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1. Zooplankton Community Response to Seasonal Hypoxia: A Test of Three Hypotheses

   https://doi.org/10.3390/d12010021  

   Keister, Julie E. ; Winans, Amanda K. ; Herrmann, BethElLee ( January 2020 , Diversity)

   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 leadsmore »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.« less
2. Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

   https://doi.org/10.5194/hess-25-1009-2021  

   Ladwig, Robert ; Hanson, Paul C. ; Dugan, Hilary A. ; Carey, Cayelan C. ; Zhang, Yu ; Shu, Lele ; Duffy, Christopher J. ; Cobourn, Kelly M. ( January 2021 , Hydrology and Earth System Sciences)

   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, inmore »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.« less
3. Pyrite-lined shells as indicators of inefficient bioirrigation in the Holocene–Anthropocene stratigraphic record

   https://doi.org/10.5194/bg-18-5929-2021  

   Tomašových, Adam ; Berensmeier, Michaela ; Gallmetzer, Ivo ; Haselmair, Alexandra ; Zuschin, Martin ( January 2021 , Biogeosciences)

   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 thusmore »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.« less
4. Species Composition and Distribution of Jellyfish in a Seasonally Hypoxic Estuary, Hood Canal, Washington

   https://doi.org/10.3390/d12020053  

   Herrmann, BethElLee ; Keister, Julie E. ( February 2020 , Diversity)

   Seasonal hypoxia (≤2 mg dissolved oxygen L−1) can have detrimental effects on marine food webs. Recent studies indicate that some jellyfish can tolerate low oxygen and may have a competitive advantage over other zooplankton and fishes in those environments. We assessed community structure and distributions of cnidarian and ctenophore jellyfish in seasonally hypoxic Hood Canal, WA, USA, at four stations that differed in oxygen conditions. Jellyfish were collected in June through October 2012 and 2013 using full-water-column and discrete-depth net tows, concurrent with CTD casts to measure dissolved oxygen (DO). Overall, southern, more hypoxic, regions of Hood Canal had highermore »abundances and higher diversity than the northern regions, particularly during the warmer and more hypoxic year of 2013. Of fifteen species identified, the most abundant—the siphonophore Muggiaea atlantica and hydrozoan Aglantha digitale—reached peak densities > 1800 Ind m−3 and 38 Ind m−3, respectively. M. atlantica were much more abundant at the hypoxic stations, whereas A. digitale were also common in the north. Vertical distributions explored during hypoxia showed that jellyfish were mostly in the upper 10 m regardless of the oxycline depth. Moderate hypoxia seemed to have no detrimental effect on jellyfish in Hood Canal, and may have resulted in high population densities, which could influence essential fisheries and trophic energy flow.« less
5. Limited biogeochemical modification of surface waters by kelp forest canopies: Influence of kelp metabolism and site‐specific hydrodynamics

   https://doi.org/10.1002/lno.11999  

   Traiger, Sarah B. ; Cohn, Brian ; Panos, Demetra ; Daly, Margaret ; Hirsh, Heidi K. ; Martone, Maria ; Gutierrez, Isabella ; Mucciarone, David A. ; Takeshita, Yuichiro ; Monismith, Stephen G. ; et al ( December 2021 , Limnology and Oceanography)

   Climate change is causing decreases in pH and dissolved oxygen (DO) in coastal ecosystems. Canopy-forming giant kelp can locally increase DO and pH through photosynthesis, with the most pronounced effect expected in surface waters where the bulk of kelp biomass resides. However, limited observations are available from waters in canopies and measurements at depth show limited potential of giant kelp to ameliorate chemical conditions. We quantified spatiotemporal variability of surface biogeochemistry and assessed the role of biological and physical drivers in pH and DO modification at two locations differing in hydrodynamics inside and outside of two kelp forests in Montereymore »Bay, California in summer 2019. pH, DO, dissolved inorganic carbon (DIC), and temperature were measured at and near the surface, in conjunction with physical parameters (currents and pressure), nutrients, and metrics of phytoplankton and kelp biological processes. DO and pH were highest, with lower DIC, at the surface inside kelp forests. However, differences inside vs. outside of kelp forests were small (DO 6–8%, pH 0.05 higher in kelp). The kelp forest with lower significant wave height and slower currents had greater modification of surface biogeochemistry as indicated by larger diel variation and slightly higher mean DO and pH, despite lower kelp growth rates. Differences between kelp forests and offshore areas were not driven by nutrients or phytoplankton. Although kelp had clear effects on biogeochemistry, which were modulated by hydrodynamics, the small magnitude and spatial extent of the effect limits the potential of kelp forests to mitigate acidification and hypoxia.« less