Water Column Respiration Research Articles

Benthic and pelagic sources of carbon dioxide in boreal lakes and a young reservoir (Eastmain‐1) in eastern Canada

Lakes and reservoirs typically emit CO2to the atmosphere as a result of respiration in excess of primary production. For reservoirs, this respiration is often assumed to be primarily of benthic origin, associated with drowned vegetation and soils. This study took measurements from a young hydroelectric reservoir (Eastmain‐1) in boreal Canada and nearby natural lakes to compare the rates of water column respiration and hypolimnetic CO2 accumulation during a stratified period. Reservoir water column respiration rates were significantly higher than those in natural lakes, with surface means of 61.4 ± 4.6 mg C m−3 d−1in Eastmain‐1 and 40.2 ± 3 mg C m−3 d−1 in lakes and profundal means of 58.1 ± 11 mg C m−3 d−1in Eastmain‐1 and 17.6 ± 7.5 mg C m−3 d−1 in lakes. Reservoir hypolimnetic CO2 accumulation rates were also higher than those in natural lakes, with means of 78.8 ± 16 mg C m−3 d−1in Eastmain‐1 and 19.8 ± 2.7 mg C m−3 d−1 in lakes. Our findings further indicate that the relative contribution of benthic respiration (estimated as the difference between hypolimnetic CO2 accumulation and water column respiration) toward total net CO2production during stratified periods was similar in both aquatic systems (23% in lakes, 27% in Eastmain‐1). By identifying the major role of the water column in the reservoir's CO2 production, this study calls for a significant paradigm shift away from the common assumption that reservoir CO2 emissions are predominantly the result of decaying vegetation in the benthic environment.

Assessing constructed wetland functional success using diel changes in dissolved oxygen, pH, and temperature in submerged, emergent, and open-water habitats in the Beaver Creek Wetlands Complex, Kentucky (USA)

We assessed the functional success of restored wetlands by determining if the patterns in dissolved oxygen (DO), temperature, and pH were similar to those conditions observed in natural wetlands. The Beaver Creek Wetlands Complex consists of dozens of marshes and ponds built in a former Licking River floodplain, in the hills of east Kentucky, USA. In natural wetland ecosystems, aquatic primary production is highest in emergent and submerged vegetations zones; where daybreak dissolved oxygen (DO) is often near zero, and DO may rise to well over 100% saturation past mid-day. Open-water areas, dominated by phytoplankton, have less dramatic diel DO fluctuations—often without pre-dawn anoxia. Compared to open water, temperatures fluctuate less dramatically in vascular vegetation, due to shading and suppression of wind and waves. Measurements of ecosystem metabolism (diel changes in DO and pH) in three aquatic habitats of the constructed wetlands (emergent vegetation, submerged vegetation, open water) were compared to these natural ideals. In Beaver Creek Wetlands, water temperature patterns were not as dramatic as in natural habitats, nor did they did follow a similar trend. Waters in emergent vegetation (29.5 °C) were warmest; submerged vegetation coolest (26.5 °C); open-water intermediate (27.4 °C). Diel DO and pH patterns were not similar to natural habitats. Highest net primary production (NPP) and gross primary production (GPP) were measured in emergent vegetation waters (mean GPP = 7.58 g m −2 d −1); lowest in submerged vegetation (mean GPP = 5.48 g m −2 d −1); and intermediate in open-water (mean GPP = 6.95 g m −2d −1). Diel pH changes were greatest in the highly productive emergent waters (median maximum daily difference of 0.36), and not as pronounced in submerged vegetation and open-water (median maximum change = 0.16 and 0.22, respectively). Water-column respiration was generally about double NPP. Like natural ecosystems, near anoxic DO concentrations were consistently measured in emergent and submerged plants before dawn; whereas open-water zones were generally >4 mg l −1. These restored wetland systems may need more time to be functionally equivalent to natural marshes.

