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The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (210Pb and 137Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year−1) compared to historical averages (0.24–0.60 cm year−1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year−1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41 ± 0.07 cm year−1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.

Measurements of groundwater-dissolved inorganic nitrogen (nitrate + nitrite + ammonia) and phosphate concentrations were combined with recent, radium-based, submarine groundwater discharge (SGD) fluxes and prior estimates of SGD determined from Darcy’s Law, a hydrologic model, and total recharge to yield corresponding SGD nutrient fluxes to Ninigret, Point Judith, Quonochontaug, and Winnapaug ponds, located in southern Rhode Island. Results range from 80 to279 mmol N m−2 year−1 and 4 to 15 mmol P m−2 year−1 for Ninigret, 48 to 265 mmol N m−2 year−1 and 4 to 23 mmol P m−2 year−1 for Point Judith, 31 to 62 mmol N m−2 year−1 and 1 to 2 mmol P m−2 y−1 for Quonochontaug, and 668 to 1,586 mmol N m−2 year−1 and 29 to 70 mmol P m−2 year−1 for Winnapaug ponds, respectively. On a daily basis, the SGD supply of dissolved inorganic nitrogen and phosphorus is estimated to represent ∼1–6 % of the total amount of these nutrients in surface waters of Ninigret, Point Judith, and Quonochontaug ponds and up to 84 and 17 % for Winnapaug, respectively, which may reflect a greater SGD nutrient supply to this pond because of the proximity of fertilized golf courses. With regard to the total external input of these essential nutrients, SGD represents 29–45 % of dissolved inorganic nitrogen input to Ninigret, Point Judith, and Quonochontaug ponds and as much as 93 % for Winnapaug pond. For phosphorus, the contribution from SGD represents 59–85 % of the total external input for Ninigret, Point Judith, and Quonochontaug ponds and essentially all of the phosphorus input to Winnapaug pond. Estimated rates of primary productivity potentially supported by the average supply of dissolved inorganic nitrogen from SGD range from 10 g C m−2 year−1 for Ninigret, 13 g C m−2 year−1 for Point Judith, 4 g C m−2 year−1 for Quonochontaug, and as high as 84 g C m−2 y−1 for Winnapaug pond. The imputed SGD-derived rates of primary productivity represent 4–9 % of water column primary production for Ninigret, Point Judith, and Quonochontaug ponds, and 74 % for Winnapaug pond, a result that is reasonably comparable to several other coastal environments where estimates of SGD nutrient supply have been reported. The implication is that SGD represents an ecologically significant source of dissolved nutrients to the coastal salt ponds of southern Rhode Island and, by inference, other coastal systems.

Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export, via formation and gravitational settling of aggregates and/or consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (>0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, >53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to >53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates likely by gravitational settling and/or convective mixing. Using available pigment and ancillary data from these cruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export via formation of aggregates in this oligotrophic ocean gyre. We suggest that aggregate formation and fate should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.

Measurements of the naturally occurring radioisotopes 223 Ra ( t 1/2 = 11.4 days) and 224 Ra ( t 1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L − 1 and 5–885 dpm 100 L − 1 for 223 Ra and 224 Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L − 1 for 223 Ra, and 57–4926 dpm 100 L − 1 for 224 Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).

Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234 Th activity distribution (in order to calculate 234 Th fluxes) and an estimate of the C / 234 Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C / 234 Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / 234 Th either increases or is relatively invariant with increasing particle size (size classes > 1 to 100s μm). Observations also suggest that C / 234 Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234 Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2–3). We recommend sampling for C / 234 Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234 Th profiles and C / 234 Th samples during the course of longer observation periods to better sample temporal variations in both 234 Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column.

