Document Type

Conference Paper

Publication Date

1991

Publication Title

Proceedings of the Back Bay Ecological Symposium

Pages

11-19

Conference Name

Back Bay Ecological Symposium

Abstract

All available salinity and water clarity data for Back Bay, Virginia were edited for this manuscript. Quantitative salinity records commence in 1925. These are comprehensive and extend to 1989 except for a major interruption in the 1940's and 1950's. Quantitative water clarity records (Secchi disc visibility) commence in 1959 and are continuous to 1989 with only a few years missing. Since 1925 the water in Back Bay has fluctuated from fresh (less than 0.5 ppt) to brackish (generally oligohaline, 0.5-3.0 ppt). Fresh to slightly brackish (less than 1.0 ppt) conditions existed from the late 1930's to early 1962, from 1975 to late 1978, and in 1989. For the remainder of this 65-year period, the salinity generally ranged from 2.0 to 4.0 ppt. The higher salinity periods were 1933-34, 1936 and 1962. These were caused by voluminous intrusions of ocean water induced by hurricanes or northeastern storms. Secchi disc visibility was generally 20 to 30 inches from 1959-60 and 1965-80. During most of this period, the frequency of occurrence of submerged aquatic vegetation (SAV) in transect samples was more than 50%. From 1981 to 1989, water clarity greatly deteriorated with Secchi disc readings of only 6 to 12 inches. This increase in turbidity is attributed to the suspension of soil particles by increased wave action following a decline in SAV. Since 1980 the frequency of occurrence of SAV has been less than 5%. Without rooted, aquatic plants to stabilize the substrate, the sediment is kept in suspension by wave action.

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