Vertical distributions of dissolved organic carbon and nitrogen in the Southern Ocean

Detailed vertical profiles of dissolved organic carbon (DOC), nitrogen (DON) and DOC/DON ratios in the Australian sector of the Southern Ocean (56°–65°S around 140°W transect) were obtained by a high-temperature catalytic oxidation method with a modified Shimadzu TOC-5000 unit. The simultaneous analyses of DOC and total dissolved nitrogen (TDN) gave high precision results (±0.7 μMC and ±0.3 μMN). In surface layers, DOC concentrations were lower (45–55 μM) than those generally obtained from other oceanic environments (60–90 μM). The surface concentrations of DON varied from 4 to 9 μM, but within the range generally reported for other ocean regions. Surface excesses of DOC and DON were calculated against nearly constant subsurface concentrations. A consistent contribution of low C/N ratio (2.7–5.0) was found in the mid-surface layer (30–75 m), suggesting more extensive degradation of carbon-enriched materials and/or enhanced supply of nitrogen-enriched ones.     

Marine dissolved organic matter: can its C : N ratio explain carbon overconsumption?

Carbon overconsumption, i.e. the consumption of inorganic carbon relative to inorganic nitrogen in excess of the Redfield ratio at the sea surface, was examined in relation to the dynamics of dissolved organic carbon and nitrogen (DOC and DON) in the northeast Atlantic. We observed the presence of N-poor dissolved organic matter (DOM) in surface water during summer, requiring the consumption of inorganic carbon and nitrogen in a ratio exceeding the Redfield ratio. The C : N ratio of bulk DOM is not only different from the Redfield ratio but also variable, i.e. no fixed conversion factor of C and N exists where DOM is important in C and N transformations. The existence of N-poor DOM is recognized as a feature typical of oligotrophic systems. At the same time, the C : N ratios of particles conform to Redfield stoichiometry as does deep-ocean chemistry. The implications of this finding are discussed, the conclusion being that, while DOM buildup contributes to CO₂ drawdown seasonally, its impact on long-term carbon and nitrogen balance of the ocean is small.     

Dissolved organic matter in the subterranean estuary of a volcanic island,Jeju: Importance of dissolved organic nitrogen fluxes to the ocean

We observed the origin, behavior, and flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), colored dissolved organic matter (CDOM), and dissolved inorganic nitrogen (DIN) in the subterranean estuary of a volcanic island, Jeju, Korea. The sampling of surface seawater and coastal groundwater was conducted in Hwasun Bay, Jeju, in three sampling campaigns (October 2010, January 2011, and June 2011). We observed conservative mixing of these components in this subterranean environment for a salinity range from 0 to 32. The fresh groundwater was characterized by relatively high DON, DIN, and CDOM, while the marine groundwater showed relatively high DOC. The DON and DIN fluxes through submarine groundwater discharge (SGD) in the groundwater of Hwasun Bay were estimated to be 1.3 × 10⁵ and 2.9 × 10⁵ mol d^− 1, respectively. In the seawater of Hwasun Bay, the groundwater-origin DON was almost conservative while about 91% of the groundwater-origin DIN was removed perhaps due to biological production. The DON flux from the entire Jeju was estimated to be 7.9 × 10⁸ mol yr^− 1, which is comparable to some of the world's large rivers. Thus, our study highlights that DON flux through SGD is potentially important for delivery of organic nitrogen to further offshore while DIN is readily utilized by marine plankton in near-shore waters under N-limited conditions.     

