Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine ...nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.
Although hypoxia has been well-studied in Chesapeake Bay, little attention has been given to the origin of the particulate organic matter (POM) that potentially contributes to early summer hypoxia. A ...combination of a high-resolution baroclinic physical model and a Lagrangian particle tracking model was used to study the sources of POM to the deep channel of the mesohaline Chesapeake Bay. The circulation model reasonably reproduced the salinity structure and circulation compared with observations. The particle tracking model was improved with a fast search algorithm, and a predict-correct method was developed in the backward tracking model to ensure a similar trajectory with the forward tracking. Backward tracking results revealed that the pathway of the POM has a strong dependence on the tracking initiation time and location of the particles, as well as the magnitude of the sinking speed. In general, the behavior of the POM trajectories can be explained by the three-dimensional residual circulation of the mesohaline mainstem Chesapeake Bay. The particles that accumulated in the deep channel of the upper mesohaline Chesapeake Bay where the onset of hypoxia occurs mainly come from downstream when particles stop tracking when they backtracked to the surface layer. Downstream sources accounted for roughly 83.5% of particles for a sinking speed of 1 m/day, and the proportion from the downstream decreased to 60.5% as the sinking speed increased to 25 m/day. In addition, the source from the eastern shore was larger than that from the western shore for particle sinking speed less than 8 m/day. Moreover, particles from the Potomac estuary with a sinking speed of 1 m/day can contribute 7.4% of the bottom POM accumulation in the upper mesohaline Chesapeake Bay. When particles were allowed to stay on the surface for a period before they sink, they backtracked to the upper Chesapeake Bay suggesting that more long-lived refractory organic particles originating from the upper Bay can contribute to organic matter accumulation in the deep channel. These results help to explain why hypoxia in the deep channel of the upper mesohaline Bay occurs to the north of the primary production maximum and also why hypoxia is earlier and more severe in this region compared to the rest of hypoxic zone. Overall, the model results suggest that more local sources of POM with relatively high sinking rates are most important in driving oxygen depletion. This study improves understanding of the origin and pathways of the POM that can potentially contribute to the development of hypoxia in the deep channel of the mesohaline Chesapeake Bay. Future studies need to be undertaken to better understand the relative importance of remote versus more local sources of organic matter in driving oxygen drawdown.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Pelagic Sargassum (S. fluitans and S. natans) is endemic to the tropical and subtropical North Atlantic, where it provides habitat for a diverse and economically important ecosystem. Here, we ...investigate what controls the Sargassum seasonal distribution using a coupled modelling approach that integrates output from a data-assimilating 1/12° HYCOM simulation, a 1/4° coupled HYCOM–biogeochemical model, and individual-based Lagrangian Sargassum growth models. Passively advected, buoyant particles with no Sargassum physiology aggregate in the central North Atlantic Subtropical Gyre at annual time scales and do not show distributions consistent with satellite observations of Sargassum. However, at shorter time scales, advection alone can explain up to 60% of the following month observed distribution during some periods of the year. Connectivity between the tropical Atlantic and Sargasso Sea is largely one-way, with the Sargasso Sea acting as a ‘dead end’ for Sargassum. Adding growth, mortality and a simple formulation of reproduction through fragmentation to the passive advection of Sargassum particles generates distributions that match observations with 65 to 75% accuracy across all seasons. Incorporating both ocean circulation and Sargassum physiology appears to be key in successfully reproducing the seasonal distribution of biomass. We propose a conceptual model of the Sargassum seasonal cycle that incorporates new information about a population in the tropical Atlantic. Additionally, we suggest that the Gulf of Mexico and Western Tropical Atlantic are regions whose Sargassum populations may disproportionately influence the basin-wide biomass.
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BFBNIB, NUK, PNG, UL, UM, UPUK
Routine monitoring of shellfish growing waters for bacteria indicative of human sewage pollution reveals little about the bacterial communities that co-occur with these indicators. This study ...investigated the bacterial community, potential pathogens, and fecal indicator bacteria in 40 water samples from a shellfish growing area in the Chesapeake Bay, USA. Bacterial community composition was quantified with deep sequencing of 16S rRNA gene amplicons, and absolute gene abundances were estimated with an internal standard (
genomes). Fecal coliforms were quantified by culture, and
and
with quantitative PCR. Fecal coliforms and
were detected in most samples, and a diverse assemblage of potential human pathogens were detected in all samples. These taxa followed two general patterns of abundance. Fecal coliforms and 16S rRNA genes for Enterobacteriaceae,
, and
increased in abundance after a 1.3-inch rain event in May, and, for some taxa, after smaller rain events later in the season, suggesting that these are allochthonous organisms washed in from land. Clostridiaceae and
16S rRNA gene abundances increased with day of the year and were not positively related to rainfall, suggesting that these are autochthonous organisms. Other groups followed both patterns, such as
. Fecal coliform abundance did not correlate with most other taxa, but were extremely high following the large rainstorm in May when they co-occurred with a broad range of potential pathogen groups.
were absent during the large rainstorm, and did not correlate with 16S rRNA abundances of
spp. or most other taxa. These results highlight the complex nature of bacterial communities and the limited utility of using specific bacterial groups as indicators of pathogen presence.
