12.5.2 Evidence from Nutrient Enrichment Bioassays
In situ nutrient enrichment bioassays have shown summer Microcystis blooms to be
largely N-limited in another eutrophic lake, Lake Dianchi (Paerl et al. 2011; Xu et al.
2010). During the summer and fall bloom periods, N additions alone revealed a
significant positive effect on phytoplankton growth, and P additions only stimulated
phytoplankton growth once N had been added, suggesting that N was the primary
limiting nutrient. For Lake Dianchi, soluble reactive phosphorus (SRP) concentrations remained quite high in the water column, while dissolved inorganic N
decreased rapidly during the summer Microcystis blooms in Lake Dianchi,
(Fig. 12.14). N availability therefore controlled biomass production if having excess
SRP in the lake (Paerl et al. 2011; Xu et al. 2010). This temporal linkage implies that
atmospheric N deposition, as a highly significant N source, may support Microcystis
growth during the critical initial proliferation period and for sustaining summer
blooms in Lake Dianchi. It should be noted that this toxic bloom-forming genus is
not a nitrogen (N 2 ) fixer and hence has a strong requirement for externally supplied
N to support growth (Xu et al. 2010; Paerl et al. 2015, 2011). This places even more
weight on timely and quantitatively significant inputs of atmospheric N deposition
during a period of maximum bloom potential and N demand by this genus.
12.5.3 Limitations
At present, the contribution made by atmospheric N deposition to the total N inputs
contains some uncertainties and is influenced by factors related to datasets used in
calculation and scope of sampling. V d of gaseous N over the water was calculated
using deposition velocity model based on local meteorological data, which was not
validated by direct observations. Chemical form of gaseous and particulate N also
depends on meteorological conditions and atmospheric composition (e.g., humidity,
temperature, oxygen radicals). Any change in these factors will result in differences
in atmospheric N deposition over the lake surface.
In addition, runoff of atmospheric N deposition from watersheds (i.e., indirect
deposition) to Lake Dianchi was not measured or estimated in this study. A classical
recession method (Gonzales et al. 2009) was used to separate the baseflow from a
total streamflow, resulting in the portion directly generated from the excess rainfall
during a storm event (defined hereafter as ΔQ). Our results showed that NO 3
À -N,
which is highly soluble in water (Erickson et al. 2016; Jury and Nielsen 1989), was a
dominant N constituent in precipitation (Fig. 12.3c). However, reduced N or organic
N can be further absorbed or decayed in the transport process (Lopez-Bellido et al.
2014). The runoff of atmospheric N deposition from watersheds was then calculated
by multiplying ΔQ to NO 3
À
-N concentration in each of rainfall events. Finally, we
assumed that there were no differences in NO 3
À -N concentrations in precipitation
between the sites around and away from the lake. N deposition flowing into the lake
12 Impacts of Nitrogen Deposition on China’s Lake Ecosystems. . .
287
In situ nutrient enrichment bioassays have shown summer Microcystis blooms to be
largely N-limited in another eutrophic lake, Lake Dianchi (Paerl et al. 2011; Xu et al.
2010). During the summer and fall bloom periods, N additions alone revealed a
significant positive effect on phytoplankton growth, and P additions only stimulated
phytoplankton growth once N had been added, suggesting that N was the primary
limiting nutrient. For Lake Dianchi, soluble reactive phosphorus (SRP) concentrations remained quite high in the water column, while dissolved inorganic N
decreased rapidly during the summer Microcystis blooms in Lake Dianchi,
(Fig. 12.14). N availability therefore controlled biomass production if having excess
SRP in the lake (Paerl et al. 2011; Xu et al. 2010). This temporal linkage implies that
atmospheric N deposition, as a highly significant N source, may support Microcystis
growth during the critical initial proliferation period and for sustaining summer
blooms in Lake Dianchi. It should be noted that this toxic bloom-forming genus is
not a nitrogen (N 2 ) fixer and hence has a strong requirement for externally supplied
N to support growth (Xu et al. 2010; Paerl et al. 2015, 2011). This places even more
weight on timely and quantitatively significant inputs of atmospheric N deposition
during a period of maximum bloom potential and N demand by this genus.
12.5.3 Limitations
At present, the contribution made by atmospheric N deposition to the total N inputs
contains some uncertainties and is influenced by factors related to datasets used in
calculation and scope of sampling. V d of gaseous N over the water was calculated
using deposition velocity model based on local meteorological data, which was not
validated by direct observations. Chemical form of gaseous and particulate N also
depends on meteorological conditions and atmospheric composition (e.g., humidity,
temperature, oxygen radicals). Any change in these factors will result in differences
in atmospheric N deposition over the lake surface.
In addition, runoff of atmospheric N deposition from watersheds (i.e., indirect
deposition) to Lake Dianchi was not measured or estimated in this study. A classical
recession method (Gonzales et al. 2009) was used to separate the baseflow from a
total streamflow, resulting in the portion directly generated from the excess rainfall
during a storm event (defined hereafter as ΔQ). Our results showed that NO 3
À -N,
which is highly soluble in water (Erickson et al. 2016; Jury and Nielsen 1989), was a
dominant N constituent in precipitation (Fig. 12.3c). However, reduced N or organic
N can be further absorbed or decayed in the transport process (Lopez-Bellido et al.
2014). The runoff of atmospheric N deposition from watersheds was then calculated
by multiplying ΔQ to NO 3
À
-N concentration in each of rainfall events. Finally, we
assumed that there were no differences in NO 3
À -N concentrations in precipitation
between the sites around and away from the lake. N deposition flowing into the lake
12 Impacts of Nitrogen Deposition on China’s Lake Ecosystems. . .
287