Particulate Organic Matter In Water Column Research Articles

Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification

Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6–17.4 with the greatest effect at CO2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

Arctic lagoon and nearshore food webs: Relative contributions of terrestrial organic matter, phytoplankton, and phytobenthos vary with consumer foraging dynamics

Characterizing energy flow and trophic linkages is fundamental to understanding the functioning and resilience of Arctic ecosystems under increasing pressure from climate change and anthropogenic exploitation. We used carbon and nitrogen stable isotopes to examine trophic dynamics and the relative contribution of terrestrial organic matter, water column phytoplankton, and phytobenthos (benthic micro- and macro-autotrophs as well as sea ice algae) to the food webs supporting 45 macroconsumers in three Arctic coastal lagoon ecosystems (Krusenstern, Sisualik, Akulaaq) and the adjacent Kotzebue Sound with varying degrees of connectivity in Cape Krusenstern National Monument, Alaska. A two-source (water column particulate organic matter and benthic sediment organic matter), two-isotope trophic dynamics model informed by a Bayesian isotope mixing model revealed that the Lagoon-Kotzebue Sound coastal ecosystem supported consumers along a trophic position continuum from primary consumers, including amphipods, copepods, and clams to trophic level five predators, such as seastars, piscivorous fishes, seals, and seabirds. The relative contribution of the three primary producer end members, terrestrial organic matter (41 ± 21%), phytoplankton (25 ± 21%), and phytobenthos (34 ± 23%) varied as a function of: 1) consumer foraging ecology and 2) consumer location. Suspension feeders received most of their carbon from food webs based on phytoplankton (49 ± 11%) and terrestrial organic matter (23 ± 5%), whereas herbivores and detritivores received the majority of their carbon from phytobenthos-based food webs, 58 ± 10% and 60 ± 8%, respectively. Omnivores and predators showed more even distributions of resource reliance and greater overall variance among species. Within the invertebrates, the importance of terrestrial organic matter decreased and phytobenthos increased with increasing trophic position. The importance of terrestrial organic matter contribution increased with lagoon proximity to major rivers inputs and isolation from Kotzebue Sound. Several taxa with cultural and subsistence food importance to local communities showed significant reliance (30–90% of baseline carbon) on food chains linked to fresh terrestrial organic matter. Our study indicates that terrestrial-marine linkages are important to the function of Arctic coastal lagoon ecosystems and artisanal fisheries. These linkages are likely to strengthen in the future with regional changes in erosion and runoff associated with climate change and anthropogenic disturbance.

Lipid markers and compound-specific carbon isotopes as diet and biosynthesis reflectors in the northern Neptune whelk Neptunea heros

A suite of lipid biomarkers plus compound-specific carbon isotopes of major sterols were determined in muscle tissues across increasing sizes of northern Neptune whelks Neptunea heros, developing eggs and potential diets to link trophic patterns, metabolism and carbon sources on the Chukchi Sea shelf. Analysis of primary prey included the northern clam Astarte borealis, water column particulate organic matter (POM) and surface sediments near the collection sites. Sterols specific to major algal groups along with algal-derived polyunsaturated fatty acids (C20:5n-3, C20:4n-3, C22:6n-3) in whelk muscle tissue reflected the importance of algal primary production to benthic consumers and its direct incorporation. Compound-specific carbon isotope values of cholesterol and Δ5,7-sterols present in foot muscle of N. heros also suggest a transition from scavenging/detrital feeding in smaller juveniles to predatory consumption by larger adults. Sexually mature adult female N. heros (shell lengths >10 cm) contained lower sterol concentrations compared to smaller whelks, which may reflect reduced consumption rates or lipid translocation during the annual reproductive cycle. Analysis of N. heros eggs at 3 developmental stages ranging from internal egg masses to mature egg cases showed significant transfer of algal-derived sterols and fatty acids which were conserved during egg maturation. The suite of individual lipid signatures and δ13C sterol values observed in N. heros suggest that the northern Neptune whelk relies on both direct lipid incorporation as well as metabolic modification of algal-derived food sources for sterol requirements that transition as animals mature and become predatory consumers.

