Net Ecosystem Metabolism Research Articles

Dissolved nutrient balance and net ecosystem metabolism in a Mediterranean-climate coastal lagoon: San Diego Bay

The temporal and spatial variability of dissolved inorganic phosphate (DIP), nitrogen (DIN), carbon (DIC) and dissolved organic carbon (DOC) were studied in order to determine the net ecosystem metabolism (NEM) of San Diego Bay (SDB), a Mediterranean-climate lagoon. A series of four sampling campaigns were carried out during the rainy (January 2000) and the dry (August 2000 and May and September 2001) seasons. During the dry season, temperature, salinity and DIP, DIC and DOC concentrations increased from oceanic values in the outer bay to higher values at the innermost end of the bay. DIP, DIC and DOC concentrations showed a clear offset from conservative mixing implying production of these dissolved materials inside the bay. During the rainy season, DIP and DOC increased to the head, whereas salinity decreased toward the mouth due to land runoff and river discharges. The distributions of DIP and DOC also showed a deviation from conservative mixing in this season, implying a net addition of these dissolved materials during estuarine mixing within the bay. Mass balance calculations showed that SDB consistently exported DIP (2.8–9.8 × 10 3 mol P d −1), DIC (263–352 × 10 3 mol C d −1) and DOC (198–1233 × 10 3 mol C d −1), whereas DIN (5.5–18.2 × 10 3 mol N d −1) was exported in all samplings except in May 2001 when it was imported (8.6 × 10 3 mol N d −1). The DIP, DIC and DOC export rates along with the strong relationship between DIP, DIC or DOC and salinity suggest that intense tidal mixing plays an important role in controlling their distributions and that SDB is a source of nutrients and DOC to the Southern California Bight. Furthermore, NEM ranged from −8.1 ± 1.8 mmol C m −2 d −1 in September to −13.5 ± 5.8 mmol C m −2 d −1 in January, highlighting the heterotrophic character of SDB. In order to explain the net heterotrophy of this system, we postulate that phytoplankton-derived particulate organic matter, stimulated by upwelling processes in the adjacent coastal waters, is transported into the bay, retained and then remineralized within the system. Our results were compared with those reported for the heterotrophic hypersaline coastal lagoons located in the semi-arid coast of California–Baja California, and with those autotrophic hypersaline systems found in the semi-arid areas of Australia. We point out that the balance between autotrophy and heterotrophy in inverse estuaries is dependent on net external inputs of either inorganic nutrients or organic matter as it has been indicated for positive estuaries.

Spatial and temporal variability and drivers of net ecosystem metabolism in western Gulf of Mexico estuaries

Net ecosystem metabolism (NEM) is becoming a commonly used ecological indicator of estuarine ecosystem metabolic rates. Estuarine ecosystem processes are spatially and temporally variable, but the corresponding variability in NEM has not been properly assessed. Spatial and temporal variability in NEM was assessed in four western Gulf of Mexico shallow water estuaries. NEM was calculated from high-frequency dissolved oxygen measurements. Interbay, intrabay, and water column spatial scales were assessed for NEM, gross primary production (GPP), and respiration (R) rate variability. Seasonal, monthly, and daily temporal scales in NEM, GPP, and R were also assessed. Environmental conditions were then compared to NEM to determine which factors were correlated with each temporal and spatial scale. There was significant NEM spatial variability on interbay, intrabay, and water column spatial scales. Significant spatial variability was ephemeral, so it was difficult to ascertain which environmental conditions were most influential at each spatial scale. Significant temporal variability in NEM on seasonal, monthly, and daily scales was found and it was correlated to temperature, salinity, and freshwater inflow, respectively. NEM correlated strongly with dissolved oxygen, temperature, and salinity, but the relationships where different in each bay. The dynamics of NEM on daily scales indicate that freshwater inflow events may be the main driver of NEM in the semiarid estuaries studied. The variable nature of NEM found here is further evidence that it is not valid to use single station monitoring deployments for assessment of whole estuarine ecosystem metabolic rates in large ecosystems. The relationship between NEM and temperature, salinity, and freshwater inflow events could drive predictive models assessing the potential influence of projected climate change and watershed development scenarios on estuarine metabolic rates.

