Chapter 24 Human Impacts on Seagrasses
571
1990; Powell et al., 1989, 1991), most assessments
have concluded that increased nutrient loading of
estuarine and nearshore marine waters is more of
a detriment than a benefit to seagrass ecosystems
and the fisheries they support. Studies in Denmark
(Borum, 1985), France (Bourcier, 1986), Australia
(Larkum, 1976; Cambridge et al., 1986; Silberstein
et al., 1986), Mexico (Flores-Verdugo et al., 1988;
Reyes and Merino, 1991), and the USA (Kemp et al.,
1983; Orth and Moore, 1984; Lapointe et al., 1994;
Short and Burdick, 1996; Tomasko et al., 1996;
Valiela et al., 1997; Tomasko et al., 2001) are but
a few of the many assessments that have concluded
that anthropogenic increases in nutrient loading of
the water column are inversely related to various indices of seagrass health.
1. Sources of Nutrients
Eutrophication is one of the most widely reported
causes of anthropogenic seagrass decline and continues to pose the greatest threat to seagrass populations (Short and Wyllie-Echeverria, 1996). Increased urbanization worldwide had led to greater
discharges of domestic and industrial waste into
rivers, estuaries and coastal waters. Nitrogen and
phosphorus are the primary nutrients causing eutrophication, which in seawater, most commonly occurs in the form of excess nitrate (NO
−
3 ), ammonium
(NH
+
4 ), and phosphate (PO
3−
4 ). Major anthropogenic
sources of these nutrients include sewage effluent,
septic system seepage, stormwater outfalls, industry
(abattoirs, steel works, fertilizer processing plants),
aquaculture (particularly sea cage-culture and prawn
farms), and agricultural runoff. For example, point
sources (both wastewater treatment plants and industry) account for approx. 10 and 13% of the present
day nitrogen load into Sarasota and Tampa Bays,
with stormwater runoff accounting for 56 and 45%
of the load, respectively (Heyl, 1992; Zarbock et al.,
1996). In contrast, point sources were not a significant source of nitrogen loads into Lemon Bay
(Florida), where stormwater runoff is thought to account for ∼76% of the annual N load, followed by
septic tank systems (14%) (Tomasko et al., 2001).
Strong causal links have been established between
seagrass loss and eutrophication. For example, between 1938 and 1976, nitrogen loads into Tampa Bay
increased from 1,737 kg Total Nitrogen (TN) year
−1
to 8,985 kg TN year
−1 , a more than five-fold increase
(Tampa Bay National Estuary Program [TBNEP],
1996). In adjacent Sarasota Bay, nitrogen loads in
1988 were estimated to be 905 kg TN year
−1 , a value
nearly five-times higher than estimated loads prior
to development of the watershed (Sarasota Bay National Estuary Program [SBNEP] 2000). In response
to both the direct impact from dredging activities, as
well as reduced water clarity due to nutrient-induced
increases in phytoplankton abundance, seagrass coverage in both Tampa Bay and Sarasota Bay decreased
precipitously from early estimates (Tomasko et al.,
1996; Johansson and Greening, 1999, respectively).
2. Process of Nutrient Contamination
and Eutrophication
Seagrass beds exposed to eutrophication have been
reported to exhibit symptoms such as high epiphytic
loading, low shoot densities, low leaf area indices,
and low biomass (Tomasko and Lapointe, 1991;
Delgado et al., 1999; Walker et al., Chapter 23),
although the mechanisms of decline can vary both
spatially and temporally. The cause/effect relationships between nutrient enrichment and seagrass
response are complex and are likely related to both
the rate at which nutrients are made available to seagrass beds, as well as the absolute concentrations.
Factors such as the flushing rate of receiving waters
have a significant role in determining the persistence
of nutrients within a system. Estimates of residence
time are considered a major factor for determining
the susceptibility of an estuary to nutrient enrichment (Bricker et al., 1999). In an hypothetical
example where water volumes and nutrient loads
into an estuary would be similar, the degree of
expression of eutrophic conditions would be less in
the better-flushed estuary, and greater in the estuary
with the longer residence time (Bricker et al., 1999).
Moore and Wetzel (2000) have also suggested that
in some situations, pulsed high levels of nitrogen
into waters with limited turnover may have a greater
impact than long-term moderate concentrations.