Sediment-water fluxes of dissolved inorganic carbon, O2, nutrients, and N2 from the hypoxic region of the Louisiana continental shelf

Globally, hypoxic areas (<63 mmol O2 m−3) in coastal waters are increasing in number and spatial extent. One of the largest coastal hypoxic regions has been observed during the summer in the bottom-water of the Louisiana continental shelf. The shelf receives the sediments, organic matter, and nutrients exported from the Mississippi River watershed, and much of this material is ultimately deposited to the sea floor. Hence, quantifying the rates of sediment-water dissolved inorganic carbon (DIC), oxygen (O2), and nutrient fluxes is important for understanding how these processes relate to the development and maintenance of hypoxia. In this study, the sediment-water fluxes of DIC, O2, nutrients, and N2 (denitrification) were measured on the Louisiana shelf during six cruises from 2005 to 2007. On each cruise, three to four sites were occupied in or directly adjacent to the region of the shelf that experiences hypoxia. DIC fluxes, a proxy for total sediment respiration, ranged from 7.9 to 21.4 mmol m−2 day−1 but did not vary significantly either spatially or as a function of bottom-water O2 concentration. Overall, sediment respiration and nutrient flux rates were small in comparison to water-column respiration and phytoplankton nutrient demand. Nitrate fluxes were correlated with bottom-water O2 concentrations (r = 0.69), and there was evidence that decreasing O2 concentrations inhibited coupled nitrification-denitrification. Denitrification rates averaged 1.4 mmol N m−2 day−1. Scaled to the area of the shelf, the denitrification sink represented approximately 39% of the N load from the Mississippi River watershed. The sediment-water fluxes reported from this study add substantial information on the spatial and temporal patterns in carbon, O2, and nutrient cycling available for the Louisiana continental shelf and, thus, improve the understanding of this system.

Linking forest fires to lake metabolism and carbon dioxide emissions in the boreal region of Northern Québec

AbstractNatural fires annually decimate up to 1% of the forested area in the boreal region of Québec, and represent a major structuring force in the region, creating a mosaic of watersheds characterized by large variations in vegetation structure and composition. Here, we investigate the possible connections between this fire‐induced watershed heterogeneity and lake metabolism and CO2 dynamics. Plankton respiration, and water–air CO2 fluxes were measured in the epilimnia of 50 lakes, selected to lie within distinct watershed types in terms of postfire terrestrial succession in the boreal region of Northern Québec. Plankton respiration varied widely among lakes (from 21 to 211 μg C L−1 day−1), was negatively related to lake area, and positively related to dissolved organic carbon (DOC). All lakes were supersaturated in CO2 and the resulting carbon (C) flux to the atmosphere (150 to over 3000 mg C m2 day−1) was negatively related to lake area and positively to DOC concentration. CO2 fluxes were positively related to integrated water column respiration, suggesting a biological component in this flux. Both respiration and CO2 fluxes were strongly negatively related to years after the last fire in the basin, such that lakes in recently burnt basins had significantly higher C emissions, even after the influence of lake size was removed. No significant differences were found in nutrients, chlorophyll, and DOC between lakes in different basin types, suggesting that the fire‐induced watershed features influence other, more subtle aspects, such as the quality of the organic C reaching lakes. The fire‐induced enhancement of lake organic C mineralization and C emissions represents a long‐term impact that increases the overall C loss from the landscape as the result of fire, but which has never been included in current regional C budgets and future projections. The need to account for this additional fire‐induced C loss becomes critical in the face of predictions of increasing incidence of fire in the circumboreal landscape.

Influence of river discharge on plankton metabolic rates in the tropical monsoon driven Godavari estuary, India

To examine the influence of river discharge on plankton metabolic balance in a monsoon driven tropical estuary, daily variations in physico-chemical and nutrients characteristics were studied over a period of 15 months (September 2007 to November 2008) at a fixed location (Yanam) in the Godavari estuary, India. River discharge was at its peak during July to September with a sharp decrease in the middle of December and complete cessation thereafter. Significant amount of dissolved inorganic nitrogen (DIN, of 22–26 μmol l −1) and dissolved inorganic phosphate (DIP, of 3–4 μmol l −1) along with suspended materials (0.2–0.5 g l −1) were found at the study region during the peak discharge period. A net heterotrophy with low gross primary production (GPP) occurred during the peak discharge period. The Chlorophyll a (Chl a) varied between 4 and 18 mg m −3 that reached maximum levels when river discharge and suspended loads decreased by >75% compared to that during peak period. High productivity was sustained for about one and half months during October to November when net community production (NCP) turned from net heterotrophy to autotrophy in the photic zone. Rapid decrease in nutrients (DIN and DIP by ∼15 and 1.4 μmol l −1, respectively) was observed during the peak Chl a period of two weeks. Chl a in the post monsoon (October–November) was negatively related to river discharge. Another peak in Chl a in January to February was associated with higher nutrient concentrations and high DIN:DIP ratios suggest possible external supply of nitrogen into the system. The mean photic zone productivity to respiration ratio (P:R) was 2.38 ± 0.24 for the entire study period (September 2007–November 2008). Nevertheless, the ratio of GPP to the entire water column respiration was only 0.14 ± 0.02 revealing that primary production was not enough to support water column heterotrophic activity. The excess carbon demand by the heterotrophs could be met from the allochthonous inputs of mainly terrestrial origin. Assuming that the entire phytoplankton produced organic material was utilized, the additional terrestrial organic carbon supported the total bacterial activity (97–99%) during peak discharge period and 40–75% during dry period. Therefore, large amount of terrestrial organic carbon is getting decomposed in the Godavari estuarine system.

