CONDITIONS THAT STIMULATE HARMFUL AND TOXIC ALGAL BLOOMS
Particulates of decayed phytoplankton may sink several meters a day. These particulates are mineralized by bacterial activities before they are deposited to the ocean bed. The nutrients released by decomposition of these particulates can drift below photic zone. For phytoplankton forming red tides, therefore, eutrophication of surface water by supplying nutrients, especially phosphorous is necessary. Eutrophication in the open sea is affected by upwelling whereby transport of deep water to the euphotic zone is driven by a force mainly origi-nated from an eddy, enriching the coastal water column and initiating plankton blooms.
This is the amount of plant nutrients introduced with river discharge, effluents from aquaculture farms or a terrestrial run off from heavy rains that might provide enough nutrients to stimulate phytoplankton blooms.
The increase in activities of sulfate reducing bacteria due to ascending tem-perature of bottom water that will lead to the production of large amount of H2S, liberating P043- from Fe bound P in the sediments, could induce phy-toplankton red tides.
Surf zones are reasonably broad and shallow, cellular circulation predominates that tend to retain nutrients generated by the macrofauna and interstitial mi-crofauna of the beaches. These nutrients may then cause blooms of surf zone phytoplankton, which in turn serves as food for macrofauna filter feeders. With the perimeters of the circulation cells of the surf zone forming its marine boundary, beach plus surf zone may be denoted as an ecosystem with surf
phytoplankton as the primary producers, beach macrofauna the consumers, and the interstitial fauna the decomposers.
In brackishwater, N2 fixation by the blue-green algae takes place. But the fixed N is mostly retained by the nitrogen fixers themselves, forming blooms. N2 fixation decreased with depth in response to light, although other factors are involved. Rates of fixation decrease concurrently with bloom age, total soluble inorganic nitrogen and salinity. Maximum daily fixation occurs early morning.
Unlike most centric diatoms studied, none has an absolute Vitamin B12 requirement. However, Vitamin B12 (5 mg l-1) stimulates growth of most clones by eliminating or shortening the lag phase and increasing the growth rate. High
population densities developed 4-54 days with Vitamin B12 present. Several clones grown with Vitamin B12 removed more than 80% of the Vitamin from the medium. Clearly, B12 is important for quick growth and high density of the
Stratification of water prolongs the residence period of the surface water, allowing enough growth for the red tide algae. But sometimes stratification inhibits growth because of lack of ingress of nutrients from outside. Thus, destratification, in some occasions, favors introduction of nutrients from outside and indirectly causes red tides.
Onshore winds carry offshore surface waters to the coast. The buoyancy of the surface water maintains phytoplankton at the surface for a longer period, causing dense blooms along the coast where terrestrial nutrients have been furnished.
Downward migration of phytoflagellates appears essential to maintain red tide blooms in estuarine embayment having intensive tidal flushes.
Grazing pressure due to zooplankton on phytoplankton is regarded as a great deterrent of phytoplankton blooms. Sometimes the grazing pressure by macrozooplankton depletion is exhibited on zooplankton, resulting in zooplankton
depletion. Occasionally, zooplankton will avoid certain species and reduce grazing response to Gymnodinium splendens. Consequently, such phytoplankton grows massively, forming red tides.
These behavioral responses may help explain formation and persistence of dinoflagellate blooms such as red tides in coastal waters often dominated by diatoms with higher maximum growth rates.
In summer, phytoflagellates and diatoms stay temporarily in sediment as resting cells such as hypocites or cysts. In spring and for autumn, their germination is initiated with increase and or decrease in sediment temperature, and their swimming cells appear in the upperlying water.
Marine phytoflagellates and diatoms grow in marine waters with fairly high salinity and also in brackishwater with considerably low salinity. In coastal waters, estuarine surface waters are enriched with nutrients by river water,
resulting in lower salinity and consequent phytoplankton stress. Species which can tolerate the stress may grow utilizing these nutrients and develop into red tide blooms. Table 8-1 shows the tolerance of phytoflagellates and diatoms to lowered salinity stress.
Evidently, diatoms can tolerate lower salinity levels more than the dinoflagel-lates. They exploit this difference in utilizing nutrients in coastal waters. This may account for the succession of dinoflagellate blooms.
Diatoms can make their frutules with less amounts of silicate than usual when silicate in external media is depleted. Dinoflagellate blooms have so often ap-peared after diatom blooms that there must be a certain rule of succession between diatoms and dinoflagellates.
In waters containing metals in very low concentrations, phytoplankton accu-mulates these metals 103–104 times higher than they would in ambient water. This might decrease the grazing pressure of zooplankton on phytoplankton having a high content of heavy metals.
Ciliates called “Mesodinium” rubrum cause red tide and may die under poor nutritive conditions. However, not all of these ciliates may die, some of them may adapt themselves to the said situation, and settle to the bottom for some time. When there is a high supply of nutrient due to eutrophication of marine waters, these ciliates will utilize the nutrients for their growth and eventually will grow dense.
Reports of harmful algal blooms associated with human illnesses or damage toaquaculture operations are receiving increased attention in newspapers, theelectronic media and the scientific literature. As a result, more and more re-searchers are surveying their local waters for the causative organisms.
Aquaculture operations act as sensitive bioassay systems for harmful algal spe-cies and can bring to light the presence of problem organisms in waters notknown to exist before. The increase in shellfish farming worldwide is leading to more reports of paralytic, diarrhetic and neurotoxic or amnesic shellfish poi-soning. On the other hand, increased finfish culture is drawing attention toalgal species, which can cause damage to delicate gill tissues of fishes.
While some organisms such as the dinoflagellates Gymnodinium breve, Alexandrium and Pyrodinium appear to be unaffected by coastal nutrient en-richments, many other algal species appear to be stimulated by cultural eutrophication from domestic, industrial and agricultural wastes.
The coincidental occurrence of Pyrodinium blooms and El Niño-Southern Os-cillation (ENSO) climatological events presented strong circumstantial evidence on the possible impacts on algal bloom. El Niño is caused by an imbalance in atmospheric pressure and sea temperature between the eastern and western parts of the Pacific Ocean, which results in a shoaling of the thermocline. The 1991-1994 ENSOevent and the recurrence of dinoflagellate blooms in the Philip-pines tend to substantiate these claims.
Cargo ballast water was first suggested as a vector in the dispersal of non-indigenous marine plankton some 90 years ago. However, in the 1980s theproblem of ballast water transport of plankton species gained considerable in-terest when evidence was brought forward that non-indigenous toxic di-noflagellate species had been introduced into Australian waters including sen-sitive aquaculture areas, without disastrous consequences for commercial shellfish farm operations. Another vector for the dispersal of algae (especially their resting cysts) is with the translocation of shellfish stocks from one area to another. The feces and digestive tracts of bivalves can be loaded with viabledinoflagellate cells and sometimes can also contain resistant resting cysts.
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