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1.
In January 1982, sediment microbial N transformations and inorganic N fluxes across the sediment/water interface were studied at nine sites off the South Island West Coast, New Zealand. The sediments showed a great variety in physical, chemical and biological properties. The sediment organic matter had a molar CN ratio of 5.9–10.9, and the total NP ratio was 1.2–4.0. The denitrification capacity in the top 7.5 cm of sediment was 0.1–77.2 mmol N m?2 day?1 and generally declined with increasing sediment depth. The in situ denitrification rate was 0.02–1.84 mmol N m?2 day?1 and highest activities were generally found in surface sediments and at 6–7.5 cm depth. Denitrification accounted for 82–100% of total nitrate reduction. Net N mineralization was indirectly estimated at 0.6–2.4 mmol N m?2 day?1, and the experimental determination of this N transformation gave 0.6–3.2 mmol N m?2 day?1. Denitrification accounted for 3–75% of net N mineralization. The diffusive flux of ammonium and nitrate across the sediment/water interface was 0.1–0.7 and 0.1–0.6 mmol N m?2 day?1, respectively.  相似文献   

2.
A preliminary mass balance for organic carbon in the Fly Delta was constructed to determine the quantity and source of organic matter exported to the adjacent Gulf of Papua and Coral Sea. Total organic carbon input from the river to the delta is 1.7 × 1012 g C yr–1, composed almost equally of DOC and POC. Benthic and pelagic respiration in the delta accounts for 1.0 × 1012 g C yr–1, being a major sink for riverine organic carbon. Benthic flux measurements indicate that one third of all DOC entering the delta is taken up by sediments there. Mangrove forests export >3.0 × 1011 g C yr–1 POC to delta waters, and it appears that this mangrove carbon is exported to the adjacent shelf and deep sea. These results imply that little of the riverine supply of organic carbon reaches the Gulf of Papua, but that mangrove forests in the Fly and other rivers lining the gulf play a major role in river-shelf carbon exchange.  相似文献   

3.
The mangrove, Avicennia marina var. resinifera in a tidally-flooded explosion crater, Tuff Crater, near the southern latitudinal limit of mangroves in New Zealand adopts two distinct growth forms, taller tree-like mangroves up to 4 m tall along the banks of the tidal creek, and low stunted shrub mangroves less than 1 m tall on the mudflats. Twelve trees were felled and on the basis of a biomass/height relationship for the taller trees and a biomass/canopy width relationship for the lower, above-ground biomass (excluding pneumatophores) was estimated. Average above-ground biomass for the taller mangrove was estimated to be 104·1 t ha?1 and for the lower 6·8 t ha?1. While the value for the taller mangroves is similar to figures reported for more complex tropical mangroves, the fact that 94% of the basin is covered by low generally sparse mangroves means that total biomass for the basin is estimated to be 153 t, an average of only 7·6 t ha?1. Litter-fall beneath the taller mangroves is estimated as 7·6±2·5 t ha?1 a?1 and beneath the lower mangroves 3·3±0·5 t ha?1 a?1. The value for the taller mangroves is similar to that reported from mangroves in many other parts of the world, but because of the extensive low sparse mangroves the total for the basin is estimated as 53·7 t a?1, an average rate of 2·7 t ha?1 a?1, a very low rate of litter-fall when compared with elsewhere. Decomposition of mangrove leaves occurs relatively rapidly with leaves losing half their dry weight in 10 weeks and then continuing to degrade but at a slower rate. Substrate sediment samples contain high organic matter content, and although some organic matter appears to be exported via the tidal creek, a proportion of the detrital production is evidently recycled in situ.  相似文献   

