The Possible Importance of Silicon in Marine Eutrophication
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Vol. 3: 83-91. 1980 MARITUE ECOLOGY - PROGRESS SERIES Published July 31 Mar. Ecol. hog. Ser. REVIEW The Possible Importance of Silicon in Marine Eutrophication C. B. Officer1 and J. H. Ryther2 ' 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. THESIS To our knowledge all excessive marine phytoplank- ton growths which have led to undesirable eutrophica- We can delineate several phytoplankton-based tion effects have been related to flagellate blooms. ecosystems in the coastal zone which may be altered These eutrophication effects can take several forms. by human introduction of nutrients and other bio- One, the excessive growth, which is not grazed, can stimulatory chemicals into the ocean. Two such sys- lead to oxygen deficiencies when the organic particu- tems are of particular importance. One is the ecosys- late matter sinks and subsequently consumes oxygen tem dominated by diatoms which are the usual food for by respiration and decay. Such anoxic conditions can filter feeding fishes and zooplankton and contribute in lead directly to fish and shellfish kills. Two, the a direct way to the large fishable populations in coastal toxic dinoflagellates, including some red tides, can zones. Diatoms grow very rapidly, have short lifetimes, adversely effect the marine ecosystem and can poison are grazed heavily, and are rarely a nuisance. The man through the consumption of shellfish which have other is the nondiatom ecosystem usually dominated filtered out the toxic components. Three, the flagellate by flagellates, including dinoflagellates, chrysophytes, blooms can reach proportions which discolor the water chlorophytes and coccolithophoridae, though it also and make it unsightly and malodorous, reducing its may contain large proportions of nonmotile green esthetic and recreational value. and bluegreen algae, particularly in brackish and In general, diatoms are able to grow at least as estuarine environments. For convenience, the latter rapidly as other kinds of unicellular algae (Jitts et al., will be referred to here as the 'flagellate' ecosystem. 1964; Thornas et al., 1978; Brand, 1979). A reasonable Flagellates persist for longer periods of time, many are question, then, is what environmental conditions will known to be poor foods for most grazers, and the motile lead to a flagellate growth dominant over a diatom species are able to concentrate to undesirable concen- growth. Several possibilities exist, which include ini- trations due to their ability to swim and respond to tial seed population, trace elements and chemicals, light. Certain dinoflagellate epidemics, for example, sinking and motility, preferential grazing, and nutrient are serious pollution events that must be understood to concentrations. We should like to discuss one such be predicted and controlled. possibility - the importance of the nutrient silicon. O by Inter-Research 84 Mar. Ecol. Prog. Ser. 3: 83-91, 1980 Diatoms require the major nutrients nitrogen, phos- Table 1. Concentrations of inorganic nitrogen phorus and silicon for their photosynthesis; diatoms (NH; +NO~+NO~),silicon (SiO;) and phosphorus (PO~,and use silicon in approximately a one-to-one atomic ratio N:Si ratios in secondary sewage effluent (pg~l-l).Sources: (1) Garside et al. (1976),(2) Dunstan and Tenore (1972),(3) with nitrogen (Redfield et al., 1963). The flagellates Ryther (1977) associated with coastal eutrophication effects need only nitrogen and phosphorus, together with the trace Locations N Si P N:Si elements and micronutrients that all autotrophs re- quire. Tallman Island, NY (1) 1200 80 130 15 : 1 In temperate latitudes silicon in the form of dissolved Cranston, RI (2) 2166 110 213 20:l 100 295 13.1 silicate is usually supplied in plentiful amounts Warwick, RI (2) 1342 Plymouth, MA (2) 1176 70 243 17:1 through land weathering to the estuarine and coastal Otic AFB, MA (2) 1265 110 264 11:l zones. Nitrogen and phosphorus are also available Wareham, MA (3) 1205 173 142 7:1 from river runoff inputs including sewage, industrial and agricultural sources. When the solar conditions are appropriate, which usually occurs in late spring or confined estuarine or coastal water body could lead not early summer but also in midwinter in some shallow only to an increased plankton bloom but also to a estuaries, a common event is a diatom dominated change in the initial blooms from a diatom to a flagel- bloom. The organic nitrogen and phosphorus compo- late dominated population. nents