Residence time of pollutants discharged in the Gulf of Kachchh, northwestern Arabian Sea

A 2D Hydrodynamic-Particle Analysis model was applied to the Gulf of Kachchh (GoK) to estimate the residence time of pollutants. The tidal currents in the Gulf have a strong E-W component, which prevents the material in the north being transported towards south. In the regions situated very close to the open boundary, where the GoK waters exchange freely with the northern Arabian Sea, dilution takes place rapidly with the incoming waters and hence, the residence time is on the order of 1 day. Influence of eddies and a dynamic barrier across the Sikka-Mundra section on the residence time is apparent. Eastern GoK shows a relatively large residence time, on the order of 2-4 days, warranting caution while releasing industrial wastes in the northeastern Gulf. The region around location-5 behaves like a bay; the dissolved matter gets trapped in this bay and the residence time increases by 3-4 days.     

Thermohaline structure of an inverse estuary--The Gulf of Kachchh: measurements and model simulations

The Gulf of Kachchh (GoK) is situated in the northeastern Arabian Sea. The presence of several industries along its coastal belt makes GoK a highly sensitive coastal ecosystem. In the present study, an attempt is made for the first time to study GoK thermohaline structure and its variability, based on field measurements and model simulations. Though GoK is considered as a well-mixed system, the study reveals that only the central Gulf is well mixed. Vertical gradients in temperature and salinity fields are noticed in the eastern Gulf, where a cold and high saline tongue is observed in the subsurface layers. Salinity indicates the characteristic feature of an inverse estuary with low values (37.20 psu) near the mouth and high values (>40.0 psu) near the head of the Gulf. The model simulated temperature and salinity fields exhibit semidiurnal oscillations similar to that of field observations. Model results show cold, high saline waters advecting from the east during ebb forming a transition zone, which oscillates with tides. A high salinity tongue is seen in the bottom layer, indicating a westward flowing bottom current. The transient zone acts as an dynamic barrier, and plays a vital role in the pollutant transport.     

Environmental hypoxia as a metabolic constraint on fish: the case of Atlantic cod, Gadus morhua

Hypoxia is known to provoke a wide range of effects on aquatic animals. Here we use laboratory and field data on Atlantic cod, Gadus morhua, to illustrate that many of these responses can be explained within the metabolic scope (MS) framework, i.e. taking into account the directive and limiting effects of dissolved oxygen (DO) on the ability of animals to acquire energy for growth and activity. A MS model for cod shows that scope for activity (swimming, feeding, etc.) is proportional to DO and becomes nil, jeopardising survival, when DO is < approximately 20% air saturation. Laboratory studies have confirmed this lethal threshold and demonstrated that growth and food ingestion were significantly reduced below 70% sat. This loss of appetite has been linked to a reduction of the peak value and an increase in duration of postprandial metabolism, in agreement with the MS model. Dwindling MS during hypoxia imposes an upper limit to swimming performance. Cod may also opt to reduce spontaneous swimming activity to spare oxygen for other activities such as digestion. In the Kattegat, the Baltic Sea, and the Gulf of St. Lawrence, eastern Canada, cod completely avoid waters where their MS is near zero. Furthermore, cod density increases exponentially with DO up to approximately 70% sat in the Gulf of St. Lawrence. Although hypoxia results in other direct and indirect effects as well, the MS framework allows modelling of many of the responses to hypoxia for individual cod that ought to be reflected at the population and community levels. The MS framework is also useful to compare species responses. We show that the impact of hypoxia on MS is similar, when expressed as a proportion of MS in normoxia, in cod, European sea bass (Dicentrarchus labrax), the common sole (Solea solea) and turbot (Psetta maxima). Data are required for other species to evaluate how general these findings are.     

