Officer, C B., Ryther, J H (1980).
The possible importance of silicon in marine eutrophication.
Mar. Ecol. Prog. Ser 3: 83-91
Earth Sciences Department, Dartmouth College, Hanover, New Hampshire 03755, USA
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
ABSTRACT: Diatom phytoplankton populations are the usual food for zooplankton and filter feeding fishes and contribute in a direct way to the large fishable populations in coastal zones. Flagellates, on the other hand, are frequently poor foods for most grazers and can lead to undesirable eutrophication effects. Arguments are presented that silicon is often the controlling nutrient in altering a diatom to a flagellate community. The alteration is governed by the relative magnitudes of the natural fluxes of the nutrients nitrogen, phosphorus and silicon to the receiving water body and the recycled fluxes of nitrogen and phosphorus from zooplankton grazing and phytoplankton respiration and decomposition. Examples of such alterations are presented for oceanic, estuarine and inland water bodies.
We can delineate several phytoplankton-based ecosystems in the coastal zone which may be altered by human introduction of nutrients and other biostimulatory chemicals into the ocean. Two such systems are of particular importance. One is the ecosystem dominated by diatoms which are the usual food for filter feeding fishes and zooplankton and contribute in a direct way to the large fishable populations in coastal zones. Diatoms grow very rapidly, have short lifetimes, are grazed heavily, and are rarely a nuisance. The other is the nondiatom ecosystem usually dominated by flagellates, including dinoflagellates, chrysophytes, chlorophytes and coccolithophoridae, though it also may contain large proportions of nonmotile green and bluegreen algae, particularly in brackish and estuarine environments. For convenience, the latter will be referred to here as the 'flagellate' ecosystem.
Flagellates persist for longer periods of time, many are known to be poor foods for most grazers, and the motile species are able to concentrate to undesirable concentrations due to their ability to swim and respond to light. Certain dinoflagellate epidemics, for example, are serious pollution events that must be understood to be predicted and controlled.
To our knowledge all excessive marine phytoplankton growths which have led to undesirable eutrophication effects have been related to flagellate blooms. These eutrophication effects can take several forms. One, the excessive growth, which is not grazed, can lead to oxygen deficiencies when the organic particulate matter sinks and subsequently consumes oxygen by respiration and decay. Such anoxic conditions can lead directly to fish and shellfish kills. Two, the toxic dinoflagellates, including some red tides, can adversely effect the marine ecosystem and can poison man through the consumption of shellfish which have filtered out the toxic components. Three, the flagellate blooms can reach proportions which discolor the water and make it unsightly and malodorous, reducing its esthetic and recreational value.
Diatoms require the major nutrients nitrogen, phosphorus and silicon for their photosynthesis; diatoms use silicon in approximately a one-to-one atomic ratio with nitrogen (Redfield et al., 1963). The flagellates associated with coastal eutrophication effects need only nitrogen and phosphorus, together with the trace elements and micronutrients that all autotrophs require.
H. Peterson (personal communication) states that San Francisco Bay does not at present have excessive or undesirable phytoplankton concentrations or conditions that might lead to the development of predominantly nuisance species or to serious dissolved oxygen deficiencies except locally, as in tributary streams along the margins. He cautions, however, that a significant reduction in the amount of available silicate that would accompany large scale diversions of freshwater inflow could alter this situation.
We suggest that the silicon in the bloom area was removed during the spring diatom bloom and that the recycled nitrogen and phosphorus provided the nutrient pool for the summer algal bloom.
Arguments have been presented as to the importance of the nutrient silicon in altering a generally desirable, diatom phytoplankton population to a frequently undesirable, flagellate phytoplankton population and consequent eutrophication effects. If these arguments are accepted, several possible conclusions follow. We mention three. One, rather than considering treatment procedures which remove the nutrients nitrogen and phosphorus from a sewage discharge into a eutrophied region, one might consider the addition, if feasible, of silica in quantity at the discharge site to alter the receiving waters to a diatom population and a consequent fertile and productive region. Two, regions with substantial natural silica inputs can toler ate larger sewage inputs of nitrogen and phosphorus before undesirable eutrophication effects occur. Three, as in Lake Michigan silica measurements are a critical key to the determination of the onset of undesirable eutrophication effects.