bluuebunny/crossref_metadata_2025 huggingface dataset:
Do wetland lakes exhibit alternative stable states? Submersed aquatic vegetation and chlorophyll in western boreal shallow lakes
Authors: Bayley Suzanne E. , Prather Colleen M.
Published: July 2010

Shallow lakes often exhibit alternative vegetative states, one a clear‐water state dominated by submersed aquatic vegetation (SAV) and the other a turbid state dominated by pelagic phytoplankton. We determined the nutrient and vegetation status of 148 wetland lakes in boreal Alberta, Canada. The lakes were very shallow (mean depth, 1.3 m), rich in phosphorus (123 µg total P L−−1), and relatively low in available nitrogen (18 µg L−−1 NH4+ + NO3−−), and 62% of them exhibited alternative states. The results of principal components analysis suggested that these lakes could be divided into four categories: (1) phytoplankton‐dominated lakes (25%) with higher concentrations of chlorophyll a (>20 µg L−−1), are more turbid, and have low densities of SAV; (2) SAV‐dominated lakes (37%), with high densities of submersed aquatic plants (>25% cover), are clearer, and have lower phytoplankton concentrations; (3) high SAV and high phytoplankton lakes (12%), with dense populations of both SAV and phytoplankton; and (4) low SAV and low phytoplankton lakes (26%), with low densities of both SAV and phytoplankton. The phytoplankton‐dominated lakes are usually found in hypereutrophic conditions (mean = 205 µg TP L−−1), whereas the SAV‐dominated lakes primarily exist in eutrophic and mesotrophic conditions (mean = 82 µg total P L−−1) and have lower available N (11 µg L−−1 NH4+ + NO3−−). Because most of these lakes lack fish, we expect that nutrient status, depth, and invertebrate predators are probably the most important determinants of vegetative structure and alternative states.
The interplay between upwelling and deep convective mixing in determining the seasonal phytoplankton dynamics in the Gulf of Aqaba: Evidence from SeaWiFS and MODIS
Authors: Labiosa Rochelle G. , Arrigo Kevin R. , Genin Amatzia , Monismith Stephen G. , van Dijken Gert
Published: July 2010

In the Gulf of Aqaba, the northeasternmost segment of the Red Sea, phytoplankton blooms are more intense than in other oligotrophic regions (e.g., the Sargasso Sea). In this study, we use multiyear in situ (1988–2000) and Sea viewing Wide Field of View Sensor (SeaWiFS) (1999–2001) chlorophyll a (Chl a) data to describe the dynamics of phytoplankton biomass throughout the Gulf of Aqaba. The temporal pattern of phytoplankton biomass in the Gulf of Aqaba includes a strong spring bloom and a somewhat weaker autumn bloom, the length, intensity, and timing of which vary from year to year. In addition, highly positive west‐to‐east (W to E) gradients in Chl a were found throughout the Gulf of Aqaba. A corresponding negative gradient in sea surface temperature (SST) obtained from MODerate resolution Imaging Spectroradiometer (MODIS) in 2001 indicates that the gradient of Chl a across the Gulf of Aqaba is an outcome of an Ekman‐driven upwelling along the eastern side. Calculations of the upwelling and convective fluxes that are based on meteorological data from Eilat, Israel, indicate that upwelling is comparable to or exceeds convection during much of the year. We present a conceptual model demonstrating how upwelling and convection can either support or oppose each other, thereby jointly controlling mixed‐layer depth and the development of phytoplankton blooms. Coastal upwelling plays a larger role in controlling phytoplankton dynamics than was previously thought, and it explains much of the observed spatial and temporal variability in phytoplankton distributions.
Lateral injection of oxygen with the Bosporus plume—fingers of oxidizing potential in the Black Sea
Authors: Konovalov Sergey K. , Luther George III. W. , Friederich Gernot E. , Nuzzio Donald B. , Tebo Bradley M. , Murray James W. , Oguz Temel , Glazer Brian , Trouwborst Robert E. , Clement Brian , Murray Karen J. , Romanov Alexander S.
Published: July 2010

Saline and warm Mediterranean water flowing through the Bosporus Strait maintains a permanent pycnocline with vertical separation of oxic (O2), suboxic (absence of O2 and H2S), and anoxic (H2S) zones in the Black Sea. The stable suboxic zone implies restricted vertical mixing of the upper oxic and lower anoxic layers and limited vertical flux of oxygen that cannot balance the upward flux of sulfide. We report data that directly confirm massive lateral injections (>200 km from the Bosporus) of oxygen‐enriched waters of the Bosporus plume, created by the mixing of shallow, cold, intermediate‐layer Black Sea water with Mediterranean water. These plume waters are laterally injected into the oxic layer and, more importantly, into the suboxic and anoxic layers over several small vertical scales (“fingers” of ~5 m) at water densities (σt) from 15.0 to 16.4. O2 injection oxidizes Mn(II) to Mn(III,IV), which then oxidizes H2S. The onset of H2S detection occurs in deeper waters in the southwest (>170 m; σt ~= 16.4) relative to the west central Black Sea (110 m; σt ~= 16.2) and coincides with increased MnO2 and S8 formation in the southwest.
Contributions of phytoplankton and other particles to inherent optical properties in New England continental shelf waters
Authors: Green Rebecca E. , Sosik Heidi M. , Olson Robert J.
Published: July 2010

