Porewater Nutrient Concentrations Research Articles

Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments

We measured porewater profiles of inorganic (NH + ,N O − (+NO − ), PO 3− 4 (here- after referred to as DIP)) and organic (DON, DOP) nutrients in seagrass-vegetated sediments at two sites in a shallow bay in Bermuda within close proximity (200 m) but subject to different nutrient loading. At both sites, total dissolved and inorganic nutrient concentrations were usually 1-2 orders of magnitude higher in the sediments than in the water column, with the exception of NO − . Organic N and P were significant components of the total dissolved nutrient pools both in the sediment porewater and in the overlying water column (up to 75% for DON and 40% for DOP), and may be important in meeting plant nutrient demands. We used two approaches to examine how well porewater nutrient concentrations reflected the relative availabilities of N and P for seagrasses: (1) a simple stoichiometric nutrient regeneration model based on the N:P ratio of decomposing organic matter and porewater NH + concentrations to predict porewater DIP, and (2) fitting of the porewater profiles to estimate rates of net nutrient production (or consumption), which reflects the balance between nutrient sources and sinks in the rhizosphere. The stoichiometric model indicated that sediment porewaters were depleted in P relative to N in the low-nutrient outer bay site, and enriched in P relative to N in the higher- nutrient inner bay site. These results are consistent with the mechanism of carbonate sediments in oligotrophic tropical environments being a strong sink for dissolved inorganic P and our previous work suggesting that nutrient enrichment causes P to become disproportionately more available than N. Net nutrient production rates of porewater P at both sites and N at the inner bay site were low (typically < 2%) relative to the nutrient demands of the seagrasses. The implications of the profile interpretation are two-fold: (1) the low rates of net nutrient production indicate diffusive losses from the root zone were insignificant and that nutrient turnover rates were high, except in the P-limited outer bay where N accumulated in sediment porewaters; and (2) because standing stock nutrient concentrations often represent a small fraction of the total nutrients cycled in the sediments, they are in many cases a poor indicator of nutrient availability. Based on our estimates of losses from the root zone, decomposition, and plant uptake we have constructed a rough budget for the cycling of P and N at our two sites.

Close and delayed benthic–pelagic coupling in coastal ecosystems: the role of physical constraints

We aimed to analyse the temporal scales of the variability of benthic ammonium flux using data from an estuarine bay (Alfacs Bay, N. W. Mediterranean). Several nitrogen (N) pools in the sediment, their reactivities and their associated fluxes were estimated. Decomposition of labile buried N (4.5 mol N m-2) was found to cause an ammonium flux of 0.1 mmol N m-2 d -1, referred to as background flux. The fluxes measured from bell-jar incubations were usually higher, between 2 and 6 mmol N m-2 d -1, which reflects mineralization of recent sedimentation. A typical sedimentation event was found to account for 25 – 75 mmol m-2 of freshly settled N, which should bring on an ammonium flux of about 1.7–5.0 mmol N m-2 d -1, referred to as fast flux. The concordance between measured and computed benthic fluxes is associated with the coupling of benthic fluxes to production and sedimentation. Close benthic–pelagic coupling was observed in winter and early spring, while a delayed flux response to sedimentation, with transient variations of pore water ammonium profiles, showing surface peaks and decreasing concentrations with depth, was found in autumn. Structures, such as viscous layers, which develop over the sediment–water interface, were found to be essential to the regulation of benthic processes and to lead to transient variations of pore water nutrient concentrations and associated fluxes. The temporal scales of the benthic flux response to sedimentation were discussed in terms of the physical structures involved in decomposition (the bulk sediment, the viscous layers or the fresh settled layer), the processes controlling kinetics and diffusion laws. Several scenarios for the benthic- pelagic coupling in Alfacs Bay, in which local (estuarine circulation) and climatic components combine to yield the variability observed, were examined.

