Chapter 24 Human Impacts on Seagrasses
575
interfere with N metabolism (Pregnall et al., 1984),
as well as leading to the production of sulfide compounds which are toxic to seagrasses (Delgado et al.,
1999). Sediment sulfides can diffuse through the rhizosphere directly affecting the below-ground root
tissues (Erskine and Koch, 2000). High sulfide levels have also been linked to decreases in the photosynthetic capacity and efficiency of species such
as Zostera marina (Goodman et al., 1995) and these
impacts may compound the effects of other stressors
to seagrass such as reduced light availability, salinity and temperature (Koch and Erskine, 2001). There
has been a direct relationship found between sediment sulfide and mortality in Thalassia testudinum
(Carlson et al., 1994), and this needs to be examined
in other enriched systems. In this model, sediments
enriched in organic production results in increased
sulfide levels that decrease seagrass production, lead
to decreased sediment sulfide re-oxidation and subsequently even higher levels of sulfide, that further
decrease seagrass production.
8. Recovery
Of the relatively few studies where recovery from
eutrophication has been documented, it has been
reported that seagrasses take many years to recover.
Not only are seagrasses notoriously slow to recolonize following a decline, particularly if seedling
recruitment is the only means of re-colonization
(Orth and Moore, 1986; Olesen and Sand-Jensen,
1994; Kirkman, 1997), but there may also be a significant nutrient load stored in the sediments which
is available to be re-mineralized and released into the
water column for years after the source of the nutrients is ameliorated. Several case studies in Australia
and France following recovery after the removal or
upgrade of sewage outfalls found little evidence of
successful recovery of Posidonia spp. (Hillman et al.,
1991; Pergent-Martini and Pergent, 1996; see also
Walker et al., Chapter 22). Similarly, the complete
loss of seagrass adjacent to a sludge outfall in South
Australia described by Neverauskas (1987) was revisited 8 years after the outfall was decommissioned.
This recent work revealed that seagrass cover had increased from 0 to 33% in a 2 ha study area adjacent to
the outfall; however, this was comprised almost entirely of fast-growing Halophila australis, with only
4% cover of the original species (Amphibolis antarctica, Posidonia angustifolia, and Posidonia sinuosa)
(Bryars and Neverauskas, 2001).
In Florida, reductions in point source nitrogen
loads during the 1980s and early 1990s led to a decline in bay-wide nitrogen loads by approximately
60% in Tampa Bay (TBNEP, 1996; Johansson and
Greening, 1999) and 50% in Sarasota Bay (SBNEP,
2000). In response, seagrass coverage in Tampa Bay
increased by 15% between 1982 and 1999, while
coverage in Sarasota Bay increased by 7% between
1988 and 1999 (Tomasko, 2002). Overall, a national
survey of eutrophication in US estuaries, Bricker
et al. (1999) found trends of increasing seagrass coverage in only 16 out of 138 estuaries surveyed: and
noted that seagrass coverage was increasing in 2 for
the North Atlantic coast, 8 for the Middle Atlantic
coast, 1 for the South Atlantic coast, 4 for the Gulf
of Mexico, and 1 for the Pacific coast. In contrast,
seagrass coverage had declined in 49 of the 138 surveyed estuaries (36% of the total).
B. Sediments
1. Sources of Sediments
Sediments can be terrigenous or carbonate in origin,
but it is generally the terrigenous sediments from
human-induced perturbations that cause most of the
problems via river runoff, storm water outfalls and
dredging activities. This may be due, in part, to the
concentration of suspended sediment derived from
terrigenous sources, as well as the effects of the
sediment type on light attenuation. Typically, specific light absorption or light attenuation is greater
with sediment derived from organic-rich terrestrial
sources than with carbonate sediments (Kirk, 1994)
and wavelength specific attenuation increases with
distance up the estuary (Moore et al., 1997). Carbonate sediments tend to be produced in situ from
calcareous animals and epiphytic algae and may have
less impact on seagrasses due to potentially higher
backscatter to absorbance ratios than terrigenous
sediments.
