Oxygen Content In Water Column Research Articles

Ferromanganese crusts of the Vietnam margin, central South China Sea: Composition and genesis

Ferromanganese (FeMn) crusts collected from two seamounts of the Vietnam continental margin, central South China Sea were analyzed for mineralogical and chemical compositions, and dated using 230Thexcees technique and cobalt chronometry. The studies of dissolved oxygen content in water column at the area by CTD-profiling display Oxygen Minimum Zone locates at the depth 420 to 1100 m with O2 concentration of 1.6–1.8 ml/l. The composition and morphology of studied crusts are different from Prime Crust Zone crusts in Pacific Ocean and partially similar to ferromanganese crusts and nodules from northern South China Sea due to unique environment of basin. The FeMn crusts from Vietnam margin show high ratios of Fe relative to Mn and Ni relative to Co, high Th and Pb concentrations and comparable to PCZ deposits low contents of REE. Few main mechanisms were dominated in crust formation at Vietnam margin including a permanent input of detrital material sourced by river discharge, shelf sediments and dust plume while Fe and Mn (oyxhydr)oxides slow precipitate from seawater as initially colloidal particles within oxygen-minimum zone predominantly at the surface of biogenic limestone substrate. Upwelling and high coastal primary productivity are important factors in Fe, Ni, Th and Pb transportation and accumulation. The main factor of high Fe/Mn ratio is the significant flux of Fe released by redox cycling in the sediment. SCS crusts are enriched in Ni relative to Co due to influx of Ni from coastal laterite weathering and optimum conditions for its transportation to crust formation area. Thorium and lead related to significant continental detrital input and eolian supply, especially Asian dust. Due to diversity of crust thicknesses local hydrographic current regime is also suggested as crucial factor in crust formation. Calculated growth rate using 230Thexcees technique is 11 ± 1 mm/Myr and 4.4 ± 0.5 mm/Myr for two measured crusts. The crust growth in South China Sea might be initiated around 1.5 Myr ago, the period which coincide with the Southern Ocean glacial-induced restructuring dated 1.6 Myr.

A high-resolution record of Holocene primary productivity and water-column mixing from the varved sediments of Lake Żabińskie, Poland

Eutrophication and anoxia are increasing in lakes worldwide. However, our understanding of variations of primary productivity and anoxia in lakes over thousands of years is limited. Long-term records are needed to understand the natural variability of lake ecosystems and to improve our understanding of drivers of productivity and anoxia. In this study, we used the varved sediment record of Lake Żabińskie, Poland to answer the following research questions: 1) How have primary production and water column oxygen concentrations varied during the past 10,800 years?; 2) what role did natural and anthropogenic forces have in driving changes in primary production or lake mixing regime? Recently developed hyperspectral imaging (HSI) techniques were used to quantify sedimentary chloropigments-a and bacteriopheopigments-a (Bphe-a) at sub-annual resolution. These data, combined with elemental data from micro X-ray fluorescence (μ-XRF) and pigment assemblage data from high-performance liquid chromatography (HPLC) measurements, were used to reconstruct paleolimnological conditions. Bphe-a was used as an indicator of anoxia, and its presence suggests that an extensive anoxic zone was present nearly continuously from 10.8 to 2.8 ka BP. Anoxic conditions, driven by thermal stratification, were promoted by closed forest cover during that time, which limited wind-driven mixing of the water column. After 2.8 ka BP, water column oxygenation occurred more frequently, particularly during periods of increased human agricultural activity and forest opening. Pronounced anoxia was again present continuously from ~610 to 1470 CE, concurrent with a period of reforestation. After ~1610 CE, deforestation caused increases in erosion rates, algal production, and water column oxygenation. Pigment assemblages indicate that the algal community during the past 150 years was different from any other time during the Holocene. This study demonstrates a clear link between lake biogeochemical processes and forest cover and shows the potential of HSI to produce extremely high-resolution records of past productivity and redox conditions from varved lake sediments.

