-cm depth in the sediment, which is well below the depths at which nitrate and sulfate are completely exhausted, as well as below the zone of methanogenesis. Iron-dependant AOM was verified by Fe(III)-amended mesocosm studies using intact sediment cores, and native iron oxides were detectable throughout the sediments. Because anaerobic Fe(III) respiration is thermodynamically more favorable than both sulfate-dependent methanotrophy and methanogenesis, its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited. Similar conditions are likely to prevail in other incompletely pyritized aquatic sediments, indicating that AOM with Fe(III) is an important global sink for methane.

Dissimilatory reduction of nitrate to ammonium, not denitrification or anammox, dominates benthic nitrate reduction in tropical estuaries

Abstract: We measured benthic denitrification (DN) and dissimilatory reduction of nitrate to ammonium (DNRA) using the isotope-pairing technique in three tropical estuaries in Thailand (Mae Klong), Indonesia (Cisadane), and Fiji (Vunidawa-Rewa) during rainy, dry, and intermediate seasons along the salinity gradient of each estuary. DNRA dominated. Anammox (AN) was measured initially but neither AN activity nor AN bacteria-related 16S ribosomal RNA genes were detected in any of the estuaries. DN was either zero or extremely low, driven by water column nitrate and not from benthic nitrification-DN. N2O was not formed during DN. N2O saturations in estuary water were low, except in the nutrified Indonesian estuary, and tropical estuaries are therefore likely to be only small sources of N2O. Benthic nitrate reduction was nitrate limited; when nitrate was enhanced experimentally, DN increased slightly, but DNRA increased proportionately much more. Predominance of DNRA over DN in tropical estuaries may be due both to an energetic advantage (greater standard free energy change, ΔG°) of nitrate ammonifiers over denitrifiers when competing for limited nitrate, and also to higher affinity for nitrate by the nitrate ammonifiers. At tropical temperatures the three processes occur in the order DNRA > DN > AN. In contrast, temperate estuaries, at lower temperature and higher nitrate concentrations, exhibit proportionately greater levels of AN and DN. The Cisadane estuary became anoxic during the dry season, with high ammonium and sulfide, but no nitrate reduction because of lack of nitrate. Addition of nitrate stimulated high rates of autotrophic DN driven by sulfide, but not DNRA.

Seabed foraging by Antarctic krill: Implications for stock assessment, bentho‐pelagic coupling, and the vertical transfer of iron

Abstract: A compilation of more than 30 studies shows that adult Antarctic krill (Euphausia superba) may frequent benthic habitats year-round, in shelf as well as oceanic waters and throughout their circumpolar range. Net and acoustic data from the Scotia Sea show that in summer 2-20% of the population reside at depths between 200 and 2000 m, and that large aggregations can form above the seabed. Local differences in the vertical distribution of krill indicate that reduced feeding success in surface waters, either due to predator encounter or food shortage, might initiate such deep migrations and results in benthic feeding. Fatty acid and microscopic analyses of stomach content confirm two different foraging habitats for Antarctic krill: the upper ocean, where fresh phytoplankton is the main food source, and deeper water or the seabed, where detritus and copepods are consumed. Krill caught in upper waters retain signals of benthic feeding, suggesting frequent and dynamic exchange between surface and seabed. Krill contained up to 260 nmol iron per stomach when returning from seabed feeding. About 5% of this iron is labile, i.e., potentially available to phytoplankton. Due to their large biomass, frequent benthic feeding, and acidic digestion of particulate iron, krill might facilitate an input of new iron to Southern Ocean surface waters. Deep migrations and foraging at the seabed are significant parts of krill ecology, and the vertical fluxes involved in this behavior are important for the coupling of benthic and pelagic food webs and their elemental repositories.

Reconstructing the various facets of dissolved organic carbon bioavailability in freshwater ecosystems

Abstract: We explored various aspects of freshwater dissolved organic carbon (DOC) lability by comparing short-term (, 2 d) bacterial C consumption (STCC; derived from bacterial respiration measurements) with long-term (28 d) C consumption (LTCC) in DOC bioassays in lakes, rivers, and marshes located within the same complex drainage basin in southern Quebec. We also combined STCC and LTCC measurements to estimate the proportion of DOC removed, and to derive a first-order decay constant (k). STCC rates were, on average, 25% higher than LTCC, and both parameters showed distinct patterns, reaching their lowest and highest values in lakes and marshes, respectively. STCC and LTCC were correlated to DOC concentration across these freshwater ecosystems, whereas in lakes, STCC was positively correlated to chlorophyll and LTCC to terrestrial C inputs. k showed no ecosystem-specific patterns but was negatively correlated to chlorophyll across systems. The size of the DOC pools supporting STCC and LTCC, as well as k, were related to distinct components of the DOC pool, as revealed by a parallel factor analysis of fluorescent dissolved organic matter excitation-emission spectra. Short- and long- term lability and C consumption, and the resulting k, are shown to be complementary facets of DOC bioavailability, which may play very different roles on aquatic ecosystem functioning.