of the diatoms will be recycled rapidly back to an inorganic form through zooplankton grazing, depend- ing on the size of the bloom and the zooplankton CONCEPTUAL RELATIONS population, and through respiration and decay. The silicon is largely confined to the tests, skeletal mate- The proposed nutrient cycle relations for a diatom rial, of the diatoms. The animals which feed on the bloom followed by a flagellate bloom are shown in diatoms have no use for silicon and reject it directly. Figure 1. In the discussion above we have considered a The tests dissolve and recycle silicon at a much slower riverine source of nutrients, which is the usual case in rate than the organic nitrogen and phosphorus compo- coastal eutrophication. We could alternatively have nents. We may, then, expect following the initial considered an oceanic source either of a continuous or diatom bloom period that a new nutrient pool will be an intermittent nature related to coastal upwelling or produced which is rich in inorganic nitrogen and phos- seasonal overturn, as shown in the figure. phorus but depleted in silicon. The relative amount of The fluxes, as shown by the arrows in the figure, silicon depletion in this second nutrient pool will, of depend both on the magnitude, or concentration level, course, also depend on the amounts and rate process of the nutrient involved in the given process and the relations between the organic nutrient cycling and the continuing river nutrient inputs. We now have condi- OCEANIC SOURCE or RIVERINE SOURCE tions suitable for a flagellate bloom, and depending on INTERMlTTEYT OR CONTtNUOUS R:VER RUNOCF N2TR:EN: FLUXES the nutrient levels this could lead to an excessive, TO CQASTAL UPWELLING OR eutrophic, growth. This sequence of events from a late spring-early N,D,Si r' summer diatom dominated bloom to a later surnmer- i ~HUTRIENTPOOL - N, P, ~i 1 I early fall flagellate dominated bloom is a common AWROcPIL-E SCLbR fNSDUT'3N, I WATER TEVDERATUQE A93 TUR- OEP.ETIOtI D' NU'PIENT PML. 4 occurrence in temperate coastal waters (Margalef, BlDlTY F09 BLOOMS. RAP10 OlATOU BLOOMS 1958). We suggest that this sequence from one popula- GROWTH RATES FOR Dlb'3WS I tion to the other is controlled by the silicon regenera- I N, P PARTIAL RECYCLING I 1, 1, GRAZING AN2RESP7RATION OF ,.I tion cycle. ORjAC.: FRACTION. OEF:CIINCY Also, it is important to note that sewage effluents OF SI which invariably contain high nitrogen and phos- phorus nutrient levels usually do not have a corre- APPROPRIATE SOLAR INSCLATION, DEPLETICN OC SECOND %UTRIENT WATER TEMPERATURE AND TUR 1 N, P P03-. FLAGE-LATE BLOOMS, sponding level of silicon, i. e. equivalent to nitrogen, as 3131-" FOR BLCOVS SLCWER Al3ED BY MCTIL'-Y Arit LOWEP required by diatoms. Table 1 is a summary of observed GROWTH RATFS F35 FLPGEL- NJTPIENT NEEDS secondary sewage effluents for the East Coast of the United States. The silicon levels are principally deter- NUTRIENT RELEASE, PRINCIPALLY mined by the land weathered, municipal water input BY RESPIRATION PNO DECAY levels. We would, then, expect that increased sewage Fig. 1. Proposed nutrient (N, P, Si) cycIes related to diatom inputs, whether primary or secondary treated, to a blooms and possible, undesirable flagellate blooms Officerand Ryther: Importance of silicon in eutrophication 85 rate constant for the process. In each case the flux may supply for continued phytoplankton production and be represented by a term of the form cV/t, where c is that this recycling process is rapid. Dugdale and Goe- the concentration of the nutrient involved in the proc- ring (1967) and Eppley et al. (1973) discuss the impor- ess, Vthe volume of water defining the nutrient pool or tance of this regeneration process in the euphotic zone plankton bloom, and zthe time constant of the process. of oceanic areas. Carpenter et al. (1969) argue that A summary of these time constants is given in Table zooplankton grazing is the determining process for the 2. In the original investigations from which these continuation of the steady state phytoplankton popula- values were taken, rate coefficients k, or doubling tion in Chesapeake Bay during the summer. From times Tm, have sometimes been given. They have all measurements of radiocarbon productivity they give a been converted to time constants z, for convenience in value of 0.2 mgl-' d-l of organic carbon being produced comparison, by the usual relation t = Ilk = T/0.693. in the euphotic zone during the summer, correspond- The time constants for diatom production under opti- ing to an uptake of 4 pg at 1-' d-' of nitrogen.