Upper-water Column Research Articles

Spawning aggregation behavior and reproductive ecology of the giant bumphead parrotfish, Bolbometopon muricatum, in a remote marine reserve.

The giant bumphead parrotfish (Bolbometopon muricatum) has experienced precipitous population declines throughout its range due to its importance as a highly-prized fishery target and cultural resource. Because of its diet, Bolbometopon may serve as a keystone species on Indo-Pacific coral reefs, yet comprehensive descriptions of its reproductive ecology do not exist. We used a variety of underwater visual census (UVC) methods to study an intact population of Bolbometopon at Wake Atoll, a remote and protected coral atoll in the west Pacific. Key observations include spawning activities in the morning around the full and last quarter moon, with possible spawning extending to the new moon. We observed peaks in aggregation size just prior to and following the full and last quarter moon, respectively, and observed a distinct break in spawning at the site that persisted for four days; individuals returned to the aggregation site one day prior to the last quarter moon and resumed spawning the following day. The mating system was lek-based, characterized by early male arrival at the spawning site followed by vigorous defense (including head-butting between large males) of small territories. These territories were apparently used to attract females that arrived later in large schools, causing substantial changes in the sex ratio on the aggregation site at any given time during the morning spawning period. Aggression between males and courtship of females led to pair spawning within the upper water column. Mating interference was not witnessed but we noted instances suggesting that sperm competition might occur. Densities of Bolbometopon on the aggregation site averaged 10.07(±3.24 SE) fish per hectare (ha) with maximum densities of 51.5 fish per ha. By comparing our observations to the results of biennial surveys conducted by the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Ecosystem Division (CRED), we confirmed spatial consistency of the aggregation across years as well as a temporal break in spawning activity and aggregation that occurred during the lunar phase. We estimated the area encompassed by the spawning aggregation to be 0.72 ha, suggesting that spawning site closures and temporal closures centered around the full to the new moon might form one component of a management and conservation plan for this species. Our study of the mating system and spawning aggregation behavior of Bolbometopon from the protected, relatively pristine population at Wake Atoll provides crucial baselines of population density, sex ratio composition, and productivity of a spawning aggregation site from an oceanic atoll. Such information is key for conservation efforts and provides a basic platform for the design of marine protected areas for this threatened iconic coral reef fish, as well as for species with similar ecological and life history characteristics.

Coccolithophore productivity and surface water dynamics in the Alboran Sea during the last 25 kyr

Coccolithophore productivity and surface water dynamics for the last 25kyr in the Alboran Sea (Western Mediterranean) are described in a study of high-resolution sedimentary records from two cores, HER-GC-T1 and CEUTA10PC08, whose locations are currently characterized by different hydrographic conditions. Fossil coccolithophore assemblages and oxygen isotopes and alkenone- and planktonic foraminifera-derived sea surface temperature (SST) records allowed a reconstruction of the properties of the inflowing Atlantic Water (AW), which have proved to be a primary control of the variations in productivity in the neighborhood of the Strait of Gibraltar. Other local factors, such as fluvial discharge, wind-induced and eddy-induced upwelling, are proposed to have influenced marine productivity in more distant areas. The entrance of cold and less saline AW during the stadials associated with Heinrich events 2 and 1 prevented primary productivity, which increased along the Last Glacial Maximum, probably due to a greater fluvial discharge. During Terminations 1a and 1b, the upper water column was affected by stratification, although wind-induced upwelling occurred locally. The Bølling–Allerød was characterized by a gradual increase in productivity and the development of the organic-rich layer. Two phases of the Younger Dryas are recognized: a first phase, which was colder, followed by a second phase, which was warmer and wetter. Differences in productivity between both locations during these two phases can be attributed to fluvial discharge and the changing properties of the AW. Local hydrography, such as the dynamics of the western anticyclonic gyre, gained greater importance in determining productivity and its variations during the Holocene, which was the most productive period.