4.
This study shows results on litterfall dynamics and decay in mangrove stands of Avicennia germinans distributed along a latitudinal gradient (three forest sites) in the Gulf of California, in order to assess whether internal sources could support the observed mangrove ecosystem organic deficit in this arid tropic. Total mean annual litterfall production increased southward (712.6 ± 53.3, 1501.3 ± 145.1 and 1506.2 ± 280.5 g DW m−2 y−1, in the Yaqui, Mayo and Fuerte areas respectively), leaves being the main component of litter in all locations during the entire year, followed by fruits. The wet season (June–September) showed the highest litterfall rates through fruits. The temporal trend of litterfall production was significantly explained through mean air temperature (R2 = 68%) whilst total annual litter production in the entire region showed a statistically significant relationship with total soil phosphorus, salinity, total nitrogen, organic matter and tree height (R2 = 0.67). Throughout 117 days of the decomposition experiment, the litter lost 50% of its original dry weight in 5.8 days (average decay rate of 0.032 ± 0.04 g DW d−1) and there were not significant differences in the remaining mass after 6 days. The percentage of both C and P released from the litter correlated significantly with the ratio of tidal inundated days to total experiment days (R2 = 0.62, p = 0.03 and R2 = 0.67, p = 0.02, respectively); however, the frequency of tidal inundation only showed a significant increase in C release from Avicennia litter after 6 and above 48 days of decomposition. Whereas the total C content of litter bags decreased linearly over the decomposition to (% Total C = 5.52 − 0.46 days, R2 = 0.81, p = 0.0005), N content displayed an irregular pattern with a significant increase of decay between 48 and 76 days from the beginning of the experiment. The pattern for relative P content of litter revealed reductions of up to 99% of the original (%tot-P = −9.77 to 1.004 days, R2 = 0.72, p = 0.01) although most of the P reduction occurred between 17 and 34 days after the experiment started. Soil N and P contents, which exhibited significant differences in the course of the decomposition experiment, appeared to show significant differences between sampling sites, although they were not related to tidal influence, nor by leaf and nutrient leaching. In a global basis, C/N litter ratios decreased linearly (C/N = 32.86 − 0.1006 days, R2 = 0.62, p = 0.02), showing a strong and significant correlation with meteorological variables (R2 = 0.99, p = 0.01). C/P ratios of litter increased through an exponential function (C/P = 119.35e0.04day, R2 = 0.89, p < 0.001). Changes in the remaining percentage of litter mass during the experiment were significantly correlated with soil C/N ratio (R2 = 0.56, p = 0.03) as well as with the soil C/P ratio (R2 = 0.98, p < 0.001). Our results of litter decomposition dynamics in this mangrove support the fact of null net primary productivity of the arid mangrove wetlands: fast litter decomposition compensates the ecosystem organic deficit in order to sustain the mangrove productivity. Litter decomposition plays a key role in the ecosystem metabolism in mangroves of arid tropics.  相似文献   

5.
The dynamics of methane (CH4) flux in relation to populations of methanogenic and methanotrophic bacteria was studied under the different biophysical conditions of the Indian Sundarban mangrove ecosystem. Soil depth profile analysis (up to 60 cm) in the lower littoral zone (LLZ) revealed that a methanogenic population of 6.45 ± 0.19 × 104 cells/g dry weight (dry wt) of soil accounted for a CH4 production rate of 6.23 ± 3.53 × 103 µmol m?2 day?1, whereas in the surface soil, a methanogenic population of 3.34 ± 0.37 × 104 cells/g dry wt of soil accounted for a CH4 production rate of 31.6 ± 0.57 µmol m?2 day?1. The CH4 oxidation rate at 60 cm depth in the LLZ was 24.42 ± 1.28 µmol m?2 day?1, with an average methanotrophic population of 1.33 ± 0.43 × 104 cells/g dry wt of soil, whereas in the surface soil, the oxidation rate and average population were 3.38 ± 1.43 × 103 µmol m?2 day?1 and 12.80 ± 2.54 × 104 cells/g dry wt of soil, respectively. A similar soil profile in terms of CH4 dynamics and the populations of methanogenic and methanotrophic bacteria was found in the mid‐littoral and upper littoral zones of the studied area. The results demonstrate that most of the produced CH4 (approximately 60%) was oxidized by methanotrophic bacteria present in the soil, thus revealing their principal role in regulating the CH4 flux from this unique ecosystem.  相似文献   