Net community production in the northeastern Chukchi Sea

To assess the magnitude, distribution and fate of net community production (NCP) in the Chukchi Sea, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), and particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured during the spring and summer of 2004 and compared to similar observations taken in 2002. Distinctive differences in hydrographic conditions were observed between these two years, allowing us to consider several factors that could impact NCP and carbon cycling in both the Chukchi Shelf and the adjacent Canada Basin. Between the spring and summer cruises high rates of phytoplankton production over the Chukchi shelf resulted in a significant drawdown of DIC in the mixed layer and the associated production of DOC/N and POC/N. As in 2002, the highest rates of NCP occurred over the northeastern part of the Chukchi shelf near the head of Barrow Canyon, which has historically been a hotspot for biological activity in the region. However, in 2004, rates of NCP over most of the northeastern shelf were similar and in some cases higher than rates observed in 2002. This was unexpected due to a greater influence of low-nutrient waters from the Alaskan Coastal Current in 2004, which should have suppressed rates of NCP compared to 2002. Between spring and summer of 2004, normalized concentrations of DIC in the mixed layer decreased by as much as 280 μmol kg⁻¹, while DOC and DON increased by ∼16 and 9 μmol kg⁻¹, respectively. Given the decreased availability of inorganic nutrients in 2004, rates of NCP could be attributed to increased light penetration, which may have allowed phytoplankton to increase utilization of nutrients deeper in the water column. In addition, there was a rapid and extensive retreat of the ice cover in summer 2004 with warmer temperatures in the mixed layer that could have enhanced NCP. Estimates of NCP near the head of Barrow Canyon in 2004 were ∼1500 mg carbon (C) m⁻² d⁻¹ which was ∼400 mg C m⁻² d⁻¹ higher than the same location in 2002. Estimates of NCP over the shelf-break and deep Canada Basin were low in both years, confirming that there is little primary production in the interior of the western Arctic Ocean due to near-zero concentrations of inorganic nitrate in the mixed layer.     

C : N ratios in the mixed layer during the productive season in the northeast Atlantic Ocean

Redfield stoichiometry has proved a robust paradigm for the understanding of biological production and export in the ocean on a long-term and a large-scale basis. However, deviations of carbon and nitrogen uptake ratios from the Redfield ratio have been reported. A comprehensive data set including all carbon and nitrogen pools relevant to biological production in the surface ocean (DIC, DIN, DOC, DON, POC, PON) was used to calculate seasonal new production based on carbon and nitrogen uptake in summer along 20°W in the northeast Atlantic Ocean. The 20°W transect between 30 and 60°N covers different trophic states and seasonal stages of the productive surface layer, including early bloom, bloom, post-bloom and non-bloom situations. The spatial pattern has elements of a seasonal progression. We also calculated exported production, i.e., that part of seasonal new production not accumulated in particulate and dissolved pools, again separately for carbon and nitrogen. The pairs of estimates of `seasonal new production’ and `exported production’ allowed us to calculate the C : N ratios of these quantities. While suspended particulate matter in the mixed layer largely conforms to Redfield stoichiometry, marked deviations were observed in carbon and nitrogen uptake and export with progressing season or nutrient depletion. The spring system was characterized by nitrogen overconsumption and the oligotrophic summer system by a marked carbon overconsumption. The C : N ratios of seasonal new as well as exported production increase from early bloom values of 5–6 to values of 10–16 in the post-bloom/oligotrophic system. The summertime accumulation of nitrogen-poor dissolved organic matter can explain only part of this shift.     

DOC dynamics in the meso and bathypelagic layers of the Mediterranean Sea

Seven years (2001–2008) of dissolved organic carbon (DOC) vertical profiles were examined in order to assess the main processes determining DOC concentration and distribution in the meso- and bathypelagic layers of the Mediterranean Sea. As expected, DOC showed high and highly variable concentrations in the surface layer of 57–68 μM (average values between 0 and 100 m), with a decrease to 44–53 μM between 200 and 500 m. Deep DOC distribution was strongly affected by deep-water formation, with a significant increase to values of 76 μM in recently ventilated deep waters, and low concentrations, comparable to those observed in the open oceanic waters (34–45 μM), where the oldest, deep waters occurred. In winter 2004/2005 a deep-water formation event was observed and the consequent DOC export at depth was estimated to range between 0.76–3.02 Tg C month^–1. In the intermediate layer, the main path of the Levantine Intermediate Water (LIW) was followed in order to estimate the DOC consumption rate in its core. Multiple regression between DOC, apparent oxygen utilization (AOU), and salinity indicated that 38% of the oxygen consumption was related to DOC mineralization when the effect of mixing was removed. In deep waters of the southern Adriatic Sea a DOC decrease of 6 μM, together with an AOU increase of 9 μM, was observed between the end of January 2008 and the end of June 2008 (5 months). These data indicate a rate of microbial utilization of DOC of about 1.2 μM C month⁻¹, with 92% of the oxygen consumption due to DOC mineralization. These values are surprisingly high for the deep sea and represent a peculiarity of the Mediterranean Sea.     