Alkenone Paleothermometry in Coastal Settings: Evaluating the Potential for Highly Resolved Time Series of Sea Surface Temperature

AbstractThe unsaturation ratios of alkenones have been applied widely to reconstruct pelagic sea surface temperatures (SSTs). However, applications to costal settings have been hampered by the effects of salinity and nutrient dynamics. Here we present a 4.5‐year‐long record of alkenones in water column particulate organic matter from sites in Narragansett Bay (RI, USA) spanning wide ranges in salinity and nutrients, and a 300‐year alkenone record from two Narragansett Bay sediment cores. Particulate organic matter results suggest that there are two distinct alkenone producing populations. One is a widespread population that blooms in summer (July–September) and displays alkenones typical of Group III producers—the basis for the widely applied marine Uk’37 temperature index. The other population is confined to brackish waters in the Providence River during spring salinity minima and displays alkenone profiles consistent with Group I production—the first detection of Group I in a brackish nonlacustrine environment. The sedimentary alkenone profiles consist of only Group III alkenone distributions, allowing for the reconstruction of a 300‐year‐long record of Narragansett Bay SST. Comparison of the most recent 100 years of this reconstruction with local and regional instrumental SST records shows excellent agreement, confirming a regional pattern of SST rise in the coastal northeastern United States that exceeds global or hemispheric rates. Group III alkenone production in coastal settings may be common and may open the way to high resolution local and regional SST reconstruction.

Trophic Ecology of Sheepshead and Stone Crabs at Oil and Gas Platforms in the Northern Gulf of Mexico's Hypoxic Zone

AbstractOil and gas platforms (platforms) may provide important habitat for reef‐associated organisms in the Gulf of Mexico's hypoxic zone. While summer hypoxia typically affects the bottom 3.0 m of the water column, platforms span from the bottom to above the surface and provide hard substrate in oxygenated waters overlaying hypoxia. However, little is known about the effects of hypoxia on organisms living at these platforms. We compared diets of Sheepshead Archosargus probatocephalus and Gulf stone crabs Menippe adina during early and late summer at platforms with mild (shoal platforms) and intense hypoxia (seaward platforms) using a combination of stable isotopes (δ13C and δ15N) and gut contents analyses. We hypothesized that diets at seaward platforms would shift from bottom to platform‐dwelling prey during late summer when hypoxia had fully developed and decimated benthic communities, but no such effect was found. Barnacles (Balanidae) were the most common prey found in Sheepshead guts for all samples collected (64.1% of gut contents). Sheepshead gut contents showed high predation of mantis shrimp (Squillidae; 35.2% of gut contents) soon after the formation of hypoxia at seaward platforms, likely reflecting Sheepshead exploiting benthic prey that were vulnerable because of hypoxia. These findings indicate that hypoxia affected the behavior of mantis shrimp in a way that altered their availability for Sheepshead associated with platforms. Comparisons of Sheepshead and stone crab isotope values (δ13C and δ15N) to those of their potential prey and basal sources suggested a high dependence on barnacles (proxy for water column particulate organic matter) and filamentous algae throughout the summer. However, comparisons of isotope values in early and late summer indicated no apparent effect of hypoxia. High contributions of barnacles to Sheepshead diet showed that platform‐dwelling prey were an important resource for Sheepshead.

Quantity and biochemical composition of particulate organic matter in a highly trawled area (Thermaikos Gulf, Eastern Mediterranean Sea)

