Seismic quality factors across a bottom simulating reflector in the Makran Accretionary Prism, Arabian Sea

The hydrate-bearing sediments above the bottom simulating reflector (BSR) are associated with low attenuation or high quality factor (Q), whereas underlying gas-bearing sediments exhibit high attenuation. Hence, estimation of Q can be important for qualifying whether a BSR is related to gas hydrates and free-gas. This property is also useful for identifying gas hydrates where detection of BSR is dubious. Here, we calculate the interval Q for three submarine sedimentary layers bounded by seafloor, BSR, one reflector above and another reflector below the BSR at three locations with moderate, strong and no BSR along a seismic line in the Makran accretionary prism, Arabian Sea for studying attenuation (Q⁻¹) characteristics of sediments. Interval Q for hydrate-bearing sediments (layer B) above the BSR are estimated as 191 ± 11, 223 ± 12, and 117 ± 5, whereas interval Q for the underlying gas-bearing sediments (layer C) are calculated as 112 ± 7, 107 ± 8 and 124 ± 11 at moderate, strong and no BSR locations, respectively. The large variation in Q is observed at strong BSR. Thus Q can be used for ascertaining whether the observed BSR is due to gas hydrates, and for identifying gas hydrates at places where detection of BSR is rather doubtful. Interval Q of 98 ± 4, 108 ± 5, and 102 ± 5, respectively, at moderate, strong and no BSR locations for the layer immediately beneath the seafloor (layer A) show almost uniform attenuation.     

Mercury speciation in surface and deep waters of the Mediterranean Sea

Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during two oceanographic cruises on board the Italian research vessel URANIA in summer 2003 and spring 2004 as part of the Med Oceaneor and MERCYMS projects. The study included deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total Hg (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg) in open ocean waters. Average concentrations of measured Hg species were characterized by seasonal and spatial variations. Overall average THg concentrations ranged between 0.41 and 2.65 pM (1.32 ± 0.48 pM) and were comparable to those obtained in previous studies of the Mediterranean Sea. A significant fraction of Hg was present as “reactive” Hg (average 0.33 ± 0.32 pM). Dissolved gaseous Hg (DGM), which consists mainly of Hg⁰, represents a considerable proportion of THg (average 20%, 0.23 ± 0.11 pM). The portion of DGM typically increased towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its geotectonic origin. No dimethyl Hg was found in surface waters down to the depth of 40 m. Below this depth, its average concentration was 2.67 ± 2.9 fM. Dissolved fractions of total Hg and MeHg were measured in filtered water samples and were 0.68 ± 0.43 pM and 0.29 ± 0.17 pM for THg and MeHg respectively. The fraction of Hg as MeHg was in average 43%, which is relatively high compared to other ocean environments. The concentrations reported in this study are among the lowest found in marine environments and the quality of analytical methods are of key importance. Speciation of Hg in sea water is of crucial importance as THg concentrations alone do not give adequate data for understanding Hg sources and cycling in marine environments. For example, photoinduced transformations are important for the presence of reactive and elemental mercury in the surface layers, biologically mediated reactions are important for the production/degradation of MeHg and DGM in the photic zones of the water column, and the data for DGM in deep sea indicate the natural sources of Hg in geotectonicaly active areas of the Mediterranean Sea.     

Turbidites and their association with past earthquakes in the deep Çınarcık Basin of the Marmara Sea