Environmental hypoxia as a metabolic constraint on fish: The case of Atlantic cod, Gadus morhua

Hypoxia is known to provoke a wide range of effects on aquatic animals. Here we use laboratory and field data on Atlantic cod, Gadus morhua, to illustrate that many of these responses can be explained within the metabolic scope (MS) framework, i.e. taking into account the directive and limiting effects of dissolved oxygen (DO) on the ability of animals to acquire energy for growth and activity. A MS model for cod shows that scope for activity (swimming, feeding, etc.) is proportional to DO and becomes nil, jeopardising survival, when DO is <≈20% air saturation. Laboratory studies have confirmed this lethal threshold and demonstrated that growth and food ingestion were significantly reduced below 70% sat. This loss of appetite has been linked to a reduction of the peak value and an increase in duration of postprandial metabolism, in agreement with the MS model. Dwindling MS during hypoxia imposes an upper limit to swimming performance. Cod may also opt to reduce spontaneous swimming activity to spare oxygen for other activities such as digestion. In the Kattegat, the Baltic Sea, and the Gulf of St. Lawrence, eastern Canada, cod completely avoid waters where their MS is near zero. Furthermore, cod density increases exponentially with DO up to ≈70% sat in the Gulf of St. Lawrence. Although hypoxia results in other direct and indirect effects as well, the MS framework allows modelling of many of the responses to hypoxia for individual cod that ought to be reflected at the population and community levels. The MS framework is also useful to compare species responses. We show that the impact of hypoxia on MS is similar, when expressed as a proportion of MS in normoxia, in cod, European sea bass (Dicentrarchus labrax), the common sole (Solea solea) and turbot (Psetta maxima). Data are required for other species to evaluate how general these findings are.     

The combined effects of oxygen availability and salinity on physiological responses and scope for growth in the green-lipped mussel Perna viridis

Mussels were maintained for 4 weeks under different combinations of dissolved oxygen concentration (1.5, 3.0 and 6.0 mg O2 l(-1)) and salinity (15, 20, 25 and 30) in a 3×4 factorial design experiment. Clearance rate (CR), absorption efficiency (AE), respiration rate (RR) and scope for growth (SFG) decreased with decreasing salinity and dissolved oxygen concentration (DO), while excretion rate (ER) increased with decreasing salinity and increasing DO. The O:N ratio was <10 at salinities of 15 and 20, irrespective of DO levels. SFG was negative in most of the treatments, except for those under 6.0 mg O2 l(-1) or at a salinity of 30 when DO was lower. The results may help explain the distribution pattern of Perna viridis in Hong Kong waters and provide guidelines for mussel culture site selection.     

On the disposal of contaminated milk in coastal waters.

Estimates have been made of the reduction in dissolved oxygen levels in coastal waters that would result from the disposal of contaminated milk following a radiological accident. Two contrasting sites were chosen: the Bristol Channel near Hinkley Point and the coast of Cumbria near Sellafield. The results suggest that the dilution would be sufficiently strong near Hinkley Point, due to vigorous tidal mixing, that the impact on the DO levels of the coastal waters would be negligible. However, at both Sellafield and Heysham the disposal of milk could result in a reduction of the DO by 1-2 mg l(-1). In contrast to shallow estuarine waters, the recovery of oxygen levels due to the effects of re-aeration through surface gas exchange is unlikely to be significant due to the depth of the coastal waters. However, the recovery of the dissolved oxygen levels to ambient conditions following the completion of the discharge would occur on a time scale of about 17 days due to mixing of the DO deficit plume into the surrounding waters.     

Monitoring and modeling dissolved oxygen dynamics through continuous longitudinal sampling: a case study in Wen‐Rui Tang River,Wenzhou, China

Synoptic water sampling at a fixed site monitoring station provides only limited ‘snap‐shots’ of the complex water quality dynamics within a surface water system. However, water quality often changes rapidly in both spatial and temporal dimensions, especially in highly polluted urban rivers. In this study, we designed and applied a continuous longitudinal sampling technique to monitor the fine‐scale spatial changes of water quality conditions, assess water pollutant sources, and determine the assimilative capacity for biochemical oxygen demand (BOD) in an urban segment of the hypoxic Wen‐Rui Tang River in eastern China. The continuous longitudinal sampling was capable of collecting dissolved oxygen (DO) data every 5 s yielding a ~11 m sampling interval with a precision of ±0.1 mg L^?1. The Streeter and Phelps BOD‐DO model was used to calculate: (1) the oxygen consumption coefficient (K₁) required for calibration of water quality models, (2) BOD assimilative capacity, and (3) BOD source and load identification. In the 2014 m river segment sampled, the oxygen consumption coefficient (K₁) was 0.428 d^?1 (20°C), the total BOD discharge was 916 kg d^?1, and the BOD assimilative capacity was 382 kg d^?1 when the minimum DO level was set to 2 mg L^?1. In addition, the longitudinal analysis identified eight major drainage outlets (BOD point sources), which were verified by field observations. This new approach provides a simple, cost‐effective method of evaluating BOD‐DO dynamics over large spatial areas with rapidly changing water quality conditions, such as urban environments. It represents a major breakthrough in the development and application of water quality sampling techniques to obtain spatially distributed DO and BOD in real time. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐resolution monitoring reveals dissolved oxygen dynamics in an Antarctic cryoconite hole