An analysis of the 234 Th method for determining the export flux of particulate organic carbon ( P POC ) from the upper ocean using in situ pumps or water bottles shows that the accuracy of the method (the ratio of the experimental value of P POC divided by the true POC flux, F POC ), defined as the p -ratio, is equal to the mean settling velocity of particulate 234 Th divided by that of POC. Therefore, P POC is equivalent to the true POC flux ( F POC ) and the p -ratio is equal to unity if, and only if, POC and particulate 234 Th have the same mean sinking velocities. A simple particle settling model is discussed that invokes Stokes’ Law settling velocities, volume:surface area ( V :SA) fractionation of C: 234 Th and two assemblages of particles having different sizes and densities. The model is used to illustrate the ranges of parameter values that conform to values of the p -ratio sufficiently close to unity that the experimentally determined POC flux can be considered to be an accurate representation of the true POC flux. Despite the over-simplification of real systems implicit in the model, the results suggest that p -ratios 234 Th on particles is governed by the volume to surface area ( V :SA) ratio of the particles. This results in a greater proportion of 234 Th compared to C being associated with the smaller, more slowly settling particles and, as a result, normalization of the POC flux to the 234 Th flux provides an under-estimate of the former quantity. However, when the smaller particle assemblage dominates the vertical flux, as could occur in open ocean regimes having high aeolian inputs of dense, rapidly settling, inorganic particles, then the p -ratio could exceed unity ( p -ratio >1) resulting in an over-estimate of the POC flux using the 234 Th method. High levels of flocculation associated with phytoplankton blooms in productive regions of the ocean are likely to produce p -ratios approaching unity, because flocs would tend to preserve the V :SA partitioning of the original particle size distribution and thereby minimize differences in the mean settling speeds of POC and particulate 234 Th (Waite, A.W., Hill, P.S., 2006. Flocculation, phytoplankton and the accuracy of 234 Th-based estimates of the vertical flux of particulate organic carbon in the sea. Marine Chemistry in press). Selective sampling of the large particle fraction using, for example, 53-μm screens can produce a more accurate estimate of the true POC flux, but may not entirely shift the p -ratio to a value of unity.

Poretics® glass fiber filters of 0.7 μm nominal pore size (GF-75) were evaluated for determination of particulate organic carbon and nitrogen in seawater and freshwater. Carbon and nitrogen blanks determined by CHN analysis of precombusted 25 mm filters were, respectively, 0.8 ± 0.3 and 0.02 ± 0.01 μmol for Poretics® GF-75 filters, compared to 1.7 ± 0.7 and 0.19 ± 0.12 μmol for Whatman® GF/F filters. These results suggest that GF-75 filters are characterized by lower blanks and better analytical precision for determination of carbon and nitrogen than GF/F filters. Comparable POC and PON concentrations were obtained using GF-75 and GF/F filters on paired analyses of samples from the Gulf of Maine, Narragansett Bay, and a freshwater creek. Depth profiles of POC and PON determined using GF-75 filters are presented for the Gulf of Maine and the Canada Basin, Arctic Ocean. Results reported in this study demonstrate that the Poretics® GF-75 filter is a suitable, cost effective, alternative to the widely used Whatman® GF/F filter.

An intercomparison was conducted between 14 different cross-flow filtration (CFF) systems. Each CFF membrane had a 1000 nominal molecular weight cut-off, and five different manufacturers' membranes were tested. The goal of this exercise was to examine whether the different CFF systems were behaving in a well-defined and operationally reproducible manner in marine applications. Surface seawater from Woods Hole and mid-depth waters from Hawaii were prefiltered ( 100 μM. The variation in OC blanks between CFF systems, even of a single type, suggested that cleaning and handling protocols were critical. One of the primary features of this intercomparison was the large range in apparent retention characteristics of the different CFF systems when used on natural seawater samples. In both settings, the quantity of colloidal material retained by the CFF systems followed the order: Amicon > Filtron ≥ Osmonics > Membrex. For example, in the Hawaii sample, the Amicon CFF system retained on average 43% colloidal OC, while the Filtron, Osmonics and Membrex retained < 4%. Variations within a factor of 2–5 were found within a single membrane type. Other results from this intercomparison suggested that these same relative retention characteristics hold for Fe and Al, organic nitrogen and organic phosphorus, optical properties, and colloid standards. Time-series samples of permeate showed a generally increasing OC concentration with time or concentration factor which must be taken into account when interpreting CFF data. We recommend that considerable care be taken in quantifying CFF blanks and in assessing the CFF cut-off for each system prior to use in marine applications. Time-series sampling and the use of standard molecules in controlled experiments are encouraged in order to further our understanding of the behavior of natural compound assemblages during CFF processing.

