31
3 Effects of Eutrophication
3.2 Effects on the Ecosystem
The impact of eutrophication could appear on variable aspects and characteristics of the ecosystem including biotic
and abiotic components. Coastal marine communities are
being increasingly affected by disturbance events (Thompson
et al. 2002) such as the fast-growing phytoplankton, microphytobenthos, ephemeral macroalgae (Borum 1996), and
increase of suspended particles in the sea water which are
trapped on the sea bottom and caused loss of submersed
macrophyte beds (Rybicki et al. 1997). The suboxic conditions may enhance intensive growth of filamentous bacteria
on the sea bed which oxidizes sulfide, providing a detoxified microhabitat for eukaryotic benthic communities (Levin
et al. 2009), while the intensive algal growth resulting from
nutrient enrichment causes replacement of hermatypic corals
with coralline algae, filamentous algae, macroalgae, and a
variety of filter feeders (Foden et al. 2011).
In Chesapeake Bay, bottom-water hypoxia resulted in
diminished submerged aquatic vegetation and fisheries harvests (Flemer et al. 1983) and a steady decline of abundant
native oysters, Crassostrea virginica (Kirby and Miller
2005). In Skagerrak (Sweden) and Kattegat (Denmark) of
the North Sea, increasing eutrophication during the last
15–20 years accompanied by oxygen decrease (Andersson
and Rydberg 1993) reduced the growth depth of macroalgae
and caused an increase in biomass and change in the species composition in benthic communities (Anon 1993). The
disastrous bloom of the prymnesiophyte Chrysochromulina
polylepis occurred in the Skagerrak-Kattegat area during
spring 1988 (Ambio 1990) and caused fundamental ecological changes (Graneli and Risinger 1995). Meanwhile, in the
inshore regions of the Great Barrier Reef, acute eutrophication increased environmental risk probabilities on the ecosystem health (Brodie 1997).
Accelerated eutrophication poses a serious threat to the
long-term health and function of many coastal bays in the
mid-Atlantic, impacting essential habitats (e.g., seagrass
and shellfish beds) as well as finfish nursery areas (Kennish
2007). Also, high levels of algal growth were reported in
these shallow bays, accelerating seagrass loss, increasing
system respiration and oxygen stress, and altering biogeochemical processes (e.g., sediment anoxia, accumulation of
deleterious hydrogen sulfide, and nutrient cycling) that are
detrimental to ecosystem structure and function (Kennish
2009), in addition to nuisance and toxic algal blooms
(HABs), loss of submerged aquatic vegetation, altered benthic faunal communities, and impacted essential habitats and
harvestable fisheries (Valiela 2006).
High production of CO 2 associated with the decomposition-intensive organic matter produced in eutrophic areas
enhances water acidification. In the northern Gulf of Mexico
and the East China Sea, CO 2 produced during the decomposition (microbial respiration) of lush organic matter resulting
from eutrophicated river plumes, has already enhanced the
acidification of coastal subsurface waters (Minhazul Islam
2011). In the meantime, extensive macroalgal blooms and
severe brown tide ( Aureococcus anophagefferens) caused
low water clarity and resulted in wide seagrass dieoffs, and
severe infestations of noxious jellyfish in the Barnegat BayLittle Egg Harbor Estuary (Kennish 2009).
3.3 Effect on Phytoplankton Community
Although it is well documented that anthropogenic nitrogen
and phosphorus flux drive the proliferation of extensive phytoplankton blooms everywhere in the aquatic habitat (Brodie
and Mitchell 2006; Dorgham 2011), they also cause change
in the speciation of phytoplankton (Crosbie and Furnas
2001; Ismael and Dorgham 2003). Shifts of phytoplankton
species to larger forms of diatoms and dinoflagellates with a
contemporary increase of dissolved inorganic compounds of
both nitrogen and phosphorus (Furnas et al. 2005) were most
obviously documented in several tropical locations, such as
Singapore (Gin et al. 2000), Japan (Tada et al. 2003), Curacao (Van Duyl et al. 2002), New Caledonia (Jacquet et al.
2006), Hawaii (Cox et al. 2006), and Moorea (Delesalle et al.
1993). In the Great Barrier Reef lagoon near Low Isles, eutrophication has occurred over the past 65 years, and caused
a change from a system characterized by both centric and
pennate diatoms to one that is now dominated by pennate
diatoms and small flagellates (Bell and Elmetri 1995).
The variable nutrient requirement of different phytoplankton groups is one of the crucial factors that impact the
community structure and the ratios between the different
nutrients (N:P, Si:N and Si:P) and lead to a shift in phytoplankton community. The suitable ratio of silicon to nitrogen
(Si:N) for diatoms growth is about 1 (Redfield et al. 1963;
Dortch and Whitledge 1992); change of this ratio causes a
pronounced drop in the growth rate of diatoms, and results in
consequent change in the predominance of other algal groups
that usually require small amounts of silicon (Conley et al.
