An hypoxia-tolerant flatfish: consequences of sustained stress on the slender sole Lyopsetta exilis (Pleuronectidae) in the context of a changing ocean

Slender sole Lyopsetta exilis is an abundant groundfish on the continental shelf and inner waters of British Columbia, Canada, where it reaches a maximum standard length of 44 cm. Benthic image surveys coupled with oxygen measurements in Saanich Inlet document a dense population in bottom conditions near anoxia (0.03 ml l⁻¹ oxygen) where diel migrating zooplankton intersect the bottom; we confirm this species is a planktivore, which limits its depth range to the base of the migration layer. In a comparison with slender sole from a nearby well-oxygenated habitat, several probable effects of living in severe hypoxia emerge: both sexes are significantly smaller in Saanich and the sex ratio is male-skewed. Otoliths from the Saanich fish were difficult to read due to many checks, but both sexes were smaller at age with the largest female (20 cm) from the hypoxia zone registering 17 years. Hypoxia appears to have a direct consequence on growth despite good food supply in this productive basin. Hyperventilation, a low metabolic rate and a very low critical oxygen tension help this fish regulate oxygen uptake in severely hypoxic conditions; it will be particularly resilient as the incidence of hypoxia increases on the continental shelf. Data from small-mesh bottom-trawl surveys over four decades reveal an increase in mean annual catch per unit effort in southern regions of the province, including the outer shelf and the Strait of Georgia. The California Cooperative Oceanic Fisheries Investigations (CalCOFI) ichthyoplankton database records a general decline in fish larvae on the Oregon–California shelf since 1990, but slender sole larvae are increasing there, as they are in the Strait of Georgia. We project that the slender sole will gain relative benefits in the future warming, deoxygenated northeast Pacific Ocean.     

Oxygen limitations on marine animal distributions and the collapse of epibenthic community structure during shoaling hypoxia

Deoxygenation in the global ocean is predicted to induce ecosystem‐wide changes. Analysis of multidecadal oxygen time‐series projects the northeast Pacific to be a current and future hot spot of oxygen loss. However, the response of marine communities to deoxygenation is unresolved due to the lack of applicable data on component species. We repeated the same benthic transect (n = 10, between 45 and 190 m depths) over 8 years in a seasonally hypoxic fjord using remotely operated vehicles equipped with oxygen sensors to establish the lower oxygen levels at which 26 common epibenthic species can occur in the wild. By timing our surveys to shoaling hypoxia events, we show that fish and crustacean populations persist even in severe hypoxia (<0.5 mL L^?1) with no mortality effects but that migration of mobile species occurs. Consequently, the immediate response to hypoxia expansion is the collapse of community structure; normally partitioned distributions of resident species coalesced and localized densities increased. After oxygen renewal and formation of steep oxygen gradients, former ranges re‐established. High frequency data from the nearby VENUS subsea observatory show the average oxygen level at our site declined by ~0.05 mL L^?1 year^?1 over the period of our study. The increased annual duration of the hypoxic (<1.4 mL L^?1) and severely hypoxic periods appears to reflect the oxygen dynamics demonstrated in offshore source waters and the adjacent Strait of Georgia. Should the current trajectory of oxygen loss continue, community homogenization and reduced suitable habitat may become the dominant state of epibenthic systems in the northeast Pacific. In situ oxygen occurrences were not congruent with lethal and sublethal hypoxia thresholds calculated across the literature for major taxonomic groups indicating that research biases toward laboratory studies on Atlantic species are not globally applicable. Region‐specific hypoxia thresholds are necessary to predict future impacts of deoxygenation on marine biodiversity.     

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.     