Photosynthesis, water respiration and growth performance of Litopenaeus vannamei in a super-intensive raceway culture with zero water exchange: Interaction of water quality variables

Gross and net photosynthesis, and water column respiration of a super-intensive Litopenaeus vannamei raceway culture with zero water exchange were measured over 21 weeks using the light and dark bottle method. Spearman correlation analysis was used to explore the relationship of these parameters with other water quality parameters (O 2, temperature, pH, alkalinity, TAN, NO 2, NO 3, dissolved orthophosphate, turbidity, TSS and VSS) and of shrimp performance based on growth rate and feed conversion rate. Net photosynthesis generally had negative values, indicating that the system was predominantly heterotrophic. Gross photosynthesis was significantly related only with VSS, and water column respiration was related only with turbidity. Shrimp growth rate increased with higher temperature and dropped with higher pH. Nitrite had a strong inverse relationship with shrimp growth rate ( R 2 = 0.8044, p < 0.001) and a weak relationship with FCR ( R 2 = 0.3319, p = 0.0284). VSS has a significant inverse relationship with shrimp growth rate ( R 2 = 0.4989, p = 0.0102) and a direct relationship with FCR ( R 2 = 0.5356, p = 0.0162). Photosynthesis and the water column respiration did not seem to have a significant impact on shrimp productivity.

Surface microlayers on temperate lowland lakes

At the air–water interface material, organisms accumulate and form a thin layer of organic and inorganic material called the surface microlayer (SML). In order to investigate the development, composition, and metabolism of SML on lakes, samples were collected using a screen sampler along with subsurface water (SSW) in an eutrophic and a mesotrophic lake from April to September 2007. Wind, solar irradiance, and lake temperature were followed continuously. Samples were analyzed for organic and inorganic compounds as well as for photosynthesis and respiration. Most compounds were enriched in the SML relative to the SSW. Enrichment was small, however, probably because sampling was performed on nonslick areas. Most compounds correlated closely between the SML and the SSW, confirming the hypothesis that most SML material originates from the bulk water. Correlations were strongest in the eutrophic lake, probably because external sources had a greater effect on SML concentrations in the mesotrophic lake. Enrichment of compounds and metabolic rates in the SML had similar seasonality and dependency of climatic conditions in the two lakes, suggesting common regulating mechanisms of enrichment and production. Enrichment factors of several compounds were higher at low bulk water concentrations, suggesting that atmospheric deposition then contributed relatively more to concentrations in the SML. Increasing temperature significantly decreased SML enrichment of TOC (total organic carbon), related to changes in TOC composition and higher heterotrophic activity, while wind and solar irradiance had no pronounced enrichment effect on any compound. Net photosynthesis was significantly lower in the SML, experiencing photoinhibition in one-third of the samples. In contrast, respiration was much elevated in the SML. Nonetheless, respiration in the SML never contributed by more than 0.3% of water column respiration, but the combination of enhanced degradation rates of organic carbon in the SML and strong interaction with water below suggests that the SML, nonetheless, may play an important role in degradation of refractory organic carbon. Combining these results, we found that the SML of nonslicked areas on lakes are enriched in organic and inorganic pools and constitute a strong heterotrophic environment, albeit of minor importance for whole lake pelagic metabolism.