1993). In the German Bight, the decline in the Si:N ratio to
< 0.1 during the period 1962–1984 caused the decrease of the
abundance of diatoms and increase of flagellate/dinoflagellates (Radach et al. 1990), with the pronounced abundance
of some toxic species or harmful blooms such as the flagellate Phaeocystis sp. which extended its seasonal duration
blooms in the Wadden Sea (Richardson 1997). Numerical
displacement was reported of diatoms with other different
phytoplankton groups (Paerl 1997), particularly in diverse
regions of the North Sea, which experienced decreasing Si:N
and Si:P ratios (Riegman et al. 1992). In contrast, decreased
Si:N with increasing nutrient fluxes of nitrate led to an increase in abundance of marine planktonic diatom Pseudo-
3 Effects of Eutrophication
3.2 Effects on the Ecosystem
The impact of eutrophication could appear on variable aspects and characteristics of the ecosystem including biotic
and abiotic components. Coastal marine communities are
being increasingly affected by disturbance events (Thompson
et al. 2002) such as the fast-growing phytoplankton, microphytobenthos, ephemeral macroalgae (Borum 1996), and
increase of suspended particles in the sea water which are
trapped on the sea bottom and caused loss of submersed
macrophyte beds (Rybicki et al. 1997). The suboxic conditions may enhance intensive growth of filamentous bacteria
on the sea bed which oxidizes sulfide, providing a detoxified microhabitat for eukaryotic benthic communities (Levin
et al. 2009), while the intensive algal growth resulting from
nutrient enrichment causes replacement of hermatypic corals
with coralline algae, filamentous algae, macroalgae, and a
variety of filter feeders (Foden et al. 2011).
In Chesapeake Bay, bottom-water hypoxia resulted in
diminished submerged aquatic vegetation and fisheries harvests (Flemer et al. 1983) and a steady decline of abundant
native oysters, Crassostrea virginica (Kirby and Miller
2005). In Skagerrak (Sweden) and Kattegat (Denmark) of
the North Sea, increasing eutrophication during the last
15–20 years accompanied by oxygen decrease (Andersson
and Rydberg 1993) reduced the growth depth of macroalgae
and caused an increase in biomass and change in the species composition in benthic communities (Anon 1993). The
disastrous bloom of the prymnesiophyte Chrysochromulina
polylepis occurred in the Skagerrak-Kattegat area during
spring 1988 (Ambio 1990) and caused fundamental ecological changes (Graneli and Risinger 1995). Meanwhile, in the
inshore regions of the Great Barrier Reef, acute eutrophication increased environmental risk probabilities on the ecosystem health (Brodie 1997).
Accelerated eutrophication poses a serious threat to the
long-term health and function of many coastal bays in the
mid-Atlantic, impacting essential habitats (e.g., seagrass
and shellfish beds) as well as finfish nursery areas (Kennish
2007). Also, high levels of algal growth were reported in
these shallow bays, accelerating seagrass loss, increasing
system respiration and oxygen stress, and altering biogeochemical processes (e.g., sediment anoxia, accumulation of
deleterious hydrogen sulfide, and nutrient cycling) that are
detrimental to ecosystem structure and function (Kennish
2009), in addition to nuisance and toxic algal blooms
(HABs), loss of submerged aquatic vegetation, altered benthic faunal communities, and impacted essential habitats and
harvestable fisheries (Valiela 2006).
High production of CO 2 associated with the decomposition-intensive organic matter produced in eutrophic areas
enhances water acidification. In the northern Gulf of Mexico
and the East China Sea, CO 2 produced during the decomposition (microbial respiration) of lush organic matter resulting
from eutrophicated river plumes, has already enhanced the
acidification of coastal subsurface waters (Minhazul Islam
2011). In the meantime, extensive macroalgal blooms and
severe brown tide ( Aureococcus anophagefferens) caused
low water clarity and resulted in wide seagrass dieoffs, and
severe infestations of noxious jellyfish in the Barnegat BayLittle Egg Harbor Estuary (Kennish 2009).
3.3 Effect on Phytoplankton Community
Although it is well documented that anthropogenic nitrogen
and phosphorus flux drive the proliferation of extensive phytoplankton blooms everywhere in the aquatic habitat (Brodie
and Mitchell 2006; Dorgham 2011), they also cause change
in the speciation of phytoplankton (Crosbie and Furnas
2001; Ismael and Dorgham 2003). Shifts of phytoplankton
species to larger forms of diatoms and dinoflagellates with a
contemporary increase of dissolved inorganic compounds of
both nitrogen and phosphorus (Furnas et al. 2005) were most
obviously documented in several tropical locations, such as
Singapore (Gin et al. 2000), Japan (Tada et al. 2003), Curacao (Van Duyl et al. 2002), New Caledonia (Jacquet et al.
2006), Hawaii (Cox et al. 2006), and Moorea (Delesalle et al.
1993). In the Great Barrier Reef lagoon near Low Isles, eutrophication has occurred over the past 65 years, and caused
a change from a system characterized by both centric and
pennate diatoms to one that is now dominated by pennate
diatoms and small flagellates (Bell and Elmetri 1995).
The variable nutrient requirement of different phytoplankton groups is one of the crucial factors that impact the
community structure and the ratios between the different
nutrients (N:P, Si:N and Si:P) and lead to a shift in phytoplankton community. The suitable ratio of silicon to nitrogen
(Si:N) for diatoms growth is about 1 (Redfield et al. 1963;
Dortch and Whitledge 1992); change of this ratio causes a
pronounced drop in the growth rate of diatoms, and results in
consequent change in the predominance of other algal groups
that usually require small amounts of silicon (Conley et al.
1993). In the German Bight, the decline in the Si:N ratio to
< 0.1 during the period 1962–1984 caused the decrease of the
abundance of diatoms and increase of flagellate/dinoflagellates (Radach et al. 1990), with the pronounced abundance
of some toxic species or harmful blooms such as the flagellate Phaeocystis sp. which extended its seasonal duration
blooms in the Wadden Sea (Richardson 1997). Numerical
displacement was reported of diatoms with other different
phytoplankton groups (Paerl 1997), particularly in diverse
regions of the North Sea, which experienced decreasing Si:N
and Si:P ratios (Riegman et al. 1992). In contrast, decreased
Si:N with increasing nutrient fluxes of nitrate led to an increase in abundance of marine planktonic diatom Pseudo-