Chapter 24 Human Impacts on Seagrasses
585
Quantification of atmospheric loads of nutrients
into estuarine and/or nearshore marine waters require, at a minimum, information on the quantity
and quality of rainfall. By knowing the area of open
water, and the amount of precipitation that falls upon
the water, the quantity of water deposited onto the
open waters of the ecosystem can be determined. If
the concentration of nutrients in rainfall is known,
estimates can be made of the amount of nutrients
delivered to the open waters of the system via direct precipitation (e.g. Tomasko et al., 2001). However, the above-described technique does not take
into account the impact of ‘dry’ deposition of pollutant loads (i.e. atmospheric deposition fluxes not
associated with precipitation).
Alexander et al. (2001) presented results of an empirical modeling effort that estimates both the direct
and indirect (i.e. via stormwater runoff) impacts of
atmospheric deposition of total nitrogen to estuarine
and nearshore marine waters of the USA. When direct atmospheric deposition onto open waters is excluded, indirect impacts of atmospheric deposition
of total nitrogen accounts for between 4 and 35% of
the nitrogen load to 42 US estuaries studied, with a
mean value of 20% (Alexander et al., 2001).
The implications of estimating the ‘indirect’ impact of atmospheric deposition vary between the
estuaries examined. For example, direct wet and
dry deposition of nitrogen onto the open waters
of Narragansett Bay (Rhode Island, USA) was estimated to contribute 4.6% of the total nitrogen
load to this system (Nixon et al., 1995). However,
Alexander et al. (2001) estimate that 10% of the
non-direct atmospheric load (i.e. stormwater runoff,
point sources, baseflow, etc.) is due to indirect
impacts from wet and dry deposition onto the watershed. Combined, the total impact of both direct and indirect deposition of atmospheric nitrogen
would equal approximately 14.1% of the total nitrogen load (i.e. 4.6% + 0.10 × 95.4%). Additionally, Alexander et al. (2001) estimated that 19% of
non-direct atmospheric loads to Barnegat Bay were
atmospherically derived, so that 53.3% of the total
load would be derived either directly or indirectly
from atmospheric deposition (i.e. 42.3% + 0.19 ×
57.7%).
In general, estuaries with relatively small watersheds, as compared to the amount of open water of
the estuary, are more strongly influenced by direct atmospheric deposition of pollutants such as nitrogen.
However, the proportion of non-point source loads
(i.e. stormwater runoff) due to atmospheric deposition of pollutants onto the watershed varies as a
function of land use, air quality, and stream characteristics (Alexander et al., 2001). Estuaries in the
Northeastern and Mid-Atlantic portions of the US
had the highest percentage of total nitrogen loads
attributed to indirect impacts of atmospheric deposition onto the watershed (Alexander et al., 2001).
B. Pollutant Loads and Pollutant Yields
Nitrogen loads to estuarine and/or nearshore marine waters, minus the contribution from direct atmospheric deposition onto the open water, can be
used to compare the relative pollutant loads of various portions of the contributing watershed. Nitrogen
yields for Southwest Florida (USA) estuaries vary
from a low value of 2.6 kg Total Nitrogen (TN) ha
−1
year
−1 in Charlotte Harbor (Squires et al., 1998)
to 10.9 kg TN ha
−1 year
−1 in Sarasota Bay (Heyl,
1992). For estuaries in Southwest Florida, the degree of ‘urbanization’ of an estuary’s watershed accounts for the majority of the variation in nitrogen
yields (Tomasko, 2000). Similarly, Howarth et al.
(1996) found nitrogen yields from both large rivers
and small coastal watersheds discharging into the Atlantic Ocean were positively correlated with human
population density. The amount of variation found in
this correlation can be further reduced by examining
land use patterns within various watersheds, as per
capita total nitrogen loads are positively correlated
with the percent of the watershed used as harvestable
cropland (Turner et al., 2001).
Based on the above-described relationship between human population density and watershedlevel nitrogen yields, it is possible to estimate a ‘natural’ watershed-level nitrogen yield, by deriving the
yield associated with an undisturbed watershed (i.e.
the calculated yield when human population equals
zero). Turner et al. (2001), using a data set derived
from 27 US estuaries, estimated that a natural nitrogen yield would be 1.08 kg TN ha
−1 year
−1 . If the
percent of the watershed that is urbanized is plotted against watershed nitrogen yields for estuaries
in Southwest Florida (e.g. Tomasko, 2000), the calculated natural watershed nitrogen yield comes out
to 1.31 kg TN ha
−1 year
−1 . Estimates of natural watershed nitrogen yields derived by Howarth et al.
