Tracking oxy-thermal habitat compression encountered by Chesapeake Bay striped bass through acoustic telemetry

In many coastal ecosystems, habitat compression is caused by seasonal combinations of hypoxia and supraoptimal temperatures. These conditions commonly induce avoidance behaviours in mobile species, resulting in the concentrated use of marginal habitats. Using 3 years of acoustic telemetry and high-resolution water quality data recorded throughout Chesapeake Bay, we measured the seasonal movements and exposure of striped bass (Morone saxatilis) to oxy-thermal habitat compression. Striped bass moved to tidal freshwaters in spring (March–May), mesohaline waters in summer (June–August) and fall (September–November), and mesohaline and polyhaline waters in winter (December–February): seasonal patterns consistent with known spawning, foraging, and overwintering migrations. Analyses of habitat selection suggest that during conditions of prevalent sub-pycnocline hypoxia (June–September), striped bass appeared to select surface waters (i.e. they may avoid bottom hypoxic waters). Striped bass detections indicated tolerance of a wide range of surface water temperatures, including those >25°C, which regional regulatory bodies stipulate are stressful for this species. Still, during summer and fall striped bass selected the lowest-available temperature and avoided water temperature >27°C, demonstrating that Chesapeake Bay striped bass can encounter habitat compressions due to the behavioural avoidance of bottom hypoxia and high temperatures.

Bottom hypoxia alters the spatial distribution of pelagic intermediate consumers and their prey

While recent research has shed insight into how bottom hypoxia affects pelagic food webs in coastal marine ecosystems and natural lakes, its effects on man-made lake (reservoir) food webs remains more incomplete. To address this gap, we conducted a study in two Midwestern USA reservoirs to examine how the spatial overlap and vertical distributions of dominant zooplanktivores (i.e., pelagic fish, the bentho-pelagic macroinvertebrate Chaoborus) and their prey vary between periods of normoxia and hypoxia. Surprisingly, we found high levels of spatial overlap between zooplankton and both intermediate consumers (pelagic fish and Chaoborus) during both normoxia and hypoxia, though the extent of spatial overlap was higher during hypoxia at night relative to day. As expected, pelagic fish and zooplankton avoided hypoxic waters, and Chaoborus moved from hypoxic waters during the day to the well-oxygenated surface waters at night. Using our findings, we discuss the potential influence of bottom hypoxia and Chaoborus on the function and structure of north-temperate reservoir food webs.

Hypoxia changes the shape of the biomass size spectrum of planktonic communities: a case study in the eastern Mediterranean (Elefsina Bay)

Hypoxia is a major stressor on biological communities in many oceanic and coastal ecosystems. Various size-dependent processes (e.g. growth and reproduction rates, predator–prey interactions) are adversely affected by hypoxia. We hypothesized that the impacts of hypoxia on planktonic communities would also be reflected in their Normalized Biomass Size Spectra (NBSS) as steeper slopes and lower intercepts. To explore this hypothesis, we studied the planktonic communities (from bacteria to mesozooplankton) of Elefsina, an enclosed bay that exhibits near bottom hypoxia during summer, and Aghios Kosmas, an adjacent coastal site outside the bay. Bottom layer hypoxia formed during the stratification period in Elefsina Bay significantly altered the distribution of planktonic organisms in the water column. Several unicellular and mesozooplanktonic groups avoided the hypoxic layer, in which the biomass of autotrophic picoeukaryotes was markedly higher. Community changes related to hypoxia were clearly reflected in the NBSS. The slope was significantly steeper in the hypoxic layer (−1.330 vs −1.193) and the intercept was lower (−2.222 vs −0.972, hypoxic vs oxic layer). This result can be interpreted as reduced trophic transfer efficiency to the higher trophic levels due to hypoxia.