High temperatures cause reduced growth, plant death and metabolic changes in eelgrass Zostera marina

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 604:121-132 (2018) - DOI: https://doi.org/10.3354/meps12740 High temperatures cause reduced growth, plant death and metabolic changes in eelgrass Zostera marina Kathrine Jul Hammer1, Jens Borum1,*, Harald Hasler-Sheetal2,3, Erin C. Shields4, Kaj Sand-Jensen1, Kenneth A. Moore4 1Freshwater Biological Laboratory, Biological Institute, University of Copenhagen, 2100 Copenhagen, Denmark 2Nordcee, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark 3Villum Center for Bioanalytical Sciences, University of Southern Denmark, 5230 Odense M, Denmark 4Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, VA 23062, USA *Corresponding author: jborum@bio.ku.dk ABSTRACT: We investigated temperature effects on eelgrass Zostera marina L. growing close to its southern distribution limit along the eastern coast of North America in Virginia, USA. We combined growth and survival experiments with microelectrode measurements of internal meristematic oxygen and analyses of metabolic compounds. Eelgrass shoots were grown at 3 different temperatures (22, 26 and 30°C) and field equivalent light levels (23% of sea surface insolation) for 28 d while water column oxygen concentration was kept at air saturation. Meristematic oxygen concentrations did not vary significantly with temperature, and meristems maintained a relatively high oxygen concentration (average: 38% air saturation) during dark hours. Despite high meristematic oxygen concentrations, shoots growing at 30°C exhibited increased mortality, reduced growth and reduced leaf production compared to shoots growing at 22 and 26°C. The leaf metabolome was significantly altered at 30°C, indicating an increase of reactive oxygen species. In addition, total nitrogen and metabolites related to the nitrogen cycle (amino acids, urea and γ-aminobutyric acid [GABA]) were low in the heat-stressed shoots, whereas soluble sugars increased. In conclusion, high temperature (30°C) has strong negative effects on eelgrass in the lower Chesapeake Bay, affecting growth, tissue integrity, nitrogen metabolism and protein/enzyme synthesis. Future global warming may likely deteriorate populations of eelgrass at its present southern distribution limit. KEY WORDS: Zostera marina · Temperature stress · Growth · Metabolomics · Climate change Full text in pdf format Supplementary material PreviousNextCite this article as: Hammer KJ, Borum J, Hasler-Sheetal H, Shields EC, Sand-Jensen K, Moore KA (2018) High temperatures cause reduced growth, plant death and metabolic changes in eelgrass Zostera marina. Mar Ecol Prog Ser 604:121-132. https://doi.org/10.3354/meps12740 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 604. Online publication date: October 04, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

The interplay of methane and ammonia as key oxygen consuming constituents in early stage development of Base Mine Lake, the first demonstration oil sands pit lake

The efficacy of oil sands water capped tailings technology is currently being assessed in the first pilot pit lake in the Athabasca Oil Sands region (AOSR). Base Mine Lake (BML, Syncrude Canada Mildred Lake mine, Fort McMurray AB Canada) was commissioned in December of 2012 and consists of circa 40 m depth of fluid fine tailings (FFT) overlain by circa 10 m of a freshwater cap. As this is the first oil sands pit lake, it is unknown to what extent known oxygen consuming constituents (OCC) such as methane, hydrogen sulfide and ammonia will mobilize from the underlying FFT layer in BML and impact oxygen concentrations within the water cap. Thus, the field objectives here were to characterize the physico-chemistry and geochemistry of the BML water cap from the FFT water interface (FWI) to the surface over two summers (2015 and 2016). Results identify that the ∼10 m water cap thermally stratifies and that oxygen persists, albeit at low levels (i.e. <5% saturation) to the FWI during the summer season for both years. Consistent with the FFT acting as an OCC source, aqueous CH4, ∑H2S and NH4+ concentrations were highest closer to the FWI, decreasing upwards into the water cap. As O2 persists to the FWI, ∑H2S was rapidly removed within this region, with little mobilized to the overlying water as evidenced by nondetectable ∑H2S within 1 m of the FWI. In contrast, CH4 and NH4+ concentrations were detectable higher up into the water cap, indicating incomplete oxidation at the FWI most likely due to oxygen limitation. In 2015, CH4 was the only identified variable significantly negatively related to BML water cap oxygen concentrations. However in 2016, NH4+ emerged as an important OCC negatively related to water cap oxygen concentrations in addition to CH4. Mass balance of nitrogen redox species throughout the water column are consistent with active nitrification occurring in 2016; which was not evident in 2015 results. Thus, within 4 years of commissioning, nitrification has become an active and important process affecting water column oxygen concentrations within this pit lake. This is in contrast to an absence of detectable microbial nitrification in active oil sands tailing ponds, despite high levels of ammonia. While the absence of nitrification occurring in oil sands tailings ponds has previously been suggested to reflect naphthenic acids toxicity, results here identify that some lower limit of oxygen may also be required for this metabolism to flourish and that at very low oxygen levels (i.e. <5% saturation), nitrifying microbes appear to be outcompeted by methanotrophs, at least during early establishment stages within a pit lake.

