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1.
Land-based pollutants such as fertilizers and wastewater can infiltrate into aquifers and discharge into surrounding coastal water bodies as submarine groundwater discharge (SGD). Oceanic islands, with a large coast length to land area ratio, may be hot spots of SGD into the global ocean. Although SGD may be a major pathway of dissolved nutrients, carbon and metals to coastal waters, studies have been limited due to the difficulties in measuring this often diffuse process. This study used radium isotopes (223Ra, 224Ra, 226Ra) to investigate SGD and the associated fluxes of nutrients into Tauranga Harbour, New Zealand. We calculated the apparent water mass ages of the harbour to be between ~4.1 and 7.8 days, which was similar to a previous numerical model of ~2–8 days. A 226Ra mass balance was constructed to quantify SGD fluxes at the harbour scale. A minimum SGD flux rate of 0.53 cm day?1 was calculated by using the maximum groundwater end-member value from 22 sample sites. However, using the geometric mean from these samples as a representative end-member, a final value of 2.83 cm day?1 or a flux of 3.09 × 106 m3 day?1 was calculated. These values were between ~1 and 2.8 times greater than all the major river and creeks discharging into the harbour during the sampling period. Due to the higher observed nutrient concentrations in groundwater, the SGD-derived dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and total dissolved phosphorus (TDP) fluxes were calculated to be 1.07, 0.87 and 0.05 mmol m2 day?1, respectively. These SGD inputs were ~5 times (for nitrogen) and ~8 times (for phosphorus) greater than the input from surrounding rivers and streams. The average N:P ratio in groundwater samples was 36:1 (which was greatly in excess of the Redfield ratio of 16). The harbour water had a N:P ratio of ~17:1. A positive relationship between radium isotopes and N:P ratios in the harbour further supported the hypothesis that SGD can have major implications for primary production, including recurrent algal bloom events which occur in the harbour. We suggest SGD as a major driver of nutrient dynamics in Tauranga Harbour and potentially other similar coastal lagoon systems and estuaries on oceanic islands.  相似文献   

2.
Measurements of groundwater-dissolved inorganic nitrogen (nitrate?+?nitrite?+?ammonia) and phosphate concentrations were combined with recent, radium-based, submarine groundwater discharge (SGD) fluxes and prior estimates of SGD determined from Darcy’s Law, a hydrologic model, and total recharge to yield corresponding SGD nutrient fluxes to Ninigret, Point Judith, Quonochontaug, and Winnapaug ponds, located in southern Rhode Island. Results range from 80 to279 mmol N m?2 year?1 and 4 to 15 mmol P m?2 year?1 for Ninigret, 48 to 265 mmol N m?2 year?1 and 4 to 23 mmol P m?2 year?1 for Point Judith, 31 to 62 mmol N m?2 year?1 and 1 to 2 mmol P m?2 y?1 for Quonochontaug, and 668 to 1,586 mmol N m?2 year?1 and 29 to 70 mmol P m?2 year?1 for Winnapaug ponds, respectively. On a daily basis, the SGD supply of dissolved inorganic nitrogen and phosphorus is estimated to represent ～1–6 % of the total amount of these nutrients in surface waters of Ninigret, Point Judith, and Quonochontaug ponds and up to 84 and 17 % for Winnapaug, respectively, which may reflect a greater SGD nutrient supply to this pond because of the proximity of fertilized golf courses. With regard to the total external input of these essential nutrients, SGD represents 29–45 % of dissolved inorganic nitrogen input to Ninigret, Point Judith, and Quonochontaug ponds and as much as 93 % for Winnapaug pond. For phosphorus, the contribution from SGD represents 59–85 % of the total external input for Ninigret, Point Judith, and Quonochontaug ponds and essentially all of the phosphorus input to Winnapaug pond. Estimated rates of primary productivity potentially supported by the average supply of dissolved inorganic nitrogen from SGD range from 10 g C m?2 year?1 for Ninigret, 13 g C m?2 year?1 for Point Judith, 4 g C m?2 year?1 for Quonochontaug, and as high as 84 g C m?2 y?1 for Winnapaug pond. The imputed SGD-derived rates of primary productivity represent 4–9 % of water column primary production for Ninigret, Point Judith, and Quonochontaug ponds, and 74 % for Winnapaug pond, a result that is reasonably comparable to several other coastal environments where estimates of SGD nutrient supply have been reported. The implication is that SGD represents an ecologically significant source of dissolved nutrients to the coastal salt ponds of southern Rhode Island and, by inference, other coastal systems.  相似文献   