Toward a more comprehensive theory of zooplankton diel vertical migration: integrating ultraviolet radiation and water transparency into the biotic paradigm

Abstract: The current prevailing theory of diel vertical migration (DVM) of zooplankton is focused largely on two biotic drivers: food and predation. Yet recent evidence suggests that abiotic drivers such as damaging ultraviolet (UV) radiation and temperature are also important. Here we integrate current knowledge on the effects of abiotic factors on DVM with the current biologically based paradigm to develop a more comprehensive framework for understanding DVM in zooplankton. We focus on “normal” (down during the day, up at night) DVM of holoplanktonic, primarily herbivorous zooplankton. This new transparency-regulator hypothesis differentiates between structural drivers, such as temperature and food, that vary little over a 24-h period and dynamic drivers, such as damaging UV radiation and visual predation, that show strong variation over a 24-h period. This hypothesis emphasizes the central role of water transparency in regulating these major drivers of DVM. In less transparent systems, temperature and food are often optimal in the surface waters, visual predators are abundant, and UV radiation levels are low. In contrast, in more transparent systems, vertical thermal gradients tend to be more gradual, food quality and quantity are higher in deeper waters, and visual predator abundance is often lower and damaging UV radiation higher in the surface waters. This transparency-regulator hypothesis provides a more versatile theoretical framework to explain variation in DVM across waters of differing transparency. This hypothesis also enables clearer predictions of how the wide range of ongoing transparency-altering local, regional, and global environmental changes can be expected to influence DVM patterns in both inland and oceanic waters of the world.

Dissolved organic matter composition and photoreactivity in prairie lakes of the U.S.Great Plains

Abstract: Dissolved organic matter (DOM) of 27 prairie saline lake ecosystems was investigated in the Northern and Central Great Plains of the United States using absorbance, fluorescence, lignin concentration, and stable C isotope values. The majority of variation in DOM fluorescence among lakes was due to humic (peak C) and microbially formed (peak M) fluorescent components, which appear to be derived from autochthonous primary production. Strong correlations between peak M and nutrients allow us to model total phosphorus (TP) concentration using peak M fluorescence and chromophoric dissolved organic matter (CDOM) absorption. The rate of primary production (PP) was positively correlated with peak M fluorescence and negatively with lignin concentration. Lignin phenol yields in the DOM were generally smaller than those of most freshwater systems. δ13C values of dissolved organic carbon (DOC) ranged from −25.0‰ to −20.1‰ and were generally enriched relative to typical freshwaters (ca. −27‰). Terrestrial DOM is degraded in prairie lakes, spanning a gradient from mixotrophic to eutrophic, as determined by a color–nutrient model. The photodegradation of autochthonous DOM was significant: CO2 fluxes from these prairie lakes, modeled from peak M fluorescence, ranged from 5 to 228 mmol C m−2 d−1 (median, 37 mmol C m−2 d−1) and was similar to community respiration estimated from protein fluorescence (median, 50 mmol C m−2 d−1). The combined estimates were about 50% of the global mean total C release previously reported for saline lake ecosystems. The implication of these new results is that the global C release from saline lake ecosystems is likely underestimated.

Comparative effects of urea, ammonium, and nitrate on phytoplankton abundance, community composition, and toxicity in hypereutrophic freshwaters

Abstract: Dissolved nitrogen (N) as urea ([NH2]2CO), nitrate (NO { ), and ammonium (NH z ) was added to naturally phosphorus (P)-rich lake water (up to 175 m gPL 21) to test the hypotheses that pollution of hypereutrophic lakes with N increases total algal abundance, alters community composition, and favors toxic cyanobacteria that do not fix atmospheric N2. Monthly experiments were conducted in triplicate in polymictic Wascana Lake, Saskatchewan, Canada, during July, August, and September 2008 using large (. 3140 liters) enclosures. Addition of all forms of N added at 6 mg N L21 increased total algal abundance (as chlorophyll a) by up to 350% relative to controls during August and September, when soluble reactive P (SRP) was . 50 m gPL 21 and dissolved N:P was , 20:1 by mass. In particular, NH z and urea favored non-heterocystous cyanobacteria and chlorophytes and NO { , urea promoted chlorophytes, some cyanobacteria, and transient blooms of siliceous algae, whereas N2-fixing cyanobacteria and dinoflagellates exhibited little response to added N. Added N also increased microcystin production by up to 13-fold in August and September, although the magnitude of response varied with N species and predominant algal taxon (Planktothrix agardhii, Microcystis spp.). These findings demonstrate that pollution with N intensifies eutrophication and algal toxicity in lakes with elevated concentrations of SRP and low N:P, and that the magnitude of these effects depends on the chemical form, and hence source, of N.

