High Particulate Organic Matter Research Articles

Impact of stress and disturbance factors on the phytoplankton communities in Northeastern Brazil reservoir

The ecology of phytoplankton reservoir communities plays a pivotal role in their management and in the development of inland fisheries in the water scarce semi-arid regions of Rio Grande do Norte State, Brazil. The hydrological conditions characterizing these reservoirs include infrequent rainfall and high evaporative loses. The phytoplankton assemblages in tropical climate are structured primarily on dry and wet cycles, not the annual temperature and light regime. The purpose of the current study was to investigate the ecological aspects of phytoplankton communities from three reservoirs during the period of July 2003–June 2004, which include atypical summer rainfall and reservoir overflow. The environmental data broadly divided into two categories, stress factors and disturbance factors. The stress factors deals with the impact of high particulate organic matter and low dissolved oxygen concentrations and disturbance factors, linked to water level fluctuation through flushing and reservoir drawdown, and these events are associated with the phytoplankton assemblages. Results indicated that the three distinct hydroperiods determined the structure of phytoplankton and chlorophyll levels and limited the presence and relative abundance of cyanobacterial species. Phytoplankton succession accompanied changes in clear and turbid water phase represented by the alternate dominance between diatoms and chlorophytes. Inundation and complete filling of reservoir stimulated “s” and “r” strategists species (<20 um) of unicellular and some colonial members of chlorophytes and cryptomonads in highly turbid water environment, and “c” strategist species in rest of the wet period following disturbance gradients. Statistical analysis elicited a significant relationship between particulate organic matter and relative abundance of smaller chlorophytes, which was deemed as stress factor. The flushing and reservoir drawdown indicate disturbance gradient and as a consequence high diversity. Furthermore, a substantial reduction in chlorophyll levels was registered in turbid water phase is related to the reduction of light penetration and re-suspension of sediments. Considering these results, it can be suggested that the well-managed reservoir drawdown can possibly maintain an environment free of eutrophication and cyanobacterial dominance.

Mechanical Grinding for Particulate Organic‐Matter Analysis

Particulate organic matter (POM) is recognized as a valuable measure of labile soil organic matter. The method usually requires hand‐sieving through a 2‐mm sieve. Hand‐sieving has not been widely adopted by soil testing laboratories, where samples are generally mechanically ground. Composites of 20–25 cores (2×15 cm) were collected from a long‐term crop rotation×fertility treatment study in central Pennsylvania and from 11 central and eastern Pennsylvania farms. Subsamples of each sample were hand‐sieved through a 2‐mm sieve or mechanically ground and analyzed for POM carbon (C). The POM‐C of hand‐sieved and ground samples were similar except for one site with large POM concentrations; at this site, grinding generated lower POM‐C yields than hand‐sieving. Grinding soil samples may be an effective means to increase the availability of POM analysis, but additional work is necessary to determine if this method decreases POM yield, particularly in high POM samples.

Dissolved organic matter along the eutrophication gradient of the Darβ–Zingst Bodden Chain, Southern Baltic Sea: I. Chemical characterisation and composition

The Darβ–Zingst Bodden Chain (DZBC) is a shallow tideless estuary of 4 separate basins containing extremely high contents of particulate organic matter (POM) because of an eutrophication for more than 30 years. Therefore the open question to be addressed in this study was whether and how high POM is reflected in amounts and composition of dissolved organic matter (DOM). The pelagial was sampled at different stations along the eutrophication gradient during the growing season 1996, as well as in winter 1995/96 and 1996/97 to assess seasonal variations. The chemically utilisable compounds (uDOM) like dissolved free amino acids (DFAA) and dissolved free carbohydrates (DFCHO), as well as the corresponding polymer fractions dissolved combined amino acids (DCAA) and dissolved combined carbohydrates (DCCHO) were analysed together with the total dissolved organic carbon (DOC) and total dissolved organic nitrogen (DON). The concentrations of these fractions were used to calculate the respective percentage contribution of the DOC and DON and to compare with the C:N ratio in the dissolved pool. In the DZBC very high absolute amounts of DOM components of up to 30 μmol C l − 1 for DFAA, 288 μmol C l − 1 for DCAA, 67 μmol C l − 1 for DFCHO, 196 μmol C l − 1 for DCCHO were determined. On average, proportions of 0.8% DFAA, 3.6% DFCHO, 6.5% DCAA and 12.6% DCCHO of DOC were measured in the growing season of 1996. Consequently, at least 23.5% of DOC in the DZBC were classified as readily utilisable. The concentrations of these bio-available compounds fluctuated strongly over the course of the year, but the dissolved monomers more than the corresponding polymers. With the exception of DFAA at significantly higher concentrations in winter, a clear seasonality in DOM compounds was absent. In spite of the high amounts of uDOM, most likely being able to sustain many microheterotrophs, the quality of the total DOM has to be assessed as recalcitrant due to an average molar C:N ratio of ca. 19. This fact makes the Darβ–Zingst Bodden Chain quite unique concerning persistent high amounts of utilisable and refractory organic matter. So far, it was not possible to prove whether the high DOM originates directly from primary production, is mostly the result of microbial degradation in the pelagial or stems from anaerobic degradation in the sediment. However, the high DOM concentrations, its element composition and the accumulation of glycine and serine in the dissolved amino acid pool suggest that the degradation of DOM was advanced. Such analyses seem to be an additional tool to describe the ecological state of highly polluted waters and estimate the origin and fate of organic matter.