The effect of freshwater inflow on net ecosystem metabolism in Lavaca Bay, Texas

Estuaries and other coastal ecosystems depend on freshwater inflow to maintain the gradients in environmental characteristics that define these transitional water bodies. Freshwater inflow (FWI) rates in many estuaries are changing due to changing land use patterns, water diversions for human consumption, and climate effects, but there are no standard criteria to determine minimum inflow rates. An ecological indicator is required so models can be produced to predict how changing hydrology might affect estuarine metabolic rates, productivity, or impairment. One indicator of estuarine metabolic rates is net ecosystem metabolism (NEM). It is hypothesized that metabolic rates (as indicated by NEM) will depend on recently delivered FWI. To test this hypothesis, daily NEM was calculated from high frequency changes in dissolved oxygen measurements in a shallow water estuary, Lavaca Bay, Texas, USA and related to FWI and other environmental conditions. There was a significant relationship between NEM and cumulative ten-day FWI in upper Lavaca Bay, with more heterotrophic conditions occurring after high FWI events. No significant relationship existed between NEM and FWI in lower Lavaca Bay, where other environmental conditions, such as tidal forcing, may be more influential. An empirical model simulating NEM's response to FWI in upper Lavaca Bay was more accurate during moderate to high flows than during low base-flow conditions. Thus, quantity of FWI and distance from the inflow point source constrained the dependence of NEM on FWI. NEM can be quite variable over time and within a bay dominated by a river at one end and ocean exchange at the other end. The range of environmental gradients over time and within an estuary will determine how representative NEM at a single location is for an entire estuary. However, during high flow period pulses NEM can accurately predict shifts from balanced to heterotrophic conditions. Therefore, use of the NEM model as an indicator of FWI effects is constrained to regions in estuaries that are most effected by FWI because of proximity to the source or the size of FWI pulse events. The unique geologic, geographic, and climate signature's of individual estuaries will dictate the fit of this model in space and time. Inclusion of other environmental factors, such as temperature or irradiance, is necessary to improve the NEM model during low base-flow conditions. It is concluded that the NEM model does provide a useful indicator of FWI effects on ecosystem metabolic rates as it works well in the upper regions of Lavaca Bay and during freshwater inflow pulses where and when freshwater inflow has more influence than other environmental factors.

Net autotrophy and heterotrophy in the Pontevedra Ria upwelling system (NW Iberian margin)

A non-stationary state mass balance of the type advocated by the International Geosphere-Biosphere Programme/Land-Ocean Interactions in the Coastal Zone (IGBP/LOICZ) has been used to assess net ecosystem metabolism in the Pontevedra Ria (NW Spain), a large embayment subject to transient upwelling of Eastern North Atlantic Central Water (ENACW) under favourable meteorological conditions. Nitrate (NO3–) and phosphate (PO43–) data spanning 12 months were resolved at the oceanic boundary by treating the ria as a single box of constant volume. The nutrient budget was scaled up to an estimate of net community production (NCP) using the compositional relationship of local phytoplankton. The mean annual NCP predicted by the NO3– balance was 24.0 mgC m–2 h–1, and 35.8 mgC m–2 h–1 by the PO43– budget. Nutrient fluxes into the reservoir were strongly driven by the incoming oceanic flow throughout the year, which supplied 88% and 98% of the total NO3– and PO43– load, respectively. A high NCP (NO3–) of 57.2 mgC m–2 h–1 was calculated for the height of the growth season contemporaneous with ENACW intrusion, whereas a slightly higher value of 61.4 mgC m–2 h–1 corresponded to PO43–. Nutrient limitation was variable throughout the sampling, switching from PO43– to NO3– from winter to spring concurrent with a drop in riverine PO43– input. Scaling the inorganic nutrients to carbon units, the Pontevedra Ria may be considered as net autotrophic, although the NO3– budget suggests alternation between autotrophy and heterotrophy over an annual cycle.

Net ecosystem metabolism in a micro-tidal estuary (Randers Fjord, Denmark): evaluation of methods

The metabolic status, the difference between organic matter production and consump- tion of an estuary (Randers Fjord, Denmark) has been assessed based on 2 field cruises in April and August 2001 and a number of approaches: (1) the oxygen (O2) incubation method, (2) dissolved inor- ganic carbon (DIC) budgets, (3) the response surface difference (RSD) method based on diel O2 changes and (4) land-ocean interaction in the coastal zone (LOICZ) budgets based on dissolved inor- ganic phosphorus (DIP). Although each method has its own associated limitations and uncertainties, the above approaches converged most of the time in consistent metabolic estimates, both in sign and magnitude, and revealed that this system was near metabolic balance in spring (net ecosystem pro- duction: NEP ~ 0) and net heterotrophic in summer (NEP ~ -50 mmol C m -2 d -1 ). In this shallow estu- ary (mean depth = 1.6 m), the benthic compartment was very active and represented 70 and 30% of the total gross primary production in April and August, respectively. NEP rates measured during this study are in the range of previously reported rates in estuaries.