Kinetic experiments were conducted using batch systems to investigate the removal of dissolved Al and 234Th tracer by dead Phaeodactylum tricomutrnm diatoms in seawater. Experiments were conducted at constant temperature (2°C), pH (7.8), and salinity (30 psu), using realistic oceanic concentrations of dissolved Al (50 nM), and 1, 2.5, 5, and 10 mg/L suspensions of dead diatoms in ultrafiltered (

Vertical profiles of dissolved aluminum (A1) at five stations in the Weddell Sea are presented. Concentrations of dissolved A1 in the Weddell Sea range from 1 to 5 nM, considerably lower than A1 levels in the Atlantic, typically in the 10–40 nM range, and similar to levels of 0.06–6 nM reported for the Pacific. The slight maximum in dissolved A1 (4–5 nM) in the surface waters is suggested to be derived primarily from the partial dissolution of atmospheric dust. Minimum concentrations of dissolved A1 (1–1.6 nM) are observed at intermediate depths (1500–3000m). The slight increase in dissolved A1 of about 2 nM with proximity to the sediment-water interface implies that sediments are not a significant source of dissolved A1 to the deep waters. The combination of the low atmospheric input and the seasonally high rate of primary productivity that characterize the Southern Ocean can account for the low concentrations of dissolved A1 in the Weddell Sea. The results reported in this study suggest that the low A1 levels in the deep North Pacific can be explained by advection of low-A1 source waters originating in the Weddell Sea.

[1] Particulate organic carbon (POC) export fluxes were estimated in the shelf-slope region of the Chukchi Sea using measurements of 234Th−238U disequilibria and the POC/234Th ratio in large (>53-μm) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf-slope region. Samples were collected along a series of shelf-basin transects in the spring (May–June) and summer (July–August) of 2004. These stations were previously occupied during the ice covered (spring) and open water (summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice-covered period, POC export fluxes in 2004 were similar during the spring (average = 19.7 ± 24.8 mmol C m−2 d−1) and summer (average = 20.0 ± 14.5 mmol C m−2 d−1). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 ± 88.1 mmol C m−2 d−1). The shelf-slope budget of particulate organic carbon indicates that 10–20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column−sediment budget of 234Th indicates that particulate material is retained in shelf sediments on a seasonal basis.

Since the 1960s, thorium isotopes occupy a special place in the oceanographer’s toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234 Th (as well, in principle, other Th-nuclides) to carbon (and other elements), and linking this ratio to the parent–daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its bparticle- or surface-activityQ. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234 Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium. D 2005 Elsevier B.V. All rights reserved.

The goal of this proposal was to develop new sampling and analytical techniques which could be used to understand the transformation rates and mechanisms of exchange between dissolved, colloidal, small particle and large particle size classes of organic carbon in ocean margins. To meet this goal, the authors focused on the development of cross-flow filtration (CFF) for the isolation of colloidal material from the dissolved phase in seawater. In addition, they tested and optimized high sensitivity techniques for the measurement of thorium isotopes using thermal ionization mass spectrometry (TIMS) and carbon isotopes using accelerator mass spectrometry (AMS) and stable C mass spectrometric techniques. In this final report, they highlight some of these new sampling and analytical developments as well as preliminary results from the first DOE cruise this past April. The report is broken down into 4 sections, namely (1) colloidal sampling strategies, (2) TIMS analytical developments, (3) carbon isotopic measurements and (4) results from the R/V Columbus Iselin cruise. For more detailed discussion of the findings, they have included as an appendix to this final report manuscripts which have been published or will be submitted during this funding cycle.

Dissolved and acetic acid leachable particulate Al ([Al]HAc) concentrations have been measured as a time series during a spring phytoplankton bloom in Bedford Basin off Nova Scotia, Canada. Two factors were observed to influence Al levels: the spring bloom and river runoff. Changes in dissolved Al and [Al]HAc associated with intense biological activity resulted in a 55–65 n m (40%) decrease in dissolved Al and a coincident similar increase in [Al]HAc over 15–20 days. The intensity of Al removal is apparently a function of biogenic particle loads. A pulse in river runoff caused dissolved Al to increase roughly 10 fold (up to 1130 n m ) and [Al]HAc by nearly 4 fold (up to 175 n m ) in the surface 5 m. These data indicate that biological activity can significantly alter concentrations of Al in coastal waters and distributions among dissolved and particulate forms. In the presence of strong sources, such as river runoff, these effects can be obscured.