Major impacts of climate change on deep-sea benthic ecosystems

The deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. Projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep-water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L–1 by 2100. Bathyal depths (200–3000 m) worldwide will undergo the most significant reductions in pH in all oceans by the year 2100 (0.29 to 0.37 pH units). O2 concentrations will also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century. Unfortunately, how these major changes will affect deep-seafloor ecosystems is, in some cases, very poorly understood. In this paper, we provide a detailed overview of the impacts of these changing environmental parameters on deep-seafloor ecosystems that will most likely be seen by 2100 in continental margin, abyssal and polar settings. We also consider how these changes may combine with other anthropogenic stressors (e.g., fishing, mineral mining, oil and gas extraction) to further impact deep-seafloor ecosystems and discuss the possible societal implications.

Environmental impacts on the distribution of microbial tetraether lipids in Chinese lakes with contrasting pH: Implications for lacustrine paleoenvironmental reconstructions

Glycerol dialkyl glycerol tetraethers (GDGTs) in lake sediments are useful biomarkers for the continental paleoclimatic reconstruction. However, the environmental controls on the distribution of these compounds, in particular the 6-methyl isomers of bacterial branched GDGTs (bGDGTs), in the lakes with contrasting pH, are still unknown, hindering their application for paleo-reconstructions. Here, we investigated the environmental impacts on the distribution of GDGTs in 17 alkaline lakes and 1 acid lake in China. It was found that the dissolved oxygen content in water column may have an impact on the distribution of archaeal isoprenoid GDGTs (iGDGTs) by causing the change in archaeal communities. The ratio of GDGT-0/crenarchaeol increases with decreasing oxygen content, indicating that the relative abundance of anaerobic methanogenic archaea or Miscellaneous Crenarchaeotic Group (MCG) vs. aerobic Thaumarchaeota is controlled by the oxygen content dissolved in water of these lakes. Thaumarchaeota are likely to contribute only a small proportion of iGDGTs in the relatively oxygen-depleted lakes, and thus TEX86 is not suitable for the reconstruction of the surface temperature of these lakes. The abundance ratio of iGDGTs to bGDGTs (R i/b ) appears to show no relationship with water pH in all the lakes, but exhibits a significant positive correlation with the water depth of the acid Lake Qinghai in Tengchong. As expected, the methylation degree of bGDGTs (MBT′) was found to correlate with both mean annual air temperature (MAT) and water pH, and the cyclization degree of bGDGTs (CBT) correlates only with water pH in these lakes. However, the MBT′5ME, an index to measure the methylation degree of 5-methyl bGDGTs, exhibits no relationship with MAT, whereas MBT′6ME, the methylation degree of 6-methyl bGDGTs, was found to correlate significantly with MAT. This is opposite to the situation observed in the global soils, pointing to a different adaptation of bGDGT-producing bacteria to environmental variables or different microbial sources of bGDGTs in these lakes. The relative abundance of 6- vs. 5-methyl bGDGTs is controlled by pH in these lakes, similar to that observed in worldwide soils. Hence, the isomer ratio (IR) of 6-methyl bGDGTs or CBT′ can be used as a proxy for water pH, although they might be influenced by other environmental factors including temperature in the lakes with a narrow range of pH.