Variability in upper ocean optical properties is often driven by changes in the particle pool. We investigated the effects of such changes by characterizing individual particles. For particles in natural assemblages, we used a combination of Mie theory and flow cytometry to determine diameter (D), complex refractive index (n + in'), and optical cross‐sections at 488 nm. Particles were grouped into categories of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals to interpret variability in concurrently measured bulk inherent optical properties (IOPs) in New England continental shelf waters during two seasons. The summed contributions of individual particles to phytoplankton absorption and particle scattering were close to values for these properties measured independently using bulk methods (87% and 107%, respectively). In surface waters during both seasons, eukaryotic phytoplankton were responsible for the majority of both total particle absorption and total particle scattering. Mineral particles contributed the most to backscattering (bb) in the spring, whereas in the summer both mineral and detrital particles were important. Synechococcus and heterotrophic prokaryotes never contributed more than 14% to IOPs. Our findings emphasize that the measurement of nonliving particles, including detritus and minerals, is necessary for understanding variability in bb in the ocean, an important quantity in the interpretation of satellite ocean color.
Origin and diagenesis of polyphosphate in lake sediments: A 31PߚNMR study
Authors: Hupfer Michael , Ruübe Bernhard , Schmieder Peter
Published: July 2010

Polyphosphate (polyߚP) was detected with the use of 31P nuclear magnetic resonance (NMR) spectroscopy in sediments from a large variety of lakes with different trophic state and morphometry. In the top 0.5 cm of sediment, polyߚP was 1.5 to 11.4% of total P. Nonreactive phosphorus (NRP) in the NaOH fraction (often classified as organically bound phosphate) was up to 46%inorganic polyߚP. In some surface sediments, the polyߚP content equalled the ironߚfixed phosphorus determined by chemical phosphorus fractionation. Sediments were probably supplied with polyߚP by sedimentation because there were substantial amounts of polyߚP in plankton and settling seston. As demonstrated with sediments of Lake Petersdorf, benthic organisms can also contribute to the formation of polyߚP (up to 0.11 mg P [g dry weight]ߚ1) under favorable aerobic conditions. PolyߚP is more rapidly transformed into single orthophosphate during diagenesis than other inorganic and organic P species. The transformation of organic P compounds and polyߚP can contribute significantly to the release of P during diagenesis and should be considered along with the reductive dissolution of P sorbed to iron oxihydroxides.
Use of Keeling plots to determine sources of dissolved organic carbon in nearshore and open ocean systems
Authors: Mortazavi Behzad , Chanton Jeffrey P.
Published: July 2010

We apply a Keeling mixing model commonly used in atmospheric sciences to oceanic dissolved organic carbon (DOC) 14C and concentrations to demonstrate that DOC distribution across spatial transects or depth profiles can be modeled as two‐component mixtures; one component consists of an old refractory pool to which a newly synthesized component pool is added. We then use the Keeling approach to determine the Δ14C signature of excess DOC added to background concentrations in nearshore and open ocean systems. Our analysis indicates enrichment in 14C of excess DOC in coastal waters relative to offshore waters, particularly in spring when river discharge increases. These findings are consistent with the export of 14C‐enriched terrestrial DOC to the ocean and inconsistent with the idea of selective degradation of young terrestrial DOC in estuaries, resulting in export of old refractory component to the ocean. In the middle Atlantic Bight region, during the spring, excess DOC has a 14C signature similar to that of DOC in the major rivers, whereas during the peak primary productivity of the summer season, excess DOC has a signature more similar to that of dissolved inorganic carbon. Variability in the Δ14C of excess DOC in the open ocean was lower than that of the coastal ocean. Minima in Δ14C values of excess DOC in the open ocean were measured during periods of high particulate organic matter (POC) flux, indicating that POC can interact with DOC and selectively remove 14C‐enriched components from the surface ocean. The Keeling plot provides a powerful approach for determining the isotopic composition of excess DOC from an entire DOC profile or transect.
Degradation of dissolved organic matter in oxic and anoxic lake water
Authors: Bastviken David , Persson Linn , Odham Göran , Tranvik Lars
Published: July 2010

Decades of conflicting results have fueled a debate about how O2 affects organic matter (OM) degradation and carbon cycling. In a laboratory study, using both OM taken directly from a humic lake and chemically isolated fulvic acid, we monitored the mineralization of dissolved OM in freshwater under purely oxic and anoxic conditions, under oxic then anoxic conditions, and under anoxic then oxic conditions, for 426 d. Between 5% and 24% of the initial OM was mineralized, with most extensive mineralization occurring under purely oxic and anoxic—oxic conditions. A sequential change in the O2 regime did not result in greater overall degradation, but initially anoxic conditions favored subsequent oxic mineralization. A substantially greater fraction of the OM was degraded than in previous shorter studies, with as much as 50% of the total OM degradation occurring after 147 d into the experiment. Three fractions of the degradable OM were identified: OM degraded only under oxic conditions (68–78%), OM degraded more rapidly under anoxic conditions than under oxic conditions (16–18%), and OM degraded at equal rates under both oxic and anoxic conditions (6–14%). The degradation patterns of natural dissolved OM from a humic lake and chemically isolated fulvic acid were very similar, which indicates a similar level of bioavailability. The difference between anoxic and oxic degradation was greater in our long‐term studies than in previous short‐term experiments, which indicates that the oxic and anoxic degradation potentials vary with increasing overall OM recalcitrance and that similar oxic and anoxic degradation rates can be expected in short‐term experiments in which <30% of the long‐term degradable OM is allowed to decompose.
Bacterial metabolism and growth efficiency in lakes: The importance of phosphorus availability
Authors: Smith Erik M. , Prairie Yves T.
Published: July 2010