Sediments as a source of nutrients to hypereutrophic marshes of Point Pelee, Ontario, Canada

Elevated concentrations of nutrients, resulting in prolific algal growth, were reported in some open-water ponds in one of the finest surviving wetlands on the Lower Great Lakes, Point Pelee National Park, Ontario, Canada. A study was undertaken to identify the sources of nutrients to the most impacted pond in the complex, Sanctuary Pond. Sediment chemistry and chemistry of interstitial water were examined to determine the role of benthic sediments in nutrient dynamics. The results indicate high concentrations of dissolved nutrients (about 4 mg/l of P and over 20 mg/l of N) in sediment porewater, a consequence of organic matter decomposition. No spatial differences were observed between porewater profiles, nor in the levels of dissolved constituents. Likewise, sediment P concentrations were similar at both investigated sites. Thermodynamic calculations, carried out to determine mineral equilibria of phosphate minerals potentially controlling the concentrations of dissolved P in sediment porewater, suggest that P generated from organic matter decomposition should be reacting with Fe, Ca and Al to form authigenic mineral phases vivianite, hydroxyapatite and variscite. The high levels of nutrients in porewater, the lack of spatial differences in sediment and porewater nutrient concentrations and the absence of known external nutrients sources suggest that at the Sanctuary Pond internal regeneration of nutrients from sediments is responsible for the hypereutrophic conditions in the pond.

Nutrient fluxes on an intertidal mudflat in Marennes-Oléron Bay, and influence of the emersion period

Fluxes of nutrients (NH4+, NO3-, PO43- and Si(OH)4) were studied on an intertidal mudflat in Marennes-Oleron Bay, France, at two different seasons and at different times of the emersion period. Fluxes through the sediment-water interface were both calculated from vertical profiles of nutrient concentration in pore-water (diffusive fluxes, JD) and measured in light and dark benthic mini-chambers (measured fluxes, J0). Results indicate that ammonia was mainly released in summer while nitrate was mainly taken up in late winter. This uptake from the overlying water was probably due to the coupling of nitrification-denitrification within the sediment. The J0/JD ratio further indicates that bioturbation likely enhanced ammonia release in summer. Concerning phosphate, the comparison of diffusive and measured fluxes suggests that PO43- could be assimilated by the biofilm in winter while it was released in summer at a high rate due to both bioturbation and desorption because of the relative anoxic conditions in summer. Silica was always released by the sediment, but at a higher rate in summer. Statistically significant differences in measured fluxes were detected in dark chambers at different times of low tide, thus suggesting a short-term variability of fluxes. Microphytobenthos preferred ammonia to nitrate, but assimilated nitrate when ammonia was not available. It also turned out that benthic cells could be limited in nitrogen during low tide in late winter. In summer, ammonia was not limiting and microphytobenthic activity significantly decreased the measured flux of NH4+ in the middle of low tide when its photosynthetic capacity was highest.

The dynamics of benthic nutrient pools and fluxes in tropical mangrove forests

Variations in benthic nutrient pools and rates of dissolved nutrient exchange between the forest floor and tidal waters were examined over a 5-yr period in mixed Rhizophora forests lining Coral Creek on Hinchinbrook Island in Queensland, Australia. Seasonal and spatial changes in redox status, porewater and solid-phase nutrients, and in exchange rates were not consistent and did not correlate with temperature. Below-ground roots, on average, accounted for ∼79%, 37% and 26% of bulk sediment TOC, total N and total P pools, respectively. Porewater nutrient concentrations were dominated by Si(OH) 4 + and DON with consistently low levels of NO 2 - + NO 3 - . At most sampling periods, porewater NH 4 + and PO 4 3- concentrations were higher in creek bank sediments than in mangrove sediments indicating uptake by trees. These sediments have low adsorption capacity (K = 0.17-0.47) for NH 4 + , but a moderate capacity (K = 0.8-4.8) for PO 4 3- adsorption. Most measured benthic fluxes of dissolved nitrogen and phosphorus showed uptake by sediments, prop roots and timber lying on the forest floor. Relative (per ha) estimates indicate that low-intertidal Rhizophora forests import ∼2220 mmol N ha -1 d -1 and ∼496 mmol P ha -1 d -1 , with sediments accounting for nearly all uptake while Si is exported (∼2475 mmol ha -1 d -1 . Mid-intertidal forests import ∼1385, 93 and 4720 mmol ha -1 d -1 of N, P and Si, respectively ; sediments, prop roots and timber respectively account for 36%, 62% and 2% of the N import. Mid-intertidal sediments account for all net P uptake, but prop roots and sediments account for 60% and 40% of total Si uptake. On an absolute basis, low-intertidal forests (78 ha total area) in Coral Creek import ∼881 kgN yr -1 , 436 kgP yr -1 and export 1963 kgSi yr -1 , and the mid-intertidal forests (338 ha total area) import ∼2392 kgN yr -1 , 356 kgP yr -1 and 16300 kgSi yr -1 . The sum of these estimates equates to ∼95% of the net annual import of total dissolved N and ∼66% of the net annual import of total dissolved P into the Coral Creek tidal basin from adjacent coastal waters. By difference, ∼14337 kgSi yr -1 is imported into the system. This indicates that mangrove forests are a very efficient sink of dissolved nitrogen, phosphorus and silicon in this tidally-driven coastal ecosystem. This import may be driven by the consistently high rates of microbial and plant growth and productivity within the forests.