Effects of changing pCO2 and phosphate availability on domoic acid production and physiology of the marine harmful bloom diatom Pseudo-nitzschia multiseries

Abstract: Some members of the diatom genus Pseudo-nitzschia produce the toxin domoic acid (DA), which through trophic transfer causes mass mortalities of wildlife, shellfish harvesting closures, and risks to human health. Nutrient and micronutrient limitation have been shown to regulate DA production. This study tested the hypothesis that changing partial pressure of CO2 (pCO2) can interact with nutrient limitation to help determine cellular DA levels, an environmentally relevant issue in light of current increases in atmospheric pCO2. Cultures of the toxic species Pseudo-nitzschia multiseries were incubated using semicontinuous methods under a matrix of three pCO2 conditions: , 22 Pa (220 ppm), , 41 Pa (400 ppm), and , 74 Pa (730 ppm), and two phosphate concentrations: 20 mmol L21, P-replete; and 0.5 mmol L21, P-limited. DA production was regulated by both pCO2 and phosphate availability. DA concentrations were 30–50 times higher in P-limited cultures compared to Preplete ones, at the same pCO2 levels. Increasing CO2 levels stimulated DA production under both nutrient conditions, but especially in P-limited cultures, where DA levels increased approximately four times over the pCO2 range examined. Growth rates, primary productivity, photosynthesis vs. irradiance parameters, and cellular elemental ratios also responded interactively to the availability of both CO2 and phosphate. Our results raise the possibility that growth rates and toxicity of the diatom Pseudo-nitzschia multiseries could increase substantially in the future high-CO2 ocean, suggesting a potentially escalating negative effect of this harmful algal bloom species on the future marine environment.

Mobilization and transport of coarse woody debris to the oceans triggered by an extreme tropical storm

Abstract: A significant consequence of Typhoon Morakot in August 2009 was the production of vast volumes of driftwood in Pacific Asia. We have quantified the flux of this coarse woody debris (CWD) to the oceans from typhoontriggered landslides in Taiwan, where Morakot made landfall, by combining remote sensing (using FORMOSAT-2 imagery and aerial photography), analysis of forest biomass, and field observations. A total of 3.8–8.4 TgCWD was transported to the oceans, carrying 1.8–4.0 Tg of organic carbon. In addition to the local effects on the marine and coastal environment from such a highly concentrated flux of carbon and nutrients, storm-driven mobilization of CWD may represent a significant, if infrequent, transfer of terrestrial biomass to the oceans. If the frequency of relatively rare, extreme storms such as Morakot increases in a changing climate, this transport mechanism may play an important role in feedbacks between global climate, storm intensity, and carbon cycling.

Zooplanktivory ameliorates the effects of ocean acidification on the reef coral Porites spp

Abstract: I tested the hypothesis that the effects of high pCO2 and temperature on massive Porites spp. (Scleractinia) are modified by heterotrophic feeding (zooplanktivory). Small colonies of massive Porites spp. from the back reef of Moorea, French Polynesia, were incubated for 1 month under combinations of temperature (29.3°C vs. 25.6°C), pCO2 (41.6 vs. 81.5 Pa), and feeding regimes (none vs. ad libitum access to live Artemia spp.), with the response assessed using calcification and biomass. Area-normalized calcification was unaffected by pCO2, temperature, and the interaction between the two, although it increased 40% with feeding. Biomass increased 35% with feeding and tended to be higher at 25.6°C compared to 29.3°C, and as a result, biomass-normalized calcification statistically was unaffected by feeding, but was depressed 12–17% by high pCO2, with the effect accentuated at 25.6°C. These results show that massive Porites spp. has the capacity to resist the effects on calcification of 1 month exposure to 81.5 Pa pCO2 through heterotrophy and changes in biomass. Area-normalized calcification is sustained at high pCO2 by a greater biomass with a reduced biomass-normalized rate of calcification. This mechanism may play a role in determining the extent to which corals can resist the long-term effects of ocean acidification.

Complex interactions between marine sponges and their symbiotic microbial communities

Abstract: To investigate the importance of symbiont-derived nutrition to host sponges, we coupled manipulative shading experiments with stable isotope analyses of isolated symbiont and host cell fractions. Experiments were conducted with four common reef sponges: Aplysina cauliformis, A. fulva, Neopetrosia subtriangularis, and Niphates erecta. The sponge N. erecta lacks photosymbionts, had a higher growth rate under shaded conditions, and displayed no difference in chlorophyll a (Chl a) concentrations across treatments. Isotope values suggested that this sponge obtains nutrition from particulate organic matter in the water column. In contrast, sponges hosting cyanobacterial symbionts (Aplysina spp. and Neopetrosia) had lower growth rates and lower Chl a concentrations under shaded conditions, suggesting that these hosts rely on photosymbiont nutrition. d15N and d13C values of sponge and microbial cell fractions demonstrated that, while both carbon and nitrogen are transferred from symbionts to host cells in A. cauliformis, only carbon is transferred in N. subtriangularis, and only nitrogen is transferred in A. fulva. Under shaded conditions, shifts in symbiont d13C values were coupled to shifts in host d13C values in some, but not all, host species, suggesting that the stability of these interactions varies across host species. Symbiont-derived nutrients are transferred to the cells of host sponges, and the variability observed among host species indicates that these interactions are more complex than originally hypothesized.