(1996) and Nixon (1997) ranged between 0.76 and
1.10 kg TN ha
−1 year
−1 . While such estimates must
be viewed with an appropriate degree of caution, they
585
Quantification of atmospheric loads of nutrients
into estuarine and/or nearshore marine waters require, at a minimum, information on the quantity
and quality of rainfall. By knowing the area of open
water, and the amount of precipitation that falls upon
the water, the quantity of water deposited onto the
open waters of the ecosystem can be determined. If
the concentration of nutrients in rainfall is known,
estimates can be made of the amount of nutrients
delivered to the open waters of the system via direct precipitation (e.g. Tomasko et al., 2001). However, the above-described technique does not take
into account the impact of ‘dry’ deposition of pollutant loads (i.e. atmospheric deposition fluxes not
associated with precipitation).
Alexander et al. (2001) presented results of an empirical modeling effort that estimates both the direct
and indirect (i.e. via stormwater runoff) impacts of
atmospheric deposition of total nitrogen to estuarine
and nearshore marine waters of the USA. When direct atmospheric deposition onto open waters is excluded, indirect impacts of atmospheric deposition
of total nitrogen accounts for between 4 and 35% of
the nitrogen load to 42 US estuaries studied, with a
mean value of 20% (Alexander et al., 2001).
The implications of estimating the ‘indirect’ impact of atmospheric deposition vary between the
estuaries examined. For example, direct wet and
dry deposition of nitrogen onto the open waters
of Narragansett Bay (Rhode Island, USA) was estimated to contribute 4.6% of the total nitrogen
load to this system (Nixon et al., 1995). However,
Alexander et al. (2001) estimate that 10% of the
non-direct atmospheric load (i.e. stormwater runoff,
point sources, baseflow, etc.) is due to indirect
impacts from wet and dry deposition onto the watershed. Combined, the total impact of both direct and indirect deposition of atmospheric nitrogen
would equal approximately 14.1% of the total nitrogen load (i.e. 4.6% + 0.10 × 95.4%). Additionally, Alexander et al. (2001) estimated that 19% of
non-direct atmospheric loads to Barnegat Bay were
atmospherically derived, so that 53.3% of the total
load would be derived either directly or indirectly
from atmospheric deposition (i.e. 42.3% + 0.19 ×
57.7%).
In general, estuaries with relatively small watersheds, as compared to the amount of open water of
the estuary, are more strongly influenced by direct atmospheric deposition of pollutants such as nitrogen.
However, the proportion of non-point source loads
(i.e. stormwater runoff) due to atmospheric deposition of pollutants onto the watershed varies as a
function of land use, air quality, and stream characteristics (Alexander et al., 2001). Estuaries in the
Northeastern and Mid-Atlantic portions of the US
had the highest percentage of total nitrogen loads
attributed to indirect impacts of atmospheric deposition onto the watershed (Alexander et al., 2001).
B. Pollutant Loads and Pollutant Yields
Nitrogen loads to estuarine and/or nearshore marine waters, minus the contribution from direct atmospheric deposition onto the open water, can be
used to compare the relative pollutant loads of various portions of the contributing watershed. Nitrogen
yields for Southwest Florida (USA) estuaries vary
from a low value of 2.6 kg Total Nitrogen (TN) ha
−1
year
−1 in Charlotte Harbor (Squires et al., 1998)
to 10.9 kg TN ha
−1 year
−1 in Sarasota Bay (Heyl,
1992). For estuaries in Southwest Florida, the degree of ‘urbanization’ of an estuary’s watershed accounts for the majority of the variation in nitrogen
yields (Tomasko, 2000). Similarly, Howarth et al.
(1996) found nitrogen yields from both large rivers
and small coastal watersheds discharging into the Atlantic Ocean were positively correlated with human
population density. The amount of variation found in
this correlation can be further reduced by examining
land use patterns within various watersheds, as per
capita total nitrogen loads are positively correlated
with the percent of the watershed used as harvestable
cropland (Turner et al., 2001).
Based on the above-described relationship between human population density and watershedlevel nitrogen yields, it is possible to estimate a ‘natural’ watershed-level nitrogen yield, by deriving the
yield associated with an undisturbed watershed (i.e.
the calculated yield when human population equals
zero). Turner et al. (2001), using a data set derived
from 27 US estuaries, estimated that a natural nitrogen yield would be 1.08 kg TN ha
−1 year
−1 . If the
percent of the watershed that is urbanized is plotted against watershed nitrogen yields for estuaries
in Southwest Florida (e.g. Tomasko, 2000), the calculated natural watershed nitrogen yield comes out
to 1.31 kg TN ha
−1 year
−1 . Estimates of natural watershed nitrogen yields derived by Howarth et al.
(1996) and Nixon (1997) ranged between 0.76 and
1.10 kg TN ha
−1 year
−1 . While such estimates must
be viewed with an appropriate degree of caution, they