6.
An intra-annual investigation of the fugacity of CO2 (fCO2) has been conducted in surface waters of the north-eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula) in four cruises made in 2006 and 2007. Intra-annual variability of fCO2 was assessed and is discussed in terms of mixing, temperature and biology. In the study area of the shelf, thermodynamic control over fCO2 predominates from early May to late November, and this is opposite and similar in magnitude to the net biological effect. However, biological control over fCO2 predominates during winter. The results suggest that surface waters in the coastal area are under-saturated with respect to atmospheric CO2 during most of the year; therefore they represent a sink for atmospheric CO2 between November and May (? 1.0 mmol m? 2 day? 1), but a weak source in June (1.3 mmol m? 2 day? 1). In contrast, the coastal ecosystems studied (the lower estuary of Guadalquivir Estuary and Bay of Cádiz) acted as a weak sink for atmospheric CO2 during February (? 1.3 mmol m? 2 day? 1) and as a source between May and November (2.6 mmol m? 2 day? 1). The resulting mean annual CO2 flux in the north-eastern shelf of the Gulf of Cádiz was ? 0.07 mol m? 2 year? 1 (? 0.2 mmol m? 2 day? 1), indicating that the area acts as a net sink on an annual basis.  相似文献   

7.
We measured dissolved isoprene (2-methyl-1,3-butadiene; C5H8) concentrations in a broad area of the southern Indian Ocean and in the Indian sector of the Southern Ocean from 35°S to 64°S and from 37°E to 111°E during austral summer 2010–2011. Isoprene concentrations were continuously measured by use of a proton-transfer-reaction mass spectrometer combined with a bubbling-type equilibrator. Concentrations of isoprene and its emission flux throughout the study period ranged from 0.2 to 395 pmol L?1 and from 181 to 313 nmol m?2 day?1, respectively, the averages being generally higher than those of previous studies. Although we found a significant linear positive relationship between isoprene and chlorophyll-a concentrations (r 2 = 0.37, n = 36, P < 0.001), the correlation coefficient was lower than previously reported. In contrast, in the high-latitude area (>53°S) we identified a significant negative correlation (r 2 = 0.59, n = 1263, P < 0.001) between isoprene and the temperature-normalized partial pressure of carbon dioxide (n-pCO2), used as an indicator of net community production in this study. This suggests that residence times and factors controlling variations in isoprene and n-pCO2 are similar within a physically stable water column.  相似文献   

8.
Myanmar is tenth among the world’s fish-producing countries and third in ASEAN (Association of Southeast Asian Nations). To understand the mechanisms underlying the high production, oceanographic and phytoplankton surveys, including primary productivity measurements based on pulse amplitude modulation fluorometry, were conducted near an active fishing ground near Myeik City. Three surveys, one in each of the representative seasons and covering the characteristic coastal environments, showed well-defined seasonality in primary production and phytoplankton occurrence. End of the dry season was the most productive, with productivity of 2.59 ± 1.56 g C m?2 day?1 and high concentration of chlorophyll a (3.14 ± 2.64 µg L?1). In this season, the phytoplankton population was dominated by high densities of the diatoms Bellerochea horologicalis and Chaetoceros curvisetus, whereas primary productivity was low at the onset of the dry season, 1.36 ± 0.77 g C m?2 day?1. However, this low primary production might be compensated by activation of microbial food chains originating from high dissolved organic carbon. The rainy season exhibited the lowest production, 6.6% of the end of the dry season, due to the extensive discharge of turbid water from the rivers which lowered euphotic layer depth and resulted in an unusually high diffuse attenuation coefficient of 2.30 ± 1.03 m?1. This incident of turbid water may be related to soil erosion from deforestation and mangrove deterioration. This research reveals the seasonal trend in Myanmar’s coastal productivity and its relationship to the tropical monsoon climate as well as emphasizing the importance of tropical coastal environments to the sustainability of the fisheries.  相似文献   