Two gravity cores (CAG-3 and C-15) from the tectonically active, 1,276-m deep Çınarcık Basin of the Marmara Sea each include three sandy turbiditic mud units (1 mm–2 cm thick) with sharp basal contacts. The high benthic foraminifer content of these units suggests that the sediments were transported by turbidity currents from the upper slope region. These units represent the thin edges of turbidites thickening towards the subsiding north-eastern part of the basin, and contain quartz, detrital calcite, intact shells and shell fragments, smectite, pyrite framboids, muscovite, biotite, epidote and garnet. Their clay fractions are more enriched in smectite than those of adjacent layers. AMS 14C ages (957±43 a.d. and 578±31 a.d.) of two upper and middle turbiditic units in core C15 overlap with the historical İstanbul-Thrace (intensity=10) and İstanbul-Kocaeli (intensity=9) earthquakes of 26 October 986 and 15 August 553, respectively. This overlap, together with sedimentological characteristics, strongly suggests that the turbiditic units are related to the tectono-seismic activity of the North Anatolian Fault. The age of the lowest turbiditic unit in core C-3 was found to be 6,573±87 a b.p. (calendar) by AMS 14 C. In terms of chronostratigraphic relationships and lithological composition, the turbiditic units in core CAG-3 cannot be correlated with those in C15. This can be explained by gravity-controlled sedimentation causing wedging out of turbidites towards the edge of the basin.     

Variability in the Relative Penetration of Ultraviolet Radiation to Photosynthetically Available Radiation in Temperate Coastal Waters, Japan

UVR and PAR wavelengths are attenuated to different extents within the water column, causing variations in spectral composition with depth. The present investigation (a) describes the variability of UVR and PAR penetration at a station in the temperate coastal waters of Sagami Bay and determines (b) the characteristics of relative UVR penetration to the euphotic zone. Examination of the seasonal irradiance profile measurements indicated eight measurements displaying two distinct attenuation coefficients (K _d) for specific UVR wavelengths and PAR. The two attenuation coefficients observed from specific wavelengths in the water column may be caused not only by chlorophyll pigments, but also by dissolved organic material in the upper layer. The 1% depth of surface UVR at 305, 320, 340, and 380 nm averaged 10.8 ± 5.7, 14.9 ± 9.5, 19.8 ± 12.1, and 30.4 ± 17.6 m, respectively. The depth of euphotic layer displayed less variability averaging 62 ± 15 m throughout the entire study. Relative UVR penetration within the euphotic zone averaged 17.8 ± 8.1, 22.9 ± 10, 30.5 ± 13.8, and 46 ± 46.9% for 305, 320, 340, and 380 nm, respectively. A large variation of the relative transmission of UVR within the euphotic zone was found although the spectral composition was relatively stable in the air throughout the study.     

Eddy-resolving ocean circulation in the Asian-Australian region inferred from an ocean reanalysis effort

The first global ocean reanalysis with focus on the Asian-Australian region was performed for the period October 1992 to June 2006. The 14-year experiment assimilated available observations of altimetric sea-level anomaly, satellite SST and quality-controlled in situ temperature and salinity profiles from a range of sources, including field surveys and the Argo float array. This study focuses on dominant circulation patterns in the South-East Asian/Australian region as simulated by an eddy-resolving and data-assimilating ocean general circulation model. New estimates of the ocean circulation are provided which are largely in agreement with the limited number of observations. Transports of key currents in the region are as follows: The total (top-to-bottom) annual mean Indonesian Throughflow transport and its standard deviation are 9.7 ± 4.4 Sv from the Pacific to the Indian Ocean with a minimum in January (6.6 Sv) and a maximum in April (12.3 Sv). The Leeuwin Current along the west coast of Australia is dominated by eddy structures with a mean southward transport of 4.1 ± 2.0 Sv at 34°S. Along the southern coast of Australia a narrow shelf edge current known as the South Australian Current advects 4.5 ± 2.6 Sv eastward at 130°E. The South Australian Current converges east of Tasmania with the eddy-rich extension of East Australian Current. At 32°S this current transports 36.8 ± 18.5 Sv southward. A dominating feature of the circulation between north-eastern Australia and Papua-New Guinea is the strong and quasi-permanent Coral Sea Gyre. This gyre is associated with the highly variable Hiri Current which runs along the south coast of Papua-New Guinea and advects 8.2 ± 19.1 Sv into the Western Pacific Ocean. All of these transport estimates are subject to strong eddy variability.     