Phytoplankton size structure in the southern Bay of Bengal modified by the Summer Monsoon Current and associated eddies: Implications on the vertical biogenic flux

The present study combines field and satellite observations to investigate how hydrographical transformations influence phytoplankton size structure in the southern Bay of Bengal during the peak Southwest Monsoon/Summer Monsoon (July–August). The intrusion of the Summer Monsoon Current (SMC) into the Bay of Bengal and associated changes in sea surface chemistry, traceable eastward up to 90°E along 8°N, seems to influence biology of the region significantly. Both in situ and satellite (MODIS) data revealed low surface chlorophyll except in the area influenced by the SMC. During the study period, two well-developed cyclonic eddies (north) and an anti-cyclonic eddy (south), closely linked to the main eastward flow of the SMC, were sampled. Considering the capping effect of the low-saline surface water that is characteristic of the Bay of Bengal, the impact of the cyclonic eddy, estimated in terms of enhanced nutrients and chlorophyll, was mostly restricted to the subsurface waters (below 20m depth). Conversely, the anti-cyclonic eddy aided by the SMC was characterized by considerably higher nutrient concentration and chlorophyll in the upper water column (upper 60m), which was contrary to the general characteristic of such eddies. Albeit smaller phytoplankton predominated the southern Bay of Bengal (60–95% of the total chlorophyll), the contribution of large phytoplankton was double in the regions influenced by the SMC and associated eddies. Multivariate analysis revealed the extent to which SMC-associated eddies spatially influence phytoplankton community structure. The study presents the first direct quantification of the size structure of phytoplankton from the southern Bay of Bengal and demonstrates that the SMC-associated hydrographical ramifications significantly increase the phytoplankton biomass contributed by larger phytoplankton and thereby influence the vertical opal and organic carbon flux in the region.

Copper speciation and distribution in the Atlantic sector of the Southern Ocean

The distribution and speciation of dissolved copper (Cu) was investigated in the Atlantic sector of the Southern Ocean in austral autumn of 2008 as part of the IPY GEOTRACES expedition ZERO & DRAKE. Distribution measurements focused on two transects across the major frontal systems along the Zero Meridian and across the Drake Passage whereas speciation work was investigated in the Drake Passage and the ice covered Weddell Sea. Along the two transects the dissolved Cu concentration exhibited a gradient in the surface with Cu values increasing poleward. Vertical profiles of Cu showed in general monotonic increases with depth, which correlated slightly with silicate but poorly with phosphate. Benthic sources of Cu were observed along the Zero Meridian transect and close to the Antarctic Peninsula for the Drake Passage. Dissolved Cu (DCu) appears to be removed from surface waters through adsorption/complexation by particulate matter. Speciation measurements on DCu indicated a relatively uniform distribution of 25–50nM of L2 (log K~11.8) class ligands. No L1 class ligands (log K>13) were detected in any of the samples examined. Estimates of free Cu (pCu=13.5 to 12.5) showed only small variations in the upper water column and was generally uniform in deep waters. These findings indicate that there is insufficient free copper to account for the rapid reactivity of superoxide (O2−) with Cu in these waters (Heller and Croot, 2010c) suggesting that the organic Cu complexes present in seawater can undergo rapid redox reactions.

Foraging and predation risk for larval cisco (Coregonus artedi) in Lake Superior: A modelling synthesis of empirical survey data