3.
The spatial and temporal variations of the flux of CO2 were determined during 2007 in the Recife estuarine system (RES), a tropical estuary that receives anthropogenic loads from one of the most populated and industrialized areas of the Brazilian coast. The RES acts as a source of nutrients (N and P) for coastal waters. The calculated CO2 fluxes indicate that the upstream inputs of CO2 from the rivers are largely responsible for the net annual CO2 emission to the atmosphere of +30 to +48 mmol m?2 day?1, depending on the CO2 exchange calculation used, which mainly occurs during the late austral winter and early summer. The observed inverse relationship between the CO2 flux and the net ecosystem production (NEP) indicates the high heterotrophy of the system (except for the months of November and December). The NEP varies between ?33 mmol m?2 day?1 in summer and ?246 mmol m?2 day?1 in winter. The pCO2 values were permanently high during the study period (average ~4,700 μatm) showing a gradient between the inner (12,900 μatm) and lower (389 μatm) sections on a path of approximately 30 km. This reflects a state of permanent pollution in the basin due to the upstream loading of untreated domestic effluents (N/P?=?1,367:6 μmol kg?1 and pH?=?6.9 in the inner section), resulting in the continuous mineralization of organic material by heterotrophic organisms and thereby increasing the dissolved CO2 in estuarine waters.  相似文献   

4.
Increased nitrogen (N) input to ecosystems could alter soil organic carbon (C) dynamics, but the effect still remains uncertain. To better understand the effect of N addition on soil organic C in wetland ecosystems, a field experiment was conducted in a seasonally inundated freshwater marsh, the Sanjiang Plain, Northeast China. In this study, litter production, soil total organic C (TOC) concentration, microbial biomass C (MBC), organic C mineralization, metabolic quotient (qCO2) and mineralization quotient (qmC) in 0–15 cm depth were investigated after four consecutive years of N addition at four rates (CK, 0 g N m?2 year?1; low, 6 g N m?2 year?1; moderate, 12 g N m?2 year?1; high, 24 g N m?2 year?1). Four-year N addition increased litter production, and decreased soil organic C mineralization. In addition, soil TOC concentration and MBC generally increased at low and moderate N addition levels, but declined at high N addition level, whereas soil qCO2 and qmC showed a reverse trend. These results suggest that short-term N addition alters soil organic C dynamics in seasonally inundated freshwater marshes of Northeast China, and the effects vary with N fertilization rates.  相似文献   

5.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude ?31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (233Ra and 224Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m?2 estuary day?1; 20 mmol m?2 catchment day?1, respectively), with a large export of alkalinity (23 mmol m?2 catchment day?1). Average water to air flux of CO2 (869 mmol m?2 day?1) and CH4 (26 mmol m?2 day?1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO2- and CH4-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO2, and CH4 was 22, 41, 3, 75, and 100 %, respectively. The results show that CO2 and CH4 evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.  相似文献   

6.
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7.
A nutrient mass balance for the tidal freshwater segment of the James River was used to assess sources of nutrients supporting phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to marine waters. Monthly mass balances for 2007–2010 were based on riverine inputs, local point sources (including sewer overflow events), ungauged inputs, riverine outputs, and tidal exchange. The tidal freshwater James River received exceptionally high areal loads (446 mg TN m?2 day?1 and 55 mg TP m?2 day?1) compared to other estuaries in the region and elsewhere. P inputs were principally from riverine sources (84 %) whereas point sources contributed appreciably (54 %) to high N loads. Despite high loading rates and short water residence time, areal mass retention was high (143 mg TN m?2 day?1 and 33 mg TP m?2 day?1). Retention of particulate fractions occurred during high discharge, whereas dissolved inorganic fractions were retained during low discharge when chlorophyll-a concentrations were high. On an annualized basis, P was retained more effectively (59 %) than N (32 %). P was retained by abiotic mechanisms via trapping of particulate forms, whereas N was retained through biological assimilation of dissolved inorganic forms. Results from a limited suite of stable isotope determinations suggest that DIN from point sources was preferentially retained. Combined inputs from diffuse and point sources accounted for only 20 % and 36 % (respectively) of estimated algal N and P demand, indicating that internal nutrient recycling was important to sustaining high rates of phytoplankton production in the tidal freshwater zone.  相似文献   