Quantifying ecosystem metabolism in the middle reaches of Murrumbidgee River during irrigation flow releases

The relative importance of floodplain carbon inputs and in-stream metabolic processes have not been well quantified in major Australian rivers. We quantified seasonal phytoplankton primary production and net ecosystem production during irrigation flow regimes at four sites each located ~100 km apart in the middle Murrumbidgee River. During flow periods dominated by storage release, ecosystem gross primary productivity, system respiration and phytoplankton chlorophyll concentrations all increased downstream so that overall net ecosystem metabolism was strongly net heterotrophic upstream and closer to balanced downstream. Phytoplankton production dominated ecosystem production throughout the entire reach and was likely to have been phosphorus limited throughout the study. Additionally, phytoplankton biomass was limited by short residence times at the upstream sites and nitrogen limited downstream in summer, despite an increase in turbidity. Both production and respiration rates were generally lower in winter, as expected, owing to lower temperatures.

Shallow-water oceans: a source or sink of atmospheric CO2?

The shallow-water ocean environment is of great importance in the context of global change and is heavily impacted by human activity. This study evaluates the effects of human activity on the CO2 exchange between the atmosphere and the surface water of shallow-water oceans. The evaluation is based on changes in net ecosystem metabolism, net ecosystem calcification, and atmospheric CO2 concentrations, as seen in a process-driven biogeochemical box model. Numerical simulations show that this air–sea interface has probably served as a net source of CO2 to the atmosphere for much of the past 300 years, but has recently switched, or will switch soon, to a net sink of CO2, because of rising atmospheric CO2 and increasing inorganic nutrient load.

New and remineralised nutrient supply and ecosystem metabolism on the northeastern New Zealand continental shelf

The nutrient dynamics of the NE New Zealand shelf and adjacent Hauraki Gulf are described, with the aim of relating the along- and cross-shelf physical circulation to co-occurring supplies of new and remineralised nutrients, and ecosystem metabolism. Chemical and biological properties were studied over four research voyages in early spring to late summer in 1996–1997, coincident with the physical oceanography (Zeldis et al. this issue). Upwelling prevailed in early spring to early summer, but between early and late summer upwelling ceased and there was a marked transition toward downwelling and strongly stratified conditions on the shelf. The upwellings caused extensive loading of the inner-shelf water column and Hauraki Gulf bottom waters with nitrate. However, significant increases in phytoplankton biomass were not observed, probably because of dilution of shelf phytoplankton stocks by upwelling, and mismatch of voyages with delayed bloom development. Between spring and late summer, the shelf and Hauraki Gulf evolved strongly from net autotrophic toward net heterotrophic ecosystem metabolism, as indicated by decreasing ratios of new-to-regenerated nitrogen (N) stocks and of chlorophyll- a to phaeopigment, and increases in apparent oxygen utilisation (AOU). The effect of this transition on supply rates of new and remineralised N and silica in shelf waters was examined using a three end-member mixing model. Nitrate was the dominant N species remineralised near-bed, whereas ammonium remineralisation dominated shallower in the water column. Silica trapping and denitrification on the inner shelf were evident from stoichiometries of remineralisation. Vertical and along-shelf patterns in remineralisation were found, apparently related to benthic remineralisation, current speed and sediment type. Remineralised nutrients and AOU were concentrated by strong stratification in bottom waters in late summer, but on a water-column integrated basis, no seasonal change in remineralisation rates was evident. Nevertheless, with the large decrease in advected (i.e. upwelled) nutrients in late summer, the maintenance of significant remineralised nutrient supply appeared to be important for sustaining the chlorophyll- a stocks in late summer over the shelf.