In the Indian Ocean, mid-depth oxygen minimum zones (OMZs) occur in the Arabian Sea and the Bay of Bengal. The lower part of the Arabian-Sea OMZ (ASOMZ; below 400m) intensifies northward across the ...basin; in contrast, its upper part (above 400m) is located in the central/eastern basin, well east of the most productive regions along the western boundary. The Bay-of-Bengal OMZ (BBOMZ), although strong, is weaker than the ASOMZ. To investigate the processes that maintain the Indian-Ocean OMZs, we obtain a suite of solutions to a coupled biological/physical model. Its physical component is a variable-density, 612-layer model, in which each layer corresponds to a distinct dynamical regime or water-mass type. Its biological component has six compartments: nutrients, phytoplankton, zooplankton, two size classes of detritus, and oxygen. Because the model grid is non-eddy resolving (0.5°), the biological model also includes a parameterization of enhanced mixing based on the eddy kinetic energy derived from satellite observations. To explore further the impact of local processes on OMZs, we also obtain analytic solutions to a one-dimensional, simplified version of the biological model. Our control run is able to simulate basic features of the oxygen, nutrient, and phytoplankton fields throughout the Indian Ocean. The model OMZs result from a balance, or lack thereof, between a sink of oxygen by remineralization and subsurface oxygen sources due primarily to northward spreading of oxygenated water from the Southern Hemisphere, with a contribution from Persian-Gulf water in the northern Arabian Sea. The northward intensification of the lower ASOMZ results mostly from horizontal mixing since advection is weak in its depth range. The eastward shift of the upper ASOMZ is due primarily to enhanced advection and vertical eddy mixing in the western Arabian Sea, which spread oxygenated waters both horizontally and vertically. Advection carries small detritus from the western boundary into the central/eastern Arabian Sea, where it provides an additional source of remineralization that drives the ASOMZ to suboxic levels. The model BBOMZ is weaker than the ASOMZ because the Bay lacks a remote source of detritus from the western boundary. Although detritus has a prominent annual cycle, the model OMZs do not because there is not enough time for significant remineralization to occur.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Low-lattude waters of the Indian Ocean are warming faster than other major oceans. Most models predict a zooplankton decline due to lower productivity, enhanced metabolism and phytoplankton size ...shifts that reduce trophic transfer efficiency. In May−June 2019, we investigated mesozooplankton biomass and grazing along the historic 110° E transect line from the International Indian Ocean Expedition (IIOE) of the 1960s. Twenty sampling stations from 39.5 to 11.5° S spanned latitudinal variability from temperate to tropical waters and a pronounced 14°C gradient in mean euphotic zone temperature. Although mesozooplankton size structure was similar along the transect, with smaller (<2 mm) size classes dominant, total biomass increased 3-fold (400 to 1500 mg dry weight m−2) from high to low latitude. More dramatically, gut-fluorescence estimates of grazing (total ingestion or % euphotic zone chl a consumed d−1) were 14- and 20-fold higher, respectively, in the low-latitude warmer waters. Biomass-normalized grazing rates varied more than 6-fold over the transect, showing a strong temperature relationship (r² = 0.85) that exceeded the temperature effects on gut turnover and metabolic rates. Herbivory contributed more to satisfying zooplankton energetic requirements in low-chl a tropical waters than chl a-rich waters at higher latitude. Our unexpected results are inconsistent with trophic amplification of warming effects on phytoplankton to zooplankton, but might be explained by enhanced coupling efficiency via mixotrophy. Additional implications for selective herbivory and topdown grazing control underscore the need for rigorous field studies to understand relationships and validate assumptions about climate change effects on the food webs of tropical oceans.