Live benthic foraminiferal faunas along a bathymetrical transect (140-4800 m) in the Bay of Biscay (NE Atlantic)

In a 10-stations bathymetrical transect in the Bay of Biscay, we observed important changes in the density, composition and microhabitats of live foraminiferal faunas from the outer continental shelf to the abyssal plain. Four zones are recognised: (1) at the upper continental shelf (140 m water depth), foraminiferal densities are very high and the superficial sediment is occupied by Bolivina subaenariensis and Valvulineria bradyana. Globobulimina spp., Chilostomella oolina and Nonion fabum dominate the infaunal niches, which are positioned close to the sediment-water interface due to a strong compaction of the vertical succession of redox zones. (2) At the upper continental slope stations (300-1000 m), foraminiferal densities are high and the superficial sediments are dominated by Uvigerina mediterranea/peregrina. Deeper in the sediment, intermediate infaunal niches are occupied by Melonis barleeanus. Due to a deeper oxygen penetration, the deep infaunal taxa Globobulimina spp. and C. oolina live at a considerable depth in the sediment. (3) At the mid and lower slope stations (1000-2000 m) in the superficial sediment Cibicidoides kullenbergi and Hoeglundina elegans progressively replace U. mediterranea. U. peregrina is still a dominant taxon, reflecting its preference for a somewhat intermediate organic flux level. Deep infaunal taxa become increasingly rare. (4) At the lower slope and abyssal plane stations (deeper than 2000 m), faunal densities are very low and the fauna is composed exclusively by shallow infaunal species, such as Nuttallides umboniferus and Melonis pompilioides. The foraminiferal data together with the pore water data in the sediment give evidence of the presence of a trophic gradient from very eutrophic settings at the upper continental shelf towards oligotrophic settings at the abyssal area.     

Changes in northern Gulf of Mexico sediment bacterial and archaeal communities exposed to hypoxia

Biogeochemical changes in marine sediments during coastal water hypoxia are well described, but less is known about underlying changes in microbial communities. Bacterial and archaeal communities in Louisiana continental shelf (LCS) hypoxic zone sediments were characterized by pyrosequencing 16S rRNA V4‐region gene fragments obtained by PCR amplification of community genomic DNA with bacterial‐ or archaeal‐specific primers. Duplicate LCS sediment cores collected during hypoxia had higher concentrations of Fe(II), and dissolved inorganic carbon, phosphate, and ammonium than cores collected when overlying water oxygen concentrations were normal. Pyrosequencing yielded 158 686 bacterial and 225 591 archaeal sequences from 20 sediment samples, representing five 2‐cm depth intervals in the duplicate cores. Bacterial communities grouped by sampling date and sediment depth in a neighbor‐joining analysis using Chao–Jaccard shared species values. Redundancy analysis indicated that variance in bacterial communities was mainly associated with differences in sediment chemistry between oxic and hypoxic water column conditions. Gammaproteobacteria (26.5%) were most prominent among bacterial sequences, followed by Firmicutes (9.6%), and Alphaproteobacteria (5.6%). Crenarchaeotal, thaumarchaeotal, and euryarchaeotal lineages accounted for 57%, 27%, and 16% of archaeal sequences, respectively. In Thaumarchaeota Marine Group I, sequences were 96–99% identical to the Nitrosopumilus maritimus SCM1 sequence, were highest in surficial sediments, and accounted for 31% of archaeal sequences when waters were normoxic vs. 13% of archaeal sequences when waters were hypoxic. Redundancy analysis showed Nitrosopumilus‐related sequence abundance was correlated with high solid‐phase Fe(III) concentrations, whereas most of the remaining archaeal clusters were not. In contrast, crenarchaeotal sequences were from phylogenetically diverse lineages, differed little in relative abundance between sampling times, and increased to high relative abundance with sediment depth. These results provide further evidence that marine sediment microbial community composition can be structured according to sediment chemistry and suggest the expansion of hypoxia in coastal waters may alter sediment microbial communities involved in carbon and nitrogen cycling.     

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.     

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ¹³C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ¹³C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1 mg l^?1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ¹³C levels (>???40‰). In contrast, larvae were markedly ¹³C–depleted (down to ??60‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.     