We investigated bacterial responses to variations in dissolved organic carbon (DOC) and nutrient availability by a comparative analysis of bacterial metabolism in lakes ranging from oligotrophic to eutrophic. Bacterial growth, respiration, and growth efficiency were quantified in lake water dilution cultures performed in 20 lakes located in eastern Quebec, Canada, which varied with respect to both DOC and nutrient concentrations. Intrinsic growth rates of the bacteria ranged from 0.1 to 1.4 d−, bacterial cell‐specific respiration rates ranged from 0.4 to 7.2 fg C cell−1 h−1, and growth efficiencies ranged from 6.7% to 51.6%. These variations were unrelated to bulk DOC concentrations. Instead, growth rate and efficiency were positively related to total phosphorus concentrations. Specific respiration rate, on the other hand, decreased with increasing phosphorus concentrations, and the magnitude of respiration, on a per‐cell basis, strongly influenced observed growth efficiencies. In a series of substrate enrichment experiments, additions of glucose alone failed to stimulate a response in growth rate, mean cell biovolume, or the potential biomass yield in dilution cultures, but all responded positively to phosphorus additions. Our results show that bacterial metabolism and the fate of DOC input to lake microbial communities are strongly dependent on phosphorus availability, rather than total carbon availability. Extreme oligotrophy appears to place high respiratory demands on the bacterioplankton, resulting in very low bacterial growth efficiencies and consequently greater DOC flow to CO2 than to biomass available for transfer to higher trophic levels.
Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component
Authors: Worden Alexandra Z. , Nolan Jessica K. , Palenik B.
Published: July 2010

We assess population dynamics of picophytoplankton groups (≤2 µm diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument‐specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 6 12% of the total picoplankton cells over time. It had moderately high growth rates (0.52–0.86 d−1) and was subject to low grazing mortality (−0.14 to −0.39 d−1). Prochlorococcus growth and mortality rates were roughly balanced (0.33 ± 0.14 d−1 and −0.36 ± 0.06 d−1, respectively). Picoeukaryotes had the highest growth rates (0.71–1.29 d−1) and were responsible for, on average, 76% of net carbon production (NCP), amounting in up to 32.05 ± 1.31 µg C L−1 d−1 produced and 28.31 ± 2.61 µg C L−1 d−1 consumed. In order to better define the eukaryotic component of these populations, an isolate was characterized via small subunit rRNA gene sequencing, transmission electron microscopy, and growth experiments and was identified as the prasinophyte Ostreococcus, not previously known to the Pacific Ocean. Our results show that although picoeukaryotes do not stand out as particularly important players in this system on the basis of cell abundance, they dominate in terms of picophytoplankton biomass and trophic transfer potential of carbon in this size class.
Frequency, composition, and causes of summer phytoplankton blooms in a shallow coastal ecosystem, the Kattegat
Authors: Carstensen Jacob , Conley Daniel J.
Published: July 2010

We propose a new operational definition for summer phytoplankton blooms based on station‐specific chlorophyll a distributions. This definition has been applied to a large monitoring data set from the Kattegat, a shallow marginal sea affected by man‐induced eutrophication, in order to describe spatial and temporal variations of summer blooms as well as their underlying cause. Blooms were associated with relatively higher salinity, nutrient concentrations, and wind speed prior to observation as well as relatively lower temperature, which suggests that entrainment and the upwelling of nutrient‐rich bottom water are the main causes of summer blooms. The station‐specific frequency of blooms was, on average, 8.7%, with higher frequencies in the frontal area toward the Skagerrak as well as in the shallow western area. Blooms generally had a limited extent in localized regions, and we suggest that blooms could be initiated in hydrodynamically active regions and subsequently transported to other parts of the Kattegat. There were no trends in the bloom frequency over the study period (1989–1999), but interannual variations were linked to external nitrogen input, through upwelling of regenerated nutrients, which suggests that blooms are increasing in frequency over longer timescales. Summer blooms were dominated by large species (Ceratium spp. and Rhizosolenia spp.), which indicates that size‐dependent grazing affects the development and probably also the fate of summer phytoplankton blooms. This bloom definition can be generally applied to other data sets to investigate the properties of high phytoplankton biomass.
Seasonal cycle of phytoplankton UV absorption at the Bermuda Atlantic Time‐series Study (BATS) site
Authors: Morrison J. Ruairidh , Nelson Norman B.
Published: July 2010