Molecular weight and chemical reactivity of dissolved trace metals (Cd,Cu, Ni) in surface waters from the Mississippi River to Gulf of Mexico

It is generally assumed that estuarine mixing is continuous for metals from terrestrial sources, gradually decreasing towards the open ocean endmember. Here we show that, chemical reactivity, determined by ion exchange method, and molecular weight distributions, obtained using cross-flow ultrafiltration, of dissolved Cd, Cu, and Ni in the surface waters of the Gulf of Mexico varied systematically across the estuarine mixing zone of the Mississippi River. Most size or chemical affinity fractions of dissolved metals (<0.4 μm) were linearly related to salinity (10.8–36.6), suggesting that the distribution of these elements was mainly controlled by continuous mixing processes. Dissolved concentrations across the salinity gradient ranged for Cd: 87–187 pM; Cu: 1.4–18.3 nM; and Ni: 2.6–18.8 nM, with highest values near the Mississippi river mouth, and lowest concentrations in a warm core ring in the Gulf of Mexico. Dissolved Cd was mostly present as a truly dissolved (<10 kDa, 97 ± 1%) and cationic fraction (Chelex-100 extractable, 94 ± 4%). A novel observation across the estuarine mixing zone was that colloidal metal concentrations were identical to either inert (for Cu, Ni) or AMPG-labile anionic (Cu, Cd) fractions. The difference in behavior between Cu and the other two metals might indicate differences in the biopolymeric nature of the metal–organic chelates. In particular, the anionic-organic Cd fractions accounted for just 3 ± 1%, on average. However, for Cu, it was 24 ± 4%, and for Ni, it was 9 ± 6%. The fractions of the total dissolved metal fractions that were “inert” averaged 31 ± 10% for Cu and 29 ± 12% for Ni. Small but noticeable amounts (6 ± 3%) of dissolved inert Cd fractions were also present. Apparent non-local transport processes, likely associated with cross-shelf sediment resuspension processes, could have been responsible for the relatively high concentrations of ‘inert’ and ‘anionic’ metal fractions in high salinity coastal waters, and accounting for the persistence of metals bound to humic substances in the Gulf of Mexico.     

The role of the deep mixing in the Storfjorden shelf water plume

Hydrographic observations in deep Fram Strait evidence a plume of Storfjorden Brine-enriched Shelf Water in 1986, 1988 and 2002. The plume spreads along the continental slope over 600 km away from its formation area and reaches 2000 m depth. The plume is 30 to 80 m thick in the deep layer of Fram Strait; it is almost 0.4 °C warmer and 0.06 more saline than the ambient water. The velocity of the plume, observed by a moored current meter in Fram Strait, is 12.60±4.70 cm s⁻¹ .The hydrographic properties of the plume are used to study entrainment. A streamtube model with four entrainment parameterizations is applied. Two Froude-number dependent parameterizations lead to mixing mostly happening over the shelf break, where the Froude number is large. This is in agreement with the traditional view, but is inconsistent with the observed temperature and salinity of the Storfjorden plume. Therefore further entrainment assumptions (a constant and a volume-dependent entrainment) are tested. The volume-dependent entrainment scheme yields the best representation of entrainment in the Storfjorden plume. Our results emphasize the necessity of strong mixing in the deep layers in Fram Strait to achieve an agreement with observed properties of the plume.     

Possible mixotrophy of pigmented nanoflagellates: Microbial plankton biomass, primary production and phytoplankton growth in the NW Iberian upwelling in spring

