2.5 Threats to Coral Reefs
33
which promotes the growth of many blue green algae responsible for black band
disease (El-Naggar 2020). Excessive production of mucus is attributed to natural
and anthropogenic influences such as global warming, toxic chemicals, pollution,
eutrophication and increased sedimentation (El-Naggar 2020). Coral bleaching or
heat stress expedites coral diseases.
Endolithic borers such as photosynthetic algae, heterotrophic fungi and bacteria,
sponges, polychaete worms and mollusks cause bioerosion, which is the biological
breakdown of limestone skeleton and reef frameworks. Baker et al. (2008) highlighted
that disturbance such as eutrophication, sedimentation, epizootics and bleaching
cause significant declines in coral cover and result in a loss of reef structures.
Bleaching can be triggered by high sea surface temperatures (SSTs), which are
associated with climate change; solar radiation and El-Nino events (Douglas 2003),
and bacterial infection as well as contamination from chemicals and pesticides
(Souter and Lindén 2000). Bleaching is associated with high irradiance environments
experiencing unusually warm conditions (1.0–1.5 °C above seasonal maximum mean
temperatures) (Baker et al. 2008) and thus this may impair the symbiosis between the
coral and its zooxanthallae, causing its breakdown (Putnam et al. 2017; Weijerman
et al. 2018; Allemand and Osborn 2019).
It is well documented that increased solar energy [both in visible (400–700 nm)
and ultraviolet region (290–400 nm) regions of the electromagnetic spectrum] have
been associated in mass coral bleaching (Hoegh-Guldberg and Smith 1989; Brown
et al. 1994; Fitt and Warner 1995; Shick et al. 1996; Brown 1997a, b; Lesser 1997,
Brown et al. 1999; Dunne and Brown 2001; Coles and Brown 2003; Lesser 2004;
Smith et al. 2005).
With a steady increase of ocean temperatures, many shallow water, tropical, reefbuilding corals live in excess of their upper thermal tolerance limits (Fitt et al. 2001),
which varies geographically. Thermal anomalies repeatedly lead to the breakdown
of the symbiotic relationship of corals with the Symbiodinium spp. Global coral
bleaching events have intensified and are happening at greater frequencies over
the last two decades and therefore coral reef communities have less recovery time,
leading to widespread losses of coral cover and species diversity from tropical coral
reef systems (Hughes et al. 2017, 2018).
Localized bleaching normally occurs and results from factors such as chemical
spills, sedimentation, ship grounding and decrease in oceanic salinity by heavy rains
or even from flood plumes (UNEP 2005). For instance, bleaching can be induced by
cyanide exposure resulting in symbiont expulsion (Jones et al. 1999).
2.5.2 Anthropogenic Causes of Coral Reef Degradation
Coral reefs are exposed to anthropogenic disturbances which can range from local
(e.g. overfishing, coastal development, changes in water quality and pollution) to
global in scope (e.g. climate change, ocean acidification and hypoxia) (Pörtner et al.
2005; Hoegh-Guldberg et al. 2011, 2017; Putnam et al. 2017). Activities such as
33
which promotes the growth of many blue green algae responsible for black band
disease (El-Naggar 2020). Excessive production of mucus is attributed to natural
and anthropogenic influences such as global warming, toxic chemicals, pollution,
eutrophication and increased sedimentation (El-Naggar 2020). Coral bleaching or
heat stress expedites coral diseases.
Endolithic borers such as photosynthetic algae, heterotrophic fungi and bacteria,
sponges, polychaete worms and mollusks cause bioerosion, which is the biological
breakdown of limestone skeleton and reef frameworks. Baker et al. (2008) highlighted
that disturbance such as eutrophication, sedimentation, epizootics and bleaching
cause significant declines in coral cover and result in a loss of reef structures.
Bleaching can be triggered by high sea surface temperatures (SSTs), which are
associated with climate change; solar radiation and El-Nino events (Douglas 2003),
and bacterial infection as well as contamination from chemicals and pesticides
(Souter and Lindén 2000). Bleaching is associated with high irradiance environments
experiencing unusually warm conditions (1.0–1.5 °C above seasonal maximum mean
temperatures) (Baker et al. 2008) and thus this may impair the symbiosis between the
coral and its zooxanthallae, causing its breakdown (Putnam et al. 2017; Weijerman
et al. 2018; Allemand and Osborn 2019).
It is well documented that increased solar energy [both in visible (400–700 nm)
and ultraviolet region (290–400 nm) regions of the electromagnetic spectrum] have
been associated in mass coral bleaching (Hoegh-Guldberg and Smith 1989; Brown
et al. 1994; Fitt and Warner 1995; Shick et al. 1996; Brown 1997a, b; Lesser 1997,
Brown et al. 1999; Dunne and Brown 2001; Coles and Brown 2003; Lesser 2004;
Smith et al. 2005).
With a steady increase of ocean temperatures, many shallow water, tropical, reefbuilding corals live in excess of their upper thermal tolerance limits (Fitt et al. 2001),
which varies geographically. Thermal anomalies repeatedly lead to the breakdown
of the symbiotic relationship of corals with the Symbiodinium spp. Global coral
bleaching events have intensified and are happening at greater frequencies over
the last two decades and therefore coral reef communities have less recovery time,
leading to widespread losses of coral cover and species diversity from tropical coral
reef systems (Hughes et al. 2017, 2018).
Localized bleaching normally occurs and results from factors such as chemical
spills, sedimentation, ship grounding and decrease in oceanic salinity by heavy rains
or even from flood plumes (UNEP 2005). For instance, bleaching can be induced by
cyanide exposure resulting in symbiont expulsion (Jones et al. 1999).
2.5.2 Anthropogenic Causes of Coral Reef Degradation
Coral reefs are exposed to anthropogenic disturbances which can range from local
(e.g. overfishing, coastal development, changes in water quality and pollution) to
global in scope (e.g. climate change, ocean acidification and hypoxia) (Pörtner et al.
2005; Hoegh-Guldberg et al. 2011, 2017; Putnam et al. 2017). Activities such as