Factors controlling net ecosystem metabolism in U.S. estuaries

High frequency dissolved oxygen data were analyzed to calculate primary production, respiration and net ecosystem metabolism (NEM) from 42 sites within 22 National Estuarine Research Reserves (NERR), 1995–2000. NERR sites are characterized by a variety of dominant plant communities including phytoplankton, salt marsh, seagrass, macroalgae, freshwater macrophyte, and mangrove, and are representative of the coastal bioregions of the United States. As expected from the wide diversity of sites, metabolic rates were temporally and spatially variable with the highest production and respiration occurring during the summer in Southeastern estuaries. Sites within different regions exhibited consistent seasonal trends in production, respiration, and NEM. Temperature was the most important environmental factor explaining within-site variation in metabolic rates; nutrient concentrations were the second most important factor. All but three of the 42 sites were heterotrophic (respiration was greater than production) on an annual basis. Habitat adjacent to the monitoring site, estuarine area, and salinity explained 58% of the variation in NEM. Open water sites or sites adjacent to mangroves or in marsh creeks were heterotrophic, while sites in or adjacent to submerged aquatic vegetation (eelgrass or macroalgal beds) were either autotrophic or near balance. Estuarine area was also a significant factor explaining variability in NEM; larger systems were closer to balance than smaller systems that trended toward heterotrophy. Freshwater sites were more heterotrophic than saline sites. Nutrient loading explained 68% of the variation in NEM among some of the sites. When these estimates were compared to the literature, metabolic rates from the NERR sites were much larger, often two to five times greater than rates from other estuarine and coastal systems. One explanation is that these small, generally shallow sites located near shore may have greater allochthonous organic inputs as well as significant benthic primary production than the large, deeper systems represented by the literature.

Nutrient dynamics in the west arm of San Quintín Bay, Baja California, Mexico, during and after El Niño 1997/1998

Time series of 8 h (May, September and November 1997, and March 1998) and 24 h (June and September 1999, and February 2000) were carried out at two (1997–1998) or three sites (1999–2000) of the west arm (Falsa Bay, FB) of San Quintín Bay, in order to assess the nutrient dynamics during and after the 1997/1998 El Niño event. Chlorophyll a concentrations were also determined during each time series. Non-conservative fluxes of dissolved inorganic phosphorus (∆DIP), dissolved inorganic nitrogen (∆DIN), nitrates (NO3–) and ammonium (∆NH4+) were estimated using a nutrient budget model. High NH4+ peaks (18–45 µM) were detected during El Niño 1997/1998, and the mean concentration values were higher than those of 1999–2000. Ammonium concentration was consistently less than 8 µM during 1999–2000. Significant correlations (r = 0.60 to 0.80) were found between tidal height and NO3– concentration in 1999–2000, as well as an inverse relationship between nitrates and salinity, thus reflecting the input of new DIN from the ocean. Chlorophyll a concentration was up to two times higher in 1999–2000 than during El Niño. There were no significant correlations between tidal height and nutrient concentration during 1997–1998, suggesting that: (a) the high NH4+ concentrations found in 1997–1998 were due to in situ remineralization of the organic matter and, possibly, the excretion by cultured mollusc bivalves in FB, and (b) the increase of chlorophyll a and NO3– during 1999–2000 was mainly due to new inputs from the adjacent ocean. The positive ∆DIP values (0.1 to 0.57) and negative values of the net ecosystem metabolism (NEM = –1 to –60 mmol m–2 d–1) observed throughout 1997–2000 indicate that FB permanently exported DIP towards the ocean and behaved as a net heterotrophic system, with or without the presence of El Niño.

Non-conservative P and N fluxes and net ecosystem production in San Quintin Bay, México