Impact of land clearing methods and cropping systems on labile soil C and N pools in the humid zone Forest of Nigeria

Abstract Labile soil C and N play vital roles in soil–plant nutrient dynamics, especially in the low input cropping system and are vulnerable to perturbation. Surface (0–0.15 m) soils from three land clearing methods (slash and burn, bulldozed non-windrowed and bulldozed windrowed) and each with two cropping systems (5-and 4-year cropping/2-year cassava fallow) were collected in the humid forest ecosystem of Nigeria. The soils were analysed for total C and N, microbial biomass C and N (SMB C and N), particulate organic matter C and N (POM C and N), water-soluble C, potentially mineralizable N (PMN) and mineral N. The size of the labile C and N and their relative contributions to the organic C and total N differed significantly among land clearing methods, irrespective of the cropping system. Soils under slash and burn had a significantly ( p > 0.05) higher particulate organic matter C, N (10.80 and 0.16 g kg −1 , respectively) and microbial biomass C and N (1.07 and 0.12 g kg −1 ) compared to the bulldozed windrow, regardless of the cropping system. Four years cropping/2-year cassava fallow resulted in a significant higher labile C and N, relative to 5-year cropped plots across the land clearing methods. Effect of the treatments on the concentration of PMN and mineral N mirrored the SMB N and POM N. However, the quantity of most of the labile C and N pool and crop yield obtained from the slash and burn and bulldozed non-windrowed treatment did not differ significantly. Hence, bulldozed non-windrowed clearing could be a viable alternative to slash and burn in the case of large-scale farming in ensuring reduced losses of soil organic matter and nutrient during land clearing in the humid tropics.

Internal tide—shelf topography interactions as a forcing factor governing the large-scale distribution and burial fluxes of particulate organic matter (POM) in the Benguela upwelling system

This study investigates the role of internal tides in driving the sedimentation and re-suspension of biogenic POM on the Namibian shelf and give rise to stable 500–800 km long shore alternating bands of high and low POM concentrations. Temperature time series data (September 2000–March 2001) from the benthic boundary layer at three sites are used to hypothesise that the dominant forcing mechanism are internal tides and their interaction with the shelf break zones. Vertical displacements of the temperature structure by 100–150 m at the outer shelf break (depth 450 m) are shown to occur through out the 6-month time series. In contrast at non-shelf break sites the vertical displacements of temperature are negligible. The shear–stresses predicted landward of the shelf break zone from 100–150 m vertical displacements of the temperature structure are significantly higher (>0.1 Pa) than the critical shear–stresses which govern the re-suspension of biogenic particles and fine sediments (0.05–0.1 Pa). Short-term ADCP data was used to show that critical shear–stress distribution at the different POM areas is consistent with the predicted net accumulation and net erosional zones of POM across the Namibian shelf. This study hypothesises that the barotropic–baroclinic tidal coupling governs vertical particle flux dynamics whereas Ekman and inertial flows are thought to govern the horizontal advection scales that result in the observed POM distribution. The importance of this improved dynamical understanding has implications for both carbon burial efficiency as well as the variability in the suitability of benthic fisheries habitats.

