The water quality control system designed for the Monterey Bay Aquarium Sea Otter display consists of re-circulating display water through a pair of high-pressure sandfilters at a rate of 500 gpm and through an ultra-violet sterilizing unit rated at a maximum flow of 800 gpm. Return water quality is monitored weekly for coliform content and routinely measures coliform bacteria at less than 2 MPN (most probable number). Incoming or new seawater is added continuously to the display at a rate of 250 gpm, the measured coliform content for MBA intake water is also less than 2 MPN. Surface skimming removes water from the system at a rate equal to the incoming new water.

The combined flow of these two systems is calculated to be 750 gpm which give a theoretical turn over once every 1.1 hours for the 50,000 gallon display. Weekly analysis for coliform content of the display water were started in October 1984, when the display first received three sea otters. The results from the early coliform water tests indicated that a potential problem existed in water quality control. On three separate occasions, one each in January, March and June 1985, coliform counts between 1000-2400 MPN were received. (Maximum allowable coliforms not to exceed 1000 MPN, USDA regulations). In spite of weekly cleaning by draining and hosing the tank down with seawater and physically removing all remaining food particles and organic debris, coliform counts as high as 2400 MPN were being reported. To better understand the problem, a brief description of the display tank is needed.

The display is 15.3 feet deep and 29 feet long and 23 feet wide at its widest points and has a water volume of approximately 50,000 gallons. The backwalls and portions of the bottom of the display are covered with artificial rockwork. In addition to the artificial rockwork, large boulders cover the sub-flooring, a fiberglass grate which extends over the bottom and is raised four inches from the actual bottom surface. Two conical shaped floor drains lead to the filter-sterilization system. Incoming and return water is added through manifold system designed to mix the water and created a circular current.

Due to our desire to raise subtidal kelp and algae in the otter exhibit direct cleaning of the rock surfaces was restricted to seawater sprayed onto the rockwork. The development of an algal turf on the rock surfaces did not succeed. Diatoms, filamentous algae and some red algae did grow but were not considered desirable.

Several theories exist to why we were experiencing water quality problems; the addition of a fourth otter in the summer of 1985; high food consumption, 14-18% of their combined body weights; and the possible retention of coliform. Type bacteria on the algal mat and rock surfaces were considered as potential contributing factors.

The extent of the problem did not become known until spring of 1986. The weekly test reports in March and continuing through June exceeded the maximum concentration allowed by the USDA. Reports of 1600-9200 MPN being common in spite of daily cleaning and refilling the display. At this time the combined weights of the four display sea otters was 167 lbs and they were eating 24 lbs per day of food or 14.4% of their body weight.

During the period of time that the coliform counts were exceeding the maximum allowable number the return water was being tested and continued to be less than two coliform MPN. In spite of filtration-sterilization the coliforms counts continued to increase. The original water system design took into account the organic and bacterial load that would be encountered in keeping four adult sea otters in a display of this size. Additional contributing factors were examined.

A theory that the high coliform counts were directly related to the algal mat was proposed. A new tank cleaning procedure where sodium hypochlorite, 12.5% active chlorine, was sprayed onto all rock surfaces and left for several hours to re-act with the organic coating. The tank was then washed down with a high pressure hose, the chlorine neutralized and the tank drained, re-filled and flushed overnight. This procedure resulted in lowering the coliform counts to levels well below the maximum allowable number.

This procedure was modified from direct spraying of sodium hypochlorite to adding it directly into the display tank water. Thirty gallons of 12.5% active chlorine solution is added and allowed to circulate through the entire system including the sand filters. The concentration of chlorine is calculated to be approximately 30 ppm. After a two hour exposure time, the chlorine is neutralized with sodium thiosulfate, the tank emptied, hosed out with a high pressure hose, rinsed and then re-filled. The sea otters are returned to the exhibit the following morning. This procedure has been in effect since June 1986, on three occasions during the next year, or 3 out of 52 weekly water samples, coliform counts exceeded 1000 MPN. Between June 1987 and March 1988 four coliform samples exceeded 1000 MPN. During this last period of time, October through March, the cleaning schedule was modified from weekly to Bi-weekly as a test.

It is interesting to note that the high coliform counts for the last three years have occurred during early spring. It is during this time period that the Sea otter display starts receiving extra sunlight and nutrient levels as the incoming seawater increases. Consequently, it is during this time period when we first notice a sharp increase in diatom and filamentous algae growth, a time marked by increased sunlight and increased nutrient levels in the incoming seawater.