Seasonal Variation of Dissolved Oxygen in the Southeast of the Pearl River Estuary

Dissolved oxygen (DO) concentration in estuaries is highly variable at different spatial and temporal scales, which is affected by physical, chemical and biological processes. This study analyzed the spatial–temporal distributions of dissolved oxygen concentration and bottom hypoxia in the southeast of the Pearl River Estuary (PRE) using monthly water quality monitoring and hydrographic data covering the period 2000–2017. The seasonal spatial–temporal variation of DO concentration was studied using various methods, such as rotated empirical orthogonal functions, harmonic analysis, and correlation analysis. The results showed that DO stratification was significant in summer, but it was not distinct in winter, during which DO concentration peaked. DO stratification exhibited a significantly positive correlation with water stratification. In the south and west of Hong Kong (SHK and WHK, respectively), DO concentration fields exhibited distinct seasonal changes in the recent 18 years. In SHK, the main periods of the surface DO variation were 24, 12, and 6 months, whereas the main period was 12 months in WHK. The main period of the bottom DO variation was 12 months in both SHK and WHK. In SHK, the spatial–temporal variations in surface and bottom DO were highly related to the variations of salinity, dissolved inorganic nitrogen (DIN), and active phosphorus, and the variation of surface DO was also connected to the variation of temperature and chlorophyll a. In WHK, the variations in surface and bottom DO were highly related to the variations of salinity and temperature, and the variation of surface DO was also connected to the variation of DIN. The river discharge and wind had a different important influence on the temporal variability of DO in WHK and SHK. These findings suggested that the variations of DO may be controlled by coupled physical and biochemical processes in the southeast of PRE. From 2000 to 2017, bottom hypoxia in the southeast of PRE occurred in the summers of 7 years. SHK appeared to be more vulnerable to hypoxia than WHK.

The Near Bottom Hypoxia and Hydrogen Sulphide Formation on the Black Sea Shelf

Purpose. Estimation of the Northwestern part of the Black Sea Shelf in modern period. Methods. The sample of average daily measurements of the temperature, salinity on the surface, level, wind velocity and direction during 2007, 2012 and 2017 had been done. The analyses of the cruise investigation parameters and NASA satellite photos had been done in this region as well. Results. Increasing of nutrient, heavy metals, oil concentration in the Danube, Dnieper and Dniester of water runoff was fixed during the last 50 years. It was the reason of its permanent accumulation in marine ecosystem. Also it was the reason of anthropogenic eutrophication development in the sea water in spring and at the beginning of summer time. Later, at the end of summer and in autumn the dissolved oxygen is decreasing in the bottom layers because of destruction of organic matter. In the last years, decreasing of nutrient from the rivers input was marked. It provided the increasing the transparency in the sea column and made the water condition more positive. But for assessment of the whole ecosystem state the complexes monitoring is absolutely necessary. In September of 2017 the special investigation cruise was done. The result shown the deficit of the dissolved oxygen – hypoxia in the near bottom layer is spreading in the center of the shelf ecosystem (the depths are more than 20 m). The oxygen concentrations were less than 2,0 ml/l.. The reason of this negative phenomena was provided by NASA satellite photos of eutrophication process in summer and marking of upwelling at the shallow waters during the warm period in 2007, 2012 and 2017. Conclusions. Anthropogenic eutrophication development in the sea water was fixed as well as the near bottom hypoxia and hydrogen sulphide formation in the Ukrainian part of the Northwestern shelf of the Black sea in the modern period. Spatial scale of this phenomena is comparable with the scales from 70's of last century.

A numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the Pearl River estuary