The relative importance of predation and food availability as contributors to larval cisco (Coregonus artedi) mortality in Lake Superior were investigated using a visual foraging model to evaluate potential predation pressure by rainbow smelt (Osmerus mordax) and a bioenergetic model to evaluate potential starvation risk. The models were informed by observations of rainbow smelt, larval cisco, and zooplankton abundance at three Lake Superior locations during the period of spring larval cisco emergence and surface-oriented foraging. Predation risk was highest at Black Bay, ON, where average rainbow smelt densities in the uppermost 10 m of the water column were >1000ha−1. Turbid conditions at the Twin Ports, WI-MN, affected larval cisco predation risk because rainbow smelt remained suspended in the upper water column during daylight, placing them alongside larval cisco during both day and night hours. Predation risk was low at Cornucopia, WI, owing to low smelt densities (<400ha−1) and deep light penetration, which kept rainbow smelt near the lakebed and far from larvae during daylight. In situ zooplankton density estimates were low compared to the values used to develop the larval coregonid bioenergetics model, leading to predictions of negative growth rates for 10mm larvae at all three locations. The model predicted that 15mm larvae were capable of attaining positive growth at Cornucopia and the Twin Ports where low water temperatures (2–6°C) decreased their metabolic costs. Larval prey resources were highest at Black Bay but warmer water temperatures there offset the benefit of increased prey availability. A sensitivity analysis performed on the rainbow smelt visual foraging model showed that it was relatively insensitive, while the coregonid bioenergetics model showed that the absolute growth rate predictions were highly sensitive to input parameters (i.e., 20% parameter perturbation led to order of magnitude differences in model estimates). Our modelling indicated that rainbow smelt predation may limit larval cisco survival at Black Bay and to a lesser extent at Twin Ports, and that starvation may be a major source of mortality at all three locations. The framework we describe has the potential to further our understanding of the relative importance of starvation and predation on larval fish survivorship, provided information on prey resources available to larvae are measured at sufficiently fine spatial scales and the models provide a realistic depiction of the dynamic processes that the larvae experience.

I/Ca evidence for upper ocean deoxygenation during the PETM

AbstractAnthropogenic global warming affects marine ecosystems in complex ways, and declining ocean oxygenation is a growing concern. Forecasting the geographical and bathymetric extent, rate, and intensity of future deoxygenation and its effects on oceanic biota, however, remains highly challenging because of the complex feedbacks in the Earth‐ocean biota system. Information on past global warming events such as the Paleocene‐Eocene Thermal Maximum (PETM, ~55.5 Ma), a potential analog for present and future global warming, may help in such forecasting. Documenting past ocean deoxygenation, however, is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column. As yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones (OMZs). We apply a novel proxy for paleoredox conditions, the iodine to calcium ratio (I/Ca) in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans, and compared our reconstruction with modeled oxygen levels. The reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the Atlantic, Indian Oceans, and possibly the Pacific Ocean, as well during the PETM, due to vertical and potentially lateral expansion of OMZs.

Cadmium enriched stable isotope uptake and addition experiments with natural phytoplankton assemblages in the Costa Rica Upwelling Dome

Cadmium (Cd) can function as either a nutrient or toxin in the marine environment. This duality has been demonstrated in phytoplankton cultures where Cd has been shown to have toxic effects to cyanobacteria, but acts as a nutrient in the marine diatom Thalassiosira weissflogii by biochemically replacing zinc (Zn). In July of 2005, Cd bioavailability and uptake in the Costa Rica Upwelling Dome in the eastern Pacific Ocean were examined using Cd addition and enriched stable isotope uptake experiments. This dome is known to support particularly high densities of the cyanobacterium, Synechococcus. Bottle incubation experiments with Cd additions ranging from 0.5 to 5 nM resulted in reduced chlorophyll a outside and at the edge of the dome relative to control treatments, but showed no reduction in chlorophyll a inside the dome. Total dissolved Cd showed depletion of Cd in the surface waters and increased concentrations with depth. 110Cd stable isotope tracer uptake experiments were conducted at stations inside and outside the dome, in which variations with depth and time were examined. Cd uptake was greatest within the upper 40m of water inside the dome, decreased with depth, and increased with time. Uptake trended positively with chlorophyll a concentrations. Together, these experiments demonstrate Cd uptake into the microbial loop in the upper water column both inside and outside of the dome, but show that Cd toxicity was not induced within the dome. This greater Cd tolerance within the Costa Rica Dome relative to oligotrophic waters was likely due to a combination of higher quantities of biomass, resultant greater ligand production inside the dome, metallothionein production by Synechococcus, and different toxicity thresholds and coping mechanisms of the microbial communities.

