Nutrients in seawater — good and bad

My daughter has a problem with her aquarium. She tends to overfeed her fish which causes algae to grow, and if not cleaned, will cause the fish to die. It seems Nature likes a certain state of balance, and when this balance is disturbed, Nature will adjust to a new balance. From a human perception, sometimes these adjustments are for the better, or quite often as in the case of excessive food (nutrients) in my daughter’s aquarium, for the worse.

Nutrients are defined as substances needed for growth. They are, of course, present in food and from supplementary sources such as vitamins for animals, including humans. They are also found in soil and fertilizers for plants. Further, they may be present in organic wastes (such as sewage) and inorganic wastes (such as fertilizer runoff) that result from human production and consumption activities. In the marine environment, nitrogen and phosphorus are the "traditional" nutrients of interest. Typically in the marine environment, the limiting nutrient is nitrogen. Nutrients find their way to the sea from land via river discharge and non-point source inputs such as runoff from agriculture and unsewered areas. They can also be introduced through atmospheric deposition, as in the case of nitrogen.

Nutrients are good. Being food for plants and animals, nutrients are essential for life. Nutrients feed the phytoplankton (tiny plants) of the sea, the phytoplankton feed the fish, and the fish feed man. Intuitively, more nutrients should be great for the food chain. But then again, too much of a good thing could be bad. Here’s why.

Nutrients and human activities are linked. The continued increase in human population brings in attendant problems associated with human activities. With dwindling space for waste disposal in land, the sea has become a convenient repository. For example, domestic sewage, mostly untreated, often ends up in coastal waters such as Manila Bay. The organic matter in sewage decomposes and releases the inorganic constituents, among which are nitrogen and phosphorus, which now become available as food for the phytoplankton. Plenty of nutrients translate to more phytoplankton produced, often resulting in bloom conditions. The occurrence and die-off of blooms are actually part of the cycle of life in the sea. Increased frequency of blooms, however, quite often larger in magnitude, disrupts the balance and affects the cycle. Furthermore, the presence and abundance of toxin-forming phytoplankton, such as in red tide blooms, is a problem with negative impact on the economy and health of the populace.

Nutrients and fish kills are linked. When the phytoplankton bloom dies, decomposition sets in, requiring oxygen for the process to proceed. A large amount of organic material to decompose would require substantial amounts of oxygen, thus compromising the supply of oxygen in the water column. Fish need oxygen to live, and with low levels or no oxygen, they die. Massive fish kills have been associated with depleted oxygen in the water column. A case in point is the fish kill of February 2002 in Bolinao, Pangasinan that caused major losses in bangus production amounting to approximately P500 million. This fish kill event had been preceded by the occurrence of a bloom of phytoplankton Prorocentrum minimum, toxic to shellfish, and possibly to fish and man. Low oxygen, and quite likely, the phytoplankton bloom, may be attributed principally to the proliferation of fishcage and fishpen structures far exceeding the carrying capacity of the area. Furthermore, some of these structures were sited in places not designated as appropriate, for instance, in poorly flushed locations.

More than half of the protein requirement of Filipinos comes from fish. The demand for fish has been increasing but the ability of the capture fishery to meet this demand has been pushed to the limit as catch per unit effort has shown drastic declines in many fishing grounds. Fish farming through aquaculture (fish ponds) and mariculture (fishpens and fishcages) will need to play an increasingly significant role in fish production. A recent trend has been toward expansion of mariculture, particularly fishcages, in our coastal waters. This fast development, however, has lacked parallel investigation into the environmental impact of the activity. Intensive mariculture releases large amounts of nutrients to the environment in the form of wasted feed and fecal materials. These wastes have the potential to enrich the sea when they are located in semi-enclosed coastal basins that have restricted exchange with the more open coastal waters. Anticipated problems associated with the discharge of these wastes include algal blooms, oxygen depletion and fish kills. Isn’t this a macro view of what happens in my daughter’s aquarium?

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Dr. Maria Lourdes San Diego-McGlone is an associate professor at the University of the Philippines, Marine Science Institute. She is into chemical oceanography with interests in nutrient biogeochemistry and marine pollution. Her daughter Miya is doing better with her fish. E-mail your comments and reactions to [email protected].