San Quintin Bay, Mexico, is a hypersaline coastal lagoon where the main external forcing of physical and biogeochemical processes is oceanic. Non-conservative fluxes of inorganic N (ΔDIN) and P (ΔDIP), and aspects of net ecosystem metabolism were studied in this lagoon during August 1995, August 1996, and February 1996, by following the LOICZ budgetary modeling approach. The whole-system water exchange time during summer (≈13 d) was shorter than in winter (≈26 d) as northwesterly winds enhancing mixing with the ocean are more intense during the spring-summer upwelling season. Whole-bay ΔDIP values of +0.2 to +0.3 mmol m−2 d−1 in August, and <+0.01 mmol m−2 d−1 in February indicate that the system is a net source of dissolved inorganic phosphorus (DIP). DIP fluxes from the Bay to the ocean during August are probably balanced by a net import of particulate organic matter between 1,000–1,300 × 103 mol C d−1, equivalent to a net ecosystem production (NEP) between −24 and −31 mmol C m−2 d−1. ΔDIN showed opposite trends in August 1995 and August 1996, with a net import of 13×103 mol N d−1 and a net export of 30× 103 mol N d−1, respectively. However, N fixation minus denitrification (“apparent denitrification”) estimates of ≈−4 mmol N m−2 d−1 in both periods indicate that San Quintin Bay is a net sink of nitrogen. Results from a 3-box model indicate that during summer Box C, adjacent to the ocean, contributed 70–80% of the excess DIP produced in the whole-system. This observation and high apparent denitrification values of ≈−7 mmol N m−2 d−1 at the entrance of the Bay, suggest that the net heterotrophic condition of San Quintin Bay in summer is largely determined by imports of labile phytoplanktonic carbon generated in the adjacent ocean during upwelling. A net flux of organic carbon of 30×106 mol C yr−1 was estimated from Box C, adjacent to the ocean, to Box B, locally known as Bahia Falsa, which is the area designated for oyster aquaculture in the lagoon. It is estimated that this net organic carbon supply is almost equivalent to the annual oyster food demand; our estimate is that oyster aquaculture in San Quintin Bay accounts for the vast majority of the net heterotrophy of Bahia Falsa.

The effects of multiple stressors on the balance between autotrophic and heterotrophic processes in an estuarine system

Responses of autotrophic and heterotrophic processes to nutrients and trace elements were examined in a series of experimental estuarine food webs of increasing trophic complexity using twenty 1-m3 mesocosms. Nutrients (nitrogen and phosphorus) and trace elements (a mix of arsenic, copper, cadmium) were added alone and in combination during four experimental runs spanning from spring 1997 to spring 1998. Diel changes in dissolved oxygen were used to examine whole system gross primary production (WS-GPP), respiration (WS-RESP), and net ecosystem metabolism (NEM). Nutrient and trace element additions had the greatest effect on WS-GPP, WS-RESP, and NEM; trophic complexity did not significantly affect any of these parameters (p . 0.3). Effects of trophic complexity were detected in nutrient tanks where bivalves significantly (p 5 0.03) reduced WS-GPP. Nutrient additions significantly enhanced WS-GPP and to a lesser extent WS-RESP during most mesocosm runs. The system shifted from net heterotrophy (217.2 6 1.8 mmol C m23 d21) in the controls to net autotrophy (29.1 6 7.6 mmol C m23 d21) in the nutrient tanks. The addition of trace elements alone did not affect WS-GPP and WS-RESP to the same extent as nutrients, and their effects were more variable. Additions of trace elements alone consistently made the system more net heterotrophic (224.9 6 1.4 mmol C m 23 d21)

Production, respiration and net ecosystem metabolism in U.S. estuaries.

Primary production, respiration, and net ecosystem metabolism (NEM) are useful indicators of ecosystem level trophic conditions within estuaries. In this study, dissolved oxygen data collected every half hour between January 1996 to December 1998 by the National Estuarine Research Reserve System Wide Monitoring Program were used to calculate primary production, respiration, and net ecosystem metabolism. Data from two sites at each of 14 Reserves were analyzed. On average, three quarters of the data available could be used to calculate metabolic rates. Data from two of the Reserves were used to evaluate the assumption of homogeneity of water masses moving past the oxygen sensor. Temperature was the single most important factor controlling metabolic rates at individual sites, although salinity was also important at about half the sites. On an annual basis, respiration exceeded gross primary production demonstrating that all but 4 of the 28 sites were heterotrophic.

Carbon, nitrogen, and phosphorus budgets for a shallow subtropical coastal embayment (Moreton Bay, Australia)