8.
Oxygen fluxes across the sediment–water interface reflect primary production and organic matter degradation in coastal sediments and thus provide data that can be used for assessing ecosystem function, carbon cycling and the response to coastal eutrophication. In this study, the aquatic eddy covariance technique was used to measure seafloor–water column oxygen fluxes at shallow coastal sites with highly permeable sandy sediment in the northeastern Gulf of Mexico for which oxygen flux data currently are lacking. Oxygen fluxes at wave-exposed Gulf sites were compared to those at protected Bay sites over a period of 4 years and covering the four seasons. A total of 17 daytime and 14 nighttime deployments, producing 408 flux measurements (14.5 min each), were conducted. Average annual oxygen release and uptake (mean ± standard error) were 191 ± 66 and ?191 ± 45 mmol m?2 day?1 for the Gulf sites and 130 ± 57 and ?152 ± 64 mmol m?2 day?1 for the Bay sites. Seasonal variation in oxygen flux was observed, with high rates typically occurring during spring and lower rates during summer. The ratio of average oxygen release to uptake at both sites was close to 1 (Bay: 0.9, Gulf: 1.0). Close responses of the flux to changes in light, temperature, bottom current velocity, and wave action (significant wave height) documented tight physical–biological, benthic–pelagic coupling. The increase of the sedimentary oxygen uptake with increasing temperature corresponded to a Q10 temperature coefficient of 1.4 ± 0.3. An increase in flow velocity resulted in increased oxygen uptake (by a factor of 1–6 for a doubling in flow), which is explained by the enhanced transport of organic matter and electron acceptors into the permeable sediment. Benthic photosynthetic production and oxygen release from the sediment was modulated by light intensity at the temporal scale (minutes) of the flux measurements. The fluxes measured in this study contribute to baseline data in a region with rapid coastal development and can be used in large-scale assessments and estimates of carbon transformations.  相似文献   

9.
This paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ 13C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m?2 day?1 in the Elorn estuary and 10 mmol m?2 day?1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O2 fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO x ? (>200 μM) and the large NH4 + increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11–15 % d.w. and 4–6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ 13C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ 13C values in the Aulne watershed. A high increase of δ 13C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter.  相似文献   

10.
Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum’s open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O2 m?2 day?1) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O2 m?2 day?1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O2 m?2 day?1. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem ? water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O2 m?2 day?1 but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O2 m?2 day?1 in spring to 86.7 mmol O2 m?2 day?1 in summer. The summer rates were much higher than could be realistically attributed to benthic processes and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.  相似文献   

11.
The present analysis adjusts previous estimates of global ocean CaCO3 production rates substantially upward, to 133 × 1012 mol yr?1 plankton production and 42 × 1012 mol yr?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO3 production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO3 mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO3 sediment accumulation rates remain about the same as they have been for some years: ~20 × 1012 mol yr?1 on shelves and 11 × 1012 mol yr?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 1012 mol yr?1 (~50 %) of shelf benthonic carbonate production and 122 × 1012 mol yr?1 (>90 %) of planktonic production. Most CaCO3 production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO2 release to CaCO3 precipitation (CO2↑/CaCO3↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO2 associated with CaCO3 reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 1012 mol C yr?1, at a time scale of 102–103 years. The CaCO3-mediated influx of CO2 is approximately offset by CO2 release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO3 and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.  相似文献   

12.
The carbonate system of waters of the inner, middle, and outer shelves was studied for a long period in the eastern part of the Laptev Sea. It is shown that the inner and middle shelves is a heterotrophic province and is a source of CO2 to the atmosphere during August–September. The average CO2 flux is 7.9 mmol m–2 day–1 and during one month waters of inner and middle shelves release into the atmosphere up to 0.7 Tg of carbon (C). The outer part of the shelf absorbs up to ~0.1 Tg C at an average rate of 3.9 mmol m–2 day–1 during the month. Generally, the ice-free zone of the eastern shelf of the Laptev Sea is a source of CO2 in the studied season: the amount of C released into the atmosphere within a month is ~0.6 Tg.  相似文献   