Gulf of Mexico Hypoxia: Alternate States and a Legacy

A 20+ year data set of the size of the hypoxic zone off the Louisiana-Texas coast is analyzed to reveal insights about what causes variation in the size of the hypoxic zone in summer, the accumulation of carbon storage in sediments, and pelagic and sediment oxygen demand. The results of models support the conclusion that some of this variation can be explained by a higher sedimentary oxygen demand, which may be larger than water column respiration rates in summer. Proxies for organic loading to sediments reveal that carbon losses continue after accumulation, and results from other studies indicate that sediment oxygen demand is directly related to surface water phytoplankton production, which has increased because of higher nutrient loading from the Mississippi River watershed. The potential size of the hypoxic zone for a given nitrogen load has increased as a result and has doubled from 1980 to 2000. The development of widespread hypoxia after the early 1980s and its consequences could, therefore, be considered a shift to an alternate ecosystem state. The Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico goal of reducing the size of the hypoxic zone to an average of 5000 km2 by 2015 becomes more difficult to achieve for every year there is no significant reduction in nutrient loading. The decisions made to reduce the size of the hypoxic zone must incorporate these nonlinear responses and, we think, err on the side of caution in assuming that existing management efforts are sufficient to restore water quality on this shelf. The legacy of a higher sediment respiratory demand following eutrophication should apply to other coastal systems.

How does the character of oxygen demand control the structure of hypoxia on the Texas–Louisiana continental shelf?

A realistic hydrodynamic model of the Texas–Louisiana shelf is configured with various simple oxygen respiration models to isolate the effects of stratification and circulation on the formation and maintenance of hypoxia. Biological activity is parameterized through various forms of respiration rather than using a complex biogeochemical model. The model domain covers the region that has historically been observed to be affected by seasonal hypoxia, and is forced with observed fresh water fluxes from the Mississippi and Atchafalaya Rivers and winds. Three simple parameterizations of biological respiration are compared. Results of the numerical simulations indicate that water column respiration (dependent on the position of the two regional fresh water plumes) reproduces observed temporal and spatial structures of seasonal hypoxia in Louisiana Bight, whereas benthic respiration (dependent on local temperature and oxygen concentrations) reproduces the temporal and spatial structure of hypoxia west of Terrebonne Bay. The differences in the structure of hypoxia are related to the differences in vertical stratification east and west of Terrebonne Bay, which are controlled by the outflow characteristics from the two major river plumes. The model shows two dynamically distinct plumes. The Mississippi River plume enters the shelf near the shelf edge, forms a recirculating gyre in Louisiana Bight, and typically does not interact directly with topography. Conversely, the Atchafalaya River plume enters a broad shelf at the coast, is more diffuse, and interacts more with the shallow coastal topography. Both plumes are strongly affected by winds, and tend offshore during the mean summer conditions of upwelling winds. The principle conclusion of this study is that the biological processes responsible for producing hypoxia change from east to west, with the shelf region south of Terrebone Bay being the approximate dividing line between water column respiration (predominantly causing hypoxia to the east) and benthic respiration (causing hypoxia to the west). Also, the formation and destruction of hypoxia is a primarily vertical process; hypoxia formed in one region is not advected laterally to different shelf regions.

Pelagic production and respiration in the Gulf of Papua during May 2004

The metabolic balance between production and respiration in plankton communities of the Gulf of Papua was investigated in May 2004. Water samples taken at 19 stations were allocated to groups on the basis of physico-chemical characteristics. Oxygen consumption and production in flasks incubated in the dark and in the light was determined by micro-Winkler titration. Dark bottle respiration in samples influenced by the estuarine plume averaged 3.09±1.92 (SD) mmol O 2 m −3 d −1 and production within surface light bottles averaged 7.63±3.36 (SD) mmol O 2 m −3 d −1. Corresponding values in stations more typical of the central Gulf of Papua were 1.68±1.30 (SD) mmol O 2 m −3 d −1 and 1.08±2.25 (SD) mmol O 2 m −3 d −1. Despite a shallow (<10 m) euphotic zone within the plume stations, phytoplankton production in the surface layers was sufficiently high to subsidise total water column respiration. Integrating production and respiration over the water column resulted in a calculation of net community production (NCP) of 626±504 (SD) mg C m −2 d −1, and community respiration (CR) of 712±492 mg C m −2 d −1 at the plume stations, with an average P: R ratio of 1.97. In the offshore group NCP was 157±450 (SD) mg C m −2 d −1 and CR was 1620±1576 mg C m −2 d −1. The average P: R ratio was 1.27. Three of the 7 stations allocated to the offshore group were net heterotrophic. In contrast to earlier studies in the area indicating that the Gulf of Papua waters is heterotrophic [Robertson, A.I., Dixon, P., Alongi, D.M., 1998. The influence of fluvial discharge on pelagic production in the Gulf of Papua, Northern Coral Sea. Estuarine, Coastal and Shelf Science 46, 319–331], our data indicate that in May 2004 the Gulf was in positive metabolic balance, but by only ∼120 mg C m −2 d −1. We conclude that waters of the Gulf of Papua under riverine influence are net autotrophic, but that within the central Gulf there is a fine metabolic balance alternating between autotrophy and heterotrophy.