Measurements of the phytoplankton absorption coefficient, aph(Λ), at the Bermuda Atlantic Times‐series Study (BATS) site demonstrated a seasonal pattern of absorption in the ultraviolet (UV) range. This was evidenced as peaks between 313 and 335 nm in aph(Λ) from the surface waters (≪50 m) in the summer months (April‐October) that were smaller or absent in the winter (November‐March). These peaks were most probably caused by pigments such as mycosporine‐like amino acids. UV pigment expression in the surface samples, approximated from aph(319)/aph(365), was linearly related to the irradiance exposure at 324 nm (r2 = 0.81). Irradiance exposure was estimated by a layered mixing model parameterized with the calculated mixed layer depth and modeled surface irradiance. Modeled UV irradiance, calculated with satellite‐derived atmospheric data and an atmospheric radiative transfer model, was not significantly different from the measured monthly mean surface irradiance. These results suggest that UV pigments are potentially ubiquitous throughout the world’s oceans and could be an important source of highly colored dissolved organic matter. A peak in the monthly averaged aCDOM for both surface and deeper waters occurred during the period of maximal UV pigment expression at the surface, suggesting either a direct link or similar physical forcing of the two. UV pigments can also be important in higher trophic levels if bioaccumulation, which has been demonstrated in other regions, occurs.
Entrainment and advection in an island's tidal wake, as revealed by light attenuance, zooplankton, and ichthyoplankton
Authors: Suthers Iain M. , Taggart C. T. , Kelley D. , Rissik D. , Middleton J. H.
Published: July 2010

Spatial and temporal patterns of light attenuance, zooplankton abundance, and larval fish assemblages observed at night in the flood tide wake of a 2‐km‐wide, steep‐sided island within the Great Barrier Reef lagoon (40–45‐m local depth) are compared with two simple models. Eddy upwelling is shown to be slow relative to erosion, vertical entrainment, and advection (EA) arising near the flanks of the island, where currents were accelerated to 1–2 m s−1, approximately twice that in the free stream. Turbidity (particles <300‐µm equivalent spherical diameter [ESD], inferred from increased water column light attenuance of an optical plankton counter) and medium‐sized zooplankton (700–1,000‐µm ESD) appeared to be entrained toward the surface, to form a V‐shaped plume. The plume originated near the island's flanks and converged 4 km downstream. Here, light attenuance returned to the free stream conditions, presumably as sediments settled, leaving a patch of medium‐sized zooplankton that had a three‐ to fourfold greater biomass concentration than the free stream. A decrease in the concentration of small zooplankton (300–500‐µm ESD, generally found in surface waters) is also consistent with vertical mixing by EA. Neuston net collections across the wake revealed two larval fish assemblages that were correlated with either the small surface zooplankton or with the deeper, medium‐sized zooplankton, which included epibenthic taxa. If EA is a common process for patch formation in tidal waters, then the geometry of the associated plumes may predict larval settlement (recruitment “hotspots”) in shallow tidal waters.
Bacterial roles in the formation of high‐molecular‐weight dissolved organic matter inestuarine and coastal waters: Evidence from lipids and the compound‐specificisotopic ratios
Authors: Zou Li , Wang Xu-Chen , Callahan Julie , Culp Randolph A. , Chen Robert F. , Altabet Mark A. , Sun Ming-Yi
Published: July 2010

High‐molecular‐weight dissolved organic matter (HMW‐DOM, > 1,000 Daltons) is actively involved in the global biogeochemical cycling of many elements, but its carbon sources and detailed formation pathways are still not well understood. In this study, we measured bulk stable carbon and nitrogen isotopic ratios, lipid composition, and compound‐specific carbon isotopic ratios of HMW‐DOM samples collected from four U.S. estuaries (Boston Harbor/Massachusetts Bay, Delaware/Chesapeake Bay, San Diego Bay, and San Francisco Bay). Analytical results show (1) a fraction of HMW‐DOM (lipid associated) in estuarine and coastal waters is derived from bacteria and phytoplankton; (2) this fraction of HMW‐DOM is formed by various release processes of bacterial membrane components and bacterial reworking of phytoplanktonderived material; (3) this fraction of HMW‐DOM is generally present in all samples from different coastal systems despite variable organic matter inputs and environmental conditions, suggesting an important bacterial role in HMW‐DOM formation.
Predation by calanoid copepods on the appendicularian Oikopleura dioica
Authors: López‐Urrutia Ángel , Harris Roger P. , Smith Tania
Published: July 2010

Appendicularians have some of the highest growth rates among metazoans but they are usually outnumbered by the slower growing copepods in mesozooplankton communities. We present experimental evidence that the eggs and juveniles of the appendicularian Oikopleura dioica are actively consumed by various copepod species. Clearance rates of Calanus helgolandicus, Candacia armata, Centropages typicus, Eucalanus crassus, and Temora longicornis on eggs of O. dioica were usually above 300 ml copepod‐1 d‐1. The ingestion rates of C. helgolandicus on O. dioica eggs did not reach saturation even at the highest egg concentrations (4,560 eggs L‐1). Although C. helgolandicus, C. typicus, and C. armata preyed actively on 1‐mm‐long O. dioica (approximately 0.2 mm in trunk length), predatory pressure decreased with increasing appendicularian size. We suggest that the association of dense appendicularian populations with phytoplankton blooms could be explained by their opportunistic response to beneficial conditions before copepod densities become high enough to cause a decline in appendicularian populations due to predation. Our results indicate that, in addition to the direct shunt of biomass from picoplankton to fish, appendicularians can occupy a similar role to that of microzooplankton in the ocean. They represent an intermediate step in the less efficient, picoplankton‐appendicularia‐copepod‐fish food chain.
Postindustrial enhancement of aragonite undersaturation in the upper tropical andsubtropical Atlantic Ocean: The role of fossil fuel CO2
Authors: Chung Sook‐Nye , Park Geun‐Ha , Lee Kitack , Key Robert M. , Millero Frank J. , Feely Richard A. , Sabine Christopher L. , Falkowski Paul G.
Published: July 2010