Phosphorus Speciation in the Coastal Sediments of Khawr Ash Shaibah Al- Masdudah: Coastal Lagoon in the Eastern Red Sea, Kingdom of Saudi Arabia

The total phosphorus concentrations and its speciation were studied in the surface sediment of Khawr ash Shaibah al Maftuhah. The distribution pattern of total phosphorus in the sediment showed east-west gradient indicating different depositional environment of the lagoon. The largest pool of the sedimentary phosphorus was apatitic phosphorus (57%) followed by refractory organic phosphorus (42%). The impact of loosely adsorbed and exchangeable phosphorus (0.9 % of total phosphorus) and Metal oxide Phosphorus (0.6 % of total phosphorus) on the budget of total phosphorus in the sediments was minor. The concentration of dissolved phosphorus in the upper water column is most probably buffered by apatitic phosphorus. The low concentration of metal oxide phosphorus is suggested to be influenced by the calcareous nature of the lagoon. Possible transformation between apatitic and refractory organic phosphorus is inferred from the correlation of both parameters. The correlation of refractory organic carbon with total phosphorus in the sediments suggests in situ production due to biological activities.

Further insights into how sediment redox status controls the preservation and composition of sedimentary biomarkers

Sedimentary biomarker distributions can record ocean productivity and community structure, but their interpretation must consider alteration during organic matter (OM) export and burial. Large changes in the water column redox state are known to impact on the preservation of biomarkers, but more subtle variation in sediment redox conditions, characteristic of major modern ocean basins, have been less thoroughly investigated. Here we evaluate changes in biomarker distributions during sinking and burial across a nearshore to offshore transect in the southwestern Cape Basin (South East Atlantic), which includes a range of sedimentary environments. Biomarker concentrations and distributions in suspended particulate matter from the upper water column were determined and compared with underlying sedimentary biomarker accumulation rates and distributions. Biomarker distributions were similar in surface and subsurface waters, indicating that the OM signature is exported from the ocean mixed layer with minimal alteration. We show that, while export production (100m) is similar along this transect, 230Thxs-corrected biomarker accumulation rate varies by over an order of magnitude in sediments and is directly associated with sedimentary redox conditions, ranging from oxic to nitrogenous–ferruginous. Biomarker distributions were dominated by sterols in surface water, and by alkenones in underlying sediments, which we propose to be primarily the result of selective preservation. Notably, the difference in sediment O2 penetration depth was associated with relative biomarker preservation. Subtle variation in sedimentary redox conditions has a dramatic impact on the distribution of preserved biomarkers. We discuss mechanisms for preferential degradation of specific biomarkers within this setting.

Phytoplankton community composition in the south-eastern Black Sea determined with pigments measured by HPLC-CHEMTAX analyses and microscopy cell counts

The phytoplankton community structure and abundance in the south-eastern Black Sea was measured from February to December 2009 using and comparing high performance liquid chromatography pigment and microscopy analyses. The phytoplankton community was characterized by diatoms, dinoflagellates and coccolithophores, as revealed by both techniques. Fucoxanthin, diadinoxanthin, peridinin and 19′-hexanoyloxyfucoxanthin were the main accessory pigments showing significant correlation with diatom-Cr2 = 0.56–0.71,P &lt; 0.05), diatom-C (r2 = 0.85–0.91,P &lt; 0.001), dinoflagellate-C (r2 = 0.39–0.88,P &lt; 0.05) and coccolithophore-C (r2 = 0.80–0.71,P &lt; 0.05), respectively. Microscopy counts indicated a total of 89 species, 71% of which were dinoflagellates, 23% were diatoms and 6% other species (mainly coccolithophores). Pigment-CHEMTAX analysis also indicated the presence of pico- and nanoplankton. Phytoplankton carbon (phyto-C) concentrations were highest in the upper water column, whereas chlorophyll-a (Chl-a) showedadeep maximum. Average phyto-C was higher at the coastal station (291 ± 66 µg l−1) than at the offshore station (258 ± 35 µg l−1), not statistically different (P &gt; 0.05). The coastal station also had higher Chl-aconcentrations (0.52–3.83 µg l−1) compared to the offshore station (0.63–2.55 µg l−1), not significant (P &gt; 0.05). Our results are consistent with other studies and indicate that the southern Black Sea is shifting towards mesotrophy with the increasing prevalence of dinoflagellates compared to diatoms.