Water column oxygen dynamics within the coastal gradient in the northeastern Gulf of Mexico inner shelf

This study quantified the coastal gradients of salinity, temperature, nutrients, chlorophyll and oxygen along a transect from 5 to 18m depth in the northeastern Gulf of Mexico (NEGOM) through measurements of water column profiles over 4.5-years and recordings of probes mounted at 0.5m above the seafloor over 2-years. The main goal was to determine the influences of the coastal gradients on water column oxygen concentration distributions. Apalachicola River discharge affected the study region as reflected in low-salinity surface layers. Decreasing rates of oxygen production and consumption from the shallowest station to the deepest station were attributed to measured coastal gradients of nutrients and the decreasing influence of seafloor processes with increasing distance from shore. Water column photosynthetic oxygen production at a realistic irradiance of 200μEm−2s−1 ranged from 0.5 to 4.5mmolO2m−3h−1, and dark consumption rates from 0.2 to 2.6mmolO2m−3h−1. Based on July irradiance profiles and associated production and consumption rates, the water column in July 2009 produced 0.9 (1SD=0.2)gCm−2d−1 with rates remaining relatively constant over the length of this coastal transect. Benthic boundary layer production reaching 26–43% of the surface layer production underlines the role of the benthic boundary layer for production in the inner shelf. Oxygen in the benthic boundary layer never decreased below 73% air saturation, and hypoxic zones, as reported from shelf areas west of the study region, did not occur. Oxygen production exceeded consumption suggesting a net autotrophic system.

Coupled changes in oxygen concentration and pH caused by metabolism of benthic coral reef organisms

Benthic marine primary producers affect the chemistry of their surrounding environment through metabolic processes. Photosynthesis and respiration will elevate or depress the concentration of oxygen in the diffusive boundary layer. Likewise, acid–base regulation and biomineralization/dissolution for calcifying species can alter the relative concentration of inorganic carbon species and thus pH. Here, we measured the relative ability of several common benthic primary producers from coral reef systems of the central Pacific and the Caribbean to simultaneously affect seawater oxygen concentration and pH values. Repeated measurements over a diel cycle confirmed that several primary producers substantially alter surrounding seawater chemistry over time. The majority of fleshy algae exhibited a stoichiometric ratio of oxygen to hydrogen ions not significantly different from one during daylight hours. In contrast, calcifiers exhibited significantly lower oxygen to hydrogen ion ratios that were unique for each species and were inversely correlated with known rates of calcification. These data provide the first quantitative estimates of the simultaneous influence of several species of benthic primary producers on water column oxygen concentrations and pH across different tropical reef systems. Finally, because more productive fleshy taxa have the potential to raise both oxygen and pH during the day to a greater extent than calcified species, our results suggest that some fleshy taxa may provide a buffering capacity to future ocean acidification scenarios.

Seacage aquaculture in a World Heritage Area: The environmental footprint of a Barramundi farm in tropical Australia

The fate of aquaculture wastes from a seacage farm within a pristine mangrove environment was studied. Seasonal and tidal differences were most important in determining water quality within receiving waters and obscured any nutrient enrichment effect by the farm. Farm wastes added significantly to the N budget status of the creek system, but overall water quality conformed to Queensland EPA Water Quality standards. Mangrove trees throughout the creek system contained 15N signatures traceable to aquaculture feeds, but the footprint of the farm itself was best indicated by the ratio of Zn:Li in sediments. The creek became hypoxic (<2mgl−1) during wet season low tides. Consequently, we recommended monitoring of water-column oxygen concentrations to warn of hypoxic conditions threatening to fish health, as well as Zn:Li ratios in sediment accumulation zones to determine the area of influence of the farm.

