ZOOPLANKTON AND SOME ENVIRONMENTAL CONDITIONS IN THE BOT RIVER ESTUARY

SUMMARY A zooplankton study was commenced during July 1980 while the Bot River estuary was closed off from the sea, and had a mean surface salinity of 13,5%. The latter declined to 2,3‰ in September 1981 before the estuary was opened to the sea at the Sonesta Holiday Resort, after which it increased again to means of, respectively, 28,9 and 30,8‰ during November and December 1981, the last two months of the survey. Zooplankton numbers and biomass fluctuated seasonally with maximum mean values occurring during the spring of 1980. It would appear that the estuarine zooplankton population in the Bot River estuary was adversely affected in terms of both numbers and biomass by the decrease in salinity prior to the opening of the estuary at Sonesta. Zooplankton composition, however, changed relatively little. The nauplii, copepodites and adults of the calanoid copepod Pseudodiaptomus hessei dominated numerically, together forming an average of approximately 90% of the zooplankton during the study. Only two marine zooplankton organisms, Oithona similis and Paracalanus crassirostris, were collected in the estuary, appearing after it was opened at Sonesta. The prevailing salinities in the estuary prior to its opening at Sonesta appear to have inhibited the breeding of an important estuarine bait organism, the sandprawn Callianassa kraussi.