First autonomous bio-optical profiling float in the Gulf of Mexico reveals dynamic biogeochemistry in deep waters.

Profiling floats equipped with bio-optical sensors well complement ship-based and satellite ocean color measurements by providing highly-resolved time-series data on the vertical structure of biogeochemical processes in oceanic waters. This is the first study to employ an autonomous profiling (APEX) float in the Gulf of Mexico for measuring spatiotemporal variability in bio-optics and hydrography. During the 17-month deployment (July 2011 to December 2012), the float mission collected profiles of temperature, salinity, chlorophyll fluorescence, particulate backscattering (bbp), and colored dissolved organic matter (CDOM) fluorescence from the ocean surface to a depth of 1,500 m. Biogeochemical variability was characterized by distinct depth trends and local “hot spots”, including impacts from mesoscale processes associated with each of the water masses sampled, from ambient deep waters over the Florida Plain, into the Loop Current, up the Florida Canyon, and eventually into the Florida Straits. A deep chlorophyll maximum (DCM) occurred between 30 and 120 m, with the DCM depth significantly related to the unique density layer ρ = 1023.6 (R2 = 0.62). Particulate backscattering, bbp, demonstrated multiple peaks throughout the water column, including from phytoplankton, deep scattering layers, and resuspension. The bio-optical relationship developed between bbp and chlorophyll (R2 = 0.49) was compared to a global relationship and could significantly improve regional ocean-color algorithms. Photooxidation and autochthonous production contributed to CDOM distributions in the upper water column, whereas in deep water, CDOM behaved as a semi-conservative tracer of water masses, demonstrating a tight relationship with density (R2 = 0.87). In the wake of the Deepwater Horizon oil spill, this research lends support to the use of autonomous drifting profilers as a powerful tool for consideration in the design of an expanded and integrated observing network for the Gulf of Mexico.

Temperature-dependent remineralization and carbon cycling in the warm Eocene oceans

Metabolic rates in heterotrophic bacteria are more sensitive to temperature than rates of primary production (e.g., López-Urrutia et al., 2006; Regaudie-de-Gioux and Duarte, 2012). Consequently, faster bacterial respiration rates in a warmer ocean may result in more efficient remineralization of sinking organic matter higher in the water column, with implications for carbon and nutrient cycling during warm climate states. This is supported by a series of reconstructed δ13CDIC: depth profiles based on well-preserved planktonic foraminifera assemblages from Tanzania from the warm Eocene epoch (55.5–33.7Ma) when global surface and deep ocean temperatures exceeded those of the modern day (John et al., 2013). These results indicate relatively sharp δ13CDIC gradients in the upper water column which supports the hypothesis that high metabolic rates in warm Eocene oceans led to more efficient recycling of organic matter and reduced burial rates of organic carbon (Olivarez Lyle and Lyle, 2006). Shallower remineralization depths would also cause an upward displacement and intensification of the oxygen minimum zone which is consistent with evidence for a pelagic ecosystem that was focused in a narrow depth range near the surface during the warm early and middle Eocene. Here we use the Earth System model, cGENIE, that incorporates a new temperature dependence of remineralization rates to illustrate the potential effects of temperature on particulate organic carbon fluxes and hence vertical δ13CDIC gradients. Modeled δ13CDIC vertical profiles off the coast of Tanzania for the Eocene agree well with the reconstructed δ13CDIC profiles, supporting our interpretations based on temperature-dependent remineralization.