Eco-evolutionary differences in light utilization traits and distributions of freshwater phytoplankton

Abstract: We compiled light utilization traits for 56 species of freshwater phytoplankton to analyze group differences, trait trade-offs, and allometric scaling relationships. We also used these traits to explain differences in major group distributions along the light availability gradient in 527 lakes in the continental United States. Major taxonomic groups differed significantly in their light utilization traits. Cyanobacteria had the highest initial slope of the growthirradiance curve (a) and low irradiance at the onset of photoinhibition, indicating adaptation to low light environments. Green algae had the highest maximal growth rates and low a, indicating adaptation to higher light environments. Groups capable of mixotrophy had traits indicative of poor light competitive abilities and high light requirements. Key light utilization traits scaled allometrically with cell size and exhibited trade-offs leading to contrasting ecological strategies; a and cell size were conserved at the highest taxonomic level (domain), indicating a fundamental trait divergence between prokaryotic and eukaryotic phytoplankton. In line with these trait differences, major groups showed different responses to light availability in natural conditions. The relative abundances of low light–adapted groups declined with increasing light availability and vice versa. The genera mean values of the initial slopes of the growth-irradiance curves were significantly negatively correlated with the slopes of the relationships between the genus’s relative abundance and light availability characterized by Secchi depth in 527 lakes. This indicates that light utilization traits can be used to explain phytoplankton distributions in nature.

Differential effects of ocean acidification on growth and photosynthesis among phylotypes of Symbiodinium (Dinophyceae)

Abstract: We investigated the effect of elevated partial pressure of CO2 (pCO2) on the photosynthesis and growth of four phylotypes (ITS2 types A1, A13, A2, and B1) from the genus Symbiodinium, a diverse dinoflagellate group that is important, both free-living and in symbiosis, for the viability of cnidarians and is thus a potentially important model dinoflagellate group. The response of Symbiodinium to an elevated pCO2 was phylotype-specific. Phylotypes A1 and B1 were largely unaffected by a doubling in pCO2; in contrast, the growth rate of A13 and the photosynthetic capacity of A2 both increased by , 60%. In no case was there an effect of ocean acidification (OA) upon respiration (dark- or light-dependent) for any of the phylotypes examined. Our observations suggest that OA might preferentially select among free-living populations of Symbiodinium, with implications for future symbioses that rely on algal acquisition from the environment (i.e., horizontal transmission). Furthermore, the carbon environment within the host could differentially affect the physiology of different Symbiodinium phylotypes. The range of responses we observed also highlights that the choice of species is an important consideration in OA research and that further investigation across phylogenetic diversity, for both the direction of effect and the underlying mechanism(s) involved, is warranted.

Adaptive strategies by Southern Ocean phytoplankton to lessen iron limitation: Uptake of organically complexed iron and reduced cellular iron requirements

Abstract: We report results of laboratory studies examining the effect of low levels of iron (Fe) availability on the intracellular Fe concentrations and specific growth rates in Southern Ocean diatoms (Fragilariopsis kerguelensis, Eucampia antarctica, Proboscia inermis, and Thalassiosira antarctica) and Phaeocystis antarctica. All species grew on Fe complexed to the siderophore desferrioxamine B (DFB). Concentrations of DFB up to 100-fold in excess of Fe were required to limit growth rates by $ 50%. Southern Ocean phytoplankton also grew on Fe complexed by $ 10-fold excess concentrations of the siderophores ferrichrome, enterobactin, or aerobactin, whereas the temperate coastal diatoms Thalassiosira weissflogii and Thalassiosira pseudonana did not. Intracellular Fe concentrations and Fe:C ratios of all Southern Ocean species were exceptionally low and decreased with decreasing Fe availability. However, large diatoms had significantly lower cell-volume–normalized Fe content and Fe:C ratios than Phaeocystis. Short-term Fe uptake and extracellular Fe(II) production measurements provided evidence that Phaeocystis possesses a reductive Fe transport pathway. Our findings demonstrate that the largediatom Fe requirements are at least 2-fold lower than currently reported for oceanic algal species and suggest that bioreduction may enable resident phytoplankton to directly use Fe bound to strong organic ligands.

Sedimentary phosphorus dynamics and the evolution of bottom-water hypoxia: A coupled benthic-pelagic model of a coastal system

Abstract: The present study examines oxygen and phosphorus dynamics at a seasonally hypoxic site in the Arkona basin of the Baltic Sea. A coupled benthic–pelagic reactive-transport model is used to describe the evolution of bottomwater solute concentrations, as well as pore-water and sediment profiles. Aerobic respiration dominates remineralization, with iron reduction, denitrification, and sulphate reduction playing secondary roles, while other pathways are negligible. Sediments represent a significant oxygen sink chiefly due to the aerobic degradation of organic matter, as well as nitrification and iron oxyhydroxide precipitation. Most phosphorus deposited in sediments is in organic matter, yet cycling is dominated by iron-bound phosphorus due to rapid dissimilatory iron reduction coupled with aerobic iron oxyhydroxide formation. Sustained hypoxia results in an initial decrease in sediment phosphorus content due to dissolution of phosphorus-bearing iron oxyhydroxides, resulting in a pulse of phosphate to overlying waters. Although an organic-rich layer is formed under low-oxygen conditions, enhanced remineralization of organic phosphorus relative to organic carbon tempers sedimentary phosphorus accumulation. Upon reoxygenation of bottom waters after a decade of sustained hypoxia, oxygen concentrations do not immediately achieve values observed prior to hypoxia because the organic-rich layer creates a higher benthic oxygen demand. Artificial reoxygenation of bottom waters leads to a substantial increase in the ironbound phosphorus pool; the total phosphorus content of the sediment, however, is unaffected. A relapse into hypoxia would consequently produce a large pulse of phosphate to the overlying waters potentially exacerbating the situation.