Two important processes have been linked to
eutrophication-related decline in seagrasses; shading by phytoplankton and overgrowth by epiphytes,
both result in a reduction in the light reaching the
seagrass. The dominant impact of eutrophication is
mostly due to phytoplankton-induced reductions in
water clarity, which in some instances is further compounded by direct impacts such and dredge and fill
571
1990; Powell et al., 1989, 1991), most assessments
have concluded that increased nutrient loading of
estuarine and nearshore marine waters is more of
a detriment than a benefit to seagrass ecosystems
and the fisheries they support. Studies in Denmark
(Borum, 1985), France (Bourcier, 1986), Australia
(Larkum, 1976; Cambridge et al., 1986; Silberstein
et al., 1986), Mexico (Flores-Verdugo et al., 1988;
Reyes and Merino, 1991), and the USA (Kemp et al.,
1983; Orth and Moore, 1984; Lapointe et al., 1994;
Short and Burdick, 1996; Tomasko et al., 1996;
Valiela et al., 1997; Tomasko et al., 2001) are but
a few of the many assessments that have concluded
that anthropogenic increases in nutrient loading of
the water column are inversely related to various indices of seagrass health.
1. Sources of Nutrients
Eutrophication is one of the most widely reported
causes of anthropogenic seagrass decline and continues to pose the greatest threat to seagrass populations (Short and Wyllie-Echeverria, 1996). Increased urbanization worldwide had led to greater
discharges of domestic and industrial waste into
rivers, estuaries and coastal waters. Nitrogen and
phosphorus are the primary nutrients causing eutrophication, which in seawater, most commonly occurs in the form of excess nitrate (NO
−
3 ), ammonium
(NH
+
4 ), and phosphate (PO
3−
4 ). Major anthropogenic
sources of these nutrients include sewage effluent,
septic system seepage, stormwater outfalls, industry
(abattoirs, steel works, fertilizer processing plants),
aquaculture (particularly sea cage-culture and prawn
farms), and agricultural runoff. For example, point
sources (both wastewater treatment plants and industry) account for approx. 10 and 13% of the present
day nitrogen load into Sarasota and Tampa Bays,
with stormwater runoff accounting for 56 and 45%
of the load, respectively (Heyl, 1992; Zarbock et al.,
1996). In contrast, point sources were not a significant source of nitrogen loads into Lemon Bay
(Florida), where stormwater runoff is thought to account for ∼76% of the annual N load, followed by
septic tank systems (14%) (Tomasko et al., 2001).
Strong causal links have been established between
seagrass loss and eutrophication. For example, between 1938 and 1976, nitrogen loads into Tampa Bay
increased from 1,737 kg Total Nitrogen (TN) year
−1
to 8,985 kg TN year
−1 , a more than five-fold increase
(Tampa Bay National Estuary Program [TBNEP],
1996). In adjacent Sarasota Bay, nitrogen loads in
1988 were estimated to be 905 kg TN year
−1 , a value
nearly five-times higher than estimated loads prior
to development of the watershed (Sarasota Bay National Estuary Program [SBNEP] 2000). In response
to both the direct impact from dredging activities, as
well as reduced water clarity due to nutrient-induced
increases in phytoplankton abundance, seagrass coverage in both Tampa Bay and Sarasota Bay decreased
precipitously from early estimates (Tomasko et al.,
1996; Johansson and Greening, 1999, respectively).
2. Process of Nutrient Contamination
and Eutrophication
Seagrass beds exposed to eutrophication have been
reported to exhibit symptoms such as high epiphytic
loading, low shoot densities, low leaf area indices,
and low biomass (Tomasko and Lapointe, 1991;
Delgado et al., 1999; Walker et al., Chapter 23),
although the mechanisms of decline can vary both
spatially and temporally. The cause/effect relationships between nutrient enrichment and seagrass
response are complex and are likely related to both
the rate at which nutrients are made available to seagrass beds, as well as the absolute concentrations.
Factors such as the flushing rate of receiving waters
have a significant role in determining the persistence
of nutrients within a system. Estimates of residence
time are considered a major factor for determining
the susceptibility of an estuary to nutrient enrichment (Bricker et al., 1999). In an hypothetical
example where water volumes and nutrient loads
into an estuary would be similar, the degree of
expression of eutrophic conditions would be less in
the better-flushed estuary, and greater in the estuary
with the longer residence time (Bricker et al., 1999).
Moore and Wetzel (2000) have also suggested that
in some situations, pulsed high levels of nitrogen
into waters with limited turnover may have a greater
impact than long-term moderate concentrations.
Two important processes have been linked to
eutrophication-related decline in seagrasses; shading by phytoplankton and overgrowth by epiphytes,
both result in a reduction in the light reaching the
seagrass. The dominant impact of eutrophication is
mostly due to phytoplankton-induced reductions in
water clarity, which in some instances is further compounded by direct impacts such and dredge and fill