An article on reef balls, "Scientists hope to get Bay's oyster restoration rolling with reef balls," (December 2006) fascinated me.
The concrete balls are intended to promote the settling of benthic organisms, including oysters. "Before going in the water, 30 of the balls were placed in tanks filled with oyster larvae." After a year those balls "...had become so overgrown with oysters and colonizing organisms that they had become almost unrecognizable as reef balls."
In contrast "The balls that did not have larvae before going into the water still had no oysters growing on them." Why did the balls not exposed to larvae in the laboratory fail to attract oyster "strike" naturally?
Each year, I pile up the shells from the oysters grown at my pier that I have eaten. In early June, I throw the clean shell in the water at one location, a couple dozen shells at a time, with the hope of starting a reef. I have had no success despite there being thousands of oysters in floats nearby to provide larvae.
Why doesn't the clean shell attract natural strike? Last spring, I took clean shells, drilled a hole in each one, and suspended the shells concave-side down off my pier, adding two shells every three days. The photographs on this page show the result. After a little more than a week, the bottom two shells were densely coated, top and bottom, with "slime" or, as it is properly called, biofilms.
It is well known that oysters require clean substrate for spat to attach or "set." The classic book, "The Oyster," by William K. Brooks, published more than a century ago, clearly states this in several places. For example, on page 99 "The first thing found out was that the floating spawn would not attach itself to or 'set' upon anything which had not a clean surface; smoothness did not hinder-but the surface of the object must not be slimy." And on p. 116, to attract spat for commercial purposes "...it is important that the shells or other substances which are employed be perfectly clean, and that they be not put into the water until spawning has commenced."
The owner of a nearby marine railway told me that when he was young, he designed a scraper to remove the oysters and barnacles from the rails because they became so fouled it was difficult to pull a boat out of the water. He hasn't used the tool for many years because the fouling of the rails no longer occurs.
Other people, including his adult son, have told me that several decades ago they could pole a skiff around the shoreline of the creek where I live and harvest enough oysters for a meal off fallen trees. No oysters attach to fallen trees today.
Why don't oysters settle on the rails, the fallen trees and my clean shell?
Dr. Jeremy Jackson, a scientist at the Scripps Institute of Oceanography, has advocated the "rise of slime" as one of the consequences of nutrification (nitrogen and phosphorus pollution) of the coastal ocean. In his 2001 paper in the Proceedings of the National Academy of Sciences (Vol. 98, pages 5411-5418) he states "Today Chesapeake Bay is a bacterially dominated ecosystem with a totally different trophic structure from a century ago." Nutrification of coastal environments causes the prolific growth of suspended algae (phytoplankton) that cloud the water, die and settle to the bottom, creating anoxic (without oxygen) black muck, dominated by microbial activity. Biofilms, composed of both plants and the microbes that consume them (slime) are just another result of nutrification, as microbes colonize available substrate to decompose the over-abundant organic material. Humankind has changed the Bay from an ecosystem dominated by clear water, and inhabited by fish, crabs, oysters and the myriad of creatures associated with healthy oyster reefs, to cloudy water dominated by bacteria. The inhibition of oyster settling because of slime adds to the litany of human-induced activities, including overharvesting, habitat destruction by dredging and the introduction of a disease (MSX), that are responsible for oyster populations today roughly a percent of their original abundance.
The "rise of slime" is real, and provides a reasonable hypothesis for the different behavior of the reef balls. The problem is more likely the inability of spat to set, and not just too few spat in the water as the authors of the article suggested. The reef ball results suggest an important lesson about encouraging oyster settling. Much more success will be realized if the substrate is colonized before it can be covered in slime, which means before it is released to the environment.
The practice of settling oysters on a substrate prior to placing the substrate in the environment (spat-on-shell) is gaining popularity, and will likely be very useful for aquaculture and for repopulating oyster beds. But until the "rise of slime" is curtailed, natural oyster spat set will not improve.
Not only does the slime cover hard substrates, and prevent benthic organisms like oysters from settling, it also covers the leaves of what little submerged aquatic vegetation (SAV or sea grass) remains in the Bay and further restricts the amount of light the plants can receive.
The "slime" is caused by over-fertilization of the Bay because our government values agricultural profits, cheap food and low wastewater bills over satisfactory water quality.
Until nitrate and phosphate pollution is reduced by improved agricultural fertilization efficiency, along with upgraded wastewater treatment facilities, water quality in the Bay will not improve, the ability of oyster larvae to attach naturally will not improve and the Chesapeake Bay ecosystem will remain on a downward spiral, dominated by bacteria.