Diagenetic processes and nutrient fluxes at the sediment-water interface, northern Adriatic Sea, Italy

Early diagenetic processes that control pore-water chemistry and nutrient fluxes at the sediment-water interface were studied in late summer 1989 and spring 1990 at four sites influenced by fine sediments of the Po River. Parameters on the solid fraction (grain size and C, N and P) and pore- water profiles of SO42-, NO3-, NH3, alkalinity, PO43-, Fe, Mn, Ca and Mg were determined. Data interpretation was supported by application of kinetic and stoichiometric modelling. Nutrient fluxes were calculated by Fick's first law. In the prodelta station the pore-water profiles showed large depletion in SO42- ions, jointly with the increase with depth of alkalinity, NH3 and PO43- concentrations, thus indicating anaerobic decomposition of organic matter, mainly due to bacterial sulfate reduction. At the other three sites the results were completely different. Nutrient concentrations in pore water were one order of magnitude lower. The peaks of alkalinity, NH3 and PO43- values near the interface and the constant presence of SO42- ions indicate aerobic decomposition of organic matter in the uppermost sediment. Nutrient fluxes showed much higher values in the prodelta station, whereas fluxes in the remaining stations decreased southward. In addition, seasonal variations produced higher summertime fluxes. The clear-cut difference in diagenetic processes as a function of the distance from the main Po River mouth can be explained by the rapid burial of sediments rich in organic matter and inorganic compounds in the prodelta station. At the other stations, the lower sedimentation rate and more efficient bioturbation and resuspension processes cause a prolonged exposure of sediments to aerobic metabolization reactions before burial.

Biogeochemistry of N, P and Si in Baltic Sea sediments:response to a simulated deposition of a spring diatom bloom

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 122:265-276 (1995) - doi:10.3354/meps122265 Biogeochemistry of N, P and Si in Baltic Sea sediments: response to a simulated deposition of a spring diatom bloom Conley DJ, Johnstone RW Experimental studies of intact cores from the Baltic Sea were conducted to determine the response of sediment nutrient recycling processes to varied inputs of organic matter. A 2 mo enrichment experiment was carried out in the laboratory on sediment cores held at 4*C using a flow-through system where overlying waters were continuously replaced at a rate of 1 d-1. The experiments were designed to simulate the deposition of organic matter that occurs during a typical spring diatom bloom (1x) and under enriched conditions with eutrophication at approximately 3 times (3x) a normal spring bloom utilizing added organic matter from a natural phytoplankton assemblage collected in a eutrophic coastal fjord during the spring diatom bloom. Low and constant sediment-water fluxes were observed throughout the duration of the experiment in control cores with no added organic matter. In all cases an immediate response was noted when a single pulsed addition of algal material was added to the sediment surface. Sediment-water fluxes of ammonium (NH4+), and dissolved inorganic phosphate (DIP) increased significantly (ANOVA, p &LT 0.01). For nitrite + nitrate (NO2- + NO3-) and dissolved silicate (DSi) sediment-water fluxes, differences were initially observed; however, only the NO2- + NO3- fluxes were significantly different over time (ANOVA, p &LT 0.01). Fluxes of NO2- + NO3- were into the sediment for 3 to 10 d after addition of organic material, followed by small fluxes out of the sediment. The addition of algal material proportionate to a normal spring bloom (1x) had only a minor effect on porewater nutrient concentrations, whereas the 3x treatment substantially modified both the short- and long-term response of sediments. A greater proportion of anaerobic decomposition products, e.g. NH4+ and DIP, were observed with an expansion of more reducing conditions resulting from the addition of organic matter. The percentage of Si remineralized decreased as the flux of material to the sediment increased. Deposition rates similar to a typical spring bloom did not have long-term effects on the nutrient recycling processes; however, increases in the present level of deposition (as simulated in this study), which are forecasted with further eutrophication in the Baltic Sea, may have a significant impact on nutrient biogeochemical cycles. None Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 122. Publication date: June 15, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