Bottom trawling represents nowadays one of the most severe anthropogenic disturbances at sea, and determines large impacts on benthic communities and processes. Bottom trawling determines also local sediment resuspension and the effects of the injection of large amounts of surface sediments into the water column have been repeatedly investigated. Few studies have assessed the consequences of sediment resuspension caused by bottom trawling on the quantity, biochemical composition and bioavailability of suspended organic particles and how these eventually rival those exerted by natural storms. To provide insights on this poorly addressed issue, we investigated concentrations and biochemical composition of total and enzymatically digestible pools of particulate organic matter (POM) in the Thermaikos Gulf (Mediterranean Sea) under calm sea conditions, during intensive trawling activities, and after a severe storm. We show here that sediment resuspension caused by trawling can cause large effects on POM quantity, biochemical composition and bioavailability. Both during trawling and after the storm, the relative importance of the carbohydrate pools increased (in the upper water column) and the total lipid concentrations decreased (in the intermediate and bottom layers) when compared to values measured during calm conditions. These results would suggest that bottom trawling could inject in the upper water column POM pools more refractory in nature (<em>e.g</em>., carbohydrates) than those present in calm or after-storm conditions. By contrast, we show also that the bioavailable fraction of biopolymeric C increased significantly during trawling in the upper water column of the shallowest stations and in the bottom water column layer of the deepest ones. These results provide evidence that bottom trawling can influence the overall trophic status of coastal waters, exerting effects similar or stronger than those caused by natural storms, though of variable amplitude depending on the water depth. Since bottom trawling is carried out worldwide and natural storms at sea can be frequent and intense, we claim for the need of assessing new adapting management strategies of bottom trawling in order to mitigate the synergistic impacts of anthropogenic and natural sediment resuspension on coastal biogeochemical cycles.

Elevated 15N/14N in particulate organic matter, zooplankton, and diatom frustule-bound nitrogen in the ice-covered water column of the Bering Sea eastern shelf

We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ15NDB), and zooplankton from water column material collected with net tows across the eastern Bering Sea shelf in late winter of 2007 and 2008, to investigate the N dynamics of primary and secondary production in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1–14.7‰; frustule-bound N: 4.9–20.7‰; zooplankton: 6.4–18.0‰), with POM δ15N reaching ~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28‰ and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic–pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.

An automated purification method for archaeal and bacterial tetraethers in soils and sediments

Isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) are archaeal and bacterial polar lipids increasingly used as environmental biomarkers, and are studied in a wide range of settings: lacustrine and oceanic sediments, water column particulate organic matter, soils, peats, sedimentary rocks and extracts from archaeal and bacterial cultures. In paleoclimatology, for example, typical work on a sediment core of several tens of m consists of several hundreds to more than a thousand HPLC–MS (high performance liquid chromatography–mass spectrometry) analyses. The measurements therefore require purification steps from total lipid extracts. We propose an automated procedure for obtaining the GDGT core lipid fraction. We first evaluate both the yield and efficiency of the separation using different cartridges. We then compare the results from automated and “classical manual” procedures for a soil and a marine sediment, as well as for a sedimentary paleosequence.

A multiple biomarker approach to tracking the fate of an ice algal bloom to the sea floor

In ice-covered Arctic seas, the ice algal production can be the main input of organic matter to the ecosystem. Pelagic–benthic coupling is thought to be particularly tight in those areas. The increase in ice algal production in Franklin Bay from January/February to April/May 2004 paralleled an increase in benthic oxygen demand. However, sedimentary chlorophyll a, which is usually an indicator of “fresh” organic matter inputs to the sea floor, did not increase. Consequently, it was asked what was the fate of the ice algal phytodetritus arriving at the sea floor? To answer this question, photosynthetic pigments from the sea ice, water column particulate organic matter, and sediment, as well as diatom frustules in the sediment, were studied from January to May 2004. The number of ice diatom cells in the sediment showed an increase in April/May, confirming higher inputs of fresh ice algae to the sediment. Changes in sedimentary pigment profiles in the first 10 cm suggested an increase in bioturbation due to enhanced benthic activities. Finally, the decrease in the ratio of chlorophyll a to phaeophorbide a implied an increase in macrobenthic activity. Benthic macrofauna consumed some of the deposited material and mixed some within the top five cm of sediment. The response of sedimentary pigments to an ice algal input can be studied at different levels and it is only the combination of these studies that will allow an understanding of the overall fate of phytodetritus in the benthic compartment.