The dissolution of aragonite particles in the ocean primarily depends on the degree of undersaturation of seawater with respect to that mineral. Most of the upper Atlantic Ocean, particularly north of 308S and at depths of less than 2000 m, is supersaturated with respect to aragonite, whereas much of the deep Atlantic is undersaturated. Here we report, for the first time, shallow layers of aragonite‐undersaturated water between 20°S and 15°N in the eastern tropical Atlantic. These layers are centered at 800 m and are surrounded by aragonite‐supersaturated water above and below. This feature most likely results from a combination of chemical and biological processes including the uptake of anthropogenic CO2 and the oxidation of organic matter falling from the highly productive overlying surface water. Reaction with protons resulting from these processes decreases the carbonate ion concentration and consequently the saturation state of the waters with respect to aragonite. The oceanic uptake of anthropogenic CO2 during the industrial era has caused a significant increase in the size of the undersaturated layers. Future expansion will likely occur laterally to the west and south, where the degree of supersaturation is low compared to waters to the north. This expansion of the undersaturated layers is a prime example of how human activity during the industrial era has altered the upper ocean chemistry by injecting fossil fuel CO2 into the ocean.
Carbon dioxide degassing and inorganic carbon export from a marsh‐dominated estuary (the Duplin River): A marsh CO2 pump
Authors: Wang Zhaohui Aleck , Cai Wei‐Jun
Published: July 2010

We investigated the seasonal changes and the interactions of the CO2 system in a marsh‐dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO2, total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256‐306 g C m‐2 (of water) yr‐1 of CO2 to the atmosphere and exported 109 g C m‐2 (of water) yr‐1 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt marshes was 156 g C m‐2 (of marsh) yr‐1. Extrapolation of this value to all southeastern U.S. salt marshes indicates that they cumulatively export 0.7 X 1012 g inorganic carbon annually to coastal waters. This export rivals that of riverine inorganic carbon flux. We propose that the CO2 fixation of marsh grasses and the subsequent export of inorganic and organic carbon is one major mechanism that causes the marsh‐influenced nearshore and offshore waters in the southeastern U.S. to be annual net sources of atmospheric CO2. We also suggest that this process provides an efficient and unique means for ocean carbon sequestration of atmospheric CO2.
A comparison of the speciation and fate of mercury in two contaminated coastal marine ecosystems: The Venice Lagoon (Italy) and Lavaca Bay (Texas)
Authors: Bloom Nicolas S. , Moretto Ligia M. , Ugo Paolo
Published: July 2010

We compared the speciation of Hg in two estuaries, Lavaca Bay (Texas) and the Venice Lagoon (Italy). Both are large, shallow embayments that are contaminated by direct discharges from mercury‐cell chlor‐alkali plants and dominated by rapid tidal flushing. Although contaminated by discharges into Lavaca Bay from 1966 to 1979 and into the Venice Lagoon from 1951 to 1989, Hg concentrations have dwindled to similar moderate levels in sediments (200‐2,000 ng g‐1) and water (1‐3 ng L‐1 dissolved) since the advent of pollution abatement. The Venice Lagoon contains lower levels of CH3Hg than Lavaca Bay, which is surprising, given the much larger degree of wetlands coverage in the lagoon. A narrow temporal peak in methylation was seen in early spring at both sites, although it was much stronger in Lavaca Bay. Data indicated that sediment CH3Hg is more strongly correlated with variables such as habitat and seasonality than with total Hg. In Lavaca Bay, because of high sedimentation rates and low remixing, the highest levels of Hg are buried 10‐30 cm below the surface. Combined with previous observations of strong Hg methylation after dredging activities, this argues for leaving Hg‐contaminated sediment in place, to be buried by the deposition of cleaner sediments. In Venice Lagoon, decisions may be complicated by continuous remixing of the upper sediment layers, moving the highest levels of Hg toward the surface and distributing it over a far larger area than has been seen in Lavaca Bay.
Seasonal variation of the δC and δN of particulate and dissolved carbon and nitrogen in Lake Lugano: Constraints on biogeochemical cycling in a eutrophic lake
Authors: Lehmann Moritz F. , Bernasconi Stefano M. , McKenzie Judith A. , Barbieri Alberto , Simona Marco , Veronesi Mauro
Published: July 2010