9.
Roving creel surveys and aerial surveys of shore-angling were undertaken as part of a national investigation into linefishing in South Africa. Shore patrols utilized a random stratified sampling procedure to collect catch-and-effort data, and a questionnaire provided information on fishing effort, angler demographics, economics and attitudes towards current regulations. A total of 1 677 patrols, covering 19 616 km, was conducted between April 1994 and February 1996, during which period 9 523 anglers had their catches checked and 4 490 were interviewed. A further 16 497 km were covered by aerial surveys, when 22 609 anglers were counted. From the aerial surveys, angler densities were highest on the KwaZulu-Natal coast (4.65 anglers·km?1), followed by the Southern Cape coast (2.29 anglers·km?1), the Eastern Cape coast (0.36 anglers·km?1) and the West Coast (0.12 anglers·km?1). Catch rates varied from 1.5 kg·angler?1·day?1 on the Southern Cape coast to 0.45 kg·angler?1·day?1 on the KwaZulu-Natal coast. Total effort was estimated at 3.2×106 angler days·year?1 and the total catch was estimated at 4.5×106 fish·year?1 or 3 000 tons·year?1. Targeted species varied regionally, with elf Pomatomus saltatrix (29%) being the most sought after species on the KwaZulu-Natal coast, kob Argyrosomus japonicus (18%) on the Eastern Cape coast and galjoen Dichistius capensis on the Southern Cape coast (30%) and along the West Coast (50%). The catch composition by mass was dominated by P. saltatrix on the KwaZulu-Natal coast (29%), the Eastern Cape coast (26%), and on the Southern Cape coast (56%) and by white stumpnose Rhabdosargus globiceps (40%) on the West Coast. Although anglers generally supported the regulations currently governing the linefishery, the questionnaire results showed that knowledge and compliance of them was poor. A low level of law enforcement was found, except for in KwaZulu-Natal. Management of the linefishery is discussed in relation to the findings of this study.  相似文献   

10.
The littoral zone of Swartvlei, an estuarine lake on the southern Cape coast, was characterized by extensive beds of submerged aquatic macrophytes. May 1979 marked the beginning of a regression phase which lasted more than three years. The senescence of the Potamogeton pectinatus and charophyte beds resulted in a 60% decline in primary production, a 74% slump in littoral invertebrate biomass and a 54% decline in the abundance of the fishes Monodactylus falciformis (Lacepede) and Rhabdosargus holubi (Steindachner) associated with the macrophytes. Gravimetric and calorific analyses of their food revealed that invertebrates and filamentous algae from the littoral zone were of major importance. The two fish species consumed approximately 2% of the primary production during the Potamogeton canopy phase and 3% during the Potamogeton senescent phase. Invertebrate consumption declined from 33 mg m?2 day?1 during the canopy phase to 8 mg m?2 day?1 during the senescent phase. The disappearance of the Potamogeton and charophyte beds also resulted in a decrease in the condition of both species but the effect was indirect and related mainly to the collapse of invertebrate stocks associated with the plants.  相似文献   