Temperature-induced stress leads to bleaching in larger benthic foraminifera hosting endosymbiotic diatoms

Abstract: Physiological mechanisms of bleaching were studied on larger benthic foraminifera (LBF) hosting endosymbiotic diatoms. Amphistegina radiata, Heterostegina depressa, and Calcarina hispida were exposed to increasing temperatures in static temperature experiments (23u Ct o 33uC, 6 d). Photosynthetic activity (Fv:Fm, measured with a pulse-amplitude modulated fluorometer), chlorophyll a (a proxy for symbiont biomass), and motility (a proxy for overall fitness of the foraminifera) were reduced in specimens at 32u Ct o 33uC, and cytoplasm color changes associated with bleaching were observed. A 30-d flow-through experiment at three temperatures (26u Ct o 31uC) and three levels of inorganic nitrate concentration (0.5 to 1.4 mmol L21) confirmed negative effects of temperature at 31uC for A. radiata (including growth) and H. depressa. Another Calcarina species, Calcarina mayorii, was not affected. This suggests that temperature effects are species-specific. However, elevated nutrient concentrations did not affect any of the parameters measured. Temperatures . 30uC stress the foram–diatom endosymbiosis in some LBF species, which may lead to subsequent bleaching of the host. Given that a 2–3uC increase led to rapid bleaching of most species, we propose that, similar to corals, these species are threatened by sea-surface temperature increase predicted for tropical reef waters in the near future. Benthic foraminifera are unicellular eukaryotes (Rhizaria) with external shells, which are typically produced by active biomineralization of calcite. Together with calcare

Disentangling the spatial patterns in community composition of prokaryotic and eukaryotic lake plankton

Abstract: We sampled 100 small lakes in Finland for bacterio-, phyto-, and zooplankton. The lakes were located in five drainage systems, 20 lakes for each system. We tested two main predictions: that the correlation between community similarity and geographical distance (spatial distance decay) is stronger at the across-drainage than at the within-drainage system scale, and that spatial distance decay is strongest for zooplankton and weakest for bacteria. We used a combination of direct ordination, multivariate statistical tests, and distance-based approaches to examine spatial patterns in our data. Our analyses confirmed both of our predictions. Spatial distance decay was scale-dependent; communities were overall weakly spatially structured within the drainage systems, yet distance decay was significant for all planktonic groups across drainage systems. Spatial distance decay was stronger for zooplankton, with higher slopes and shorter halving distances, than for phytoplankton and bacteria. These results provide evidence that distance decay of similarity is related to study scale, environment, and organism characteristics. Planktonic communities may be controlled by both dispersal-driven assembly and local ecological determinism, with the balance between these two forces depending on study scale.

Thermal tolerance of two seagrass species at contrasting light levels: Implications for future distribution in the Great Barrier Reef

Abstract: This study assessed metabolism, growth, and survival of two seagrass species at three different seawater temperatures (27°C, 30°C, and 33°C) under saturating (400 μmol photons m-2 s-1) and limiting (40 μmol photons m-2 s-1) light over 1 month. Halodule uninervis grown at 33°C was within its physiological optimum temperature range, exhibiting 2.3× higher photosynthetic rates than at 27°C, and increased net shoot carbon (C) production (up to 10× higher) at saturating light levels. In contrast, 33°C exceeded the optimum temperature threshold for Zostera muelleri, resulting in critical metabolic imbalances with large reductions in photosynthesis and increases in leaf respiration. This led to substantially lower growth rates (0-2% of those at 27°C) and lower final biomass (only 10% of that at 27°C) in the 33°C treatment after 1 month. This decline at higher temperatures occurred at both light levels, but it was more severe in limiting light, where the C balance went into deficit. H. uninervis in the Great Barrier Reef (GBR) exists well within its optimal temperature range and should continue to thrive at projected future temperatures, at least under saturating light levels. In contrast, Z. muelleri currently exists near its upper thermal threshold, and future temperature increases of the magnitude investigated here would likely lead to the contraction of the range of this species from the northern GBR - potentially by more than 1000 km. This could have ecologically significant ramifications, because Z. muelleri is often the only GBR species that currently inhabits muddy estuarine areas, which are critical fisheries habitats.