Respiration, mineralization, and biochemical properties of the particulate matter in the southern Nansen Basin water column in April 1981

Determinations of the activity of the respiratory electron transport system (ETS), during the FRAM III expedition permit us to estimate oxygen utilization rates ( R O 2 ) from the surface to 2000 m under the polar pack ice in the Nansen Basin just north of Svalbard (83°N, 7°E) during April 1981. We found R O 2 at in situ temperatures ranging from 20 pM O 2 min −1 just below the ice to 0.2 pM O 2 min −1 at 2000 m. These rates are low compared to most other ocean regions, but they could decrease particulate organic carbon and nitrogen by 76% and 74%, respectively, over a period of ∼6 months. The R O 2 calculations based on measurements made at 0 °C yielded a power function of R O 2 vs. depth ( Z) of R O 2 =67 Z −0.5534. When this R O 2 profile was superimposed on a more recent oxygen utilization rate profile made using the 3He– 3H–AOU method (OUR), in the same vicinity of the Nansen Basin during 1987 (OUR=52 Z –0.4058, [Zheng, Y., Schlosser, P., Swift, J.W., Jones, E.P., 1997. Oxygen utilization rates in the Nansen Basin, Arctic Ocean: implications for new production. Deep Sea Research I 44, 1923–1943]), the agreement of the two profiles was close. On one hand, this was to be expected because R O 2 is the biological basis of OUR, on the other hand, it was a surprise because the methodologies are so different. Nitrate mineralization obtained from ETS activities also compared favorably with calculations based on the data of Zheng et al. [1997. Oxygen utilization rates in the Nansen Basin, Arctic Ocean: implications for new production. Deep Sea Research I 44, 1923–1943]. Chlorophyll ranged from 6 ng L −1 at 5 m to 0.06 ng L −1 at 2000 m. Particulate organic carbon (POC) decreased from 0.93 μM C just below the ice to less than 0.4 μM C at 500 m. Particulate organic nitrogen (PON) was not detectable below 70 m, however in the upper 70 m it ranged from 0.16 to 0.04 μM N. The C/N mass ratio over these depths ranged from 5.8 to 11.3. Annual carbon productivity as calculated to balance the total water column respiration was 27 g C m −2 y −1. The integrated respiration rate between 50 and 4000 m suggests that exported production and carbon flux from the 50 m level was 24 g C m −2 y −1. These are minimal estimates for the southern Nansen Basin because they are based on measurements made at the end of the Arctic winter.

Evaluation of respiration rates and mechanical aeration requirements in semi-intensive shrimp Litopenaeus vannamei culture ponds

To increase the efficiency of dissolved oxygen management in semi-intensive shrimp Litopenaeus vannamei culture in Santa Catarina, Southern Brazil, the present study aimed at characterizing the respiration rates in three ponds (1.2 ha) and estimating the approximate calculation of the actual aeration requirements (HP ha −1) during 9 weeks of a production cycle (February–April, 2003). The sediment respiration rate (SR) and the water column respiration rate (WR) were characterized using the column method, by which ponds initial and final dissolved oxygen concentrations (surface and bottom) were determined, then incubated for 24 h in PVC tubes fixed in the experimental units. SR was significantly higher than WR, except for the third week of culture. SR showed a wide variation (0.07–0.16 g m −2 h −1) during the experiment and the highest respiration rate occurred in the last week of culture. Sediment analyses were done in the beginning and in the end of the experiment, and they also showed differences in the concentration of organic matter (3.4 and 6.3%). On the other hand, WR varied little (0.04–0.07 g m −2 h −1) and presented a pattern similar to photosynthesis, determined by the dark and clear bottles method. Temporal aeration requirements were characterized from SR and WR data, and also from the shrimp mean respiration rate described in the literature. More aeration was required in the last week of culture (2.62 ± 0.06 HP ha −1). However, only the aeration calculated for the first week was close to that used in the Yakult Experiment Farm (UFSC), where the study was carried out (1.08 and 1.70 HP ha −1, respectively). All aeration estimates from this study were below those used in cultures at UFSC, especially after the third week of experiment.