A three-dimensional (3-D) physical–biogeochemical coupled model was applied to explore the mechanisms controlling the dissolved oxygen (DO) dynamics and bottom hypoxia during summer in the Pearl River estuary (PRE). By using the numerical oxygen tracers, we proposed a new method (namely the physical modulation method) to quantify the contributions of boundary conditions and each source and sink process occurring in local and adjacent waters to the DO conditions. A mass balance analysis of DO based on the physical modulation method indicated that the DO conditions at the bottom layer were mainly controlled by the source and sink processes, among which the sediment oxygen demand (SOD) at the water–sediment interface and the re-aeration at the air–sea interface were the two primary processes determining the spatial extent and duration of bottom hypoxia in the PRE. The SOD could cause a significant decrease in the bottom DO concentrations (averaged over July–August 2006) by over 4 mg L−1 on the shelf off the Modaomen sub-estuary, leading to the formation of a high-frequency zone of hypoxia (HFZ). However, the hypoxia that occurred in the HFZ was intermittent and distributed in a small area due to the combined effects of re-aeration and photosynthesis, which behaved as sources for DO and offset a portion of the DO consumed by SOD. The bottom DO concentrations to the west of the lower Lingdingyang Bay (i.e. the western shoal near Qi'ao Island) were also largely affected by high SOD, but there was no hypoxia occurring there because of the influence of re-aeration. Specifically, re-aeration could lead to an increase in the bottom DO concentrations by ∼ 4.8 mg L−1 to the west of the lower Lingdingyang Bay. The re-aeration led to a strong vertical DO gradient between the surface and the lower layers. As a result, the majority (∼ 89 %) of DO supplemented by re-aeration was transported to the lower layers through vertical diffusion and ∼ 28 % reached the bottom eventually. Additional numerical experiments showed that turning off re-aeration could lead to an expansion of the hypoxic area from 237 to 2203 km2 and result in persistent hypoxia (hypoxic frequency > 80 %) to the west of the lower Lingdingyang Bay. Compared to re-aeration and SOD, photosynthesis and water column respiration had relatively small impacts on the DO conditions; turning off these two processes increased the hypoxic area to 591 km2. In summary, our study explicitly elucidated the interactive impacts of physical and biogeochemical processes on the DO dynamics in the PRE, which is critical to understanding hypoxia in this shallow and river-dominated estuarine system.

Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?

The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which the seasonal stratification of the water column isolates the bottom waters from the atmosphere. This prevents ventilation from counterbalancing the large consumption of oxygen due to respiration in the bottom waters and in the sediments, and sets the stage for the development of seasonal hypoxia. A three-dimensional (3-D) coupled physical–biogeochemical model is used to investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at seasonal and then at interannual scales (1981–2009), and to differentiate its driving factors (climatic versus eutrophication). Model skills are evaluated by a quantitative comparison of the model results to 14 123 in situ oxygen measurements available in the NOAA World Ocean and the Black Sea Commission databases, using different error metrics. This validation exercise shows that the model is able to represent the seasonal and interannual variability of the oxygen concentration and of the occurrence of hypoxia, as well as the spatial distribution of oxygen-depleted waters. During the period 1981–2009, each year exhibits seasonal bottom hypoxia at the end of summer. This phenomenon essentially covers the northern part of the NWS – which receives large inputs of nutrients from the Danube, Dniester and Dnieper rivers – and extends, during the years of severe hypoxia, towards the Romanian bay of Constanta. An index H which merges the aspects of the spatial and temporal extension of the hypoxic event is proposed to quantify, for each year, the intensity of hypoxia as an environmental stressor. In order to explain the interannual variability of H and to disentangle its drivers, we analyze the long time series of model results by means of a stepwise multiple linear regression. This statistical model gives a general relationship that links the intensity of hypoxia to eutrophication and climate-related variables. A total of 82% of the interannual variability of H is explained by the combination of four predictors: the annual riverine nitrate load (N), the sea surface temperature in the month preceding stratification (Ts), the amount of semi-labile organic matter accumulated in the sediments (C) and the sea surface temperature during late summer (Tf). Partial regression indicates that the climatic impact on hypoxia is almost as important as that of eutrophication. Accumulation of organic matter in the sediments introduces an important inertia in the recovery process after eutrophication, with a typical timescale of 9.3 yr. Seasonal fluctuations and the heterogeneous spatial distribution complicate the monitoring of bottom hypoxia, leading to contradictory conclusions when the interpretation is done from different sets of data. In particular, it appears that the recovery reported in the literature after 1995 was overestimated due to the use of observations concentrated in areas and months not typically affected by hypoxia. This stresses the urgent need for a dedicated monitoring effort in the Black Sea NWS focused on the areas and months concerned by recurrent hypoxic events.