Submarine groundwater discharge from oceanic islands standing in oligotrophic oceans: Implications for global biological production and organic carbon fluxes

Abstract: We investigated submarine groundwater discharge (SGD)–associated nutrient fluxes and budgets in two coastal embayments, Hwasun Bay and Bangdu Bay, off the volcanic island of Jeju, Korea. SGD in Hwasun Bay is a composite of marine and meteoric groundwater, while that in Bangdu Bay mainly includes marine groundwater. The submarine inputs of groundwater into Hwasun and Bangdu Bays were approximately 0.12 and 0.27 m3 m22 d21, respectively, on the basis of the 222Rn mass balance models. The nitrogen:phosphorus ratios in coastal groundwater (85 6 96) were considerably larger than those in the seawater (3.8 6 1.6) of both bays. Fluxes of dissolved inorganic nitrogen (DIN) through SGD were more than 90% of the net DIN input into both bays; approximately 93% and 39% of SGD-driven DIN was consumed inside Hwasun and Bangdu Bays, respectively. The discharge of DIN through SGD from the entire island was approximately 2.1 3 109 mol yr21, which is equivalent to that of some large rivers, potentially supporting approximately 1.6 3 1011 g carbon yr21 of new primary production. Because Jeju accounts for less than 1% of the total land mass of the volcanic islands, SGD-driven nutrient fluxes from highly permeable islands standing in oligotrophic oceans could be very important for global nutrient budgets.

Warmer more acidic conditions cause decreased productivity and calcification in subtropical coral reef sediment-dwelling calcifiers

Abstract: The effects of elevated CO2 and temperature on photosynthesis and calcification in the calcifying algae Halimeda macroloba and Halimeda cylindracea and the symbiont-bearing benthic foraminifera Marginopora vertebralis were investigated through exposure to a combination of four temperatures (28uC, 30uC, 32uC, and 34uC) and four CO2 levels (39, 61, 101, and 203 Pa; pH 8.1, 7.9, 7.7, and 7.4, respectively). Elevated CO2 caused a profound decline in photosynthetic efficiency (FV:FM), calcification, and growth in all species. After five weeks at 34uC under all CO2 levels, all species died. Chlorophyll (Chl) a and b concentration in Halimeda spp. significantly decreased in 203 Pa, 32uC and 34uC treatments, but Chl a and Chl c2 concentration in M. vertebralis was not affected by temperature alone, with significant declines in the 61, 101, and 203 Pa treatments at 28uC. Significant decreases in FV:FM in all species were found after 5 weeks of exposure to elevated CO2 (203 Pa in all temperature treatments) and temperature (32uC and 34uC in all pH treatments). The rate of oxygen production declined at 61, 101, and 203 Pa in all temperature treatments for all species. The elevated CO2 and temperature treatments greatly reduced calcification (growth and crystal size) in M. vertebralis and, to a lesser extent, in Halimeda spp. These findings indicate that 32uC and 101 Pa CO2, are the upper limits for survival of these species on Heron Island reef, and we conclude that these species will be highly vulnerable to the predicted future climate change scenarios of elevated temperature and ocean acidification.

Relating sediment phosphorus mobility to seasonal and diel redox fluctuations at the sediment–water interface in a eutrophic freshwater lake

Abstract: Relationships between phosphorus cycling and redox conditions in the sediments of eutrophic Missisquoi Bay, Lake Champlain were investigated over diel and seasonal timescales in three consecutive summers (2007–2009), one of which (2007) did not experience a cyanobacteria bloom for the first time in a decade. Sediment extraction data showed that reactive phosphorus (RP) is strongly correlated to reactive iron (RFe), suggesting that the mobility of a large portion (30–40%) of the total sediment phosphorus pool is associated with nanocrystalline iron oxide minerals that may be influenced by redox conditions. RP concentrations in the top sediments increased throughout 2007 but decreased throughout 2008; calculations indicate that ~ 1 mm of sediment could account for the increased total dissolved phosphorus observed in overlying water. Redox conditions were measured over 24 h in situ within sediment cores and at the sediment–water interface (SWI) at different stages of each season using voltammetry. SWI redox conditions became progressively more reduced across the season and overnight and were significantly more reducing in the presence of a bloom. Soluble RP and cyanobacteria cell counts measured at five depths through the water column strongly correlated with the most reducing conditions at the SWI. Observations suggest that redox controlled nutrient flux between the sediments and the water column is variable over diel and seasonal cycles. Cyanobacteria blooms most significantly affect SWI redox conditions, suggesting that blooms may enhance RP flux from sediments, setting up a positive feedback loop that can propagate and sustain blooms in shallow freshwater systems.

Phytoplankton community composition can be predicted best in terms of morphological groups

Abstract: We explored how well the aggregated biovolume of groups of species can be predicted from environmental variables using three different classification approaches: morphology-based functional groups, phylogenetic groups, and functional groups proposed by Reynolds. We assessed the relationships between biovolume of each group and environmental conditions using canonical correlation analyses as well as multiple linear regressions, using data from 211 lakes worldwide ranging from subpolar to tropical regions. We compared the results of these analyses with those obtained for single species following the same protocol. While some species appear relatively predictable, a vast majority of the species showed no clear relationship to the environmental conditions we had measured. However, both the multivariate and the regression analyses indicated that morphology-based groups can be predicted better from environmental conditions than groups based on the other classification methods. This suggests that morphology captures ecological function of phytoplankton well, and that functional groups based on morphology may be the most suitable focus for predicting the composition of communities.