Experimental manipulation of sediment organic content and water column aeration reduces Zostera marina (eelgrass) growth and survival

The effect of synergy between sediment organic enrichment and lack of night oxygen renewal in the water column on growth and survival of Zostera marina, and how it is reflected in the sulfur parameters in the plants ( δ 34 S, TS and S 0) was studied experimentally. An experiment consisting of Z. marina mesocosms with different levels of organic enrichment and water column aeration was established, and the effects on sediment conditions, sulfide invasion and growth and survival of Z. marina were examined over a 4 week period. Shoots growing in Ambient Organic matter–sediments showed signs of sulfide invasion, as TS increased in all plant compartments and δ 34 S of the plant tissues decreased during the experiment, but neither growth rate nor survival were significantly affected. The lack of night oxygen renewal had no evident effects in non-enriched sediments as porewater sulfide concentrations, AVS- and CRS-pools were not different from the corresponding 24 h aeration treatment. Plant growth rate and survival were neither different from the corresponding 24 h aeration treatment. On the contrary, shoots growing on High Organic matter-sediments suffered a massive sulfide invasion and it was directly correlated to the observed decrease in growth rates. Even though the lack of night oxygen renewal had no evident effects on sediment variables there were, however, strong indications that the different aeration levels affected plant performance, suggesting a lower sulfide oxidation capacity and confirming that low water column oxygen concentrations reduces the defense capacity of the shoots against sulfide invasion. Although δ 34 S, TS and S 0 concentrations together provided a powerful set of indicators to detect the invasion of sulfide in Z. marina shoots, this study enlightens the need for a deeper investigation of sulfide intrusion in seagrasses and the relation between plant sulfur parameters and sediment conditions.

Effects of hypoxia on animal burrow construction and consequent effects on sediment redox profiles

We conducted a laboratory experiment to investigate the effects of mild hypoxia on the burrowing behavior of three marine species (the hard clam Mercenaria mercenaria, the polychaete worm Neanthes virens, and the amphipod Leptocheirus plumulosus) and consequent effects on sediment redox profiles. Animals were introduced into defaunated sediment and allowed to burrow for four months at mildly hypoxic (2 mg l − 1 ) and normoxic (7 mg l − 1 ) dissolved oxygen (DO) levels. Sediment redox profiles were measured 10 times during the course of the experiment. At the end of the experiment, the sediment was imaged using computer-aided tomography to quantify burrow volume and location. For the three species, burrow volume remained constant over DO treatments, but amphipod and clam burrows were shallower in hypoxic treatments compared with normoxic treatments. Redox profile discontinuity (RPD) depth was shallower in hypoxic treatments compared with normoxic treatments for experiments without animals, indicating that water column oxygen concentration alone influences diffusion of oxygen into the sediment. Worms, but neither clams nor amphipods, increased the RPD depth relative to no-animal controls in both hypoxic and normoxic treatments, but the effect was greater in normoxic conditions. These results suggest that although hypoxia can reduce burrowing depth, infauna can still increase the depth to which oxygen penetrates the sediment, but not to the same degree as they would under normoxic conditions.

A novel 2D model of internal O 2 dynamics and H 2S intrusion in seagrasses

Seagrasses provide a physical connection between the water column and sediments by transporting photosynthetic- and seawater-derived oxygen to their roots and rhizomes. In this paper, we present a single-shoot reaction-transport model that incorporates the biological, chemical and physical processes in the water column, seagrass plant, and sediments and that simulates oxygen and hydrogen sulfide dynamics in the system. The model reproduces oxygen and sulfide patterns observed in laboratory manipulations and field measurements of Thalassia testudinum and Zostera marina. Model results reinforce experimental conclusions that (1) meristem oxygen is tightly coupled to water column oxygen and diel patterns of sunlight, (2) sediment sulfide enters the plant when plant tissues are hypoxic, and (3) internal sulfide is rapidly depleted once oxic seawaters are re-established or with the onset of photosynthesis. Sensitivity analysis further emphasizes that water column oxygen concentration has a strong influence on the minimum oxygen concentration and maximum sulfide concentration in the meristem at night. The model indicates that diffusion is the dominant transport process in the lacunae, though advective mass flow can account for nearly a quarter of oxygen transport during periods of increasing sunlight. In the model, biological sulfide oxidation and plant dissolved organic carbon exudation both play significant roles in determining patterns of sediment oxygen consumption and sulfide intrusion into the plant.