Temporal dynamics of dissolved combined neutral sugars and the quality of dissolved organic matter in the Northwestern Sargasso Sea

The dynamics of dissolved combined neutral sugars (DCNS) were assessed in the upper 250 m at the Bermuda Atlantic Time-series Study (BATS) site between 2001 and 2004. Our results reveal a regular annual pattern of DCNS accumulation with concentrations increasing at a rate of 0.009–0.012 μmol C L^?1 d^?1 in the surface 40 m from March to July and reaching maximum mean concentrations of 2.2–3.3 μmol C L^?1. Winter convective mixing (between January and March) annually exported surface-accumulated DCNS to the upper mesopelagic zone (100–250 m), as concentrations increased there by 0.3–0.6 μmol C L^?1. The exported DCNS was subsequently removed over a period of weeks following restratification of the water column. Vertical and temporal trends in DCNS yield (% of DOC) supported its use as a diagenetic indicator of DOM quality. Higher DCNS yields in surface waters suggested a portion of the DOM accumulated relatively recently compared to the more recalcitrant material of the upper mesopelagic that had comparably lower yields. DCNS yields and mol% neutral sugar content, together, indicated differences in the diagenetic state of the surface-accumulated and deep pools of DOM. Seasonally accumulated, recently produced DOM with higher DCNS yields was characterized by elevated mol% of galactose and mannose+xylose levels. Conversely, more recalcitrant DOM from depths >100 m had lower DCNS yields but higher mol% of glucose. Lower DCNS yields and elevated mol% glucose were also observed in the surface waters during winter convective mixing, indicating an entrainment of a diagenetically altered DOM pool into the upper 100 m. A multivariate statistical analysis confirms the use of DCNS as an index of shifts in DOM quality at this site.     

Copper complexation by fulvic acid affects copper toxicity to the larvae of the polychaete Hydroides elegans

Copper toxicity is influenced by a variety of environmental factors including dissolved organic matter (DOM). We examined the complexation of copper by fulvic acid (FA), one of the major components of DOM, by measuring the decline in labile copper by anodic stripping voltammetrically (ASV). The data were described using a one-site ligand binding model, with a ligand concentration of 0.19 μmol site mg⁻¹ C, and a log K′ of 6.2. The model was used to predict labile copper concentration in a bioassay designed to quantify the extent to which Cu–FA complexation affected copper toxicity to the larvae of marine polychaete Hydroides elegans. The toxicity data, when expressed as labile copper concentration causing abnormal development, were independent of FA concentration and could be modeled as a logistic function, with a 48-h EC₅₀ of 58.9 μg l⁻¹. However, when the data were expressed as a function of total copper concentration, the toxicity was dependent on FA concentration, with a 48-h EC₅₀ ranging from 55.6 μg l⁻¹ in the no-FA control to 137.4 μg l⁻¹ in the 20 mg l⁻¹ FA treatment. Thus, FA was protective against copper toxicity to the larvae, and such an effect was caused by the reduction in labile copper due to Cu–FA complexation. Our results demonstrate the potential of ASV as a useful tool for predicting metal toxicity to the larvae in coastal environment where DOM plays an important role in complexing metal ions.     

Seasonal variations of dissolved organic matter and nutrients in sediment pore water in the inner part of Tokyo Bay

At four stations in Tokyo Bay, pore water profiles of dissolved organic carbon (DOC), nitrogen (DON), phosphorus (DOP), and inorganic nutrients were determined at 3-month intervals over 6 years. Concentrations of dissolved organic matter (DOM) and nutrients were significantly higher in pore waters than in the overlying waters. Pore water DOC, DON, and DOP concentrations in the upper most sediment layer (0–1 cm) ranged from 246 to 888 μM, from 14.6 to 75.9 μM, and from 0.02 to 9.83 μM, respectively. Concentrations of DOM and nutrients in pore waters occasionally showed clear seasonal trends and were highest in the summer and lowest in the winter. The seasonal trends in the pore water DOM concentrations were coupled with trends in the overlying water temperature and dissolved oxygen concentration. Benthic effluxes of DON and DOP were low compared with those of inorganic nutrients, accounting for only 1.0 and 1.5 % of the total benthic effluxes of nitrogen and phosphorus, respectively; thus benthic DOM fluxes were quantitatively insignificant to the inorganic nutrient fluxes in Tokyo Bay. The DOM fluxes represented about 7, 3, and 10 % of the riverine discharge of DOC, DON, and DOP to Tokyo Bay, respectively.     

Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 6°S to 63°N along longitudes 20°W (CLIVAR line A16), 52°W (A20) and 66°W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr⁻¹ from the epipelagic zone into the mesopelagic and bathypelagic zones. Model II regression and multiple linear regression models applied to pairwise measures of DOC and chlorofluorocarbon (CFC-12) ventilation age, retrieved from major water masses within the main thermocline and NADW, indicate decay rates for exported DOC ranging from 0.13 to 0.94 μmol kg⁻¹ yr⁻¹, with higher DOC concentrations driving higher rates. The contribution of DOC oxidation to oxygen consumption ranged from 5 to 29% while mineralization of sinking biogenic particles drove the balance of the apparent oxygen utilization.     

Microbial availability and degradation of dissolved organic carbon and nitrogen in two coastal areas

Microbial availability and degradation rates of dissolved organic carbon (DOC) and nitrogen (DON) were estimated at two coastal stations (Horsens Fjord and Darss Sill) in Denmark, by measuring the decrease in DOC and DON concentrations during long-term laboratory incubations (150 days). The experiments used two different treatments: one only receiving a microbial inoculum and another additionally to the inoculum, carbon and phosphate to ensure nitrogen limitation. The additions of carbon and phosphate led to increased DON bioavailability in all experiments. The incubations showed that bioavailable DOC (BDOC) accounted for 22 ± 13% of total DOC in Horsens Fjord and 14 ± 5% at Darss Sill. Bioavailable DON (BDON) accounted for 43 ± 10% (Horsens Fjord) and 28 ± 12% of DON (Darss Sill). The linear relations between BDOM and DOM suggested that the DOC variations in Horsens Fjord were controlled by the available fraction, while this was only partly the case for DOC at Darss Sill and DON (both stations), showing that the refractory pool to some degree controlled the seasonal variations in DOM at these coastal stations. Additionally we found that DOC and DON were cycled at approximately the same speed, probably due to a high carbon demand of the microbial community. Calculating the amounts of DON degraded within the two areas using the obtained decay rates showed that compared with the ambient inorganic nitrogen levels BDON contained a large proportion (52 ± 37%, Horsens Fjord and 74 ± 19%, Darss Sill) of the bioavailable nitrogen (BDON + DIN). These calculations further suggested that bioavailable DOM was washed out of the respective areas and could contribute to heterotrophic growth in adjacent waters.     

Temporal variability in the composition and abundance of terrestrially-derived dissolved organic matter in the lower Mississippi and Pearl Rivers

Here we report on temporal changes in the concentration and composition of lignin phenols in high molecular weight (< 0.2 μm, > 1 kDa) dissolved organic matter (HMW DOM) collected from the lower Mississippi and Pearl Rivers (MR and PR) (USA). Monthly water samples were collected at a station in the lower reach in each river from August 2001 to August 2003. Significantly higher concentrations of lignin and Λ₈ values (mg lignin phenols in 100 mg organic carbon) in the Pearl River than in the Mississippi River, reflected sporadic inputs of terrestrial DOM during rainstorm events from wetlands and forest soils. Larger seasonal variations in lignin concentration and composition in the Pearl River, compared to the Mississippi River, were attributed to shifts in organic matter sources from topsoil inputs during rainstorm events to groundwater inputs and in situ production during base flow in this small river. Conversely, lower Λ₈ and vanillic acid to vanillin ratios [(Ad/Al)v] in the HMW DOM of the lower Mississippi River may be a result of a lower export rate of lignin from agricultural soils due to lower carbon storage in the expansive agricultural systems of the Mississippi River watershed, as well as dilution of phytoplankton DOM inputs. Large seasonal changes in lignin concentration and Λ₈ (linked at times with river discharge), and minimal variability in the composition of lignin phenols, likely represented an integrated signal of soil-derived vascular inputs from the upstream drainage basin. If we are to better understand the controls of organic matter delivery to the coastal zone from both small and large rivers, sampling strategies need to be adjusted to account for the different scales of hydrologic response time and in situ processing associated with different residence times.     

Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface waters of the western North Atlantic Ocean have been constrained because ambient concentrations are typically at or below the detection limits of standard colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods—determinations of nitrate plus nitrite (N+N) by chemiluminescence and soluble reactive phosphorus (SRP) by the magnesium induced co-precipitation (MAGIC) protocol—to surface waters along a transect from the Sargasso Sea at 26°N through the Gulf Stream at 37°N, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compared with data from the BATS program, and the HOT station in the Pacific Ocean, permitting cross-ecosystem comparisons. Microbial populations were analyzed along the transect, and an attempt was made to interpret their distributions in the context of the measured nutrient concentrations.Surface concentrations of N+N and SRP during the March 1998 transect separated into 3 distinct regions, with the boundaries corresponding roughly to the locations of the BATS station (∼31°N) and the Gulf Stream (∼37°N). Although N+N and SRP co-varied, the [N+N] : [SRP] molar ratios increased systematically from ∼1 to 10 in the southern segment, remained relatively constant at ∼40–50 between 31°N and 37°N, then decreased again systematically to ratios <10 north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (⩾90%) the total dissolved N (TDN) and P (TDP) pools except in the northern portion of the transect. The [DON] : [DOP] molar ratios were relatively invariant (∼30–60) across the entire transect.Heterotrophic prokaryotes (operationally defined as “bacteria”), Prochlorococcus, Synechococcus, ultra- and nanophytoplankton, cryptophytes, and coccolithophores were enumerated by flow cytometry. The abundance of bacteria was well correlated with the concentration of SRP, and that of the ultra- and nanophytoplankton was well correlated with the concentration of N+N. The only group whose concentration was correlated with temperature was Prochlorococcus, and its abundance was unrelated to the concentrations of nutrients measured at the surface.We combined our transect results with time-series measurements from the BATS site and data from select depth profiles, and contrasted these North Atlantic data sets with time-series of N and P nutrient measurements from a station in the North Pacific subtropical gyre near Hawaii [Hawaii Ocean Time-series (HOT) site]. Two prominent differences are readily observed from this comparison. The [N+N] : [SRP] molar ratios are much less than 16 : 1 during stratified periods in surface waters at the BATS site, as is the case at the HOT site year round. However, following deep winter mixing, this ratio is much higher than 16 : 1 at BATS. Also, SRP concentrations in the upper 100 m at BATS fall in the range 1–10 nM during stratified periods, which is at least one order of magnitude lower than at the HOT site. That two ecosystems with comparable rates of primary and export production would differ so dramatically in their nutrient dynamics is intriguing, and highlights the need for detailed cross ecosystem comparisons.     

Role of colloidal material in the removal of 234Th in the Canada basin of the Arctic Ocean

The phase partitioning of ²³⁴Th between dissolved (<10-kiloDalton, kD), colloidal (10 kD—0.4 μm), and particulate (⩾0.5 μm) matter across a horizontal transect, from a coastal station to the deep Canada Basin, and a vertical profile in the deep Canada Basin of the western Arctic Ocean was investigated. Concentrations of suspended particulate matter (SPM), dissolved, colloidal and particulate organic carbon, particulate organic nitrogen and nutrients (silicate, phosphate and nitrate) were also measured to assess transport and scavenging processes.Total ²³⁴Th (colloidal+particulate+dissolved) indicated deficiencies relative to secular equilibrium with its parent, ²³⁸U in the upper 100 m, which suggests active scavenging of ²³⁴Th onto particle surfaces. In contrast, at depths >200 m, general equilibrium existed between total ²³⁴Th and ²³⁸U. The inventory of SPM and the specific activity of particulate ²³⁴Th in the Canada Basin was about an order of magnitude higher than the profile reported for the Alpha Ridge ice camp station. This higher concentration of SPM in the southwestern Canada Basin is likely derived from ice-rafted sedimentary particles. Inventories of nutrients, and dissolved organic carbon and nitrogen in the upper 100 m of the Canada Basin are comparable to the other estimates for the central Arctic Ocean. Comparison of the mass concentrations of colloidal and filter-retained particulate matter as well as the activity of ²³⁴Th in these phases indicates that only a very small component of the colloidal material is actively involved in Th scavenging. Lower values of the conditional partition coefficient between the colloidal and dissolved phase indicate that the Arctic colloids are less reactive than colloidal material from other regions. The conditional partition coefficient between the filter-retained and dissolved phases (K_f) is generally higher than that for other regions, which is attributed to the higher complexation capacity of glacio-marine sedimentary particles in these waters. The ²³⁴Th-derived export of POC for the shelf and deep Canada Basin ranges between 5.6 and 6.5 mmol m⁻² d⁻¹, and is in agreement with other estimates reported for the central Arctic Ocean and Beaufort Sea.     