Quantifying Hypoxia Impacts on an Estuarine Demersal Community Using a Hierarchical Ensemble Approach

In coastal marine ecosystems, hypoxia and anoxia are emerging as growing threats whose ecological impacts are difficult to ascertain because of the frequent lack of adequate references for comparison. We applied conventional and hierarchical ensemble analyses to evaluate the weight of evidence in support of hypoxia impacts on local densities of individual and groups of demersal fish and invertebrate species in Hood Canal, WA, which is subject to seasonal hypoxia in its southern reaches. Central to our approach was a sample design and analysis scheme that was designed specifically to consider multiple alternative hypotheses regarding factors that dictate local species’ densities. We anticipated persistent effects of hypoxia (felt even when seasonal hypoxia was absent) on species densities would be most pronounced for sessile species, but that immediate effects (felt only when seasonal hypoxia was present) would dominate for mobile species. Conventional analysis provided strong evidence that densities of sessile species were persistently reduced in the hypoxic-impacted site, but did not indicate widespread immediate density responses during hypoxic events among mobile species. The absence of strong weights of evidence for hypoxia effects was partly a consequence of alternative hypotheses that better explained spatial-temporal variation in species’ densities. The hierarchical ensemble analysis improved the precision of species-specific effect sizes, and also allowed us to make inferences about the response of aggregated groups of species. The estimated mean density reductions during hypoxic events (dissolved oxygen ~2 mg/l) ranged from 73 to 98% among mobile invertebrates, benthic, and benthopelagic fishes. The large reduction in benthic and benthopelagic species suggests substantial effects of hypoxia in Hood Canal even at oxygen levels that were marginally hypoxic. Understanding the full ecological consequence of hypoxia will require a greater knowledge on the spatial extent of distributional shifts and their effects on competitive and predator–prey interactions.     

Hypoxia and Acidification Have Additive and Synergistic Negative Effects on the Growth,Survival, and Metamorphosis of Early Life Stage Bivalves

Low oxygen zones in coastal and open ocean ecosystems have expanded in recent decades, a trend that will accelerate with climatic warming. There is growing recognition that low oxygen regions of the ocean are also acidified, a condition that will intensify with rising levels of atmospheric CO₂. Presently, however, the concurrent effects of low oxygen and acidification on marine organisms are largely unknown, as most prior studies of marine hypoxia have not considered pH levels. We experimentally assessed the consequences of hypoxic and acidified water for early life stage bivalves (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria), marine organisms of significant economic and ecological value and sensitive to climate change. In larval scallops, experimental and naturally-occurring acidification (pH, total scale  = 7.4–7.6) reduced survivorship (by >50%), low oxygen (30–50 µM) inhibited growth and metamorphosis (by >50%), and the two stressors combined produced additively negative outcomes. In early life stage clams, however, hypoxic waters led to 30% higher mortality, while acidified waters significantly reduced growth (by 60%). Later stage clams were resistant to hypoxia or acidification separately but experienced significantly (40%) reduced growth rates when exposed to both conditions simultaneously. Collectively, these findings demonstrate that the consequences of low oxygen and acidification for early life stage bivalves, and likely other marine organisms, are more severe than would be predicted by either individual stressor and thus must be considered together when assessing how ocean animals respond to these conditions both today and under future climate change scenarios.     

Hypoxia,Blackwater and Fish Kills: Experimental Lethal Oxygen Thresholds in Juvenile Predatory Lowland River Fishes

Hypoxia represents a growing threat to biodiversity in freshwater ecosystems. Here, aquatic surface respiration (ASR) and oxygen thresholds required for survival in freshwater and simulated blackwater are evaluated for four lowland river fishes native to the Murray-Darling Basin (MDB), Australia. Juvenile stages of predatory species including golden perch Macquaria ambigua, silver perch Bidyanus bidyanus, Murray cod Maccullochella peelii, and eel-tailed catfish Tandanus tandanus were exposed to experimental conditions of nitrogen-induced hypoxia in freshwater and hypoxic blackwater simulations using dried river red gum Eucalyptus camaldulensis leaf litter. Australia''s largest freshwater fish, M. peelii, was the most sensitive to hypoxia but given that we evaluated tolerances of juveniles (0.99±0.04 g; mean mass ±SE), the low tolerance of this species could not be attributed to its large maximum attainable body mass (>100,000 g). Concentrations of dissolved oxygen causing 50% mortality (LC₅₀) in freshwater ranged from 0.25±0.06 mg l⁻¹ in T. tandanus to 1.58±0.01 mg l⁻¹ in M. peelii over 48 h at 25–26°C. Logistic models predicted that first mortalities may start at oxygen concentrations ranging from 2.4 mg l⁻¹ to 3.1 mg l⁻¹ in T. tandanus and M. peelii respectively within blackwater simulations. Aquatic surface respiration preceded mortality and this behaviour is documented here for the first time in juveniles of all four species. Despite the natural occurrence of hypoxia and blackwater events in lowland rivers of the MDB, juvenile stages of these large-bodied predators are vulnerable to mortality induced by low oxygen concentration and water chemistry changes associated with the decomposition of organic material. Given the extent of natural flow regime alteration and climate change predictions of rising temperatures and more severe drought and flooding, acute episodes of hypoxia may represent an underappreciated risk to riverine fish communities.     

