Because salinity (together with temperature) affects the
density of seawater, it can play an important role in its vertical stratification. Pycnoclines (layers of rapid change of
density with depth) are often coupled to haloclines and
thermoclines (layers where the temperature gradient is
greater than that of the warmer layer above and the colder
layer below).
The formation of a pycnocline restricts vertical mixing
and leads to oxygen depletion in deeper waters and the formation of anoxia. This process occurs, for example, in the
Black Sea, many marine deeps (e.g., Gotland Deep in the
Baltic Sea), anoxic fjords, and some deeper estuaries.
Bibliography
Halocline, 2013. Encyclopædia Britannica Online. Retrieved from
27 August 2013. http://www.britannica.com/EBchecked/topic/
252980/halocline
Yakushev, E. V., Debolskaya, E. I., Kuznetsov, I. S., and Staalstrøm, A.,
2013. Modelling of the meromictic Fjord Hunnbunn (Norway) with
an oxygen depletion model (OxyDep). In Yakushev, E. V. (ed.),
Chemical Structure of Pelagic Redox Interfaces: Observation and
Modeling. Berlin/Heidelberg: Springer. Handbook of Environmental Chemistry, Vol. 22, pp. 235–252.
Cross-references
Anoxia, Hypoxia, and Dead Zones
Oxygen Depletion
HALOGENATED HYDROCARBONS
Michael J. Kennish
Department of Marine and Coastal Sciences,
School of Environmental and Biological Sciences,
Rutgers University, New Brunswick, NJ, USA
Synonyms
Halocarbons
Definition
Halogenated hydrocarbons are chemical compounds
containing carbon and hydrogen atoms and one or more
halogens (bromine, chlorine, fluorine, iodine) in their
chemical structure to form organobromine, organochlorine, organofluorine, and organoiodine compounds. The
organochlorine compounds (i.e., chlorinated hydrocarbons) are a particularly well-known and hazardous group
of halogenated hydrocarbons in estuarine and marine
environments. As a group, the halogenated hydrocarbons
consist of some of the most persistent, ubiquitous, and
toxic environmental pollutants. They commonly originate
from agricultural activities, industrial operations, and
urbanization (Kennish, 1997; Kennish, 2001; Kennish
et al., 2008).
Description
Among the wide array of halogenated hydrocarbons found
in estuarine and marine environments, the highermolecular-weight compounds pose the most serious threat
to estuarine and marine organisms. This is so because the
higher-molecular-weight halogens tend to accumulate in
biota (notably in lipid-rich tissues) and can then concentrate up food chains. The organochlorine contaminants
(e.g., DDT, chlordane, toxaphene, PCBs) are noteworthy
because they have unique properties that can increase their
negative impacts. For example, they are chemically stable,
highly mobile and generally toxic, hydrophobic, resistant
to degradation, and persistent in the environment. In addition, they have an affinity for living organisms, are lipophilic, and exhibit considerable bioaccumulative
capacity. They may also biomagnificate through food
webs. Hence, organisms at the uppermost trophic levels
(i.e., dolphins, porpoises, seals, whales, and other mammals) typically carry the highest contaminant residues
(Kennish, 1997; Kennish et al., 2008).
Bibliography
Kennish, M. J., 1992. Ecology of Estuaries: Anthropogenic Effects.
Boca Raton: CRC Press.
Kennish, M. J. (ed.), 1997. Practical Handbook of Estuarine and
Marine Pollution. Boca Raton: CRC Press.
Kennish, M. J. (ed.), 2001. Practical Handbook Marine Science,
3rd edn. Boca Raton: CRC Press.
Kennish, M. J., Livingston, R. J., Raffaelli, D., and Reise, K., 2008.
Environmental future of estuaries. In Polunin, N. V. C. (ed.),
Aquatic Ecosystems: Trends and Global Prospects. Cambridge:
Cambridge University Press, pp. 188–206.
Cross-references
Anthropogenic Impacts
Nonpoint Source Pollution
Oil Pollution
Polycyclic Aromatic Hydrocarbons
Trace Metals in Estuaries
HEADLAND BREAKWATERS
C. Scott Hardaway, Jr.
1 and James R. Gunn
2
1
Virginia Institute of Marine Science, College of
William & Mary, Gloucester Point, VA, USA
2
Coastal Design & Construction, Gloucester, VA, USA
Synonyms
Bay beach; Embayment; Pocket beach
Definition
Headland breakwaters are coastal structures placed
roughly parallel to the coast and attached to the land with
a sand spit (i.e., tombolo), thereby forming a headland.