Flow velocity affects internal oxygen conditions in the seagrass Cymodocea nodosa

The internal oxygen status of seagrass tissues, which is believed to play an important role in events of seagrass die-off, is partly determined by the rates of gas exchange between leaves and water column. In this study, we examined whether water column flow velocity has an effect on gas exchange, and hence on internal oxygen partial pressures ( pO 2) in the Mediterranean seagrass, Cymodocea nodosa. We measured the internal pO 2 in the horizontal rhizomes of C. nodosa in darkness at different mainstream flow velocities, combined with different levels of water column oxygen pO 2 using an experimental flume in the laboratory. Flow velocity clearly had an effect on the internal oxygen status. In stagnant, but fully aerated water the mean internal pO 2 was 6.9 kPa, corresponding to about 30% of air saturation. The internal pO 2 increased with increasing flow velocity reaching saturation of around 12.2 kPa (60% of air saturation) at flow velocities ≥7 cm s −1. Flow had a relatively larger influence on internal pO 2 at lower water column oxygen concentrations. By extrapolating linear relationships between internal and water column pO 2 in this experimental setup, rhizomes would become anoxic at a water column oxygen pO 2 of 4–4.5 kPa (∼20% of air saturation) in flowing water, but already at 6.4 kPa (∼30% of air saturation) in stagnant water. Water flow may play an important role for seagrass performance and survival in areas with poor water column oxygen conditions and may, in general, be of importance for the distribution of submerged rooted plants.

Sulphide intrusion in eelgrass (Zostera marinaL.)

ABSTRACTSudden events of seagrass die‐off have been suggested to be induced by invasion of the phytotoxin sulphide under environmental stress generating low oxygen supply in seagrass tissues. Laboratory experiments were conducted with eelgrass (Zostera marinaL.) to measure intra‐plant changes in oxygen and sulphide by means of microelectrodes at different oxygen concentrations in the water column. The objectives were to examine whether sulphide intrusion into seagrass tissues can be induced, to determine the role of plant oxygen status for sulphide intrusion and to determine how fast internal sulphide pools are depleted after internal oxygen supplies have been restored. Under conditions with oxygen partial pressures (pO2) above 7.4 kPa (&gt; 35% of air saturation) within eelgrass rhizomes or meristematic tissues no intrusion of sulphide occurred in spite of high sediment concentrations of gaseous sulphide (&gt; 1000 µm). Lack of sulphide intrusion at high internal pO2suggested that oxygen release from the roots ensured complete re‐oxidation of sulphide in the rhizosphere. Under oxygen stress, however, the experiments clearly demonstrated intrusion of sulphide in eelgrass rhizomes and meristematic tissues. Rates of sulphide intrusion were controlled by internal pO2, which in turn was controlled by water column oxygen concentrations. Maximum internal sulphide content reached 325 µmwhich by far exceeded the 1–10 µmknown to inhibit mitochondrial activity in eukaryotic cells. Sulphide and low levels of oxygen could coexist in the eelgrass tissues reflecting fast internal transport of sulphide and slow rates of sulphide re‐oxidation. Upon re‐establishment of high internal oxygen concentrations the depletion of the sulphide pool was slow (half‐life = 20–30 min) indicating, that sulphide re‐oxidation within the eelgrass tissue was not bacterially or enzymatically facilitated but occurred by simple chemical oxidation. The results of this study are consistent with the proposed detrimental role of sulphide intrusion in events of sudden seagrass die‐off.