Average annual carbon, nitrogen, and phosphorus budgets were constructed for Moreton Bay. Primary production was the dominant source of carbon (by two orders of magnitude), N fixation was the dominant source of nitrogen, and point sources were the dominant source of phosphorus to the bay. About 41% of the nitrogen and 70% of the phosphorus entering Moreton Bay was exported to the ocean, and about 56% of the nitrogen was lost to denitrification. The high percentage loss of phosphorus to the ocean was directly related to the short residence time of the bay (46 d), which was consistent with other shallow coastal ecosystems. In contrast, the percentage loss of nitrogen to the ocean was low compared to other coastal systems due to the high percentage loss through denitrification associated with autotrophic sediments in the bay that enhance denitrification. Because most denitrification studies have been carried out using only dark incubations, the importance of denitrification to the nitrogen budgets of coastal systems in general may be underestimated. Carbon loss from Moreton Bay was dominated (by two orders of magnitude) by atmospheric exchange of CO2 associated with benthic and pelagic respiration. The distinct difference between Moreton Bay (subtropical) and temperate systems was the dominance of biological (microbial: N fixation and denitrification) over physical inputs and losses of nitrogen. High N fixation in turn fuels a positive annual mean net ecosystem metabolism (NEM) of 21 g C m-2 yr-1 and suggests that primary production in the bay is phosphorus limited at the whole ecosystem scale.

Latitudinal variation of the balance between plankton photosynthesis and respiration in the eastern Atlantic Ocean

A knowledge of the balance between plankton gross primary production (GPP) and community respiration (CR) in the open ocean is vital to the accurate determination of the global carbon cycle, yet the paucity of open ocean measurements severely limits our understanding. This study measured GPP, net community production, dark CR, and size‐fractionated primary production in the upper 200 m of a 12,100 km latitudinal (32°S‐48°N) transect in the Eastern Atlantic Ocean during May and June 1998. This comprehensive data set, which spans five contrasting plankton regimes, including two open ocean oligotrophic provinces, is used to derive a GPP:CR relationship, which suggests that net heterotrophy (GPP , CR) prevails in the eastern Atlantic when primary production falls below ~100 mmol O2 m−2 d−1. The predictive capability of this relationship is compared with that of the only other published relationship based on similar methodologies and is found to give a more representative description of the autotrophic (GPP &gt; CR) to heterotrophic seasonal cycle in the Bay of Biscay. This improved predictive power is attributed to the increased representativeness of the current data set. Specifically, the interpretation suggests that the influence of community structure on net ecosystem metabolism implies that prediction of GPP:CR balances in pelagic ecosystems can be best achieved by use of a data set that covers a wide range of community structure and not only a wide range in the magnitude of primary production.

Stoichiometry of the net ecosystem metabolism in a coastal inlet affected by upwelling. The Rı́a de Arousa (NW Spain)

Abstract The net uptake of inorganic carbon and nitrogen, phosphate and silicate and the net production of dissolved oxygen and organic carbon, nitrogen and phosphorus have been examined in the Ria de Arousa, a large coastal embayment in the NW Iberian upwelling system. Fluxes and net budgets were estimated with a non-stationary 2-D box model [Roson, G., Alvarez-Salgado, X.A., Perez, F.F., 1997. A non-stationary box-model to determine residual flows in a partially mixed estuary, based on both thermoline properties. Application to the Ria de Arousa (NW Spain). Estuarine, Coastal Shelf Sci., 44, 249–262] and the distributions of the different species acquired twice a week between May and October 1989 [Roson, G., Perez, F.F., Alvarez-Salgado, X.A., Figueiras, F.G., 1995. Variation of both thermohaline and chemical properties in an estuarine upwelling ecosystem: Ria de Arousa: 1. Time Evolution. Estuarine, Coastal Shelf Sci., 41, 195–213]. High N/P and N/Si net uptake ratios of 21 and 3.2 were observed during the upwelling season. The rapid recycling of phosphorus compared to nitrogen and the recurrent succession from pioneer diatoms (Si/N∼1) to red-tide forming species (Si/N=0) following the periodic upwelling pulses are the reasons behind the observed ratios. The molar ratios of dissolved oxygen production to inorganic carbon (−1.48) and nitrogen uptake (−10.2) during the upwelling season agree with the Redfield stoichiometry. On the contrary, net nutrient regeneration occurred with N/P, N/Si and O2/C ratios of 7.4, 1.0 and −1.02 during an intense autumn downwelling event. These low ratios are due to the release of an excess of phosphate, silicate and CO2 from the sediments. Conversely, the production of inorganic nitrogen is associated to the consumption of dissolved oxygen following a Redfield ratio of −10.0. Whereas the C/N ratio of the suspended organic matter produced during the upwelling season and consumed during the autumn downwelling event is 6.3–6.7, the N/P ratio changes from 11 during the upwelling season to 15 during the autumn downwelling. About 1/5 of the dissolved oxygen produced during the upwelling season and consumed during the autumn downwelling is delivered to and came from the atmosphere, respectively. Despite the C/N/P/O2 ratios differ from the Redfield values, the high correlation between nutrient salts consumption and dissolved oxygen production (r2=0.74–0.86) allow to estimate an average net ecosystem production (NEP) from the individual elements. The 3–4 d time-scale variability of the average NEP depends on the 2-week periodicity of upwelling pulses, the heat exchange across the sea surface and the stability of the water column. As much as 70% of the total variability can be explained with a linear combination of these parameters.