Rapid removal of terrigenous dissolved organic carbon over the Eurasian shelves of the Arctic Ocean

The fate of terrigenous dissolved organic carbon (tDOC) delivered to the Arctic Ocean by rivers remains poorly constrained on both spatial and temporal scales. Early reports suggested Arctic tDOC was refractory to degradation, while recent studies have shown tDOC removal to be an active but slow process. Here we present observations of DOC, salinity, δ¹⁸O, and ²²⁸Ra/²²⁶Ra in the Polar Surface Layer (PSL) over the outer East Siberian/Chukchi shelf and the adjacent Makarov and Eurasian basins of the eastern Arctic Ocean. This off-shelf system receives meteoric water, introduced by rivers, after a few years residence on the shelf. Elevated concentrations of DOC (> 120 μM C) were observed in low salinity (~ 27) water over the Makarov Basin, suggesting inputs of tDOC-enriched river water to the source waters of the Transpolar Drift. The regression of DOC against salinity indicated an apparent tDOC concentration of 315 ± 7 μM C in the river water fraction, which is significantly lower than the estimated DOC concentration in the riverine sources to the region (724 ± 55 μM C). To obtain the timescale of removal, estimates of shelf residence were coupled with measurements of dissolved ²²⁸Ra/²²⁶Ra, an isotopic tracer of time since shelf residence. Shelf residence time coupled with DOC distributions indicates a first order tDOC removal rate constant, λ = 0.24 ± 0.07 yr^-1, for the eastern Arctic, 2.5–4 times higher than rates previously observed in the western Arctic. The observed removal of tDOC in the eastern Arctic occurs over the expansive shelf area, highlighting the initial lability of tDOC upon delivery to the Arctic Ocean, and suggests that tDOC is composed of multiple compartments defined by reactivity. The relatively rapid remineralization of tDOC on the shelves may mitigate the strength of the Arctic Ocean atmospheric CO₂ sink if a projected increase in labile tDOC flux occurs.     

Distributions of particulate and dissolved organic and inorganic phosphorus in North Pacific surface waters

Several operationally defined fractions of phosphorus (P) were measured along a surface water transect in the North Pacific. The P content in all fractions was found to increase northward from the edge of the subtropical to the subarctic region. Particulate organic P (POP) concentrations increased from 9 to 110 nmol L^− 1, whereas the particulate inorganic P (PIP) concentrations increased from 1 to 13 nmol L^− 1. A significant correlation between POP, PIP and chlorophyll a suggested that these P pools are associated directly or indirectly with phytoplankton cells. PIP comprised 10–20% of the total particulate P pool across the transect, indicating it is an important component of the marine P cycle in this region. Dissolved non-reactive P (assumed to consist predominantly of non-reactive organic P compounds, thus referred to as DOP) concentration increased from 0.10 to 0.22 μmol L^− 1, whereas soluble reactive P (SRP) concentration increased from 0.01 to 1.42 μmol L^− 1 along the transect. The proportion of DOP and SRP varied widely, with a large proportion of DOP in areas with low total dissolved P concentrations in lower latitudes and a large proportion of SRP in areas with high total dissolved P concentrations in higher latitudes. High demand for DOP in the lower latitudinal region would diminish the concentration of this pool relative to higher latitudinal regions where SRP is more abundant and would be preferentially utilized. The availability of SRP could have a significant impact on the concentration and probably on the composition of DOP. We show that P fractionation provides an important insight for discussing the marine P cycle.     