Microbial plankton biomass, primary production (PP) and phytoplankton growth rates (μ) were estimated along the NW Iberian margin during an upwelling relaxation event. Although the interaction between wind forcing and coastline singularities caused high spatial variability in PP (0.4-8.4 g C m⁻² d⁻¹), two domains (coastal and oceanic) could be distinguished regarding microbial plankton biomass and μ. At the coastal domain, with higher influence of upwelling, diatoms showed an important contribution (27 ± 17%) to total autotrophic biomass (AB). Nonetheless, AB was dominated by autotrophic nanoflagellates (ANF) at both realms, accounting for 62 ± 16% and 89 ± 6% of the integrated AB at the coastal and oceanic domain respectively. AB and heterotrophic biomass (HB) were significantly higher at the oceanic than at the coastal domain, with both biomasses covarying according to HB:AB = 0.33. Whereas the low phytoplankton carbon to chlorophyll a ratio (C_ph:chl a = 38 ± 3) and the high μ = 0.54 ± 0.09 d⁻¹ registered at the coastal stations suggest that phytoplankton was not nutrient limited at this domain, the values (C_ph:chl a = 157 ± 8; μ = 0.17 ± 0.02 d⁻¹) recorded at the oceanic domain point to severe nutrient limitation. However, the high Fv/Fm fluorescence ratios (0.56 ± 0.09) measured at the sea surface in the oceanic domain suggest that nutrient limitation did not occur. To reconcile these two apparently opposite views, it is suggested the occurrence of mixotrophic nutrition of ANF, with heterotrophic nutrition supplying about 75% of carbon requirements.     

Carbon sources supporting a diverse fish community in a tropical coastal ecosystem (Gazi Bay,Kenya)

Interlinked mangrove–seagrass ecosystems are characteristic features of many tropical coastal areas, where they act as feeding and nursery grounds for a variety of fishes and invertebrates. The autotrophic carbon sources supporting fisheries in Gazi bay (Kenya) were studied in three sites, two located in the tidal creeks flowing through extensive mangrove forests, another site located in the subtidal seagrass meadows, approximately 2.5 km away from the forest. Carbon and nitrogen stable isotope composition of 42 fish species, 2 crustacean species and a range of potential primary food sources (e.g., mangroves, seagrasses and epiphytes, macroalgae) were analysed. There was considerable overlap in the δ¹³C signatures between fish (−16.1 ± 2.1‰), seagrasses (−15.1 ± 3.0‰), seagrass epiphytes (−13.6 ± 3.3‰), and macroalgae (−20.4 ± 3.1‰). Nevertheless, the signatures for most primary producers were sufficiently distinct to indicate that the dominant carbon sources for fish were mainly derived from the seagrass and their associated epiphytic community, and possibly macroalgae. Mangrove-derived organic matter contributes only marginally to the overall fish food web. Carbon supporting these fish communities was derived directly through grazing by herbivorous and some omnivorous fishes, or indirectly through the benthic food web. Fishes from the mangrove creeks had distinctly lower δ¹³C signatures (−16.8 ± 2.0‰) compared to those collected in the adjacent seagrass beds (−14.7 ± 1.7‰). This indicated that these habitats were used as distinct sheltering and feeding zones for the fishes collected, with minimal degree of exchange within the fish communities despite their regular movement pattern.     

Heat flow and quantity of methane deduced from a gas hydrate field in the vicinity of the Dnieper Canyon,northwestern Black Sea

Seismic reflection data document for the first time the existence of a BSR in a limited area west of the Dnieper Canyon in the northwestern Black Sea. Seismic wide-angle data suggest that gas hydrates occupy in average 15±2% of the pore space in a zone of 100 m in thickness. A conservative quantification of the amount of methane associated with this gas hydrate occurrence is about 12±3×10¹¹ m³ (0.6±0.2 Gt of methane carbon). Conductive heat flow deduced from the BSR depth is in the range of 21±6 to 55±15 mW m^–2.     

The influence of shrimp farms organic waste management on chemical water quality

We evaluated the physical–chemical properties and nutrient concentrations in two shrimp ponds under conventional management, intensive (I-P) and semi-intensive (SI-P), and one under organic management (O-P), three days after loading and before stoking. The mean’s total alkalinity concentrations were 2.26 ± 0.04 mM, 2.28 ± 0.01 mM and 2.59 ± 0.01 mM, respectively for I-P, SI-P and O-P. Bicarbonate and carbonate accounted for 62% and 37% in I-P, 64% and 36% in SI-P and 83–17% in O-P, respectively. Aragonite and calcite were oversaturated around Ω = 5. Mean total phosphate (TP) concentrations were 441.37 ± 92.06 μg/L, 449 ± 48 μg/L and 473.64 ± 84.17 μg/L, under I-P SI-P and O-P management respectively. Following this sequence, NO₃⁻ concentration was 2.98 ± 0.7 μg/L, 1.16 ± 0.16 μg/L and 0.32 ± 0.12 μg/L, under I-P, SI-P and O-P management respectively. Thus, the data suggest that different management of farm organic waste leads to different chemical water quality.     

