Effects of flood waters on the Planktonic Community of the Hellshire Coast, Southeast Jamaica

Between 24 May and 4 June 1986, the island of Jamaica experienced up to 635 mm of rainfall producing island-wide flooding. Planktonic communities along the south coast were studied before and after the flooding. Flood waters from Kingston Harbour flowed southwest along the Hellshire coast as far as Wreck Reef, but were confined within 4 km of the shore. Water in the bays along the lower Hellshire coast remained clear and unaffected by Harbour water, although planktonic communities there showed marked changes. The Port Royal Cays area and much of the shelf was unaffected. During the flood, the normal gradient of decline in planktonic indices (biomass, abundance, and community similarity) with distance from the Harbour was disrupted. The extent of flooding was evident from dramatic reductions in salinity and increases in extinction coefficient; temperatures were lowered by only 1 to 2$^\circ$C. Flood effects were confined to the upper water column; whereas, surface salinity and phytoplankton were dramatically changed, they remained largely unaffected at 7 m depth. In surface waters, the number of phytoplankton cells increased up to 4 \times 10$^6$ per liter (5 times greater than normal) while biomass increased to 30 mg Chl. $a$/m$^3$ (10 times normal) and primary production reached 20.5 mg C/m$^3$/hr (20 times normal). These large increases in both biomass and primary production occurred primarily in the nanoplankton size fraction (2-20 $\mu$ m) which was dominated by Protoperidinum spp. Within a week however, the larger net plankton (>20 $\mu$ m) assumed dominance. These high values were short-lived as phytoplankton communities returned to normal within two weeks of the flood. Zooplankton abundances at first decreased sharply in response to flooding (e.g.. from $\sim$20,000 to 200/m$^3$), but reattained normal values within four weeks following the flood. In some areas (e.g., Harbour mouth, Great Salt Pond) there was a subsequent 10-fold increase in zooplankton abundance (up to 209,000/m$^3$) comprising mainly larval stages. Thus, return to nonflood conditions was achieved at different rates for the various physical and biological parameters.