This study presents the first high‐resolution dataset of dissolved oxygen (DO) measurements in an ice‐lidded cryoconite hole on Canada Glacier, McMurdo Dry Valleys, Antarctica. Fibre optic DO minisensors were installed in a cryoconite hole prior to seasonal internal melting and hydrological connection to the subsurface drainage system. Oxygen air saturation in the cryoconite hole typically ranged from 50 to 80%, in broad agreement with previous single measurements, indicating net respiration (R). This is consistent with results of simple incubation experiments performed in the field. Simultaneous time series for electrical conductivity, water temperature, and DO over the four‐week study period provide information regarding the connectivity of cryoconite holes with the near‐surface drainage system. The main driver of the observed variations in DO is likely to be periodic melt‐freeze cycles. We conclude that automated sensing techniques, such as those described here, when used in conjunction with physical measurements, have great potential for high‐resolution monitoring of the factors that perturb biogeochemical processes in cryospheric surface aquatic ecosystems. Copyright © 2011 John Wiley & Sons, Ltd.     

珠江流域东江（东莞段）溶解氧时空变化及其影响因素

溶解氧是水生生态系统健康程度的重要度量指标,研究溶解氧时空变化及其影响因素对揭示河流溶解氧动态过程和理解水体生物地球化学循环具有重要意义.当前,许多河流均存在溶解氧偏低问题,严重影响河流水质.基于此,收集了珠江流域东江(东莞段)上游桥头、中游樟村和下游沙田泗盛3个水质监测站的实测资料,采用折线图和箱型图分析溶解氧在月、季节和年际时间尺度下的变化规律及其空间差异;采用最大信息系数、交叉小波转换和多元线性回归3种方法解析溶解氧时空变化的影响因素.结果表明:空间分布上,溶解氧浓度从上游到下游逐渐降低,低值区分布在入海口处的沙田泗盛站,多年平均溶解氧浓度仅为3.11 mg/L.时间变化上,2011-2019年溶解氧主要在11.64个月的周期下循环变化.溶解氧展现出明显的季节性.3个站点丰水期溶解氧浓度比枯水期分别降低1.68、2.03和1.77 mg/L.驱动因素上,溶解氧的时空变化受到多个因素的综合作用.桥头站和樟村站水质较好,水温是这两个站点溶解氧时空变化的主要影响因素.其主要在8～16个月的周期上调控溶解氧变化,在两个站点分别可以解释溶解氧整体变化的70%和57%.沙田泗盛站受多条支流汇...     

Predicting summer hypoxia in the northern Gulf of Mexico: redux

We report on the evolution and accuracy of a model used to predict the mid-summer area of hypoxia (oxygen ≤2 mg l(-1)) in the northern Gulf of Mexico, use it to test for impacts from the Deepwater Horizon oil spill (2010), and estimate the N loading that would meet a management goal. The prediction since 2000 were 100%±6% (μ±1 SE) of the actual value. The predicted in 2010 was 99% of that actual value, suggesting that the net effect of the 2010 oil spill on the hypoxic zone size was negligible. A tropical storm, however, may have reduced the potential size of the hypoxic zone. Lowering the May nitrogen load to about 70,000 mton N nitrate+nitrite would bring the model's predicted hypoxic zone size down to the management goal of 5000 km(2) and restore hypoxic waters to normoxic conditions.     