The interannual variability of megafaunal assemblages in the Arctic deep sea: Preliminary results from the HAUSGARTEN observatory (79°N)

Although megafaunal organisms play an important role in deep benthic ecosystems and contribute significantly to benthic biomass in the Arctic little is known about their temporal dynamics. Here, we assessed the interannual dynamics of megafaunal organisms from the HAUSGARTEN observatory in the Fram Strait, an area where the effects of climatic forcing are particularly evident. We analysed three congruent camera transects taken in 2002, 2004 and 2007. Environmental parameters were measured in order to be able to put our faunal results into an environmental context.Our results indicate that although the densities of megafaunal species show different patterns over time, most exhibit an overall decrease between 2002 and 2007 and total megafaunal densities decreased regularly from 2002 to 2004 to 2007 (12.16±0.96 to 7.41±0.43 ind m⁻²). This concurs with a steady increase in bottom-water temperatures and a decrease in the total organic content and microbial biomass of surficial sediments at the same time period. Although suspension feeder densities also decreased, predator/scavenger and deposit feeder densities have declined to such an extent that suspension feeders accounted for almost 100% of the megafauna in 2007. It could thus be argued that the trophic diversity at the central HAUSGARTEN station (2500 m) has decreased. Temperature-related changes in the production of the surface layers may lead to changes in the quality and/or quantity of particles exported to the deep seafloor. The densities of deposit feeders (i.e. holothurians) peaked (1.14±0.13 ind m⁻²) in 2004, the year following the longest ice cover. These results indicate the importance of ice-related export of particles to the deep seafloor and highlight the need for time-series transects, especially in an era when productive marginal ice zones tend to disappear with the receding sea ice. Although there is a general consensus that the Arctic is in a transition towards a warmer state, only continued observation will allow us to assess if the interannual changes observed are a result of decadal cycles related to the Arctic and North Atlantic Oscillation or if they are indicators of long-term change.     

Secondary production in a Laminaria hyperborea kelp forest and variation according to wave exposure

The secondary production of mobile invertebrate fauna in the Laminaria hyperborea (Gunn.) Foslie kelp forest increases with wave exposure level. This faunal group has a key function in transferring kelp carbon to higher levels in the food web. By using a size-frequency method the calculated production was 68 (±18) g D.W. m⁻² yr⁻¹ (±S.E.) at low, 250 (±57) at medium and 308 (±64) at high exposure levels. The calculations included 30 macrofauna species, which accounted for 96% of the specimens registered, with Gastropods, amphipods and bivalves being the most abundant taxa. The calculated secondary production is high, but comparable to that previously reported from other macrophyte systems and was 3%, 8% and 8% of the total primary production at low, medium and high exposure levels, respectively. Our results indicate that large quantities of Laminaria kelp are exported from the system, although the production of sessile animals was not taken into account. The most important factor in determining faunal densities and secondary production was probably habitat size but at low exposure levels the percentage of egg-carrying crustacean females and juveniles were lower than at medium and high exposure levels, thereby indicating lower fitness for animals at low exposure stations.     

Young age bias of radiocarbon dates in pre-holocene marine deposits of Hong Kong and implications for Pleistocene stratigraphy

Radiocarbon dates of pre-Holocene marine deposits in Hong Kong ranging from 21.580±1.210 to 45.700±2.000 years BP are found to be younger than uranium-series dates of mollusks and other indirect age evidence. Two mollusk samples yielded last interglacial ages of 130.500±5.300 and 142.000±20.000 years BP. respectively. Palynological and oxygen-isotope evidence shows that the marine deposits containing the mollusks were formed under marginally warmer temperature conditions than in the present day, which is consistent with a last interglacial age. Since old radiocarbon dates are likely to be minimum age estimates, similar studies carried out elsewhere would be of value to Pleistocene stratigraphy.     