11.
The biomass and productivity of phytoplankton populations inshore on the west coast of South Africa were investigated towards the end of the upwelling season, a period when high-biomass dinoflagellate blooms are common. Productivity was estimated from natural fluorescence measurements (PNF ), using photosynthesis (P) v. irradiance (E) relationships (PE ) and by means of the in situ 14C-method (PC ) A linear regression of PNF productivity against PC and PE productivities yielded a slope of 0.911 and an r 2 of 0.83 (n = 41). Physical and biological variability was high inshore, reflecting alternating periods of upwelling and quiescence. Mean chlorophyll inshore (within a 12 m water column) ranged from 0.7 to 57.8 (mean = 8.9) mg·m&minus3, mean PNF productivity ranged from 8.4 to 51.0 (mean = 24.6) mgC·m?3·h?1 and daily integral PNF productivity from 0.8 to 4.8 (mean = 2.3) gC·m?2·day?l. Transects sampled during active and relaxation phases of upwelling had different chlorophyll distributions. High chlorophyll concentrations (sometimes >50 mg·m?3) were associated with surface blooms within the region of the upwelling front. Estimates of daily water-column PNF productivity within these frontal blooms ranged from 4.0 to 5.6 gC·m?2·day?1. With relaxation of wind stress, blooms dominated by dinoflagellates flooded shorewards and often formed red tides. Chlorophyll concentrations of > 175 mg·m?3 and productivity rates > 500 mgC·m?3·h?1 and 12 gC·m?2·day?1 were measured during a particularly intense red tide. Offshore, the water column was highly stratified with a well-defined subsurface chlorophyll maximum layer within the pycnocline region. Estimates of daily water-column PNF productivity ranged from 2.4 to 4.0 gC·m?2·day?1 offshore. The high productivity of shelf waters on the West Coast in late summer can be ascribed largely to dinoflagellate populations and their success in both upwelling systems and stratified conditions.  相似文献   

12.
Tropical estuaries are under increasing pressure worldwide from human impacts, but are poorly studied compared with temperate systems. This study examined a tropical macrotidal estuary, Darwin Harbour, in northern Australia, using a combination of direct measurements and literature values to determine the main sources of primary production and the sources of nutrients supporting growth. The main source of primary production was calculated to be the extensive area of fringing mangroves and resulted in a net autotrophic system (PG:R = 2.1). Much of the carbon in the mangrove forests appears to be retained within the forests or respired, as the water column was also net autotrophic despite the carbon inputs. Phytoplankton were the second largest primary producer on a whole-of-harbour basis, with low biomass constrained by light and nutrient availability. The phytoplankton were likely to be nitrogen (N) limited, based on low N:phosphorus (P) ratios, low dissolved bioavailable N concentrations (ammonium (NH4+), nitrate (NO3−), urea), and evidence that phytoplankton growth in bioassays was stimulated by NH4+ addition. The largest new source of N to the system was from the ocean due to higher N concentrations in the incoming tides than the outgoing tides. Atmospheric inputs via N fixation on the intertidal mudflats and subtidal sediments were substantially lower. The rivers feeding into the harbour and sewage were minor N inputs. Nitrogen demand by primary producers was high relative to available N inputs, suggesting that N recycling within the water column and mangrove forests must be important processes. Darwin Harbour is adjacent to the rapidly growing urban area of Darwin city, but overall there is no evidence of anthropogenic nutrient inputs having substantial effects on primary production in Darwin Harbour.  相似文献   

13.
Rates of nitrogen assimilation by phytoplankton were measured at 13 stations along a transect in the northwestern Indian Ocean, from the Gulf of Oman, southwards to approximately 8°N, during November and December 1994. Nitrate (NO3), ammonium (NH4) and urea assimilation were measured using simulated in situ 15N incubation techniques. These measurements were supported by simultaneous rate measurements of primary production using 14C incubation techniques and detailed vertical distributions of temperature and chlorophyll concentrations. Euphotic zone integrated nitrogen assimilation rates varied between 1.1 and 23.6 mmol N m-2 day-1, with generally higher rates occurring at the northern and southern ends of the transect. At the majority of stations ammonium was the preferred nitrogen substrate assimilated; the average integrated assimilation rate of ammonium being 3.7 mmol N m-2 day-1 compared to 1.6 and 1.8 mmol N m-2 day-1 for urea and nitrate respectively. This general preference is reflected in the low f-ratios, which were ⩽0.52 for all stations and in the relative preference indices (RPI) values which were consistently >1 for ammonium and <1 for nitrate. A further examination of the data has lead to an apparent partitioning of the northwestern Indian Ocean into 2 regions; a region north of 17°30′N and a region south of this, to about 8°N. This division is based on: (i) the relationship between the f-ratio and ambient nitrate levels; (ii) nitrogen assimilation and primary production and (iii) the biomass distribution. It is suggested that this partitioning should be investigated further with the development of biogeochemical provinces in mind and the estimation of f-ratios on much larger, horizontal scales.  相似文献   

