The seasonal variation of dissolved nutrients, chlorophyll a and suspended sediments at Nelly Bay, Magnetic Island

Pink shrimp (Farfantepenaeus duorarum) postlarvae were collected monthly with moored channel nets from October 1997 to June 1999 at Whale Harbor (WH) and Long Key (LK) Channels, two channels at the east and west ends of the Middle Florida Keys that connect the Atlantic Ocean with Florida Bay, the main nursery ground of this species in south Florida. The density influx of pink shrimp postlarvae through these two channels differed in magnitude and seasonal patterns. The monthly influx of postlarvae at LK was lower in magnitude but steadier than at WH. LK showed the highest postlarval influx in late spring–summer. Postlarval influx through WH further downstream showed both spring–summer and winter peaks. The spring–summer peaks correspond with favorable wind and current conditions. In contrast, the winter peaks occurred when cyclonic eddies were detected passing through the Middle Florida Keys. These cyclonic eddies induced coastal countercurrent flow that favors onshore larval transport. The position of the leading edge of the eddy relative to the channels may cause the temporal variability observed in the temporal postlarval influx patterns between channels. A seasonal size variation in postlarvae was also observed at both sampling sites. Postlarvae captured during the winter months were significantly larger than those captured during the summer months. The number of rostral spines, a morphological feature generally correlating with the number of moults and age, also varied seasonally but with a trend opposite to that of the body size, which may indicate a lengthening of the intermoult period during wintertime. The reasons for the seasonal differences in growth rates and, possibly, different lengths of time in the currents, are yet to be resolved.