The limited effect of in situ phosphorus and nitrogen additions to seagrass beds on carbonate and terrigenous sediments in South Sulawesi, Indonesia

Seagrass response to in situ sediment nutrient enrichment with slow-release fertilizers was studied (4 to 5 months after fertilization) in three tropical seagrass beds in South Sulawesi, Indonesia. Nitrogen-addition and P-addition had no significant effects on seagrass biomass, shoot density, and leaf production in mixed seagrass beds in a terrigenous sandy bay and a carbonate sedimentary reef flat environment, simultaneously fertilized in 1990. An additional experiment using a slightly adapted methodology (1991) at another carbonate site, characterized by a more homogeneously vegetated, and largely monospecific seagrass meadow of Thalassia hemprichii, again showed no significant response of seagrass growth, biomass, and shoot density to fertilization. C and N contents of plant tissue, however, had increased significantly ( p<0.05) to 12–25% above the controls in response to N-addition at this locality. The relatively high ambient porewater nutrient concentrations (about 10 μM PO 4, about 100 μM NH 4) and rather low C:N:P ratios (565:18:1) in seagrass tissue, indicate that nutrient supply meets seagrass demand and provide an explanation for the lack of response to the manipulations. The relatively high porewater phosphate concentrations in carbonate sediments are attributed to the relatively coarse particle-size distribution of these sediments in the study area, which limits their adsorption capacity and prevents P-limitation of seagrass growth, which is generally considered to be characteristic of tropical carbonate-rich environments.

Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment

The primary source of nutrients for seagrass growth is considered to be the sediment porewater. Porewater nutrient concentrations were measured in 18 seagrass beds across Florida Bay, USA, a shallow, seagrass-dominated subtropical embayment, during the summers of 1987 and 1988. Concentrations of nutrients in porewater varied widely, with median values of 0.34 μM for soluble reactive phosphorus (SRP) and 78.6 μM for NH 4 + . SRP and NH 4 + concentrations were positively correlated. Due to spatial heterogeneity, there were no apparent trends with sediment depth (down to 40 cm) in the porewater nutrient concentrations. The SRP concentration of the porewater was highest in areas supporting Halodule wrightii, intermediate in areas of Thalassia testudinum, and lowest in sediments without seagrasses. There was no similar relationship with NH 4 + . Porewater SRP, but not NH 4 + , was significantly correlated with total seagrass standing crop. Elemental content (both N and P) of green leaves of T. testudinum was a function of the concentration of the nutrients in the porewater. Standing crop of T. testudinum was correlated with phosphorus content, but not with N content, of the seagrass leaves. The results support the hypothesis that sediment porewaters are the most important source of nutrients for seagrass growth. In this subtropical carbonate environment, the availability of phosphorus in the porewater limits the development, and controls the species composition, of seagrass beds.

Nutrient concentrations in pore waters of intertidal sediments in a coastal lagoon: Patchiness and temporal variations

We generated ∼ 6 h time series data by means of in-situ samplings to study the variations in the properties of pore waters in a low salt marsh with tall Spartina foliosa. The location was a coastal lagoon on the northwestern coast of Baja California, Mexico. Salinity (S‰), reactive phosphate (PO 4), and ammonia (NH 3) displayed great temporal and spatial variation and irregular variations with depth. The most marked variation in salinity, from 32.5 to 44‰ in 3 h, was at 10 cm depth. Low salinity values were attributed to underground freshwater input, and high values to excess evaporation from the surface. Within a line ∼ 8 m parallel to the water front, at 30 cm depth, we obtained PO 4 variation with time from as low as 20–29 μM to as high as 103 μM at two locations, and NH 3 from 19–55 μM to 220–1970 μM. In some cases, with samplers only 7 cm apart, for the same depth and sampling time, salinity differed by as much as 6‰; PO 4 concentrations differed by a factor of five. Patchiness, mostly created by biota, and percolation with tides, enhanced by movements through plants' roots and animals' burrows, are mainly responsible for these changes in the properties of pore waters.