Measures of extreme climate variability around Chesapeake Bay have changed over the past century (1895–2014), resulting in the need to establish new baselines for understanding future change. Here, ...observed climate variability is compared with Coupled Model Intercomparison Project fifth assessment climate models to evaluate ensemble model skill in this region. Observed trends include annual and seasonal declines in the percentage of cold days. Similarly, increases in the annual and seasonal percentages of warm days occurred in all seasons of the North Chesapeake (>38.2°N) but were only significant in spring and summer in the South (<38.2°N) demonstrating regional differences even at local scales. Precipitation intensity increased over the past century. Models using emissions scenarios RCP4.5 and 8.5 project these trends will continue, though they have little skill in precipitation extremes. Pacific and Atlantic climate modes are generally more correlated with climate extremes than mean temperature and precipitation suggesting potential predictability in the synoptic patterns underlying the extremes. However, they also drive the need for time series at least 60‐years long for establishing climate trends. This local analysis differs from regional, state level, and local 2.5° × 3.75° grid analyses, highlighting the importance of local climate assessments that consider topographic and regional weather patterns.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Despite strong control over marine plankton dynamics and negative impacts on human activities, jellyfish are not well quantified due primarily to sampling difficulties with nets. Therefore, some of ...the longest records of jellyfish are visual shore-based surveys. As surface counting is inexpensive and simple, it is of interest to determine what can be learned from such records as well as the usefulness of the method. We analyzed a 4-year high-frequency time series of
Chrysaora quinquecirrha
medusa counts collected using three sampling methods in the Choptank River, Chesapeake Bay. Medusa abundance was modeled by change points and was highly correlated between the sampling methods. The remaining signal was random, and indices of aggregation fit to the Poisson distribution, Taylor’s Power Law (TPL), and Morisita’s Index indicated that medusae were aggregated. TPL suggested that patches grew in the number of individuals as abundance increased. Additionally, a simple conceptualization of where the time series sampled in space revealed that the upper bound of patch size was on the order of kilometers. Our results enhance the knowledge of local
C. quinquecirrha
abundance and patchiness, alluding to processes that generate these patterns. This study also provides direction for improving population monitoring from visual shore-based surveys.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Photochemical degradation (PD) of colored dissolved organic matter (CDOM) is a key transformational process for both natural and anthropogenic DOM. A fully mechanistic model is presented that can ...simulate laboratory incubations of the controlled PD of marsh and estuarine derived CDOM. The model was designed and optimized to recreate the loss of absorbance for marsh low tide and estuarine samples, representing high molecular weight allochthonous and mid molecular weight estuarine CDOM. In the model, high specific absorbance fractions representative of marsh and estuarine CDOM are transformed into a low specific absorbance fraction representative of coastal ocean CDOM as well as non-colored fractions. The various transformations in the model have maximum apparent quantum yields (at 284 nm) that range from 3.22 × 10−8 ± 1.75 to 56.05 ± 21.5 mmol C (mol photons)−1, with non-colored DOM/inorganic carbon production outpaced by inter-molecular organic carbon transformations. Model performance was tested using an independent incubation data set whereby experimental results of photobleaching of spectral absorbance at 300 nm were recreated with a Willmott model skill of 0.98 and mean percent error of −3.66%. The production of the low molecular weight photodegraded end member ranged from 0.52 to 4.86 μmol C L−1 h−1.
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•Photobleaching of CDOM was simulated with a mechanistic model.•The model is well constrained and recreated a test data set with high skill.•Transformation of dissolved organic carbon peaked at 4.86 μmol C L−1 h−1.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 185.Indian Ocean Biogeochemical Processes and Ecological Variability provides a synthesis of current ...knowledge on Indian Ocean biogeochemistry and ecology and an introduction to new concepts and topical paradigm challenges. It also reports on the development of more extensive/frequent observational capacity being deployed in the Indian Ocean. This represents the first collection of syntheses that emphasize a basin-wide perspective, and the contributing authors include some of the most esteemed oceanographers and Indian Ocean experts in the world. The volume is derived from invited plenary talks that were presented at the initial Sustained Indian Ocean Biogeochemistry and Ecosystem Research (SIBER) workshop held at the National Institute of Oceanography (NIO) in Goa, India, in October 2006. The volume discusses The overlying physical processes set by monsoonal forcing and how these control biological production and variability Nutrient cycling and limitation Pelagic carbon cycling and air-sea exchange Benthic biogeochemistry and ecology The impact of climate and human activities on biogeochemistry and ecosystems. The readership for this book will consist of academic and governmental researchers interested in exploring how oceanographic, atmospheric, and hydrological processes combine to establish the environmental setting that supports and drives the pelagic system and which are especially relevant to understanding the complex biogeochemical and ecological interactions in the Indian Ocean.
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