Modelling spatial variations in dissolved oxygen in fine‐grained streams under uncertainty

This paper presents a novel triple‐layer model, called VART DO‐3L, for simulation of spatial variations in dissolved oxygen (DO) in fine‐grained streams, characterized by a fluid mud (fluff or flocculent) layer (an advection‐dominated storage zone) as the interface between overlying stream water and relatively consolidated streambed sediment (a diffusion‐dominated storage zone). A global sensitivity analysis is conducted to investigate the sensitivity of VART DO‐3L model input parameters. Results of the sensitivity analysis indicate that the most sensitive parameter is the relative size of the advection‐dominated storage zones (A_s/A), followed by a lumped reaction term (R) for the flocculent layer, biological reaction rate (μ_o) in diffusive layer and biochemical oxygen demand concentration (L) in water column. In order to address uncertainty in model input parameters, Monte Carlo simulations are performed to sample parameter values and to produce various parameter combinations or cases. The VART DO‐3L model is applied to the Lower Amite River in Louisiana, USA, to simulate vertical and longitudinal variations in DO under the cases. In terms of longitudinal variation, the DO level decreases from 7.9 mg l at the Denham Springs station to about 2.89 mg l^?1 at the Port Vincent station. In terms of vertical variation, the DO level drops rapidly from the overlying water column to the advection‐dominated storage zone and further to the diffusive layer. The DO level (C_F) in the advective layer (flocculent layer) can reach as high as 40% of DO concentration (C) in the water column. The VART DO‐3L model may be applied to similar rivers for simulation of spatial variations in DO level. Copyright © 2014 John Wiley & Sons, Ltd.     

Sensitivity analysis of generalized Streeter-Phelps models

Sensitivity theory is applied in this paper to a class of generalized Streeter-Phelps models in order to predict the variations induced on BOD and DO by the variations of some parameters characterizing the river system.The paper shows how simple and elegant this technique is and, at the same time, proves that many relatively complex phenomena can be explained by Streeter-Phelps models.     

Sediment resuspension events within the (microtidal) coastal waters of Thermaikos Gulf, northern Greece

High-frequency flow, pressure and suspended sediment concentration (SSC) measurements are presented from the Paralia-Katerinis coastal area, in Thermaikos Gulf. The data were collected along a cross-shore transect, between the 6 and 12 m water depth contours. The relative importance of wave- and tidally-induced resuspension is examined. Resuspension events are shown to be dominated by wind-generated waves, especially under storm conditions. Some evidence is provided for tidal resuspension, but the overall impact of this process is minimal, compared to wave resuspension. Such resuspension, under storm conditions, increased the SSC levels in the waters of the nearshore zone to 35 mg/l; this is a >15-fold increase over the ambient levels (1–2 mg/l) of turbidity.     

EVALUATION OF WATER AND SEDIMENT QUALITIES AT RIVER MOUTHS IN THE HAIHE RIVER SYSTEM

Water and sediment qualities are studied by analyzing samples taking from the mouths of the Haihe, Duliujian, New Ziya and Beipai rivers in the Haihe river basin in north China in 2005 and 2001, in order to find the changes of water and sediment pollutions. The concentrations of heavy metals, arsenic, total nitrogen (TN) and total phosphorus (TP) are analyzed and results have been compared for the two times. The in-situ measurement for Dissolved Oxygen (DO) and Sediment Oxygen Demand (SOD) rates were carried at the Haihe and Duliujian river mouths in 2006. The results show that the waters of the 4 river mouths are still seriously polluted, though much improved in the case of the Haihe and Duliujian rivers. The main pollutants are TP and TN in the New Ziya and Beipai rivers and mercury (Hg) at all 4 river mouths. Compared with those in 2001, the concentrations of almost all metals and arsenic in the 4 river mouths have decreased. Water quality at Haihe and Duliujian shows an improving trend, while the water quality at Beipai is similar to that of 2001. In contrast, water at the New Ziya river mouth is more severely polluted. The sediments in the 4 river mouths are not seriously polluted by heavy metals but are polluted by nitrogen and phosphorus. Most of the pollutant contents in the sediments show little change between 2001 and 2005. The in-situ DO and SOD measurement shows that the waters at the Haihe river mouth is in the state of oxygen depletion, and SOD is important consumer of DO at the river mouths. The overall analysis shows that increasing water pollution and eutrophication in waters far from cities are ongoing causes of concern.     