Using stable isotope analyses to assess carbon dynamics in a shallow subtropical estuary

The sources of carbon, which fuel water column respiration, remain unresolved for most estuaries; our objective was to examine carbon dynamics in a shallow subtropical estuary. We sampled the Sabine-Neches estuary, Texas, during low (November 1999) and high (May 2000) freshwater inflow and measured stable carbon isotope ratios of the dissolved inorganic and orgnaic carbon (δ13C-DIC, δ13C-DOC), as well as quantifying accessory parameters (salinity, nutrients, total suspended solids, and photosynthetic pigments). Pigment analysis indicated that diatoms were the predominant phytoplankton. Data from the May 2000 sampling event exhibited conservative mixing, indicating that the system was acting as a conduit between the watershed and the Gulf of Mexico. During November, mixing was generally nonconservative indicating extensive recycling of allochthonous and autochthonous carbon sources. Our data imply that both carbon sources had similar isotope, ratios that made it impossible to unambiguously determine the dominant source supporting respiration. The nonconservative DIC concentration data indicating an autotrophic sink as well as the strong relationship between δ13C-DOC and chlorophylla, suggest that in situ production was an important component of the DOC pool. We hypothesize that uncharacteristically calm wind conditions during sampling may have promoted phytoplankton settling, removing autotrophs, from the water column, but leaving behind a dissolved biogeochemical signature. Interpretation of carbon dynamics may be confounded by spatial and temporal decoupling of producers and consumers from biogeochemical indicators.

Hypolimnetic oxidation rates in Lake Superior: Role of dissolved organic material on the lake's carbon budget

On average, the water column of Lake Superior is undersaturated with respect to dissolved oxygen and supersaturated with respect to carbon dioxide during the summer‐stratified period. On the basis of temporal changes in water column dissolved oxygen, we calculate rates of oxygen consumption that range from 0.19 to 0.75 mmol m−3 d−1. These rates are a factor of 5–10 times larger than can be supported by the particulate carbon settling rates and benthic oxygen consumption rates. In addition, on the basis of the limited information available, dissolved allochthonous carbon inputs are insufficient to account for the calculated rates of carbon oxidation. Rates of nitrate and total CO2 ΣCO2) production are 0.019 ± 0.012 and 0.13 ± 0.06 mmol m−3 d−1, respectively, and are consistent with the oxidation of a dissolved organic component that is similar in composition (C :N ratio) to the settling particulate material. Previously published estimates of total primary production were smaller but similar in magnitude to our integrated water column respiration rates. We interpret the observed imbalance between particulate carbon delivered to the deep lake and the calculated rate of carbon oxidation to be the result of the decomposition of dissolved organic carbon that appears to have both an autochthonous and an allochthonous component.

Organic carbon distribution and water column respiration in the NW Africa-Canaries Coastal Transition Zone