Vertical and horizontal larval distribution of an offshore brachyuran crab, &lt;em&gt;Monodaeus couchii&lt;/em&gt;, off the south coast of Portugal

A strong understanding of larval distribution and abundance is of major value in delineating the location and size of a breeding population of deep-sea species such as Monodaeus couchii . In this study, vertical distribution of the larvae of a brachyuran crab, M. couchii , was assessed during two week-long cruises conducted at the end of January 2006 and 2007 off the South Coast of Portugal. Larvae were collected by oblique plankton hauls with a Longhurst-Hardy Plankton Recorder from the surface to 300 m. Abundance and distribution of zoeae I and II were correlated during both years. For all stages, abundance decreased with depth during the day while it increased with depth at night; the larvae thus displayed reverse diel vertical migration. Abundance of zoeae I and II was correlated with chlorophyll a levels, whereas those of later stages were correlated with neither physical parameters (chlorophyll a , temperature or salinity) nor each other. An ontogenic shift in vertical distribution explained the results; earlier zoeal stages remain in the food-rich upper water column while later stages were not correlated with any physical parameters (i.e. chlorophyll a , salinity or temperature) and migrated to the bottom for settlement.

Polyphosphate goes from pedestrian to prominent in the marine P-cycle.

Phosphorus plays a key role in all life (1), from providing the energy that runs the cell machinery, linking nucleotides in RNA and DNA that form the “blueprint” of life, to being an integral part of the cell membrane lipid bilayer, making life possible by compartmentalizing biochemical processes. Polyphosphates (poly-P) are linear or cyclical polymers of a few to many hundreds of phosphate molecules linked with the same type of high-energy bond found in ATP, the key molecule in energy transactions within living cells, and poly-P may in fact act as a substitute for ATP in some enzymatic reactions (2). Poly-P was most likely present in prebiological times of the Earth’s history as these polymers spontaneously form during volcanic, or hydrothermal vent, activity (3). Additionally, it has been speculated that poly-P may have played a role in the origins of life on our planet (4). Although, poly-P appears to be ubiquitous throughout the natural world (5) and has long been recognized as a storage molecule for phosphate, relatively little has been known about its role and function in cells until quite recently (6). As a result, poly-P has sometimes been called a molecular “fossil” (7) due to its obscure and presumably pedestrian role in cell metabolism and hence has not garnered much scientific interest in the past. Nevertheless, over the last decades, it has become increasingly clear that poly-P may in fact be essential for a multitude of cellular functions, including, but not limited to, gene expression regulation, energy storage, motility, and ultimately the survival of the cell (4). However, the presence, distribution, and significance of poly-P in the marine environment and to its microbial inhabitants have been largely lacking, although recent metagenomic studies have suggested their potential importance to microorganisms in the oligotrophic oceans (8). Here, Martin et al. (9) present, to date, the most comprehensive study of poly-P content and distribution in particulate material in the marine environment. Their study transects the temperate western North Atlantic (TWNA) Ocean, which is relatively rich in nutrients, across the Gulf Stream and into the severely phosphorus-limited subtropical Sargasso Sea. Along this transect, which provides a natural gradient in nutrient regimes, they measure the poly-P content, among other phosphorus-relevant parameters, of the microbial communities in the upper water column, as well as the amount of poly-P in particles sinking out of the upper ocean into its deep interior.

Flocculent layers and bacterial mats in the mudstone interbeds of the Primary Lower Gypsum unit (Tertiary Piedmont basin, NW Italy): Archives of palaeoenvironmental changes during the Messinian salinity crisis

