Having determined that the Ulva was capable of affecting the pH, I moved on to using the three sources of water to do a bioassay with oyster larvae. I took four 10 liter samples each from the three tank system, the large tank system and from the incoming filtered HMSC water. I stocked each 10 liter sample with 50,000 larvae. I did periodic water changes, using water from the corresponding sources, and feed each sample with a calculated amount of micro algae. After 6 days I sieved the larvae out and restocked the samples with 10,000 larvae each to remove any excess dead to avoid fouling the water. After 10 days I again sieved the larvae out of the samples. I used a 60 micrometre sieve and put each sample into 800 ml of water, from that I took three 20 ml from each 800 ml sample. I then took photos, did counts and took measurements form each 20 ml sample.
In the samples taken from the filtered HMSC water, 48% of those found survived, and of those alive the average length was 116 micrometers across.
In the Three Tank System, 57% of those found survived, and of those alive the average length was 120 micrometres across.
There doesn't appear to be a significant difference between the filtered HMSC water and the Three tank system treatments. But...
In the Large Tank System, 83% of those found survived, and of those alive the average length was 150 micrometres across. This is a significant difference between the filtered HMSC water and the large tank system.
This bioassay is the conclusion to my research. it shows that there is a potential for Ulva to be used as a biofilter to improve the health and survival of oyster larvae.
~David L. Hubert