EFFECTS OF LAND USE ON STREAM METABOLISM AND ORGANIC MATTER TURNOVER

We measured ecosystem metabolism and organic matter transport seasonally in five streams draining catchments dominated by native forest, exotic pine plantation, grazed tussock grassland, or developed pasture. All streams are tributaries of the Taieri River in southeastern New Zealand. Whole-stream metabolism was estimated by both two-station and single-station open-channel methods, allowing comparison between these techniques. Transfer of oxygen across the stream surface was estimated using reaeration coefficients determined from three different procedures: tracer gas injections (propane), analysis of the oxygen record, and empirical velocity–depth equations. Measurements of gross primary production (GPP) and community respiration (CR) showed differences among streams that reflected method rather than ecosystem process. The ratio of GPP:CR and net ecosystem metabolism, however, showed strong concordance among methods, suggesting that bias involved in standardizing estimates to areal units may be more impo...

Short-term changes and longitudinal distribution of carbon metabolism in the Piauí River estuary (Sergipe, Brazil)

Net pelagic metabolism (NPM) and net ecosystem metabolism (NEM) were assessed by sigmaCO2 changes in three sampling stations along Piauí River estuary. At the upper estuary station, sampling was carried out over 48 h. Samples exhibited high DIN:DIP ratios. Chlorophyll-a ranged from 0.2 to 2.5 mug.l-1, being higher at the upper estuarine station than marine ones. Net pelagic metabolic rates ranged from -13.2 to 61.2 mgC.m7-2.d-1. Presence of mucilaginous algal material can explain the net mineralization. In the photic period, NPM ranged from-0.05 to 3.04 mgC.m-2h-1. NEM ranged from -7.77 to 6.65 mgC.m-2.d-1. Bottom ressuspension due to microphytobenthic flotation and a high turbidity plume inside de estuary reflected in negative metabolic rates (mineralization). Benthic metabolism was inferred by net system minus pelagic metabolism. Microphytobenthic community seems to be the main responsible for system metabolism, specially in the upper estuary, although the anthropogenic inputs exert strong long term influence.

Changes in production and respiration during a spring phytoplankton bloom in San Francisco Bay, California, USA:implications for net ecosystem metabolism

We present results of an intensive sampling program designed to measure weekly changes in ecosystem respiration (oxygen consumption in the water column and sediments) around the 1996 spring bloom in South San Francisco Bay, California, USA. Measurements were made at a shallow site (2 m, where mean photic depth was 60% of the water column height) and a deep site (15 m, mean photic depth was only 20% of the water column). We also estimated phytoplankton primary production weekly at both sites to develop estimates of net oxygen flux as the sum of pelagic production (PP), pelagic respiration (PR) and benthic respiration (BR). Over the 14 wk period from February 5 to May 14, PP ranged from 2 to 210, PR from 9 to 289, and BR from 0.1 to 48 mmol O 2 m-2 d-1, illustrating large variability of estuarine oxygen fluxes at the weekly time scale. Pelagic production exceeded total respiration at the shallow site, but not at the deep site, demonstrating that the shallow domains are net autotrophic but the deep domains are net heterotrophic, even during the period of the spring bloom. If we take into account the potential primary production by benthic microalgae, the estuary as a whole is net autotrophic during spring, net heterotrophic during the nonbloom seasons, and has a balanced net metabolism over a full annual period. The seasonal shift from net autotrophy to heterotrophy during the transition from spring to summer was accompanied by a large shift from dominance by pelagic respiration to dominance by benthic respiration. This suggests that changes in net ecosystem metabolism can reflect changes in the pathways of energy flow in shallow coastal ecosystems

Organic carbon balance and net ecosystem metabolism in Chesapeake Bay

Organic carbon balance and net ecosystem metabolism in Chesapeake Bay