The Canary Current (CC) is characterised by strong disruption of its flow by the Canaries Archipelago, which extends across the prevailing flow. In the Coastal Transition Zone (CTZ) downstream of the islands, a region of high mesoscale activity connects the eutrophic waters of the NW Africa coastal upwelling (UW) system with the oligotrophic open ocean waters of the eastern subtropical North Atlantic gyre (ESNA). We studied the distribution of particulate (POC) and dissolved organic carbon (DOC) and carbon remineralization rates in the 0 to 1000 m water column of this transitional region to assess its role in horizontal and vertical export of organic matter. The epipelagic waters (0 to 200 m) of the CTZ are characterised by higher mean surface integrated concentrations of POC (0.8 ± 0.4 mol C m -2 ) and DOC (17 ± 7 mol C m -2 ), and by higher community respiration (R >100 mmol C m -2 d -1 ) than the ESNA or UW waters. Integrated R was much lower (7 ± 3 mmol C m -2 d -1 ) in the mesopelagic zone (200 to 1000 m), indicating that most of the organic mat- ter was respired in the upper 200 m, with little being transported downwards. Nevertheless, peaks in DOC and R were observed at 300 to 600 m depth, coinciding with the depth of the deep scattering layer. These relative maxima were particularly important in the case of DOC, reaching concentra- tions comparable to surface waters (> 80 µM). In spite of this, DOC contributed only 26.5% to the apparent oxygen utilisation (AOU) in the mesopelagic zone. Correlations between R and particulate- and dissolved organic matter suggest that respiration was mainly supported by particulate material at surface and deep waters. We compared plankton metabolic balances along the UW, CTZ and ESNA. Our analyses show that during summer time, and probably during most of the year, the ESNA and CTZ behave as net heterotrophic regions. In spite of a relatively high primary production (P) favoured by mesoscale activity in the CTZ, plankton R clearly exceeds P as result of the excess carbon advected from the coastal UW. However, a significant amount of non-respired carbon must be trans- ported from the CTZ to the open ocean to account for part of the surface metabolic imbalance between P and R reported for the ESNA.

Seasonal hypoxia in the bottom water off the Mississippi River delta.

Hypoxia (oxygen concentration less than 2 mg L-1 or 62.5 mmol m-3) occurs on the Louisiana continental shelf during summer when the consumption of oxygen by sediment and water column respiration exceed resupply by photosynthesis and mixing. Biological processes that consume or produce oxygen have been summarized in a budget that can be used to quantify the degree to which consumption in deep water and in the sediments exceeds net production and thus the time it takes to reach hypoxic conditions following the spring onset of stratification. The net consumption rate by the sea floor biota (sediment oxygen consumption, SOC) is inversely related to oxygen concentration and directly related to temperature. Photosynthesis is of potential importance throughout the deep water column and on the sea floor when light is adequate. A non-steady state, time-dependent numerical simulation model is used to compare biological and physical processes with shipboard measurements and continuous near-bottom records. The simulations illustrate possible variations in oxygen concentration on time scales of hours to months, and these in general match much of the variability in the direct observations at time scales of days to weeks. The frequently observed unremitting anoxia lasting weeks at some locations is not produced in the present simulations. A possible explanation is the chemical oxidation in the water column of reduced metabolic end-products produced in the sediments by anaerobic metabolism. Direct measurements of biological processes could lead to better understanding of how extrinsic forcing functions can best be managed to improve water quality.

Effects of solar radiation on dissolved organic matter cycling in a subtropical seagrass meadow

The influence of sunlight on bacterioplankton production was investigated over a 12‐month period in Laguna Madre, a shallow, subtropical, seagrass‐dominated lagoon in south Texas. Two types of experiments were conducted every 6 to 8 weeks from June 1996 through June 1997 to assess the effect of photochemical transformations of dissolved organic matter (DOM) on bacterial production and the net effect of sunlight exposure on bacterioplankton and phytoplankton production in surface waters. Photobleaching of DOM was observed; however, estimates of the photomineralization of DOM in May and July were very low and indicated that this process could represent at most 3% of water column respiration. Photochemical transformations of DOM did not have any significant effect on bacterioplankton activity. However, bacterial production measured during exposure of water samples to natural sunlight in the summer months, with and without ultraviolet light, was enhanced relative to dark controls. Phytoplankton release of bioavailable substrates appeared to be responsible for light‐mediated increases in bacterioplankton production. Bacterio‐ and phytoplankton were not photoinhibited by exposure to ultraviolet radiation in surface waters. Overall, we observed no indication of photoinhibition of microbial processes in Laguna Madre, and photochemical transformations appeared to play a minor role in the cycling of DOM. The low photoreactivity of DOM in combination with its relatively high bioreactivity appeared responsible for the minor role of photochemical transformations in the cycling of DOM in Laguna Madre.