14.
The flux of ammonia, phosphate, silica and radon-222 from Potomac tidal river and estuary sediments is controlled by processes occurring at the sediment-water interface and within surficial sediment. Calculated diffusive fluxes range between 0·6 and 6·5 mmol m?2 day?1 for ammonia, 0·020 and 0·30 mmol m?2 day?1 for phosphate, and 1·3 and 3·8 mmol m?2 day?1 for silica. Measured in situ fluxes range between 1 and 21 mmol m?2 day?1 for ammonia, 0·1 and 2·0 mmol m?2 day?1 for phosphate, and 2 and 19 mmol m?2 day?1 for silica. The ratio of in situ fluxes to diffusive fluxes (flux enhancement) varied between 1·6 and 5·2 in the tidal river, between 2·0 and 20 in the transition zone, and from 1·3 to 5·1 in the lower estuary. The large flux enhancements from transition zone sediments are attributed to macrofaunal irrigation. Nutrient flux enhancements are correlated with radon flux enhancements, suggesting that fluxes may originate from a common region and that nutrients are regenerated within the upper 10–20 cm of the sediment column.The low fluxes of phosphate from tidal viver sediments reflect the control benthic sediment exerts on phosphorus through sorption by sedimentary iron oxyhydroxides. In the tidal river, benthic fluxes of ammonia and phosphate equal one-half and one-third of the nutrient input of the Blue Plains sewage treatment plant. In the tidal Potomac River, benthic sediment regeneration supplies a significant fraction of the nutrients utilized by primary producers in the water column during the summer months.  相似文献   

15.
An assessment of catch and effort in the Eastern Cape shore and skiboat linefisheries was undertaken between 1994 and 1996 by means of roving creel and access point surveys. Catch-and-effort data were obtained from direct observation of 3 273 shore-fishers, 172 recreational and 223 commercial skiboat outings. Total effort in the region was high at 903 186 fisher-days year?1 in the shore fishery and 64 266 and 24 357 fisher-days year?1 in the commercial and recreational skiboat sectors respectively. The fisheries are multispecies in nature. The shore fishery consisted of 66 species, the recreational skiboat fishery 44 species and the commercial skiboat fishery 48 species. Just 10 species accounted for 75, 83 and 90% of the catch of the shore, recreational and commercial skiboat fishery respectively. The average catch per unit effort (cpue) was low in all sectors, 1.15 kg fisher?1 day?1 in the shorefishery, 9.4 kg fisher?1 day?1 in the recreational skiboat fishery and 21.5 kg fisher?1 day?1 in the commercial skiboat fishery. Catch data showed that professional and club anglers are more successful fishers. Comparisons with historic records for Port Elizabeth revealed that the cpue in the shore fishery had declined markedly, whereas the total effort increase was negligible (0.1%). In addition, the species composition of the fishery has changed.  相似文献   

16.
The distribution patterns of saury eggs, larvae and early juveniles have been investigated over the continental shelf off the South-Western Cape from August 1977 to August 1978. Young saury tend to occupy water of 18°C or warmer in all seasons and are found offshore of the Benguela oceanic front when inshore upwelling is evident. Their distribution in the region appears to be linked to seasonal hydrography. Spawning in the study area occurs mainly between Cape Columbine and Cape Point, and to a lesser extent east of Cape Point. The ratio of large larvae and early juveniles to small larvae is greater off the West Coast than off the South Coast. Most spawning appears to take place in summer, with a peak abundance of larvae and early juveniles towards the end of this season. Instantaneous natural mortality was calculated: 5 – 30 mm, Z = 0,14·day?1; 30 – 100 mm, Z = 0,02·day?1. There is therefore a marked but unexplained decline in mortality shortly after the transition from the larval to the juvenile phase.  相似文献   