Abstract The recognition of peculiar laminated layers atypically rich in a biogenic intrabasinal component in the mudstone intervals from the Messinian (late Miocene) Primary Lower Gypsum unit (5.97–5.60 Ma) of the Piedmont basin (NW Italy) provides information on the palaeoenvironmental evolution at precessional insolation maxima. These cyclic layers consist of irregular alternation of cm-thick grey terrigenous laminae and whitish composite packets; the latter is in turn composed of sub mm-thick wrinkled dolomite-rich laminae that alternate with terrigenous ones. Two types of layers can be distinguished: i) peloidal layers, composed of faecal pellets and irregular diatom-rich aggregates, interpreted as marine snow floccules; and ii) filament bearing layers, composed of interwoven filaments up to 150 μm across, corresponding to remains of Beggiatoa -like giant sulphide-oxidising bacteria. By comparison with present-day settings, the peloidal layers are interpreted as flocculent layers, deposited on anoxic sea bottoms, following episodes of phytoplankton bloom in the upper water column. The filament-bearing layers are considered as chemotrophic microbial mats, growing on dysaerobic sea bottoms. The uncommon preservation of both kinds of layers is the result of extensive dolomite precipitation in the shallow subsurface, induced by bacterial sulphate reduction. The laminated layers are the first reported example of a high frequency climate-driven cyclicity in the non-evaporitic portion of the Primary Lower Gypsum unit. Their stacking pattern within the mudstone intervals reflects the gradual increase of the oxygen content of bottom waters during the humid precessional hemicycle, culminating with gypsum deposition at the onset of the arid phase.

Phytoplankton blooms beneath the sea ice in the Chukchi sea

In the Arctic Ocean, phytoplankton blooms on continental shelves are often limited by light availability, and are therefore thought to be restricted to waters free of sea ice. During July 2011 in the Chukchi Sea, a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to >100km into the pack. The bloom was composed primarily of diatoms, with biomass reaching 1291mg chlorophyll am−2 and rates of carbon fixation as high as 3.7gCm−2d−1. Although the sea ice where the bloom was observed was near 100% concentration and 0.8–1.2m thick, 30–40% of its surface was covered by melt ponds that transmitted 4-fold more light than adjacent areas of bare ice, providing sufficient light for phytoplankton to bloom. Phytoplankton growth rates associated with the under-ice bloom averaged 0.9d−1 and were as high as 1.6d−1. We argue that a thinning sea ice cover with more numerous melt ponds over the past decade has enhanced light penetration through the sea ice into the upper water column, favoring the development of these blooms. These observations, coupled with additional biogeochemical evidence, suggest that phytoplankton blooms are currently widespread on nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in waters where under-ice blooms develop are ~10-fold too low. These massive phytoplankton blooms represent a marked shift in our understanding of Arctic marine ecosystems.

Laboratory measurements of high-frequency, broadband acoustic scattering of growing sea ice and oil beneath sea ice

The morphology of sea ice during the early stages of growth is strongly dependent on environmental conditions. Under calm conditions, congelation ice forms through downward growth of ice crystals from the water surface. Under turbulent conditions (surface waves), rapid freezing of ice crystals occurs in the upper water column (frazil ice), eventually consolidating into pancake ice through repeated collisions and agglomeration of the loose frazil crystals. It is expected that high-frequency scattering from the basal layer of the ice varies for different sea ice types and can reveal structural information that governs the behavior of the ice and its interactions with the environment. Broadband scattering measurements of sea ice are presented beginning with ice-free conditions and through initial stages of growth in laboratory experiments for both congelation and frazil ice. With increased interest in drilling for hydrocarbon resources in the Arctic and the associated environmental concerns of an oil spill in ice-covered waters, improved methods for detection of crude oil both under or frozen within sea ice are needed. Acoustic scattering data are presented demonstrating how the scattering changes when crude oil is spilled beneath the ice.