13.
The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on 15N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8 ± 0.4 to 75.9 ± 8.7 nmol N2 L?1 h?1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N year?1, which compensates for the difference between sediment cores incubation (~ 3 Tg-N year?1) and geochemical estimation based on N deficit relative to phosphorous (~ 16 Tg-N year?1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.  相似文献   

14.
Coastal wetlands are hotspots for biodiversity and biological productivity, yet the hydrology and carbon cycling within these systems remains poorly understood due to their complex nature. By using a novel spatiotemporal approach, this study quantified groundwater discharge and the related inputs of acidity and CO2 along a continuum of a modified coastal acid sulphate soil (CASS) wetland, a coastal lake and an estuary under highly contrasting hydrological conditions. To increase the resolution of spatiotemporal data and advance upon previous methodologies, we relied on automated observations from four simultaneous time-series stations to develop multiple radon mass balance models to estimate groundwater discharge and related groundwater inputs of acidity and dissolved inorganic carbon (DIC), along with surface water to atmosphere CO2 fluxes. Spatial surveys indicated distinct acid hotspots with minimum surface water pH of 2.91 (dry conditions) and 2.67 (flood conditions) near a non-remediated (drained) CASS area. Under flood conditions, groundwater discharge accounted for ～14.5 % of surface water entering the lake. During the same period, acid discharge from the acid sulphate soil section of the continuum produced ～4.8 kg H2SO4?ha?1 day?1, a rate much higher than previous studies in similar systems. During baseflow conditions, the low pH water was rapidly buffered within the estuarine lake, with the pH increasing from 4.22 to 6.07 over a distance of ～250 m. The CO2 evasion rates within the CASS were extremely high, averaging 2163?±?125 mmol m?2 day?1 in the dry period and 4061?±?259 mmol m?2 day?1 under flood conditions. Groundwater input of DIC could only account for 0.4 % of this evasion in the dry conditions and ～5 % during the flood conditions. We demonstrated that by utilising a spatiotemporal (multiple time-series stations) approach, the study was able to isolate distinct zones of differing hydrology and biogeochemistry, whilst providing more reasonable groundwater acid input estimates and air–water CO2 flux estimates than some traditional sampling designs. This study highlights the notion that modified CASS wetlands can release large amounts of CO2 to the atmosphere because of high groundwater acid inputs and extremely low surface water pH.  相似文献   

15.
Submarine groundwater discharge (SGD) is an important pathway for groundwater and associated chemicals to discharge to the sea. Groundwater levels monitored along a transect perpendicular to the shoreline are used to calculate SGD flux from the nearshore aquifer to Tolo Harbor, Hong Kong (China). The calculated SGD flux—recharge/discharge measured with Darcy’s Law methods—agrees well with estimates based on geo-tracer techniques and seepage meter in Tolo Harbor during previous studies. The estimated freshwater SGD is 1.69–2.0 m2/d at the study site and 0.3?±?0.04 cm/d for the whole of Tolo Harbor, which is comparable to the river discharge (0.25?±?0.07 cm/d) and precipitation (0.45?±?0.15 cm/d). The tide-driven SGD in the intertidal zone is 13.98–17.59 m2/d at the study site and 2.42?±?0.56 cm/d for the whole of Tolo Harbor. The SGD occurring in the subtidal zone and the bottom of Tolo Harbor is 3.12?±?4.63 cm/d. Fresh SGD accounts for ~5% of the total SGD, while the rest (~95%) is contributed by saline SGD driven by various forces. About 96% of the tide-driven SGD in the intertidal zone occurs in the ebbing tide period because the head difference between the groundwater level and sea level is great during this period. Tide-driven SGD in the spring tide is ~1.2 times that during neap tide. The tidal fluctuation amplitude and tide-driven SGD in the intertidal zone are positively correlated to each other; thus, a spring neap variation of the tide-driven SGD is observed.  相似文献   