17.
An integrated telephone and on-site questionnaire survey was used to estimate total shark fishing effort and specific catch of the ragged-tooth shark Carcharias taurus by coastal club-affiliated shore-anglers, primarily along the east coast of South Africa. Mean total shark fishing effort was estimated to be 37 820 fisher-days year?1 (95% CI = 28 281–47 359 fisher-days year?1) with a mean cpue of 0.073 C. taurus fisher?1 day?1 (95% CI = 0.068–0.078 fisher?1 day?1). Cpue for C. taurus varied significantly along the coast and ranged from a high of 0.260 fisher?1 day?1 in the Eastern Cape to zero fisher?1 day?1 on the West Coast. The total number of C. taurus caught annually by coastal club anglers was estimated at 1 764 fish year?1 (95% CI = 321–3 207 fish year?1). The majority (92.1%) of young-of-the-year sharks (<1.2m total length, TL) were recorded between East London and Jeffreys Bay, suggesting that this may be the primary nursery area for C. taurus. Post-release mortality ranged from 3.8% for young-of-the-year sharks to 18.5% for adult sharks (>2.4m TL). The majority (76.2%) of anglers interviewed stated that they now fished less for sharks since the banning of vehicles on beaches in December 2001. As a result of this ban, almost half (49.2%) of interviewed anglers stated that they now fished more for non-cartilaginous species.  相似文献   

18.
Sea-spawned saury eggs collected 20 km off the west coast of the Cape Peninsula were hatched in laboratory tanks and the larvae reared. The larvae were fed initially on nauplii of cultured copepods, Tisbe holothuriae Humes and Pseudodiaptomus hessei (Mrázek). Growth rate during the first 47 days post-hatch averaged 0,62 mm·day?1 (at 18—19°C) and between the 47th and 120th day averaged 0,45 mm·day?1 (at 14,8—15,5°C). Some observations on feeding behaviour are included.  相似文献   

19.
The bioavailability of nutrients is important in controlling ecological processes and nitrogen cycling in oligotrophic mangrove forests, yet the variation of diazotrophic community structure and activity with nutrient availability in sediments remains largely unexplored. To investigate for the first time how nutrients in sediments affect spatial and temporal patterns of diazotrophic community structure and activity, the sedimentary environment of Twin Cays, Belize, was examined with respect to the effects of long‐term fertilization [treatments: control (Ctrl), nitrogen (N), and phosphorus (P)] on N2 fixation rates and nifH gene community structure. We found that N2 fixation rates were significantly higher at the P‐treatment, intermediate at the Ctrl‐treatment and lower in the N‐treatment (P: 4.2 ± 0.5, Crtl: 0.8 ± 0.1, N: 0.4 ± 0.1 nmol·N·g?1·h?1; P < 0.001) with spatial (Ctrl‐ and P‐treatments) and temporal (only P‐treatment) variability positively correlated with live root abundance (r2 = 0.473, P < 0.001) and concentration (r2 = 0.458, P < 0.0001). The community structure of diazotrophs showed larger spatial and temporal variability in the fertilized treatments than in the Ctrl‐treatment, with the relative abundance of OTUs (nifH operational taxonomic units) at the fertilized treatments inversely related to live root abundance. Overall, long‐term fertilization (with either N or P) affects not only nutrient levels in mangrove sediments directly, but also spatial and temporal patterns of both community structure and activity and likely plant‐microbe interactions as well. Our findings suggest that the maintenance of natural nutrient conditions in mangrove sediments is important to ensure the stability of microbial functional groups like diazotrophs.  相似文献   

20.
Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N2) fixation is estimated to supply 8×1012 moles N y?1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H2), but the extent to which it leads to supersaturation of H2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H2 concentration (nmol L?1), and rates of N2 fixation (μmol N m?3 d?1), determined using 15N2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation (r=0.98) between dissolved H2 and rates of N2 fixation, with the greatest supersaturation of H2 and highest rates of N2 fixation being observed in the subtropical gyres at the southern (～18°S) and northern (18°N) reaches of the transect. The lowest H2 saturation and N2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H2 supersaturations and N2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements.  相似文献   

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