Seed bank,biomass, and productivity of Halophila decipiens, a deep water seagrass on the west Florida continental shelf

One of the largest contiguous seagrass ecosystems in the world is located on the shallow continental shelf adjacent to the west coast of Florida, USA and is comprised of seasonally ephemeral Halophila decipiens meadows. Little is known about the demography of the west Florida shelf H. decipiens, which may produce 4.56 × 10⁸ g C day⁻¹ or more during the peak growing season. We documented seagrass distribution, biomass, and productivity, and density of sediment seed reserves, seedlings, flowers and fruits on the southeastern portion of the west Florida shelf by sampling along a transect at three stations in 10, 15, and 20 m water depth. Biomass, flower, fruit, seedling, and seed bank densities tended to be highest at stations in 10–15 m water depth and lowest at 20 m. Flowers and fruit were most prevalent during summer cruises (June and August 1999, July 2000). Seedling germination occurred during summer, fall (October 1999), and winter (January 2000) sampling events, with the highest seedling densities present during the winter. Seed bank density remained consistent through time. A Category I hurricane with sustained winds of 120 km h⁻¹ passed over the stations, but only limited impact on H. decipiens biomass was observed. The presence of a persistent seed bank provides for recovery after storm disturbance, annual reestablishment of populations, and continual maintenance of the 20,000 km² of deep water seagrass habitat present on the west Florida shelf.     

Responses of intertidal nematodes to short-term anoxic events

We investigated tolerance to, and activity of, nematodes in hypoxic and anoxic/partially sulphidic conditions and their ability for recovery after reoxygenation of anoxic sediment. To this end, sediments from an intertidal flat were incubated under oxic, suboxic and anoxic/partially sulphidic conditions for a 14-day period and the final density of nematodes, as a group, and of the most abundant species were assessed. In one treatment, oxygen was restored after anoxic incubation. The incorporation of ¹³C, originating from labeled algae added on top of the sediment, was taken as an indication of nematode activity.Short-term suboxic and anoxic/partially sulphidic conditions had similar structuring impact on the nematode community, reducing total densities by about one third. Survival in suboxic and anoxic/partially sulphidic conditions was species-specific. Daptonema setosum, D. tenuispiculum and Chromadora macrolaima, dominant in the oxic incubation, disappeared when the oxygen level was reduced. The density of the other dominant species was slightly reduced (Sabatieria pulchra), similar (Terschellingia communis) or even increased in the suboxic and anoxic conditions (Metachromadora vivipara). The activity level of these three species was, however, reduced under oxygen limitation. Our results are discussed in terms of the life-history strategies of these species.     

Seasonal dynamics of chironomids in the profundal zone of a mountain lake (L’adové pleso,the Tatra Mountains,Slovakia)

The profundal community of L’adové pleso (an oligotrophic high mountain seepage lake at an altitude of 2,057 m with a max. depth of 18 m and an ice-cover period from October–July) was studied from December 2000–October 2001. Chironomidae, the most significant part of the studied community, are represented by four taxa and dominated by Micropsectra radialis Goetghebuer, 1939 and Pseudodiamesa nivosa (Goethgebuer, 1928). These two species showed a 1-year life cycle. The total densities of chironomids varied from 0 to 5,927 ind. m^?2; no chironomids, or very low densities, were found during the winter/spring period, probably due to low oxygen concentrations in the medial part of the lake. These low oxygen concentrations probably caused the relocation of larvae from the medial part of the sedimentary area at the same time.     

Direct and indirect effects of hypoxia on benthos in Corpus Christi Bay, Texas, U.S.A.