Meristematic oxygen variability in eelgrass (Zostera marina)

We examined the variability in oxygen content of meristematic tissues in eelgrass in order to evaluate its potential role in events of sudden mass mortality within eelgrass beds. Oxygen content within intact eelgrass plants could be described by use of microelectrode techniques at high temporal and spatial resolution in the laboratory and in the field. Under both situations, the meristematic oxygen content was highly variable, ranging from 0 to 200% of air saturation depending on environmental conditions. Changes from steady‐state maximum oxygen content to steady‐state minimum content occurred within 30 min following experimental manipulation. The internal oxygen content exceeded water column oxygen concentration in the light and was intimately coupled to changes in irradiance because of the photosynthetic oxygen release within the leaves. The photosynthetically produced pool of oxygen could, however, not function as an efficient storage to support nighttime respiration. In the dark, oxygen was primarily supplied from the water column via diffusion into leaves, and the meristem quickly turned anoxic if the water column was anoxic. Experimental reduction of oxygen conditions immediately around the basal plant meristem had no major effect on internal oxygen content. High temperatures had a dramatic effect on the internal oxygen balance of eelgrass. Increasing temperature stimulated plant respiration more than photosynthesis, and the meristem went anoxic, even in the light, at water temperatures above 30°C. We hypothesize that low meristematic oxygen content is a key factor in events of seagrass die‐off.

Effect of sampling frequency on measurements of seasonal primary production and oxygen status in near-shore coastal ecosystems

New automated in situ techniques were used for high-resolution time series measurement of primary production and water column oxygen concentration in near-shore coastal marine embayments. Results revealed 2-fold day-to-day fluctuations in measured phytoplankton production, and periodic changes in water column oxygen concentration as great as 100 pM occurring in a matter of a few hours. Effects of sampling frequency upon the determination of seasonal primary production and water column oxygen status were assessed by subsampling the high-resolution measurements at various intervals and reanalyzing of the deslred parameters using the lower-resolution data. Results indicate that ecosystem studies using classic measurement intervals (e.g. 30 d) may be undersampled 5to 30-fold with respect to both primary production and oxygen concentration, resulting in serious inaccuracies in data crucial to ecological and water quality studies.

Effects of hypoxia on estuarine benthos: the lower Rappahannock River (Chesapeake Bay), a case study

The effects of seasonal hypoxia on population dynamics of macrobenthic organisms were studied at a meso-polyhaline, deep sandy-mud habitat in the lower Rappahannock River, a tributary of Chesapeake Bay (Virginia, U.S.A.). Water quality was monitored on a monthly to weekly basis, spring to autumn, to determine the extent, intensity and duration of hypoxic events. Core samples collected along a five-station transect were used to identify species, discern patterns of spatial and temporal variation of abundance, and determine vertical distribution patterns in the sediment relative to dissolved oxygen fluctuations. A significant decrease in macrobenthic abundance followed by defaunation at some stations was attributed to intermittent hypoxia. Abundance and species number were low during summer, decreasing with increasing water depth and upriver direction in relation to lowest dissolved oxygen concentrations. Species composition appeared to be influenced by hypoxia. Annelid polychaetes comprised the largest percentage of the population, with dominance of ‘opportunists’. The magnitude of the responses to hypoxia appeared to be species-specific. Some polychaetous species such as the spionid Streblospio benedicti and the capitellid Mediomastus ambiseta became locally extinct with the onset of severe hypoxia. In contrast, the terebellid Loimia medusa and the spionid Paraprionospio pinnata were present in affected habitats during subsequent intermittent hypoxia. The vertical distribution of macrobenthos in sediment changed significantly with season, with overall lower abundance at depth in sediment during hypoxia than in normoxia; however, this trend was not always clear or consistent with changes in water-column oxygen concentrations. Alternative factors that may account for such patterns (e.g., recruitment pulses) are discussed. The intensity of hypoxia was correlated with changes in benthic community structure and distribution. Results suggest that the periodicity of hypoxic events may lead to long-term alterations of benthic populations, largely as a reflection of the varying species tolerances and life-histories.