Knowledge gaps in tropical Southeast Asian seagrass systems

Seagrasses are habitats with significant ecological and economic functions but we have limited knowledge of seagrasses in Southeast Asia, the hypothesized centre-of-origin for tropical seagrasses. There have been only 62 ISI-cited publications on the seagrasses of Southeast Asia in the last three decades and most work has been in few sites such as Northwest Luzon in the Philippines and South Sulawesi in Indonesia. Our understanding of the processes driving spatial and temporal distributions of seagrass species here has focussed primarily on backreef and estuarine seagrass meadows, with little work on forereef systems. We used Pulau Tinggi, an island off the southeast coast of Peninsular Malaysia, as an example of a subtidal forereef system. It is characterized by a community of small and fast growing species such as Halophila ovalis (mean shoot density 1454.6 ± 145.1 m⁻²) and Halodule uninervis (mean shoot density 861.7 ± 372.0 m⁻²) growing in relatively low light conditions (mean PAR 162.1 ± 35.0 μmol m⁻² s⁻¹ at 10 m depth to 405.8 ± 99.0 μmol m⁻² s⁻¹ at 3 m water depth) on sediment with low carbonate (mean 9.24 ± 1.74 percentage dry weight), organic matter (mean 2.56 ± 0.35 percentage dry weight) and silt-clay content (mean 2.28 ± 2.43 percentage dry weight). The literature reveals that there is a range of drivers operating in Southeast Asian seagrass systems and we suggest that this is because there are various types of seagrass habitats in this region, i.e. backreef, forereef and estuary, each of which has site characteristics and ecological drivers unique to it. Based on our case study of Pulau Tinggi, we suggest that seagrasses in forereef systems are more widespread in Southeast Asia than is reflected in the literature and that they are likely to be driven by recurring disturbance events such as monsoons, sediment burial and herbivory.     

Cover stones on liquefiable soil bed under waves

The paper describes the results of an experimental study on the behavior of cover stones on a liquefiable soil bed exposed to a progressive wave. The soil was silt with d₅₀ = 0.098 mm. Stones, the size of 4 cm, were used as cover material. The effect of packing density of stones, and that of number of stone layers (including the effect of an intermediate filter layer) were investigated. Pore pressure was measured across the soil depth. The experiments show that the soil liquefaction depended mainly on two parameters: the packing density of stones, and the number of stone layers. When the liquefaction occurs, stones sink in the soil. Mechanisms of liquefaction and sinking are described, and practical guidelines are recommended.     

Occurrence and Activity of Lipolytic Bacterioneuston and Bacterioplankton in the Estuarine Lake Gardno

The number and lipolytic activity of neustonic and planktonic bacteria inhabiting estuarine Lake Gardno were determined. Lipolytic bacteria were very numerous in investigated layers of water, accounting for 10–88% of the total number of culturable heterotrophic bacteria. Significant differences were found in the decomposition of individual lipid substrates by bacteria. The highest percentage of neustonic and planktonic strains were able to hydrolyse tributyrin and Tween 85. The least numerous bacteria group was microflora hydrolysing Tween 20 and Tween 40. The activity of lipases synthesized by bacteria from the subsurface layer was higher than that of lipases produced by bacteria isolated from the surface layers. A significant effect of salinity on the activity of lipases has been shown.     

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using ⁷Be and ²³⁴Th_XS, the sediment-mixing coefficient (D_b) was 4.3 ± 1.8 cm² y⁻¹, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates (S_a), estimated using ¹³⁷Cs and ²¹⁰Pb_XS, were 0.16 ± 0.02 g cm⁻² y⁻¹. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m⁻² d⁻¹, of which ∼10% or 2.9 ± 0.44 mmol C m⁻² d⁻¹was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O₂ were 26 ± 3.8 mmol m⁻² d⁻¹ and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m⁻² d⁻¹, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m⁻² d⁻¹. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m⁻² d⁻¹. The flux of DOC out of the sediments (DOC_efflux) was 5.6 ± 1.3 mmol C m⁻² d⁻¹. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O₂ reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.     