Use of Keeling plots to determine sources of dissolved organic carbon in nearshore and open ocean systems

We apply a Keeling mixing model commonly used in atmospheric sciences to oceanic dissolved organic carbon (DOC) 14C and concentrations to demonstrate that DOC distribution across spatial transects or depth profiles can be modeled as two‐component mixtures; one component consists of an old refractory pool to which a newly synthesized component pool is added. We then use the Keeling approach to determine the Δ14C signature of excess DOC added to background concentrations in nearshore and open ocean systems. Our analysis indicates enrichment in 14C of excess DOC in coastal waters relative to offshore waters, particularly in spring when river discharge increases. These findings are consistent with the export of 14C‐enriched terrestrial DOC to the ocean and inconsistent with the idea of selective degradation of young terrestrial DOC in estuaries, resulting in export of old refractory component to the ocean. In the middle Atlantic Bight region, during the spring, excess DOC has a 14C signature similar to that of DOC in the major rivers, whereas during the peak primary productivity of the summer season, excess DOC has a signature more similar to that of dissolved inorganic carbon. Variability in the Δ14C of excess DOC in the open ocean was lower than that of the coastal ocean. Minima in Δ14C values of excess DOC in the open ocean were measured during periods of high particulate organic matter (POC) flux, indicating that POC can interact with DOC and selectively remove 14C‐enriched components from the surface ocean. The Keeling plot provides a powerful approach for determining the isotopic composition of excess DOC from an entire DOC profile or transect.

Biodiversity and trophic structure of nematode assemblages in seagrass systems: evidence for a coupling with changes in food availability

Nematode assemblage composition, trophic structure and biodiversity were followed over an annual cycle in a Posidonia oceanica bed of the NW Mediterranean to test the response to temporally changing food availability (sediment organic matter, bacterial and microphytobenthic biomass). The sediment–water interface of the seagrass meadow was characterised by high particulate organic matter concentrations. Also, seagrass sediments had high organic matter content, chloropigment concentrations and bacterial biomass. All organic matter components (i.e. carbohydrates, proteins and lipids) changed temporally, with higher concentrations in winter and spring and lower concentrations in summer; however, overall, large amounts of organic compounds were potentially available for (seagrass) benthic consumers throughout the year. Nematode assemblage in the P. oceanica bed maintained a high genus number (88 genera), and a trophic structure tightly coupled with the composition of the potential food sources. In agreement with the relevance of microphytobenthos, epigrowth-feeder nematodes (2A) represented the dominant trophic guild. The biomass of predator nematodes (2B) was significantly correlated with the biomass of other nematodes. Non-selective deposit feeders (1B) displayed a close relationship with the concentration of proteins, carbohydrates and biopolymeric carbon (i.e. labile organic detritus). Conversely to what was expected, epigrowth feeders were not correlated with chloropigment concentrations, but showed a significant relationship with the number of dividing bacteria, possibly suggesting a trophic plasticity of the 2A feeding guild. The coupling between temporal changes in food sources and temporal variability of the nematode trophic structure was highlighted by a CANOCO analysis, which allowed us to identify and associate, at each sampling time, nematode genera and their feeding habits, with specific environmental variables and food indicators. Furthermore, species diversity (H′) and evenness (J) calculated on nematodes identified to the genus level displayed evident temporal changes, also reflected by the index of trophic diversity (ITD). Both structural and functional diversity were coupled with high concentration and highly heterogeneous composition of the food sources (including organic detritus, microphytobenthic algae and bacteria) potentially available to nematodes. These data suggest that temporal changes in quantity and quality of food sources do not only influence nematode dynamics and trophic composition, but also influence nematode structural and functional diversity.

Comparative models of mussel bioenergetics and their validation at field culture sites