Hypoxia (low oxygen conditions) has been found in the southeastern region of Corpus Christi Bay, Texas, U.S.A. every summer since 1988. The objectives of the current study were to determine direct and indirect effects of hypoxia on macrofauna. Direct physiological effects of hypoxia include reduction of benthic abundance, biomass, diversity, species richness and species evenness because of physiological intolerance. Indirect ecological effects of hypoxia include predation of emerging benthic fauna from the sediment. Macrofaunal community characteristics were compared vertically within sediments in caged and uncaged sediment samples in hypoxic and normoxic areas. Cage effects were determined with partial cages, which had reduced flow and no predator exclusion. Dissolved oxygen concentrations during the experiment was monitored in water column profiles and continuous measurement of bottom water in the hypoxic and normoxic areas. Hypoxia in Corpus Christi Bay in 1999 occurred as transient events, many of which were of short duration (less than 1 h) and moderate intensity (around 2 mg l^− 1). The macrobenthic community characteristics (i.e., abundance, biomass, species richness, diversity, and evenness) were directly affected by hypoxia as indicated by depressed levels and few deeper-dwelling organisms in the hypoxic area. Community structure was also different between the hypoxic and normoxic areas because of loss of species (presumably due to intolerance to low oxygen) in the hypoxic areas. Benthic invertebrates were found primarily in the surface in the hypoxic area, but there was no significant indication of indirect effects, i.e., increased predation pressure in the hypoxic area. The increased exposure to predation risk may be mitigated by predator avoidance of hypoxic areas. In conclusion, hypoxia in Corpus Christi Bay has negative direct effects on benthic organisms, but no indirect effects, such as increased predation pressure. The most significant finding is the interaction between hypoxia and vertical distributions of infauna, which drive hypoxia intolerant organisms to the surface and out of sediments.     

Effect of the mitochondrial unfolded protein response on hypoxic death and mitochondrial protein aggregation

Mitochondria are the main oxygen consumers in cells and as such are the primary organelle affected by hypoxia. All hypoxia pathology presumably derives from the initial mitochondrial dysfunction. An early event in hypoxic pathology in C. elegans is disruption of mitochondrial proteostasis with induction of the mitochondrial unfolded protein response (UPR^mt) and mitochondrial protein aggregation. Here in C. elegans, we screen through RNAis and mutants that confer either strong resistance to hypoxic cell death or strong induction of the UPR^mt to determine the relationship between hypoxic cell death, UPR^mt activation, and hypoxia-induced mitochondrial protein aggregation (HIMPA). We find that resistance to hypoxic cell death invariantly mitigated HIMPA. We also find that UPR^mt activation invariantly mitigated HIMPA. However, UPR^mt activation was neither necessary nor sufficient for resistance to hypoxic death and vice versa. We conclude that UPR^mt is not necessarily hypoxia protective against cell death but does protect from mitochondrial protein aggregation, one of the early hypoxic pathologies in C. elegans.Subject terms: Necroptosis, Energy metabolism     

Linking Molecular Microbial Ecology to Geochemistry in a Coastal Hypoxic Zone

Multiple environmental mechanisms have been proposed to control bottom water hypoxia (<2 mg O₂ L^?1) in the northern Gulf of Mexico Louisiana shelf. Near-bottom hypoxia has been attributed to a direct consumption of oxygen through benthic microbial respiration and a secondary chemical reaction between oxygen and reduced metabolites (i.e. ferrous iron and total sulfide) from these populations. No studies to date have examined the metabolically active microbial community structure in conjunction with the geochemical profile in these sediments. Temporal and spatial differences in dissolved and solid phase geochemistry were investigated in the upper 20 cm of the sediment column. Pyrosequencing of reverse transcribed small subunit (SSU) ribosomal ribonucleic acid (rRNA) was used to determine population distribution. Results indicated that populations shallower than 10 cm below surface were temporally variable yet uniform between sites, while below this depth, populations were more site-specific. This suggests a potential interaction between the water column and the benthic microbial population limited to a shallow depth. The presence of dissolved reduced iron in the upper sediment column was indicative of low oxygen concentration, yet sulfide was at or below detection limits. Putative sulfate and iron reducing and oxidizing populations were metabolically active at similar depths suggesting potential recycling of products. Results from this study indicate low carbon concentrations in the shallow sediments limit general metabolic activity, reducing the potential for microbial respiration. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.     