2. Process of Sedimentation and Turbidity
It is often difficult to separate the contributions of
high levels of turbidity and sedimentation to seagrass decline, because sources of particulate matter, such as terrestrial runoff, industrial effluents or
dredging, usually simultaneously increase both turbidity and sedimentation. Once in the system, fine
sediments can be continually re-suspended and re-
575
interfere with N metabolism (Pregnall et al., 1984),
as well as leading to the production of sulfide compounds which are toxic to seagrasses (Delgado et al.,
1999). Sediment sulfides can diffuse through the rhizosphere directly affecting the below-ground root
tissues (Erskine and Koch, 2000). High sulfide levels have also been linked to decreases in the photosynthetic capacity and efficiency of species such
as Zostera marina (Goodman et al., 1995) and these
impacts may compound the effects of other stressors
to seagrass such as reduced light availability, salinity and temperature (Koch and Erskine, 2001). There
has been a direct relationship found between sediment sulfide and mortality in Thalassia testudinum
(Carlson et al., 1994), and this needs to be examined
in other enriched systems. In this model, sediments
enriched in organic production results in increased
sulfide levels that decrease seagrass production, lead
to decreased sediment sulfide re-oxidation and subsequently even higher levels of sulfide, that further
decrease seagrass production.
8. Recovery
Of the relatively few studies where recovery from
eutrophication has been documented, it has been
reported that seagrasses take many years to recover.
Not only are seagrasses notoriously slow to recolonize following a decline, particularly if seedling
recruitment is the only means of re-colonization
(Orth and Moore, 1986; Olesen and Sand-Jensen,
1994; Kirkman, 1997), but there may also be a significant nutrient load stored in the sediments which
is available to be re-mineralized and released into the
water column for years after the source of the nutrients is ameliorated. Several case studies in Australia
and France following recovery after the removal or
upgrade of sewage outfalls found little evidence of
successful recovery of Posidonia spp. (Hillman et al.,
1991; Pergent-Martini and Pergent, 1996; see also
Walker et al., Chapter 22). Similarly, the complete
loss of seagrass adjacent to a sludge outfall in South
Australia described by Neverauskas (1987) was revisited 8 years after the outfall was decommissioned.
This recent work revealed that seagrass cover had increased from 0 to 33% in a 2 ha study area adjacent to
the outfall; however, this was comprised almost entirely of fast-growing Halophila australis, with only
4% cover of the original species (Amphibolis antarctica, Posidonia angustifolia, and Posidonia sinuosa)
(Bryars and Neverauskas, 2001).
In Florida, reductions in point source nitrogen
loads during the 1980s and early 1990s led to a decline in bay-wide nitrogen loads by approximately
60% in Tampa Bay (TBNEP, 1996; Johansson and
Greening, 1999) and 50% in Sarasota Bay (SBNEP,
2000). In response, seagrass coverage in Tampa Bay
increased by 15% between 1982 and 1999, while
coverage in Sarasota Bay increased by 7% between
1988 and 1999 (Tomasko, 2002). Overall, a national
survey of eutrophication in US estuaries, Bricker
et al. (1999) found trends of increasing seagrass coverage in only 16 out of 138 estuaries surveyed: and
noted that seagrass coverage was increasing in 2 for
the North Atlantic coast, 8 for the Middle Atlantic
coast, 1 for the South Atlantic coast, 4 for the Gulf
of Mexico, and 1 for the Pacific coast. In contrast,
seagrass coverage had declined in 49 of the 138 surveyed estuaries (36% of the total).
B. Sediments
1. Sources of Sediments
Sediments can be terrigenous or carbonate in origin,
but it is generally the terrigenous sediments from
human-induced perturbations that cause most of the
problems via river runoff, storm water outfalls and
dredging activities. This may be due, in part, to the
concentration of suspended sediment derived from
terrigenous sources, as well as the effects of the
sediment type on light attenuation. Typically, specific light absorption or light attenuation is greater
with sediment derived from organic-rich terrestrial
sources than with carbonate sediments (Kirk, 1994)
and wavelength specific attenuation increases with
distance up the estuary (Moore et al., 1997). Carbonate sediments tend to be produced in situ from
calcareous animals and epiphytic algae and may have
less impact on seagrasses due to potentially higher
backscatter to absorbance ratios than terrigenous
sediments.
2. Process of Sedimentation and Turbidity
It is often difficult to separate the contributions of
high levels of turbidity and sedimentation to seagrass decline, because sources of particulate matter, such as terrestrial runoff, industrial effluents or
dredging, usually simultaneously increase both turbidity and sedimentation. Once in the system, fine
sediments can be continually re-suspended and re-