Chlorophyll-a patterns and mixing processes in the Yucatan Basin, Caribbean Sea

Daily images collected with the MODIS sensor on the NASA Aqua satellite were used to describe chlorophyll-a (chl-a) concentrations before, during, and after synoptic-scale meteorological pulses in the Yucatan Basin, Caribbean Sea. The relative influence of vertical diffusive and vertical advective transport on mixing of the near surface layer was quantified with wind data from the QuikSCAT satellite. Computation of vertical density eddy diffusivity and gradient Richardson numbers was done with data from the three-dimensional MERCATOR model. The model evidenced the importance of vertical shear (i.e., vertical diffusive processes) in generating mixing during autumn and winter (2007–2009). During moderate meteorological pulses (e.g., cold fronts, easterly tropical waves, and low-pressure systems with wind speeds of 9–15 m s–1 sustained over 2 days), mixing caused by diffusive transport (eddy viscosity of 10 × 10–3 m2 s–1) was at least one order of magnitude higher than upward advective mixing (0.3 × 10–3 m2 s–1). During the passage of hurricanes Ivan (September 2004) and Wilma (October 2005), upward advective mixing (5–7 m2 s–1) dominated mixing of the upper water column and was nearly four orders of magnitude higher than during moderate meteorological events. Background chl-a concentrations of 0.03–0.08 mg m–3 were observed during the July–October period. During synoptic weather pulses, three different patterns in the chl-a concentrations were observed: first, chl-a concentrations in the range of 0.05 to 0.12 mg m–3 followed moderate meteorological pulses; second, higher regional chl-a concentrations (0.5–2.0 mg m–3) followed the passage of hurricanes; and third, the formation of filaments showing apparent high chl-a (0.3–1.5 mg m–3) south of Cuba, possibly caused by freshwater discharge from land. The changes in chlorophyll patterns following meteorological events illustrate typical patterns of connectivity among different parts of the Yucatan Basin.

Turbulence structure in the upper ocean: a comparative study of observations and modeling

Observations of turbulent dissipation rates measured by two independent instruments are compared with numerical model runs to investigate the injection of turbulence generated by sea surface gravity waves. The near-surface observations are made by a moored autonomous instrument, fixed at approximately 8 m below the sea surface. The instrument is equipped with shear probes, a high-resolution pressure sensor, and an inertial motion package to measure time series of dissipation rate and nondirectional surface wave energy spectrum. A free-falling profiler is used additionally to collect vertical microstructure profiles in the upper ocean. For the model simulations, we use a one-dimensional mixed layer model based on a k–ε type second moment turbulence closure, which is modified to include the effects of wave breaking and Langmuir cells. The dissipation rates obtained using the modified k–ε model are elevated near the sea surface and in the upper water column, consistent with the measurements, mainly as a result of wave breaking at the surface, and energy drawn from wave field to the mean flow by Stokes drift. The agreement between observed and simulated turbulent quantities is fairly good, especially when the Stokes production is taken into account.

Assessment of the potential for copper limitation of ammonia oxidation by Archaea in a dynamic estuary

The distribution and speciation of copper (Cu) in Hood Canal, a fjord in western Puget Sound, Washington, were studied over a 2-year period. Cu is required as a cofactor for many enzymatic pathways, including ammonia oxidation. In Hood Canal, ammonia oxidation is largely dominated by ammonia-oxidizing archaea (AOA), which have high Cu requirements for other processes as well. Dissolved Cu was slightly depleted in the upper water column, and concentrations were almost unchanged from measurements made in the late 1970s, ranging from 4.08 to 6.12nM. Although this implies that the biological demand is small relative to the large and relatively constant inventory of dissolved Cu, and that Cu limitation is therefore unlikely to influence rates of biological processes, speciation measurements indicated that dissolved Cu is strongly complexed by organic ligands. As a result, bioavailable Cu2+ concentrations were considerably lower, varying from 6.14×10–15M to 1.36×10–12M. This is a range that encompasses the threshold (<~2×10–13M) for Cu limitation of ammonia oxidation by Nitrosopumilus maritimus SCM1, a representative AOA, in culture (Amin et al., 2013). Furthermore, Cu2+ displayed a clear trend over most sampling periods, with Cu2+ concentrations one to two orders of magnitude higher below 20m in the deeper, saline waters, and exhibiting minima in the upper 15m. The major freshwater input to Hood Canal is not an important source of ligands, which suggests that the ligands are likely produced biologically in the water column and have slow turnover times. In general, ammonia oxidation rates varied considerably but were lowest in the upper water column where Cu2+ concentrations were also lowest. Thus, these findings will facilitate further work to ascertain the relative importance of Cu bioavailability in limiting ammonia oxidation rates versus light inhibition, which has frequently been invoked to account for low rates of nitrification in the upper water column.