We determined the Δ13C and Δ15N of water‐column particulate organic matter (POM), dissolved inorganic carbon, and nitrate, together with water chemistry and phytoplankton biomass and species composition every month in eutrophic Lake Lugano. As primary productivity increased during spring, the Δ13C of photic‐zone POM increased from −34‰ to −24‰. This 13C enrichment reflects decreasing C‐isotope fractionation between organic and inorganic carbon pools in response to decreasing surface water [CO2(aq)]. Variations in the Δ15N of surface‐water POM (+2‰ to +8‰) collected during the productive period were attributed to isotope effects associated with nitrate uptake, nitrogen fixation, and mixing of different organic matter sources. The apparent N‐isotope enrichment (ɛ) associated with nitrate assimilation varied with ɛ = −1.0‰ ± 0.9 for diatoms and ɛ = −3.4‰ ± 0.4 for green algae. The mechanisms controlling the N‐isotopic composition of surface‐water nitrate include the combined processes of nitrate assimilation, nitrification, mixing of water masses, and external nitrate loading. There was no consistent relation between the Δ15N of POM, the Δ15N of nitrate, and the nitrate concentration in surface waters. Low Δ13C (−60‰) of POM from the benthic nepheloid layer (BNL) indicated that as much as 80% of the POM in the BNL consisted of methanotrophic bacteria. The Δ13C and Δ15N of near‐bottom POM allowed tracing a “bacteriocline”, which rose with the expanding volume of bottom‐water suboxia. Associated with the development of anaerobic conditions and coupled to decreasing [NO3+], the Δ15N of bottom‐water nitrate progressively increased from +8‰ to +27‰, indicating active denitrification in the hypolimnion. This study demonstrates the potential of natural‐abundance level measurements of stable carbon and nitrogen isotopes to trace biogeochemical processes in lakes. However, the Δ15N of POM in eutrophic lakes is of limited use as an indicator of the degree of nitrate utilization and trophic state.
Do brominated natural products defend marine worms from consumers? Some do, most don’t
Authors: Kicklighter C. E. , Kubanek J. , Hay M. E.
Published: July 2010

Worms and other marine invertebrates living in soft sediments commonly produce brominated natural products that have been hypothesized to function as defenses against consumers, but this hypothesis has not been tested directly. When 16 species of worms from a Georgia mud flat were fed to two sympatric fishes (Fundulus heteroclitus, Leiostomus xanthurus) and a crab (Callinectes similis), 15 species (94×;) were palatable to all three predators. Only the hemichordate Saccoglossus kowalevskii was unpalatable to both fishes, but even it was readily consumed by the crab. Bioassay‐guided chemical investigations demonstrated that Saccoglossus kowalevskii was rejected by fishes because it contained 2,3,4‐tribromopyrrole at 0.2×; of worm dry mass. This is the first direct test of brominated worm metabolites as defenses against sympatric consumers. The deterrence of 2,3,4‐tribromopyrrole in S. kowalevskii may explain why densities of this worm increase 40‐fold during seasons when predation is high and densities of palatable worms decline sharply. To more broadly examine the effects of brominated metabolites on worm palatability, we collected from North Carolina, Georgia, and Florida 14 species reported to produce brominated metabolites. These species were fed to sympatric consumers to assess palatability and also analyzed for brominated metabolites by gas chromatography mass spectrometry (GC/MS). Nine of the fourteen species contained brominated metabolites, but only two were unpalatable. In a final test, five additional brominated metabolites produced by marine worms were added to palatable foods at natural concentrations and at up to 20× natural concentrations. None deterred feeding at natural concentrations, one was deterrent at 5−15× natural concentration, and four had no effect at even 20× natural concentration. Thus, while one worm was defended by a brominated compound, most worms containing brominated metabolites were palatable, and brominated natural products seldom functioned as chemical defenses against consumers.
Phototransformation of riverine dissolved organic matter (DOM) in the presence of abundant iron: Effect on DOM bioavailability
Authors: Kaiser1 Edith , Sulzberger Barbara
Published: July 2010

We conducted studies with high‐molecular‐weight (HMW) and low‐molecular‐weight (LMW), hydrophobic, and hydrophilic dissolved organic matter (DOM) from an oligotrophic alpine river, the Tagliamento River (Italy), to assess the effect of light on DOM utilization by riverine bacterioplankton. Immediately after the exposure of the DOM fractions to simulated or natural sunlight, short‐term (1 h) bacterial utilization of all DOM fractions decreased by up to 80%, compared with the uptake of the corresponding nonirradiated fractions. The addition of scavengers suggests that reactive oxygen species caused this bacterial growth inhibition. After the long‐term growth of bacteria on irradiated DOM, uptake was unchanged for HMW DOM, considerably lower for LMW and hydrophilic DOM, and much higher for hydrophobic DOM, compared with the nonirradiated fractions. These results suggest that the phototransformation of HMW, LMW, hydrophobic, and hydrophilic DOM results in contrasting effects on their bioavailability. Size‐exclusion chromatography showed that bacteria preferably used the larger molecular sizes of all nonirradiated fractions. Light induced no significant shifts in the apparent molecular weight distribution of the four DOM fractions. However, the highly bioavailable LMW DOM (especially the portion ~5 kDa) was no longer taken up after irradiation. We conclude that light overall leads to a strong decrease in microbial DOM transformation during hydrological transport in the Tagliamento River.
Identification and enumeration of bacteria assimilating dimethylsulfoniopropionate (DMSP) in the North Atlantic and Gulf of Mexico
Authors: Malmstrom Rex R. , Kiene Ronald P. , Kirchman David L.
Published: July 2010

