In a video regarding shellfish farming, the now infamous “they clean the water” claim is made. Two 1-gallon glass containers are shown side by side. They each contain seawater and “300,000 cells of algae/mL.” One container holds just the seawater and the other includes 60 manila clams suspended in a net. After 28 minutes, the water in the container with the manila clams is obviously clearer.
The Water’s Clearer – But Is it Cleaner?
Actually the water was never “dirty” and the clams are not cleaning anything – they are eating. Like all filter feeders, they eat phytoplankton (minute plants). They also ingest other suspended matter including minute animals (zooplankton), fish larvae and other larvae, and anything else small enough to pass through their siphons.
To rephrase an old saying… “What goes in must come out.” That’s right, they poop! Scientists don’t call it poop – they call it biodeposits (the process is called biodeposition). The biodeposits contain feces (waste from digested plant matter) and pseudofeces (dead zooplankton and other non-digestible matter).
What is not shown on the video is the bottom of the tank where biodeposits would have accumulated. What happens to these biodeposits? It really depends on the amount of accumulated biodeposits. At low density they are decomposed via an anaerobic bacterial process (one that does not use oxygen). This process also converts dissolved nitrogen into nitrogen gas that bubbles up and into the air (called denitrification). This is a positive process since one of the issues in our saltwater areas is an excess of dissolved nitrogen.
However, if the density of biodeposits gets too high, the “good bacteria” are overwhelmed and the decomposition changes to an aerobic process (uses up dissolved oxygen in the water column). Instead of denitrification, oxygen is depleted and nasty stuff like Beggiatoa is formed. Scientifically speaking, “Beggiatoa can grow chemoorgano-heterotrophically by oxidizing organic compounds to carbon dioxide in the presence of oxygen.”
So the question of positive impact versus negative impact depends on the density of the biodeposits. The density of biodeposits is related to the density of animals and the speed of the current. Shellfish planted in low-densites, and in high current almost never cause problems. However, aquaculture often places animals in densities 10 to 100 times greater than found in nature. In addition, shellfish are often grown in protected coves and inlets that have very slow currents. Thus the likelihood of high-density related biodeposition problems associated with aquaculture is significant.
Sharing the Wealth
As the video points out, filter feeders eat a lot phytoplankton (an adult oyster can filter 65 gallons/day). Unfortunately, filter feeders are not the only consumers of phytoplankton. The truth is EVERYTHING in the sound is dependent, either directly or indirectly on phytoplankton to survive. This ranges from microscopic zooplankton, up to and including the Orca.
It is common to hear shellfish people (and some state agency people) say that there is an abundance of phytoplankton in Puget Sound. They point to algae blooms as evidence of this. The truth however is there is no science that we know of that substantiates this claim. Yes, there are algae blooms in some parts of the sound during some times of the year, but this really isn’t the issue. The issue is what happens in small, protected coves and inlets where currents are low, when the shellfish farmer places 10 to 100 time more animals than found in nature on the shoreline? What happens to the food web in these areas? What if these areas also support the spawning activities of forage fish (including their larvae).
Summary
“Cleaning the Water” is a misleading public relations term that the shellfish industry has used for decades to justify its expansion. It is time for regulators to sit up and take into account the density and placement of shellfish operations to insure that the shoreline ecosystem is not harmed.
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