Herbicides increase the vulnerability of corals to rising sea surface temperature

Abstract: In order to examine the potential interactive pressures of local pollution and global climate change, we exposed corals and crustose coralline algae (CCA) to three agricultural photosystem II (PSII) herbicides at four temperatures (26–32uC). The coral Acropora millepora was 3- to 10-fold more sensitive to the three herbicides than the CCA Neogoniolithon fosliei. While the photosynthesis of CCA was not affected by the herbicide concentrations used (, 1 m gL 21), temperatures of 31uC and 32uC alone significantly inhibited photosynthetic efficiency (DF:F 9 ) and caused chronic photoinhibition (reduced Fv:Fm) and substantial bleaching. Environmentally relevant concentrations of each herbicide increased the negative effects of thermal stress on coral at 31uC and 32uC. Mixed model analyses of variance showed that the effects of elevated sea surface temperatures (SST) and herbicide on photosynthetic efficiency of coral symbionts were additive. Furthermore, the effect of either diuron or atrazine in combination with higher SST (31uC and 32uC) on chronic photoinhibition was distinctly greater than additive (synergistic). Reducing the herbicide concentration by 1 m gL 21 diuron above 30uC would protect photosynthetic efficiency by the equivalent of 1.8uC and reduce chronic photoinhibition by the equivalent of a 1uC reduction. Reduced water quality increases the vulnerability of corals to elevated SSTs, and effective management of local water quality can reduce negative effects of global stressors such as elevated SST.

Carbon gas fluxes from a brown‐water and a clear‐water lake in the boreal zone during a summer with extreme rain events

Abstract: We studied CO2 and CH4 fluxes from two boreal lakes with differing trophic status (chlorophyll a 17.8 vs. 48.7 mg m−2) and water color (100 vs. 20 mg Pt L−1) throughout an open-water period when summer precipitation doubled, using both floating chambers and concentration gradients. Fluxes measured in chambers were higher, but irrespective of the method, both lakes were heterotrophic and were annual sources of carbon gases to the atmosphere. However, with the annual CO2 flux of 6.85 (chambers) or 5.43 mol m−2 (gradients), the humic lake had notably higher emissions than the clear-water lake, where the fluxes were 3.97 and 3.38 mol m−2, respectively. The annual CH4 flux from the clear-water lake was 28.5 (chambers) or 20.5 mmol m−2 (gradients) and from the humic lake 20.7 or 16.2 mmol m−2, respectively. There were interlake differences in seasonal patterns, but the most obvious changes in fluxes occurred during or just after the rains. In the humic lake, the resulting peak in CO2 and CH4 flux was responsible for 46% and 48% of the annual flux, respectively. Before the rains, the clear-water lake was a small sink of CO2 or had near-zero efflux but afterwards became a source of CO2. In the humic lake, biological mineralization explained the majority of the fluxes, whereas in the clear-water lake the association between the biological processes and fluxes was less pronounced.

Settlement of vegetative fragments of Ulva prolifera confirmed as an important seed source for succession of a large-scale green tide bloom

Abstract: Ulva prolifera green tides in the Yellow Sea of China in the past 3 yr are among the largest macroalgal blooms ever recorded. Satellite images, as well as wind and current patterns, provide a clear picture of the formation of these green tides; however, their place of origin remains unclear. Terrestrial investigation along the coastline of the Jiangsu Province and two shipboard surveys in the Yellow Sea in 2009 showed that filamentous Ulvaceae algae were prevalent in various environments in Jiangsu Province including floating rafts for Porphyra cultivation, land-based bivalve culturing ponds, estuaries, embankments, intertidal stones, wharfs, and hard muddy coasts. Vegetative fragments in sediment samples were found in both shipboard surveys during the bloom and postbloom periods, and all of the samples were U. prolifera. Phylogenetic analysis of 5S spacer sequences showed that the green tide in the Yellow Sea was not of terrestrial origin. However, settled vegetative fragments of U. prolifera were an important seed source for the successive and more serious green tide blooms.

Distinguishing between the effects of ocean acidification and ocean carbonation in the coccolithophore Emiliania huxleyi