Seasonal and short-time-scale dynamics of microplankton community production and respiration in an inshore upwelling system

An intensive study of pelagic primary production and microplankton community respiration was carried out during an entire upwelling season in the Ria de Vigo (NW Spain). From April to November measurements of oxygen production and respiration using the light-dark bottle technique were made twice a week at the surface, 1% light depth (1 % LD, 12 ± 4 m) and 40 m (8 m above sea floor) alongside routine physical, chemical and biological measurements. During the major part of the survey period intermittent intrusions of cold, nutrient-rich upwelled water were observed in the ria with a periodicity of about 2 wk. Rates of gross primary production (GPP) were high but variable averaging 37.3 ± 30.7 μM O 2 d -1 and 3.6 ± 4.8 μM O 2 d -1 at the surface and 1% LD respectively over the period of survey (n = 50). Rates of dark community respiration (DCR) were also high and variable with maximum values being observed in the surface layer where the seasonal average was 12.2 ± 9.8 μM O 2 d -1 . At the 1% LD and 40 m, DCR averaged 5.3 ± 4.4 and 2.8 ± 3.0 μM O 2 d -1 respectively. Although seasonal average and maximal DCR (up to 46.5 μM O 2 d -1 ) were among the highest reported for coastal areas, microplankton production over the period of survey was dominated by autotrophic processes. Respiration losses by the microplankton community in the euphotic zone represented on average 43% of estimated mean seasonal water column GPP (2.1 to 2.7 g C m -2 d -1 ). Net heterotrophy in the aphotic layer consumed the equivalent of a further 25 % of estimated water column GPP. The degree of coupling between primary production and respiration was primarily controlled by upwelling. During upwelling events respiration was generally low in the water column but it increased as a linear function of chlorophyll a concentration (R 2 = 0.55, n = 13) and GPP (R 2 = 0.47, n = 13) in the surface layer. Under such condition phytoplankton appears as the dominant component of community respiration consuming 14% of GPP. During periods of upwelling relaxation respiration was high relative to GPP. High water column respiration rates extending occasionally down to 40 m took place at the expense of organic matter trapped inside the bay. The seasonal breakdown of thermal stratification in autumn presented a relationship between surface respiration and chlorophyll a or GPP similar to that observed during upwelling events. The large excess primary production during this period was not remineralised inside the ria, suggesting that a large fraction may be exported towards the shelf.

Origin and fate of organic carbon in the freshwater part of the Scheldt Estuary as traced by stable carbon isotope composition

We investigated the seasonal and geographical variation in the stable carbon isotope ratios of total dissolved inorganic carbon (δ13 cdic) and suspended matter(δ13 CPOC) in the freshwater part of the River Scheldt. Two major sources of particulate organic matter (POM) occur in this riverine system: riverine phytoplankton and terrestrial detritus. In winter the lowest δ13 cdic values are observed due to enhanced input of CO2 from decomposition of13C-depleted terrestrial plant detritus (average δ13Cdic=-14.3‰). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of terrestrial origin is also the lowest as evidenced by less negative δ13Cdic values (average δ13Cdic =−9.9‰). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a 9) suggesting that in winter a major fraction of POC is derived from allochthonous matter. In summer δ13 cpoc exhibits a very wide range of values, with the most negative values coinciding with high Chl a concentrations and low C/N ratios (C/N< 8). This suggests predominance of phytoplankton carbon in the total particulate carbon pool, utilising a dissolved inorganic carbon reservoir, which is already significantly depleted in13C. Using a simple two source mixing approach a reconstruction of the relative importance of phytoplankton to the total POC pool and of13C/12C fractionation by phytoplankton is attempted.

Dissolved organic carbon cycling in a subtropical seagrass-dominated lagoon

The cycling of dissolved organic matter (DOM) and its significance to ecosystem metabolism was studied over a 16 mo period in a Thalassia testudinum dominated meadow. The benthos was usually net autotrophic (annual gross primary production to respiration ratio [P:R] = 1.3) while water column respiration (R) exceeded gross primary production (annual P:R = 0.3). Net fluxes of dissolved organic carbon (DOC) from the benthos primarily occurred in the light (0 to 18 mmol C m -2 d -1 ) and from seagrass-dominated areas, suggesting that release of DOC was mainly due to seagrass exudation. Net benthic DOC fluxes measured in the light were significantly correlated (p 50 %) of the DOC released from the benthos on a daily basis. Annual estimates of bacterioplankton C demand, based on water column R (∼8 mol C m -2 yr -1 ), represented >50% of the benthic NPP (∼14 mol C m -2 yr -1 ) These measurements indicate a stronger linkage between benthic and water column processes than previously believed, and it appears that water column heterotrophic processes are largely dependent upon seagrass exudation.