Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 93

are less than 16:1 have been used to indicate that nitrogen is less abundant than
phosphorus with respect to algal (usually phytoplankton) metabolic demand (Red-
field Ratio). Boynton et al. 42 report that 22 of 27 estuaries surveyed were nitrogen
limited with N:P ratios in the water column well below 16:1 during the time of peak
algal growth. In the systems having low N:P ratios, blue-green algae may have a
competitive advantage over other phytoplankton groups due to their ability to fix
atmospheric nitrogen. 43
There are, however, some problems with using the N:P ratio as an indicator of
nutrient limitation. First, different types of algae have different N:P ratios ranging
from 10:1 to 30:1. 41,42 Second, nutrient limitation is often assumed without testing,
and other factors may be limiting. Third, the use of the water column N:P ratio is
based on the assumption that nutrient loading is constant or at steady state; however,
nutrients are often supplied in pulses and the N:P ratio is constantly altered, depend-
ing on both the pulse and uptake rates. Thus, nutrient ratios in the water column are
insufficient for determining the limiting nutrient for algal growth, especially when
more than one algal group is present. 44 Fourth, the limiting nutrient can change
temporally in a system. For example, during winter, when algal crops are sparse and
growth is slow, the amount of phosphorus present may be sufficient to be nonlimiting.
As the growth of algae proceeds during spring, phosphorus is removed from the
water by the algae and the supply becomes progressively depleted. 41 Fifth, in the
case of phosphorus, the past history of cells must be considered. Many algae seem
to be able to store phosphorus in excess of their present requirements. For this reason,
sometimes algae may grow at dissolved phosphate concentrations that seem to be
limiting. The problem is complicated further by the fact that phytoplanktons are
free-moving plants, and thus, they may not have grown in and derived their nutrients
from the water in which they were found (spatio-temporal organization). The time
dimension is another problem, as discussed in Chapter 2. Nutrients are recycled
rapidly through mineralization, and without additional input of phosphorus from
external sources the recycled nutrients become available for growth. 41

4.1.3 S ILICON C YCLE

Although considered a minor nutrient, silicon is significant in the dynamics of
phytoplankton because of its importance as a major structural element in the cells
of diatoms, an important phytoplankton group in coastal waters. 7 Because silicon is
used in large quantities for diatom cell walls, it can be a limiting element for
phytoplankton growth where diatoms are the predominant algae. 23,45 Silicon limita-
tion impacts the diatoms and silicoflagellates among the phytoplankton, and the
radiolarians among the zooplankton. 25
Silicon is present in coastal waters in three principal forms: detrital quartz,
aluminosilicate clays, and dissolved silicon. Similar to phosphorus, silicon occurs
primarily in one oxidation state (

4). 27 At the pH and ionic strength of seawater, the
dominant dissolved species of silicon is silicic acid (H 4 SiO 4 ). 12
The dominant input of dissolved silicate to most aquatic systems occurs as
riverine inputs, as a consequence of weathering reactions in the watershed. In order
to become available for biological activity, silicate rocks must be broken down. 46-48

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