Egg production of the copepod Acartia tonsa: The influence of hypoxia and food concentration

Low dissolved oxygen conditions, or hypoxia, occur in estuaries and impact more than just the obvious commercially important species. Copepods are an important link in the food web, and the influence of hypoxia upon them is relatively unstudied. Using the copepod Acartia tonsa, a study of the impact of hypoxia on egg production was conducted. A. tonsa had reduced egg production at low dissolved oxygen concentrations (DO), with the lowest egg production occurring at 0.53-ml/l O₂ concentration. Another experiment was conducted to determine if, by increasing food concentration, the affect of hypoxia could be mitigated. The results indicate that increased food did not offset the impact that hypoxia had on egg production of A. tonsa. These results suggest that as A. tonsa experiences hypoxia in the wild, population numbers will decrease. Thus, if hypoxic conditions increase in scope and duration, declines in copepod abundance may lead to a decline in the abundance of species that depend on them as food. These species may be of commercial importance (e.g., fish, crabs, and oysters).     

Dependence of Ethanolic Fermentation, Cytoplasmic pH Regulation, and Viability on the Activity of Alcohol Dehydrogenase in Hypoxic Maize Root Tips

We examined the role of alcohol dehydrogenase (ADH) in the metabolism and survival of hypoxic maize (Zea mays L.) root tips. The dependence of the rate of ethanolic fermentation, cytoplasmic pH, and viability on the activity of ADH in maize root tips during extreme hypoxia was determined. Maize lines with ADH activities differing over about a 200-fold range were studied. Effects of genetic background were controlled by comparing pairs of F4 progeny of crosses between mutant (low ADH activity) and reference inbred lines. The capacity of hypoxic root tips to perform ethanolic fermentation exhibited a dependence on ADH activity only at activities found in Adh 1 nulls. The ability of maize root tips to withstand prolonged and extreme hypoxia was like-wise independent of ADH activity, except at the lowest activities. Root tips that exhibited lower tolerance of hypoxia had more acidic cytoplasm during extreme hypoxia. We conclude that the activity of ADH in normal maize root tips does not limit the capacity for energy production via fermentation, and does not determine viability under extreme hypoxia. The significance of the induction of ADH activity in plants by hypoxia is discussed.     

Effects of hypoxia on the behavior and physiology of kelp forest fishes

Forecasts from climate models and oceanographic observations indicate increasing deoxygenation in the global oceans and an elevated frequency and intensity of hypoxic events in the coastal zone, which have the potential to affect marine biodiversity and fisheries. Exposure to low dissolved oxygen (DO) conditions may have deleterious effects on early life stages in fishes. This study aims to identify thresholds to hypoxia while testing behavioral and physiological responses of two congeneric species of kelp forest fish to four DO levels, ranging from normoxic to hypoxic (8.7, 6.0, 4.1, and 2.2 mg O₂/L). Behavioral tests identified changes in exploratory behavior and turning bias (lateralization), whereas physiological tests focused on determining changes in hypoxia tolerance (pCrit), ventilation rates, and metabolic rates, with impacts on the resulting capacity for aerobic activity. Our findings indicated that copper rockfish (Sebastes caurinus) and blue rockfish (Sebastes mystinus) express sensitivity to hypoxia; however, the strength of the response differed between species. Copper rockfish exhibited reduced absolute lateralization and increased escape time at the lowest DO levels, whereas behavioral metrics for blue rockfish did not vary with oxygen level. Both species exhibited decreases in aerobic scope (as a function of reduced maximum metabolic rate) and increases in ventilation rates to compensate for decreasing oxygen levels. Blue rockfish had a lower pCrit and stronger acclimation response compared to copper rockfish. The differences expressed by each species suggest that acclimatization to changing ocean conditions may vary, even among related species that recruit to the same kelp forest habitat, leading to winners and losers under future ocean conditions. Exposure to hypoxia can decrease individual physiological fitness through metabolic and aerobic depression and changes to anti‐predator behavior, with implications for the outcome of ecological interactions and the management of fish stocks in the face of climate change.