Seasonal influences on size-fractionated chlorophyll a concentrations and primary production in the north-west Indian Ocean

Chlorophyll a (chl a) concentrations and primary production by the 0.2–2, 2–18 and >18 μm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the ¹⁴C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m^-2 and 3800 mg C m^-2 d^-1, respectively. The maxima, which were distributed patchily in this region, were dominated by the >18 μm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m^-2 and 1500 mg C m^-2 d^-1, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2–2 μm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2–2 μm size-fraction, with average chl a and production values of the order of 20 mg m^-2 and 750 mg C m^-2 d^-1, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon.     

Impact of lugworms (Arenicola marina) on mobilization and transport of fine particles and organic matter in marine sediments

We investigated the impact of sediment reworking fauna and hydrodynamics on mobilization and transport of organic matter and fine particles in marine sediments. Experiments were conducted in an annular flume using lugworms (Arenicola marina) as model organisms. The impact of lugworms on sediment characteristics and particle transport was followed through time in sediments experimentally enriched with fine particles (< 63 μm) and organic matter. Parallel experiments were run at low and high water current velocity (11 and 25 cm s^− 1) to evaluate the importance of sediment erosion at the sediment–water interface. There was no impact of fauna on sediment composition and particle transport at current velocity below the sediment erosion threshold. At current velocity above the erosion threshold, sediment reworking by lugworms resulted in dramatic particle transport (12 kg dry matter m^− 2) to an adjacent particle trap within 56 days. The transported matter was enriched 6–8 times in fine particles and organic matter when compared to the initial sediment. This study suggests that sediment reworking fauna is an important controlling factor for the particle composition of marine sediments. A. marina mediated sediment reworking greatly increases the sediment volume exposed to hydrodynamic forcing at the sediment–water interface, and through sediment resuspension control the content of fine particles and organic matter in the entire reworked sediment layer (> 20 cm depth).     

Seasonal and interannual variability in particle fluxes of carbon,nitrogen and silicon from time series of sediment traps at Ocean Station P, 1982–1993: relationship to changes in subarctic primary productivity

An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO₃ and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m⁻² yr⁻¹, 1.1±0.5 g POC m⁻² yr⁻¹, 0.15±0.07 g PON m⁻² yr⁻¹, 5.9±2.0 g BSi m⁻² yr⁻¹ and 1.7±0.6 g PIC m⁻² yr⁻¹. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d⁻¹, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.     

Bioavailability and bacterial degradation rates of dissolved organic matter in a temperate coastal area during an annual cycle

The bioavailability and bacterial degradation rates of dissolved organic matter (DOM) were determined over a seasonal cycle in Loch Creran (Scotland) by measuring the decrease in dissolved organic carbon (DOC), nitrogen (DON) and phosphorous (DOP) concentrations during long-term laboratory incubations (150 days) at a constant temperature of 14 °C. The experiments showed that bioavailable DOC (BDOC) accounted for 29 ± 11% of DOC (average ± SD), bioavailable DON (BDON) for 52 ± 11% of DON and bioavailable DOP (BDOP) for 88 ± 8% of DOP. The seasonal variations in DOM concentrations were mainly due to the bioavailable fraction. BDOP was degraded at a rate of 12 ± 4% d^– 1 (average ± SD) while the degradation rates of BDOC and BDON were 7 ± 2% d^– 1 and 9 ± 2% d^– 1 respectively, indicating a preferential mineralization of DOP relative to DON and of DON relative to DOC. Positive correlations between concentration and degradation rate of DOM suggested that the higher the concentration the faster DOM would be degraded. On average, 77 ± 9% of BDOP, 62 ± 14% of BDON and 49 ± 19% of BDOC were mineralized during the residence time of water in Loch Creran, showing that this coastal area exported C-rich DOM to the adjacent Firth of Lorne. Four additional degradation experiments testing the effect of varying temperature on bioavailability and degradation rates of DOM were also conducted throughout the seasonal cycle (summer, autumn, winter and spring). Apart from the standard incubations at 14 °C, additional studies at 8 °C and 18 °C were also conducted. Bioavailability did not change with temperature, but degradation rates were stimulated by increased temperature, with a Q₁₀ of 2.6 ± 1.1 for DOC and 2.5 ± 0.7 for DON (average ± SD).