The algal‐derived compound dimethylsulfoniopropionate (DMSP), which is the precursor of the climatically active gas dimethylsulfide, is potentially an important source of carbon and sulfur to marine bacterioplankton. Currently, bacteria of the Roseobacter clade, a subgroup of ά;‐proteobacteria, are hypothesized to be the key participants in the metabolism of DMSP. To test this hypothesis, we used a combination of microautoradiography and fluorescence in situ hybridization (Micro‐FISH) to identify the bacteria assimilating 35S DMSP in the Gulf of Mexico, the Gulf of Maine, and the Sargasso Sea. On average, half of the bacterial community assimilated DMSP in these environments. Members of the ά‐proteobacteria dominated DMSP assimilation, accounting for 35–40% of bacteria assimilating DMSP. Cytophaga‐like bacteria andγ‐proteobacteria each accounted for 15–30% of DMSP‐assimilating cells. The ά‐proteobacteria accounted for a greater fraction of the DMSP‐assimilating community than expected based on their overall abundance, whereas Cytophaga‐like bacteria were typically underrepresented in the DMSP‐assimilating community. Members of the Roseobacter clade assimilated more DMSP on a per‐cell basis than any other group, but they did not account for most of the DMSP assimilation, nor were they always present even when DMSP turnover was high. These results indicate that the biogeochemical flux of dissolved DMSP is mediated by a large and diverse group of heterotrophic bacteria.
Relating cell‐level swimming behaviors to vertical population distributions in Heterosigma akashiwo (Raphidophyceae), a harmful alga
Authors: Bearon R. N. , Grünbaum D. , Cattolico R. A.
Published: July 2010

Cell motility may facilitate the formation of harmful algal blooms (HABs) by enabling algal cells to swim to favorable microenvironments that support explosive growth. Motility also augments the formation of algal cell aggregations that are often associated with ecological and economic consequence. In this study, we used computerized video analysis to quantify cell‐level swimming characteristics by reconstructing cell trajectories in the motile raphidophyte Heterosigma akashiwo, a unicellular alga that forms toxic surface slicks in temperate coastal waters worldwide. Heterosigma cells are capable of rapid changes between at least two active swimming modes, distinguishable by the magnitude of the oscillatory component of motion. Swimming direction varied during a diurnal photoperiod, with swimming direction changing from random to upward directed shortly after the start of the light phase. Motility assays performed 6–8 h into the light phase showed that two Heterosigma strains from geographically distant locations differed significantly in gross swimming speeds, with mean values of 49–66 µm s−1 for strain CCMP452 (West Atlantic, USA), and 88–119 µm s−1 for strain CCAP934–1 (North Sea, Norway). A spatially explicit model of vertical distribution of Heterosigma cells based on strain‐specific motility data suggests that cells of the two strains may diverge in water‐column position within a few hours and that CCAP934–1 develops dense surface aggregations more rapidly and more robustly than CCMP452. Propensity to form toxic surface slicks, and therefore frequency and severity of HAB impacts, may vary substantially among Heterosigma strains, mediated by differences in cell‐level motility.
Life cycle plasticity and differential growth and development in marine and lacustrine populations of an Antarctic copepod
Authors: Swadling K. M. , McKinnon A. D. , De'ath G. , Gibson J. A. E.
Published: July 2010

We examined life cycle plasticity in two populations of the copepod Paralabidocera antarctica, one of which inhabits the coastal sea ice belt of Antarctica and the other of which has been isolated in a nearby saline lake for several thousand generations. Similarities in the life cycles of the two populations included long overwintering phases (.5 months) by late‐stage nauplii, rapid development through the copepodid stages, and a short adult life span of 2–3 weeks. Adults appeared in late spring or early summer and spawned and died soon after. However, the life cycle of the lacustrine population was much less tightly regulated than at the marine site; animals were rarely found living within the lake ice, and synchrony in the developmental cycle was diminished. It is likely that a combination of factors, including ice hardness, a lack of predation threat, and a consistent food supply has freed the lacustrine population from the constraints imposed by living within the ice cover. Instantaneous growth rates calculated for the marine site showed a variable growth rate (0.04–0.14 d21). The lacustrine population in general had faster growth rates than the marine population (0.10–0.26 d21) and reached maturity at a smaller size. This is attributed, in part, to the higher environmental temperatures experienced by the lacustrine population.
Dissolution mortality of juvenile bivalves in coastal marine deposits
Authors: Green Mark A. , Jones Michael E. , Boudreau Carrie L. , Moore Richard L. , Westman Benjamin A.
Published: July 2010

We evaluate experimentally the effect of carbonate saturation state at the sediment‐water interface (SWI) on survivorship of various size classes of the juvenile bivalve Mercenaria mercenaria. Populations of 0.2‐mm, 0.3‐mm, 1‐mm, and 2‐mm M. mercenaria were introduced to sediments realistically undersaturated (experimental, saturation state with respect to aragonite = Ώaragonite = IMP/K′sp = ~0.3) and saturated (control, Ώaragonite = ~1.5) with respect to aragonite in order to evaluate the impact of saturation state and dissolution on survivorship. Linear regression analysis was used to examine mortality within each treatment over time and show significant mortality for each size class in experimental‐undersaturated treatments only (P < 0.05). Mortality rates in experimental undersaturated sediments were ‐11.8, ‐4.8, ‐1.9, and ‐1.1% d−1 for the 0.2‐, 0.3‐, 1.0‐, and 2.0‐mm bivalves, respectively. Analysis of covariance (ANCOVA) was used to examine differences in mortality between treatments over time and show significantly different mortality rates for the 0.2‐, 0.3‐, and 1‐mm individuals (P < 0.05). Dissolution may represent a previously unrecognized yet significant source of mortality for "just‐set" juvenile bivalves, particularly the very small individuals that have been largely ignored in recruitment studies to date. Dissolution‐induced mortality may help explain the exponential losses of juvenile bivalves following their transition from the pelagic larval phase to the benthic juvenile phase.
Ascending marine particles: Significance of transparent exopolymer particles (TEP) in the upper ocean
Authors: Azetsu-Scott Kumiko , Passow Uta
Published: July 2010