Models of bioenergetics of the blue mussel (Mytilus edulis) were constructed to simulate growth in suspended culture at field sites in Upper South Cove (Nova Scotia) and Marennes-Oléron Bay (France). Two models with contrasting complexity of feeding behaviour were used to test whether simple empirical formulations of the energy budget (statistical model) were adequate to simulate growth compared to a more fully mechanistic model. In the statistical model, ingestion was related to a single food source (particulate organic matter, POM) and absorption efficiency via laboratory feeding studies from the literature. In the mechanistic model, filtration, particle rejection and selection, and absorption efficiency were related to phytoplankton, detrital food and total particulate load. Respiration terms were identical between models to facilitate comparisons between feeding behaviour. Measured tissue trajectories of cultured mussels were used to groundtruth model predictions. The Nova Scotia grow-out site was characterized by low turbidity and particulate organic matter (POM), and seasonally high chlorophyll, while the Marennes-Oléron site had high turbidity (up to 180 mg l −1) and POM, but similar chlorophyll to the Nova Scotia site. Results of the simulations indicated that for Nova Scotia, the statistical model provided a realistic growth trajectory, whereas the mechanistic model underpredicted growth during the first half of the year due to low phytoplankton biomass. Use of fluorometer records rather than water samples as a measure of chlorophyll improved the accuracy of the growth prediction. For Marennes-Oléron, the statistical model was unsuccessful since its POM-ingestion relationship did not allow for limitation by turbidity. The mechanistic model applied to this site was reasonable in predicting a growth trajectory, but it was sensitive to the functional curve of particle rejection as well as the maximum daily ingestion. Simulations at both sites were sensitive to the POC content of POM and the absorption efficiency of detritus. These simulations demonstrate that simple formulations of food and feeding will suffice in predicting growth where seston values are not extreme (e.g., high turbidity). Although the separation of suspended food source into POM and chlorophyll may be convenient, it is not clear how organic particle selection occurs, considering the aggregated nature of particles in the field, and the close association of POM with silt grains. The sensitivity of models to both food quality and feeding formulation requires better knowledge of variance in suspended food sources and the ability of bivalves to deal with natural particle mixtures.

Methanogenesis in Arizona, USA dryland streams

Methanogenesis was studied in five streams of central and southern Arizona by examining the distribution of methane in interstitial water and evasion of methane in three subsystems (hyporheic, parafluvial and bank sediments). In Sycamore Creek, the primary study site (studied during summer and early autumn), methane content of interstitial water exhibited a distinct spatial pattern. In hyporheic (sediments beneath the wetted channel) and parfluvial zones (active channel sediments lateral to the wetted channel), which were well oxygenated due to high hydrologic exchange with the surface stream and had little particulate organic matter (POM), interstitial methane concentration averaged only 0.03 mgCH4-C/L. Bank sediments (interface between the active channel and riparian zone), in contrast, which were typically vegetated, had high POM, low hydrologic exchange and concomitantly low dissolved oxygen levels, had interstitial concentration averaging 1.5 mgCH4-C/L. Methane emission from Sycamore Creek, similar to methane concentration, averaged only 3.7 mgCH4-C·m−2·d−1 from hyporheic and parafluvial zones as opposed to 170 mgCH4-C·m−2·d−1 from anoxic bank sediments. Methane in four additional streams sampled (one sampling date during late winter) was low and exhibited little spatial variation most likely due to cooler stream temperatures. Interstitial methane in parafluvial and bank sediments of all four streams ranged from only 0.005 to 0.1 mgCH4-C/L. Similarly methane evasion was also low from these streams varying from 0 to 5.7 mgCH4-C·m−2·d−1. The effects of organic matter and temperature on methanogenesis were further examined by experimentally manipulating POM and temperature in stoppered flasks filled with hyporheic sediments and stream water. Methane production significantly increased with all independent variables. Methane production is greatest in bank sediments that are relatively isolated hydrologically and lowest in hyporheic and parafluvial sediments that are interactive with the surface stream.

Quantity and quality of the particulate organic matter in Concepcion Bay, during the spawning season of the scallop Argopecten circularis (Sowerby, 1835)

In this study we describe the spatial variability of the particulate organic matter (POM), expressed in terms of proteins, carbohydrates and chlorophylls, and the specific abundance of phytoplankton at Concepción Bay during the maximum reproductive peak of Argopecten circularis. The distribution of these variables shows two zones of high concentration of particles: one located in the first “third” of the bay and the other in the upper end. High values of chlorophyll a were due to the nannophytoplankton size class, which is the major contributor to the total phytoplankton abundance. The most abundant species of microphytoplankton were Chaetoceros curvisetus, C. lorenzianus C. teres, C. cinctus and Lithodesmiun undulatum. The phytoplanktonic-POM and non-phytoplanktonic-POM contribution estimated at the surface shows that the highest protein percentage (69.5%) was due to phytoplankton, whereas at 10 m depth it corresponded to non-phytoplankton-POM or detritus. This indicates that a high quality POM in the water column represents a potential source of food for the different development stages of A. circularis.