16.
The effects of fish farm activities on sediment biogeochemistry were investigated in Loch Creran (Western Scotland) from March to October 2006. Sediment oxygen uptake rates (SOU) were estimated along an organic matter gradient generated from an Atlantic salmon farm using a combination of in situ techniques: microelectrodes, planar optode and benthic chamber incubations. Sulphide (H2S) and pH distributions in sediment porewater were also measured using in situ microelectrodes, and dissolved inorganic carbon (DIC) fluxes were measured in situ using benthic chambers. Relationships between benthic fluxes, vertical distribution of oxidants and reduced compounds in the sediment were examined as well as bacterial abundance and biomass. Seasonal variations in SOU were relatively low and mainly driven by seasonal temperature variations. The effect of the fish farm on sediment oxygen uptake rate was clearly identified by higher total and diffusive oxygen uptake rates (TOU and DOU, respectively) on impacted stations (TOU: 70 ± 25 mmol O2 m?2 day?1; DOU: 70 ± 32 mmol O2 m?2 day?1 recalculated at the summer temperature), compared with the reference station (TOU: 28.3 ± 5.5 mmol O2 m?2 day?1; DOU: 21.5 ± 4.5 mmol O2 m?2 day?1). At the impacted stations, planar optode images displayed high centimetre scale heterogeneity in oxygen distribution underlining the control of oxygen dynamics by small-scale processes. The organic carbon enrichment led to enhanced sulphate reduction as demonstrated by large vertical H2S concentration gradients in the porewater (from 0 to 1,000 μM in the top 3 cm) at the most impacted site. The impact on ecosystem functions such as bioirrigation was evidenced by a decreasing TOU/DOU ratio, from 1.7 in the non-impacted sediments to 1 in the impacted zone. This trend was related to a shift in the macrofaunal assemblage and an increase in sediment bacterial population. The turnover time of the organic load of the sediment was estimated to be over 6 years.  相似文献   

17.
We developed a synthesis using diverse monitoring and modeling data for Mattawoman Creek, Maryland, USA to examine responses of this tidal freshwater tributary of the Potomac River estuary to a sharp reduction in point-source nutrient loading rate. Oligotrophication of these systems is not well understood; questions concerning recovery pathways, threshold responses, and lag times remain to be clarified and eventually generalized for application to other systems. Prior to load reductions Mattawoman Creek was eutrophic with poor water clarity (Secchi depth <0.5 m), no submerged aquatic vegetation (SAV), and large algal stocks (50–100 μg L?1 chlorophyll-a). A substantial modification to a wastewater treatment plant reduced annual average nitrogen (N) loads from 30 to 12 g N m?2 year?1 and phosphorus (P) loads from 3.7 to 1.6 g P m?2 year?1. Load reductions for both N and P were initiated in 1991 and completed by 1995. There was no trend in diffuse N and P loads between 1985 and 2010. Following nutrient load reduction, NO2?+?NO3 and chlorophyll-a decreased and Secchi depth and SAV coverage and density increased with initial response lag times of one, four, three, one, and one year, respectively. A preliminary N budget was developed and indicated the following: diffuse sources currently dominate N inputs, estimates of long-term burial and denitrification were not large enough to balance the budget, sediment recycling of NH4 was the single largest term in the budget, SAV uptake of N from sediments and water provided a modest seasonal-scale N sink, and the creek system acted as an N sink for imported Potomac River nitrogen. Finally, using a comparative approach utilizing data from other shallow, low-salinity Chesapeake Bay ecosystems, strong relationships were found between N loading and algal biomass and between algal biomass and water clarity, two key water quality variables used as indices of restoration in Chesapeake Bay.  相似文献   