The high abundance of transparent exopolymer particles (TEP) in marine and freshwater greatly affects particle dynamics. TEP act as glue for colliding particles and form the matrix in aggregates, thereby altering aggregation dynamics. We studied the sinking behavior of freshly produced, particle‐free TEP and of aggregates composed of TEP and latex spheres in a laboratory using water collected from Santa Barbara Channel, California. Particle‐free TEP ascend and accumulate in the surface layer of a settling column at an average velocity of 1.6 x 10−4 cm s−1. The estimated density of TEP ranges from 0.70 to 0.84 g cm−3. TEP also transported latex spheres of 45.6 and 1.82 µm in diameter and a density of 1.05 g cm−3 to the surface layer. We describe a simple model illustrating the role of TEP for the vertical transport of solid particles. The densities and relative proportions of TEP, solid particles, and interstitial water within an aggregate determine its sinking or ascending velocity. High ratios of TEP to solid particles retard the sinking of aggregates, prolonging their residence time in the surface ocean. Our results demonstrate that TEP can provide a vehicle for the upward flux of biological and chemical components in the marine environment, including bacteria, phytoplankton, carbon, and reactive trace elements.
The rate of inflow and mixing during deep‐water renewal in a sill fjord
Authors: Arneborg Lars , Erlandsson Carina P. , Liljebladh Bengt , Stigebrandt Anders
Published: July 2010

We obtained high‐resolution data on a deep‐water renewal in the basin of Gullmar Fjord, Sweden, using an autonomous profiling platform. In the middle of the fjord, where the platform is anchored, renewal starts with the passage of a gravity current front and continues with a steady thickening of the new, oxygen‐rich, low‐nitrate bottom layer and an associated lifting of the old, oxygen‐depleted, high‐nitrate bottom water. The basin continuously fills to sill level during a period of 10 d. At the mouth of the fjord, a three‐layer structure develops. Renewal is driven by the density difference between the intermediate water inside and the new deep water outside the fjord. The volume flux is well predicted by a hydraulic exchange model in which the upper layer plays a passive role. Local upwelling and downwelling of the upper halocline cause fluctuating baroclinic currents during renewal, but these seem to have little influence on the average volume flux of new deep water. Entrainment rates are small, and the associated volume flux increase seems to be balanced by detrainment.
Response of Lake Kivu stratification to lava inflow and climate warming
Authors: Lorke Andreas , Tietze Klaus , Halbwachs Michel , Wüest Alfred
Published: July 2010

During the eruption of Nyiragongo Volcano in January 2002 about 106 m3 of lava entered Lake Kivu. The high concentrations of CO2 and CH4 dissolved in the deep waters of Lake Kivu raised serious concerns about a potential gas outburst with catastrophic consequences for the population in the Kivu‐Tanganyika region. Therefore, 3 weeks after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification. Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth; however, there was no substantial warming or destratification of the gas‐containing deep layers below. The deep double‐diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition, the recent measurements allowed for an update and gave new insight into the stratification and double‐diffusive mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part of the lake of up to 0.5°C within the last 30 yr, which could be attributed to climate variability.
Response of bacterioplankton to iron fertilization in the Southern Ocean
Authors: Arrieta Jesús M. , Weinbauer Markus G. , Lute Carolien , Herndl Gerhard J.
Published: July 2010

We studied the bacterial response to Fe fertilization over 3 weeks during the second iron‐enrichment experiment (EisenEx) in the Southern Ocean. Bacterial abundance in the Fe‐fertilized patch increased over the first 12 d following Fe release and remained about twice as high as outside the Fe‐fertilized patch until the end of the experiment. Bacterial production peaked a few days after each of the three Fe releases inside the Fe‐fertilized patch, reaching rates two to three times higher than outside the patch. Besides the peaks in leucine and thymidine incorporation following Fe release, bacterial production was not significantly higher inside the patch than outside, suggesting direct limitation of bacterial growth by Fe. Bacterial aminopeptidase activity roughly followed the increase in bacterial abundance, whereas cell‐specific α‐ and β‐glucosidase were higher inside the Fe‐fertilized patch. The diversity of β‐glucosidases was determined by capillary electrophoresis zymography. The different β‐glucosidases showed much higher activity levels inside the patch than in the surrounding waters, and three additional β‐glucosidases constituting ~55% of the total β‐glucosidase activity were present inside the Fe‐fertilized patch from day 9 onward. No major changes in response to Fe fertilization were detected in the phylogenetic composition of the bacterioplankton community, as determined by 16S rDNA fingerprinting, indicating a remarkable adaptation of the bacterioplankton community to episodic iron inputs. This stability on the phylogenetic level is contrasted by the dramatic qualitative and quantitative changes in ectoenzymatic activity.