Redefining the trophic importance of seagrasses for fauna in tropical Indo-Pacific meadows

Fauna species living in seagrass meadows depend on different food sources, with seagrasses often being marginally important for higher trophic levels. To determine the food web of a mixed-species tropical seagrass meadow in Sulawesi, Indonesia, we analyzed the stable isotope ( δ 13C and δ 15N) signatures of primary producers, particulate organic matter (POM) and fauna species. In addition invertebrates, both infauna and macrobenthic, and fish densities were examined to identify the important species in the meadow. The aims of this study were to identify the main food sources of fauna species by comparing isotopic signatures of different primary producers and fauna, and to estimate qualitatively the importance of seagrass material in the food web. Phytoplankton and water column POM were the most depleted primary food sources for δ 13C (range −23.1 to −19.6‰), but no fauna species depended only on these sources for carbon. Epiphytes and Sargassum sp. had intermediate δ 13C values (−14.2 to −11.9‰). Sea urchins, gastropods and certain fish species were the main species assimilating this material. Seagrasses and sedimentary POM had the least depleted values (−11.5 to −5.7‰). Between the five seagrass species significant differences in δ 13C were measured. The small species Halophila ovalis and Halodule uninervis were most depleted, the largest species Enhalus acoroides was least depleted, while Thalassia hemprichii and Cymodocea rotundata had intermediate values. Fourteen fauna species, accounting for ∼10% of the total fauna density, were shown to assimilate predominantly (>50%) seagrass material, either directly or indirectly by feeding on seagrass consumers. These species ranged from amphipods up to the benthic top predator Taeniura lymma. Besides these species, about half of the 55 fauna species analyzed had δ 13C values higher than the least depleted non-seagrass source, indicating they depended at least partly for their food on seagrass material. This study shows that seagrass material is consumed by a large number of fauna species and is important for a large portion of the food web in tropical seagrass meadows.

Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)

Suspended particles from surface and bottom waters and surficial sediments from the seabed were collected throughout the Fly River subaqueous delta region during the monsoon season in January 2003. Because of the unusually low river discharge associated with a strong El Niño, water-column salinities were relatively high (10 to 32) throughout most of the delta, with brackish salinities (<10) only measured within the main distributary channel of the Fly River. The concentration and composition of particulate organic matter (POM) in these samples showed distinct spatial differences and marked contrasts between water column and seabed samples. Overall, relatively low concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured in surface (27 ± 50, 0.83 ± 1.2, and 0.05 ± 0.05 mg/L, respectively) and bottom (400 ± 743, 7.1 ± 7.3, and 1.7 ± 7.2 mg/L, respectively) waters throughout the delta. Particles in both surface and bottom waters displayed elevated organic carbon contents (%OC > 4 wt.%), relatively high organic carbon:nitrogen molar ratios (OC:N > 20 mol:mol) and quite depleted stable isotopic compositions of organic carbon (δ 13C OC < −27‰). In contrast, surface sediments in the seabed displayed spatially uniform compositions that were characterized by markedly lower %OC contents (1.1 ± 0.8 wt.%), lower OC:N ratios (17 ± 9 mol:mol) and relatively enriched δ 13C OC compositions (−25.5 ± 1.1‰). The radioisotopic compositions of OC from a selected set of seabed samples (Δ 14C OC of −408 ± 82‰) indicate OM in surface sediments is old ( 14C ages of 2800 to over 6000 years before present). The ratios of organic carbon to mineral surface area exhibited by these sediments are within the typical (mono-layer equivalent) ranges characteristic of shelf sediments and do not reflect the preferential removal of terrigenous OM. Overall, these compositions indicate that while water-column POM appears to be derived mainly from terrigenous vascular plant debris and riverine/estuarine phytoplankton, the source of most of the sedimentary POM is aged, soil OC ultimately derived from C3 vegetation. We speculate that the concentrations and sources of suspended POM in the water column of the Fly River delta region reflect the conditions of low river discharge, low wave energy and neap tides encountered at the time of sampling. In contrast, POM compositions in surface sediments are consistent with the transport and deposition of old, mineral-bound OC most likely eroded from the upland regions of the Fly River watershed, which is characterized by steep slopes, high precipitation and C3 tropical forests and grasslands.

