Graduate Thesis Or Dissertation
 

The effects of sewage on the distribution and abundance of benthic animals in a Puget Sound bay

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  • The effects of discharged sewage on physico-chemical conditions and the distribution and abundance of marine benthic animals were studied in Shilshole Bay, a part of Puget Sound, off Seattle, Washington. For over 50 years prior to 1965, when this study was initiated, 87,300 pounds (32,400 pounds BOD) per day of mostly raw, domestic sewage with some undefined industrial waste and storm drainage had entered the Bay via a submarine outfall. As a result of the prolonged period of discharge, different habitat types were established. These ranged from relative clean sediment to the sludge bed itself. Four areas representing the two extremes and two intermediate conditions were selected and sampled, first in 1965 when sewage was being discharged and then in 1968, three years after discharge ceased. In the water at the sediment-water interface, salinity, temperature, pH, dissolved oxygen, hydrogen sulfide, chromium, and dissolved organics were not found to be responsible for the observed differences of animal distribution and abundances. Current velocity and amount of inorganic material were interrelated in that the former prevented or allowed the latter to settle and seal off the sediment from the water column, thus helping to drive some sediments anoxic. Although relatively low in concentration, copper and lead may have had some subtle effects on the animals, especially larvae or sensitive species. In the interstitial water, salinity, temperature, pH, and ammonia were similar at the four sampling stations and were not important factors in determining the distribution of the animals. Very low oxygen levels in the sediments may not have been of much direct importance, but resultant increases in hydrogen sulfide at the two most polluted stations probably affected many species. The amount of organic material in the water column did not appear to be of direct importance in determining the distribution and abundance of animals. Upon settling, however, it was probably the greatest factor in determining different habitat types and affecting other environmental variables, which collectively determined which species would be present. A reasonable variety of infauna (83 species exclusive of nematodes) was found in 1965 among the four sampling sites in Shilshole Bay, but adjacent Puget Sound areas exhibited a greater number of species. Some species were present in the Bay in extraordinarily great numbers, this suggesting enrichment had favored these species. In 1965 the intermediate condition sampling station, closest in enrichment to the clean site, exhibited the greatest number of species and the highest total biomass. On the basis of number of species and total biomass, the sludge bed exhibited the poorest biota. Species number and total biomass in most areas were lower in 1968 than in 1965, presumably due to overall loss of organic enrichment.
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