Benthic nutrient regeneration and denitrification on the Washington continental shelf

Benthic nutrient regeneration on the Washington continental shelf was investigated using vertical profiles of pore-water nutrient concentrations and whole sediment sulfate reduction rates. In August, carbon oxidation rates by sulfate reduction frequently exceeded those calculated from previously reported oxygen consumption rates. Total carbon oxidation rates averaged 18.9 pmol CO 2 cm −2 s −1, and using the 106 C: 16 N atomic ratio, 2.86 pmol N cm −2 s −1 would have been regenerated. Nutrient fluxes were calculated from pore-water profiles using diagenetic equations containing terms for vertical molecular diffusion, sedimentation, adsorption and macrobenthic irrigation which was estimated by modeling published radon distributions. In August, the calculated nitrate influx was 0.81 pmol N cm −2 s −1. The outward ammonium flux of 0.53 pmol N cm −2 s −1, calculated from pore-water ammonium profiles, was only 19% of the N regeneration rate. The missing ammonium could be accounted for by coupled nitrification and denitrification (1.16 pmol N cm −2 s −1) and by consumption of organic matter with high C:N ratios (9.8 by atoms). A comparison of annual regeneration and burial rates indicated that 84% of the deposited carbon is regenerated. For nitrogen, the annual outward ammonium flux was about half of the sum of the organic N burial rate and nitrate influx suggesting that these shelf sediments were a net sink for nitrogen. If the annual nitrate influx (0.57 pmol NO 3 − cm −2 s −1) to this shelf approximated that in other shelves, the global denitrification rate in continental shelf sediments wound be 69 Tg N y −1, a value similar to the oceanic water-column denitrifications rate.

Examination of the connection between the patchiness of benthic nutrient efflux and epiphytobenthos patchiness on intertidal flats

Intertidal epiphytobenthos activity (expressed in mmol oxygen evolution·m −2·h −1) and nutrient efflux out of the sediment (computed from pore water nutrient concentration profiles and expressed in μmol·m −2·h −1) were studied at the same places. Increased ammonia or phosphate efflux stimulated epiphytobenthos activity, and an optimum of epiphtobethos activity was observed when the molar N:P ratio in the efflux agreed with the N:P ratio in diatom biomass. It is concluded that epiphytobenthos patchiness depends on the spatial distribution of nutrient efflux and on the N:P ratio in the efflux. The conditions of nutrient efflux were further studied by fertilizing the sediment with urea and phosphate. Phosphate could be traced in the pore water over two months; ammonia over more than two months. The formation of a diatom bed at the fertilized are was observed once, but efforts to reproduce it failed, indicating the importance of other growth-determining conditions. Nutrient efflux was prevented by a plastic foil to stimulate a possible efflux inhibition by diatom mats. Concentrations below the foil were examined and compared with results from an untreated area. An increase of nutrient concentrations in the pore water was observed.

Effects of a decomposing drift algal mat on sediment pore water nutrient concentrations in a Florida seagrass bed

Effects of a decomposing drift algal mat on sediment pore water nutrient concentrations in a Florida seagrass bed

In situ Measurements of Nutrient and Oxygen Fluxes in a Coastal Marine Benthic Community

Benthic nutrient and oxygen fluxes were measured during summer in the nearshore zone of the Georgia Bight, USA. Ammonium, nitrite, nitrate and phosphate were released by the sediments to the overlying water, while dissolved organic nitrogen was taken up by sediments. Oxygen uptake was equivalent to 1.1 g Cm-* d-'. Based on other data available for this area the benthos accounted for the remineralization of approximately 55 % of net primary production and 25 % of total community (benthic and pelagic) metabolism. To balance a simple carbon budget for the nearshore region during summer an outside source of organic matter, most likely originating in adjacent estuaries, is required. The benthos supplied 16 % of the N and 53 % of the P required by phytoplankton. Pore water nutrient concentrations suggested substantial denitrification (estimated at 4.9 mg Nm-2 d by mass balance) in sediments which may have caused the low N : P benthic release ratios we observed. The results support the hypothesis that benthic processes in shallow marine areas control the availability of N and P for water column production.

Nutrient regeneration in sediments off Cap Blanc, Spanish Sahara

Abstract Fluxes of nutrients out of the bottom sediments were measured off Cap Blanc, Spanish Sahara, by in situ incubation of the bottom under bell jars. The relationship of Si/N/P regenerated averaged 11/8/1. The average total nitrogen flux out of the sediments (410 μg-atoms M n −2 h −1 ) would account for approximately 30 to 40% of the nitrogen presumed to be required for photosynthesis. Pore water nutrient concentrations were high over the 60 nautical miles square area investigated during the JOINT-I expedition, but tended to decrease offshore in deep water, and it can be presumed that sediment to bottom water fluxes of nutrients followed a similar trend.