100
Large marine and estuarine areas frequently have nitrogen as the limiting nutrient, especially in summer. Intermediate areas such as river plumes are often
phosphorus- limited during spring, but may turn to silica or nitrogen limitation in
summer. When phosphorus is the limiting factor, a phosphate concentration of
0.01 mg l
−1
is enough to support plankton and concentrations from 0.03 to 0.1 mg l
−1
or higher will be likely to promote blooms.
In coastal areas, the growth and proliferation of diatoms is promoted by the presence of silica. When the silica concentration is low diatoms cannot develop. Then
other opportunistic toxic algal species, which are no longer submitted to competition, can grow and form blooms. Species from the genus Phaeocystis and several
dinoflagellates (Prorocentrum, Dinophysis, Gymnodynium) are known to proliferate under such conditions.
In this study, the N: P ratios in all the selected stations and seasons are greater
than 10 (except monsoon season in Namkhana) (Table 4.2). This implies that the
aquatic phase of the present study area is P limiting (WHO 2003). A high N: P ratio
normally increases the standing stock of dinoflagellates and diatoms.
Significant variations in the level of dissolved nitrate, phosphate and silicate
between years and between stations were observed (p < 0.01) which reveal the
impacts of season and anthropogenic pressure in the present study area.
However, mangrove flora can absorb nutrients from ambient media preferably N
and P as has been documented here (Tables 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10,
4.11, and 4.12). In nutrient-limited mangrove wetlands where most of the leaf litter
is exported by tidal flushing, internal nutrient levels in mangrove floral species
speaks in favour of the mangrove ecosystem as potential retention box of nutrients,
which otherwise may results in adverse impacts like eutrophication, low dissolved
oxygen level etc. if leaches to adjacent waterbodies due to tidal actions.
The present chapter has great relevance in the mangrove panorama as very little
is known about how nutrient availability affects plant-mediated nutrient-cycling
processes in coastal wetlands that are N (Valiela and Teal 1979) or phosphorus (P)
limited (Davis 1994; Feller 1995).
In low-nutrient environments, some plant species are more competitive because
they possess mechanisms for nutrient conservation that acquire, use, retain, and
recycle nutrients efficiently (Berendse et al. 1987; Aerts and Berendse 1988; Aerts
1990). Adaptations for nutrient conservation in low-fertility soils include evergreenness, sclerophylly, low photosynthetic capacity, long life-span for plant tissue, low
levels of leaching from plant tissue, high investment in chemical and structural
defense, low losses to herbivory, slow growth rates, high resorption efficiency of
nutrients from senescing tissues, high nutrient-use efficiency, high nutrient- uptake
efficiency, high root: shoot ratio, symbiotic mycorrhizal relationships, and low
decomposability of litter (Loveless 1961; Beadle 1967; Janzen 1974; Grubb 1977;
Bryant et al. 1983; Shaver and Melillo 1984; Medina and Cuevas 1989; Medina
et al. 1990; Hobbie 1992; Aerts 1995).
Mangrove species that flourish in low-nutrient environments have very efficient mechanisms for retaining and recycling nutrients (Boto 1982; Twilley et al.
1986). According to Alongi et al. (1992), biological and environmental factors
4 Mangroves: A Nutrient Retention Box
Large marine and estuarine areas frequently have nitrogen as the limiting nutrient, especially in summer. Intermediate areas such as river plumes are often
phosphorus- limited during spring, but may turn to silica or nitrogen limitation in
summer. When phosphorus is the limiting factor, a phosphate concentration of
0.01 mg l
−1
is enough to support plankton and concentrations from 0.03 to 0.1 mg l
−1
or higher will be likely to promote blooms.
In coastal areas, the growth and proliferation of diatoms is promoted by the presence of silica. When the silica concentration is low diatoms cannot develop. Then
other opportunistic toxic algal species, which are no longer submitted to competition, can grow and form blooms. Species from the genus Phaeocystis and several
dinoflagellates (Prorocentrum, Dinophysis, Gymnodynium) are known to proliferate under such conditions.
In this study, the N: P ratios in all the selected stations and seasons are greater
than 10 (except monsoon season in Namkhana) (Table 4.2). This implies that the
aquatic phase of the present study area is P limiting (WHO 2003). A high N: P ratio
normally increases the standing stock of dinoflagellates and diatoms.
Significant variations in the level of dissolved nitrate, phosphate and silicate
between years and between stations were observed (p < 0.01) which reveal the
impacts of season and anthropogenic pressure in the present study area.
However, mangrove flora can absorb nutrients from ambient media preferably N
and P as has been documented here (Tables 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10,
4.11, and 4.12). In nutrient-limited mangrove wetlands where most of the leaf litter
is exported by tidal flushing, internal nutrient levels in mangrove floral species
speaks in favour of the mangrove ecosystem as potential retention box of nutrients,
which otherwise may results in adverse impacts like eutrophication, low dissolved
oxygen level etc. if leaches to adjacent waterbodies due to tidal actions.
The present chapter has great relevance in the mangrove panorama as very little
is known about how nutrient availability affects plant-mediated nutrient-cycling
processes in coastal wetlands that are N (Valiela and Teal 1979) or phosphorus (P)
limited (Davis 1994; Feller 1995).
In low-nutrient environments, some plant species are more competitive because
they possess mechanisms for nutrient conservation that acquire, use, retain, and
recycle nutrients efficiently (Berendse et al. 1987; Aerts and Berendse 1988; Aerts
1990). Adaptations for nutrient conservation in low-fertility soils include evergreenness, sclerophylly, low photosynthetic capacity, long life-span for plant tissue, low
levels of leaching from plant tissue, high investment in chemical and structural
defense, low losses to herbivory, slow growth rates, high resorption efficiency of
nutrients from senescing tissues, high nutrient-use efficiency, high nutrient- uptake
efficiency, high root: shoot ratio, symbiotic mycorrhizal relationships, and low
decomposability of litter (Loveless 1961; Beadle 1967; Janzen 1974; Grubb 1977;
Bryant et al. 1983; Shaver and Melillo 1984; Medina and Cuevas 1989; Medina
et al. 1990; Hobbie 1992; Aerts 1995).
Mangrove species that flourish in low-nutrient environments have very efficient mechanisms for retaining and recycling nutrients (Boto 1982; Twilley et al.
1986). According to Alongi et al. (1992), biological and environmental factors
4 Mangroves: A Nutrient Retention Box