Several hypotheses of how zooplankton communities respond to coastal hypoxia have been put forward in the literature over the past few decades. We explored three of those that are focused on how zooplankton composition or biomass is affected by seasonal hypoxia using data collected over two summers in Hood Canal, a seasonally-hypoxic sub-basin of Puget Sound, Washington. We conducted hydrographic profiles and zooplankton net tows at four stations, from a region in the south that annually experiences moderate hypoxia to a region in the north where oxygen remains above hypoxic levels. The specific hypotheses tested were that low oxygen leads to: (1) increased dominance of gelatinous relative to crustacean zooplankton, (2) increased dominance of cyclopoid copepods relative to calanoid copepods, and (3) overall decreased zooplankton abundance and biomass at hypoxic sites compared to where oxygen levels are high. Additionally, we examined whether the temporal stability of community structure was decreased by hypoxia. We found evidence of a shift toward more gelatinous zooplankton and lower total zooplankton abundance and biomass at hypoxic sites, but no clear increase in the dominance of cyclopoid relative to calanoid copepods. We also found the lowest variance in community structure at the most hypoxic site, in contrast to our prediction. Hypoxia can fundamentally alter marine ecosystems, but the impacts differ among systems.
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Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities
In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.
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- NSF-PAR ID:
- 10127245
- Date Published:
- Journal Name:
- Water
- Volume:
- 11
- Issue:
- 10
- ISSN:
- 2073-4441
- Page Range / eLocation ID:
- 2179
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Coastal hypoxia—harmfully low levels of oxygen—is a mounting problem that jeopardizes coastal ecosystems and economies. The northern Indian Ocean is particularly susceptible due to human‐induced impacts, vast naturally occurring oxygen minimum zones, and strong variability associated with the seasonal monsoons and interannual Indian Ocean Dipole (IOD). We assess how
natural factors influence the risk of coastal hypoxia by combining a large set of oxygen measurements with satellite observations to examine how the IOD amplifies or suppresses seasonal hypoxia tied to the Asian Monsoon. We show that on both seasonal and interannual timescales hypoxia is controlled by wind‐ and coastal Kelvin wave‐driven upwelling of oxygen‐poor waters onto the continental shelf and reinforcing biological feedbacks (increased subsurface oxygen demand). Seasonally, the risk of hypoxia is highest in the western Arabian Sea in summer/fall (71% probability of hypoxia). Major year‐to‐year impacts attributed to the IOD occur during positive phases along the eastern Bay of Bengal (EBoB), where the risk of coastal hypoxia increases from moderate to high in summer/fall (21%–46%) and winter/spring (31%–42%), and along the eastern Arabian Sea (i.e., India, Pakistan) where the risk drops from high to moderate in summer/fall (53%–34%). Strong effects are also seen in the EBoB during negative IOD phases, when the risk reduces from moderate to low year‐round (∼25% to ∼5%). This basin‐scale mapping of hypoxic risk is key to aid national and international efforts that monitor, forecast, and mitigate the impacts of hypoxia on coastal ecosystems and ecosystem services. -
null (Ed.)Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.more » « less
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Abstract. Although the depth of bioturbation can be estimated on the basisof ichnofabric, the timescale of sediment mixing (reworking) and irrigation(ventilation) by burrowers that affects carbonate preservation andbiogeochemical cycles is difficult to estimate in the stratigraphic record.However, pyrite linings on the interior of shells can be a signature of slowand shallow irrigation. They indicate that shells of molluscs initiallyinhabiting oxic sediment pockets were immediately and permanentlysequestered in reduced, iron-rich microenvironments within the mixed layer.Molluscan biomass-stimulated sulfate reduction and pyrite precipitation wasconfined to the location of decay under such conditions. A high abundance ofpyrite-lined shells in the stratigraphic record can thus be diagnostic oflimited exposure of organic tissues to O2 even when the seafloor isinhabited by abundant infauna disrupting and age-homogenizing sedimentaryfabric as in the present-day northern Adriatic Sea. Here, we reconstructthis sequestration pathway characterized by slow irrigation (1) by assessingpreservation and postmortem ages of pyrite-lined shells of theshallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) byevaluating whether an independently documented decline in the depth ofmixing, driven by high frequency of seasonal hypoxia during the 20thcentury, affected the frequency of pyrite-lined shells in the stratigraphicrecord of the northern Adriatic Sea. First, at prodelta sites with a highsedimentation rate, linings of pyrite framboids form rapidly in the upper5–10 cm as they already appear in the interiors of shells younger than 10 yearsand occur preferentially in well-preserved and articulated shells withperiostracum. Second, increments deposited in the early 20th centurycontain < 20 % of shells lined with pyrite at the Po prodelta and30 %–40 % at the Isonzo prodelta, whereas the late 20th centuryincrements possess 50 %–80 % of shells lined with pyrite at both locations.At sites with slow sedimentation rate, the frequency of pyrite linings islow (< 10 %–20 %). Surface sediments remained well mixed by depositand detritus feeders even in the late 20th century, thus maintainingthe suboxic zone with dissolved iron. The upcore increase in the frequencyof pyrite-lined shells thus indicates that the oxycline depth was reducedand bioirrigation rates declined during the 20th century. Wehypothesize that the permanent preservation of pyrite linings within theshells of V. gibba in the subsurface stratigraphic record was enabled by slowrecovery of infaunal communities from seasonal hypoxic events, leading tothe dominance of surficial sediment modifiers with low irrigation potential.The presence of very young and well-preserved pyrite-lined valves in theuppermost zones of the mixed layer indicates that rapid obrution by episodicsediment deposition is not needed for preservation of pyrite linings whensediment irrigation is transient and background sedimentation rates arenot low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organismsdie at their living position within the sediment. Abundance ofwell-preserved shells lined by pyrite exceeding ∼ 10 % perassemblage in apparently well-mixed sediments in the deep-time stratigraphicrecord can be an indicator of inefficient bioirrigation. Fine-grainedprodelta sediments in the northern Adriatic Sea deposited since themid-20th century, with high preservation potential of reducedmicroenvironments formed within a mixed layer, can represent taphonomic andearly diagenetic analogues of deep-time skeletal assemblages with pyritelinings.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/w11102179