Abstract: The coccolithophore Emiliania huxleyi was cultured under a broad range of carbonate chemistry conditions to distinguish the effects of individual carbonate system parameters on growth, primary production, and calcification. In the first experiment, alkalinity was kept constant and the fugacity of CO2 (fCO2) varied from 2 to 600 Pa (1 Pa < 10 matm). In the second experiment, pH was kept constant (pHfree 5 8) with fCO2 varying from 4 to 370 Pa. Results of the constant-alkalinity approach revealed physiological optima for growth, calcification, and organic carbon production at fCO2 values of , 20 Pa, , 40 Pa, and , 80 Pa, respectively. Comparing this with the constant-pH approach showed that growth and organic carbon production increased similarly from low to intermediate CO2 levels but started to diverge towards higher CO2 levels. In the high CO2 range, growth rates and organic carbon production decreased steadily with declining pH at constant alkalinity while remaining consistently higher at constant pH. This suggests that growth and organic carbon production rates are directly related to CO2 at low (sub-saturating) concentrations, whereas towards higher CO2 levels they are adversely affected by the associated decrease in pH. A pH dependence at high fCO2 is also indicated for calcification rates, while the key carbonate system parameter determining calcification at low fCO2 remains unclear. These results imply that key metabolic processes in coccolithophores have their optima at different carbonate chemistry conditions and are influenced by different parameters of the carbonate system at both sides of the optimum. With the beginning of the industrial revolution in the late 18th century, humankind started to considerably increase the atmospheric carbon dioxide (CO2) concentration mainly due to burning of fossil fuels and deforestation of vast regions. This increase would have been even more pronounced if the ocean had not absorbed , 30% of anthropogenic CO2 (Sabine et al. 2004). The flip side of the invasion of anthropogenic CO2 into the ocean (i.e., ocean carbonation) is a decreasing seawater pH—a problem that is referred to as ocean acidification. The surface seawater pH has already decreased by 0.1 pH units since preindustrial times and might further decrease about 0.3–0.4 pH units by the year 2100 (Wolf-Gladrow et al. 1999; Caldeira and Wickett 2005). Ocean acidification and ocean carbonation lead to changes in the concentrations of CO2, HCO { , CO 2{ 3 ,a nd H + in seawater. The effect of these projected changes on different cellular processes such as photosynthesis and calcification is not yet fully understood. Phytoplankton produces organic carbon via photosynthesis, thereby forming the basis of the marine food web. Coccolithophores constitute an important group of phytoplankton. Their characteristic feature is the production of small scales (coccoliths) made of calcium carbonate (CaCO3) which cover the cell. The response of coccolithophores to projected changes in carbonate chemistry has been studied extensively in the past (reviewed in Riebesell and Tortell in press). Most of these studies examined the response of coccolithophores to projected future ocean CO2 scenarios (IPCC 2001) and found profound changes in growth rates, photosynthesis, and carbon fixation in response to increasing CO2 levels. The study presented here focuses on the physiology of the coccolithophore Emiliania huxleyi (strain B92/11). The primary motivation was to determine optimum carbonate chemistry conditions for growth, primary production, and calcification rates of E. huxleyi and to elucidate which parameters of the carbonate system were responsible for the observed sensitivities.

Crenarchaeol tracks winter blooms of ammonia-oxidizing Thaumarchaeota in the coastal North Sea

Abstract: We followed the abundance and distribution of ammonia-oxidizing Archaea (AOA) in the North Sea from April 2003 to February 2005 and from October 2007 to March 2008 by quantification of archaeal genes and core glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipids in suspended particulate matter, to determine whether their abundance in the North Sea is seasonal. GDGT and gene abundance increased during winters and was low during the summer. Crenarchaeol—a GDGT specific to AOA—was a major fraction of the GDGTs and varied in concert with AOA gene abundance, indicating that AOA are the predominant source of crenarchaeol. The presence of crenarchaeol-based intact polar lipids (IPLs) confirmed that the GDGTs recovered were derived from living AOA, as IPLs are rapidly degraded upon cell senescence and thus their occurrence represents living biomass more robustly than their fossil (i.e., core GDGT) counterparts. Dark incubations of North Sea water sampled during the 2007–2008 seasonal cycle with 13C-labeled bicarbonate revealed incorporation of inorganic carbon into IPL-derived GDGTs, directly showing autotrophic production of Thaumarchaeota biomass during the winter. Inhibition of 13C uptake by nitrification inhibitors confirmed that ammonia oxidation was the main source of energy for carbon fixation. Winter blooms of planktonic AOA in the North Sea were recurrent and predictable, occurring annually between November and February, emphasizing the potential importance of AOA in nitrogen cycling in the North Sea.

Source, biogeochemical cycling, and fluorescence characteristics of dissolved organic matter in an agro‐urban estuary

Abstract: Seasonal measurements of dissolved organic matter (DOM) fluorescence characteristics were combined with weekly to bi-weekly measurements of carbon (C), nitrogen (N), and phosphorus (P) (including particulate, dissolved organic and inorganic forms) and chlorophyll to determine how riverine inputs and autochthonous production influence DOM and nutrient dynamics in the Swan-Canning estuary, Western Australia. Estuarine concentrations of C, N, and P were influenced by multiple factors, including the seasonal riverine flux of DOM, N, and P, the regeneration of mainly inorganic N and P from benthic anoxia, and the delivery of DOM and nutrients from urban drains. Parallel factor analysis of excitation-emission matrices identified eight fluorescence components that were used to fingerprint three distinct sources of estuarine DOM: (1) riverine DOM derived mainly from vascular plant material, (2) autochthonous DOM recently produced within the estuary (indicated by tryptophan-like fluorescence), and (3) autochthonous DOM originating from within the lower estuary or coastal marine environment enriched in marine humic-like and tyrosine-like fluorescence. Overall, fluorescence DOM characteristics shifted from humic-like in the upper and mid-estuary to protein-like in the lower estuary, which likely reflects the increased contribution of autochthonous relative to terrigenous DOM closer to the estuary mouth. Our findings show that DOM composition varies with season and estuary position as riverine DOM inputs, mainly from terrestrial plant material, are supplemented by autochthonous DOM contributions associated with discrete inputs of inorganic nutrients.