Distribution and contribution of major phytoplankton groups to carbon cycling across contrasting conditions of the subtropical northeast Atlantic Ocean

The relation between trophic regime and phytoplankton composition and function in oceanic systems is well accepted in oceanography. However, the relative dynamics and carbon cycling contributions of different phytoplankton groups across gradients of ocean richness are not fully understood. In this work we investigated phytoplankton dynamics along two transects from the NW African coastal upwelling to open-ocean waters of the north Atlantic subtropical gyre. We adopted a pigment-based approach to characterize community structure and to quantify group-specific growth and grazing rates and associated carbon fluxes. Changes in pigment cell concentration during the incubation experiments due to photoadaptation were corrected to obtain reliable rates. The oceanic region was dominated by Prochlorococcus (PRO) (45±7% of total chlorophyll a) while diatoms dominated in upwelling waters (40±37%). Phytoplankton grew faster (μ=0.78±0.26 d⁻¹) and free of nutrient limitation (μ/μ_n=0.98±0.42) in the coastal upwelling region, with all groups growing at similar rates. In oceanic waters, the growth rate of bulk phytoplankton was lower (μ=0.52±0.16 d⁻¹) and nutrient limited (μ/μ_n=0.68±0.19 d⁻¹). Diatoms (0.80±0.39 d⁻¹) and Synechococcus (SYN) (0.72±0.25 d⁻¹) grew faster than Prymnesiophyceae (PRYMN) (0.62±0.26 d⁻¹) and PRO (0.46±0.18 d⁻¹). The growth rates of PRO and SYN were moderately nutrient limited (μ/μ_n=0.81 and 0.91, respectively), while the limitation for diatoms (μ/μ_n=0.71) and PRYMN (μ/μ_n=0.37) was more severe. Microzooplankton grazing rate was higher in upwelling (0.68±0.32 d⁻¹) than in oceanic waters (0.37±0.19 d⁻¹), but represented the main loss pathway for phytoplankton in both systems (m/μ=0.90±0.32 and 0.69±0.24, respectively). Carbon flux through phytoplankton, produced and grazed, increased from offshore to coastal (∼2 to ∼200 μg C L⁻¹ d⁻¹), with diatoms dominating the flux in the upwelling region (52%) while PRYMN (40%) and PRO (30%) dominated in the open ocean.     

Burial tolerances of reef-building Sabellariid worms from the east coast of Florida

Sabellariid worms, such as Phragmatopoma lapidosa, are sessile suspension feeders that attach to exposed hard bottom and serve as foundation species for worm reefs which are complex, multifaceted habitats. While worm reefs are adapted to dynamic sedimentary environments, burial of these habitats by beach nourishment projects is a concern. This study determined duration and depth of burial that can be tolerated by P. lapidosa without death. Worm rock samples were buried in sand at 1–10 cm (1-cm intervals), and at 15, 25 and 40 cm for the duration of 72, 144, and 216 h and then surveyed for initial mortality after burial and one week after removal of sediment (latent effects). Initial mortality was similar across all burial depths for the 72-h duration with values ranging from 8.3% (±0.8 SE) for 1 cm to 24.0% (±8.0 SE) for 10 cm of sediment. As burial duration increased to 144 h, mortality generally increased as burial depth increased with an average mortality for 2 cm of sediment of 23.5% (±5.3 SE) increasing to 96.0% (±14.3 SE) with 40 cm of sediment. The mean percent mortality for burial samples in the 216 h treatment varied from a low of 71.2% (±3.3 SE) for 1 cm depth to a high of 100% (±0 SE) for 10, 15, 25, and 40 cm depths. Mortality for most treatments also increased over time after removal of sediment indicating latent effects of burial stress.