Distributions of UK37 and UK37′ in the surface waters and sediments of the Nordic Seas: Implications for paleoceanography

In this paper we revise the application of the UK37′ and UK37 indices as sea surface temperature (SST) proxies in the Nordic Seas. In the summer of 1999 and 2000 we obtained samples of filtered particulate organic matter (POM) from surface waters (∼6 m depth) of the Nordic Seas. A number of samples were collected from polar waters with up to 80% of sea ice cover. Alkenones were detected in all of the major water masses of the Nordic seas, across a spectrum of SST values from −0.5 to 13°C and SSS values from 29.6 to 35.6 (psu). Concentrations of alkenones were similar in magnitude to those reported previously for the North Atlantic and the Southern Ocean. Values of UK37′ from the new Nordic Seas POM data show no correlation with SST below 8°C. In contrast, below this temperature a linear correlation exists between UK37 and regional SST, supporting previous suggestions that, overall, UK37 may be a more appropriate SST index for the region. It must be noted however that UK37 is calculated using the tetraunsaturated alkenone, and the dominant control on this compound is not yet fully understood. The new data highlight major differences between distributions of UK37 and UK37′ in the water column POM and surficial sediments of the Nordic Seas. We also examine the geographical dependence of the UK37′ versus SST relationship in the region's surficial sediments. Some areas are associated with unreliable SST estimates, whereas in others the UK37′ versus SST relationship falls within the range of a global core top calibration. It is suggested that the breakdown of the UK37′‐SST relationship in some regions is due to ecological and sedimentological factors. The latter relates to the extension of the habitats of alkenone producers in polar waters. The former to the resuspension of sediments and laterally transported alkenone inputs. This suggests that accurate alkenone derived SST estimates in the Nordic seas are geographically constrained.

A novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids

We propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high-performance liquid chromatography/mass spectrometry (HPLC/MS). Analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol, the characteristic membrane lipid of non-thermophilic crenarchaeota, which is especially abundant in the marine and lacustrine environment. Based on these findings, an index was developed, the so-called Branched and Isoprenoid Tetraether (BIT) index, based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol. This BIT index was applied to surface sediments from the Angola Basin, where it was shown to trace the outflow of the Congo River. Furthermore, analyses of particulate organic matter from the North Sea showed relatively higher BIT indices in water column particulate organic matter near large river inputs. A survey of globally distributed marine and lacustrine surface sediments shows that the BIT index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable. The new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 Ma old.

CO2 Sequestration and Fate of Organic Matters within Seagrass (Zostera marina) Ecosystem

The purpose of this study was to clear the fate of organic carbon and nitrogen in seawater and sediment, especially the difference between organic carbon and nitrogen in dissolved and particulate forms and to determine carbon sequestration by seagrass ecosystem. We had carried out four types experiments: the diel variation for 24 hour, and seasonal changes of DOC (Dissolved Organic Carbon)/DON (Dissolved Organic Nitrogen) and POC (Particulate Organic Carbon)/PON (Particulate Organic Nitrogen) in outside and inside of seagrass communities, and the enclosed chamber experiment for evaluation of release of organic matters from seagrass exuadation and the sediment, decomposition experiments in laboratory and in situ. POM (Particulate Organic Matter) of seagrass leaf litter is resistance to the biological degradation under 25°C. (The rate constant of decomposition is 0.0083 d -1 , namely remineralization time is about 120 days.) While POM in water column shows POC is more rapidly consumed to PON, and POC/PON ratio decresed with time of decomposition. DOM in water column shows DON is more rapidly consumed to DOC and DOC/DON ratio increased with time of decomposition. This suggests that rate of solubilization of DON from PON may dominate a rate of remineralization of nutrients in seagrass communities. We developed a kinetic model of the seagrass decomposition process (SDP) changes in POC/PON ratio with time as well as phytoplankton decomposition model (Fujii et al., 2002). The SDP model was adapted to the results of three kinds of decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by more than a factor of three. The apparent zeroth-order decrease in POC and the constant PON in the first labile decomposition process are well expressed by the SD model, and consequently, the calculated POC/PON ratio shows good agreement with the experimental result. A realistic model for CO 2 sequestration was also developted using net organic production (net organic production=gross organic production-respiration), sedimentation inside seagrass bed and sedimentation at the open ocean (=export flux). We calculated net sequestration of carbon into deep water after 100 years under two different sinking velocities which are for 6 m/d and 24 m/d. These results showed that 120 t-C for 6 m/d and 150 t-C for 24 m/d during 100 years were accumulated into deepwater, suggesting that 1.2 to 1.5 t-C/year for 31 t-C of gross production of seagrass/km 2 are accumulating into deep water. Seagrass ecosystem is very effective as one of the CO 2 sequestration.