18.
Long time-series studies are critical to assessing impacts of climate change on the marine carbon cycle. A 27-year time-series study in the abyssal northeast Pacific (Sta. M, 4000 m depth) has provided the first concurrent measurements of sinking particulate organic carbon supply (POC flux) and remineralization by the benthic community. Sediment community oxygen consumption (SCOC), an estimate of organic carbon remineralization, was measured in situ over daily to interannual periods with four different instruments. Daily averages of SCOC ranged from a low of 5.0 mg C m?2 day?1 in February 1991 to a high of 31.0 mg C m?2 day?1 in June 2012. POC flux estimated from sediment trap collections at 600 and 50 m above bottom ranged from 0.3 mg C m?2 day?1 in October 2013 to 32.0 mg C m?2 day?1 in June 2011. Monthly averages of SCOC and POC flux correlated significantly with no time lag. Over the long time series, yearly average POC flux accounted for 63 % of the estimated carbon demand of the benthic community. Long time-series studies of sediment community processes, particularly SCOC, have shown similar fluctuations with the flux of POC reaching the abyssal seafloor. SCOC quickly responds to changes in food supply and tracks POC flux. Yet, SCOC consistently exceeds POC flux as measured by sediment traps alone. The shortfall of ~37 % could be explained by sediment trap sampling artifacts over decadal scales including undersampling of large sinking particles. High-resolution measurements of SCOC are critical to developing a realistic carbon cycle model for the open ocean. Such input is essential to evaluate the impact of climate change on the oceanic carbon cycle, and the long-term influences on the sedimentation record.  相似文献   

19.
Fine sediment inputs can alter estuarine ecosystem structure and function. However, natural variations in the processes that regulate sediment transport make it difficult to predict their fate. In this study, sediments were sampled at different times (2011–2012) from 45 points across intertidal sandflat transects in three New Zealand estuaries (Whitford, Whangamata, and Kawhia) encompassing a wide range in mud (≤63 μm) content (0–56 %) and macrofaunal community structure. Using a core-based erosion measurement device (EROMES), we calculated three distinct measures of sediment erosion potential: erosion threshold (? c ; N m?2), erosion rate (ER; g m?2 s?1), and change in erosion rate with increasing bed shear stress (m e ; g N?1 s?1). Collectively, these measures characterized surface (? c and ER) and sub-surface (m e ) erosion. Benthic macrofauna were grouped by functional traits (size and motility) and data pooled across estuaries to determine relationships between abiotic (mud content, mean grain size) and biotic (benthic macrofauna, microbial biomass) variables and erosion measures. Results indicated that small bioturbating macrofauna (predominantly freely motile species <5 mm in size) destabilized surface sediments, explaining 23 % of the variation in ? c (p ≤ 0.01) and 59 % of the variation in ER (p ≤ 0.01). Alternatively, mud content and mean grain size cumulatively explained 61 % of the variation in m e (p ≤ 0.01), where increasing mud and grain size stabilized sub-surface sediments. These results highlight that the importance of biotic and abiotic predictors vary with erosion stage and that functional group classifications are a useful way to determine the impact of benthic macrofauna on sediment erodibility across communities with different species composition.  相似文献   

20.
As part of the National Oceanic and Atmospheric Administration’s (NOAA) Nutrient Enhanced Coastal Ocean Productivity program, we have conducted four research cruises, July–August 1990, March 1991, September 1991, and May 1992, in the Mississippi River plume and adjacent shelf regions. Over this time period, photic-zone-integrated primary production varied significantly in both the river plume and shelf study regions, with greatest variability observed in the river plume region. In the river plume and the adjacent shelf, highest production occurred during July–August 1990 (8.17 g C m?2 d?1 for the plume and 1.89–3.02 g C m?2 d?1 for the shelf) and the lowest during March 1991 (0.40–0.69 g C m?2 d?1 for the plume and 0.12–0.45 g C m?2 d?1 for the shelf). The vertical export of POC from the euphotic zone, determined with free-floating MULTITRAP sediment trap systems, also varied temporally in both study regions, with highest values occurring in May 1992 (1.80±0.04 g C m?2 d?1 for the plume and 0.40±0.02 g C m?2 d?1 for the shelf) and the lowest values occurring during July–August 1990 (0.29±0.02 g C m?2 d?1 for the plume and 0.18±0.01 g C m?2 d?1 for the shelf). The fraction of production exported out of the photic zone was highly variable and was dependent, in part, on phytoplankton species composition and on the grazing activities of microzooplankton and mesozooplankton. The lowest ratio of export to production coincided with the time when production was greated and the highest ratios occurred when production was the lowest.  相似文献   

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