The use of surface-enhanced Raman scattering (SERS) for detection of PAHs in the Gulf of Gdańsk (Baltic Sea)

A field operable surface enhanced Raman scattering (SERS) sensor system was applied for the first time under real conditions for the detection of polycyclic aromatic hydrocarbons (PAHs) as markers for petroleum hydrocarbons in the Gulf of Gdańsk (Baltic Sea). At six stations, seawater samples were taken, and the sensor system was applied in situ simultaneously. These measurements were compared to the results of conventional GC/MS laboratory analysis of the PAH concentrations in the seawater samples. For a PAH concentration above 150 ng(12PAH)l(-1), there was agreement between the SERS sensor and the GC/MS determinations. A standard addition experiment yielded a PAH concentration of 900 ng l(-1) at the Gdańsk Harbor, which was of the same order as the GC/MS determinations of 12PAHs (200 ng(12PAH)l(-1)). The high SERS detection limit for seawater samples is explained by the competition for PAHs between the sensor membrane and particulate matter surfaces. Thus, the SERS sensor can be applied, e.g., as a non-quantitative alarm sensor for relatively high PAH concentrations in heavily polluted waters. The spectral unmixing procedure applied for Gdańsk Harbor water confirmed the presence of phenanthrene at the highest concentration ([Phe]=140 ngl(-1)) and of Chr (2.7 ng l(-1)), but it did not detect the other PAHs present in the Gdańsk Harbor water, as determined by GC/MS. When compared to the past literature and databases, the SERS spectra indicated the presence of a mixture of molecules consisting of carotenoids, n-alkanes, amines or fatty acids, and benzimidazoles at the coastal station ZN2. The spectra in the offshore direction indicated carboxylic acids. Interpretation of the farthest offshore in situ SERS measurements is difficult, principally due to the limited availability of reference spectra. The detection of the lower PAH concentrations commonly found in Baltic coastal water needs further research and development to obtain better sensitivity of the SERS sensor. However, the high analytical specificity of the SERS sensor also allows the detection of other chemical species that require the development of a SERS/Raman library for specific in situ spectral interpretation.     

Seasonal and spatial dynamics of nutrients and phytoplankton biomass in Victoria Harbour and its vicinity before and after sewage abatement

This study investigated the seasonal and spatial dynamics of nutrients and phytoplankton biomass at 12 stations in Hong Kong (HK) waters during a three year period from 2004 to 2006 after upgraded sewage treatment and compared these results to observations before sewage treatment. Pearl River estuary (PRE) discharge significantly increased NO(3) and SiO(4) concentrations, particularly in western and southern waters when rainfall and river discharge was maximal in summer. Continuous year round discharge of sewage effluent resulted in high NH(4) and PO(4) in Victoria Harbour (VH) and its vicinity. In winter, spring and fall, the water column at all stations was moderately mixed by winds and tidal currents, and phytoplankton biomass was relatively low compared to summer. In summer, the mean surface phytoplankton chl biomass was generally > 9 microL(-1) in most areas as a result of thermohaline stratification, and high nutrients, light, and water temperature. In summer, the potential limiting nutrient is PO(4) in the most productive southern waters and it seldom decreased to limiting levels ( approximately 0.1 microM), suggesting that phytoplankton growth may be only episodically limiting. The mean bottom dissolved oxygen (DO) remained > 3.5 mg L(-1) at most stations, indicating that the eutrophication impact in HK waters was not as severe as expected for such a eutrophic area. After the implementation of chemically enhanced primary sewage treatment in 2001, water quality in VH improved as indicated by a significant decrease in NH(4) and PO(4) and an increase in bottom DO. In contrast, there were an increase in chl a and NO(3), and a significant decrease in bottom DO in southern waters in summer, suggesting that hypoxic events are most likely to occur in this region if phytoplankton biomass and oxygen consumption keep increasing and exceed the buffering capacity of HK waters maintained by monsoon winds, tidal mixing and zooplankton grazing. Therefore, future studies on the long-term changes in nutrient loading from PRE and HK sewage discharge will be crucial for developing future strategies of sewage management in HK waters.     