350
HALOGENATED HYDROCARBONS
density of seawater, it can play an important role in its vertical stratification. Pycnoclines (layers of rapid change of
density with depth) are often coupled to haloclines and
thermoclines (layers where the temperature gradient is
greater than that of the warmer layer above and the colder
layer below).
The formation of a pycnocline restricts vertical mixing
and leads to oxygen depletion in deeper waters and the formation of anoxia. This process occurs, for example, in the
Black Sea, many marine deeps (e.g., Gotland Deep in the
Baltic Sea), anoxic fjords, and some deeper estuaries.
Bibliography
Halocline, 2013. Encyclopædia Britannica Online. Retrieved from
27 August 2013. http://www.britannica.com/EBchecked/topic/
252980/halocline
Yakushev, E. V., Debolskaya, E. I., Kuznetsov, I. S., and Staalstrøm, A.,
2013. Modelling of the meromictic Fjord Hunnbunn (Norway) with
an oxygen depletion model (OxyDep). In Yakushev, E. V. (ed.),
Chemical Structure of Pelagic Redox Interfaces: Observation and
Modeling. Berlin/Heidelberg: Springer. Handbook of Environmental Chemistry, Vol. 22, pp. 235–252.
Cross-references
Anoxia, Hypoxia, and Dead Zones
Oxygen Depletion
HALOGENATED HYDROCARBONS
Michael J. Kennish
Department of Marine and Coastal Sciences,
School of Environmental and Biological Sciences,
Rutgers University, New Brunswick, NJ, USA
Synonyms
Halocarbons
Definition
Halogenated hydrocarbons are chemical compounds
containing carbon and hydrogen atoms and one or more
halogens (bromine, chlorine, fluorine, iodine) in their
chemical structure to form organobromine, organochlorine, organofluorine, and organoiodine compounds. The
organochlorine compounds (i.e., chlorinated hydrocarbons) are a particularly well-known and hazardous group
of halogenated hydrocarbons in estuarine and marine
environments. As a group, the halogenated hydrocarbons
consist of some of the most persistent, ubiquitous, and
toxic environmental pollutants. They commonly originate
from agricultural activities, industrial operations, and
urbanization (Kennish, 1997; Kennish, 2001; Kennish
et al., 2008).
Description
Among the wide array of halogenated hydrocarbons found
in estuarine and marine environments, the highermolecular-weight compounds pose the most serious threat
to estuarine and marine organisms. This is so because the
higher-molecular-weight halogens tend to accumulate in
biota (notably in lipid-rich tissues) and can then concentrate up food chains. The organochlorine contaminants
(e.g., DDT, chlordane, toxaphene, PCBs) are noteworthy
because they have unique properties that can increase their
negative impacts. For example, they are chemically stable,
highly mobile and generally toxic, hydrophobic, resistant
to degradation, and persistent in the environment. In addition, they have an affinity for living organisms, are lipophilic, and exhibit considerable bioaccumulative
capacity. They may also biomagnificate through food
webs. Hence, organisms at the uppermost trophic levels
(i.e., dolphins, porpoises, seals, whales, and other mammals) typically carry the highest contaminant residues
(Kennish, 1997; Kennish et al., 2008).
Bibliography
Kennish, M. J., 1992. Ecology of Estuaries: Anthropogenic Effects.
Boca Raton: CRC Press.
Kennish, M. J. (ed.), 1997. Practical Handbook of Estuarine and
Marine Pollution. Boca Raton: CRC Press.
Kennish, M. J. (ed.), 2001. Practical Handbook Marine Science,
3rd edn. Boca Raton: CRC Press.
Kennish, M. J., Livingston, R. J., Raffaelli, D., and Reise, K., 2008.
Environmental future of estuaries. In Polunin, N. V. C. (ed.),
Aquatic Ecosystems: Trends and Global Prospects. Cambridge:
Cambridge University Press, pp. 188–206.
Cross-references
Anthropogenic Impacts
Nonpoint Source Pollution
Oil Pollution
Polycyclic Aromatic Hydrocarbons
Trace Metals in Estuaries
HEADLAND BREAKWATERS
C. Scott Hardaway, Jr.
1 and James R. Gunn
2
1
Virginia Institute of Marine Science, College of
William & Mary, Gloucester Point, VA, USA
2
Coastal Design & Construction, Gloucester, VA, USA
Synonyms
Bay beach; Embayment; Pocket beach
Definition
Headland breakwaters are coastal structures placed
roughly parallel to the coast and attached to the land with
a sand spit (i.e., tombolo), thereby forming a headland.
350
HALOGENATED HYDROCARBONS