Source and dispersal of suspended sediment in the macro-tidal Gulf of Kachchh

The macro-tidal Gulf of Kachchh, covering nearly 7000 km(2), is located about 150 km south of the Indus River mouth. In spite of semi-arid climate and lack of major rivers flowing into it, the Gulf is highly turbid with suspended sediment concentrations (SSC) during October-November 2002 ranging between 0.5 and 674 mgl(-1). Highly turbid waters are observed towards the northern portion of the mouth of the Gulf, at the head of the Gulf and adjacent to the numerous shoals present within the Gulf. Perennial high SSC in the Gulf is due to resuspension of sediments by strong tidal currents, shallow bathymetry and presence of fine-grained sediments on the sea floor. Numerical model studies show that there is a dynamic barrier in the central Gulf, which prevents the exchange of water and suspended sediments between the outer and inner Gulf. This dynamic barrier associated with strong east-west tidal currents restricts the turbid waters mainly to the northern Gulf, resulting in relatively clear waters (SSC<10 mgl(-1)) in the southern and central portions of the Gulf. Laser particle size distribution, clay mineralogy and geochemistry of the suspended matter show that the main source of sediments to the Gulf of Kachchh is the Indus River. Although the Indus discharge has been severely curtailed in the recent decades due to construction of numerous dams and barrages, the Gulf of Kachchh continues to receive resuspended sediments from the numerous meso and macro-tidal creeks of the Indus delta. The sediments at the head of the Gulf appear to be a mixture of sediments derived from the Indus as well as the numerous seasonal rivers draining the Rann of Kachchh.     

Magnetic monitoring of marine particulate pollution in the Elefsis Gulf,Greece

Measurements of magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) on both filtered particulates and sediment samples provide a cheap, rapid and non-destructive method of particulate pollution monitoring in marine environments where ferrimagnetic iron oxides are important components of the discharge from industrial or urban complexes. The value of this approach is illustrated by reference to results obtained from the Elefsis Gulf, Greece, where the major source of particulate metal pollution is an iron and steel works discharging waste into a settling lagoon connected to the waters of the Gulf.     

Radon as an indicator of limited cross-shelf mixing of submarine groundwater discharge along an open ocean beach in the South Atlantic Bight during observed hypoxia

Hypoxic conditions (dissolved oxygen (DO)<2 mg l⁻¹) have been documented in the nearshore coastal waters of Long Bay, South Carolina, United States of America, during summer months over the past several years. Hypoxia was documented in August 2009 in the nearshore (<500 m offshore) for ten consecutive days and four days in September 2009 corresponding with spring tides. This study measured radon activities of shallow beachface groundwater and nearshore bottom waters to estimate mixing rates and submarine groundwater discharge (SGD) in the nearshore waters of central Long Bay. Statistical analyses demonstrate significant correlations between high bottom water radon activities, low DO, and cooler bottom water temperatures during hypoxic conditions. Elevated radon activities during hypoxia were significantly influenced by upwelling favorable conditions which severely limited cross-shelf mixing. Model results indicate mixing of nearshore and offshore waters was limited by up to 93% (range: 43-100%) relative to non-hypoxic conditions. Data suggests previously overlooked natural phenomena including limited cross-shelf mixing and SGD can significantly influence nearshore water quality.     

Effect of prolonged hypoxia on food consumption, respiration, growth and reproduction in marine scavenging gastropod Nassarius festivus

The effects of prolonged exposure to reduced oxygen levels (3.0 and 1.5 mg O(2)l(-1)) on marine scavenging gastropods Nassarius festivus were studied for 8 weeks. The percentages of individuals engaged in feeding and amount of food consumed were reduced as oxygen level decreased; absorption efficiency, however, did not vary significantly with oxygen level. Oxygen consumption rates and specific oxygen consumption rates were lower at reduced oxygen levels. Reproduction occurred at all oxygen levels with less egg capsules being produced at lower oxygen levels. Egg size and number of eggs per capsule, however, were not significantly affected by oxygen level. The increase in shell length was 12%, 6% and 5% at 6.0 mgO(2)l(-1) (normoxia), 3.0 mgO(2)l(-1) and 1.5 mgO(2)l(-1), respectively. At the end of the experiment, the amount of energy allocated to growth and reproduction decreased at reduced oxygen levels with values obtained at 3.0 mgO(2)l(-1) and 1.5 mgO(2)l(-1) being 48% and 70% lower than those at 6.0 mgO(2)l(-1). At all oxygen levels, most of the accumulated energy was allocated to shell growth and reproduction, and the amount allocated to somatic growth was relatively insignificant. The reduction in energy allocated to reproduction was greater than that to shell growth as the oxygen level was reduced, indicating a strategic energy allocation of marine scavengers under stressful conditions to enhance survival.